flaperons on a biplane
 

I bought a used HPI racing biplane with a symetrical wing design. Excelent flying plane. I have been playing with flaperons on my big stick and like what it does to th elanding speed. I decided to do the same thing to the bipe. The bipe reacts much differently to the flaps than the stick. When deployed on the stick in the down position the plane wants to lift, on the bipe i deploy them downward and the plane dives for the ground. I can compensate with up elevator to stabalize it but it seems backwards to me. If i switch the flaps up to spileron then it goes up. With that much aleron surface is it pionting the nose up and down orsomthing strange? Can you use flaps with a bipe wing design? All i really want is to slow the landing speed down a bit, this thing really comes in hot and falls fast when you drop the power back. Any thoughts would be helpful.

Sean

RE: flaperons on a biplane
 

I don't know for sure in your case, but I can say that on a short coupled plane, dropping flaperons often makes the nose go down. The shorter the tail moment, the more pronounced the "down" when flaperons are deployed. The shorter tail moments seem to result in behavior that is closer to a flying wing in this regard.

RE: flaperons on a biplane
 

Sean,

A few thoughts... Looking strictly at the wings, deploying flaps will almost always result in a nose-down pitching moment. This means that deploying the flaps trims the aircraft for a lower angle of attack. On the other hand, deploying the flaps is likely to increase the wing's maximum lift coefficient. A bigger maximum lift coefficient means that you can probably fly slower before stalling. The trick is to retrim for the desired angle of attack once the flaps come down. Unless you get it just right, you will probably experience transients in both pitch and flight path.

As you have discovered, not all airplanes pitch down with flap extension (even if their wings experience a nose-down pitching moment). The reason is that deflecting the flaps may have a big effect on the flow over the tail surfaces. A substantial increase in downwash over the tail will cause a nose-up moment that may dominate the nose-down moment experienced by the wings. As montague points out, the effect is often exaggerated with a short-coupled tail. In contrast, aircraft with T-tails will almost always experience a pitch-down because flow over the tail is less affected by the flap deflection.

Many airplanes automatically retrim when the flaps come down (the T-38 will experience an almost uncontrollable pitch-up if the interconnect between the flaps and stabilator fails). It sounds like you are already doing it manually, but if you have the ability to couple some nose-up elevator trim to flap extension, I think you should be able to slow the landing speed down. Hope this helps.

RE: flaperons on a biplane
 

Helps alot, thats sort of what i figured was happening but i like to be pretty sure with the biplane so i dont crash it. I just bought a 9c futaba so i can program the elevator into the flaperon function with only the efforet required to learn the program.

Thanks for the advise.