Need help with thrust line!

[D-Rock]

I need some advice on an aircraft I have designed. Its a twin engined, low wing aircraft. It has a flat bottom airfoil and a sheet horizontal stablizer. The thrust-line is even with the bottom of the wing. To the best of my ability, the wing and tail incidence is 0 and so is the thrust-line. When I test flew it the other day, anything beyond half throttle made it climb dramatically. My first thought was that I might need downthrust. Is this correct? If so, how much is recomended. The aircraft is balanced at about 20-25% back from the leading edge, could this be an issue as well? Thanks in advance.

D-Rock

[Sparky Paul]

It might be.... overpowered! :)
If your wing is the typical "flat bottom", it already has some incidence. No need to mess with that.
Downthrust might help.. check the prop clearance.
It can't hurt to experiment.

[Rodney]

Every conventional model I have built and flown required some downthrust, some more than others. High winged models similar to the J# Cub required the most downthrust, usually 3 to 5 degrees along with about 3 degrees right thrust. Lower winged models required less downthrust. Those unusual models, like those with the motor mounted above the wing, were the worst to set up best thrust angles on. I never did get one of those to accomodate all throttle settings without programing in a mix of elevator/throttle to tame down the pitch/dive charcteristics as throttle setting changed.

[D-Rock]

Thanks for the comments guys. It seems that I should indeed adjust the thrust-line down a little.

D-Rock

[vintage1]

also try moving the GC aft a little at a time until the model when put into a diving glide with the power off only just pulls out..that's about as near as you want to go to neutral stability on a sport model.

THEN adjust the thrust lines.

[hul]

try moving the GC aft a little at a time
agree to that, nose heavy planes climb under power. I haven't found downthrust necessary on any of my planes.

The thrust-line is even with the bottom of the wing.... the wing and tail incidence is 0 and so is the thrust-line
that sounds like you're measuring incidence to the flat bottom of the wing. Shouldn't do it that way, measure to the chord line (dotted in this drawing). The difference is about 2 degrees in a stock thickness Clark Y.

Hans

[D-Rock]

Thanks for the responses, I will have to do some experimenting with the cg as well. Does a nose heavy config climb under power because of the up-trim needed?

D-Rock

[hul]

Does a nose heavy config climb under power because of the up-trim needed?
yes

Hans

[D-Rock]

Thanks! :)