Center of Gravitiy quesiton - Page 1
Forum section: electronics / general

author: Karamel
author_rank: Regular Contributor
timestamp: September 15, 2016, 05:10:01 pm
content: Hi everyone,Nowadays, I am intelligent in with rc planes. Actually, I am fresman at this topic, too. So, I was looking for pictures of rc planes and I got a question.Here, there is a rc f16 picture which has many deprone bomb under its wind where they are left and right side.This is also compatible picture of caricature of real f16.Yes. As you guess that, my quesiton is about center of gravity problem. Let's think that plane has just dropped 24th bomb. Left side of plane will lost weight and center of gravity will shift right side? So, How do they solve this problem in real f16 planes?I investigated old war planes which has drop bomb inside. They have bombs which are center of body. But this tipe of planes has weight two sides.

author: ataradov
author_rank: Super Contributor
timestamp: September 15, 2016, 05:16:48 pm
content: The real ones are controlled by the computer, it will just compensate for this. The model ones are typically way overpowered, that they would not care either.

author: Kilrah
author_rank: Unknown
timestamp: September 15, 2016, 05:27:10 pm
content: On full scale there could be limitations e.g. you have to drop both 16 and 24 together if required, but at the speeds these fly at the weight difference will be easily compensated with just a bit of opposing aileron input, whether automatically or manually applied.Airplanes are very sensitive to CG changes along their length that affect pitch stability, but along the other 2 axes the effect is very small compared to other forces. Unsurprisingly that's why all the load is distributed closely around the longitudinal CG point.The "bombs" on the model weigh nothing, so lateral balance isn't really a problem. Drag difference would have more effect on yaw than weight on roll, but as mentioned above other aspects e.g. the rudder would stabilize that. Plus they're usually not dropped in flight in the first place.

author: NottheDan
author_rank: Frequent Contributor
timestamp: September 15, 2016, 05:30:49 pm
content: Quote from: Karamel on September 15, 2016, 05:10:01 pmTrimming.But mostly computer control.

author: Karamel
author_rank: Regular Contributor
timestamp: September 15, 2016, 05:40:15 pm
content: Quote from: Kilrah on September 15, 2016, 05:27:10 pmYes. I thought like that but I am not sure they solve problem like that because of non-constant speed. For real planes, let's think, we have just dropped a bomb which left side. Plane must be trying to turn right side and we tried to have equlibrium the structure with trimming or aileron control. And then Okay. We got an equlibrium.But at the moment, if speed changes, equlibrium will be lost so, either computer must try every time to get true center of gravity for every speed or there is another solution.

author: ataradov
author_rank: Super Contributor
timestamp: September 15, 2016, 05:42:54 pm
content: Real fighter jets are not aerodynamically stable on their own, so computer constantly balancing the plane anyway. This helps with maneuverability, but if computer is lost, then plane is basically a brick.EDIT: Here is a good starter article on aircraft stabilityhttps://en.wikipedia.org/wiki/Longitudinal_static_stability

author: Rick Law
author_rank: Super Contributor
timestamp: September 15, 2016, 11:57:16 pm
content: Coming from a Physics point of view (i.e.: not a weapon specialist)...When dropping the bomb, it is not just weight you have to consider. The aerodynamic effect is probably as significant and perhaps more significant than the mass of the bomb itself.A short hand of thinking about lift created by a wing is: air flow on the top surface is faster than the air flow on the bottom surface of the wing. If the bomb is fully under the wing (which the picture shown it is not); a bomb attached to the bottom surface would reduce the air flow speed below the wing. Change in above-the-wing flow speed verses below-the-wing flow speed changes lift.For “bomb 24” stated in the OP, bomb 24 is on the pilot’s left side. After the bomb drops, below-left-wing air flows freer. This results in a lift reduction on the left wing. So besides mass-induced lift change, there is this aerodynamic-induced reduction in left-wing lift.The reduce overall air-resistance on the left also results in a turning force toward the right. With the lost of mass on the left, Newton’s second law F=mA makes matters worst: lost of mass on the left also made the left side easier to move -- which amplifies the turning force caused by aerodynamics.The picture shows the bombs are not entirely tucked under the wing. The front/tips of the bombs protrude ahead of the wings. Thus the air flow disruption could be both above and below the wing. Without wind-tunnel data, it is purely speculative to suggest which one is disrupted more and how. However, one can reason that air flow would be different with verses without the bomb.To compensate for all that, I suspect a feedback based trimming would be far more effective over that of a static calculation of CG shift.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: September 16, 2016, 01:17:07 am
content: Quote from: ataradov on September 15, 2016, 05:42:54 pmThis reminds me of the Swedish JAS 39 Gripen; My college professor told us that this crash was caused by the PID control system being unstable. Someone had apparently made a noob error in the implementation, resulting in the excessive control surface oscillations seen before the crash:This jet fighter is inheritably unstable to increase its maneuverability...Also, the pilot survived and crashed another prototype in downtown Stockholm a few years later. I could see the crash location from where I lived a few years later.

