Unconventional Maneuvers of Fighter Jets: How Do They Defy Gravity?

[banerjeerupak]

We learn from a very early class that the aerofoil design of the aircraft wings provide it with the required thrust to keep the plane afloat. It is the only upward acting force. And the reason cited for is that the design allows air to rush faster over the upper portion of the wings compared to the under side. I accept all of it. Nothiing i can counter.
But then how do the fighter jets carry out those maneovres in which they are upside down. The same principal would force the craft to come down towards the Earth at very high velocities. Am i correct??

 

[RainmanAero]

Hello banerjeerupak,

We learn from a very early class that the aerofoil design of the aircraft wings provide it with the required thrust to keep the plane afloat. It is the only upward acting force. And the reason cited for is that the design allows air to rush faster over the upper portion of the wings compared to the under side. I accept all of it. Nothiing i can counter.
But then how do the fighter jets carry out those maneovres in which they are upside down. The same principal would force the craft to come down towards the Earth at very high velocities. Am i correct??

Well, the thing you are missing (or perhaps just forgetting about) is the angle of attack. Even a flat board moving in an airflow at some angle of attack can still produce lift. The airfoil is simply the most efficient for producing lift with a minimum amount of drag.

So when a fighter (or any aircraft with sufficient thrust) flies inverted, it is generally flying at a higher angle of attack to remain in level flight than it would fly if it were not inverted. Thrust+angle of attack are the primary factors that keep an inverted plane from falling out of the sky. But you can think of other, secondary factors... for example fuel pumps that can maintain positive fuel pressure under <0 g conditions!

Rainman

 

[paddy-boy66]

I asked a similar question. Take a look at this thread
https://www.physicsforums.com/showthread.php?t=130414
Paddy

 

[Peregrine]

Also, on newer aircraft performing extreme manuevers, the impact of thrust vectoring can have huge effects on the manuevuer since the propulsive force is no longer in line with the plane's axis.

 

[banerjeerupak]

i read about the angle of attack... but what about the time when the pilot performs a cobra...

 

[FredGarvin]

The cobra relies on the momentum of the aircraft and Newton's first law to carry them in a straight line. Make no mistake, the aircraft is very much stalled at that point.

 

[russ_watters]

...and a >1 thrust to weight ratio.

What makes the aircraft controllable at that point is simply a good computer and elevators that can rotate pretty far.

I've heard that an F-18 could do a cobra maneuver if the software would let it.

 

[banerjeerupak]

@ russ_watters
This i too have heard, but many people feel that this is just envy to russian makers

 

[russ_watters]

Well, actually, it is speculated that the Mig 29 copied a lot from the F-18 (actually, the YF-17) - [edit - too speculative].

Regardless of if that is true, the cobra maneuver is just a peculiar capability that happens when the control system allows the plane to continue to operate far beyond a stall. And since it is dangerous and tactically useless, there really isn't anything about it to envy. It looks cool at air shows, but that's it.

 

[russ_watters]

I did a little research on the cobra manuever. Interesting...

The cobra and it's horizontal counterpart, "the hook", are performed by the pilot disabling the angle of attack (alpha) limiter on the flight control system just prior to the pitch up. The fly-by-wire FLCS normally limits the aircraft to 35 degrees alpha, but by disabling the alpha limiter, the pilot can generate up to 110 degrees alpha in the single seat aircraft, while the two seater seem to stop at around 90. Neither aircraft has vectored thrust and this is simply a demonstration of the aircraft's ability to generate a tremendous pitch rate, without changing the vector of flight.

http://aeroweb.lucia.it/rap/RAFAQ/cobra.html

So their flight control systems do normally prohibit such a maneuver.

 

[AlephZero]

Yeah, but flight control systems are for wimps, and wimps don't fly high speed jets :-)

Don't forget that in dogfights, extreme slow-speed manuvers are much more useful than flying in straight lines at Mach 2, if you want to avoid being shot out of the sky. Being able to "stop in mid air" and let a missile overtake you can be a VERY neat trick.

 

[RainmanAero]

Hi AlephZero,

Yeah, but flight control systems are for wimps

Yes, but I'd like to see you or anyone fly without one! I believe what you were trying to get at was "flight envelope limiting systems", because if you have no flight control system (whether fly-by-wire or mechanical) then you won't be flying very long!

However, your comments about slow-speed flight are, of course, right on. Unless, of course, you are being followed by a heat-seeking missile. It can find your tailpipe no matter how slow you are going.

Rainman

 

[LURCH]

The Cobra is a very usefull maneuver in a very few, highly specialized situations. It is a one-Time move to cause and overshoot, but it bleeds the energy state of the aircraft down to almost nothing. If that one move does not solve every problem in the sky for you, you are now a sitting duck.

Mostly, it is an impressive move for air shows.

