Is a trimmed flight not always an unstable system? As soon as I am flying in trimmed flight condition, my pitching moment around the center of gravity becomes zero. So my center of pressure has to be located in the center of gravity, so that there is no resulting turning moment.
But assume, there comes a wind gust, and the angle of attack will increase. This causes the center of pressure to move forward in front of my gravity location. So the aircraft will become instable because the angle of attack will further increase (nose up). The only thing the pilot could do is to use the elevator to trim in pitch.
So in the end, as long as I want to use the principle of "trimmed flight" , I will always have to use control surfaces to counteract the unstable flight condition. So why to care about stability, in trimmed flight it will always come to an unstable flight condition as long as I do not use the elevators?
Can you follow me? Is this true?

But assume, there comes a wind gust, and the angle of attack will
increase. This causes the center of pressure to move forward in front
of my gravity location. So the aircraft will become instable because
the angle of attack will further increase (nose up). The only thing
the pilot could do is to use the elevator to trim in pitch.

This is not accurate.  Are you familiar with "free-flight" model airplanes?  Some of them are stable enough to be flown in the middle of the afternoon when the atmosphere is full of thermal updrafts and downdrafts, with no input from a pilot whatsoever, and no automatic control system (autopilot etc).
You are correct that if some disturbance increases the angle-of-attack of the wing, with a conventional airfoil with no "reflex", the center of lift and center of pressure of the wing will move forward, which is destabilizing.
But the tail is also seeing an increase in angle-of-attack, or if the tail is set to fly at a negative angle-of-attack in normal flight, then the angle-of-attack of the tail becomes either less negative or somewhat positive due to the disturbance.  Regardless of which of these is the case in any given instance, the change in angle-of-attack of the tail, which is far from the CG and therefore exerts lots of leverage, causes the center of lift and center of pressure of the whole aircraft to move aft of the CG.  So the aircraft noses down to a lower angle-of-attack, where the center of lift and center of pressure are again co-located with the CG.  That's how pitch stability works.
For more, see this section of John Denker's "See How It Flies" website.
Note-- for simplicity, I'm assuming that the only forces we need to consider are the "upward" or "downward" ones.  I.e., that the "center of lift" and the "center of pressure" are one and the same.  This is not exactly true.  For example, if the thrust line were way above the CG, then it would still be true that the center of pressure (including the effect of the thrust vector) was co-located with the CG when the aircraft was at equilibrium, but the center of lift would have to be a bit ahead of the CG.  If you find this simplification to be objectionable, then just (mentally) strike out "center of lift" from my answer and just leave "center of pressure".
In my answer here, I'm assuming that some sort of disturbance, such as a sudden updraft, has increased the angle-of-attack of the aircraft.  However, there's actually an additional question embedded in your question, which is "will a purely horizontal gust of wind cause a change in the angle-of-attack of an aircraft that is flying horizontally?"  That's a whole other "can of worms" that we may not really need to get into right now, but let's at least "lift the lid" and take a peek.  The answer to that appears to be that the sequence of events is as follows:  gust of wind from front -> increased lift -> upward curvature of flight path (which may temporarily decrease the angle-of-attack slightly) -> nose-up change in pitch attitude to conform to new direction of flight path -> continued upward curvature of flight path and continued nose-up pitch rotation.  All assuming that the aircraft has not yet come to equilibrium with the new velocity of the surrounding airmass at any given point in the chain of events.  It's not obvious that the angle-of-attack is ever increased above the original angle-of-attack.

I think most answers has missed phugoid oscillation which is a key word here.
Most general aviation airplanes are designed to be more or less stable in flight once trimmed for the actual situation. This means that if the flight is disturbed somehow, the plane most often returs to a new stable situtation. This goes for all the three inputs: roll, yaw and pitch. In the plane you can try this by giving a short measured input on the controls, letting go and wait out the response.
This stability generally comes at a cost of lower performance. So in high performance airplanes the "natural" stability might be intentionally lower: witness as example the extreme case of jet fighters where modern ones often cannot fly without the computer continously counter effecting the instability.
But back to "naturally" stable planes which most often shows a specific pattern called "phugoid osciallation" in the longitudual or pitch direction. This is one of several oscillation modes a plane can have, but one that is most easy to observe in a GA plane. If the plane gets a disturbance in either speed or pitch the following happens (I assume a traditional tail design + starting pitch down):

The plane pitches down below "steady horizontal".

the speed will now increase
as the speed increases, the down force of the tail will increase and start pitching the plane up (the tail is a wing that pushes down in steady flight, as the plane is pitched down, the attack angle of wind will change and the speed of air over it will increase, giving more "down lift").

The plane will now pitch up, often a bit above "horisontal".

the speed will now decrease
as the speed the decreases, the down force of tail will decrease and start pitching the plane down

Start from 1. again.

This will lead to an oscillation. In GA planes a typical period may be around 30 seconds. (There can actually be several periods concurrently). And this now continues with three possible outcomes:

The phugoid oscillation will decrease and gradually stop
The oscillation may decrease and continue at a constant amplitude. Often this is the desired state when designing airplanes, as totally stable migth cost to much performance.
The oscillation is to extreme to be comfortable or over time increases and leads to extreme pitch changes and you as pilot need to take action.

Over time the trim has to be changed as fuel is consumed and changing balance or the engine changes the output due to air temperature and so on. But for an interval, sometimes 30 minutes in my experience the plane simply flyes itself.


What is the most efficient way to get your instrument rating? For someone that has recently completed their private pilot SEL, what would be the suggested next steps/process to getting instrument rated as efficiently as possible (both time and money)?

The fastest path to any rating is intensive training.
If you can take time off from work and do an "accelerated instrument training" program you can probably knock out the rating in less than a month. That AOPA article says 8 days, I've seen a lot of two-week courses advertised.

Note however that there's a difference between being instrument rated and proficient for real-world IFR operations in actual IMC. These courses will almost certainly cram all the knowledge and procedures into your head that are required to get you to pass your instrument checkride, but you may have never flown an approach in actual IMC (if suitable weather doesn't pop up for you to fly in your time will all be simulated under the hood), and because everything is compressed and focused on "passing the test" you may come out of the program lacking the confidence to actually go out and use your newfound rating without doing some additional work with a local CFI (this is a point sometimes raised by pilots who have gone through these accelerated training programs).

Well for money, the military is the most efficient.  For time, just getting it done as fast as possible (one of those 10 week/summer programs) and showing up to each lesson prepared.  For being extremely confident in IMC flying, again, the military.


