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non-flying j-3 cub

Old 05-22-2011, 09:49 AM
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dsawyer44
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Default non-flying j-3 cub

I have a hobbyzone j-3 super cub that i cannot get to fly. After i crashed it into a tree and glued it back together, it will not fly. It takes off and when it's about 10feet off the ground, it turns right and comes down. When i try to straighten it out it will not respond. Can it be that the horizontal stabilizer is not level with the wing? It doesn't have alierons and the rudder will not straighten it out. Any help is appreciated.
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Old 05-22-2011, 10:04 AM
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Lomcevak Duck
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Default RE: non-flying j-3 cub

Are you absolutely sure that your rudder is moving in the correct direction (and with enough throw) when you push left on the transmitter?
Did you check the CG after your repairs? How about lateral CG?
Also be sure to check that your motor is still straight on the mount.
Old 05-22-2011, 05:32 PM
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dsawyer44
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Default RE: non-flying j-3 cub

I have checked cg and also made sure the rudder is operating correctly. The motor seems to have a lot of thrust angle to the right and down. Thanks
Old 05-22-2011, 07:28 PM
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Default RE: non-flying j-3 cub

If the firewall was damaged in the crash that may be contributing to your troubles. I am not that familiar with this particular airplane, so I don't know how much -if any- right or down thrust is designed into the firewall or mount from the factory.

What all got damaged in the crash?
Old 05-23-2011, 04:40 AM
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dsawyer44
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Default RE: non-flying j-3 cub

The wing broke in half when it flew into a tree. I repaired it and it looks fine. Is it possible to hand launch these planes without power to see if they will glide? That way it would eliminate the thrust from the motor. Everytime i launch it with the motor on it turns to the right and comes down hard. When this happens it breaks the prop.
Old 05-23-2011, 05:56 AM
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jester_s1
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Default RE: non-flying j-3 cub

Yes, you can toss it into the wind with some tall grass in front of you and get no damage. I would suspect wing warpage first, and thrust angle second. It doesn't take much warp in the wing of a 3 channel plane to cause a roll that you can't compensate for with the rudder.
Old 05-23-2011, 09:46 AM
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Default RE: non-flying j-3 cub

One thing you don't want is a improper mount for your motor.Your firewall must have been damaged or the mount itself? Anyway your motor should not be pointed left or right unless you are flying control lines that need to stay tight which takes right rudder and at times some outward tilt of the motor but, very little is required.Your motor should be inline with the fuselage a straight edge off the lower part of the wing and a square off that to the prop should get you close? Maybe run a string from rudder to center of firewall and beyond to nosecone point for left and right setting.Check mount for cracks or bends!Good luck
Old 05-23-2011, 09:59 AM
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Default RE: non-flying j-3 cub

On your Cub as on all planes your plane should go left on take-off requireing alot of right rudder until you rech sufficiante airspeed for the tail to work properly.It's called (p) factor air circleing the fuselage hits the tail fin pushing it right which makes you turn left.Ounce moving the vertical stabilizer holds your plane straight unless rudder is applied.As you gain airspeed right rudder can gradually be reduced. Good Luck
Old 05-23-2011, 10:51 AM
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Default RE: non-flying j-3 cub

Truthfully there just isn't enough information to go by here. Also we don't know what sort of pilot you are so there's no way to know if you're trying to climb too sharply and are causing the wing to stall just after takeoff or you may be trying to lift it off at too low an airspeed. Also "looks fine" seldom is fine unless you've got the trained eye of a good carpenter or cabinet maker for seeing twists and out of square angles. The rest of the world really needs to set up the model and use rulers or levels to check this stuff using a known flat table (checked with levels again) to ensure that you've got a truly flat and level surface as a reference. Otherwise your idea and our idea of "just a little" won't translate.

Typical basics to check for with any model since this would appear to be the case;

[ul][*] Set up the model on the leveled table so that the lower side of the wing by the fuselage is sitting level. Shim up the tail wheel with blocks or whatever until you measure the wing as level to within less than 1/16 inch or use a small level to set the model up.[*] Check that the tail is angled slightly nose down compared to the level wing. The wing's TRUE angle with the flat lower surface level will actually be about 1 degree positive. So the leading edge of the tail should be about 1/16 to 1/8 lower than the trailing edge.[*] Check the wing for warps using either the level again or by measuring with a ruler from the table to the wing leading and trailing edges at a few spots along each wing. Ideally you want BOTH wings to twist slightly for a trailing edge high twist that puts the trailing edge about 3/16 higher than the leading edge at the tips. This will aid in delaying any tip stalling. The two wing twists MUST match up to within 1/16 or less or you will have noticable tendency to roll from the difference. Your repairs need to be accurate and inline to this sort of tolerance. Again "looks OK" seldom is OK since most folks' eyes can't see this sort of tolerance.[*] Ensure that your model balances correctly. THis is super important to making the model fly well. Check your manual for how and where to set the balance point. Tolerance for "good enough" is to place the balance point to within 1/8 inch or less of where you want it to be. When suspended at this point the lower side of the wings should be level, not angled.

