Undercumber wings on bipes

[gEcky]

A few questions on undercumber wings:

1) the flat part of the undercumber (wing bottom) must be parrallel to the motor thrust line?

2) for bipes with undercumber, the flat wing bottoms are parrallel?

3) ailerons on such wings are pretty useless?


I attempted a full fuse bipe (sort of a modified micropitts scaled up to 30" WS) out of compressed foam and it simply shot for the sky on throttle Ailerons were also helpless. Broke some props but i managed a painful 3min flight by coupling aileron+rudder and elevator+throttle.

Seeking your advice and opinions Thanks!

[joe yap]

First of all, the term is spelt as 'Undercamber'.

Second of all, I'm not too sure of what you mean by undercamber with the bottom of the wing flat. I'm conveniently assuming that your plane has no airfoil and it's nothing more than a flat plate wing with the trailing edge bent to form the camber. By doing so, you are effectively increasing the angle of incidence of the wings, which brings it nearer to the stall margin. This also increases lift, as you probably know.

Assuming a flat plate wing mounted with zero degrees angle of incidence. By bending the TE to form a camber, the angle if incidence will be changed. The effective angle of incidence will be represented by a straight line drawn between the tip of the wing LE and the lowered extreme end of the TE. Unless a 'droop' on the LE is being introduced to lower the angle of incidence again, it'll bring it closer to your wings' stall angle.

Doing this has some effects on the plane. First of all, the lift is increased but the induced drag will be increased as well. By increasing the TE camber, as mentioned before, brought the angle of incidence nearer to the stall angle of the wings, meaning the flight envelope will be narrower which makes the plane tricky to fly.

Anyway, the problem you faced may be contributed by multiple factors instead of the one mentioned above. To put things to perspective, ailerons are pretty tricky and problematic at lower speeds. They are pretty ineffective at lower speeds and that explained why you need to couple your rudder to aileron controls to make control possible. To make matter worst, you had already made the wings Angle of incidence nearer to your wings stall angle, by lowering one aileron to lift the wing to acheive rolling effect, in fact induces a premature stall to that particular wing. The net result is that the wing that supposed to rise infact drops sharply, giving you OPPOSITE roll effect. To cure that, some people rig the ailerons as such that the upward throws is usually more than the downward throw, or what we call the 'differential'.

Another factor could be contributed my the propellor torque effect. Most scale and sport scale Pitts models have short wings and are very prone to prop torque effect, especially those with large prop. Looking from the front of the plane, the prop is usually turning anti-clockwise. Due to mechanical friction of the motor and the aerodynamic drag on the prop, the reaction of the motor tends to rotate the plane on the clockwise direction. Try loading up a rubber powered glider, and release the plane while holding on to the prop, the plane will rotate the opposite direction of the prop rotation.

The lower the airspeed, the more pronounce the torque effect as the wings produce less counteracting forces.

Hope this help you to understand better.

[P4cm4n2000]

Wah Joe, thats explains why when I maiden the bipe for Sinkyo this morning, realised to turn using Ailerons seems not much effect, I got to react by using Rudder to assist the turn.

On low speed, when I tried to use Aileron to turn the plane, it almost stall and the plane simple nose dive, I had to quickly again throttle up and level the plane with slight Elevator to level it and gain speed to stablize.

Do you propose bigger Ailerons will help?

[joe yap]

More aileron area will help to counter torque effect. You can increase the size or use 4 instead of 2. However, If it's stalling, I'll suggest incorporating differential aileron throws. Increasing the flight speed will help too. Use higher pitch prop. To make things easier, program rudder coupling to your aileron controls if possible. You can program it to activate it by a switch if you want.

[P4cm4n2000]

Thanks Bro!

[gEcky]

Thanks Joe!

Will post some pic of the "problem" when i get my camera back.

[MicroHeli-Nut]

There's a thread on an underchambred Biplane profile design on RCG where your issues are discussed... Read it through. given below is page 2.

Here