Why do many fighter jets have double vertical stabilizers? Many fighter jets have double vertical stabilizers, e.g. F-14, F-15, F/A-18, F-22:

On the contrary, most general aviation planes and commercial airliners do not. While there are certainly exceptions in both categories, I think it is fair to say that this design is, comparatively, far more common in fighter jets.

What is the advantage of double vertical stabilizers? Why is it common in fighter jets?

There are a few reasons many fighter/bomber designs have 2 vertical stabilizers:

one large tail produced more shear force at the root, 2 smaller tails mean less force at the vertical stabilizer root. This allows for a lighter structure
redundancy if one tail is damaged. The US navy philosophy is 2 is better than one, this makes sense given that its aircraft spend much of the time over oceans where there's few alternative landing sites
twin tails are shorter than one big tail, and vertical height is a consideration on carrier aircraft
twin engines spaced apart make for a wide fuselage back which can create a blanking effect with a single tail in the middle, 2 tails mean that there's always one vertical stabilizer that is effective
Twin tails create less of a radar cross section, and allow the tails to be angled to deflect radar signals away from airborne radar sources

the reason for two vertical tails is so the plane can maneuver during high angle of attack situations.  With a single vertical tail, you cannot turn ( maneuver ) during a high angle of attack situation, making the plane a strafing target.


Why aren't bird radars placed on the front of commercial aircraft? Unfortunately, my skills and understanding of radar and the like are very limited, but if somebody could enlighten me why there is no such thing as a dual bird/weather radar to switch to?

I've understood there there have been some (successful) attempts with ground station. It would add weight, but i'd guess that perhaps would be offset for some time by a single ruined engine.

There really is no such thing as "bird radar".  Radar comes in a few frequency bands that have certain advantages depending on the distance, resolution and attenuation you need out of it.  The band isn't the only factor here as the band will dictate what size dish you need and your aircraft nosecone will but a distinct upper limit on that.  Weather radar is geared toward detecting rain drops and anything bigger than rain (hail, bugs, birds, etc) with reasonable distance and attenuation features (you want to penetrate into a storm).  Birds will show up on this radar, though depending on smoothing and processing algorithms you may need a flock of them before they get drawn on the radar, at which point they'll look like some rain.  A radar optimized to detect bird sized objects would probably use a wavelength that did not let you see things as small as rain drops and small hail.
Some other things to keep in mind are that the radar is not always on, and it should be off on the ground except during takeoff when needed.  The reason for this is that you do not want to be putting radar energy into people, and they tend to be in the way on the ground.  We only turned the radar on for takeoff when there was significant weather around the field and wanted to be prepared for requesting diversions.  Aircraft radar tilt can also be adjusted, so we wouldn't be looking at birds in front of us, but clouds above and in front of us.
Wit that said, our SOP on fields with known heavy bird activity was to takeoff with the radar on and pointed forward because someone had the idea that the birds didn't like being in the radar beam.  I have no sources to back that idea up, but it was written into our FOM.
Now assume you can takeoff and see a flock of birds on the radar.  Do you have enough time to do anything about it?  If so, what do you do, some maneuver away from them or do you turn into them in hops they take the evasive action?
Now back to the title question: Why aren't bird radars in the nose cone?

The weather radar can see birds, it would be redundant.
A bird radar (optimized for birds) would not be good at seeing weather, so where would the weather radar be placed?
In a real situation, what would a bird radar enable you to do with a positive detection with the time you have to react to it?  Probably not much.
Weather is a much more common threat than flocks of birds, so if you can only have 1 radar system, you want the weather radar.

The weight of the bird radar would be trivial and it could easily fit in the nose cone; they aren't that big.  They don't have to look nearly as far out as weather radar,  All they have to do is indicate altitude and direction.  Aircraft can easily avoid collision by changing altitude.  They only have to miss the bird by being a few feet higher or lower, and birds oftentimes fly level at the same altitude, especially when migrating.  The computer could be programmed to avoid collision by adjusting the angle of ascent or descent.


What if all passengers pay to check in extra baggage? Suppose a given flight is fully booked and everybody shows up. Not only have they all brought the allowed fully loaded carry-on & checked-in luggage weight but exceed the free-limits. They are all willing to pay extra for, say, an additional 10 Kg per person. Let's assume average weight for all passengers.

Will this cause the flight to go over the Maximum Take-Off Weight? If so, will the flight depart?
What are the measures taken for the passengers & their cargo for such flights? Are they divided to take separate flights?
What if everyone has 20 Kg extra? How about 30 Kg? Is there any limit to it?
Does it add if everyone is overweight?

I am asking this in a general context of a non-budget airline.

To answer the questions as asked,

We won't depart over the maximum takeoff weight. We'll do something to bring the takeoff weight down to not exceed the limit.
Generally, people fly as scheduled if at all possible, and as much as possible their bags go on the same plane that they do, and other cargo is a lower priority. There are exceptions; passengers might be denied boarding when the plane is simply too heavy with every one and every thing on board, but this is a last resort and not particularly common.
Total weight is total weight, and the more of it there is, the more limited your options become.  This is explained in more detail below.
If the people are overweight, it's probably not a factor in the numbers, because for scheduled commercial flights, most airlines use average numbers for passengers and their carry-on bags.  So it doesn't matter if they're all skinny or all heavy, they all count for the same "200 lbs".  For military charters and sports charters, actual weights are used, and some non-US carriers may use actual weights as well.

Okay, that's the what; let's discuss the "why":

There are a lot of "it depends" elements here, but in most cases, what happens is that the amount of other freight that can be carried gets reduced, and in the worst case some of the bags stay behind.  But that's probably fairly uncommon.  It's a very worst-case scenario when you can't even take all the passengers -- pretty rare, in my experience.

Your maximum takeoff weight is limited by one of three limits:

a takeoff limit (how much the airplane can weigh to take off & climb out while meeting all of the various requirements),
an enroute limit (essentially, if you lose an engine at some/any point enroute, can you clear all of the terrain along the way to get to a suitable landing site), and
a landing limit (your expected landing weight is no greater than the maximum allowed landing weight).

