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J3 cub, crash on take off nearly every time.

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J3 cub, crash on take off nearly every time.

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Old 10-08-2007, 09:50 AM
  #1  
JPANN
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Default J3 cub, crash on take off nearly every time.

Some of you know of this story somewhat from the post titled "the black hole", for others I will give a brief history. My friend and I started flying with electric trainers and the proceeded directly to glow engine warbirds, and after failing miserably we purchased 50 size J3 cubs (based mainly from suggestions by other posters on these message boards). I have flown mine several times with good sucsess infact, I have never crashed (and yes I am knocking on wood as we speek My friend on the other hand has had a real hard time, crashing every time. When he first showed me the completed plane I noticed that the stab and elevator were tilted about 1/8 inch or so compared to the wing level. I told him I was not sure what this would do and he needs to ask someone that knows more then I do. That did not happen, first flight rolled left on take-off and into the pavement. The fuse broke in half just behind the wing. He was ready to "quit and sell all of his stuff on e-bay" and I talked him out of that by saying it was a blessing in disguise because now he can fix the tilted tail and elevator seeing how the reason it was wrong was because the fuse was twisted to begin with. After fixing the cub he took off and flew for about 3-5 mins "landing" in a bean field. The flight was good but the operator was a little herky jerky with the controls hence landin in the beans. Bean field landings by the way are not too bad, very lttle damage done. Now every time he goes for a take off the plane rolls left and crashes. Outside of the one time mentioned above, he has crashed on take off. He keeps asking me to take it up for him just to see if it is the pilot that is the problem or the plane. The problem is that I realy do not want to crash his airplane. What should I do?
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Old 10-08-2007, 10:19 AM
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Default RE: J3 cub, crash on take off nearly every time.

You may want to do a search on problems encountered when flying J3s. The fuse is relatively short making it more easily prone to wind and/or pilot "over corrections". Also, depending on engine placement/mounting and what direction the prop spins, there may be a torque present that makes the plane veer off to the left. This can be corrected for by shimming the engine to compensate for the torque. Suggest that you ask some of the more experienced members in your club about this. [As an after thought, did your friend make sure that the stabilizer tips are equidistant to the wing tips etc?]
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Old 10-08-2007, 07:42 PM
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Default RE: J3 cub, crash on take off nearly every time.

JPANN, the j-3 cub is a nice forgiving airplane most of the time, it does need extra attention when taking off. If you try to pull a cub off the ground it is likely to snap on you, and the secret is to be sure you have enough speed before pulling it off the ground. I like to use grass with the cubs to take off some of the workload, as it naturally keeps the plane pretty straight. As far as the stabalizer being off a little, a average pilot will never notice the difference, you would be amazed how crooked a plane can be and still fly! The only time it gets critical is on really high performance planes that get used for competing and get flown to the envelope and beyond. It sounds to me like it was just a cub snap on take off and he just needs to be careful until he gets used to the plane, and be sure the cg is close or it could make it want to snap quicker as well. Good Luck!!

If you can't see it, don't hand me the controls!!
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Old 10-08-2007, 07:56 PM
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Default RE: J3 cub, crash on take off nearly every time.


ORIGINAL: JPANN

He keeps asking me to take it up for him just to see if it is the pilot that is the problem or the plane. The problem is that I realy do not want to crash his airplane. What should I do?
Just give him the "Test Pilot Disclaimer"

You have asked me by your own volition to test fly your airplane. In doing so I am relieved of all responsability for the plane should an unfortunate incedent occure.

If he still wants you to fly it for him... do it.
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Old 10-09-2007, 07:01 AM
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Default RE: J3 cub, crash on take off nearly every time.

Thanks for the advise waydown, I think what you have told me makes a ton of sense.
Mr67stang,
Honestly I had to laugh at the "test pilot disclaimer". But that is a good idea and consider it done. I will report the results from my flight of his airplane as soon as I get it done.
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Old 10-09-2007, 07:36 AM
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Default RE: J3 cub, crash on take off nearly every time.

Make sure you have a little tow-in on your main landing gear.
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Old 10-09-2007, 07:55 AM
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Default RE: J3 cub, crash on take off nearly every time.

JPANN, I was given a Cub that seemed impossible to get in the air. It taught me what a ground loop is. The folks here at RCU gave me some good advice and led me to the main problem. The wire landing gear had been put in backwards giving it toe-out instead of toe-in. Make sure there is some right thrust as Firebird25 mentioned and as Insanemoondoggie said, check the toe-in. Toe-in will make it somewhat self-correcting when it starts to veer off line during take off. The final thing I did was to install a more powerful engine. Long scale-like takeoffs are fine when you get used to working the rudder on a Cub, but at first it is nice to just get it in the air quick.
Edited to add: If your radio will allow expo on the rudder, try about 30-40%. This will make over-correcting less likely.
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Old 10-13-2007, 11:54 PM
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timothy thompson
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Default RE: J3 cub, crash on take off nearly every time.

