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Old Aug 23, 2013, 04:15 AM
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Piper Cub 1/4 rolled on take off. PLS HELP

Hi Guys,

I have made a Piper Cub J3 1/4 scale with DLE 20 mounted on it. During my maiden flight plane was taxing well, ailerons, elevator and rudder were all levelled.
As soon as plane took off, plane took a hard left turn and rolled left.

Any expert guidance what needs to be checked. I have already balanced the plane with correct cg.

Thanks & Regards,
Anilesh
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Old Aug 23, 2013, 06:11 AM
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It may be that you took off before reaching flying speed. It may have been a stall of the left wing exacerbated by engine torque. Who's Cub model? Would there be wash-in, in the left wing? Not having been there and seen what happened makes diagnosis difficult. Where you able to recover? If so how did it fly after the recovery? Is the angle of incidence of both wings consistent at a few locations on both wings? Maybe some wash-out should be introduced.

Not much help, sorry! However a little more information would be helpful.
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Old Aug 23, 2013, 07:19 AM
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Thanks for your reply.

Model is 1/4 scale piper J3 cub from world models and dle 20 mounted on it went for a test flight

Plane was taxing well on ground, as soon as it took of nose was pitched up, then suddenly it rolled to the left, plane was recovered had a harsh landing but luckily there was no damage..i cut the throttle and managed to level the plane. Now haven't tried it again.

Please advise how to check wash in as i am new to gassers and scale planes? how to introduce wash out ?

Also advise if i need to check the thrust line of the engine ?

Thanks & Regards,
Anilesh
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Old Aug 23, 2013, 07:59 AM
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How is the airplanes balance? Wash-in/wash-out is a twist in the wing. Wash-out would be a downward twist of the wing at the wing tip (a good thing if port and starboard panels are equal). Wash-in would be an upward twist of the tips (a bad thing, as we need to keep the tips flying).

You should be able to find all the information you need on these details here on RCG.

At any rate, you need to be able to check your wing's incidences. With a flat bottomed airfoil you can check the incidence by laying straight edges on the bottom of the wing and checking to see that they are parallel. Inducing wash-out can be performed by twisting the wing in the proper direction and running and iron or heat gun over the wing to induce some twist.
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Old Aug 23, 2013, 08:20 AM
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You need to ease into the throttle, dont firewall it like a lot of sport planes or hit it too fast. Sort of like driving a clutch in a car. And stay on the rudder to keep it straight. A cubs nature is to want to swing left at least a little. Build up speed and take off with about a 30deg climbout. You have a lot of torque in that engine which is gonna want to swing left when throttling up. I have two busa 1/4 scale cubs which are comparable in size. They like a nice and easy transition for takeoff. The one and only world models cub I had was a 1/5 scale and was an absolute delight to fly. One of the few arfs that went together with no problems and flew perfect.
Edwin
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Old Aug 23, 2013, 09:34 AM
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Thanks Edwin for your detailed information. Do i need to give any right thrust to counter engine torque?

Anilesh
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Old Aug 23, 2013, 09:44 AM
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Build it per instructions. I dont remember if it has right thrust built in or not. If I were to guess, I'd say yes it does, but I could be wrong. What prop are you using? I use 16x8 on my 1.20 glows and 20cc.
Edwin
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Old Aug 23, 2013, 12:49 PM
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Not an expert, but my guess is that you had full up elevator and increase the speed too fast without loosing up the elevator. The plane took off, but stall instantly due to insufficient air speed. My suggestion will be to take it out and practice some taxiing. Try to keep it straight and progressively increase the speed till the tail lifts and come back. Do it a few times and see if you got the hang of it.

Cheers,
MarLi
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Old Aug 23, 2013, 05:28 PM
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Look like this? Ease it into the air, sounds like a tip stall.


1/4 Scale Cub Crash (0 min 10 sec)
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Old Aug 23, 2013, 08:39 PM
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While torque and a too steep of a climb-out are likely contributing factors, checking the wing incidences is a very good idea, too. Many of the folks involved in the hobby today, do not build and don't understand very much about aerodynamics and the various geometry checks that people who've built airplane models use as benchmarks for building. It doesn't hurt to familiarize yourself with this knowledge.
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Old Aug 25, 2013, 09:32 AM
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Anilesh,

I flew my clipped wings cub today (electric) and I have tried to reproduce your problem. I could. Without up elevator (TX stick pulled towards you) as I increased the ground speed the plane nosed over. So, as you described with neutral elevator it is an expected behavior. Try taxing around with elevator stick all the way back and increase throttle till the tail lifts. Retract throttle at that point. Do that a few times and learn to release elevator as the tail lifts. At that point your rudder will gain authority and you will control direction. Pushing the throttle further will build ground speed and the plane will take off.

hope it helps,
MarLi
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Old Aug 31, 2013, 12:41 PM
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The Seagull 80 inch Cub was the most difficult plane that I have ever flown. The wings and ailerons are so big, that doing aileron turns can yaw the plane, and put it into a snap roll or stall. When taking off, the tail doesn't start to "bite" until good speed is reached (the main wing will fly before the tail will grab, so you must keep the plane on the ground until you get up enough speed to fly the tail).

The tail is so close to the main wing, that it has trouble keeping the plane straight on the ground and at low speed. If you throttle up too fast, the torque can overpower the ability of the tail to keep the plane in a straight line. Right rudder may be needed on the ground, until speed gets up. Gas engines put out a lot of torque, and so make things worse on a Cub, taking off.

A scale Cub is not a beginner's airplane! I don't know why people keep advising new pilots to try a Cub. A clipped wing Cub is much better. Or a Decathlon.

Good luck taming the Cub. It will teach you a lot about flying planes that don't really want to fly.
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Old Aug 31, 2013, 12:47 PM
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Sounds very much like you stalled it.
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Old Aug 31, 2013, 03:16 PM
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Thanks guys for your tips and suggestion. Today i was able to do successful flight, you were right guys, earlier i was not giving enough power, this time i kept this thing in my mind and was able to do maiden flight....

Thank you all for your suggestions and help.

I am facing another issue with my DLE 20 as i am new to this gasser, i am not able to get linear throttle response i.e on my half stick open i am getting full rpm. I have tried hard to set my linkage and push rod and even tried my hands on throttle but this is not much difference. if anyone can help me further with that??

Thanks & Regards,
Anilesh
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Old Aug 31, 2013, 06:34 PM
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Thats the general nature of gas engines. To get a linear response you have to change your throttle curve on the radio. I just deal with it.
Edwin
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