UAV "pitcherons" 1

[JJSt]

Hi All,

As an electronic engineer my knowledge on this subject is rather lacking, so apologies if this is a simple/ridiculous question:

I recently cane across some remote control models that had variable incidence wings, i.e. both the aileron and in some cases the elevator functions were achieved by twisting the entire wing. I realize on an aircraft large enough to be manned this sounds like a mechanical nightmare, but I was wondering... Given a smaller, lighter airframe where this is mechanically more feasible, would this or would this not be a more efficient way of achieving these control inputs?

I have looked around and seen a few people (in the RC community) doing this, but no discussions about the aerodynamic effects, is it that obvious or are people ignoring it because of the mechanical complexity? Anywho, If you guys are interested, have at it!

Thanks!

 

[IRstuff]

The Wright brothers did that on their aircraft, but the concept is not supportable for planes that need stiff wings. The wings can't be both stiff and flexible. Moreover, I don't see why this is "efficient," since great force must be exerted to physically bend the wing, which would be in addition to the force required to deflect the air flow, as opposed to an aileron or flap, which has essentially no mechanical resistance to moving, other than its weight.

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[JJSt]

Thanks for your reply, I was aware of the wright brothers wing warping, but that wasn't quite what I had in mind. The implementations I saw had a stiff wing fixed via a bearing to the spar, around which the entire wing rotated and the entire wing rotated. see the last few seconds of this video for an example:

https://www.youtube.com/watch?v=ImJMAzteuK4

sorry if I wasn't clear.

 

[JJSt]

Might actually be clearer here, fair warning about the music though...

https://www.youtube.com/watch?v=-rGE0Z7zNcg

 

[IRstuff]

Oh, I see. Seems to me that it's a novelty rather than a practical thing. The wing still has to be stiff, so in addition to the weight of the wing, we would need to add the supports for the wing, which need to be equally stiff. This adds a truckload of weight. Moving an entire wing is not practical for something like a 787. Finally, most commercial aircraft need to use the wings as fuel tanks, so moving all that fluid is crazy.

For planes that are light, I think it's still a novelty item. What's the performance gain, or is there even any performance gain. Pitching the wings kills the lift, so why is that a good thing? While the plane might be able to snap roll faster, moving all that mass winds up slowing down everything down.

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[JJSt]

"Pitching the wings kills the lift" does it? I would have thought that increasing the AOA of a wing would add to the lift (while increasing drag); a similar effect to the downward deflection of an aileron. Of course, a negative AOA would kill the lift, but then that's what you want, at least on one side, if your rolling the plane to make a turn.

The performance gain as my flawed model of the world sees is; with ailerons, during the rolling motion the apparent direction of the airflow is not lined up with the wing. e.g with a downward deflection of the aileron increasing lift, as the wing starts to rise, the apparent airflow changes to decrease the AOA as the wings velocity around the fuselage increases. With a pitcheron, I believe this is less of an issue, the issue is still there as the wingtip would have a different velocity and therefore a different apparent AOA than the root, but to fix that we need wing warping...

 

[3DDave]

It works for dragon flies - about the most maneuverable things in the air. But it comes at a structural cost. The bending strength limit of the connection is set by the largest diameter pin that fits into the wing**. A fixed wing can use the almost the full chord of the wing to resist bending. Alternatives will increase the weight. For passenger liners there's the added complication of sweep which is used to lower drag at near-sonic conditions.

One model plane I like mounts the motors on pitch gimbals. Differential thrust produces yaw, differential motor pitch produces roll. I guess if you want pitch control you still need an elevator.

A pitcheron vs aileron still has the same AoA change during the roll maneuver. The main difference is the degree of control response for a certain amount of control surface deflection. Essentially a pitcheron is a wing consisting only of the aileron; not a separate case.

**Full-flying stabilizers are used on fighter aircraft and a number of them are differential.

 

[LiftDivergence]

An interesting note...

The XF-91 Thunderceptor was designed with a variable incidence wing which could be adjusted in flight. Just one of several interesting design features of that aircraft.


Keep em' Flying

"I intend to live forever, or die trying" - Groucho Marx

 

[JJSt]

Interesting, thanks!
"A pitcheron vs aileron still has the same AoA change during the roll maneuver. The main difference is the degree of control response for a certain amount of control surface deflection. Essentially a pitcheron is a wing consisting only of the aileron; not a separate case."
Is it not true that in the case of the aileron, the "fixed" part of the wing is working against the aileron when the aileron is deflected and the wing is rising/falling? or does the wing never rise/fall fast enough to have a noticeable effect on the apparent AoA?

Any thoughts on, disregarding the mechanical stuff for the moment, whether this is a more efficient way to achieve roll control for an aircraft?

 

[3DDave]

Neglecting the AoA needed to keep the plane flying and looking only at the roll component:

The angle of attack of the pitcheron tends towards zero. When it doesn't it develops lift and causes the roll rate to increase until the AoA is zero.

Taken as a whole, the wing section with the aileron is changing the camber of the airfoil, which changes it's angle of attack. Like the pitcheron the AoA will tend towards zero, until there is no lift acting to increase the roll rate.

