Why is the maximum speed of current fighter aircraft lower than those from the 1960's and 1970's? While reading about the characteristics of the newest fighter aircraft, I observed that their maximum speed is lower than of the aircraft in the same roles half a century ago.

For example, the iconic F-14 and Mig-25 have a higher maximum speed than their modern equivalents.

After the Cold War ended, the major air forces are only engaged in ground support and bombing against low-tech opponents who use asymmetrical warfare, and no modern scenario would see the top fighter aircraft directly engaging each other any time soon. However, for any fighter aircraft in the interceptor role, I would expect speed should be an important characteristic. Also, military aircraft are designed and introduced over a long time period, and are expected to be used for many decades.

Why did maximum speed become less important? Wouldn't it be easier to design even faster fighters now than with the technology available 50 years ago?

Speed was life in air combat until the jet age. The pilots who became Air Force generals in the Fifties had learned their trade in the Thirties when speed was the most-desired quality in a fighter. Naturally, the ability to be faster than any adversary was very important to them. When the requirements for new fighter aircraft were written in the Fifties, those generals made sure that a higher top speed was part of the specification.

When those supersonic-capable aircraft were used in real conflicts, something very surprising and unforeseen happened: They did hardly ever fly supersonic. When the Air Force in the late Sixties accumulated the flight data from several years of Vietnam war air combat, they found that all aircraft had accumulated just minutes at Mach 1.4 and only seconds at Mach 1.6 out of more than 100,000 combat sorties¹. Never was even Mach 1.8 flown in aircraft which had been optimized for Mach 2.4 (F-104, F-105, F-106A, F-4D/E and F-111).

To cite from this study why speed stayed mostly subsonic:

  The first of these reasons lies in the shape of the turn rate vs Mach number relationship for an aircraft. […] In combat, each pilot has the tendency to to fly his aircraft so as to maximize his turn rate. He thus gains angular position on the enemy which, in turn, may permit a missile launch or a gun firing. […] It can be seen that the pilot's urge to maximize his turn rate will unfailingly drive his Mach number to about 0.7. Thus, if the pilot is going to join in combat, […] his speed will inevitably drop to subsonic speeds. […]
  Note also, that even if the turn rate is held constant while increasing the speed, […] the turn radius and load factor increase, bringing with it increasing problems of keeping the enemy in sight.

The second reason given in the study is the dramatically smaller combat radius (the maximum distance the airplane can travel from its base, accomplish an objective and return) once the aircraft starts to fly at supersonic speeds. Even for flying into the combat arena supersonic speed was rarely advantageous. Northrop studied a multitude of intercept cases and found that speeds above Mach 1.1 were almost never helpful because they curtailed the combat radius severely.

Now you need to know that the top speed is an important driver in airframe design. Flying at Mach 2+ requires heavy and complex intakes, a heat-resistant structure, high wing sweep and heavy, low-bypass engines. This all degrades the combat qualities at high subsonic speed, which was where those aircraft were used the most. Building into them the capacity for Mach 2+ made them worse for what they were actually used for.

From the late Sixties on this lesson was incorporated into the newer designs like the F-16. Stealth again decreased the importance of supersonic capability, and the maximum sustained speed of the F-22 was actually reduced from Mach 1.8 to Mach 1.6 to reduce the heat load on the composite wing leading edge.

¹ Source: Northrop F-5 case study in aircraft design, AIAA Professional Study Series

Maximum speed is not the best speed to fight. Two major factors in a fighter must be taken into account:

Wing loading.
Thrust-to-weight ratio (T/W ratio).

Wing loading is the weight of the airplane distributed across the total area of the wing. This tells the aircraft's ability to turn sustained.

The T/W ratio is the amount of thrust developed by the engine divided by the weight of the aircraft, and this will tell the acceleration the aircraft is able to achieve in level flight. When both things are taken into account, you get a second parameter that is the aircraft ability to climb (climb rate).

When air war developed in the WW1/2, one rule of thumb for fighter pilots was that speed/altitude was life. If you examine the aircraft as a physical system of energy, you have two kinds of energy at your disposal: Kinetic energy (the actual rate of aircraft movement) and potential energy, the altitude of the aircraft. When you dive, you convert potential energy into kinetic energy. When you climb you turn kinetic energy into potential energy, until the point where your engine is the only thing that is inputting energy into the system (so you reach the stage where instantaneous climb rate ends and you start to see the sustained aircraft climb rate).

When your aircraft turns, it will be slowed a little bit, and the increase in induced drag generated by the turn will decrease your speed slowed. In the fighter pilot jargon, you are burning energy. There are two (again) kinds of turn, instantaneous and sustained.

The fighting styles where divided into two kinds: Turn and Burn and Boom and Zoom.

Turn and burn means that both fighters try to outturn each order in order to get into their six o'clock position in order to fire, it's a horizontal fight in circles. Boom and Zoom is a climb fight where the higher positioned aircraft dives into the other in order to make a pass, fire and then climb again in order to not lose airspeed (and thus energy).

Quickly the boom and zoom style was adopted. Experienced pilots only entered combat when sure to have higher altitude than their enemies, because altitude was a reservoir of energy to be exploited in combat.

A turn and burn styled fighter/pilot would quickly slow down and be unable to either attack or flee from a higher positioned aircraft.

So far so good.

After the war, all efforts went on to produce ever faster aircraft and aircraft able to reach high altitudes quickly. This has dual advantage of giving a better high altitude position and allowing such fighters to engage incoming bombers.

All changed when better missiles and onboard radars became available. Not that current fighters don't need to be able to dogfight, but because the missile became the major combat element. Two kinds of missiles come to my head at this stage. AAMs and SAMs.

The increasingly powerful radars (even onboard) meant that aircraft might very well attack things that are above it, meaning that the boom and zoom tactics might go wrong. Secondly, a radar allows one to see the enemy way before visual contact is made, giving more time to climb to a similar altitude, etc.

If you are flying very high, you will be seen on enemy radar far away, and missiles directed by those radars will come from various directions. To enter enemy airspace, you need to stay low, where air resistance is bigger and most aircraft can't reach the stated figures of speed. (See F-111 or Su-24 for example).

Bombers won't be flying at stratospheric levels like B-29 anymore, due to the same reasons. They need to enter enemy airspace at low altitude in order to have a chance of surviving. If they use standoff weapons, those will fly low too. So, if you are escorting a bunch of bombers in a mission, or you are trying to intercept those, they will not be very high.

When a onboard radar tries to lock against a low flying aircraft, there is a problem. Earlier radars where not able to do so at all. More modern versions (called look-down/shot-down) are able to lock a target against terrain, but not with the same performance as when they are looking up. Clutter (false echoes) is removed via software. Infrared detectors work better at high altitude (where air is colder and a hot fighter is more visible), than against the hot ground below, and so on.

So, all in all, fighter combat in intensive theaters will be going to be done at middle or low level, where speed is slower due to higher drag. Even then, all fighter aircraft have something called corner velocity, where the aircraft is able to sustain their most tight turns, and those speeds are usually far from being top speed.

Another factor is engine consumption. Only recently did supercruising aircraft came to be operational. Forth-generation aircraft where not able to break the sound barrier without using afterburns, which both increase aircraft detectability (due to increased heat signature) and fuel consumption.

The same aircraft that might travel 1000 km without using afterburners, might very well be unable to do more than 300 km while afterburning all along.

All things taken into account, an aircraft with more missiles, better radar and longer loiter time is preferred above fighters like the British Lightning or the F-104 which had a very high speed and climb rate, but were short-legged and lugged only a pair of missiles.


What happens if you take off with a direct tailwind? What happens if you take off with a direct tailwind?

Given how few bounding criteria you have in your question, the general result is

For fixed wing aircraft: your ground speed will be significantly
higher than if you'd taken off heading into the same wind, and thus
you'll tend to use up more runway before lifting off.

Example: with a take off speed of 100 kts, going into or downwind is the difference between 90 knots or 110 knots ground speed before reaching take off speed when the wind down the runway is 10 knots.  This difference becomes more pronounced as wind speed increases; it's a differential of 2 x wind speed.

With a long enough runway it won't matter in terms of "taking off before running out of runway."  On a short field it may matter a great deal.
For Rotary Wing aircraft: you may need a bit more power to hover when
lifting off with a tail wind.  The FAA has this guidance:

  Headwinds are the most desirable as they contribute to the greatest
  increase in performance. {snip} When taking off into a headwind,
  effective trans lational lift is achieved earlier, resulting in more
  lift and a steeper climb angle. When taking off with a tailwind, more
  distance is required to accelerate through translational lift.

For a variety of aircraft types, if the tail wind is high enough your manual will recommend against/prohibit down wind takeoffs, but that varies enough between models and types that a general answer can't encompass all of the variations.

The other answers and comments are ignoring an effect that usually does not show up in practice, but that it exists and I have witnessed first hand: as long as the air flow on the frame is reversed, so is the effect of the commands.

For take off, the effect diminishes as the aircraft gains speed, so it will probably only affect gliders and other light aircraft that rely heavily on the commands since the start of the run. On landing, the effect increases as the ground speed diminishes, and I have witnessed a serious crash (straight against a fence that run at the side of the runway) because of it.


How do civilian pilots and ATC verify that other people on the radio are who they claim to be? I am interested in authentication - how identity is verified - in civilian aviation radio transmissions. I know that when someone is speaking on the radio he tells his call sign. But is there any other system that verifies the identity of the person speaking besides just the call sign that he claims to have?

I am interested not only in how ground ATC verifies which aircraft is speaking, but also in how aircraft ensure that they are speaking to a real ATC facility.

This is a reasonable question, and to an outside the simple answer of We don't / can't might seem a little bit jarring.

After-all, we live in a time of hacking, terrorism and so on and I can certainly see how this looks like a simple vector to create some havoc.