In practice, the enroute limit is pretty rare, and if the departure runway is long enough, the landing limit is usually the most restrictive.  So, most of the time, your limit boils down to: basic weight of the aircraft, plus cargo, plus passengers, plus landing fuel, can't exceed the maximum certified landing weight.  The aircraft weighs what it weighs, and a full load of passengers is essentially a given weight (most flights use an "average" weight value rather than actual passenger weights).  So the interesting variables are the landing fuel, and the cargo weight (bags plus freight).

If it's a good weather day and no alternate is required, then the required fuel for landing is one number -- let's say 5,000 lbs in a 737, assuming a fairly "best case" sort of day.  (Various operators may have slightly different numbers, but this is a high-level explanation, not way down in the details.)  But if the weather is bad, that number can go up: allowing fuel for holding, as well as for a possible diversion to an alternate.  Depending on how bad the weather is and how far away the alternate, minimum landing fuel might be 8,000-10,000 lbs for a 737, or more.

Thus... my maximum landing weight is 144,000 lbs for a 737-800.  If the airplane and the people weigh 125,000 lbs, I have 19,000 lbs left over.  If my landing gas is 5,000, then I can take 14,000 lbs of bags/cargo/whatever -- which is a lot.  If the weather is crummy at the destination and instead my min landing gas is 12,000 lbs, then I'm limited to 7,000 lbs of bags.  If today's flight had 6,000# of bags and 3,000# of cargo, it all goes in the first case, but not in the second, and it's usually easier to let the freight take the next flight than the bags -- it gets expensive to deliver bags that the airline didn't get on the same plane as the passenger!

There can also be instances where the max takeoff weight is more limiting... a high & hot airport and/or short runways can limit how much weight you can take off with.  So perhaps "today" where I'm departing, the best that I can do (after considering current winds & temperature and the various runways available) is a max takeoff weight of 165,000#.  If I'm burning 25,000# of gas getting where I'm going, then I'll land at 140,000# -- max landing weight (144k) is no longer the limit, max takeoff weight is.  And the computation now looks at the various weights plus min TAKEOFF fuel compared to the max takeoff weight, rather than weights plus min landing fuel compared to the max landing weight.  But the basic process is essentially the same.

Okay, disclaimer, there are various other things than can come into play that I've glossed over for the sake of simplicity -- max ZFW and improved climb and wet landing runways and plenty of other stuff.  If you're studying for your dispatcher license, this is NOT the answer to address all your what-if's.  It's just an overview.

I work as a ramp agent for a major airline so I can tell you exactly what will happen in this type of scenario.

Once a checked bag gets weighed, very rarely will we get more than 5 heavy bags for an A320 or A319 type aircraft with 177 passengers max in any one plane and on average we only get 2 heavy bags out of an average of 72 checked bags daily for a full 177 passenger flight.

But! Let's play with your little fantasy of absolute perfect storm of probability here:

If ever! Ever! We get 177 Heavy! bags to load onto an aircraft no one I mean no one! Will load them. These little demon bags we call heavys are responsible for throwing the backs of many ramp agents causing serious spinal injuries that is notoriously common in this occupation.

Honestly the entire occupation reminds me of how reckless humans can be. I work in an airport where people board using an Ada ramp (outdoors boarding) so I see people all the time spitting out their gum on the turmac, littering, smoking near a plane, going under stanchions, ropes, and even jumping over fences to get on the wrong plane as though we create obstacles for them to board! I swear if I get 8 truck tires and lay them down near a plane as to create an obstacle course we will always have 2 to 3 dumbasses a day sprint through it while quickly darting their foot through each tire as though it were a real obstacle course.

So anyway it felt like I threw out my back on that last paragraph so "back" on track here: (pun intended)

The most bags we loaded onto an aircraft at one time was 130 bags with 5 heavys and let me tell you something: God forbid your out of shape when you have to load these bad boys because the pain is coming. Loading each of these bags onto a plane consists of 3 people: one person putting the bag on the conveyor belt, one person receiving the bag on top within the belly of the aircraft (feeding bags) who then slides the bags to the third who is responsible for lifting and stacking the bags neatly against the bulkhead (back wall) however! Sometimes! we are understaffed like at the airport that I work in where there is only one person in the belly receiving the bags and let me tell you my friends, even if you are a cross fit athlete at an Olympic event there is no preparing for this one for it is the ultimate ungodly workout with many having passed out in the bin from an exhaustion. Yes we have had heart attacks while carrying your bags. Vastly underpayed we are dying for your ability to fly so basically like the military we are sacrificing our lives in exchange for your freedom to see your families. God bless us.

So if ever we get 177 bags we will need 5 people to load these whereas on average we only have 2 ramp agents available.

The great news is that they actually have people like me in the world who will load all 177 heavy bags by myself if need be and I can do it all in under 30min. If you don't believe me share this and I will do it as a spectators event.

Realistically a thirdparty carrier will come into assist like fed ex or ups etc.

Hope this helps.


How are pilots trained to prepare for crashes? I once read somewhere that pilots can go to some facilities so get trained for crashes.

  I actually went to a facility in New London, CT where they actually
  dunk you in water strapped in the seat of an airplane upside down and
  with your eyes closed in a big pool.

Is there any formal training for the commercial pilots to be prepared for crashes? If yes, how far do they go in training pilots and for what kind of crashes?

From my research to date it seems that most general aviation and commercial fixed wing pilots are instead trained rigorously in crash avoidance rather than crash escape. Flight attendants and other cabin staff drill regularly in crash preparation and cabin evacuation in several situations (gear-down landing, belly landing, water landing, fires blocking escape doors X, Y and Z, etc) but in an emergency, the job of the people behind the yokes is to first make sure the plane gets back on the ground in as close to one piece as possible; nobody's walking away from a nose-down dive into a mountain.

They send you up in a small airplane and turn off the engine. Just kidding.

There is no formal training, except in the use of standard emergency equipment such as evacuation slides and the emergency oxygen system.

Every aircraft has an elaborately detailed set of emergency procedures which are kept in binders (although this is shifting to iPads). When something bad happens, you are supposed to turn to the relevant section of the binder (there are quick reference tabs) and follow the instructions. In theory, you are supposed to memorize all the procedures, but in practice most pilots just know the critical ones, like fires and engine failures. Even if you think you have the procedure memorized, you are still supposed to use the checklist.

One problem with simulating emergencies is that it is dangerous. For example, the FAA used to mandate spin training for all pilots. Then, years later, they found out more people were getting killed in the spin training, than were getting killed in real, actual spins. That put an end to the spin training.


