How do you slow down an RC airplane without making the prop or engine smaller?

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First of all, at idle, a larger prop will slow you down more than a smaller one - more drag. If you are talking about slowing down on approach, you might want to check to see if you the idle is too high. If not, try adding flaps to the trailing edges of the wings. These can be controlled with two extra servos. There are many designs on the internet you can look at. Flaps increase lift and drag, letting you fly slower, but may cause the model to pitch up or down.

Hope this helps!