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RC Helicopter: T-Rex 250 gyro ground test Align GP750 & DS420
Click 'more' to read all the technical details... Ground-testing a Align T-Rex 250 helicopter. The machine is mounted on a strong rotatable platform made from Lego to test the gyro/rudder. Any increased load from RPM or pitch will produce torque variations, the gyro detects uncontrolled movement and will send corrections to the tail servo to keep the tail in the same position. A wrong setup may cause drifting (uncontrolled rudder) or tail wag (over-correcting, the tail keeps shaking left and right) Setup: - Gyro Align GP750 - Tail servo Align DS420 (digital) Gyro setup: - Frequency 1520 (red) - Digital (green) - Reverse on (red) - Both limits set - Delay: large heli (red) to allow more gain Transmitter Graupner MX-12 (JR 6102) - Gain heading hold mode: 42%. Less will cause drift, more will cause tail wag. Mechanical setup: - Most important: The gyro is hold in place only by the double sided foam. Any attempt to tighten it with tie-wraps will make it more sensitive to vibrations, causing HEAVY tail wag! The gyro is located on the recommended position, on the gyro tray on the back. - Weights (bolts) on tail blade screws on the outside. During rotation less force is needed to change pitch, so the servo needs to apply less force too. I also tested without weights, the servo handles that fine too. - Lubricated tail belt to improve overal performance - Round servo arm because that's the strongest one. Altough tests with other servo arms performed the same. - Short ball link, because the long ball link moves a bit under load. Tests with the large ball link didn't show any increased tail wag though. - Ball link in the hole closest to the servo center (5 mm). I used glue to tighten the ball link in position. Tests with the ball link on the outside caused heavy tail wag. - Tail blades and rotor blades: stock/default, but custom airbrushed yellow/blue for visibility - Central tail rod holder glued to tail boom, so the tail rod can't flex. A flexing tail rod may produce tail wag. - Moving the tail rod by hand at full throttle feels smooth, so the rod won't flex/cause wag. - Check the tail blades and rotor blades for balance, unbalanced blades cause vibration which might disturbe the gyro.