 

[russ_watters]

Yeah, but flight control systems are for wimps, and wimps don't fly high speed jets :-)

Most modern figher planes are completely unflyable without flight control systems. Humans are simply not capable of dealing with such extremely unstable control environments. As RainmanAero said, only one small, specific piece of the flight control (the aoa limiter) is overridden for this.

Don't forget that in dogfights, extreme slow-speed manuvers are much more useful than flying in straight lines at Mach 2, if you want to avoid being shot out of the sky. Being able to "stop in mid air" and let a missile overtake you can be a VERY neat trick.

Dogfights don't happen at Mach 2 and they don't happen standing still either. As Lurch pointed out, a cobra is a one-time, snap maneuver and it is all or nothing. In a dogfight, energy is everything (whether speed or altitude) and if you use all of your energy, you can't easily get it back. When you are finished with it, you don't have much choice but to fly straight and level for a few seconds to regain energy. You might be able to climb a little (and waste a lot of fuel in full afterburner), but if you try much of a turn, you'll fall right out of the sky.

What's more, the cobra maneuver is fast and largely uncontrolled, so it doesn't really allow you to acquire missle lock on an overshooting enemy unless you are extremely lucky.

 

[shalav]

The technical marvel of the cobra manoeuvre is that the airflow to the engines on the Su27+ and the MiG 29 is not stalled with the tail-slide and reverse vector at the end.

Wether by accident or design, the MiG 29 and the Su 27+ have well engineered intakes to allow for this manoeuvre.

The F18's software limits this manouvre precisely because of the very real danger of the airflow stall within the intakes during the tailslide which is basic to the cobra manoeuvre.

In fact few jet aircraft ever try a tailslide because it can cause a stalled engine. One will notice that most jet aircraft on a vertical vector will flip over rather than attempt a tail slide when their TWR reaches <= 1.

Further on the cobra manoeuvre in a dogfight: It is performed at V* (corner velocity). In a dogfight every pilot scrambles to attain and maintain V*, this is the velocity at which the aircraft is the most manoeuvreable. Reportedly velocity at the end of the manoeuvre is ~50-60% of V*, which is a pretty good velocity considering that it is quite likely your opponent has overshot you and is desperately scrambling to slow down / reacquire lock. His confusion is your gain.

The uselessness of the cobra manoeuvre is heavily subscribed to by those who fly aircraft incapable of performing them! None of the same people say they will never use it if they had an option to do so. It may be a useful manoeuvre in some situations. Not the end all of combat manoeuvres that's all.

 

[AlephZero]

Wether by accident or design, the MiG 29 and the Su 27+ have well engineered intakes to allow for this manoeuvre.

The F18's software limits this manouvre precisely because of the very real danger of the airflow stall within the intakes during the tailslide which is basic to the cobra manoeuvre.

That I can believe, having heard a presentation from a large US aerospace company on its "leading edge" research into supersonic intakes. After listening politely for a couple of hours, we pointed out to the presenter that there was a picture of Concorde on the wall of the conference room, and we had already solved the the same problems he was discussing about 20 years before. He didn't appreciate the irony of the situation though.

I doubt the Soviets did it by accident. They might have lagged behind with computer power, but the ones I met were all VERY mean mathematicians.

 

[DaveC426913]

I doubt the Soviets did it by accident. They might have lagged behind with computer power, but the ones I met were all VERY mean mathematicians.

Yeah, but have you ever tried to pull a Cobra maneuvre with only one hand to operate your slide rule?

 

[moose]

Yeah, but have you ever tried to pull a Cobra maneuvre with only one hand to operate your slide rule?

What about the hand they use to drink their vodka?

Look mah, no hands!

 

[AlephZero]

The "Russian Test Pilots" on the UK airshow circuit had to put the vodka in the fuel tanks - they couldn't afford to buy AVTUR.

More seriously, some people are just better at designing surge-free engines than others. I don't want any libel actions against me here but there was one civil aircraft project where the flight test pilots refused to fly with one of the three engine company's engines because of the pops and bangs on rotation at evey other takeoff. They nicknamed the plane "Surge One" in honour of the President.

The most temperamental compressor I've ever had the misfortune to fight with was on a joint project with the same engine maker. And those guys make a LOT of military engines.

 

[FredGarvin]

More seriously, some people are just better at designing surge-free engines than others. I don't want any libel actions against me here but there was one civil aircraft project where the flight test pilots refused to fly with one of the three engine company's engines because of the pops and bangs on rotation at evey other takeoff. They nicknamed the plane "Surge One" in honour of the President.

The most temperamental compressor I've ever had the misfortune to fight with was on a joint project with the same engine maker. And those guys make a LOT of military engines.

Hmmm...I wonder who we could be talking about?