[/ul]

If this is the model you have;
http://secure.hobbyzone.com/HBZ7300.html
then be sure that the Anti Crash Technology is working correctly and doesn't have any glitches. If it seems to "burp" if you wiggle things around then disconnect it since it'll do more harm than good.

If your model is zooming up to 10 feet and then stalling and crashing then you're trying to use too much elevator during the takeoff and first part of the climb. The model does not have rocket like power to weight ratio. You may also be using so much up elevator in the takeoff that it lifts off prematurely with too little airspeed. You need to use lots of up initially to hold the tail down. But once the model starts to roll forward ease off the elevator smoothly so you end up with very little up elevator at lift off. Then control the elevator to hold the model to no more than a 20 degree nose up climb angle. That will prevent the model from stalling then diving back in like you described. The turn you get is more than likely just one wing stalling before the other so the "stall" also drops into the first part of a spin all at the same time. But the point is that you let it get too close to the stall speed at too low a height.

The reason you seem to suddenly lose control is that once it is close to or at the stalling point the control surfaces won't work anymore. The rudder and elevator NEED air flowing past them to have any control. That's why your corrections aren't doing any good. And with only 10 feet to get back to speed you run out of altitude before you have enough air speed to use the controls to pull out of the stall. In other words I'd have to say that this is pilot fault and not anything to do with aerodynamics. But check the models warps, alignment and balance. If they all check out then it's time to be honest with yourself and seek some help in learning to fly your model. If you insist on carrying on then at least work on the next takeoff to let off the elevator control so the model doesn't jump off the ground and immediately point strongly towards vertical. Once that happens you may as well turn off the Tx and watch the crash since not much is going to help.

Sorry if this last bit on your abilities sounds harsh but it's hard to sugar coat it. Please accept it as a frank but friendly manner aimed at helping you out.

Old 05-23-2011, 02:16 PM
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dsawyer44
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Default RE: non-flying j-3 cub

Thanks to everyone for all the help. I received a lot of useful information. I've checked everything mentioned and made some corrections, so as soon as the weather gets better i'll try her out again. I've been flying this cub for over a year. It's always flown great with no problems until one windy day when it made contact with a tree. Until then i could toss it, throw it hard or take off from a runway. It has never stalled or crashed in at least 50 or 60 times out.. This cub flies great and is very forgiving. It always takes off without the use of the elevator or rudder, until is is far up in the air. I was going to try to launch it without power because the last three times it crashed and of course broke the prop. I even changed the motor because i thought the brushed motor was just wore out and didn't have the power to launch it.. When i bought this cub i did notice that the motor had a thrust angle of down and to the right, but the problem was that was over a year ago. I'm thinking this angle has changed from a few hard landings. If i took this angle out would it still launch o.k. When it is flying, i noticed it always went to the right.
Old 05-23-2011, 04:21 PM
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Default RE: non-flying j-3 cub

It would probably be just fine. Personally, I hate thrust built into my planes in any direction. Try 0-0 and see what happens.
Old 05-23-2011, 07:58 PM
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Default RE: non-flying j-3 cub

Ah, there was nothing to indicate that you'd been flying it for some time with success.

A sort of "proper" thrust line offset for a model such as this would be about 3 to 4 degrees of down thrust relative to the wing's zero chord line and 1 or 2 degrees of right thrust. You may want to check that this is roughly what you have.

When did you change the motor? And did you change it for the same model of motor or go with a lighter brushless setup? If you went with a lighter brushless motor you may have moved the balance point way back.
Old 05-25-2011, 05:31 AM
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Default RE: non-flying j-3 cub

It sounds like a crosswind takeoff in too strong a wind for the model. It won't take much to flip a Cub in that situation.
Old 06-27-2023, 06:41 AM
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suzonka
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Default

I have a 40" mono cub balsa abd covered I hope the same down and side thrust will work for it no 0 plans

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