So, what's to stop somebody from setting themselves up with a Radio Transmitter and deciding to be Air Traffic Control for the day? Absolutely nothing. In fact, lower power handheld air-band transceivers can be purchased legally and cheaply by anyone. I have one near me - I could literally turn it on now and declare myself [Local Airport Tower]

However - the thing about Airband radio is that it's wonderful in its simplicity. At its core is AM radio which is basically unchanged since the early 1900's. This means a few things:

The strongest signal doesn't simply "win". Multiple medium strength signals will simply merge into a garbled mess. This means you don't simply get to "take over" ATC's transmissions - all you can do is ruin them.
Similiar to the above, there's no way (from outside) to stop the ATC signal. All you can do is stomp all over it - something which is totally noticeable.
Even if you do "win over" air traffic control for some aircraft, communications is a two way process, and airband radio is simplex. This means only one person can speak at a time - at some point ATC are going to key in.
Pilots aren't automatons. We listen and build up a picture, we learn normal procedures and can hear stress in somebodies voice. We also know who we were speaking to. Combined with a decent situational awareness, we don't just do things without thinking - you can't just pipe up on the radio and ask an aircraft to fly into the ground.
And even if you did, and they initially believe you, real ATC is going to hear your readback and ask what the hell is going on.

By way of analogy, imagine you've  snuck in on a conference call between an employee and a boss. And you start trying to speak on behalf of the boss - it's going to last all of 1 second before somebody realises something is going on. I think people tend to think of ATC as somewhat robotic, but it's still a human interaction and any good pilot will understand why they're being given instructions.

To attempt to interfere in Air Traffic Control in a way that is believable by the pilots is going to be incredibly difficult. You won't be able to use normal phraseology and cadence if you're attempting to step on real ATC, and that will concern the pilot. To do that for more than a few seconds and to to cause some kind of conflict with multiple aircraft is verging on the impossible.

The most likely outcome is more a denial of service, rather than some kind of spoof. It's not really a great vector to cause any damage. A nuisance, certainly and absolutely dangerous (and there's a reason it's very illegal!) but there are well established procedures for disruption in communications.

I've focused primarily on somebody trying to be ATC here, which is the way in which somebody on the ground would likely go about trying to make a mess. As for pretending to be a plane - firstly there are more technical difficulties because you'll be transmitting Ground -> Ground. I live within 2 miles of my local airport and often struggle to receive their transmissions when I'm at home.

Pretending to be a real plane that currently exists in the sky poses the same problems as above and any facility with RADAR is going to know something isn't right. Again, it might cause some initial confusion but you're unlikely to persuade somebody of something.

You could, in theory, pretend to be a light aircraft with no (or a faulty) transponder but really to what end? I guess you could cause a little disruption before somebody realises you're having a laugh but we're more into the realm of 'pranks' (Not that it's funny) now rather than serious risk.

Civilian pilots are identified and tracked in the same manner an any other pilot in controlled airspace - radio communications.  Certain airspace (class E and G) do not require radio communications with ATC.  If no flight plan is on file or activated, they will be unknown targets.
Another means to identify aircraft are, enter establishing ATC communications, they are assigned a specific transponder code which then is associate with their radar returns, making it easy for controllers to know who’s who.


Measurement discrepancy on vfrmap dot com sectional -- is the grid wrong? At Nome Alaska-- KOME-- the E2 airspace (dashed magenta line) is listed as extending 13.2 (nautical) miles east of the airport-- according to this source page E-9 -- https://www.faa.gov/documentLibrary/media/Order/JO_7400.11C.pdf  --

"Within a 4.1-mile radius of the Nome Airport and within 3.4 miles each side of the Nome
Airport 106° bearing extending from the 4.1-mile radius to 13.2 miles east of the airport..."  'AMENDMENTS 07/07/05 70 FR 22590 (Revised) Corr: 70 FR 33349""

But when I measure on the online sectional chart http://vfrmap.com/?type=vfrc&lat=64.513&lon=-165.444&zoom=10, using the tic marks on the lines of longitude (the lines running north and south) as a scale equal to nautical miles,  the far edge seems to measure more like 15.5 nautical miles from the middle of the airport symbol (as shown by the little blue square) -- now why would that be?  That's about 20% further -- about 2.3 miles different -- not insignificant.

What is going on here?

Is the scale wrong, or is the dashed magenta line shown in the wrong place, or is the airport shown in the wrong place?

I assume "east" in the above usage means "generally east, measuring along the 106-degree radial".

It's not a one-time thing, I'm getting a similar discrepancy with some other similar measurements of other similar regions of Class-E-to-surface airspace, always biased in the same direction.

All the clues were there; another member figured it out (see comments).  My computer screen was stretching out the image in the left-right sense, so when I took my nautical miles scale off the tick marks on the north-south line of longitude, and then made an east-west measurement, the measurement ran significantly high.  The question noted I had seen this elsewhere and the fact that that was also on an east-west measurement, but at much lower latitude than Alaska (neither of which were noted in the question), should have been a clue to me as to what was going on.  Nothing to do with high latitudes and nothing to do with errors in the way the airspace or the map grid were charted.

I looked at the same url on a different computer screen and the measurement came out fine.

The sectional map “ticks” and 30 mile grid squares do correspond to nautical miles. The sectional represents a 3D object (the earth) which is roughly a sphere in two dimensions. This is the reason you will find that the left and right edges of the sectional are actually not straight, but curved, and in the US (northern hemisphere)   The bottom (South) part is bigger than the top (North) part because the earth at lower latitudes is larger in diameter than it is at higher latitudes. Measure the sectional with plotting tools, like an E6B with a sectional scale, and you will also find that vertical measurements that go straight north and south (measuring across latitudes) are accurate (due to my above explanation, and the fact that degrees latitude are equidistant from each other at all such points) and that east/west measurements vary drastically as you go up in latitude (further north) because lines of longitude “scrunch together” as you approach the poles. You will find the same with WAC charts in the Southern Hemisphere, except when approaching the South Pole at lower latitudes... the same effect but on the other side of the equator. The error rather nulls out at the equator.

You can even see this curvature in the overview of sectional charts included on the front of each one:
Compare this to the higher latitude Alaska charts:

You can also see the difference in the curvature/shape of the Houston sectional at ~30 degrees north latitude vs the Green Bay sectional just 15 degrees of latitude north of Houston. This is why 2D direct measurements that do not take a 3D great circle into account are always wrong when taken east-west.

I found this when plotting flight plans several years ago and it is an interesting point to realize that when measuring with a scale, it will give you inaccurate distances unless the route of flight is straight north/south. Basically, we cannot measure a segment of a great circle while projecting the 3D earth onto a 2D map with a 2D measuring instrument that doesn’t consider scale variance between latitude and longitude in such a projection.


Why does Logan Airport's Runway 32/14 only have arrivals on the 32 heading? I was going through this PDF document on Logan International Airport's runway usage and I saw that Runway 32/14 only has arrival activity. Why is that the case? Is this the the reason or part of it why there are only stripe marks on the side of the runway close to "32"?

(Much of my answer below will be drawn from "The 30-year saga of 14/32", a 2006 Boston Globe article which is unfortunately subscription-only but which I have access to via an institutional database.)

Runway 14/32 has been a long-standing bone of contention at Boston Logan.  Like many urban airports, Boston Logan is constrained by the fact that there are many people living nearby who don't particularly like jets flying low over their houses, and who are willing to organize in opposition of any increase in operations at the airport.   Massport, the organization in charge of Boston Logan, had originally proposed to build a new runway in the late '60s.  This met fierce opposition from community members in East Boston who were opposed to additional take-offs and landings over their homes.

  [I]t's taken no less than 33 years for 14/32 to become reality, a testament to how politically charged Logan expansion has been ever since demonstrations in the 1960s, when East Boston mothers lay in the streets to protest the destruction of Eastie parks and homes.  ...

The Hyatt Harborside Hotel (now the Hyatt Regency Boston Harbor) was built in the 90s to act as an impediment to runway expansion:

  In the 1990s ... Frederick P. Salvucci engineered construction of the 270-room Hyatt Harborside Hotel, a stone's throw from the end of the new runway, largely to create as big an obstacle as possible to Massport extending 14/32 or using it for planes flying over the city.

In the end, Boston Logan was barred by court order from allowing takeoffs from 32 or landings on 14, except in emergency:

  By court order, use of 14/32 will be restricted to planes taking off and landing over Boston Harbor, so Massport will be legally forbidden - except in emergencies - from using the runway to let planes take off or land over South Boston and downtown.

This still allows for three runways to be operative when winds are from the northwest or southeast (along with 15L/R or 33L/R), so the hope was that even this "half-runway" would result in fewer delays at BOS during bad weather.  Whether or not this has panned out to be true in the decade since, I do not know.

Landings on RWY 14 and takeoffs on RWY 32 at KBOS are prohibited according to the A/FD, most likely for the obstacle clearing reasons stated above.


On a small/micro jet engine, can the turbine blades and vanes be made out of ceramic? I am researching on how to make a small jet engine, and trying to figure out the materials for rotor and stator blades just after the combustion chamber so that they can hold up to the temperatures they will encounter.

I'm looking for cast repeatable shaped blades and trying to figure out what kind of forge or kiln to use.

My questions:

Should the blades vanes be nickel alloys or can they be ceramic so that dilatation won't make them interfere with adjacent parts?
Will the ceramic be too brittle?

I can't speak for all engines, but GE has been testing ceramic blades in their LEAP engines.
Here is their report on it.


Can VOR indication be reliable if one of its antennas breaks? What will be the reliability of the VOR indication inside the cockpit if one of the two outside antenna is broken ?

Can we trust it to make a VOR approach?

What will be the consequences assuming that we are in a general aviation aircraft like cessna 150 / 172 / 182 .

Picture of antenna:

(Image Source: http://www.simmer.fr/)

No, it should not be used.

