Whole wing as an aileron ?

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by topspeed100, Jan 10, 2011.

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  1. Jan 10, 2011 #1

    topspeed100

    topspeed100

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    I was thinking this in the shower this morning;

    Could the whole wing turn a degree or two at full aileron stickforce ?

    Would there be something to be gained by this ?

    I recall from R/C models the ailerons brake a lot..this could be minimised ?
     
  2. Jan 10, 2011 #2

    Hot Wings

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    Look up Spratt control wing. Been done. It works.
     
  3. Jan 10, 2011 #3

    Dan Thomas

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    As Hotwings says, the Spratt did it, but it also moved them together to change the angle of incidence together for elevator function; the tail was fixed. It wasn't a popular design.

    The ailerons themselves have little "braking" effect. When an aileron goes down, it increases both the camber and the angle of incidence and attack of that section of wing. Since drag is created by generating lift, drag goes up if we generate more lift. The mistake comes in thinking that the aileron only deflects air one way or the other; it's not that easy, Increasing camber enlarges the differential in airspeed above and below that wing over the whole section ahead of the aileron, and deflecting the aileron up decreases the differential. Deflecting the Spratt's right wing to bank left would create more drag on the right side too.

    Dan
     
  4. Jan 10, 2011 #4

    topspeed100

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    I would have normal elevator...moving also as a whole.

    How risky would it be to operate ailerons along with feet ( using them like normal aviation brakes ) like rudder...one could yield more power to the control ( and make the coupling easier ) ?
     
  5. Jan 10, 2011 #5

    CNCRouterman

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    Dan,
    I won't pretend to be able to prove this, but is not drag due to lift a function of airfoil shape as well? I recall stories by fighter pilots relating the how the F4 Phantom would shed a lot of speed in a high G turn compared to a more modern F16. Yeah, that is pretty anecdotal for evidence. The premise that an airfoil of X shape has one L/D ratio and airfoil Y has a different L/D ratio both with respect to lift generated appears valid, as this is addressing form drag, not drag due to lift. So, it may be advantageous to change the pitch of an entire airfoil rather than just a part of one.

    The question then becomes one of benefits verses liabilities introduced by the added structure required to accomplish the task. Adding a pivot for a primary flying surface means concentrating the load to single point (presumably), the pivot mechanism. As that structure will have to handle the entire wing loading it will carry the entire bending, shear, and torsional loads, so how much more will it weigh than a fixed design, and will that added weight negate the postulated reduction in form drag? The pivoting mechanism must also be stiff enough in torsion to prevent, control, or withstand the effects of flutter.

    There would be some benefits to the concept, including improved fuselage attitude during take off and landing, spin recovery, optimizing fuselage attitude throughout the flight envelope by changing the angle of incidence to keep the fuselage in a minimum drag attitude, and probably others I am ignorant of. Being able to control the pitch and angle of incidence of the air foils might require some electronic assistance though. I guess it depends on how the controls are mixed with the tail controls (presuming they are present).

    I seems to me that some of the small planes I was around as a youngster had adjustable horizontal stabs, where the "fixed" part was adjustable, and the elevator was conventional. I think, but do not know, that this was part of the trim mechanism.

    As I am thinking about this, I can't think of a single modern day fighter jet that has a fixed horizontal stab with elevators, they are all (than I can think of at the moment) single piece airfoils that pivot, and with differential action.

    It also occurs to me there is might be one other aspect to maneuver while having the main wings in differential pitch. The roll effect will result in different angles of attack on the wing as a function of its distance from the axis of rotation. With a differential pitch or angle of incidence, and given the same rate of roll, the angle of attack at the root of wings will be slightly more (I think anyway...) for the ascending wing, and lower for the descending wing, or for high roll rates, the AOA would be "less negative" for the ascending wing and "less positive" for the descending wing at the root. Still, 1 or 2 degrees probably won't have any adverse effects unless you are at the edge of the maneuvering envelope, but if you actuate this mechanism only at full control deflection, then, you probably are at the edge of the maneuver envelope, so now what. If you are low and slow, and apply full left aileron, which also pitches the right wing up and the left wing down, under what conditions is that likely, counteracting a impending stall/spin maybe? If true, then you are probably at or approaching the critical angle of attack for the wings, with than added 1 or 2 degrees, you may go enough over on the right wing, while mitigating the marginal conditions of the left to cause a rapid roll counter to the control input, which will then accelerate the problem rapidly.