Who checks the instruments and avionics before the crew boards an aircraft? Before the crew boards the aircraft for a commercial airline flight, is there anyone who checks the instruments and avionics system? I'm sure the engines are checked  but what about the avionics systems, instruments, sensors,etc.?

There was an accident due to an incorrect reading of the pitot sensor, is there somebody to check it?

In general, nobody else does.

When pilots arrive to an aircraft for the first flight of the day, extra checks are performed by the crew to verify aircraft systems are all working properly.  These checks are done once per day and subsequent flights will perform abbreviated checks instead.  A visual walk around inspection is accomplished before and after every flight.

Maintenance will check on airplanes at required intervals but outside of specific checks (associated with the carriers approved maintenance program) the only time they'll check something like a pitot tube is if the crew notes a problem and writes it up.  At that point maintenance will address the issue to return the aircraft to airworthiness.

In theory the pilot in command (the Captain) has responsibility for making sure everything works before flight. On small aircraft responsible pilots always check all instruments and sensors. For example, a pitot tube will be checked to make sure that it is clear and that no spider or mud wasp has taken up residence therein. Another key check is to make sure the antennas are all there and firmly attached. Most checklists have at bare minimum an "instruments" line item and may have many line items for specific instruments. When a pilot does the "walk around" they will specifically examine each and every external sensor such as static ports, etc.

Unfortunately, commercial pilots rarely have time to do such thorough checks, which can be very time consuming for a large aircraft. In many cases they will often not even do a walk around, even if their airline's policy mandates it.


What is the typical value of maximum lift coefficient for aerobatic aircraft? I'm working on a conceptual design phase project for aerobatic aircraft (for university) and I can't seem to find any statistics about key aerodynamic properties (such as in the question), so I hope somebody here does know something about this subject?

First, don't take the two-dimensional value from an airfoil chart and use it for the full aircraft. Subtract 15-20% for wingtip margin and fuselage influence. Wingtip margin means that the wing is less loaded towards the tips to keep a lift margin for aileron control and to avoid a tip stall.

Next, aerobatic aircraft have lower maximum lift coefficients than GA aircraft. Their wing loading is lower, so they can afford this. A symmetric airfoil mounted at zero incidence has proven to be the best choice, and while some older aerobatic aircraft use cambered airfoils, the camber creates a distinct disadvantage in transitions from normal to inverted flight and vice versa.

And last, the maximum lift coefficient depends on quite a list of parameters. If you need a value for the calculation of structural loads, make sure you add enough strength to cover the increased lift possible with high pitch rates.

If you need a figure for a single-seater piston-powered aerobatic aircraft without flaps for the estimation of minimum speed, use a lift coefficient of 1.3.

The primary textbook for many (possibly most) University aerodynamic classes is FREE (Aerodynamics for Naval Aviators). This book explains all theoretical and practical knowledge needed to have a well rounded understanding of aerodynamics. It explains all the formulas needed to design an airplane in the real world. It does not get into the bizarre out of touch academia :)

Page 75 discusses span lift.
https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/00-80T-80.pdf

To directly answer your questions regarding aerobatic CL.  Most purely refined aerobatic airplanes will use a symmetric or modified symmetric airfoil. The exact coefficient of lift depends on shape of the leading edge, chord width, and Reynold number (~speed vs chord width).

The 1.3 given above would be close to typical - perhaps a little low, but it depends on how rounded the leading edge is and the design speed of the aircraft. Airfoil books I have show very thin sharp symmetric airfoils at 1.2 and very fat ones at 1.6. Most are 1.4 to 1.5. But remember, the exact same airfoil will show minor differences due to wind tunnel peculiarities, and now most "testing" uses computer fluid dynamic simulation!

An example of a popular aerobatic plane is Extra 300 (symmetric airfoil) often used in Red Bull Competition https://en.wikipedia.org/wiki/Extra_EA-300

You can study various CL at

http://airfoiltools.com/

Other good books I use

https://www.scribd.com/document/322901165/the-science-of-flight-w-n-hubin-1992

https://www.scribd.com/document/322676208/design-for-flying-david-b-thurston-1978

https://www.scribd.com/document/24571577/aircraft-flight-testing-design

An excellent basic series of how2 design aircraft was published in the EAA "Sport Aviation" magazine from February 1990 thru February 1991 primarily by John Roncz.


Can rotorcraft pick up a dangerous electric charge? So, I was re-watching The Hunt For Red October, (in light of current Russian shenanigans with Sweden), and there was a scene in which a crew-member on the ground was electrocuted by making contact (and presumably grounding) the helicopter, which had picked up a charge in the stormy weather. Can anyone explain more of what is going on here?

In the same way than an aircraft will build static charge on the wings from friction with the air, the same thing can happen with helicopters from fast moving blades.
From a 1962 US Army document:

Any aircraft flying in the atmosphere can be considered as a conductive body located in a highly electrically insulated environment. In such a body, electrostatic charge is generated by three principal mechanisms:

Charge transfer created by a triboelectric effect between the aircraft and the atmospheric particles, such as dust, water, water vapor, snow, etc.
Unbalance of the ionic content of the exhaust gases produced by the aircraft engine(s).
Induction charge due to high electrostatic
field gradients which are found occasionally in the aircraft flight path.

Source

Here's a more detailed photo of the US army discharging a helicopter while wearing insulating gloves:

 SGT Nathan Rodeheaven uses a static probe to discharge static electricity from a UH-60 Black Hawk before his teammate, PV2 Danny Browning, hooks up the cargo to the helicopter. The safety measure prevents the hook-up man from being electrocuted in the event there is accumulated static electricity. Photo: Kristin Molinaro

It sounds like the helicopter had no static dissipators/wicks fitted (or broken ones!), and this resulted in the normal in-flight charge buildup zapping the groundman on the submarine instead.

I'm surprised the helicopter pilots didn't notice that awful noise in their radios well before this became a problem, though!

See What are these things hanging off the trailing edge of the wing? for more details.


What are the difficulties of landing on an upslope runway? On runway like the one in Courchevel Altiport, how difficult is it to land? what is diffirent in the aircraft landing configuration when landing on an upslope runway vs normal runway?

EDIT:

Thanks for the answers guys, I didn't know that the airport had a level "touch down" point. still I am curious what should a pilot do (regarding air speed, pitch, flaps ...) if the runway itself was 10% upward (no level parts)... would the plane be in a "climbing-like" configuration? what about speed?