RUDDER MANAGEMENT PLUS TOE IN
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Old 10-16-2007, 06:31 PM
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Default RE: J3 cub, crash on take off nearly every time.

Some advice since you sound new to flying, and this tip bit me BIG TIME in the past. Have you checked the balance of the airplane? Especially if it has been rebuilt, the weight may be way off. It almost sounds like it is really tail sensitive, and that would indicate a tail-heavy plane. Just a thought...
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Old 10-21-2007, 11:17 AM
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Default RE: J3 cub, crash on take off nearly every time.

I have a Balsa USA Cub and I can tell you from experience that a cross wind can be tuff to take off in. The Cub will tend to turn into the wind (weather vane) and will need substantial rudder correction to keep going straight. I would also ask about the amount of control surface deflection. Your statment that the plane was herky jerky tells me that the plane had more control throw than the pilot could handle or that it is tail heavy. As has been said a nose heavy plane will fly poorly but a tail heavy plane won't fly long. I have many planes and I can tell you that the Cub is a pleasure to fly for an experienced pilot but as with any scale plane can be tuff to learn with. Might I sudjest a nice Sig Senior (uses the same engine and radio) to get some easy flight under his belt prior to trying the Cub again. And don't fly either planes above a 5 mile an hour wind speed (until you have become very experienced.) My other sudjestion is to get some assistance from a local club, suprising those old guys thrive on helping and are usually right 85% of the time.
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Old 11-01-2007, 03:20 PM
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Default RE: J3 cub, crash on take off nearly every time.

My friend had just completed the repairs on his cub this past week, all in all it looked good. He spent many hours re-covering and pin striping as well as tediously repairing the fuse fractures etc. The plane balanced good left to right and front to back and all of the mechanicals were in good working order. We listened to all of the advise from you fine folks regarding the toe in and everything else.

I had finaly agreed (after reading the test pilot disclaimer as described above) to take off for him as this seemed to be where he had most of his problems. Now, you must now before I go on with the story at hand that in between my last installment and this one he had fixed the airplane once and taken it out to where an old timer or two ( I mean that in a good way) fly and proceeded to crash on take off and neither off them had any real answers as to why. It has taken a very long time to get to this point as we have been routinley grounded due to the blasted wind around here, I swear at least 20 mph every stinking day!! This morning was perfect, not dead calm but very slow lazy wind with no gusts. I looked at my friend he just gives me the thumbs up with a big grin on his face. I ask again if he is sure he wants me to crash I mean fly his plane. "hell yes I do" he says as if he knows the outcome already. Its cold, maybe 40 deg. and I am shaking already from nerves so the cold does not help matters. Already dressed like an eskimo I put gloves on but I take them off because I can't be one with the radio while wearing gloves. The engine does not want to start as easy as usual my hands are going numb and as I am looking at the other end of the runway the cub goes past me! "what the heck, I was not ready for my bud to let go, too late now give it lots of gas! It gets off the ground, at this point there are about 15 people looking on which does not help either, you can hear a pin drop as everyone holds there breath. The cub is going up, left, and right all at the same time and it is a real handfull just to keep her in the air. Something is wrong and I am not sure just what the plane is just a nightmare to fly and I am in over my head. I get it about two mistakes high and somewhat level for now but continues to be a bear. I hear "JP, do you want me to trim it for you?" I can't answer because I am fixated on the airplane and its lack of respect for the pilot, as well as realizing that trimming it out will do nothing for it since it is mostly doing whatever it wants to. I am not flying this thing at all, more like just moving every control surface opposite just to keep it in the air. Finaly I just blurt out "this thing is $%@#$& up! I need to bring it down. I try to get it back on approach but it just keeps rolling over, I save it three or four times just to do it all over again. Eventualy from all of the barrel rolls it is too low to the ground and it dissapears behind some trees and I can't hear the engine anymore. Knowing the carnage is at its max, we head out into the wilderness and find it in MANY pcs. right next to a crater big enough to think a meteor had hit there. So all in all the plane was the only loss, bat and rec is good as well as all but one servo. The engine outside of some dirt on and in it looks like it never hit the ground at all.