Twin antennae, as you have shown, are balanced and fed into a balun (BALanced to UNbalanced) to feed the single transmission line to the receiver.   Together, they provide the correct impedance for the aerial system.  With an antenna broken, it is likely that the impedance of the circuit is wrong and therefore, the input to the receiver is, at best, suspect.

[GUESSWORK]

It's impossible to predict the indicated behaviour without knowing precise details of the aerial fit but a reasonable guess would be that the standing wave ratio in the feed will be altered and therefore, a phase error will be introduced.  The magnitude of the error will vary in proportion to the sine of the actual phase angle so my guess would be that the bearing will be accurate when due South or North of the VOR then increasingly inaccurate as you move round with an maximum error at 90 or 270.

[EDIT]

Early in the morning when I first wrote this but I think I am wrong about being accurate due North or due South.  It will be accurate somewhere on the compass and at 180 degrees opposite to that, with increasing inaccuracy as you move towards 90 degrees away from that somewhere.  Only calibration would tell you where the somewhere is.

Short answer: no.

In general, if a piece of equipment has an operability issue, it should not be used for navigation.

That said, if you test it and the test indicates it is functioning reasonably, it may be ok to use it on SHORT TERM BASIS. So, for example, if I were taking off from Manchester I would fly up to the Cordcord VORTAC and see if I were getting reliable readings. If so, I would probably feel ok using it on a VFR approach.

In any case, the antenna should be fixed before using the instrument long term. To use the receiver with a damaged antenna once or twice is excusable, but not for an extended time.

Note: By "reliable" readings I mean you are receiving a signal at different attitudes, or at least in the attitude you need for navigational purposes. The effect of a broken antenna will be to reduce your gain or possibly introduce a blind spot into your reception area. Note that these effects are ALWAYS present anyway. For example, in mountainous terrrain or if the VOR is straight ahead you could have blind spots in the VOR even with a perfect antenna. So, a broken antenna basically just makes standard reception problems you already have, worse.


How does turning Errors in the Air Driven Artificial Horizon work? Turning through 90°, 180°, 270° & 360°, the error is different. I can understand what is turning error and pitch error individually.

But why turning through different degree, the resulting error is different?

Rapidly becoming a nonfactor as electronic systems replace vacuum driven gyros:

https://www.flyingmag.com/g00/how-it-works-attitude-and-heading-reference-system?i10c.ua=1&i10c.encReferrer=aHR0cHM6Ly93d3cuZ29vZ2xlLmNvbS8%3d&i10c.dv=2

How It Works: Attitude and Heading Reference System

Attitude and heading reference systems (AHRS) have been providing heading and attitude information with greater accuracy and reliability than traditional mechanical gyros for years now. But how do these solid-state systems deliver this information, and how do they do so using increasingly smaller and lighter equipment?

Micro Electronics

An AHRS’s key features include micro-electronic mechanical gyros, accelerometers and a magnetometer or flux valve. The micro-electronic mechanical gyros are responsible for capturing motion data as an aircraft moves about all three axes. They do this by using vibrating components. As an aircraft changes direction, these vibrations deviate, and these deviations can be measured and used to calculate directional change data.

Gyro Drift

In order to account for gyro drift, which can produce erroneous data, an AHRS must also rely on accelerometers as well as a magnetometer or flux valve. The former uses gravity to serve as both an initial attitude reference as well as an inflight reference, whereas the latter uses Earth’s magnetic field to provide heading information.

The AHRS aggregates all the information from these different components and conducts complex algorithms and calculations to provide highly reliable attitude and heading data.

Side Note: Previously out of reach for private aircraft owners due to high costs, AHRS prices dropped remarkably as the result of the use and advancement of AHRS technology in the automotive industry. An AHRS today can be as small as a coin.


Can I fly through these military airfields' class C airspace? I am a low time private pilot now based in Pensacola. I’m a bit unsure how the Class C of PNS/NAS Whiting Field is handled. As you can see they all overlap. PNS is a civilian field while the others are military. If I was going to fly VFR to the northeast, can I fly through the Class C of Whiting field? I always get flight following when departing.

When entering Class C Airspace:

Since all the airfields are Class C you would need to contact the approach control prior to entering.  Approach control should be able to direct you appropriately from there.  Approach control would handle traffic at all three airports. If the military airfield does not want civilian aircraft to cross the field for any reason approach control will direct you around it.  If there is nothing going on at the military field then approach control could let you cross over the field.

When Departing Pensacola Field:

When departing Pensacola field you can request a Northeast departure through Clearance Delivery.  You could be granted the request to fly Northeast directly from the airport or they could make you fly East for a while then make a turn North to avoid the military airport.  Nevertheless you have to follow ATCs instructions whenever you are in Class C airspace.

Side Note:

It is fairly common that when you have military airfield close to a Class C airport, the class C airspace will be extended over the military fields as well so that one approach control would handle all the traffic and deconflict traffic for all of the fields.  A similar situation exists for the Tucson area where the Class C includes Tucson International and Davis Monthan AFB.

You're never allowed into class B or C without appropriate communication.  Contact approach, and ask them to enter the airspace.  If they clear you in, then you can.  If they tell you to remain clear, then the answer is no.

I don't know why everyone wants to make every answer into an editorial.


Are wings any more efficient at creating lift, versus orienting the engine's thrust downwards? As I understood from this article, wings on an airplane basically create lift by pushing air down.

Maintaining the lift force expends energy because air needs to be continually accelerated downwards. In the "energy budget" of the airplane, this energy cost appears in the form of extra drag due to the wing, which must be compensated by extra engine thrust.

It seems to me then that the power required to keep an airplane at a constant altitude using a wing is exactly equivalent to the power that would be needed if we would "simply" use a downwards-pointing fan instead of the wing.

Now, some posts on this website compare VTOL aircraft to airliners of similar size, and state that VTOL requires much more thrust (e.g. here - I would post more links but I'm not allowed to). But according to the reasoning above, shouldn't it require exactly the same amount of power to put a plane in the air, whether using a wing or directing the engine thrust downwards?

Please disregard, for a moment, the technical difficulties and safety issues. They are discussed in other questions related to gimbal thrust and VTOL aircraft. My question has to do with energy expenditure and power.

Interesting question. Purely empirically, it is the lift-to-drag ratio you are looking for. If you take this value as given for any particular aircraft, you have a direct answer for how much more effective wings are. It is the ratio of the lift to the total drag. The engine only needs to overcome the drag.

With L/D equal to unity you would need the same thrust as for the vertical take off. But even quite "bad" fixed wing aircraft would have L/D about 5. Gliders or similar aircraft built with a strong focus on aerodynamics can have an L/D of 50 or more (at least in some narrow range of airspeeds).

So yes, wings are more efficient. About one order of magnitude as a rule of thumb for common aircraft and optimal airspeed.

Why your reasoning with air pushed down is incorrect is more tricky to explain. I'll start with the assumption that, as air passes an airfoil, its speed relative to the airfoil is unaltered, and only the direction changes. (I know air slows down at least because of friction etc., but these are, at least theoretically, avoidable things not related directly to creating the lift. If there is something intrinsically related to lift which makes airflow change not only direction but speed too, then someone will hopefully correct me here.)

See the image. Airmass moving initially towards the airfoil with velocity $\vec{v_0}$ is deflected down by angle $\alpha$. Therefore the change in the velocity is $\vec{\Delta v}$. This change can be divided into horizontal and vertical components. To hold the aircraft in the air, the vertical component has to be equivalent to the aircraft's weight divided by the mass flow rate over the wing. The vertical component is related to the horizontal by
$$
\Delta v_{\rm horiz}=\Delta v_{\rm vert} \cdot \tan{\alpha\over 2}.
$$

So, from this simplistic view, the drag would be $\tan\alpha/2$ times the lift. Higher mass flow rate over the wing (longer wings, higher airspeed) makes it possible to keep the same lift with lower deflection ($\alpha$), thus less drag due to the generated lift.

Additional comment: how is it related to power and energy

The answer above is focused on how wings decrease necessary engine thrust, but original question could be interpreted in terms of energy efficiency too. I'll try to add some comments on this part.

simple example – rocket engine: not too typical for aircraft, but simple one. Rocket consumes same amount of fuel per second in order to generate a unit of thurst regardless of its size and regardless if it is pointing upward (and is static with respect to air) or forward (and moving through the air). You need to burn proportionally more fuel per second to generate higher thrust. So, for the rocket propulsion, you will save fuel in the same ratio as the necessary thrust decreases.
Propeller or jet engines are more complicate as their thrust and fuel consumption depends on engine movement through the air too. As David K pointed out in his answer, we can use momentum and kinetic energy of accelerated air to get power needed for unit of thrust.

With some simplifications, thrust is mass flow rate through the engine/prop multiplied by change of flow speed it causes. $T = \dot m \cdot (v_{\rm out} - v_{\rm in}) = \dot m \Delta v$. Power needed for this is $P = \dot m \cdot {1\over2}(v_{\rm out}^2 - v_{\rm in}^2)=\dot m\Delta v\cdot(v_{\rm in} + {\Delta v\over 2})$. Thus
$$
{P\over T}=v_{\rm in} + {\Delta v\over 2}\,.
$$

For stationary engine holding against gravity, higher thrust compared to flying fixed wing aircraft is needed as shown above. If we do not "cheat" by increasing mass flow rate through engine (like making it helicopter rotor or using multiple engines), $\Delta v$ has to be increased in  order to achieve the necessary thrust. So you not only need more power because of increased thrust but also more power because of increased Watts per unit of thrust. Note that even "helicopter cheat" does not work too well. To match power consumption of engine generating less thrust thanks to the wing's L/D you need to improve P/T too – by decreasing $\Delta v$, thus increasing mass flow (rotor/propeller radius even more than proportionally to increased thrust).

