FMS Giant J-3 Cub (1400mm) help

[BBCorvette18]

Hi everyone I recently purchased the FMS Giant cub in RTR fashion from Banana hobby. Was fairly easy to put together once I decided that the instructions weren't worth looking at other then getting the correct screws.

anyway fast forward to putting it in the air. I put the bird in the air and it seemed to be really twitchy. part of this I'm sure is me as I haven't flown a RC plane in years but prior to this cub I put a 1000mm Corsair in the air and was flying pretty good. Anyway from my research I think the plane is a bit tail heavy.

Now to my real question. Which has me baffled. I put the plane into a full powered steep climb then right before stall give full up elevator(back on the control) and full right Aileron (bottom right corner of joystick) and the plane does a nice "snap roll" (probably not the proper term). Now if I do the same thing to the left. Full up elevator and Left aileron the plane starts to roll to the left and then stops and just does a bit of an off axis loop. It will not roll to the left like it does the right.

So my question is why? does anyone have any idea why its doing that. During normal flight the plane turns (rolls) to each side just fine. On the shop floor the throws are equal from side to side.

The only thing I can think is it might have something to do with the thrust angle of the motor??

anyway sorry for the long post and thanks for your time reading and or replying

[BBCorvette18]

Any input is welcomed even if y'all think I'm crazy and what I'm describing is impossible lol

[JetPlaneFlyer]

If it feels twitchy then you probably just need to reduce the throw on the control surfaces. If your Tx has exponential that will help too. It doesn't necessarily mean it's tail heavy. If it was tail heavy you would have all sorts of other issues and probably have crashed.
Contrary the the impressions you might get from some sources, piling weight in the nose is not a panacea for all ills

The rolling issue is probably due to gyroscopic procession. That is the gyroscopic effect of the prop that makes the plane (among other things) yaw to the right when it is pitched sharply up. It's quite normal, a lighter (wood?) prop might reduce it a little.

http://www.free-online-private-pilot...odynamics.html

[BBCorvette18]

I've flown a few different Cubs years ago and have flown my cousins fun cub while trying my FMS out. It has crashed unfortunately I know some of the twitchyness is me on the sticks however it likes to do a viscous stall spin and I'm hoping moving the battery as far forward as possible and maybe adding a tiny amount of weight will help.

My dad and I are going to build a proper CG jig so we can get a more accurate reading for all the planes.

I've experienced the gyroscopic thing before where it wont turn as well to the left but never like this. it literally stops and just does a loop. while normal flight it banks just fine. I'm going to try to get the cg figured out a little better and then try some more maneuvers. I've had to glue the plane back together a few times already so I'm a little skidish to do much as when it decides to spin out of a stall its been very unpredictable as far as how long until it will come out of it. I've also noticed that the FMS cub is ALOT heavier then the Fun Cub and I love the way the Fun Cub flies so I have one waiting on an order from HobbyKing to show up to finish building. So I guess if in my experiments I wad up the FMS I won't be that mad lol.

To me it also looks like the plane likes to fly in a bit of a "crabbing" orientation and my rudder trim looks normal so I am wondering if I adjust the motor mount a little to change around the thrust angle if that might help?

[JetPlaneFlyer]

It should have about 3 degrees right thrust, that might help with the crabbing.

Gyroscopic procession doesn't show up in normal flight, it mainly rears it's head if you pitch up or down very violently, in which case the plane will react by yawing one way or the other. The effect is used in some aerobatic manoeuvres.

Stall spin might be slightly aggravated by a rearward CG but the root cause is probably too heavy wing loading and poor design. it wasn't this piper cub by any chance: http://www.wattflyer.com/forums/show...06&postcount=1

[BBCorvette18]

Originally Posted by JetPlaneFlyer

It should have about 3 degrees right thrust, that might help with the crabbing.

Gyroscopic procession doesn't show up in normal flight, it mainly rears it's head if you pitch up or down very violently, in which case the plane will react by yawing one way or the other. The effect is used in some aerobatic manoeuvres.

Stall spin might be slightly aggravated by a rearward CG but the root cause is probably too heavy wing loading and poor design. it wasn't this piper cub by any chance: http://www.wattflyer.com/forums/show...06&postcount=1

Thats probably the same one. Not sure if that is the 1400mm or 1030mm version tho. Compared to the fun cub my wing is flat (no dihedral) and is thinner from leading edge to trailing edge (about a 1/2 inch)

This cub seems to fly decent at higher speed but doesn't fly real scale which is why I bought the Fun Cub. I'm thinking about once I have the fun cub built dissecting the FMS cub and trying to figure out why it weighs so much

[BBCorvette18]

Update:

I added some lead to the front and she flys better to me. however it still has the issue I was talking about. Weather was foggy and I couldn't get more then 100' in the air without starting to lose visual on the plane so I haven't tried any off power spins to see if I can get it to rotate to the left.

[mclarkson]

Remember that, under power, the propeller is spinning at 10,000 RPM+. That puts a lot of torque on the plane, which causes it to act differently in different directions.

[BBCorvette18]

Originally Posted by mclarkson

Remember that, under power, the propeller is spinning at 10,000 RPM+. That puts a lot of torque on the plane, which causes it to act differently in different directions.

Yes but this is the only plane out of 15+ that I have flown that had this characteristic

[BBCorvette18]

Well flew a couple packs today and either the 2oz up front helped or im just getting better lol. I can do a barrel roll either direction but if I give full up elevator I have to use rudder and aileron to get it to snap to the left. Granted its not a stunt plane so I guess that's what I have to look forward to lol.

Another thing i found Interesting banana hobby says it should weigh 58oz but AUW after adding 2oz is 54oz