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Old Nov 05, 2010, 07:15 PM
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Joined Dec 2007
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Mechanical Gain on Tail

I want to document one of the differences between the FireFox 200 and the CopterX 250 that makes different gain settings necessary when using the PhuBar3 unit on these two helicopters. The CopterX 250 needs a much lower tail rate gain than the FireFox, mostly because of the difference in the tail rotor control arms (aka bell crank).

In the photos below, you see the FireFox control arm has a leverage ratio of about 1/2.6, where the CopterX ( and I believe the Trex 250 it is cloned from is similar) is 1/1.25. This makes for higher mechanical gain on the CopterX, which necessitates a lower electronic gain setting on the Phubar, or any other tail gyro for that matter. This is discussed in several threads on the Copter-X 250, Trex 250, HK250Gt, etc. Many people have problems with the tail wagging before they realize how different the 250 clone control arms are.

The tail servo link ball typically needs to be set further in on the horn than most people are used to, in order to compensate for the higher gain of the tail control arm.

On the CopterX, I have the tail servo ball link about 7mm out from the horn retainer screw, and tail rate gain of 17 on the Phubar3 seems to give good performance

On the FireFox, the link ball is 11mm out and the tail rate gain needs to be about 45 for good performance.

I believe tail rotor speed plays a role here too, since higher speed creates more force per degree of blade pitch. Plus the CopterX tail boom is about an inch longer, giving the tail rotor more leverage in counteracting main rotor torque.
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Last edited by Heli Hacker; Nov 28, 2010 at 11:37 PM.
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