author: Red Squirrel
author_rank: Super Contributor
timestamp: September 16, 2016, 01:53:36 am
content: You just blow up another target, then you're balanced again!But yeah I imagine you have to compensate, and the computer probably handles that. Much like quadcopers, if you unbalance it or one prop is slightly broken, the computer will still keep it straight. To some extent.Even expending fuel would require to compensate over time.

author: ataradov
author_rank: Super Contributor
timestamp: September 16, 2016, 01:57:15 am
content: Quote from: John_ITIC on September 16, 2016, 01:17:07 amDo you know the reason for ejecting? The plane looked controllable, they do worse stuff at air shows.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: September 16, 2016, 02:34:03 am
content: Quote from: ataradov on September 16, 2016, 01:57:15 amThis link just says "Pilot induced oscillation", which of course is bunk. A fly by wire system should not allow pilot input to make the aircraft unstable. I would guess they had a too simple (naïve) implementation of the FBW system.https://en.wikipedia.org/wiki/Accidents_and_incidents_involving_the_JAS_39_Gripen#August_1993

author: ataradov
author_rank: Super Contributor
timestamp: September 16, 2016, 02:53:33 am
content: And here is a topical video:Quote

author: Tomorokoshi
author_rank: Super Contributor
timestamp: September 16, 2016, 02:59:50 am
content: Perhaps they pump fuel from one wing to the other?

author: Len
author_rank: Frequent Contributor
timestamp: September 16, 2016, 03:12:59 am
content: Quote from: John_ITIC on September 16, 2016, 02:34:03 amYeah, sure, they just have to get the engineer who designed that op-amp that is stable at any frequency with any feedback network...

author: Karamel
author_rank: Regular Contributor
timestamp: September 18, 2016, 10:07:36 pm
content: Before the opening this topic, I was just wondering a classical phisics of this issue on rc planes but I saw that question is bigger than my knowledge. I am pleased to thisSo, I am a student of department of electronics engineering. This situation must be about Control theory, musn't it? We will attend control theory lesson in 3rd year of school but, I think I can read some books which has written about control. Do you suggest me anybook?Best regards,Karamel

author: John_ITIC
author_rank: Frequent Contributor
timestamp: September 18, 2016, 10:41:56 pm
content: Quote from: Len on September 16, 2016, 03:12:59 amOP-amps are of course no longer used since these are digital fly by wire systems... And the airbus systems will absolutely not allow the pilot to input any information to the system that can endanger the stability of the system. Here's a demonstration from Iron Maiden's own Bruce Dickinson!

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 18, 2016, 10:46:13 pm
content: Making sure that the airplane flies properly with the weapons on the wing (or in a weapons bay), that the weapons leave the wing (or weapons bay), and that the plane flies fine after the weapons leave is one of the very big jobs in military aircraft weapon integration. There are many aerodynamic, mechanical and electrical factors to this, some obvious, others not so obvious.I am sure if you search the internet effectively you can find videos of oops in this area. Bombs that get caught in the airflow and just sort of tag along with the dropping aircraft. Bomb bay doors that open and then rip off. Missiles that do bad things and so on. And lots of other craziness that is amusing once you are safely on the ground.All of this difficulty is why you often see new aircraft fielded with only a partial weapon complement. And one reason why costs are so high for military aircraft. You will also find that weapons are allowed on the aircraft of one nation that are not allowed on the same aircraft from another nation. The difference being that there are differences in limitations on the flight envelope of the aircraft that are acceptable to one nation and not another. Or one nation is willing to accept more danger to achieve a given result. Or one nation gave up the possibility of carrying one weapon so that another can be carried safely. Lots of rabbit holes in this world.