As far as landing in a up-sloped runway is considered, the pilot will make a lower approach if not careful as he/she may consider the ground before to be sloping up as well. From flytime.ca:

  When a runway is upsloping, the pilot thinks that the runway continues on an upslope from the terrain before it, hence thinking the terrain in front of the runway is upsloping as well.   The pilot will judge their altitude as too high, because they perceive the terrain continues on an upsloping, positive angle towards the runway  and will consequently plan a low approach that can cause landing short of the runway.

Image from flytime.ca

Another thing to note is that the landing roll will be reduced (on the other hand takeoff roll would be increased, if takeoff is on uphill direction; else, it's reduced). For example, in this airport, the actual landing point is level- shortly after the aircraft touches down, it will start traveling uphill, and the landing roll will be shorter. One (passenger) describes the landing as:

  During the approach we were below the runway and climbed up to it to land. After touchdown, we slowed down very fast due to the upslope. Very different than the normal runout after a landing.

Of course the location of this particular runway means that the landing has to be done correctly the first time around.

As you could see in the video that Ron Beyer linked (2. comment on the question) trying to land slows you down substantially as you are effectively gaining altitude when landing on an incline as you need to level yourselfes to the ground. This will make it harder for the pilot as he will have to adapt to this behauviour and land with just the right speed to not loose gliding or aproach from too low and not take to long to stop on a this short runway.

In conclusion it will take some practise to learn what speed is the best for this particular landing, but that isn't really different for other runways.
Another problem is the common ice due to the mostly cold wheather at 2000m above sea level, but that is not different to other airports in cold countries aswell.


On which point(s) in a jet engine does the reaction force act? Turbojet and turbofan engines are part of a family of engines referred to as reaction engines to emphasize the fact the aircraft is pushed forward mostly by Newton's 3rd law of motion when gases are accelerated.

Diagram of a turbofan jet engine, source
On which point(s) does the reaction force act?

The thrust produced by the engine is due to the net resultant of various forces acting on various surfaces of the engine. The thrust produced is a function of mass flow rate and the change in velocity ($T = \dot{m} (V_{e}-V_{\inf})$). So, both of these have to be taken into account, rather than acceleration alone. The location of maximum thrust generated varies with the type of engine.

For high bypass jet engines like those used by the modern commercial airliners, most of the thrust is produced by the bypass fan. Though the acceleration is not the greatest here, mass x acceleration is- so this is where the thrust is maximum. The maximum reaction forces are applied there
In case of pure turbojets, the (almost all of the) thrust is produced by the core. For low bypass turbofans, its somewhere in the middle, with the lion's share of thrust produced by the core.

Note that most of the reactive force is applied on the diffuser and the compressor due to the high pressure and the forward facing area (due to cross sectional variation) in this region. This high pressure also acts on the combustion chamber, increasing the reaction force.

In the turbine and nozzle, the variation in the cross section creates a surface area facing the rearward direction, where the gas pressure acts, resulting in an force applied in direction opposite to the compressor. The net result of all these forces gives the thrust.

Image from Aircraft Performance and Design by John Anderson; taken from quora.com

Let's take concept of energy for help. All jet engine thrust originate in combustion chamber (energy of burning fuel). Smaller or bigger part of this thrust ( depending on engine construction) is transferred from turbines blades ( high and low pressure) forward via axis to compressor / fan.
Let's take example of low pressure turbine/ fan system. Expanding gas in combustion chamber pushes on turbine blade and via axis rotates fan blades. So any forward force created by fan blades is transferred via axis and turbines blades back to its origin i.e. combustion chamber ( backward facing surfaces). So in theory if all rotating parts would be free to move forward during steadily running engine they wouldn't move.
So analogically to center of gravity of an object the central point of thrust within the jet engine lies in geometrical center of backward facing surface of combustion chamber.


How can a pilot fly a distance of 594 NM connecting two VOR's? I was researching worldwide airway designations when I stumbled upon this airway in Brazil: G678 with an MEA of 15,000 feet and a distance of 594 NM (nautical miles) between navaids.

It's drawn between the BSI and ATF VOR's. Mid-airway around S12°50.12' W52°4.38'.

Flying at 15,000' with BSI at elev ~3500', the line-of-sight is 166 NM. And as far as I know a high altitude VOR reaches out to 130 NM, maybe more, but certainly not 300 NM, right?

The "G" designation is for a non-area navigation route per ICAO annex 11. So it's a non-RNAV route. It's even colored black (vs. blue) on skyvector.

  A, B, G, R for routes which form part of the regional networks of ATS routes and are not area navigation routes.

So, how can it be flown?

You are correct that VOR's rarely have a range of more than 200 NM. However, Alta Floresta VOR (ATF) is colocated with an NDB (and DME). In spite of less accuracy, NDB's have a much greater range (partly because their signals can travel as ground waves). This makes it possible to have a very long segment on the G678 airway without other navigation aids. When leaving the coverage of Brasília VOR/DME (BSI), you should be able to pick up ATF NDB.

Several options:

1) Fly the route using GNSS navigation, as the VOR stations do not offer full coverage along that route.

2) Fly the route using INS, if the aircraft is so equipped, in the segments of the airway where VOR service is spotty.

3) Select a different route between these two points which has additional terrestrial navaids along it that all offer good signal coverage along the route.


How does the engine produce aerodynamic lift at high angle of attack? This answer asserts that "at high angle of attack the engine is producing aerodynamic lift". In the context of that answer, we are concerned about the increased torque that raises the nose of the aeroplane with respect to the center of gravity, so we should narrow down our interpretation of the otherwise vague word "lift" as the force component that is perpendicular to the wing or fuselage of an aeroplane. We define the angle of attack as the angle between the fuselage and the incoming airflow direction at infinity (shorthand word for "reasonably far away"). It is very different from the angle of the fuselage makes with the horizontal at high speed while very close to the latter at very low speed. It puzzles me in two ways.

It claims the engine itself produces fuselage-perpendicular force in addition to the thrust parallel to the fuselage and wing, despite the thrust of the engine is directed parallel to the fuselage and thus the wing. How is that perpendicular force produced?
It claims that fuselage-perpendicular force thus produced is dependent on the angle of attack. How does that work?

I would like to see a technical elucidation of this claim or some references supporting or refuting it, best if they come with mathematical derivation.