What about decrease of P/T due to the movement through air? Well, it depends on particular engine and its $\Delta v$. It will be typically in similar order of magnitude as airspeed (or even less), so we can not neglect $v_{\rm in}$ in watts-per-thrust equation above. There is a efficiency penalty when engine works on moving aircraft. But it should be still worth of it as the gain provided by lift is greater.

A simplified example: We have an engine capable of producing enough thrust to lift aircraft vertically. It can be throttled by changing $\Delta v$ without any practical problems or change in its internal efficiency. And let's assume that mass flow rate through it is fixed area $S$ multiplied by air density and multiplied by arithmetical average of speeds of entering and exiting air. For hovering aircraft and stationary engine producing thrust equal to aircraft's weight, $w$ it is
$$w=\dot m \Delta v_{\rm hover} = \rho S \Delta v_{\rm hover}^2 / 2\,;\quad \Delta v_{\rm hover}=\sqrt{2 w \over \rho S}$$
and thus
$$P_{\rm hover}=w\cdot \Delta v_{\rm hover}/2=\sqrt{w^3\over 2\rho S}\,.$$

The same aircraft flying on its wings does only need $w\over L/D$ of thrust. Airspeed is $v_{\rm air}$. Equation for the thrust: ${w\over L/D} = \dot m \Delta v_{\rm flight} = \rho S \cdot (v_{\rm air}+{\Delta v_{\rm flight} \over 2}) \cdot \Delta v_{\rm flight}$. Thus
$$\Delta v_{\rm flight}=\sqrt{{2w\over(L/D)\rho S}+v_{\rm air}^2}-v_{\rm air}$$
and
$$
P_{\rm flight}={w\over L/D}\cdot(\sqrt{{w\over 2(L/D)\rho S}+{v_{\rm air}^2\over 4}}+{v_{\rm air}\over 2})\,.
$$

Unfortunately, I do not see any way how to simplify and compare $P_{\rm hover}$ and $P_{\rm flight}$ so some concrete numbers:

Light aircraft, 1 ton, 100 knots, $S = 5\,\rm m^2$, $L/D = 15$: $P_{\rm hover} = 290\,\rm kW$, $P_{\rm flight} = 35\,\rm kW$.
Heavy aircraft, 100 ton, 200 knots, $S = 50\,\rm m^2$, $L/D = 15$: $P_{\rm hover} = 90\,\rm MW$, $P_{\rm flight} = 7\,\rm MW$.

So, based on these simplifications flying with wings with similar kind of engine should be significantly more efficient in energy terms too. And, additionally, you are already moving forward using power $P_{\rm flight}$. For vertical engine extra power would be necessary for overcoming air drag due to the movement.

Wings are an economical way to accelerate a mass of air downwards. The fact that natural selection has chosen that system over other alternatives possibly means that wings are the most economical solution...


How do pilots manage metric altitude? A minority of the world, specifically Russia and China, uses the metric system in aviation. ATC will tell the pilots to "climb to 900 meters height".

How do pilots of international flights manage this? Do they request the ATC to provide altitude in feet, or do they perform the conversion manually and dial in the altitude in feet to the autopilot?

In areas where the ATC provides the altitude in meter (China (PRC), North Korea, and Russia), depending on the equipment (altimeter), the pilot can determine what to do:

In case of older aircraft, where the altimeter is calibrated in feet, the pilots use a conversion card for determining the altitude in feet, before dialing in the altitude to the autopilot.

Source: aerosavvy.com

Note: The conversion charts are standardized for the airspace and the pilots have to use it. For Chinese airspace refer FAA document on Chinese RVSM

In case of modern aircraft displays, the altimeter can be made to display altitude in imperial (feet) or metric (meter) units.

Here is a page with all the info on this subject: http://aerosavvy.com/metric-imperial/

  Altimeter Calibration: mb, hPa & inHg
  Altimeters can be set using hPa or inHg.

  Altimeters can be set using hPa (mb) or inHg.

  In order to calibrate our altimeters for varying atmospheric pressure, we set the current pressure reported by ground stations. Pressure is reported in inches of mercury (inHg), hectopascals (hPa), or millibars (mb). North America and Japan use the inches of mercury measurement. Hectopascals dominate the rest of the aviation world (millibars and hectopascals are equal). For years, altimeter manufactures have provided two calibration windows for hPa and inHg. This makes setting our altimeters easy no matter where we fly.  Just dial the reported pressure into one of the windows and you’re all set! Newer, digital altimeters have a button to switch between units. I like the older ones, they’re easier to set.
  Measuring Altitude: Feet vs. Meters

  Here’s where things get tricky. Because of the proliferation of American and British aircraft during the early years of aviation, the imperial foot became standard for altitude measurement. China (PRC), North Korea, and Russia, however, use meters for altitude measurement.

  As luck would have it, I often fly over Chinese, Russian, and North Korean airspace. The altimeters in our older aircraft, like the 767, are calibrated in feet. When flying into metric airspace, we use a conversion card. When Shanghai Control clears us to descend to 3600 meters, we check the card and descend to the equivalent: 11,800 feet. Does this sound like a pain in the you-know-where? It is!

  During rush hour at airports like Shanghai and Beijing, the controllers rattle off clearances very fast. They will often assign us a heading, speed and metric altitude all in one shot. Doing the metric altitude conversion adds an additional task to an already challenging environment. We stay on our toes in metric airspace! The good news is that newer generation airplanes have a switch that can change the aircraft’s native altimetry display from imperial to metric units. When crossing into metric airspace, the crew hits the switch and the airplane understands the metric system. This reduces crew workload considerably.
  Distance Across The Ground

  World-wide, the nautical mile (nm) is the standard for measuring the distance an aircraft travels across the ground. That’s nice and easy and it makes me happy. :)

  Other lateral measurements are a mess. Most of the world measures runway length in meters while North America uses feet. Most of the world measures airport visibility in meters. North America? Not nautical miles, not meters, but statute miles! Huh?? Not to worry, North America changes back to feet when measuring Runway Visual Range (runway visibility measured with a laser), while the rest of the world sticks with meters. Confused? I sure am, and I do this for a living!
  Wind speed: Knots vs. Meters/Second

  When reporting weather, airports in China and Russia state the surface winds in meters per second (m/s). They love this metric stuff! The rest of the world reports wind in knots (nautical miles / hour). Converting is easy:

  1 m/s ≈ 2 kts

  Double the m/s and we are close to knots. If Shanghai tower tells me the winds are 7 meters per second, I know they are blowing just a little shy of 14 knots. I can land in that much wind. Any more than that and I’ll let my first officer do it!
  Is flying safe with all these different units?

  Yes!  Pilots flying international routes deal with this assortment of units daily. We can juggle them in our sleep. While it’s certainly an added challenge, it’s an important part of our occupation. There are very well defined standards and procedures that pilots use to work with the different units. Flying continues to be the safest form of transportation available.

Basically, it is that either the pilots have tables to convert units or the instruments can show both types. In either case, it would be very easy to follow the ATC instructions.


What methods are used to perform head tracking in the cockpit? In this question, I asked about a helmet with built-in HUD, and have a follow-up question:

What methods are used (or have been used) to perform head-tracking in the cockpit?

The website for that particular helmet (which I'm surprised exists like it does ... could I simply order one? doubt it ...) has the following (edit: here is additional information for that helmet):

  High-speed tracker with a full-spherical Field of Regard(FOR); choice of Hybrid-Inertial or Magnetic tracker

Accelerometers are used, however for lower latency and higher definition, tracking infrared emitters is the better choice1.

The A-10 as you wrote uses inertial or magnetic trackers. The Eurofighter helmet uses infrared tracking.

  ... the Typhoon’s HMSS features lower latency, higher definition, improved symbology and night vision.

It's the one on the left (1source).

You can already see it has an organic brain of its own; those bumps are the IR LEDs for tracking.

The only reasonable method is accelerometer or gyro sensor, but not only one, about 3 of them to make precise tracking without any bugs, as the plane would tilt too, it would be reasonable to have acclereometers to calculate that too to make the exclusion of any bugs and unwanted tilts


What is the first fighter jet which was built with twin engine and tail configuration? What is the very first fighter jet that pioneers the twin engine and twin vertical stabilizers over the engine just before the exhaust (like the F-14, F-15, F-22, Su-35, etc)?

One of the first was probably the unconventional XP-79.
https://en.wikipedia.org/wiki/Northrop_XP-79

It never entered service of course, but does meet your criteria.

Something that kinda meets your criteria would be the F7U Cutlass. Twin tails and twin rear mounted engines though the tails were in separate tail booms outboard of the engines, a similar configuration to that of the DH Sea Vixen and Venom of roughly the same period.

These did enter service.

Then there is the XF-90 which has twin rear mounted engines with a single vertical stabiliser over them.
The Soviet La250 and Tu28 had similar configuration but came later.

This also did not enter service.

Possibly might be the WW-2 era Me-262?  I believe the prototype had twin tails.


Do autopilot headings consider the earth's shape? How do you set correct heading while cruising across latitudes?

On a flat map, point A might be to point B at, say, heading 060 (intentionally no cardinal headings taken for example, as they don't create this situation). But on a curved surface, like spherical earth, the heading doesn't remain constant- it keeps changing as you move away from or toward the equator, sending aircraft miles off-target over long distances, as a result of pole-bound spiraling of the path.

Is there a formula to determine the optimum heading at the outset (even though it may not 'seem' correct), thereby, reaching the desired point B on the globe at the end of the cruise?

Yes, almost all modern computer navigation systems take this into account.

The track between two points along the "spherical" earth is called the great-circle track. Except for a N/S heading (or a E/W heading at the equator), the heading will vary along the track. Ed Williams has compiled a formulary for great circle navigation here, mostly derived from the Haversine formula. The formula you are interested in is "Course between points" for calculating the initial heading to fly from point A to point B.

