Electric Cub - Unstable flying
 

I have a question for anybody who might have a helpful idea. I have a Global Speed 400 Electric Cub running w/ a 2.5 gearbox and a 9/5 prop. The plane has a 45.5" span, and is 28" in length. The plane has more than enough power, and I fly it at around 50% power. But the plane doesn't want to fly straight and level. I've tried to carefully balance it at the recommended point, and it does take off and fly OK, but instead of easily flying straight ahead, it is constantly vearing off to one side or another, and it is a constant struggle to keep the wings level.

The plane starts to turn, then the turn steepens, and the plane wants to go into a spiral descent. This seems to happen in both left and right directions. At slow speeds the plane will stall and flip over either left or right.

Here are my guesses regarding the problem: first, the wings have no washout - maybe it should be introduced even though the instructions don't call for it; second, it feels like the plane isn't long enough, like it needs a longer tail moment to keep it flying straight ahead. I actually calculatied the ratio between the length and span - 28/45.5 = 0.61 and found that it was lower than other Cub models and, probably, the real Cub too.

Any thoughts or suggestions? It is so much work to fly it that it isn't much fun!

Max Jacobson

RE: Electric Cub - Unstable flying
 

Could it be a bit tail heavy?

One other thing...is the wing built flat with no dihedral?

RE: Electric Cub - Unstable flying
 

Look at the rudder when you move it from side to side. Does it center in the same spot each time?? or is it a little off. You may be trying to fly it to slowly and it is right on the edge of stalling so it seems hard to control.

Dru.

RE: Electric Cub - Unstable flying
 

It sounds like it may be tail heavy.

RE: Electric Cub - Unstable flying
 

Lynn - It wasn't tail heavy according to the specified balance point. The wing has some reasonable dihedral. But thanks for those suggestions.
Max

RE: Electric Cub - Unstable flying
 

Dru - It is true that it stalls if it is too slow. But I'm really interested in your other suggestion about the rudder basically being sticky and not returning to the same position each time. What is suggests to me is that when I've got the stick centered, the rudder is either to the right or left depending on the previous command. I'll check that out! Thanks for the thought!
Max

RE: Electric Cub - Unstable flying
 

Bipe - Thanks for the thought. See my resonse to Lynn.
Max

RE: Electric Cub - Unstable flying
 

The reason I said that is becuase I have had a couple of small planes have stiff surface movement and it did not ever fly in a straight line or stay at one altitude so I thought I would ask about it.

Dru.

RE: Electric Cub - Unstable flying
 

IF the controls are neutralizing within a very tight tolerance then I have another idea for you.

What you are describing is a classic case of spiral instability caused by having TOO BIG a vertical fin for your present CG location. Moving the CG forward will reduce but not eliminate the problem. Adding dihedral would also help and it makes me wonder if you did not misread the dihedral specification or reduced it on purpose because of adding ailerons or something like that. It's hard to imagine this company making a model that is designed wrong and flies like this. They wouldn't stay in business long.

If you did reduce the dihedral on purpose the fix to compensate is to reduce the fin and rudder area. Assuming the surfaces are a sheet design I would suggest reducing the area by cutting off a 3/16 inch band all around the outside edge so it looks the same but is smaller. Then fly and see how well it works. If you need more taken off do it in 1/8 steps but I suspect that 3/16 inch will do the trick.

I tested oversized to undersized fins on a glider a few years back to determine why some models I'd flown needed "top" aileron in turns while others did not. I learned that the balance point has a marked effect on this as well as small increments of fin area. I kept flying and moving the CG back and cutting the tail down until I learned a fair amount. I started oversized and it was not bad. I moved the CG back about 5% and the model became all but uncontrollable just as you described. I then brought the handling back to nicely balanced by removing about 10 to 15% of the tail in small increments and flying each time between cuts. I eventually removed one or two bits too many and the model then reacted in the other direction where there was noticable adverse yaw when entering turns and the tail tended to hang low in the turns and the normal flight seemed "loose". Adding on the last bit made for a very nice model to fly.

There is a measurement that takes all this into account called the tail volume coefficient. There is a TVC for both the vertical and horizontal tails and an acceptable range of values for each result. There are TVC calculators out there on the web. If you can find one and do the calc you'll see where your model sits in all this.

But PLEASE double check and eliminate the possibliblity that it's NOT something much more mundane like sticky controls or a badly routed Nyrod pushrod (those things are notorious for false neutrals if there's a lot of bends).

RE: Electric Cub - Unstable flying
 

Bruce -
Thanks for your reply. Your thoughts are a whole new area for me. It will take me some time to understand all you are suggesting. Many thanks for taking the time to present those possibilities.
Max

RE: Electric Cub - Unstable flying
 

What kind of wind are you flying in?Small slow flyers are erratic in windy conditions. Jerry

RE: Electric Cub - Unstable flying
 

Max, and anyone else curious about this...

These two bits of info are what clued me in to the spiral instability option. In particular the second part where the model wants to tighten up the turn. The fact that it does it in both directions (equally?) rules out a wing warp or other misalignment issue.

