Question, I'm building an ESM ARF and the manual states that you must drop the thrust line 1/4"/6mm. Doing so puts the crankshaft center in the middle of the cowl. Not doing so and the cosmetics are upset. I took all the measurements and the thrust line should be the first mentioned. Being somewhat familiar with thrust lines and angles would this new setting act like additional down thrust? The other odd thing I noticed, was that the fuselage, which is fiberglass, has moulding builtin to accept the horizontal stab halves. These mouldings are set at a positive angle of attack. Does this relate to the location of the engine thrustline?

Although some stabs are at a geometric angle of incidence, the downwash produced by the wing hits the stab at a negative angle, causing the stab "to feel" the AOA as negative, so the tail pushes down for dynamic balance.

Since thrust is variable during a flight, a good design should try avoiding pitch changes induced by those variations.
The ideal condition is when the thrust vector passes thru the CG (in its vertical position).

There is no much that you can change, it seems.
The designer has tested the model before massive production of the ARF, I would think, and those instructions should be OK.

Thank you, for the information. The only other ARFs I have assembled and flown where sold by "Great Planes". They were well engineered and the overall quality was excellent. This ARF is manufactured by "ESM" and leaves alot to be desired. The aircraft in question is there FW 190 and will be my first warbird. There is a build trend under the warbird forum and appears that most of the participents are kit basshers. When I posted this question on the forum I received no advice.

Moving a thrust line down won't cause extra down thrust. It would cause a small change in the thrust's relationship to the center of drag of the total airframe. Moving the thrust lower compared to the center of drag should cause a more upward pitch that increases with speed.

Want an easier way to see what the situation is? Visualize what would happen if you moved the thrust line down a couple of inches. The drag would be above the thrust and the airplane would pitch up.

Thanks for the explanation.

I'm building a Sig Spacewalker II kit and I want to "kit bash" it to look like a full size re-build of a Spacewalker II shown here. http://www.rotecradialengines.com/cu...MartinHone.htm

I intend to mount a Zenoah G26 and a fake radial inside the round cowl. So this will mean some serious bashing! It looks like I would have to lower the thrust line by about 1" or more to accomodate the round cowl and raise the fuse height to fare it all in. The plan calls for +1 degree on the stab. I think the lower thrust line will tend to pitch the plane up at cruise speed so - should I INCREASE the stab incidence to maybe +3 degrees to compensate?? I will have to do other alterations which are obvious if you look at the pix of the full size but I'll have a rarely seen scale aircraft.

Appreciate your input!

Jim,

Great project!

I believe it is the other way around.
The stab pushes down; hence, the negative incidence should be reduced, if modified, in order to produce less pitch up.
However, I prefer not to mess with original incidences, if they work fine.

I would determine the vertical location of the CG and lever of the thrust line respect to the CG of the original model plane before bashing.
Without changing stab incidence, I would try to keep the same relation between the vertical location of the new CG (probably lower) and the lever of the thrust line after bashing is complete.

In that way, you would keep the original aerodynamic balance, whatever it is.

For location of CG vertical position:

http://www.rcuniverse.com/forum/fb.asp?m=10087124

AWWWWW! Adding those pix cleared things up. I've been flying for 60 years but never heard the expression "lever of the thrust line"! But now I see what you mean. I will locate the new motor position on the plans and draw a line from it to the old lever of thrust position, as you've indicated. I can then determine the angle of downthrust I would need to compensate for lowering the motor position. Thanks for taking the trouble to explain it. Looking at the pix of the original, it appears he may have done the same thing. It is hard to detect but I seem to see a little downthrust in his motor. Cheers

You are welcome.

I call it lever, but arm maybe the proper name.

It is all about the moment or torque that acts over the CG.

As you know, Moment = Force x Arm

As for the apparent 'positive' incidence in the tailplane, you need to compare it to the wing, not the fuselage, the real FW flies 'S&L' somewhat nose down... thus giving the engine some downthrust in relation to the wings' rigged angle to the fuselage. The main wing is rigged about 5 deg positive to what would seem the fuselage horizontal line, the tailplane should be within a degree or two of that.
Evan, WB #12.