Yes. They fully take into account that the world is flat.

Just kidding. Long distance route planning is a complicated process. Usually commercial flights follow fixed, well-established routes. For example, when flying from North America to Europe, aircraft generally use North Atlantic Tracks. These tracks are designed in a way that is fully cognizant of the oblate sphericity of the Earth.


How high can propeller airplanes fly? Is there a maximum height a propeller airplane could fly?

Some lower bounds for maximum flying altitude:

12,000m (39,370ft): Antonow An-70, produced 1994
12,310m (40,387ft): Lockheed Martin C-130J Super Hercules, delivered in 2012
12,500m (41,010ft): Piaggio P.180 Avanti, production started in 1986
29,523m (96,860ft): NASA Helios HP01, record altitude reached in 2001

Just for comparison: Low earth orbit is about 160,000m.

Upper bounds: I don't know. I guess at some point there is not enough air to get lifted.
And nobody build an airplane with solar cells that flies to space.

I guess important factors for such an upper bound are:

Weight of the airplane,
Size, shape and speed of the propellers
Speed of the propellers

It is a matter of power source (and engine aspiration in case of air breathing engines), wing loading and aerodynamic efficiency. With current technology, the limit is around 100.000 ft (30 km), as proven by Pathfinder and especially Helios. I doubt that much more is possible with really useable aircraft.
Aerodynamics first: The altitude factor of $c_l \cdot Ma^2$ tells you how much lift can be produced at a given flight Mach number, and the wing loading then gives you the minimum density for sustained flight. 0.4 is a good value for subsonic $c_l \cdot Ma^2$, and 30 kg/m$^2$ is a feasible wing loading for flight at 30 km. See this answer for more detail.
If the power source needs ambient air (piston engine), the plane needs triple-stage compressors or turbochargers, which have been tested up to 20 km altitude and should be good for maybe 24 km. They are finicky devices; Boeing Condor rarely flew at its maximum power because the stages of the turbochargers would oscillate in an alternating sequence of surges. One stage would race ahead, causing the other to surge, which made the first surge and free up the other to race ahead, and so on.
Above approximately 24 km, solar-electric propulsion looks like the best option currently. In all cases you can only fly subsonic, so the minimum practical wing loading will limit the maximum altitude. Aircraft like Helios are very delicate already, so they can only be launched in calm weather and are at risk of being blown away by high altitude winds. Payload is minimal, and depending what the aircraft is supposed to do besides flying high will give you a limit on the maximum altitude between 24 and 30 km.
Going to orbit in a propeller driven device is completely illusory. There is not enough matter to push against at higher altitudes, and the theoretical propeller diameter would be measured in Kilometers (or miles, if you prefer that unit). The structural mass would be prohibitive. Also, propeller thrust is inversely proportional to flight speed, and there is no way to accelerate with a propeller to escape velocity. The thrust would be just a rounding error away from zero at 7.9 km/s.
This speed is required to escape earth's gravity by flying fast enough around it so that the centripetal force equals the aircraft's weight and is called orbital velocity. The higher the orbit becomes, the more energy is required to reach it. In order to gain enough energy, a propeller aircraft would accelerate in the atmosphere to a speed quite a bit higher than the desired orbital velocity and then convert that kinetic energy to potential energy to lift its trajectory above at least 100 km, the internationally recognized altitude where spaceflight starts. Note that this phase of the flight requires inverted flight if the acceleration takes some time. The maximum flight Mach number would need to be maybe 12 or even 15 so this maneuver is possible at all.
In short: Going to orbit with a propeller? Forget it!

They are getting ready to fly one on Mars, so the issue is more with generating power to turn the prop than the props ability to generate thrust.  Exciting advances in solar power have made very high altitude flight practical, as demonstrated by the 247 foot wingspan, 1 ton Helios at 98,000 feet.

Manned balloons have reached 125,000 feet, but the winds of Mars may have had the designers worried.  Helios might still be around if they had some old-fashioned wire bracing for the wings, but saving weight and drag is critical for maximum altitude performance.

One aspect favoring Martian flight is the molecular weight of CO2, which at 44 is around 50% greater mass than Earth's predominantly Nitrogen/Oxygen atmosphere.  Although extremely thin, heavier Martian CO2 atmosphere increases the amount of "lift" a moving airfoil generates, by bending or deflecting it, at any given "air" pressure.


How does the Pratt & Whitney Canada PT6 differ from other turboprop engines? As far as I know, all turboprop engines have mechanical continuity between the engine and the propeller except for the Pratt & Whitney Canada PT6 family of engines. What is the correct term for the setup the PT6 uses to power the propellor and how does the PT6's design differ from other turboprop engines?

Split Shaft

Several turboprop engines, including the PT6 turboprop engine, use an epicyclic gearbox only, many other turboprop engines use a multishaft gearbox. The latter usually results in the propeller axis being offset from the turbine axis.

In common with some other engines, the PT6 turbine is also reversed. This facilitates the split-shaft arrangement where the engine and propeller are driven by separate (but in-line) shafts, each connected to their own turbine wheel.

PT6

Image: P&W

Image:12Charlie

A contrasting and common design

Image:12Charlie

As described above, it's a split shaft turboprop engine.  The design isn't unique to the PT-6 (T-74) but the PT-6 variants are by far the most ubiquitous split shaft turboprops on the market today.


How does an aircraft descend without its nose pointing down? I have seen many cockpit videos of airplanes landing, and nearly none of them have their nose down for losing altitude. How does this happen? and how does an airplane, such as A320, descend without its nose pointing down?
And how does this happen in a controlled manner?

A plane descends when it does not have enough thrust to maintain its altitude. A plane can descend with its nose pointed up or down so long as there is not enough thrust to maintain altitude. Altering the pitch of an aircraft is used to control airspeed: pitching up slows an aircraft down (and may cause a climb if done from a level attitude) and pitching down generally speeds up an aircraft (and causes a descent if done from a level attitude). So if you want to descend at a slow speed you reduce power and pitch the nose up.

Since you want to land at a slow speed you will need to pitch up and slow the aircraft down while reducing power to descend nicely. This article covers it nicely in terms of flying an approach. It's also covered in this question.

The airplane flies because lift created by the wings counteracts its weight created by  gravity. To create lift the surface of the wing must be in relative forward motion through the mass of air. Thrust created by the engine generates the forward motion which is opposed by drag. As long as thrust is greater than drag, the wing can move forward and generate lift. Gravity is universal and operates in all objects. Lift, on the other hand applies only to whatever is attached to a wing. To generate lift the wing must have the air moving around it following certain rules, otherwise it is incapable of generating lift. These rules are conceptualized as the angle of attack or the angle between the cord of the wing and the relative motion of the aircraft. At high angles of attack the wing is incapable of generating lift. Gravity, experienced by the airplane as weight does not follow any particular rules, all objects fall from the sky in the absence of lift. An airplane can have its nose up or down and gravity will still apply and if the lift is less than the weight, the airplane will go down. In fact, during landing most airplanes will maintain a nose-high attitude while going down into the runway.


Why are the cockpit displays swapped between the captain's and first officer's sides? This question focuses on Airbus, like the A320, but is not restricted to those aircraft as the situation seems to be the same for other aircraft.

In front of the left seat, the navigation display is displayed at the right of the the primary flight display, and it is the inverse of the right seat.

Is there a technical reason to do so? An historical one?

It's hard to see from the angle that your example photos were taken from, but the main reason is that they want the primary flight instruments (which are on the PFD, or Primary Flight Display) directly in front of the pilot, no matter which seat they are in.
The PFD gives the most critical information which is used to actualy control and fly the aircraft, so is designed to be in the pilot's direct line of sight.
Less critical information is put on the MFD (Multi Function Display).  Since the pilots sit close to each side of the cockpit, this usually means that there isn't room to place the MFD on the outside of the PFD.  Even if there was room, the extra display is used partially for redundancy, so they don't want to put it further away from the other pilot.
Since the PFD is directly in front of each pilot, it doesn't take any effort to switch between seats because of that aspect.
This photo (it's night, so a little dark but I think that you can see what I'm referring to) was taken from my head level while sitting in the right seat, which should make it pretty clear:

own photo

It has to be symmetric because otherwise you would have to have a different arrangement of controls on either side.

The shape of the console is different left to right, so, for example, there is more room in the center of the panel than the outer edge. Everything is arranged to fit efficiently into that space. To use the same arrangement on the other side of the panel, everything has to be flipped mirror image.

It would be bad to have different arrangements of controls in the two positions, because it would be confusing to the pilots. Sometimes pilots sit in one seat, and other times in the other. They should have the same basic arrangement of controls in both cases.


Why are jet aircraft never designed with a slower cruise speed? All jet airliners have a cruise speed between Mach 0.82 and Mach 0.85.

At those speeds the aircraft are flying at their maximum subsonic speed. Any greater cruise speed would only be possible with major design changes.

My question is about fuel efficiency.

As drag increases with the square of speed, would a slower cruise speed lead to better fuel efficiency?

So why are jet aircraft never designed with a slower cruise speed?

If transonic phenomena didn't exist, would a faster speed be more fuel efficient?

Currently some airliners, after flying a 12-hour route, stay on ground for a long time before returning. Wouldn't a slow flight (if it is more fuel efficient) be better in this scenario? So not to waste time on ground?

On long-range flights, any fuel saving significantly increases the flight's profitability.

There is indeed a benefit in flying more slowly, but the aircraft needs to be designed that way to profit from slower speeds. You can see that wing sweep has been slightly reduced in the more modern airliners, but this is also due to better airfoils with higher Mach drag rise onset.

