I am flying a Dave Patrick Ultimate 40 size biplane.

Wondering if anyone can supply some aerodynamic's or trim insight to the following question.

Performing an outside loop at the top of the loop, the aircraft has a tendency to nose over. I neutralize the elevator and the airplane will complete the loop tighter than it started. I actually end up putting in a little up elevator to shallow out the loop.

I have the c.g. set at the aft end of the envelope. So I suspect it my be something else.

Thank you for your suggestions.

Mike

I don't know if this is the whole reason, but I have noticed similar behavior on a few different models, and concluded that the shifting of fuel in the tank is affecting the cg. On the up-half of the loop the fuel is at the back of the tank; at the top of the loop it starts to run to the front of the tank. So on the upside you're "tailheavy" and on the downside you're noseheavy. Add in the fact that when you come over the top of the loop, you now have gravity accelerating the model downward, whereas coming up towards the top you had gravity pulling you back. When I fly pattern, practice or competition, I am always aware of the need to manage throttle around the full circumference of the loop, and usually ease off my elevator input as I approach the top of the loop, otherwise I get a flattish topped, eliptical rather than round loop.

Thanks for the reply majortom-RCU.

That sounds like it might be part of the problem. With the light weight of the 40 size a/c, the fuel would have a greater impact.

I am leaning toward a thrust problem as this problem seems to occur at high power/low airspeed.

I need to hit the trim chart. After going through the chart once I thought it was trimmed out well. But it appears there is still something going on.

Mike

Well good luck with that. I don't know that I've ever had a plane get trimmed to the point where I felt anything I did further would make it worse instead of better. I build, I maiden, I trim, I fly for a while, I trim again, fly some more, trim again... Of course a lot of what you trim today will offset what you trimmed yesterday. All part of the fun.

Do not if this will help?
[link=http://www.rcaerobats.net/trim_chart.htm]Trimming Chart[/link]