Does a neutral Thrustline go through CG?

[shaneyee]

I converted an old FAI control line combat wing to R/C. I removed the wingtip weight and the wings are symmetrical. I have full span elevons except for the 2" wingtips which are angled up a few degrees for washout. The motor is mounted behind in a pusher configuration zero degrees to the wing 1.5" above the 48" span 480sq in wing. I have 5 degrees up on the elevons as my neutral position.

On my maiden flight it headed down straight after launch and pancaked itself on the ground. I think its because I mounted the motor 1.5" above the centreline of the wing. I want to add some upthrust to the motor for the next flight. Logically, I think I should angle the motor so that the thrustline passes through the CG. The root chord is 13" and the CG is located 2.5" from the LE. That would make the angle about arctan(1.5/10.5) = 8deg.
My gut feel tells me that its too much and 3-5 deg is enough but it seems logical to my brain.

So is my brain right or my gut? Or are they both wrong?

Thanks

Shane

[vintage1]

Oh dear. It is complex.

When you open the throttle instantaneously so to speak, the model its truw will tend to rotate around the CG, so yes, that is an imput, BUT when flying in a steady state - i..e not rotating at all, the angle between the thrustline and teh effective center of drag is what is important.

Guess why pattern plane have almost symetrical fuselalges, mid mounted zero dihedral wings and a CG somewhere plumb in the middle. An attempt to get CG, C of D and thrust all lined up nicely.

Actually I wouldn't worry TOO much - I think you probably didn't have it trimmed right or enough airspeed.

Try some glide tests over long grass if you can.

[shaneyee]

Vintage1,
Yes, makes sense....the Thrustline/CG effect is temporary and the centre of drag will eventually be the important point. But, the centre of drag for this plane will a bit above the centreline so I am guessing that the thrustline will still be above it. So, maybe I'll just change it 3-4 degrees and see how it goes.

It just didnt fly long enough for me to react.... maybe I'll get a bungee rigged up. 12oz/sqft is a bit too heavy to test glide effectively from a hand launch I think.

Thanks

Shane