Elevons or flaperons on a tandem-wing design

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cluttonfred

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I mentioned this in another thread but I wanted to explore this particular idea further. I like simplifying control systems as much as possible and have been thinking about elevons in a three-axis tandem design. Here's a quick sketch.
HBA concept sketches (19).jpg
The idea would be to use a control mixer to apply pitch and roll inputs with the front wing elevons only (shown in yellow). They are tapered to increase the incidence of the root more than the tip as they go down and so induce a safe, root-first stall progression in which the tips would remain unstalled to provide roll control and the stalled root would then allow the nose to drop and recover unstalled flight. The elevon taper could be increased and/or stall strips and/or vortex generators used to tweak the behavior as needed.

The optional trim flap on the rear wing (show in green) would probably not be needed in the simplest application, but if the elevons were to be used as flaperons, the trim flap could also be lowered to compensate thereby increasing lift and eliminating any significant fuselage nose-up change with lowered flaperons.

Feedback welcome!
 
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Aesquire

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a few thoughts...

Roll control deflection on a canard elevon limits your pitch control. So, if you're already at your limits, holding the pitch level, roll input drops the nose.

It's important the front wing stall first. Even true in conventional tailed designs with down pushing horizontal stabilizers.

Rear wing camber changing flaps/trimmers must not cause the rear wing to stall first.

It's simpler to have pitch only control on the front wing, and roll only on the rear.

None of the above says it can't work like you envision, but you need to keep the issues in mind. That probably means larger elevons than a "regular" Quickie tandem wing.

The use of tapered elevons does create apparent washout when deflected down, that's nice, simple, and elegant. And if you had separate elevators and ailerons you might avoid the pitch control loss, at the expense of complexity and larger elevators & front wing?
 
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Hot Wings

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IIRC in the early iterations of Rutans canards he thought of this method (using a mixer on the canard for both roll and pitch) but abandoned the idea because of the additional down-wash of the front deflected roll control surface reduced the lift on the trailing aft surface thus partially negating the roll input. My history may incorrect...........

I do know that the 'reflexors' on the Q's are powerful pitch devices of their own. One of the experiments I'd like to try once my Quickie flies is to use the aft surfaces for both roll and pitch and reserve the existing elevator for use as a flap.
 

cluttonfred

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Thank you both, I had forgotten about the early Rutan experiments, though I am not sure that same problem would apply when the forward wind is both substantially larger and more highly loaded then the rear wing. The idea that a roll input near the stall would cause one half the front wing to stall is more troublesome, though I wonder how dramatic that would be if the aileron taper forces the root to stall first. In some ways this concept is really closer to the Gatard Statoplan than other tandem or canard designs: Another French twist: Albert Gatard's Statoplan

To be perfectly frank, the application that appeals most to me is no flaperon function at all and no trim flap on the rear wing, so you are down to three moving surfaces (two elevons and one rudder). That would make disassembly for storage or transport pretty easy... the whole rear wing could be held on by three or four bolts or pins with zero control connections, the front wing would involve removing just two wing panels and disconnecting two controls. Even if I had to go with elevons of exaggerated size and taper to make that work, the simplicity would be worth it.

I suppose you could accomplish the same thing with separate outboard ailerons and inboard elevators (all tapered or just the elevators) on the front wing, and you could probably work it out to have just one control connection per wing, but that’s more complex than I’d prefer.
 
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Victor Bravo

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My vote (FWIW) goes for elevons on the rear wing.

This would allow a solid, simplified attachment of the front wing that is probably a lot lighter because it can use a traditional biplane "parallel inverted V" cabane assembly with no pivots, instead of the more massive single struts or pyramids that have to take drag and twisting loads and still allow the wing to pivot. Fixing the front wing allows smaller diameter, lighter V-struts or perhaps even cables to brace the outboard section as well.

The control runs are a little shorter, and the elevons can be operated by a very simple Monnett style mixer (a giant version of the old Du-Bro model airplane V-tail mixer that mounted right on the servo wheel!!! :) )

Having any control surfaces on the forward wing creates the possibility that deflecting them can affect the flow and lift on parts of the rear wing... this problem would go away if the elevons were on the back.

Having them on the rear wing gives you some amount of increased authority in the prop wash if or when that is helpful.

Elevons on the rear wing only simplifies the structure, versus one set of controls on the front and another trim flap or whatever on the back.

Some extra effort is required to have the "one-piece" elevons come a part to fold the outer wing panels on a traditional Pou configuration, but this is the same for any three-axis Pou regardless of which wing has any controls on it.
 

cluttonfred

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Thanks, VB, but you are giving me doubts about my already limited artistic skills. That's a low-wing-forward tandem in my sketch without pivoting wings along the lines of the Guerpont Autoplum but hopefully better looking. Would you care to revise any of your comments? ;-)

autplum.jpg
 
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cluttonfred

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Hehe, that won’t work on me, I am strong with the force of the Saint Patron himself, Henri Mignet! If I go high wing forward it’ll be two axis anyway, vive le Pou du Ciel! ;-p
 

Aesquire

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The idea that a roll input near the stall would cause one half the front wing to stall is more troublesome,
I wasn't worried about that, More that you would run out of deflection with roll input, and have to center the controls to maintain pitch attitude.

But now that you mention it.... stalling one half of the front wing would be a nasty roll effect added to the normal "canard nod".
 

Aesquire

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Having any control surfaces on the forward wing creates the possibility that deflecting them can affect the flow and lift on parts of the rear wing... this problem would go away if the elevons were on the back.
Since Pou uses a pivoting front wing for ( flame on! ) weight shift pitch ( hee hee ) I agree you don't want to complicate the design, ( control surfaces on front wing ) with little gain. I'd skip the elevons on the back wing of a Pou, unless you are going for a trimming aileron mutation.

My reply assumed a low wing canard/front wing.

Isn't there a NASA paper on low vs. high front wing tandems? IIRC there is a difference in stall characteristics. ( PLEASE post if you recall it )

I have an odd combination of admiration and dislike for the Quickie. It's a lovely shape, and flies with great efficiency, OTOH it's very much a hard runway bird, and I still get psychosomatic itching when I see one, having spend a little time sanding on one in a hot sweaty hanger... I mean I was sweaty, the Hanger was clean.
 

Sockmonkey

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I wasn't worried about that, More that you would run out of deflection with roll input, and have to center the controls to maintain pitch attitude.

But now that you mention it.... stalling one half of the front wing would be a nasty roll effect added to the normal "canard nod".
Have to add some washout at the tips. Maybe have your elevons only occupy the middle third of the wing.
 
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