    Am I thinking this through correctly or are my premises incorrect?
     
  6. Jan 10, 2011 #6

    Dan Thomas

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    An aileron changes the airfoil shape by increasing or decreasing camber. It's a really easy way to change airfoil shape. Anything else gets complicated and heavy.

    The Spratt was a strut-braced affair and had low speed ranges. I don't think it would work well at higher speeds. And it would get far too heavy to cantilever it all.

    The adjustable stabilizer was the trim system. Piper high-wings, Cessna 180 and 185, and others.

    The Spratt's fuselafe had a fixed V-tail that kept the fuselage pointing into the relative wind. The drawback I could see to that is the lack of reference for the pilot; we are all taught to fly with aircraft attitude relative to the horizon, and if the airplane levitates as both wings are tilted upward, the fuselage's deck angle change will be small. In the stall it wouldn't change much until the stall itself, so there would be little visual warning.


    Fighters are controlled by computers for some really exotic flight maneuvers. Heavy, expensive, really, really, expensive, and not worth it for the sport airplane. Not yet.

    Any wing that reaches stall angle will stall, whether it has ailerons or differential angle of incidence, and if it stalls before the other wing you'll get a wing drop and maybe a spin. trying to lift that wing with ailerons or angling it upward will deepen the stall and aggravate the spin. There are some things we can do little about other than simply to learn to fly and get the skills, but there seems to be a real reluctance to do that these days. The airplanes we fly will do exactly as we ask them to, as I used to tell my students, and if we tell it to spin it will spin, so let's learn what causes a spin so we recognize it when we get near it and can avoid that. Some airplanes are spin-prohibited, so we don't go there; they take special skills to recover, if they'll recover at all.


    Dan
     
  7. Jan 10, 2011 #7

    sergiu tofanel

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    There are a few problems that I could think off the top of my head

    1. Drag
    The reason airplanes do not cruise with extended flaps is parasitic drag. Highly cambered airfoils have inferior L/D characteristics at low Cl's. This means that a low camber/symmetrical airfoil will always have less drag than a highly cambered airfoil operating at a Cl of less than 0.5. So every time the ailerons enforce a roll correction, the whole wing experiences an increase in drag. In the case of conventional ailerons, only the outboard sections of the wing experience the drag increase.

    2. Control

    Non-symmetrical airfoils will exhibit non-symmetrical drag characteristics. Let's say that if the starboard aileron deflects 2 degrees up and the port aileron deflects 2 degrees down, in the case of non-symmetrical airfoils, there will be uneven parasitic drag between the left and right wings. The problem is exacerbated by the uneven induced drag which now affects the whole wing, as opposed to the conventional aileron setup. Also, the inboard aileron section does very little to contribute to the overall roll moment due to the length of the moment arm. So one pays all the penalties associated with (non-symmetrical) drag, and reaps very little benefits.

    3. Structural/Building Concerns

    These are the same as designing and building flaps, but with none of the benefits. In order to use the setup to do double duty (flaperons) one must design complicated coupling mechanisms which prove to be more hassle than they are worth.
     
  8. Jan 10, 2011 #8

    topspeed100

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    I know some cases where the use of normal flaps will cause an instand stall and a wing drop..that is when plane is close to the stall speed and banking ( lining up for the runway ).
    I bet if the wing turns as a whole ( lets say 0,7 degs plus minus )...I cannot imagine it loosing the airflow even close to the stall speed. IMHO.
     
  9. Jan 10, 2011 #9

    Dana

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    How quickly we forget...

    Search this forum, it was only about a year or so ago that Mark Stull reported extensively on his ultralight that used pivoting wings for roll control.

    -Dana

    Duct tape is like the Force. It has a light side and a dark side, and it binds the universe together.
     
  10. Jan 10, 2011 #10

    Topaz

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    In a word, no. For unswept wings, "Drag due to lift", or induced drag, is influenced solely by the lift coefficient of the wing at the given condition, the aspect ratio, and the shape of the overall lift distribution. The "L/D" for an airfoil is based on the lift coefficient and the parasite drag coefficient of the section shape.
     
  11. Jan 10, 2011 #11

    Dan Thomas

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    First time I've heard of flap application causing the stall. Flaps usually lower the stall speed. I think that you are referring to the stall/spin accident where a pilot, when turning final, gets too slow and if he's overshooting the runway centerline will tend to add more rudder while holding the inside wing up with opposite aileron, producing a skid that causes a nasty stall on the inboard wing and spins the airplane in.

