Thread: Discussion question on my 50cc ultimate
View Single Post
Old Jan 27, 2013, 09:08 PM
modestmagi is offline
Find More Posts by modestmagi
Registered User
modestmagi's Avatar
Joined Jul 2004
183 Posts

Unfortunately, bipes can be a pain to get trimmed correctly. This is mainly because the wings (and often the stab) are nowhere close to being on the engine's thrust line.

There are several things you can test that should help you diagnose and resolve the problem.

First, confirm that your plane is setup correctly. If you have the plans (or manual) verify that the thrust line, wing & tail incidence, aileron reflex, and CG all are within the designer's specs. The rudder and elevator also need to be zeroed out.

As you mentioned, there may be too much engine down thrust. To test this, fly straight and level slightly above stall speed, then slam the throttle wide open. If the plane pitches down, you will probably need to add some up thrust.

The Ulti may also be tail heavy. To find out, fly straight and level at half throttle and roll inverted. If your plane climbs, slowly move the CG forward until it holds altitude or slowly descends.

If the thrust line and CG check out and it still pitches to the belly, you might try vertically balancing it. To do this, hang the plane in the KE position by the prop shaft and at a point near the tail along the thrust line. If the plane rolls to the belly you can rearrange the internal components / add weight, as needed, to keep the wings straight up and down.

If all else fails, you can reflex the ailerons up a little and re-trim the plane. This should be a last resort, though, because it will likely introduce other handling issues.
modestmagi is offline Find More Posts by modestmagi
Last edited by modestmagi; Jan 27, 2013 at 09:20 PM.
Reply With Quote