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Big Problem with Tricopter

I chose the tricopter frame. I connected the rear tail servo motor and made the necessary settings.
But 5 seconds after takeoff, the rear engine slows down the throttle and in addition to these, the plane works in the opposite axis to balance itself. opens up gas when the nose is down.

How can i solve these problems. meanwhile the rear engine is smaller than the front two engines and the faster engine. 40A in three esc but the series of rear motor is different Is this a problem? @iampete

Servers by jDrones