Hello All

I recently put together a Old version Skymaster Hawk, on the first few flight we noticed the same pitch down issue with the flaps down. After opening the back area we found that the double pushrods with ball links were bent.

In the stock location the servos for the Old Hawk pushed on the elevator to give up. Under compression the pushrods normally do not perform as well as under tension. To fix this issue I moved the elevator servos behind the elevator pivot above the tail pipe, now the pushrods are under tension when pulling up, and the whole linkage and servos perform much better now, and although I’m careful not to put down flaps too fast, there is no tendency to pitch down what so ever. I’m using about 45 degrees of flaps on final.

If you look at the pivot of the Old version hawk and all the elevator area behind it, It does not take much force acting on the back tips of the stabs to create a huge amount of force for the servo and linkage to overcome. Although this fact does not change with the linkage under tension vs compression, the servos and linkage can handle much more force if under tension with out getting mushy, or bending.

The flaps are causing a strong downwash component when deployed, this puts a force on the stabs that trys to weathervane them into a "down elevator" condition, resulting in pitch down. We had some oil on the side of the hawk during one flight and the streamlines down the fuselage flowed down under the stab and above the wing at quite a surprising angle.

Now with dual elevator servos 8611a's on a matchbox with separate power I have positive elevator response for the whole flight envelope.

For the new ARF + version of the Hawk has a completely different setup and already has the pushrods under tension.

Aloha Duke