All my models pitch up when air speed is increased (except my 3D model type, whose pitching tendancy are at the least). Downthrust is used to help reduce this pitching tendancy by keeping the nose down, thus when thrust is increased, so is the down thrust component.
The other observation is, when the propellor's rpm is increased, the model rolls to the opposite direction of the propellor's. I understand this to be the same as torque-rolling. It is especially evident when performing rolling from hover. I normally use tractor propellors, turning counter-clockwise. It is easier to allow the model to roll left by releasing both right aileron and rudder which were held when in an hover.
There is a difference though. My 3D planes have hardly any downthrust or right thrust, whereas my ordinary model has varying degrees of downthrust and right thrust. Especially obvious is the Red Triplane that was given to me. It has ridiculous amount of down and side thrusts.
Even though it works, I don't understand how pointing the thrustline to the right help to balance torque roll. How could doing that create a rolling moment in the other direction?
Without understanding fully, I still like to consider the application on pusher models. If I have the propellor as a pusher, should I introduce thrust line deviations?
There are two possible deviations to the thrust line, whether it points upwards or downwards, and whether it points left or right. To understand the turning moment created by these deviations, the first thing I need to understand is, where is the pivoting point?
Is the pivoting point at:
- the centre of gravity; or
- the centre of drag; or
- the centre of area; or
- a combination of the above?
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