I have a Balsa USA Stick 40 with a Super Tigre S40. Currently i'm using a 11-4 prop but the plane feels like it only wants to go up unless I counter with a LOT of down trim. What is a good prop size for this combo? 10-6 maybe? Or could this plane be tail heavy?

Try a 10x5 or 10x6 and it should work a bit better. The 11x4 is a bit big for a .40. Hope this helps.

The Tigre should have at least 5* down-thrust on that Stick 40, maybe even more.
An 11-4 is OK, however on that combination, I think an 11-5 would be a bit less noisy. I would hold back on that 10-6 until I tried the above.

While not totally true for the aerodynamics purists, for the lay-person this explanation will give you a working knowledge of downthrust and what it will do for you.
At higher speed, downthrust on flat bottom winged aircraft can serve you by pulling the nose down. You will trim for a slight nose up to maintain level flight. When the throttle is retarded, you have the up trim for the machine to maintain near a straight path as there is no longer a down force. Add throttle, nose wants to go down negating the wing's additional lift with speed increase.

Another tip for the BUSA Stick is to wash out the wing tips. If money-koted, then use the heat gun and twist the wing TE tips UP about 1/8 to 3/16". Really helps, makes it fly a bit like a real airplane.

If tail heavy the aircraft will be more "squirrelly" and not just wanting to climb. Measure the wing chord on that stick. I forget the dimension, but say it's 10". If your CG is behind 3" back from the LE, you are tail heavy big time. 2.5" would be just dandy as it would then be right with the sub-sonic wing's center-of-rotation.
Some will say 3.5" is OK. Don't take that to the bank until you are doing pattern and IMAC.

Good Luck.

Not for a Tigre! An 11-4 will turn a higher RPM than a 10-6 on most any .40. It's the LOAD and not the diameter.

ok, so a 11-5 or 11-6 would be the best combo? In the past I have only flown this plane with an OS 35 fp, seems like this ST 40 is too much hp.

10x6 is the "standard" starting size for a 40 engine, i have a 40FP that works well with a 9x7 and an irvine that likes an 11x4 yon have to play with the sizes a bit to find what suites the engine,plane, flying style combination.

as for the problem of climbing that has nothing to do with the prop/engine combo all flat bottom wings want to climb the faster they go, heres what i would do.

fly the plane at 1/2 throttle and trim it so it flies level.

land the plane and see where the elevator is.
if the elevator is down try adding some down thrust on the engine.
if the elevator is straight try adding a shim under the wing TE.

but as i said flat bottom wings tend to climb as they get faster

An 11X6 will most likely be a bit much for a 40. 11X5 should be okay, as would a 10X6. 11X4 will give you more torque, less speed. (And they're right about the downthrust and CG.)
Dennis-