Cause: Insufficient engine downthrust. Engine thrustline error.
Fix: Increase Engine Downthrust.
Comment: Expected behaviour on cutting throttle to idle is a gentle nose down as power is removed. Insufficient engine downthrust creates an upward pitching moment which rotates the aircraft around it's lateral or pitch axis in a nose up direction, necessitating compensating elevator down trim settings which then pitch the aircraft downwards when the engine is throttled back to idle.
During flight testing, downthrust should be increased a little at a time until the optimised response to throttle cut is seen.
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There are two main couples at work where engine thrustline is concerned.
Couple 1. is the couple that exists between the engine thrustline and the aeroplane's center of gravity.
Couple 2. is the couple that exists between the engine thrustline and the aeroplane's center of drag.
This degree of complexity usually means that setting up downthrust accurately prior to an aircraft's maiden flight is unrealistic, and the model pilot typically expects to do some flight testing before thustline can be optimised.
In addition to this, the power configuration affects the direction of downthrust needed, e.g. pusher or puller. The aeroplane type also has considerable effect, e.g. whether a monoplane is high wing or or low wing, as this factor changes the center of drag. Typically all these factors are considered by the designer before the maiden flight, and usually totally taken care of in the case of ARTFs, unless the builder decides to fit a larger power plant which will again change things.