But some things will also cost more when you slow down: You have to pay the crew for more hours, and the aircraft cannot be utilized as often. To transport the same number of passengers requires more aircraft when they fly at a lower speed. The cost versus speed relationship is not linear, but it should have a minimum somewhere below today's operating points. Aircraft with a slower design cruise speed can be lighter and smaller for the same payload, but for now airlines don't have a choice of slow or fast jets; all manufacturers try to design their long-range airplanes for at least Mach 0.83.

This has a lot to do with marketing: The aircraft with the higher speed will show a shorter travel time for the same distance, so it will show up first in the booking systems of travel agents. Of course, now you will argue that most people book on the internet and will try to get the lowest cost, regardless of travel time. True. But this is not the sort of customer the airlines are after. Their profit comes from people in First and Business class, and these still book predominantly via travel agents. Therefore, Airbus and Boeing try to market their planes as the ones which come first on the Amadeus and Sabre screens.

If you would single-mindedly optimize a configuration for the best fuel efficiency, you would arrive at something close to the MIT D8 design or Boeing's SUGAR study. Note that both are designed to fly quite a bit slower than current airliners at Mach 0.72. These designs are for a world in which fuel costs 200 USD per barrel or more, while the current designs are based on the expected prices at the time of their design start.

When fuel prices were low, the optimum speed for the best transport performance was indeed above Mach 1. The Vickers VC-10 of 1964 sported a maximum cruise Mach number of 0.886 and still holds the record for the fastest commercial Atlantic crossing. When Concorde was developed, the general consensus was that future air travel would be supersonic. Only the jump in oil prices after the 1973 oil crisis put those plans to rest. Large business jets today can cruise at up to Mach 0.935 because their owners are less concerned with saving money by flying more slowly. Technically, there is still room for cruising above Mach 0.85; it's economics that keeps airliners back.

If airplane companies were free to pick the optimum Mach number, they would today choose a speed between Mach 0.78 and 0.82 with fuel prices around 50 USD per barrel. Note that this is the design Mach number range for regional jets which fly short hops in which the increase in trip time from the reduced speed is insignificant. Also, especially in times of high fuel prices airlines operate their gear at lower speeds in order to conserve fuel. But if you follow this link, you will learn that airplane companies don't have that freedom.

Regarding drag:

Drag does not go up with the square of speed. When plotted over speed, the drag of an airplane will first go down, reach a minimum and only then go up. Airliners fly close to this minimum, and they fly high to shift this minimum to the highest possible flight Mach number. They do this by flying in less dense air, which requires to fly at an altitude around 30.000 - 40.000 ft. An additional benefit to flying this high is the lower air temperature which makes the engines more efficient.

To understand why an aircraft will be more efficient when it flies faster you must realise that the aircraft gives the air that it passes through a downward velocity. The change in the momentum of this air is what supports the mass of the aircraft. Every kilogram of the aircraft's mass will produce a downward force of about 10 newtons. This must be balanced by a change of momentum of 10 kgms-1 every second.

Now, although the change of momentum of a given mass is proportional to the change of velocity, the energy required to produce that change is proportional to the velocity squared. (In SI units the energy is (mv2)/2.) So the 1 kg mass could be supported for 1 second by accelerating 1 kg of air to 10 ms-1 or by accelerating 2 kg of air to 5 ms-1. Both will support the same mass but the first requires 50 J of energy and the second 25 J. Thus by doubling the mass of air that supports the aircraft the power required to do so has been halved.

So to get the best possible efficiency an aircraft should displace the greatest possible mass of air. However, it can only affect the air that it passes through, so in order to move a greater mas of air it must either increase its wing span (which is why high performance gliders have very long wings) or travel faster. So if all other factors were to remain the same then if an aircraft were to double its speed it would require half as much power to keep it up. In addition it would travel twice the distance, so the energy, and hence fuel, needed to travel a unit distance would be reduced by a factor of 4.


Why do the missiles on an F-16 point slightly down? I've been wondering this for quite a while:  Why do the missiles on the F-16 point down a little bit?

I went and did a little searching and found out that the F-16 isn't the only plane that has this feature, so I'm going assume it's not the F-16s frame specifically.  I also doubt it's tactical because all the weapons are pointing down, regardless of the weapons role in combat. In fact I saw a few photos where the bombs closest to the fuselage were pretty level with the ground (and bombs pretty much always go down, so...)

Does it have to do with aerodynamics, or perhaps ease of loading?  Why is the load-out on the F-16 mostly pointed slightly downwards?

If you take a look at the picture again, you'll see that the missiles are in fact lined up quite nicely with the nose of the plane. The missiles are not pointed down, they are pointed forward. The wings and engine (and the entire back two-thirds of the plane) are pointed up, which provides the lift to keep the plane in the air during normal flight. You can see pretty well the alignment in reference to the horizon in this picture (found by @jay-carr):

Just to cut off the potential comments about how planes generate lift, reference is here.

Its so that the thing goes down (and away) from the aircraft when it is released.

For example, imagine the missile was canted up instead of down. Due to aerodynamics it might fly into the wing it was released. The armament is canted downward to prevent this.


Why does a missile have small wings but better agility than a plane? An anti air missile has often very small wings but out performs a fighter aircraft, e.g. much higher g-load, since g-load comes from lift which is the area of wing.

Why not designing an aircraft like a missile? E.g. low drag, high agility.

First of all, a missile is operating at a much higher dynamic pressure than the airplane. After all, it has to catch up with it to do its job. Since dynamic pressure scales with the square of airspeed, the wings of a missile twice as fast as the target would need only a quarter of the wing area to produce the same forces.

Secondly, the missile uses its thrust just as much as the lift from its vanes to change direction in the initial flight phase when the rocket motor runs and the biggest course changes are needed. With an airplane, this is normally not possible, since most planes need to fly in a small range of angles of attack and sideslips to operate properly. Exceeding those will produce substantially more drag and might lead to loss of control.

Lastly, the missile has to carry only the mass for a few seconds of flight. The burning time of a rocket motor is just a few seconds for short-range missiles, and their typical flight time is about 30 seconds. Since they do not need to carry the fuel to return to base, their mass can be held down, which improves agility.

Most missiles do not have better agility than planes. The maneuverability of an flight vehicle is defined typically by what is called a "maneuverability graph". Aircraft generally have much better maneuverability than traditional missiles do, but there are some newer missiles that use a technology called "Thrust-Vector Control" which can potentially make the missiles more agile in some, but not all, parts of the envelope. In general missiles achieve their lethality by greater speed, not greater maneuverability.


Are there any civilian planes with seating arranged in a round area? Recently, I heard Airbus want to make an aircraft that looks like a UFO.  Is there currently any civilian passenger plane designed with the seating in a round area rather than the traditional design with a long aisle?

The simple answer is no.

Why a round seating design is not feasible? We need to look at a few aspects.

Real Estate
An airplane fuselage is rectangular (roughly speaking) in shape (when viewed from top). Airplane seats are rectangles. If you place rectangular boxes in a circle, you can fit lesser number of boxes than if you place them in a rectangle of the same area.

Airlines have just one goal to achieve, i.e. make money. Yes they take people from point A to point B too but if demand arises for joyrides, airlines will not be hesitant to start that.

By fitting more people in lesser space (not in inhumane positions mind you, governments don't allow it), they make sure that their profits soar.
Loading/Unloading Passengers
In a circular configuration, people will be a lot more confused to find their seats. People are already so confused and disillusioned when boarding a regular airplane that flight attendants have to guide every single person about where the seat is. Don't believe me? Notice how the flight attendant will snatch the boarding pass from your hand if you try not to show it.
Servicing Meals
Trolley carts used in airplanes are designed for straight aisles. In case of circular aisles, it would be very hard to maneuver them because flying airplanes do get bumps every now and then.
Evacuation
In case passengers need to be evacuated, a circular design would not be very practical. Since all people will not facing forward, simple emergency instructions, e.g. move to the front/back/left/right of the airplane can be confusing and will waste time. Locating an emergency exit will be harder too than in a normal airplane.
History
People are used to the conventional design. The present design is in use for over half a century. In addition to airplanes, trains, buses, cars and buggies have the same configuration.

I am not sure how that (and some other) news channel found about Airbus's new design. Airbus does disclose the future innovations it is working on, and flying doughnut is not mentioned there. It could be that is it on paper only at the moment. The patent information mentioned in your referenced article cannot be found with United States Patent and Trademark Office, or at Google Patent Search.

I never saw any since I am born. I am 47 years old. In a business section, you can have rotating seats; but usually it is two seats facing two other seats. In Air Force one, I saw some pictures where the President of The United States is seated at a desk; I would remove that for safety issue.


Why does the SR-71 Blackbird sometimes have dents in the nose? On many pictures (maybe on most of the pictures) part of nose looks hammered or slightly smashed, or bent.
Please note these 2 symmetric dents just behind nose spike:

source

source

On other pictures these dents are missing:

source

Why? Is this a feature or damage?

The SR-71 had a detachable nose (photo #2), and could change between three different nose cones depending on the mission. I haven't found a lot of information on the three, other than what this site and this site mentions:

Training nose with dead weights
Radar nose containing a Side Looking Radar. This is probably the bulge you saw on the photos.
Photo nose containing the optical bar camera, taking very wide photos of the ground.

I don't have a copy with me right now, but the book Skunk Works is Ben Rich's account of taking over the Lockheed special-projects group from Kelly Johnson, and includes quite a lot of description of the SR-71 development and operation.

As I recall, he described that the SR-71 nose would often soften and deform in flight from the heat of very high speeds. I suspect that the bulges are a product of that.

As to why they are so symmetrical, I especially notice on your second picture, where the bulges appear to be propagating back from the join in sections.


Can Microsoft Flight Simulator help me learn to fly (or make me a better pilot)? Microsoft Flight Simulator has "flight lessons" with a virtual flight instructor, some of which teach concepts that are taught during actual flight training. These simulators are becoming very realistic, and I can see them being helpful as an introduction to a subject prior to running the Hobbs meter and paying for actual flight time. Will this experience help or hurt someone who decides to become a real pilot? Is it a tool which can help students/instructors in an actual training environment?

