determining proper thrust
 

I installed a motor mount on a plane that has had the front end rebuilt on it. I am not sure if the firewall was rebuilt so that the engine mount would have a few degree thrust to the right or not. Is there a way to determine if it is correct or not? If the engine is mounted with no thrust to the right or perhaps a hair to the left, how will the plane fly differently? Does a plane tend to have certain bad habits if the thrust is wrong?

RE: determining proper thrust
 

The only way to determine when the thrust is correct is to fly it. Fly away from yourself, on a calm day. Pull up until the plane is vertical. If it pulls off the the left, it needs right thrust, if it pulls off to the right, it needs left thrust. Check several times to make sure that it was not your inputs that cause it to pull off to one side, before making changes.

Typically about 2 degrees of right thrust is a good place to start. Some planes are set up with some thrust built into the firewall, and, if it has a cowl, the engine may be offset to the left so that the crank exits the center of the cowl. Engines and even props can change the necessary thrust offset.

RE: determining proper thrust
 

I use a slightly different proceddure thanb J_R does. I trim the plane out to be flying straight and level at about half throttle. Now advance the throttle and, if the plane dives,add more up thrust or delete some downthrust. If the plane zooms, increase downthrust. If it veers left, increase right thrust, if it veers right, decrease right thrust. Once it is properly adjusted, you can go from half throttle to full throttle with no change in altitude or heading, just an increase in speed with increasing throttle. You will find high wing planes usually require more right and down thrust than mid or low wings do.

RE: determining proper thrust
 

Up or down thrust is a different matter.

The incidence of the wing, relative to the tail, comes into play here. A flat bottom airfoil generally requires some positive incidence, while a symmetrical airfoil MAY be set at zero. Hopefully, the designer/manufacturer of the plane will have given the starting incidence settings. The decalage (incidence of tail relative to the wing) can be checked by putting the plane into a power off dive and noting which way the nose pulls. Adjustments to the decalage must be made before thrust is adjusted.

The accepted method for setting up and down thrust is to fly parallel to the runway and pull vertical and adjust depending on if it pulls to the canopy or to the wheels.

Ultimately, as Rodney pointed out, the plane should stay level when the throttle is advanced.

For a more complete series of changes to make in trimming a model, there is a pattern trim chart that works well at http://www.wtp.net/DBEST/trimchrt.html

There is a heck of a lot more to trimming a plane than adjusting the Tx trims.

RE: determining proper thrust
 

thanks for all the info. I flew the plane a few days ago and fortunately it flew great how the engine was (a few degrees of right thrust). I also followed Rodney's suggestions. The wind here is constantly between 10-20 mph so it is hard to tell 100%, but I flew it parallel to the runway several times and increased throttle as it passed me and I didnt notice the plane pulling consistently in any one direction. Thanks for the help!