I have a few model aircraft and know that the motor is often slightly offset. I have heard conflicting reasons as to why this is done. Some people claim it is because of motor torque and other people say that it is done because of other things, and not really because of torque. What is the most significant reason that the thrust angle is usually adjusted on model aircraft?
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2 Answers
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Typically the motor will point down and to the right. There can be various reasons for this.
I've included a plans excerpt from https://aerofred.com/details.php?image_id=91062
Downward angle
Sometimes it's to align the motor with the true flight direction, because the wings need some AOA (Angle Of AttacK) greater than 0. In the excerpt above, you can see that there's a 3 degree downward tilt. If you look at the original plan, the wings are mounted flat to the fuselage. A 3 degree AOA is pretty normal for cruise flight, so this motor is mounted so that it's pointed in the direction of movement.
If it were not pointing downward, then when you changed thrust it would have a tendency to momentarily sink/climb.
Another advantage of pointing down is that it increases pitch down tendency (or decreases pitch up tendency) when power is applied. Pitching down is safer since it drives you away from stall, instead of toward it. This is especially valuable in the case of positive pitch stability craft where there is a natural tendency to pitch up as airspeed increases.
You'll find that full-scale aircraft have the motor aligned with the fuselage, but their wings mounted at a 2-3 degree incidence angle so that the fuselage is level while the plane is flying forward. It turns out that the squishy humans seated inside prefer not to be seated on a slope.
Pointing to the right
In some models, the motor can be turned to the right so that its thrust counteracts the p-factor, which tends to turn the plane to the left. This makes the plane easier to handle when under high power at slower speeds when the rudder is less effective.
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$\begingroup$ Excellent! Could you please explain what p-factor is in your answer? Your commentary there is a little brief. $\endgroup$– ifconfig ♦Commented May 11, 2020 at 2:39
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$\begingroup$ Derp, I had meant to include a link. Thanks for pointing out my omission. $\endgroup$ Commented May 11, 2020 at 2:43
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$\begingroup$ I'd add that planes with positive pitch stability tend to pitch up when they go faster (due to more air flowing over the tail), so down thrust reduces the interaction between throttle and pitch $\endgroup$ Commented May 11, 2020 at 7:56
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Often if the motor is mounted off the centre of gravity you will adjust the thrust angle of the motor to point at the centre of gravity to reduce the torque effect on the model.
For example a model with a high mounted pusher prop mounted behind the main wing will often point up slightly as the battery will typically be mounted lower. On a flying wing where the centre of gravity aligns with the motor the motor will often be pointing directly through the model in a straight line
