Hi Guys

I was down at my local club on the weekend as my friend was test flying his new 60" scratch built ultimate bipe plane. (photo below)

The model flys realy well but it has a problem with level flight. What happens is if you fly at full power it dives when you take the power off
the models climbs , the slower the speed the model high angle of attach the model gets.

Can any one help. Is it a incidence problem or thrust problem or both?

Hope you can help.

cheers

Mike

G'day Mate,
First check balance point, sounds tail heavy, then wing incidence.
Can it be trimmed for straight & level flight, if so how much elevator trim, up or down, does it need.
More info needed.

Hi, The c of g is 25%. we can trim it for full power flight and its fine untill you drop the power and the same if we trim it for slow speed. I will give my friend a call
and ask him what the trim settings are. I don't think they were much due the model having a large elevetor.

cheers

Mike

G'day Mate,
Make it 30%, maybe it's nose heavy.
I still think it's a balance problem, the amount of elevator trim will tell the story, also how does it fly inverted, if it climbs inverted without elevator input, it's TAIL heavy.

the true test of CG is in flight...roll her over and if she climbs in inverted flight tail heavy....if you need a bunch of push (down) on the elevator then it's nose heavy...fix one thing at a time only...I'd start with CG

Sounds like there might be too much down trimmed into the elevator at full throttle to produce level flight. That down trim causes the plane to dive when throttled back. Recommend putting some down thrust into the engine. That will cause you to reset the elevator trim back up to a more neutral position producing level flight at high and low throttle settings.

It sounds like to much down thrust in the engine to me. Full power and too much down thrust will cause the plane to pitch downward, remove the power and it will pitch up.

Knowing nothing about other traits the plane has I agree that it sounds like too much down thrust.
Although since you are talking about a biplane, that involves a whole new set of problems, such as incidence of either wing or ailerons setting between the top and bottom wing being different.

After checking the thrust angle use Pete Goldsmith’s trimming guide. http://www.flagstaffflyers.com/image...thTrimming.pdf

P.S. When your friend finds the cause please post to help other modelers.

Check the relative angles between the wing's angle of attack. On all the bipes I've had (at least 15 including Phaeton 90's, Phaeton 40's, 1/4 scale Moth, 7 Aeromasters etc.) all flew best when the upper wing was about minus 1.5 degrees with respect to the lower wing. Just a degree will make a huge difference in how well they fly. Of course this all assumes that you have the CG properly set.

Engine thrust angle....

Engine is pointing down, trim it for half throttle and then hit full throttle it wants to pitch down. Let off the throttle and it wants to pitch up, a classic case of to much downward thrust. One or two washers to the bottom of each side of the engine mount should cure the problem.

I'm voting for down thrust too. If you trim the elevator for level flight under full power.... at half power you have half the power pulling the nose down.... and with all that elevator up-trim.... well, you get the idea.

I don't think the CG has anything to do with it unless it's grossly off..... And if it's set to spec like he said, it should be close enough. A little CG "tuning" isn't gonna fix this one.

I concur.

100% agree

Thanks to everyone who has taken time to consider my Ultimate Bipe trimming issues.

The plane specs are as follows;
wingspan - 1400mm
chord - 235mm
0 degrees down thrust through prop (the thrust line is halfway between the top and bottm wing)
tail plane incidence 0 degrees
Main wings are parallel (possibly 1 degrees up for the top wing if anything)

The C of G I am using is halfway between the leading edge of the top wing and trailing edge of the lower wing

On the test flight the plane was slightly tail heavy. I have attended to this but yet to re-test.