Since AoA change will be proportional to span for a given roll rate, changing the AoA by changing the camber towards the wing tips allows the maximum camber change where the maximum AoA change is needed. The inboard part of the wing has much less AoA change required; increasing the AoA of the inboard section to match that at the wing tips just increases the drag without influencing the roll rate much.

The ideal would be to twist the wing, but that's been elusive for practical airplanes.

A complicating factor for level flight is that both wings are at a comparable AoA, which also means they are producing comparable induced drag. When there is a roll input, one wing will be at a higher AoA than the other, and produce more induced drag. This produces an adverse yaw, which the Wright brothers noticed. The plane would roll one way, but the nose would turn the other, requiring the development of the moveable rudder and with it, the coordinated turn. In large aircraft spoilers are often used on the wing that should roll down. Not only does this tend to accomplish the same thing as aileron, it doesn't result in adverse yaw.

I just looked at the XF-91 Thunderceptor. Can't unsee that one. The entire wing was adjustable; or rather the fuselage had a variable incidence setting as the AoA of the wing is, for any flying condition, fixed. The articles indicate the wing incidence was changed for landing/takeoff, which really means not having to rotate the fuselage and scrape the tail. F-8 Crusader also, and a much prettier plane.

 

[LiftDivergence]

You mean to say you don't care for the inverse tapered wings? They were experimenting with solutions to spanwise flow problems with swept wings. They must have been adhering to the Republic Aviation motto: "We take all your aerodynamic issues and replace them with structural ones!"

But in all seriousness, I'm sure it was well engineered for testing. It was an intriguing choice. I just like to poke fun at it because it's so bizarre in appearance.

Keep em' Flying

"I intend to live forever, or die trying" - Groucho Marx

 

[3DDave]

The inverse taper is fine. It's the rounded ends of the wing tips that got me. I can just picture a guy with a circle template putting those on there to -make it look better-. I wonder what they thought was too much to try.

OMG Spratt Control Wing

http://www.homebuiltairplanes.com/f...ence-wing-instead-flaps-sprattcontrolwing.jpg

 

[JJSt]

"Taken as a whole, the wing section with the aileron is changing the camber of the airfoil, which changes it's angle of attack. Like the pitcheron the AoA will tend towards zero, until there is no lift acting to increase the roll rate."

Right, but with ailerons, when the AoA gets to zero (during a roll), you have a wonky airfoil, right? is that a good thing?

With pitcherons, at least at some point allong the span, you will have what you started with before the roll. i.e. zero AoA and the same section.

Also, seeing as you mentioned twisting wings (as in the tip having more of a change in angle that the root) would this not cause considerably worse induced drag related yaw problems at the start of the rotation?

Would the ideal not be: pitcherons/ailerons to start the roll, then as the speed of the roll increases, add some "twist" to keep the AoA of all parts of the wing equally positive/negative with respect to the airflow? Maybe not equally... Insert better lift distribution here

Thanks for your replies, really appreciate the input. Learning a lot here!

 

[3DDave]

With ailerons the entire wing could be matched to the roll rate. With pitcherons only a tiny part of the wing can match, with the rest of the wing at a positive AoA and the rest of the wing fighting it with a negative AoA.

With pitcherons, at the start of the roll the entire wing lift coefficient changes. With ailerons only a small part of the wing changes, with a lower induced drag penalty.

Were pitcherons an overall good idea, given the number of variations people have put into flight, they would have bubbled to the top by now. Some ideas, like flying wings or tilt-rotor craft hang on by tenuous threads where aerodynamic performance is second in importance to some other consideration. Pitcherons are even lower in utility than those. Other oddities - VTO aircraft like Pogo; parasitic fighters like the Goblin; jet and rocket propelled rotor helicopters. All interesting, but mostly impractical. Still it's interesting to look at what makes them second best on the off chance they could be useful. Though now that I think on it, they are used as the main means for adjusting lift in helicopters, but helicopters can't adjust the rotor angle of attack by altering the fuselage angle of attack and therefore change the amount of lift- so there's no choice there.

 

[tbuelna]

What about the X-29? It used a flexible composite wing that had carefully tailored torsional stiffness characteristics which were designed to respond to aerodynamic lift forces.

 

[JJSt]

"With ailerons the entire wing could be matched to the roll rate" I'm not sure I understand this, are you talking about milti-sectioned ailerons, with higher throws towards the tips? Otherwise this may be the bit of understanding I am lacking!

Thanks again!

 

[KENAT]

Tailerons have not been uncommon in high performance jet aircraft for the last few decades.

As alluded to above I suspect issues with structural efficiency & possible control sensitivity would make doing this with the main wing tricky for many applications.

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[JJSt]

"With ailerons the entire wing could be matched to the roll rate" I cant seem to get my head around this, does anyone have an idiots explanation?

Thanks

 

[moon161]

No one has yet mentioned that the wing pitch actuator would have to be sized to resist the pitching moment of the wing, and yet be delicately responsive, especially in a closed loop feedback control situation.

 

[berkshire]

JJst,
The sprat Control wing aircraft designed in the 1930s seems to be what you are talking about in your original post.
B.E.



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