    And adding any AoA, even 0.7°, when right near the stall, will bring it even closer to the stall and maybe stall it. Remember that if you change the angle of incidence of the whole wing you change the AoA of the whole wing; if you deploy flaps, you change the AoA of only the flapped (inboard) section. Wings are preferably stalled inboard first so as to maintain aileron authority for as long as possible and tame the stall; the pivoting wing might not be so forgiving.

    Dan
     
  12. Jan 10, 2011 #12

    Lucrum

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    Years ago I knew a designated examiner who thought this was true. I never really agreed with it myself. Seemed to me that while extending trailing edge flaps shifts the lift slop and reduces critical AoA the increase in pitching moment and resultant pitch over (left unchecked) simultaneously tends reduce the AoA.

    Disclaimer: I'm NOT an aerodynamics expert.
     
  13. Jan 10, 2011 #13

    Hot Wings

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  14. Jan 10, 2011 #14

    autoreply

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    Well theoretically it's true. Most profiles stall at a slightly lower aoa with flaps. At the same time though, if you apply flaps with constant aoa, you're accelerating up the aircraft, lowering the aoa. Not noticeable on a powered aircraft (flaps move too slowly), but if you can move than instantly (like in a glider), it might play a role.

    Having said that though, I'm with Dan. I've done the described maneuver many times (pitch up 20 degrees, roll in the thermal with full negative flaps, let it start to stall at 0.7G's and apply full flaps when you're at zero pitch and the right roll angle). Instantly pulling the flaps from -7 to +20 gives you a perfectly stable flight, even though you were stalled when the flaps were applied.
     
  15. Jan 11, 2011 #15

    WBNH

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  16. Jan 11, 2011 #16

    topspeed100

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    Ok dudes...flaps don't stall...but using ailerons will cause a stall near the stallspeed...right...and for that the aileronwings could be a cure...of course there is the weight penalty.

    I might be slightly ADHD case..please bear with me...I might make lesser these kinds mistakes...hopefully..I seem to picture different image of the situation in my head and say other things that I really mean...anyone experinced this kinda phenomena before ? I made similar error when discussing flat plate and frontal area earlier.
     
  17. Jan 11, 2011 #17

    Dan Thomas

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    The skidding turn stall/spin is due to the fact that there's a difference in angle of attack between the two wings when in a climbing or descending turn. In a descending turn such as base-t-final, the AoA is larger on the inside wing, and skidding the airplane increases the difference further. If the airplane is near stall, that skid is what does the bad stuff, not so much the deflected aileron, since the stall usually starts inboard anyway.

    Dan
     
  18. Jan 11, 2011 #18

    Joe Fisher

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    A stalled wing still creates a lot of lift .A Cub with its light wing loading will still maintain altitude when fully stalled. With the stick all the way back and full throttle using rudder to control it I can fly as long as I want, as long as the ailerons stays in the middle. It bucks and jumps like a bronco. It you move the ailerons it will spin opposite the stick right now. I thought my students what I call smoky hole syndrome. At altitude we would turn base and force a skid now if you did that at at 400' or less you would be just about vertical at ground level. How ever if you slip and stall it will try to spin to the high wing thus giving you plenty of warning to recover.
     
  19. Jan 12, 2011 #19

    wsimpso1

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    Regardless of which method you use to differentially adjust CL (increase Cl on one wing while decrease Cl on the other), you are also making changes to the induced drag in a differential manner. Either method will give adverse yaw.

    Next up, if you are close to stall and apply a bunch of stick to one side, either method will drive the wing making more lift to drive towards stall while the other is driven away from stall, so using lateral stick for roll control while slow is going to make wing drop and spin entry more likely.

    Next, pivoting the whole wing works Ok in slow light airplanes, but the extra flutter modes and things that have to be really stiff (and heavy) to prevent flutter could be prohibitive in a faster airplane...

    Last, fighters and jetliners use all flying tails because in transonic and supersonic regimes, an elevator becomes less effective, and the AOA of the leading edge becomes very effective. They figured this out on the Bell X-1... At 100 knots, well, an all flying tail or a trimmable horizontal stabilizer has to earn its keep based on other criteria...

    Billski
     
  20. Jan 13, 2011 #20

    topspeed100

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    I don't like adverse yaw..that is why Y-tail is also no good.
     

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