Even though I gave the pilot disclaimer speach, I have purchased a new airplane for him. He was not happy that I did and at this moment swears I will not pay for it (even though it is adone deal!) He was happy that it was not pilot error the half dozen times he crashed. I just did not want him to get bummed out and stop flying so I crashed today and purchased a new one for him today.

Thanks for the shoulder, and happy flying.
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Old 11-01-2007, 03:49 PM
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Default RE: J3 cub, crash on take off nearly every time.

Hi JPANN
I have had three J3 Cubs, all with a 71 inch wingspan. They are very different from most other models. They will snap roll in a heart beat. Rudder input is needed on takeoff, and when making aileron turns. I would not recommend a J3 Cub until you have had about 5 planes and and have been confortable with the last three. In my opinion, many have wing tip WASH IN (wing tips turn up). In order to make certain I never have wing tip wash in, I intentionally put in some wing tip WASH OUT (wing tips turn down) by adjusting the wing struts. My current J3 Cub is a SPAD, designed and built by me. It has been a hand full to get it to fly right, which is not a reflection on the J3 Cub. The problem is my design design.
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Old 11-01-2007, 06:29 PM
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Default RE: J3 cub, crash on take off nearly every time.

It really sounds as though this poor plane was never ment to fly. For some reason something was just not right. Squirlly planes instantly make me think to tail heavy and under powered. I know you checked ballance so...
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Old 11-01-2007, 10:43 PM
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Default RE: J3 cub, crash on take off nearly every time.

Sometimes the death of a plane can be a good thing, I think this is one of those times. I to am at odds with a cub at the the moment and am torn between selling it ,putting a more powerfull engine in it ,hanging it up in the family room for good and stomping the living crap out of it. I might add that this is my fault not the planes. At times like these when flying should be fun ,an easier flying plane might be the answer and lesve the cub for when you ( and I ) are better equiped to handle it. I love the look of the cubs and this is why I still have mine. Cheers the pope ( ps if mine went in I would laugh my head off )
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Old 11-02-2007, 10:27 AM
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Default RE: J3 cub, crash on take off nearly every time.

It is interesting that two of you have said that this plane was not meant to fly because as I walked away from the crash site that's exactly what I said. In my first post I mentioned that my friend was real jerky with the controls, that was obviously not the case at all. I can not help but think it almost acted as though the rudder and elevater was fluttering? I am not sure if thats how best to describe it but I can't think of any other explanation. My friends name is Kevin and from now on I will refer to him in these posts as such mainly because I am not into saying friend or bud and I realy do not want to type BF! (JP is myself as everyone I know calls me that).

There are a couple of things that I noticed about his plane and maybe you can shed some light. The first and most important thing is my J3 is a Richmodel, and his is a Nitro special (yes that was sarcasm). I imagine that the two are almost the same quality and price but they are very diffrent in size. Infact, the only thing that was the same was the wing span and wing width. For the Nitro, the fuse was shorter and smaller in diameter, the vert stab was shorter and smaller, and the elev. was alot smaller as well. Kevin also told me that the hinges for the rudder and elevater were like micro size compared to the aileron hinges. I am not saying one should not buy a plane from Nitro, instead just that they are built cheap and fast.

Last question I have is, how do they figure engine size to plane size? My J3 has a 71" wing span and calls for a .46 - .50 2cycle and it flys just fine. Kevins calls for the same but his plane overall as stated above is smaller. I have been lookin at other J3's and from what I have read so far all of the cubs that call for a .50 engine have 76.0" - 84.0" wing spans and heavier flying weight.

I lied and have one more question:
Does anyone have any ideas on better main landing gear? I am using the stock set-up which consists of two wire type with metal straps holding them to the fuse. On the sides of the wire there are wood skirts that again are straped to the wire and have plastic hinges screwed into the fuse bottom. I have yet to not destroy them on landing and can't help but think there has got to be a better way.
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Old 11-02-2007, 05:53 PM
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Default RE: J3 cub, crash on take off nearly every time.

Hi JPANN
You asked about a better landing gear for your cub. I use a DUBRO composite landing gear on my cub, available at Tower Hobbies.
My Cub with that gear can be seen at

http://www.rcuniverse.com/forum/m_44...tm.htm#4433841



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Old 11-05-2007, 11:04 AM
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Default RE: J3 cub, crash on take off nearly every time.

Thanks for the advise Villa, I have ordered the same gear (after looking at your pics) and they should arrive this week. I will let you know how they work out.

Thanks again,
JP
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Old 11-09-2007, 08:19 AM
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Default RE: J3 cub, crash on take off nearly every time.

