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Delta Wing - Natural Control Reversal

  1. Oct 17, 2006 #1
    Greetings mega-brains! :biggrin:

    A delta wing configuration, because it does not have a separation between the main wing and the horizontal tail which holds the pitch control surface(s), has a natural aerodynamic, temporary control reversal when initiating a pitch maneuver from a steady-state, trimmed, unaccelerated condition. In controls terminology this is called a "non-minimum phase zero".

    What happens is this:

    1) The generation of pitch rate via pitching moment lags behind the production of lift and drag.
    2) As such, when you deflect the elevons upwards in order to cause a pitch-up, you initially spoil the lift-generation of the main wing before you have a chance to build up enough pitching moment to overcome the pitching moment of inertia.
    3) This results in the flight path vector dipping downward (i.e. the plane falls), until sufficient angle of attack is generated to overcome pitch inertia and begin pitching the nose upward.
    4) The opposite effect occurs when trying to pitch down (you initially lift upwards before you go down).
    5) This is counter-intuitive (and somewhat alarming) to a pilot who is not aware of this natural aerodynamic tendency.

    My question (and where I am looking for help, suggestions, and references) is this:

    Short of the typical solution of adding a canard, which is deflected in opposition to the elevons to counter the initial loss of lift, is anyone familiar with any control system techniques that can minimize this tendency (with the hope of avoiding it) over the entire flight envelope.

    I've heard "tribal knowledge rumors" about manipulating the apparant aircraft instantaneous center of rotation, but no one can give me details.

    Gurus???? What do you say? Many thanks in advance,
    Rainman
     
  2. jcsd
  3. Oct 18, 2006 #2

    Danger

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    Rainman, you are so far ahead of me in this stuff that I barely dare speak in your presence. I haven't had a bird strapped on in over 30 years, and then I was only officially okay on 172's. The reality of what I could do was a bit different, but that's how it is on paper.
    Since your question is in regard to whether or not anyone knows of a compensation method other than canards, I have to answer 'no'. My approach to the issue, however, would be to consider moderate thrust vectoring.
     
  4. Oct 18, 2006 #3

    Astronuc

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    Hey RainmanAero - I figured you were the aero guru :biggrin:

    I'm sure Fred will have something to say on this subject. Meanwhile, Danger has a good point - thrust vectoring - if one wishes to avoid external control surfaces.

    Which brings me to wonder -

    I imagine one wishes to avoid canards because they can add to the radar cross-section. Similarly, tails and other control surfaces/protrusions are similarly undesirable. I imagine that the jet engines are inboard (which could make thrust vectoring more difficult/challenging), also to avoid radar signature. Would a leading edge control element (e.g. 727, or B58) be undesirable?

    Thinking of the B-58 Hustler, which is a really cool :cool: aircraft, I'm sure Convair must have enountered this issue. Perhaps Convair's or NACA's archives have someting on Delta wing stability, and would have thought that Northrop's archives would have material on this issue.
    http://en.wikipedia.org/wiki/Flying_wing

    One way may be to link engine thrust with elevons, so the thrust increases with elevon up, and decreases with elevon down.

    One question, are the elevons toward the roll (central) axis, or more outboard?

    What are the constraints on this problem?

    With regard to shift COR (CM), I've heard moving fuel or shifting mass, but that was long ago, and I don't think shifting mass is prudent. I think commercial airlines, like 747, can shift fuel between forward and aft tanks in the fuselage between the wings.


    Just can't resist -

    http://www.aviation-history.com/convair/b58.html

    http://www.xs4all.nl/~mvburen/b-58/

    http://www.b-58hustler.com/

    http://home.att.net/~jbaugher2/b58.html

    http://en.wikipedia.org/wiki/B-58_Hustler

    http://www.b58hustler.net/

    :cool: :tongue2:
     
  5. Oct 18, 2006 #4

    Danger

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    B-58 Hustler?! Man, I haven't even thought of that thing in decades. That was one of my favourite models that I built when I was about 12. Thanks for a good memory.
    The main problem that I can see with mass-shifting, which doesn't matter to a commercial plane, is response time. I don't care what kind of pumps and lines are installed; I just can't see transferring a few hundred pounds of fuel in the fraction of a second that is required in a combat situation.
    The leading-edge control sounds pretty promising, as does the auto-throttle/elevon link. That also brings to mind the concept of rear mounted speed brakes tied to the elevons. If you pop a top one when they go up, it might (and I'm not at all sure about this), impart a postive pitch. One on the underside, conversely, could hoist the tail a tad.
    As to the thrust vectoring, I was thinking of just a simple louvre system behind the exhaust, which would still work with internal engines. I don't know what that would do to normal flight characteristics or afterburners, though, and I suspect that it would play havoc with the IR signature.
     
  6. Oct 18, 2006 #5

    Mech_Engineer

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    While I am by no means an Aerospace expert, I do remeber the general idea of a project that was being worked on by several graduate students at the University of Arizona.

    Their general design was flight control of an aircraft without the use of control surfaces. The concept they were working on was directly changing the pressure above/below the wing through the use of pumps and pinholes in the surface of the wing. By either pumping air from or adding air to a side of the surface (I'm not sure which it was), it was my understanding they were actually able to gain control of pitch, yaw and roll.

    It would seem to me that if you had a system like this, which was able to actively increase or decrease the pressure in a specific area of the wing, you would be able to more actively control the lift of the wing, at least in theory.

    Being a mechanical, I only heard about this project through my aero friends, but if you would like to know more about this project, I would recommend contacting the U of A AME department, it is possible the graduate students have written papers on this projects theories, prototypes, and applications.

