ORIGINAL: Spanky1
Good day.
If I understand some of the forums, the downthrust (on a trainer) is to help overcome the lift created when adding throttle and increasing the airspeed......right?
Spanky
Actually, that's not quite right. When you add power, airspeed will initially tend to increase, and lift will increase. The model will the pitch up and climb. That's true of any airplane...ANY airplane.
Downthrust (upthrust, sidethrust, etc) is used to reduce the amount of pitch (yaw) change the airplane makes when the throttle is moved. The pitch change is dependent upon several different factors: thrustline location with respect to the aircraft's CG, wing location, tail location, wing incidence, tail incidence, and so on.
Properly speaking, if a model's in stable level flight at less than full throttle, adding throttle, and nothing else, should result in the model increasing altitude. The rate of climb would be dependend upon how much throttle was added. Reducing throttle would result in a descent. You can't avoid that. What the thrust angle will do is make the model's pitch change milder and more manageable. With out the thrust angle, the model may pitch hard nose-up when throttle's added, and vice-versa when the throttle's retarded. Of course, with too much angle, the effects would be reversed.