author: bitslice
author_rank: Frequent Contributor
timestamp: September 18, 2016, 11:55:47 pm
content: This becomes a lot simpler when they move to Flying wing designs, as the payload is internal.

author: rrinker
author_rank: Super Contributor
timestamp: September 19, 2016, 01:41:46 am
content: Except that flying wings are inherently unstable and only ended up being practical after computer control technology caught up.

author: hendorog
author_rank: Super Contributor
timestamp: September 19, 2016, 01:49:50 am
content: Quote from: rrinker on September 19, 2016, 01:41:46 amI'm sure that is not true, there are plenty of RC aircraft flying wings which are inherently stable.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 19, 2016, 04:07:37 am
content: Carrying weapons internally solves lots of problems during transit. But if they don't become external at some point there is really no point in having them along. The transition from internal to external is where all of the excitement comes. There are a couple of generic approaches. Use an elevator or crane of some sort to move the weapon out into the airflow, and then launch like an external store. The other is to use springs or explosives or something to eject the weapon from the aircraft. Both have really interesting characteristics. As in the Chinese curse "May you live in interesting times."There is no free lunch anywhere in this arena, just design choices and lots of skull sweat. By the way, the original posters concern, shifts in cg, is usually pretty low on the list of problems.

author: Monkeh
author_rank: Super Contributor
timestamp: September 19, 2016, 04:35:51 am
content: Quote from: ataradov on September 16, 2016, 01:57:15 amThe aircraft was out of the immediate control of the pilot. It placed its belly in the direction of flight at 270m. That's not enough room to recover if you know what the aircraft is doing, and he did not.The pilot made excessively harsh control inputs and induced oscillation. Yes, there were some flaws with the fly-by-wire, but the primary cause was the pilot pushing an unfamiliar (he was trained on prior prototypes, this particular aircraft had numerous changes) aircraft to the limit of its control authority.For those unfamiliar with PIO (Pilot Induced Oscillation), it doesnotimply the aircraft is oscillating for itself and unstable, the PILOT is causing the oscillation by continuously applying excessive control input in reaction to the result of his previous input. In the case of this Gripen the computer, uh, 'assisted' and forced the control surfaces to deflect at their maximum rates. If not for that effect it would likely have been a wobbly flight followed by recovery and change of underwear.Suggesting the fly-by-wire controls in a fighter aircraft should prevent this is like suggesting the traction control in your sports car should prevent you understeering out of a corner because you were going too fast. It's not an Airbus.

author: MrOmnos
author_rank: Frequent Contributor
timestamp: September 19, 2016, 06:17:41 am
content: Quote from: rrinker on September 19, 2016, 01:41:46 amAll fighter aircraft are made inherently unstable.

author: wasyoungonce
author_rank: Frequent Contributor
timestamp: September 19, 2016, 06:37:50 am
content: As said....flight control computer do all this. Pilot does have trim but its not needed for stores ejected.One FA18 I saw lost the entire inboard flap, which is a major flight control surface, pilot didn't know till spotted by wingman. The jack was still there giving position feedback...the flap was not. Another had a midair head on 1v1 radar attack and collided with wing tip. All tip to wing fold pivot all gone. Plane handled ok not great but ok enough to land safely. Not so for other plane though.Those FCS computers are unbelievable.

author: Kilrah
author_rank: Unknown
timestamp: September 19, 2016, 08:17:27 am
content: Quote from: rrinker on September 19, 2016, 01:41:46 amThat is hugely incorrect. Some flying wing designs are, just like any other type, but putting them all in the same bag is plain wrong.My record is a foamy RC flying wing that flew itself for 10 minutes without any input. The slope spot we were on and probably a lot of luck resulted in currents that somehow caused it to turn and stay in the general area instead of just flying away. At some point it seemed it would do what I wanted it to without me doing anything for a little while, so I just put the radio on the ground and watched things happen for 10 minutes. Only then I had to take back since it decided to finally drift a bit too far. Was pretty impressive.

