discussion troublesome right swerves.

[Guest]

Can anyone suggest why I need to put LEFT thrust on to a Bushwacker model I built recently when the plan shows clearly the engine mounting plate is angled to the right. The model seems to be built decently enough with everything square, no warps, recommended engine size, etc. On takeoff, halfway up the strip, it swerves to the right. Today I managed to get it in the air but as it left the ground it rolled to the right and only hard left rudder saved the day. Appreciate any comments you experienced flyers may wish to offer. Thanks.

 

[Guest]

Hi Tommygee.

I don't know the model you mention, but I assume the prop rotates in the normal "model" direction of clockwize when viewed from the tail end?

If so the right offset on the mound plate will be to counter the left pulling force produced by the torque,gyro effect and P-factor of the prop on the airframe.Usualy arround the 2 to 3 degrees should work.

What I find interesting is that the model pulls right approx halfway down the runway.This tells me that the problem you have is most likely airframe and not engine mound.I will explain - If the right thrust was to much , you will find that the plane will yaw right early during the takeoff roll as the torque and P- factor forces are stronger than the vertical stab and rudder forces being produced by the air flowing over it.

As the airspeed increaces, the aerodynamic forces become stronger and will start to override the engine forces.On some full size planes they have no right thrust but the vertical stab is actually attached scew to the fuselarge by a few degrees to offset the yaw factor, eg the Pietenpol Aircamper homebuilt.

I think that there must be an aerodynamic reason behind your problem.Things might seem "straight" but the eye can be decieved easly.

How much right thrust is in there now? I think start to reduce it by putting washers behind the right side of the engine mound and see if there is any improvement.

But to add left thrust should realy not be correct, then something else is wrong!

In the 22 odd years of flying I have never had left thrust to a model.Even on the full size planes I do, I see most have a little right thrust to the mound on "yanky" Lycoming and continental engines.

Hope it helps.

Theuns