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Fighter aircraft maneovres

  1. Nov 7, 2006 #1
    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??:rofl:
     
  2. jcsd
  3. Nov 7, 2006 #2
    Hello banerjeerupak,
    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! :wink:

    Rainman
     
  4. Nov 8, 2006 #3
  5. Nov 11, 2006 #4
    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.
     
  6. Nov 27, 2006 #5
    i read about the angle of attack... but what about the time when the pilot performs a cobra....
     
  7. Nov 27, 2006 #6

    FredGarvin

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    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.
     
  8. Nov 27, 2006 #7

    russ_watters

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    ...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.
     
  9. Nov 28, 2006 #8
    @ russ_watters
    This i too have heard, but many people feel that this is just envy to russian makers
     
  10. Nov 28, 2006 #9

    russ_watters

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    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.
     
    Last edited: Nov 28, 2006
  11. Nov 28, 2006 #10

    russ_watters

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    I did a little research on the cobra manuever. Interesting....
    http://aeroweb.lucia.it/rap/RAFAQ/cobra.html

    So their flight control systems do normally prohibit such a maneuver.
     
  12. Nov 29, 2006 #11

    AlephZero

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    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.
     
  13. Nov 29, 2006 #12
    Hi AlephZero,
    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! :rofl:

    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. :surprised

    Rainman
     
  14. Nov 29, 2006 #13

    LURCH

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    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.
     
  15. Nov 29, 2006 #14

    russ_watters

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    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.
    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.
     
  16. Dec 7, 2006 #15
    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 thats all.
     
  17. Dec 7, 2006 #16

    AlephZero

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    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.
     
  18. Dec 7, 2006 #17

    DaveC426913

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    Yeah, but have you ever tried to pull a Cobra maneuvre with only one hand to operate your slide rule?
     
  19. Dec 8, 2006 #18
    What about the hand they use to drink their vodka?

    Look mah, no hands!
     
  20. Dec 8, 2006 #19

    AlephZero

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    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.
     
  21. Dec 8, 2006 #20

    FredGarvin

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