Clarification:This was written about Microsoft Flight Simulator X (FSX).   Parts of the answer probably apply to the new Microsoft Flight Simulator (2020 release), but that isn't what I had in mind when writing this.  Maybe a new answer will be appropriate after using MSFS for some time.

@Pondlife gave a very good answer, and I'll add a few bits to that.
Even after having gotten my license, I still use flight sims (mainly Microsoft FSX and X-Plane 10) for some practice.
Before flying into an unfamiliar airport, or over an unfamiliar area, I'll often load it up in a simulator, and fly it.
Especially when I was a student, I found this incredibly helpful for my long cross country flights. You can look at a map all you want, but its still not the same as sitting in the simulator, and looking around ("I see the mountain on my left.... and the lake below me.  I can follow this valley all the way to the airport..." etc). And I've generally found the simulator, with good terrain and textures loaded, can be pretty close to reality.
The night before I did a student flight from KBFI to KVUO, I flew the entire thing in FSX. The next day, it really felt pretty much like making the same flight all over again. Based on the landmarks, timing, views, etc, I knew exactly where I was, and I was confident that everything was going right.

That said, in my opinion there are some things simulators do very poorly.
Some of these are:

TakeoffsThe application of power and left-turning tendency on the ground just doesn't feel realistic at all. The climb out doesn't feel right.

LandingsGround effect is difficult to observe in a simulator. The flare doesn't feel like reality, and touching down (or bouncing) is not realistic at all.

Cross Control, Slip, Spin & Unusual AttitudesAnything beyond basic maneuvers up to 30 degrees bank or 20 degrees climb doesn't match reality.

This is dependent on your simulator, some perform certain things more realistically than others.
Because of these limitations, I would NOT use a flight simulator to try to learn takeoffs, landings, or certain maneuvers. (You can learn the "procedure" in a simulator... when to reduce power, when to add flaps. But the "feel" will be all wrong).

If you're going to try to incorporate flight sim into your training, there are some basic things you should do:

Spend LOTS of time configuring itUnlike a video-game, this is not "plug-n-play". Spend time learning all the controls.
Turn on RealismWhen adjusting your controls, set all "null-zones" to zero, set "Sensitivity" to maximum, turn off helpers like "auto-mixture" and "auto-rudder", etc. (However, I do keep "Gyro-Drift" turned off in my sim.  Its annoying, and adds little practical value)
Turn on WeatherThe flight simulators typically come pre-setup with zero wind, good weather, standard temperature, etc. Configure yours to pull from real-world weather sources, or add some clouds, a variety of winds, and don't always fly at noon. Set the timing for early morning flights, twilight flights, night flights, etc.

Another "fun" area where flight simulators can be slightly helpful is in emergency procedures.
Typically in real-world training, my instructor told me:  "We're going to practice engine-out emergencies" and my mind immediately starts preparing for that... And naturally, we have to do them at a safe altitude in a safe area.
In a flight simulator, you can set up the computer to give you a random emergency at a random time. You might get the problem on short-final, or over a metro-downtown area. Something that you just can't do in reality.
I haven't had any real-life emergencies, so I don't know how accurate a flight simulator is. But I believe that some practice is better than no practice at all, and flight sim lets me fly into storms, icing, get lost in fog, fly approaches below minimums, have an engine seize up on me, etc, all without risking my butt or a $200,000 airframe.

Since @Lnafziger suggested more specifically addressing flight training:
I do believe that flight sim can help with certain aspects of training, but in other areas, it is no help at all, or actually harmful.
Training Areas simulators can help with:

ProceduresReviewing steps and procedures before going in the air.  For example, for a student, steep turns or stall recovery can be a little nerve wracking at first. It may be much easier (and cheaper) to do it in a sim with an instructor, discussing all the steps and reasons for actions. Then when the student gets in the air, they won't have the "feel" for it, but at least the general process is already familiar.

NavigationTuning and identifying VORs, and interpreting the needles can be done just as well be done on the ground as in the air. If the sim has good, realistic terrain (I prefer MegaSceneryEarth), it can also be used for some visual reference lessons.

Instrument interpretationScanning and cross-checking the 6-pack of instruments can be done in a simulator just fine, and a student can practice doing it for long periods of time for a fraction of the cost of flight time.

Training Areas simulators are very bad at:

On the ground & Outside the planeAnything on the ground, such as taxiing and parking, or anything outside the airplane, such as pre-flight inspection, or weather interpretation, just doesn't work in a sim.

RadiosI haven't seen any flight sims that really work for the practice of talking on or listening to the radios. (I haven't used VATSIM, which might help). I don't think there's any good substitute for actually flying in a real airspace while simultaneously engaging in real radio conversations.

FeelEven the best full-motion sim isn't a substitute for the forces a student feels in a real airplane. This is especially true on ground-reference maneuvers, takeoffs, and landings, where I feel sims fall far short of reality.  No one will ever get a "feel" for the plane from a simulator.

For much more information on the topic, I recommend these books:
Scenario-Based Training with X-Plane and Microsoft Flight Simulator

Microsoft Flight Simulator X For Pilots Real World Training

And here's a picture of my old simulator rig.
(Its been improved a bit since this picture)

I believe so. Although it is not 100% as accurate as a real flight simulator, it does help you understand a basic concepts which can help you get a boost in starting your understanding in an aviation career


Has anyone researched or built an aircraft that controls pitch and roll by pumping liquid within the airframe? I am confident I have seen an article couple years ago about a "static surface aircraft" (or at least a concept of it) with no control surfaces, which uses a "wet mass" to adjust its roll and pitch.

The container within its CG is filled with a fluid which is then transferred (or sucked) to the wing tips (to control roll) and along the fuselage (to control pitch). I'm not sure about the yaw control - it should be elevon-like.

Having spent half a day online, I have been able to only find a DEMON UAV from BAE systems, a "flapless" aircraft which uses flow injection above the control surfaces for maneuvering. That is not the same.

My question - is there something fundamentally wrong with a concept I have described?
Were there any experimental aircraft made?

The aerodynamic forces resulting from surface deflections are orders of magnitude larger and faster (in their rate of change) than the gravitational forces obtainable by shifting the centre of mass.

Additionally, as OP seems to realize, yaw control would not be feasible with this method.

So overall it would not be impossible, but it has no clear advantages to outweight the heavy disadvantages. I am also unaware of any prototypes or tech demonstrators using this method.

Nasa's Space shuttle (among many other missile systems) uses pumped liquid to control Pitch, Roll and Elevator

Not in the way that you diagrammed it, though. Instead of using hydraulic control surfaces, it uses thrust vectoring to control all three angles. By sending the Fuel out at a slight difference in angle, or by using RCS to change angles.

Thrust vectoring works by changing the angle that fuel is dejected from the nozzle of the rocket, an thus changing the direction of the thrust. This can change the pitch and yaw of an aircraft (for a rocket, pitch and roll). Controlling roll is another aspect that requires two or more thrusters, which apply a torque to the structure, similar to the way one spins a top.


Aerodynamics of double winglets I've read here on Aviation SE that the aerodynamics of winglets on an aircraft are easily described as "extending the airfoil".  Thus, as I understand it, a winglet is like stretching the airfoil a few feet and "bending" it upward.

Now, this answer mentions the various types of winglets used in contemporary aircraft, but I'm not sure the B737-800s I've seen (such as the one in the below image) with double winglets perfectly matches any of those categories1.

source

Thus, my question is:  How can one best describe the aerodynamics (and benefits) of the second, downward-pointing winglet in the context of airfoil extension?

1 I claim that the 737-MAX's "split scimitar" as mentioned in fooot's answer is different than the aftermarket double winglet seen in the United (Continental) B737-824 in the image.

Winglets don't have some mysterious magical wing tip extending property.  They don't add to the total lift of the wing (except to the extent they are canted).

The best way to conceptualize them is they extract energy from the tip flow and convert it to THRUST which can be deducted from the induced drag.  They are little wings making lift (wing..let, see?), except the lift isn't up (unless they are canted, where there is a small vertical lift component).  They work the way boat sails propel a boat forward and were originally called "tip sails".

The flow striking the winget on top is curling inboard toward the fuselage so that there is an outboard angle of attack at the winglet's leading edge that creates a lift vector (being 90 degrees to the local AOA) inboard and slightly forward.

The split scimitar doohicky just flips the effect over to take advantage of unexploited flow below and behind. The flow striking the winglet on the bottom is angling outboard, causing a lift vector to be applied outboard and slightly forward (again at 90 deg to the local AOA).

If you examine most winglets you will see that they aren't just fins that "block" the flow. They have an airfoil cross section because, as I said, they are wings making lift... just not up.

The benefit of the lift being created at the winglet is only sufficient to exceed the thrust and weight penalty of the winglet itself at lower indicated airspeeds where the vortices are stronger.  For this reason you (almost) only see them on high altitude aircraft that cruise at low indicated airspeeds.

A reasonably complex question.  What you see is call a Split Scimitar Winglet.  Lots of references online – see Wingtip device on wikipedia.  Simplest explanation: "Their intended effect is always to reduce an aircraft's drag by partial recovery of the tip vortex energy."
A better answer maybe that United believes that a cruise performance boost of 30-40% is worth the effort.

Thus, my question is: How can one best describe the aerodynamics (and benefits) of the second, downward-pointing winglet in the context of airfoil extension?

You can extend the wings many feet which reduces drag and also makes for a very ramp unfriendly airplane.  Winglets make wings behave as if they where longer.  Wingtip vortices are reduced with the one upward winglet the lower or downward scimitar tip reduces vortex loss even more.


Lift Pressure Distribution If the pressure goes always from high to low, we should have a compression, due the lower pressure of the air stream over the upper surface, and an expansion in the airstream in the lower surface.