Airplane design is a delicate balance where "if lots is good then too much is just right" often fails. In your case it sure sounds like they overdid the vertical tail area and that is what prompted me to suggest cutting down the vertical area.

RE: Electric Cub - Unstable flying
 

Jerry -
the Cub flew a little easier in very calm wind, its true, but I don't think its basic desire to tip stall and spiral dive is due to that.
Max

RE: Electric Cub - Unstable flying
 

Bruce,

I bet you meant to say moving the CG back will help. Moving it forward will make the spiral hyperstability even more pronounced. Your report about test flying, where you moved the CG back and trimmed the tail, is on the mark. I've done this, too, and it can really make a huge difference.

I'm willing to bet a whole lot of designs make their way onto the market without any of this kind of testing, and the resulting TVCvert is where it is because that's how the prototype was built, period. All the truly great Hall of Fame designs have this kind of thing dialed in, and even a beginner can sense it.

Don Bailey

RE: Electric Cub - Unstable flying
 

No, actually moving the CG BACK made it worse if you're talking about spiral instability from an oversized tail. If it was undersized and dutch rolling then it would be the opposite where moving the CG back would make the problem less.

The glider I tested was VERY demostrative of that fact.... :D

In my testing I did both but not at the same time. To determine the effect of the CG alone I first moved the CG back with the oversized tail. In the original case it had bee a bit "tippy" in the corners but controllable with a bit of "top" or corrective aileron to prevent it winding into a spiral. For the second flight I left the tail area alone and moved the CG back about 1/4 inch or 3% and it became very noticable that as soon as I turned it was a fight to keep it from winding in. For the third flight I left the tail alone and moved the CG back another 1/4 or 3% and now the model was all but uncontrollable. It was like balancing a long stick end up on your palm, It was a constant battle to keep the wings level and any turn at all required extreme top aileron. I didn't let the bank go beyond 20 degrees because even at that point I needed almost full aileron to return to level. I landed in one piece and THEN started cutting the fin and rudder down while leaving the CG in the same spot. 5% off brought me back to the same state as the second flight. Another 5 % brought me to the same condition as the original flight but now, with the CG being 6% to the rear the model flew "better" and more efficiently as I'd retrimmed the elevator of course. Dive testing for optimum balance at that point showed me to be still stable but close to what I liked. A further smaller fin trim made the model fly VERY nicely. Gentle banks would return to level with no controls, 45 degree banked turns required NO top aileron and even a small touch of pro turn control to maintain bank angle, and only at about 70 degrees of bank did the model require the odd bit of stick pressure to keep from tightening up.

In the intrests of science I kept trimming and the next 2 cuts. With the fist off the clean flying was gone and the model seemed a bit "loose". Nothing I could put my finger on but the grooviness just was not there. WIth the next peice cut off the model now tended to adverse yaw badly during aileron use and circle with the tail hanging low in the turns in a side slip. Correction to level was poor and accompanied with lots of adverse yawing that produced much side slipping. The model was fixed and cleaned up with a nice fin tip shape that replaced the last two cuts worth of area and was flown for a couple of more years until a good friend made me an offer I could not refuse. Besides, this one had flaperons that didn't work very well (another experiment) and I wanted a model with separate flaps and ailerons so it didn't take much talking to make me part with it.

Amen to that last bit. There's that other thread here about BMC or BVC with the sadly nose heavy model. It's wonderful that folks want to aid the hobby by kitting their designs but some folks let the fact that it actually flies at all cloud their judgment about what should be kitted.

I'm planning on doing more glider flying this summer. Hopefully I can get to meet you at one of the contests dgliderguy?

RE: Electric Cub - Unstable flying
 

HI MAX! The fuselage length in question sounds long enough to give you a good CG range . A flying wing can be set up hands off stable. You mentioned also about not having any washout, a plane that has a medium to low wing loading doesn't need it, and will do better without. But, you want to make sure that you aren't flying around with the opposite condition! I don't know anything about this particular plane, but if the wing is flexing, or twisting in flight, you could experience some things that are hard to explain from where you stand looking up at it. I like the idea that your controls are erratic, I should rephrase that, I like the theory that the controls could be contributing to the problem. Check them against some medium thumb pressure in push mode to make sure the linkage can't wimp out.

RE: Electric Cub - Unstable flying
 

Dear Mr. Matthews -
Thanks for the input regarding fuselage length. And it is interesting to look at the possibility of 'washin'. I also like the idea of looking whether the wings are flexing - the model is very lightly built.
Max

RE: Electric Cub - Unstable flying
 

One thing that is a little close to the heart for me at the moment, is simple radio glitches-
My last plane flew like a monster- doing just the sort of things you described- self tightening turns and also extreme elevater sensitivity.
My hunch was initially that something was wrong aerodynamicly- but it turned out to be a simple metal on metal contact of a piece of wire on the exhaust. that fixed it flew fine.

J.M

RE: Electric Cub - Unstable flying
 

Thanks for that tip about radio problems rather than aerodynamic ones. I'll check that out.
Max Jacobson