Edit:

Some answers below observe that when the plane moves, without engine turned on, against the airflow, part of the airflow curves downwards to conclude that a net force perpendicular to the fuselage is exerted. It is not that simple as looking at part of the flow stream, since part of the flow stream goes upwards above the separating point. The net force is the integration of all the pressure on the whole surface, and the pressure on the opposite side of the object presses in the opposite direction. In fact, in a potential flow, the D'Alembert's paradox  shows that the net pressure is exactly zero. Of course, real airflow is not potential flow, but this refutes the overly simple rationale of using downward airflow to derive the fuselage-perpendicular force.

However, when the engine is turned on, especially at high power. The situation may be different so long as the airflow is consistently flowing in the downward direction long after exiting the engine. The momentum of the airflow through the engine may overwhelm other parts of the airflow. But the argument must be more sophisticated than just the downward direction of the airflow.

Anything, even a metal plate creates "lift" when it is under an angle with an air flow. Lift isn't a magical attribute to "wings", it is just a physical consequence of the law of conservation of momentum: when you push a stream of air down, the reactive force is upwards.

the fuselage/engines also act as minor lifting surfaces: when they are under an angle they, inefficiently, redirect the airflow downwards.

For the engine this is also a bit due to the definition of thrust: thrust equations consider an inlet in the same direction of the thrust vector. So the inlet & compressor redirect the flow "downwards" under high angles of attack. - This effect has to show up somewhere: as a lift force generated by the engine.

It is admirable to seek the higher plane of sophistication to gain greater understanding.

First, air is compressible, so D'Alembert is out.  Probably would not be true if velocity was great enough for cavitation, but I did consider sailing battleships blunt end first to see if more energy could be recovered as flow is pulled back into the trailing edge.  There is also some evidence that air curling up and towards the front of a thin undercambered wing (or a jib sail) gives it almost magical lift properties through recovery of drag energy.

But as far as the engine creating lift force 90 degrees from its thrust (making definition of lift even more specific), we may be talking impossibility without deflection of the airstream by some other physical structure.  Tilting the engine out of the direction of travel does create some interesting ideas, but does the pitching or lifting tendency exist without the cowling around the fan?

It will be interesting to see (beyond software) what they do to solve this.  They might want to
take a look at an RB-57 Canberra.


Why do piston engines in aircraft burn fuel at a higher rate than a comparable car engine? I've never understood why piston powered plane engines burn such a large  amount of fuel per time period than car engines.

Can anyone elaborate on this?

I know that a C172M will burn ~5-9+ gph at 2200-2550 RPM between 2k-12k feet based on the POH.  This gives a decent idea of the fuel burn in a very narrow RPM range at various altitudes.  According to the POH, she came with a Lycoming 0-320-E2D. Wikipedia states that the 0-320 provides 150-160HP using a 5.23l engine.

My car on the other hand has a 2l 4 cylinder engine. At 70mph in top gear @ ~2300rpm she gets about 30mpg for a calculated 2.33gph. To me highway speeds at constant RPM provides comparable engine conditions to that of cruise phase of flight.

5.23l / 2l = 2.61

2.61* 2.33mpg = ~6gph

Does that basic analysis hold? Is the larger displacement the main factor in fuel consumption difference between the two? I consider this figure of ~6gph close enough to the 5-10gph listed in the POH for such a back of the napkin calculation if the analysis is more or less correct.

This leads to a follow up question: Is there a reason for the large displacement and low HP? For comparison a 5.7l truck engine from the 70s output 250HP for a gain of 100HP for .47l additional displacement when compared to the Lycoming. I'm in no way very knowledgeable about engine's but am guessing the answer lies in engineering trade offs for making the engine lighter or out of a different material for aviation use or for higher compression ratios for the fuel. Any detailed input would be appreciated.

I know that Volkswagen and Subaru engines have been used for aircraft.  Although my research has been very light, it still looks like the fuel consumption is in the same range.

Going back to my car vs the C172M, or the EcoTec vs Lycoming, 173hp@ 5500rpm for the EcoTec and assuming a (most probably false) linear fuel consumption based on RPM,

5500rpm /2300rpm = 2.39

2.39 * 2.33gph (@2300rpm) = 5.56gph

for comparable HP.

If my analysis provided here is close to the true answer, then I think I've cleared up my misunderstanding by writing out this question.  However, I would like someone more knoowledgeable about such things (possibly an A&P who is also a gearhead) to pipe in and verify or enlighten my analysis.

Thanks!

As others have noted, airplane engines and car engines have very different duty cycles.  An airplane engine will typically run at full power for a few minutes during takeoff & climb, then at a large fraction of max during level flight, which may last for hours, and be throttled back only during descent and landing.

An auto engine, by contrast, only operates at full power for a few seconds, as you for instance accelerate up that freeway on-ramp, then it will be throttled back to produce the small fraction of full power that's needed to maintain freeway cruise speeds¹.  Since the power output is proportional to fuel consumed², your car engine uses much less fuel.  Even though it might be rated to produce the same max HP as the airplane engine, in normal operation it produces much lower HP for the vast majority of its operating time.  You can see this in operation if you drive a car with a real-time fuel consumption gauge.  For instance, my Insight hybrid shows ~25 mpg when accelerating or climbing mountains (using near full power), but 75 mpg or better cruising on a level interstate.

Also note that the actual fuel economy of an airplane isn't that much different from say your average SUV.  I don't have the exact figures any more, but I once calculated that my Cherokee 180 got about 18 mpg.  When you figure that it's going twice as fast, and on longer trips can shorten the actual distance travelled by going in a straight line instead of having to follow roads, the comparison isn't that bad.

¹This, incidentially, is why hybrid cars can use electric assist to get much better mpg - the gas engine can be just big enough for efficient cruise, while acceleration is provided by the electric motor.

²To a first approximation.  It's a lot more complicated in the real world.  Look up BSFC maps if you're curious.

Because many of them are freaking giant like 8 liters for some reason.

That article about why car engines don't fly is BS, things have changed a lot in 20 years, Cessna was building diesel car size engines into them.

They're horribly innefficient, 2 valves per cylinder, gear driven cam that robs power, gear driven magnetos rob power, flat tappet lifters that rob power, a valve clearance that can vary because it has no setting other than changine the pushrod tube length.

Inefficient combustion chamber design, heavy engine....


Why do localizer/glideslopes use 90/150Hz specifically? Why is it that localizer and glideslope system are using 90 Hz and 150 Hz frequencies only, instead of some other LF such as 75 Hz?

Part two of my question:
How does the modulation depth differ for aircraft using localizer and GS receiver?

