My idea failed..

[mmormota]

I had an idea, checked, failed. Please help me to understand better the reasons.

I have elevon driven wings, and have some praktice in very small, extremely short aeroplanes like the Nutta. I am interested in biplanes and 3d, so started to build a foam bipe.

The idea was:
- elevons and rudder on a normal biplane, the two sides of the "elevator" plays the role of the elevons
- short body to keep the control surfaces in strong propeller wind for good 3d
- strong dd outrunner and LiPo for good hovering

The plane is flying but terrible to control it.
The elevator control (by the elevons of course) is strong, responsive.
The aileron control (by the elevons too) is terrible. It is too weak, ok, I more or less understand it. But there is a big delay too!!! I move the stick full to the left, nothing happens at all. After 2-3 seconds it starts to response, and keeps to do it after I push back the stick.

What is the reason of this extremely big delay? This is a small plane, 60 cm wingspan only.

After a crash I removed the upper wing. With one wing it is a bit better, the aileron control is stronger a bit, but the terrible delay is the same...

I attach a picture:

[Ben Diss]

The elevators, being close the fuselage, don't have enough leverage to be effective.

-Ben

[Sparky Paul]

That's a good lookin' plane!
The short leverage for the elevator halves is one source for the slow response, as mentioned.
The other is the large vertical.
Couple the rudder to the aileron channel... !!!!!

[mmormota]

Yeah, it was a cute...
And now:

[BMatthews]

And of course this is why they put the ailerons at the wingTIPS rather than inboard as you have effectively done.

The only exception to this rule is with very high speed models like pylon racers. You often see the ailerons well inboard from the tips but that's because they are trying to soften the response that comes with the very high speeds they are flying.

If you were to climb your model to a very great height and dive it at full throttle you would find that as the speed built up the response would improve and that delay would probably dissappear. But at low speeds the controls need to be at the outer reaches of the surfaces.

This method works on deltas because the controls ARE at the extreme outer edges. It also works, within reason, on twin engine format jets because the elevons are spaced out near the wingtips because of the wide space needed for the engines between the elevons.

[lensrc]

You could add airlerons to the wing, and couple them to the tailerons and get Flaperon/Taileron control like is used on fun fly types. the cool part is it still only uses two servos.

[mmormota]

Thank you for the answers.

[mmormota]

I added ailerons to the lower wing, moved the CG a little forward, and now it flies really well.

[rorywquin]

No dihedral on the wings for stability ?


Edited - posted the comment about dihedral before I saw you had added ailerons - where dihedral is not as nescessary!!

[Peter D Rieden]

Ben said:
<The elevators, being close the fuselage, don't have enough leverage to be effective.>

Not quite. Ailerons & elevators modulate the lift of the surface they're attached to. The tailplane is a very small proportion of the total surface area so it produces a comparatively small amount of lift force compared to the wing which is compounded by the short moment arm. If you put ailerons on the wing that had the same area and moment arms as your current elevons they would be more powerful because they would be modulating the lift of the much larger surface.

Note that full-size aircraft which use this kind of elevon control (eg F-18 or Tornado) use very large control throws in "aileron mode" AND are augmented by wing-mounted ailerons or spoilers.

BMathews said:
<The only exception to this rule is with very high speed models like pylon racers. You often see the ailerons well inboard from the tips but that's because they are trying to soften the response that comes with the very high speeds they are flying.>

Again, not quite. Pylon racers use small ailerons mounded inboard to provide a stiffer system that is less prone to flutter (in the case where a single aileron servo is used) and to reduce the torsional loading on the wing (whether one or two servos are used).

The "inboard aileron" concept was first used by North American on the F100 Super Sabre specificly to eliminate the "aileron reversal" problem and allow a lighter wing structure. The thin wings of these then-new high performance aircraft it was very difficult to obtain sufficient torsional stiffness in the wing structure and at high speeds the aileron tended to act as a "tab"; twisting the wing and actually providing a reversed roll moment. In more modern aircraft this is largely addressed by using either tailerons or spoilers as the primary roll control.

PDR