I am designing a bipe and i need to know how big the H and V stabs need to be in relation to the wing area.

On a monoplane i use 18 to20% of the wing area for the H stab and 9 to 10% of the wing area for the v stab.

My friend is storing his 1/4 scale tiger moth in my shop and i noticed the tail seemed small for the wing area! it has 2 71x10.5 inch wing 1491 sq inch wing area.
H stab is around 175 sq inches.

So thats like 24% of one wing or 12% of the total wing area.

So how do you determine the tail size for a bipe? I always though you did it like a mono plane

I don't completely understand all that is involved in the smaller tails on biplanes, but I believe you are correct.

First the the top wing interferes with the flow over the bottom wing, so you can probably consider the bottom wing to be 30 or 40% smaller. It seems to me that the tails are even small when you adjust for the wing efficiency, but this is a very unscientific opinion.

There are other aspects that may effect tail size selection. The tail on a biplane (or triplane) often has the air flow disturbed by extra wings. caban and interplane struts, and flying wires. The horizontal tail is not as effective at allowing the CG to be moved. Also drag is more of a factor in the performance of biplanes. I'm not sure that adding a huge tail to a biplane would give you the same handling changes that you get in a very clean and fast flying monoplane.

Also, as I recently learned, the early biplanes had very short noses (because of the heavy engines of the era). When we model these plane we tend to wind up with low pitch and yaw moments of inertia. This makes the elevator and rudder very effective.

I hope someone who understands the aerodynamics of biplanes, will comment, as I am interested in this issue, too.

As for designing, I'd look at planes that fly like the one you want to build. If you want an aerobat look at some Pitts or Ultimate models and see what they use for tail sizes etc, etc.

Carl

For a really well behaved, fully aerobatic bipe that won't bite you if you get a bit out of shape and too close to stall, I like a vertical tail with 15% of the wing area, and a horizontal tail with 20% of the wing area, for a normal tail moment in relation to wingspan. The 20% horizontal tail could be reduced to as little as 10% without too much nasty handling, but I would keep the vertical tail as large as possible. If the model is scale, just enlarging the vertical tail, while keeping the same shape is not all that detectable, appearance-wise, even with double the scale area, and your two-wing bird will be far easier to live with. Bipes do need larger tails in relation to their wing areas than monoplanes, due to all the turbulence stirred up by the extra wing and its cabane struts, open cockpits, and the like.