 

[DLinkage]

Lift doesn't necessarily always act towards outward normal from the top of the aircraft. While the upside down camber probably degrades the lift of the wing in inverted flight, the wing still probably generates upward lift pointing at some orientation from the outward normal from the bottom of the aircraft. You also need to consider the other forces (Weight, Thrust, Drag, and Intertial) as well as their moments. The overall balance of these 4 give an aircraft is flight path. The inertial terms can be very significant also due to the high velocities fighter jets travel at in combat

 

[Danger]

Hmmm...I wonder who we could be talking about?

Is that a defensive pose, or a swipe at the competition?

 

[FredGarvin]

Is that a defensive pose, or a swipe at the competition?

Well, I am taking the liberty of assuming that Aleph is talking about a competitor. I guess he could be talking about us, but I'm not aware of surge problems as bad as he was mentioning.

 

[russ_watters]

May I ask who you work for, Fred?

 

[FredGarvin]

Russ,
You've got mail.

 

[AlephZero]

Well, I am taking the liberty of assuming that Aleph is talking about a competitor. I guess he could be talking about us, but I'm not aware of surge problems as bad as he was mentioning.

"Surge One" was the PW4084 on the B777, round about 1992. Some reports suggested the problem was the active blade tip clearance system which worked fine on engine test but not on flight test.

There were plenty of reports in "Aviation Week" etc at time, and a few years later there was a BBC TV documentary series on the B777 project which had a shot of a surge on takeoff, so I don't think anybody at the time was trying to hush it up. And of course, like all such problems, it got fixed.

 

[J77]

Most modern figher planes are completely unflyable without flight control systems. Humans are simply not capable of dealing with such extremely unstable control environments.

Yeah - from seminars I've been to - the trajectories on which fighter aircraft fly are unstable solutions of a highly nonlinear system. Control mechanisms mean the aircraft don't go 'out of control'.

Search gooogle for "nonlinear flight dynamics"

 

[FredGarvin]

PW4084

That's what I was thinking, but wasn't quite sure.

 

[DaveC426913]

A while back they produced a prototype jet with its wings swept forward (popularized by GI Joe). The jet was ultra-unstable and had to be controlled by computer sixty times per second.

If I understand correctly, the rationale for this craft is that, historically, designers had always worked towards greater stability in aircraft, making them easier to control. But stability is the antithesis of manouverability, since craft that is very stable will fight rapid changes in flight direction.

Is this basically right?

 

[FredGarvin]

Dave, you're most likely thinking of the X-29 program from NASA. There was a ton of research done by that aircraft, not just maneuverability. One really cool aspect of the forward swept wings was to direct flow from the tip to the root of the wing in stead of vice versa on a rearward swept wing. This supposedly helped the AoA ability and the prevention of tip stalls.

EDIT: Just found the Dryden link for the program. Pretty cool stuff. According to this, they achieved AoA of 45°!

http://www.nasa.gov/centers/dryden/news/FactSheets/FS-008-DFRC.html

 

[LURCH]

The X-29 also greatly reduced turbulence in the trans-sonic envelope. But by the time these experiments began, most fighter aircraft were already "negatively stable". AFAIK, the F-16 Falcon (first prototype flew in 1975, I believe) was the first fighter aircraft to have the center of gravity behind the center of rotation, and therefore the first aircraft that had to be flown by computer control, earning it its nickname; "the electric jet".

 

[Danger]

Fred, I didn't read your link, but looked at it. If memory serves, the pictures that you posted are the Northrope contribution, which was a modified F-20 Tiger Shark (which was itself a modified F-5 Freedom Fighter). General Dynamics had a similar beast, which was essentially an F-16 with the wings on backwards. Grumman had one that was 'purpose-built' from the ground up, but I can't remember much about it.

 

[AlephZero]

historically, designers had always worked towards greater stability in aircraft, making them easier to control. But stability is the antithesis of manouverability, since craft that is very stable will fight rapid changes in flight direction.

Is this basically right?

The engineering is right, though the history is a bit off. Some of the first people to realize that stability and manouverability opposed each other were the Wright brothers. By design, their planes required constant human control input for stability.

Not all the early designs got the stability/manouverability balance right. The (in)famous "Flying Flea" (designed 1930 and possibly the first home-build aircraft kit) was unfortunately very stable in the stalled configuration. The control surfaces were sitting in the stalled airflow, which was not a good design concept. Few pilots ever succeeded in recovering from a stall

 

[FredGarvin]

Fred, I didn't read your link, but looked at it. If memory serves, the pictures that you posted are the Northrope contribution, which was a modified F-20 Tiger Shark (which was itself a modified F-5 Freedom Fighter). General Dynamics had a similar beast, which was essentially an F-16 with the wings on backwards. Grumman had one that was 'purpose-built' from the ground up, but I can't remember much about it.

I'm drawing a total blank on the F-16 version. Do you have any links? The only modified test version of an F-16 that I can recall is the delta wing version, the XL.

 

[Cyrus]

http://www.f-16.net/f-16_versions_article26.html