    :cool:
     
    Last edited: Oct 18, 2006
  7. Oct 18, 2006 #6

    Danger

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    Wow, Mech... that is seriously cool. Probably a bugger to implement, but elegant. :cool:
     
  8. Oct 18, 2006 #7

    Mech_Engineer

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    And a bugger to keep the holes from plugging up I would suspect. Of course, if you have enough air pressure that might not be an issue... Just throwing out ideas.
     
  9. Oct 18, 2006 #8

    Danger

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    Just make sure you stay above the birds.
    By the way, is this system by any chance integrated with 'smart skin' technology?
     
  10. Oct 18, 2006 #9

    Mech_Engineer

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    As far as I know, they implemented it on the wings of an RC plane, by making an aluminum skin with pinholes in it, and a series of ducts within the wing to supply air to the holes.
     
  11. Oct 18, 2006 #10

    LURCH

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    Regarding thrust vectoring; NASA had an x-plane called the "X-31" that used thrust vectoring for most of its manuevering. They proved the concept, but more research and testing is required. Since aerodynamic temporary control reversal only occurs during pitch manuevers, this could prove to be a very viable solution, as the X-31 showed that thrust vectoring could possibly replace controll surfaces for pitch and yaw, leaving aerodynamic controlls for only roll.
     
  12. Oct 18, 2006 #11

    Danger

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    So essentially they have a flying air-hockey table. :tongue:
     
  13. Oct 18, 2006 #12

    Mech_Engineer

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    I'm thinking that's a pretty good analogy :rofl:
     
  14. Oct 18, 2006 #13

    russ_watters

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    Same idea is used for boundary layer control and blown flaps:
    http://www.aerodyn.org/Drag/blc.html
    http://en.wikipedia.org/wiki/Boundary_layer_control_system
     
  15. Oct 18, 2006 #14

    Danger

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    Great links, Russ. Thanks.
     
  16. Oct 19, 2006 #15
    To all my Phyforum bretheren: THANKS for the fantastic replies in this thread!
    Thanks for the vote of confidence, Astro, but I am nowhere near the league of you and many others on this forum. I just feel honored to be able to contribute here and there on those few things I do know fairly well.

    Now on to some of the thoughts and suggestions:

    1) Thrust vectoring - Yep, we already have this in one of our configs. It is expensive and causes MTBF of the engine to decrease fairly significantly, so we'd like to avoid if we could. But the TVC is also ideal for lift compensation in bank maneuvers at low speed, where delta wings have a "falling off the edge of the cliff" lift loss once you get beyond a specific bank angle. :surprised

    2) Canards - You're right on when it comes to RCS, Astronuc. You can imagine that what I am working on wants to be a stealthy bird. :wink:

    3) B-58 Hustler - Actually, our human factors pilot that I work with is a former Hustler driver. He maintains "you live with it, and it is really just a training problem." But that was before the age of snazzy, non-linear, digital FBW flight control systems where you can make a rock fly like a sailplane with enough software! :smile: So I kind of feel it is my job to exhaust all possibilities.

    4) High-pressure blowers - I can't talk about details, but suffice it to say this is being looked at by the "configurators" of the project I am working on.

    5) Auto throttle "power leading pitch" - This was the first thing I added to the control system when I joined the team, mostly because I am trying to get our Cooper-Harper handling qualities scores UP, and the simplest way to do that is to take the throttle/speed task off the pilot's hands so he can focus on the stick. And in addition I have added "power lead" compensation by spooling the engine up in concert with the pilot's pitch-up stick demand (rather than waiting for the aerodynamic response and compensating with flight path angle). This helps a small amount, but not nearly enough.

    One thing I forgot to relate is how my hands are tied. You see, I am only the flight control laws/software guy. The aero and control effector configurators are another group, and it's their charter to come up with the control power means to generate the forces and moments that I can use to make it do what we want it to do. And even these configurators are "subordinated" to the Low Observables gurus. :rolleyes:

    In any event, I tip my hat in appreciation to you gents. I wanted to make sure I left no stone unturned, and your replies have at least confirmed that I have looked at the right things (so far). And I do have one of my younger engineers out doing literature searches in our own company files, as well as NASA, AIAA, CalSpan, etc.

    Thanks again!
    Rainman
     
  17. Oct 19, 2006 #16

    Danger

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    I'm just glad to see that I didn't make a fool of myself. :biggrin:
    For the hell of it, though, I'm going to keep thinking about it. If anything radical crosses my mind, I'll get back to you.
    And thank you for initiating a thread that really gave me something to chew on in an area that I do have some small knowledge of. I really enjoyed it.
     
  18. Oct 20, 2006 #17

    FredGarvin

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    The first thing that comes to mind is what they used to fix the pitch authority problem on the Bell X-1 in the transonic range; they switched from a hinged elevator to a completely moveable horizontal stabilizer. The only problem there being that the entire surface may still be effected. Is resizing or moving the horizontal stabilizer an option?
     
  19. Oct 20, 2006 #18

    Danger

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    Wouldn't that necessitate re-designing the Stealth surface angles?
     
  20. Oct 22, 2006 #19
    Hi Fred,
    I understand what you are saying, but perhaps you forgot that this is a delta wing configuration. So there is no horizontal stabilizer like a conventional "wing+tail" aircraft. In fact, this is the very reason why this initial control reversal happens. If we had a separate horizontal tail, moving the elevators would not have such a drastic effect on the lift produced by the main wing.

    However, thanks for the contribution.
    Rainman
     
  21. Oct 22, 2006 #20

    FredGarvin

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    Details, details. Hmmmm. I thought the older delta wings had horizontal stabs. A quick peek shows you're correct. Hmmm... perhaps the you are limited to control system tweaks to try to corrall the problem. Without an extra surface, I don't see how you would fix it.
     
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