The frequencies are chosen to allow the system to produce a localizer and glideslope 'beam' that provides proportional guidance that can adequately guide an aircraft on the approach.

Read through this Question and Answer.

As you can see in the description of the beam-forming, the width of the antenna array and the choice of the modulation signals (90 and 150 Hz) drive the ddm proportionally to the angle of approach to the antenna array.

There are also considerations tied to the broadcast signal. Higher frequencies would require more bandwidth which would impact channel spacing.

Ultimately, the choice of the two frequencies is an optimization of a design for the approach guidance. For any given runway the current system can be set to provide consistent guidance (+/- 350 feet == full scale deflection) at the runway threshold.  This is done by adjusting the distance between elements of the antenna array.

Answer to part 1: Radio frequencies are split up for different uses by the government (NTIA). The aviation world makes use of many of these ranges, including the range around 75 MHz, but most usable frequencies are already allocated for other purposes. This chart provides a great breakdown of the US allocation of radio frequencies.


In an emergency, can an airliner land at Pyongyang or another North Korean airport? If a flight was, say, en route to Seoul, but could only make it to the closest available airstrip and that was Pyongyang, would that be allowed? What would they be met with on the ground?

I can see at least three questions here and only one of them is fully aviation-related. First, can an airliner legally divert to Pyongyang in an emergency, under the laws of its own country? For US airlines, the answer is yes. 14 CFR 91 SFAR 79 bans flights over North Korea, but does allow an exception in an emergency:

  2. Flight Prohibition. Except as provided in paragraphs 3 and 4 of this SFAR, no person described in paragraph 1 may conduct flight
  operations through the Pyongyang FIR west of 132 degrees east
  longitude.

  [...]

  4. Emergency situations. In an emergency that requires immediate decision and action for the safety of the flight, the pilot in command
  on an aircraft may deviate from this SFAR to the extent required by
  that emergency.

Second, will North Korea allow the airliner to land? This is a political question, not an aviation one. Either they say yes or they say no. If they say no to an airliner in distress, how will they enforce that? Are they willing to shoot it down rather than let it land? Would that decision depend on the country of the airliner? Can they even make that decision within minutes, or is it already made and they simply initiate a defined procedure? The pilot is probably just going to try to land anyway; assuming the alternative is a crash, he would have little to lose.

Finally, what will the 'reception committee' on the ground look like? No idea, this is a political/military question.

But practically speaking, this situation should never arise anyway (ignoring movie plot stuff like hacked navigation systems). If your country isn't friendly or at least neutral to North Korea, your airlines simply won't plan flights where the only emergency landing possible is in Pyongyang, and for US carriers specifically it would be illegal anyway.

I think you should treat a North Korean airport as any other one. Since you're protected under international law, they can't do anything, especially if it's an emergency landing. Plus, I don't think their government would want to do something. They would probably lend a hand and help out, for their reputation. At least, if I was Kim Jiang-Un, I would do that.


What is the origin of the term "military power"? An engine producing 'dry' thrust is said to be at 'military power', a useful distinction from reheat / afterburner.

It's always seemed an odd phrase to me. There's nothing particularly military about it, and why would afterburner not be military? 'Military' has always been used as a word that always relates to soldiering in some form or another - it is a curious choice to define the operation of a jet engine.

It must, of course, be a twentieth-century phrase. Can any light be thrown on its origin?

The etiology of military power is from War Emergency Power (WEP) which in the WWII era was a higher than normal rating power (i.e. >100% rated power) setting on an aircraft engine.  Such power settings were approved for short durations (typically 5 minutes or less) such as takeoff and battle maneuvers.

The term was quickly shortened to military power.

Sometimes military power is assumed to be simply full throttle, but that may not be accurate, particularly in blown engines, where turbochargers or superchargers may be set to permit higher than normal air delivery to the combustion chamber.

Finally, military power was used for more extended periods of times, when conditions dictated, but at a reduction in reliability and service life.  Sometimes in war, service life is not a pilot's greatest concern.

"Military power" is less about the distinction between dry and wet thrust, but more about the distinction between civilian "maximum power" and "military maximum powerÄ which usually puts more strain on the engine than desired in a civilian context.


Has someone with no experience at all ever taken off in a plane? Could they? I was watching Mystery Science Theater 3000 and their showing of 1970 TV movie/pilot San Fransisco International.  One of the plots in the movie involves a twelve (or so) year old boy, who, alarmed about his parents' imminent separation, wanders across the tarmac, hops into the cockpit of an Aircoupe, and flees his mother by starting the plane and taking off before anyone quite knows what's happening.  After he unsteadily flies it a bit (the best part of the movie is some stunt pilot making the Aircoupe swerve through the air like a drunken eagle), Bonanza's Adam Cartwright talks him through a safe landing.

Plenty of questions have been asked about the possibility of planes being landed by someone with no experience at all, and, of course, talk-down landings are a real thing, even if they're rarely done by anyone with zero experience.  But I'm curious about the first part, where the kid turns a key, taxis away, and is up into the California sky without even having played Microsoft Flight Simulator.

Naturally, a civilian getting into a cockpit at SFO without being stopped seems patently absurd in the security-conscious present, but times have changed.  Has anything even similar to this ever happened?  And if not (if this doesn't over-broaden my question) could a layman get a plane, single-engine or otherwise, into the air without any experience?

The Aviation Safety Network has an article listing 11 airplanes that crashed after being stolen by a non-pilot. The most recent was the Horizon Air Dash 8 that was stolen by a ground service agent in August 2018.

There are more interesting stories such as:

The guy that tried to crash a Cessna 150 into the White House (with minimal experience and training)
Ground crew member that stole and crashed an F-16 in Norway
Mechanic that accidentally took off in an English Electric Lightning
Mechanic that took off in an F-86 Sabre
15-year-old that stole a PA-34 and took off with 4 passengers, later crashing

Whether these qualify as "amateurs" is a matter of opinion. Most of them were mechanics/engineers. It would be difficult (but not impossible) for a person not familiar at all with the aircraft to get it started and manage to take off.

Anyone who isn't employed as a pilot is by definition an amateur.

A very large number of people who fall in that category is either a pilot license holder or is (or has in the past) trained to become one.

All of those people can take off in an aircraft. And especially in the past many people did take off in aircraft without ever having had any training at all. They taught themselves to fly the hard way, by trying and see what worked. Of course a lot of those early pioneers ended up killing themselves in the process.

