unintended roll after a loop

[shaneyee]

I have a self designed flying wing roughly based on a Projeti planform but with vertical fins at the wingtips instead of a central fin.

The plane rolls nicely albeit a bit slow because I have it on low rates on its first few flights. The problem is with loops. After the first half of the loop, as it heads downwards, the plane will suddenly roll off to one side. I have tried this a few times and I cant figure why it would do this. I have checked the elevons and they dont seem to flex unduly under load nor do the servos (HS55) seem to be inadequate or damaged.

It about 260 sq in wing area and AUW of 19oz. The wing section is symetrical and the wing has zero washout. The power plant is a Cobalt 400 14T turning a 6x3.5 MAS prop off 8 Hecells.

Thanks

Shane

[EricBjorn]

It's possible you may be getting a tip stall, especially since you have no wash-out. Try taking and holding more speed into the loop and don't pull back as hard on the stick. Just a thought...

Eric

[Mitch G]

I agree with Eric, it sounds like the plane is stalling at the top of the loop.

Although this can happen with any plane design, I had the same thing happen with a Manx (another flying wing design). I found I had to ease off on the elevator as the loop progressed towards the top and then feed a bit more elevator in as the plane came back down.


Mitch

[Smash McCrash]

This exact thing was happening with my friends Wedgie and with my Raider. It turned out to be a nasty tip stall at the top of the loop. I turned down the elevator dual rate settings and it went away.

Mike

[shaneyee]

OK guys. I will try it out tomorrow. Didnt realise I could tip stall a plane on the way down...

Thanks for your help.

Shane

[SheldonYoung]

An airplane can stall at any speed, it's happens when the angle of attack of a wings causes it to lose lift. If the angle attack is severe enough it can stall no matter how fast it's going. For example, a snap roll is just a forced high speed stall.

[shaneyee]

Sheldon,
"An airplane can stall at any speed..."

Thats something I never thought of... I think the weakness of the plane is the too severe thickness reduction from root (1.75" thick) to tip ( 0.75" thick ) and the taper of wing (12" root to 5" tip). If I reduced the root rib thickness to 1.25" that would give allow the tip to stall after the root and avert this tip stall wouldn't it? It'll also give me a bit more speed....

Maybe I could add some turbulators on the outboard side for the current plane...would THAT work?

I want to keep the wing symmetrical so that inverted performance is the same as upright.

Thanks for your input. This is an education.

Shane

[SheldonYoung]

Reducing the difference in lift between the root and tip will definitely help. A less severe taper will also make a big difference, especially if there is any tip stalling tendancy in turns (there probably is). A reward taper causes the inside wing to loose lift in a turn.

The other thing to check is each side is identical and the plane is laterally balanced. It'll still stall but it'll have more of a chance of stalling straight ahead rather than off to one side.

[KCFlyBoy]

I had a Smith Mini Plane that did the samething. Check and make sure that it is truley latterly ballanced. You might need to reduce the total through of the elevator as I did on my Smith Mini Plane.

[shaneyee]

Tried it today and it did nice loops if I was careful with the stick and didnt yank it back. Looks like I'll have to modify the design if I want to really improve it. I think part of the problem is that I have a small radius on the leading edge because I wanted to use a length of leading edge stock that I had on hand. Well now I've learnt something so I'll be more concious of it in the next plane I build.

Thanks guys for your help.

Shane

[marctrudeau]

Just one more related note:

The same thing happens looping the full scale Bellanca Decathelon (and probably most other aerobatic planes). You steadily move the stick back for the first 3/8 loop, then ease up the back pressure (maybe even move the stick slightly forward) for the next 1/4 of the loop over the top, then pull back slightly again as you start down (as the speed starts to pick up), then steadily allow the stick to move forward to the nuetral position by the end of the final 3/8 loop. (Never is the stick all the way back.)

If you don't let up on the stick and float across the top, you end up stalled and in a spin, just as you've described wiht your RC flying wing.

Ain't it fun?!

Marc

[shaneyee]

My solution is to sheet the tips between the last 2 ribs on both sides of the wing from the main spar to the leading edge. This will fill out the leading edge and present a larger radius to the airflow at the tips. I hope this will delay the stall at the tips.

Thanks for all your help.

Shane