author: rrinker
author_rank: Super Contributor
timestamp: September 19, 2016, 01:01:00 pm
content: I'm talking real - check the history of the B-2 Bomber and its predecessors like the YB-35 and YB-49, not to mention the Ho 229. Model aircraft tend to be way overpowered and can fly pretty much no matter what - there are flying Snoopy doghouses even - with all the aerodynamics of a shed. With enough thrust to weight and enough wing to keep the thing from spinning on its axis, darn near anything can be made to fly. How well is a different story. Physics doesn't scale, so models almost always tend to have far better power to weight compared to the prototype. This applies to lots of scale hobbies, not just airplanes.

author: ataradov
author_rank: Super Contributor
timestamp: September 19, 2016, 03:31:40 pm
content: Quote from: Kilrah on September 19, 2016, 08:17:27 amAll REAL fighter jets are unstable. Stuff you do from foam may or may not be stable, it is irrelevant here.

author: Rick Law
author_rank: Super Contributor
timestamp: September 19, 2016, 06:19:05 pm
content: Quote from: ataradov on September 19, 2016, 03:31:40 pmStability is not necessarily a friend. In fact,stability is the enemy of agility.Imagine the situation like a book sticking out the edge of the table. If you tilt the book outward by a certain amount (angle), it will fall off the table. But if you tilt the book just a little but below that "fall off the table" angle, it will just fall back to its initial state. That there isa force to brings it back to the initial stateis what makes it stable. When the book's CG is very near the edge of the table, the "fall off the table" angle is very small. Add just a little energy (ie: tilt the book just a bit) and off it goes dropping to the ground.So, stability describes an energy to keep it at current state. On an airplane, to make a turn or a slow dive, you need to overcome (over-power) the energy that keeps it stable so you can change state. The more stable, the more energy it takes to break the stability, the more energy, the more time it takes to add that energy. So it will take more time to turn, rotate, dive -- ie: less agile.In fact, even more extreme than flying wing, stuff like forward sweep wing is of interest is because it is inherently unstable and thus inherently more agile.

author: ataradov
author_rank: Super Contributor
timestamp: September 19, 2016, 06:20:47 pm
content: Quote from: Rick Law on September 19, 2016, 06:19:05 pmI never said this. I said exact opposite long time ago in this thread.

author: Rick Law
author_rank: Super Contributor
timestamp: September 19, 2016, 06:31:07 pm
content: Quote from: ataradov on September 19, 2016, 06:20:47 pmI did not mean to imply you say it.I am the one saying it. I am adding it to the discussion - stability is an enemy to agility; along with the explanation why it is so.

author: D3f1ant
author_rank: Frequent Contributor
timestamp: September 19, 2016, 08:39:21 pm
content: In the case of the F16, I do not think those large pods are bombs, they are extended range fuel tanks, and they be kept in balance by pumps.Sent from my SM-J500Y using Tapatalk

author: Monkeh
author_rank: Super Contributor
timestamp: September 19, 2016, 08:44:11 pm
content: Quote from: D3f1ant on September 19, 2016, 08:39:21 pmIt's fully capable of carrying 2000lb bombs on those pylons. What's fitted in $IMAGE is irrelevant to the question.

author: D3f1ant
author_rank: Frequent Contributor
timestamp: September 19, 2016, 08:45:31 pm
content: Yeah I just looked it up, it's quite impressive and versatile for a relatively old plane.Sent from my SM-J500Y using Tapatalk

author: Kilrah
author_rank: Unknown
timestamp: September 20, 2016, 08:39:50 am
content: Quote from: rrinker on September 19, 2016, 01:01:00 pmYou must have missed the fact that my flying wing was unpowered since it was a glider.Those designs you refer to are not unstable because a flying wing can't be made stable, they are unstable because they were designed to be since that is a desired characteristic as it is for every plane that must be agile in combat.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 20, 2016, 09:39:18 pm
content: Aircraft cg and stability issues are not the big issue!