Hi all,
I must admit that I do not fully understand why down thrust works when the model is inverted, that and is it of any benefit with inline models?

I have not seen down thrust recommended on any plan that I have but I know of a few very good pilots that swear by it but all of their models have inverted engines with the trust line above the mean chord line and the tailplane some where in between.

So do models like the Yatsenko Shark or Beringers Gee Bee use down thrust?

Cheers.

Thanks for the reply Igor, and I have heard that before and accept it but I do not understand it when the model goes inverted.

And do you know if inline models need this down thrust phenomenon?

Thanks.

Thanks for the reply Igor, and I have heard that before and accept it but I do not understand it when the model goes inverted.

All is reversed ... all moments, and also model, so it still works well :-)

Simply put, when the plane is flying upright, the prop effects are trying to push the nose up, or towards the top of the plane. When inverted, they are still trying to push the nose towards the top of the plane, except, now, the plane is inverted, and the top of the plane is towards the ground.

The downthrust when upright, works to nuetralize the prop effects that try to raise the nose. The "downthrust", becomes upthrust when inverted, and still works to nuetralize the prop effects, which reverse when flying inverted.

Randy, Are you trolling , or just stirring the pot?   

Ok, I'll make a try at an answer to your question.

Our normal CCW rotation causes prop effects that tend to yaw the nose in, and push the nose up in level flight. Reversing the direction of the prop rotation will now change the prop effects so that the plane tends to yaw to the outside of the circle in our circular flight. This helps a bunch and contributes to tighter line tension at all times, when the engine is running. Such a change will also change the nose up force with standard CCW rotation, to nose down force with CW rotation.

So, is there still a need for thrust line adjustments? I suspect so. Simple logic says that it would be so, but I'd prefer some hands on experimentation before I fully committed to the idea. If it proves true, and I suspect it will, then with a Clock Wise rotating propellor, some upthrust may indeed be beneficial.

Further more, it might also be helpful to add a small amount of stab incidence adjustment. The amount should be small if using thrust line adjustments.

Going further;

An in-line design can be successful as has been proven in various designs over the years. Maintaining good control geometry will help a lot to cure variations mostly brought on by poor control system geometry.

It will still be necessary to make thrust line incidence corrections to help obviate the prop effects, which are not a function of whether the plane is in-line or not. These effects occur because the plane is flying with a single spinning prop, located in the front of the plane, while that plane is flying in a circular path. 

I always use a bit of up thrust on my planes.  Of course I fly clockwise.
Alan Resinger