Hey JPANN,

I have followed the thread from the start and before I'll give you some suggestions I would like you to give me some answers in my following questions...
1) Did always the plane had the tendensy on one side on its short flight? and which side was this?
2) Did this tendency increased as you increased the throttle?
3) Have you tried to idle the motor while it was 2 mistakes high to see the tendencies applied on the plane?
4) If your throttle was stable was the tendency the same when it was crosswind and downwind?
5) Did you have any side thrust and down thrust on your motor mount?
I think that I have some good answers to you if you provide the desired info...

Awaiting of your reply
BR
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Old 11-10-2007, 11:59 AM
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Default RE: J3 cub, crash on take off nearly every time.

Toe-In is the wrong way to go.

If you look at the dynamics, toe-in will make the plane more unstable.

If the plane swerves to the LEFT on the runway, the weight is transferred to the RIGHT main.
If the right main has toe-in, this makes the swerve to the left more pronounced.

Instead add just a small amount of toe-out to each main wheel.
Now the weight transfer from a swerve will act to pull the plane back into a straight line.

I didn't believe it at first either when I was first told about this.

I have since tried it on all my .60 sized WW2 fighters IT WORKS!
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Old 11-12-2007, 07:03 PM
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Default RE: J3 cub, crash on take off nearly every time.

Johnnie Red,
1. Left side more than right for sure. I had to fight it by giving it alot of right input, then it would snap.
2. No considerable change occured during throttle adjustments.
3. No, I never had a chance to because it liked the ground better!
4. It did not seem to matter.
5. Whatever thrust angle was built into the firewall. He did not add any more. (that's interesting however).

I hope this helps, I appologize for not having the exact answers you may be looking for. I had all of my attention on the task of trying to land and not realy on conditions etc.
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Old 11-12-2007, 07:11 PM
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Default RE: J3 cub, crash on take off nearly every time.

FYI,
The new Cub is all but finished now, if the weather cooperates with us it should fly soon. I also wanted to let you all know that Kevin (after being angry that I purchased the new Cub for him) purchased a new .56 engine for me. I have two planes and now have two engines and do not need to swap anymore!
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Old 11-13-2007, 12:03 PM
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Default RE: J3 cub, crash on take off nearly every time.


ORIGINAL: JPANN

Johnnie Red,
1. Left side more than right for sure. I had to fight it by giving it alot of right input, then it would snap.
2. No considerable change occured during throttle adjustments.
3. No, I never had a chance to because it liked the ground better!
4. It did not seem to matter.
5. Whatever thrust angle was built into the firewall. He did not add any more. (that's interesting however).

I hope this helps, I appologize for not having the exact answers you may be looking for. I had all of my attention on the task of trying to land and not realy on conditions etc.
Dear JPANN,

Thanks for your answer. From the answers you gave me I can think of the following.
The Left side tendency was probably from the lack of the proper right side thrust. The tendency of snaproll comes out of the throws at the elevator. (The cub has big moving tail moments so you need to trim gentle moves).
Also, it would be very helpfull if you put at your next cub 2-3deg. Down Thrust on your engine together with 2-3deg. side thrust. The plane will be tamed like this and the takeoffs will be less tricky than they are right now. Do that and you will have a perfect plane to handle.
Best regards to you and happy flights with your next Cub project
Johnnie Red
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Old 11-13-2007, 06:36 PM
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Default RE: J3 cub, crash on take off nearly every time.

JPANN,

Also, out of curiosity did you guys try putting an incidence meter on the recently deceased cub? I'm wondering if there was a warp in one of the wings that would be tough to pick up on.. If the plane wanted to turn right all the time you may have had a small amount of 'wash-in' going on in the left wing (wingtip twisted slightly so that it has a slightly higher angle of attack than the right wing)..

Just thinking out loud...

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Old 11-14-2007, 01:10 PM
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Default RE: J3 cub, crash on take off nearly every time.

Johnnie Red,
That is a valid point / idea. We checked the diff. between the new cub and the one that is no more and the new one has almost exactly the amount you described verses the spent cub. I am thinking the new one is good to go as is. Would you agree?
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Old 11-14-2007, 01:21 PM
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Default RE: J3 cub, crash on take off nearly every time.

Geardaddys,
That is also a probability. I say that mainly because of the way the wings mounted to the fuse. Normaly the halves are atttached togeter via a wing joiner as I am sure you know all to well, and then fastened in some manner to the fuse as a whole wing. The Cub that we are talking about here had the the two halves attached to tubes then the tubes themselves attached to the fuse leaving a big open space in between the wings. I looked this over about twenty times or so and about twenty times or so I did not like it. The lines on the wing halves just did not "flow" together like you would think they should if you know what I mean.
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