Now, if you're limiting your question to specific types or categories of aircraft you will limit that group further, as specific aircraft types may require knowledge that most of those people lack for how to prepare them for flight. Many may even require external equipment for them to be able to be started at all, meaning even a qualified pilot won't be able to do so without outside assistance.

But you didn't limit your question.


How does a fan differ from a propeller? Why is the fan in a turbofan different than a propeller in a small airplane? Why do propellers typically have 2 or 3 blades, but the fan has a lot more? And why is the fan shrouded but the propeller open?

I read that a ducted fan is more efficient at very slow speeds (it generates more static thrust than an open prop), but at high speeds the drag becomes too great and you're better off with an open propeller. Which is why they use a ducted fan in hovercrafts, for instance. But if that's true, shouldn't the fan in a turbofan be open (unshrouded), considering the high speeds (higher than a propeller)?

Also, how does the fan get away with supersonic tip speeds and the prop don't? Would the fan be more efficient if tip speeds were lower?

The simple answer that covers the majority of engines is that a fan has a shroud. The possible exception are unducted fans or Open Rotor Engines, which are a hybrid between a turboprop and a fan engine.
A more scientific answer is based on the difference in disk loading: How much power per disk area can be pumped into the fan for propulsion. For propellers, this is distinctively less than for fans. Some examples:

Cessna 172 with O-320 engine (150 hp) and McCauley 1C160 prop (1.9 m diameter): 39.45 kW/m².
P-51D with Packard-Merlin 1650 (1590 hp) and Hamilton-Standard prop (3.4 m diameter): 130.59 kW/m².
Lockheed C-130H Hercules with Allison T-56 engine (4590 hp) and Aeroproducts propeller (4.1 m diameter): 259.25 kW/m².

With jets, this comparison needs power when thrust is listed. Therefore, we make the heroic assumption that the speed at the fan is Mach 0.4 in static conditions and thrust is split according to bypass ratio (which is completely imprecise but will do for the purpose here):

BAe 146 with Lycoming ALF 502 (31 kN static thrust, 5.7:1 bypass ratio and 1.02 m fan diameter): 4254.35 kW/m².
Boeing 747-200 with Pratt & Whitney JT9D (213 kN static thrust, 5:1 bypass ratio and 2.34 m fan diameter): 5388.74 kW/m².
Boeing 777 with GE90 (388.8 kN thrust at take-off, 8.4:1 bypass ratio and 3.124 m fan diameter): 6077.23 kW/m².

I guess by now you see where this is heading: Turbofans are simply a different class when thrust per frontal area is concerned. And for this to be possible, three conditions must be met:

A well-designed intake which produces uniform flow over the engine face. The shroud is only the consequence of providing the fan with an intake.
A high solidity factor of the fan/propeller (the ratio of the total blade area of the propeller to the disk swept out when the prop turns)
High dynamic pressure at the engine face, so the engine power can be absorbed by the fan/propeller.

Note that those conditions depend on each other: Without the solidity, much less power could be absorbed. Without the intake, the supersonic flow at the fan tips would produce horrible losses and noise.
Now for the crazy ones. The misfits. The rebels. The troublemakers. The round pegs in the square holes:

Antonov 70 with Progress D-27 (14,000 hp and 4.5 m prop diameter): 656.4 kW/m².
MD-81 with GE36 UDF (6661 kN thrust at max climb and 3.25 m prop diameter): 803 kW/m².

Here it is hard to make a clear call whether these are fans or propellers. Their proper name "propfans" hints at that already.
A last word on terminology
Efficiency is a measure how much effort is needed for a certain result. An efficient prop needs less power per unit of thrust produced. It has few blades, turns slowly and has a big diameter. A subsonic fan would be more efficient, but would create much less thrust for a given size.
If you want to express the capacity for thrust creation at a specific diameter, use disk loading, not efficiency.

A fan is not necessarily a screw, it's any object used to move air towards something. The helix shape happens to be quite ideal but a propeller does not need to be a helix-shaped thing either, just an object used to propel something in a fluid or gas. (Propulsion through a solid is like time traveling: probably impossible. Unless we can imagine something like a tunneler spitting out whatever comes in the other end to move forward.)


Why is indicated airspeed rather than ground speed used during the takeoff roll? My instructor asked me this question during my lesson and I couldn't come up with any answers. He asked why do I need to look at the indicated airspeed rather than ground speed when in takeoff roll or as soon as we touchdown?

It would be great if you could give me the reference to back it up.

Because wings work on air moving past them, not ground moving below them.

Heck, in a 35 knot headwind, the Antonov-2 could be rolling backwards at 2 knots and still take off!

It is important to note that along with speeds such as v_rotate, predicated on IAS, there are critical speeds on takeoff that are essentially, solely or predominately functions of ground speed. Refusal speed is one. Maximum brake energy speed is another.

V1 is a speed for which ground speed is a critical component, since it is a function of runway remaining and whether it'd be possible to stop, or to be able to continue the takeoff with a lost engine.

The reason we don't use ground speed in determining when we've passed them has to do with what is available to the pilot in the cockpit to look at and safely make decisions with during the takeoff.


What are these thermal spots on this infrared image of an F-16? In this question, Hephaestus Aetnaean posted the following cool movie:

which showed the following still:

As indicated by the red circles (the yellow marking can be ignored) there seem to be a few hotspots visible on the IR image. What are these hotspots? They seem to be rather random, so I'm not sure what they exactly are.

In this picture from Wikipedia it can be clearly seen that the two along the bottom of the fuselage are both heated air data probes, as Federico pointed out. The probes are heated to prevent ice from forming and blocking air flow to the sensors. The forward one is a pitot probe, and the aft one is a total temperature probe. The AOA sensor can also be seen as a much smaller hot spot above the forward pitot probe.

I agree with Ron Beyer that the top area is probably the sun reflecting off of the canopy. The lower area is probably a sun reflection as well, off of the external fuel tank. You can see in the video that these areas are not always visible, and move as the F-16 flies past, consistent with a reflection.

I believe what you are seeing is the infrared image picking up heat sources. As the plane flies through the air friction is caused with any object sticking out from the skin of the aircraft. The nose of the aircraft has a Pitot tube which by design sticks out in the airstream. This causes a lot of friction. Additionally the pitot tube is heated to prevent ice from clogging the tube. As for the other spots, I am unsure as to the devices but they appear to be in the airstream as well. Perhaps some are small antennas.