[bcsaltchucker]

Hey I modified a nice little scale ultralight model recently. Changed it was a tepid pusher to a hotter tractor twin.

I mounted the motors (counter-rotating) on the leading edge of the wing, and I put about 10-15 degrees down angle, like you would for motor on the nose.

but this plane still doesn't handle just right yet. I think it pulls the nose down a bit much with power on, but glides power off alright.

Should I be looking at this thrust line a different way? Or just make it level with the bottom of the wing?



video of flight



thanks!

[canadianf1pilot]

10-15deg down seems a bit much.... Try bringing it back to 3-5deg down thrust
Bill

[kip51035]

Bill is right a lot less down thrust is called for. A low cruse speed will make that aircraft smoother. Going fast will make it do weird things. Set the C of G at 25% back from the leading edge of the wing.

[Flypaper]

If I was me, I'd set the downthrust at 0 relative to the wing. With all that drag below the wing, it's going to cause a nosedown tendency. Some of the twin engine seaplanes even have upthrust in the engines for this reason. Idea is to set the downthrust so there is no pitch change when you crank on the throttle.

[OLD PRAIRIE RED NECK]

Originally Posted by Flypaper

If I was me, I'd set the downthrust at 0 relative to the wing. With all that drag below the wing, it's going to cause a nosedown tendency. Some of the twin engine seaplanes even have upthrust in the engines for this reason. Idea is to set the downthrust so there is no pitch change when you crank on the throttle.

I'm 100% with Gord here!

Even with all the perceived drag below the wing, in actual fact the highest drag on an aircraft is the WING. Thrust line is relative to the distance between the center of drag (wing) and the thrust line (prop shaft). In your case it is virtually zero so there should be no thrust angle on the motors.

[JoeT]

If it was me, I'd trim the plane to fly the way you want.

To set thrust (up / down) here's what you do.
Take the plane up to a safe altitude, fly it at full throttle. Use the Tx trim to fly level hands off.

Now here's the testing part:
Chop the throttle:
If the plane balloons up, you have too much down thrust.

If the plane drops like a rock, then you have too much up thrust, adjust accordingly.

The ideal trim is very subjective, but in theory it should neither balloon nor drop, it should slow and drop very gently.

Hope this helps.

It's a simple test.

[TomGSMackay]

On a high wing twin, you generally don't require any down thrust at all due to the thrust moment being above the center of mass and center of drag. Try shimming the mounts to zero a little at a time.

[norcsnobs]

Let us know how it turns out. Great looking mod.