So What causes the "suction" on the upper surface of an airfoil and the "pushing" in the lower one?

I undestand why there is a region of high pressure and a low pressure around the airfoil. But I am a little bit confused about the effects of the external pressure (Free stream) on the airfoil stream.

The key to the lift generation is the curvature of the streamlines. Forget the popular, but unfortunately wrong explanations based on the non-equidistant particle paths.
The simplest example will be if you place a plate with an angle of attack in the subsonic flow: the streamlines will get curved on the both plate sides as the flow turns.
Therefore, even a flat plate will generate a lift once its angle of attack is different from zero. As the streamlines are turning on the upper surface, the pressure will get reduced from the free-stream to the airfoil surface.
In other words, moving away from the airfoil, the pressure will increase - in the direction of the radius of curvature of the streamlines.
Important to note is, that this pressure drop is normal to the streamlines. This change in pressure is simply an effect of turning the flow, i.e. changing its direction.
Mathematically, the pressure gradient normal to the streamline, dp/dn, can be expressed as
$$ \frac{dp}{dn} = \rho \times \frac{V^2}{R} , $$
where R is the radius of curvature the streamline. Derivation of this equation can be obtained if writing the momentum equations in the natural coordinates.

I’m not sure where to pitch this but I’ll have a go...  There are a few ways to look at it - perhaps simplest is that the top surface is longer the an the straight-line distance across the chord and so the airflow must speed up as it is effectively stretched.  If the air were stationary then the surrounding air would move in to equalise the pressure, but it’s a dynamic situation and so the surrounding air will accelerate inwards and so much of the pressure differential remains.


How do aircraft discern that an ACARS message is being sent to them? This may be a simple question, but how do airplanes know which ACARS messages are meant for them from the ground? I'm not sure how physically close planes would have to be in order for that to be an issue. Do they simply receive all ACARS transmissions in the area and the CMU checks for this information in the header and continues to route it if it has the proper header? I'm assuming the VHF doesn't perform any checks since the data is still an RF signal at that point.

A typical ACARS VHF transmission.

Mode A

Aircraft B-18722

Ack NAK

Block id 2

Flight CI5118 <<< looks like an identifier the receiver can filter on

Label B9

Msg No. L05A

Message /KLAX.TI2/024KLAXA91A1

https://en.wikipedia.org/wiki/ACARS


Is the low air pressure on top of the wing the major contributor to lift? I am kind of old time flyer. Started to fly with those primitive hang-glides with titanium frame and the yoke control, around 1966 and then after a few close calls and getting older I switched to small airplanes like Cessna 172 and Cherokee Warrior.

But from my decade of flying those hang-gliders I remember times when my wing fabric had collapsed because I must have been hit by a swift downdraft. I had to push it back up and hold it there a few seconds to stop stall. I needed to apply considerable upward force to push the fabric back up to airfoil shape. So I know how powerful the down-flow stream is.

If we consider the total pressure on top of the wing
 $ P_{wing} = P_{atm} - \frac{1}{2}\cdot\frac{dm}{dt}\cdot v_z^2$
and vertical speed $v_z = sin(\alpha)$, so by multiplying $v_z$ by the Wing surface we get the vertical down-flow and plugging it into the above equation we get a rough estimate of lift caused by low pressure on top.

I have checked many references on how lift is created and have not found any that refer to this major component of lift which I have personally experienced. Am I missing something?

I am adding a nice sketch of me rowing a boat. Using this setting as a poor man's wind-tunnel.
If you delay lifting your row at the end of its stroke it will glide back by the force of momentum of your boat. If you keep the row steady as shown on sketch and force it to drag behind you,what happens?
It creates a small swell, with water washing the outside face of the row a bit higher, shown on the sketch on red, and on the inside face of the row it creates a vortex at front and water washes the row a bit lower than lake level, shown in green.

My question is this: after reviewing many researches by labs and NASA and many other interested parties even prestigious universities, one finds less weight given to this significant part of the dynamics of lift: low pressure on top of the wing causing air to spill down to fill in by converting part of atmospheric pressure to kinetic energy, while relieving the wing top of full force of ambient atmospheric pressure?
I did a very rough estimate assuming the 176 square feet wing of Cessna  172 made of flat balsa and came up with approximately 850 lbs at 55 kn which is the rotation speed.

I am familiar with accepted theories like Newton's change of momentum of stream of air and NACA airfoils.

You must have read the wrong sources. Low pressure on the upper surface of the wing is really the major source of lift. The surrounding air sucks the wing up as much as it pushes it up from below.

On a flat plate, the contribution of suction and pressure is about equal. On a wing with a thick airfoil, some additional suction is added on both sides due to the displacement effect of the wing, so the resulting negative pressure change on the upper surface becomes bigger than the positive pressure change on the bottom. At low angles of attack, you even get suction on both sides.

Your pressure calculation is maybe helpful for the mean pressure difference between both sides of the wing, but it will not give a correct value for absolute pressure. It is better to calculate lift from the momentum change imparted on the air by the wing, as shown in this answer.

Your example with the row is well chosen: The local height difference of the water to the undisturbed sea level is equivalent to the local pressure difference to the static pressure (a higher water level signifies higher pressure) and shows the approximate conditions on a section of the wing. The row is like an inclined flat plate. You can even change the angle of the row and see the additional trailing vortex as it floats away from the row.

Now for the hang gliders: They are peculiar in that the airfoil shape depends on local pressure, and early models did not use lengthwise stiffeners so the pressure over the fabric could invert when the wing caught a negative angle of attack for a moment. You are lucky that you could press the fabric up again: Several early hang glider pilots could not recover and fell to their death. This happened when designers tried to increase the aspect ratio of the initial Rogallo shape of their gliders. I only know a German word for this phenomenon: Flattersturz.

The real cause of lift and the logical reason that an object moves up is indeed a pressure difference. But how this pressure difference is created is one of the most misunderstood things in physics.

The short answer is that a wing produces a force in upward direction (or downward in case of negative lift). This force works against the air pressure coming from above the wing, with the result a pressure decrease on top. But on the underside this force works now with the air pressure coming from below the wing, with the result a pressure increase from below. The net force is upward.
I made this video demonstration to show how pressure difference due to flow turning is the cause of lift instead of pressure difference due to Bernoulli principle.

A longer answer is hidden in this physical explanation about the center of lift I wrote in another publication:

Much is written about the center of pressure or center of lift - which is actually the same - but few people know what it actually is in a physical sense. The center of pressure, shortened as CP or CL is an important factor in aerodynamics as its position relative to the center of gravity (CG) estimates in an important way the stability of a flying system.

To really understand what the center of pressure is, it’s important to first understand the true principle behind the generation of lift which is all about acceleration of air caused by the turning of that air. Whether a flow of air is turned down along the upper side of a wing or along the lower side, in order to turn the flow, it is required that a force is acting on it. The force that is responsible to turn air along both the upper and lower side is the viscosity of air. Compare this to a viscous fluid that sticks more to your hand then a less viscous fluid. As the air sticks to the wing surface, it is able to excert a force on the wing. This is to counteract the force that the wing acts on the airflow by turning it down. Or in another words: the well-known action=reaction principle, discribed by Newtons third law.

Center of Pressure is Action=Reaction Point

This action=reaction point is the center of lift or the center of pressure. It is called the center of lift because it is the point where the lift force acts on a lifting surface (wing) or lifting configuration (aircraft). It is called the center of pressure because this is the average point of all pressure acting on the lifting surface or lifting configuration. Realize that as the air is deflected downwards by the wing, the air exerts a force on the wing in the opposite direction which means that it adds up to the pressure on the underside of the wing with the result a bigger vector in the upward direction. But on the upper side of the airfoil now we have a smaller vector as the pressure is lowered because here is a deduction of the pressure on the wing caused by the force in the upward direction. The result is a net force upward. This vertical pressure lowering is the real lift force.

Wrong Explanation

A very common wrong explanation of the center of pressure is that it is caused by the pressure lowering due to the Bernoulli principle on top of the wing. The faster airflow over the top of the wing is indeed causing a pressure drop on this location but this is by far not comparable in magnitude to the pressure lowering caused by the flow turning as discribed above. Therefore, the faster airflow over the top of the wing is not the cause of the lift generation but the flow turning is the real cause.

A more advanced answer about the real mechanism of lift I wrote here


Is a single engine failure on the HondaJet recoverable? Is it possible to recover from a single engine failure at cruising altitude on the HondaJet and safely land the aircraft?

There would not be much point in having two engines if a single engine failure would be unrecoverable.  In fact, that would make the plane twice as likely to have an unrecoverable event!

If one engine has a 1/1million chance of failure, then the chance of failure of at least one of 2 engines is 2-in-a-million.(technically, slightly less than 2-in-a-million... but close enough)  And if either engine failure is catastrophic, then you've doubled your chances of losing the plane.

I don't know about the HondaJet in particular, but by logic and statistics, I can say that a single engine failure must be recoverable.

Normally, for a dual-engine aircraft to be certifiable, it must be able to fly on a single engine.

In practice, a single engine failure is very dangerous because the dead engine generates huge drag, so the yaw effect is enormous. In many cases the pilot must reduce the power from the remaining engine to mitigate yaw imbalance, so the aircraft ends up flying at a minimal power which is dangerous. Nevertheless, in most cases if the failure occurs at altitude, there is a good likelihood of being able to land successfully if good flying and procedures are followed.

There is an old joke, "the purpose of the second engine is to fly you to the site of the crash in case of an engine failure".

Contrary to what another answer states, dual engines are not used to provide safety redundancy, they are used to provide more power. As a general rule, aircraft designs with dual engines will usually have greater power, speed and range than comparable single-engine designs. Dual engines are actually more dangerous than single engines because they are more complex to fly and you have double the chances of engine failure.