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  1. #1

    help new arf cub flying like a kite

    Ok i was given a cub arf. It's a white box chinese no name model from value hobbies

    it was free from a friend. Ok so i put this arf together this winter i finally got a chance

    To take it out. I had my trainer from the club take it up for its madden flight

    and man was I happy it was him not me. Right away he was in trouble the plane

    just flew like a kite and seemed like it was tail heavy. We checked cg before flight and it was fine.

    The plane just out of control it took 5 mins just to get it to land.

    After landing we checked the CG again it was still showing fine. It was perfectly level (NOT TAIL OR NOSE HEAVY)

    I'm wondering if it should maybe set up a little nose heavy? Couple guys at club suggested setting it up with Aliron differential?

    any input would be nice.
    Last edited by rcworld2000; 06-06-2014 at 06:21 AM.

  2. #2
    JohnBuckner's Avatar
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    Balance the airplane at 25% mac. Truly, balance at the quarter chord and you just might be amazed. Most common mistake is to treat most cubs as though they were some sort of 3d flyer and balance them at one third or more. You will find in many forums that to balance just about anything at 28% or less preferably for intial test flights is not politically correct. But do so and you just might be fascinated how docile and pleasant flyer a cub can be.

    John
    \"Keep your controllines tight\"

  3. #3
    speedracerntrixie's Avatar
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    CG is NOT a fixed position on a plan sheet or instruction manual. If the airplane is telling you the CG is off via flight performance then simply change it.

  4. #4
    im new to the hobby john i balance the plane 84mm behind the leading edge ofthe wing against the fuselage. just like the maual said? can you explain 25% mac ?

    thanks for the help john.

  5. #5

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    What were the symptoms? Often planes climb too much and people think they are tail heavy when all they really need is down trim. If you move the CG forward then you may not need down trim and you think you have solved the problem, but you would have covered up a problem that could have been corrected and created a better trainer.

    I am a club instructor and sometimes I have moved the CG forward and corrected a problem, but in those cases the student had not checked the manual and did not know what the CG was supposed to be. I have also set up many a docile trainer with the CG at the correct place and not at 25%.

    Tail heavy means the plane is unstable in pitch, i.e., if you give say, down elevator and release, it will keep going down or even increase the steepness of the dive; so you give it up and now it keeps steepening the climb. In this case, the only correction is to move the CG forward. But that is different from just climbing too much.

    Kit and ARF manufacturers are generally very conservative with their CG recommendations, so if it balanced properly it probably does not need more nose weight.

    There is no "one size fits all" CG. It depends on the wing area/stab area and tail arm/wing chord. This is physics, it's got nothing to do with "PC". The manufacturers usually err on the nose heavy side to begin with in my experience. However, for some reason many kit and arf airplanes need down trim. They shouldn't be designed that way, but they often are.

    Jim

  6. #6

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    When you measured that 84 mm did you measure by laying a tape across the wing? Or did you draw a line up from the leading edge and measure 84 mm from there.
    CG as given in the plans are a ballpark starting point. Don't forget CG is checked with an empty fuel tank
    Last edited by jetmech05; 06-06-2014 at 08:17 AM.

  7. #7

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    There's not enough information in the OP to determine if he actually has a CG problem or not. What does "flying like a kite" mean? If the elevator was too sensitive, then you may just have too much elevator throw. If you mean that the wind tossed the plane around a lot, then welcome to flying Cubs. That's what they do. On a scale plane like that (and for the initial setting on aerobats and sport planes too), I tune the CG based on how the plane lands. If I can hold flare all the way to the ground, the CG isn't too nose heavy. If the elevator feels sensitive even as the plane is settling in, the plane is tail heavy. Checking out the difference between upright and inverted is a good second test also. The less forward stick is required to maintain altitude while inverted, the closer the plane is to the neutral point.

    So how about some clarification about what exactly the plane did?
    No kid, I said break ground and fly into the wind!

  8. #8
    JohnBuckner's Avatar
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    Quote Originally Posted by rcworld2000 View Post
    im new to the hobby john i balance the plane 84mm behind the leading edge ofthe wing against the fuselage. just like the maual said? can you explain 25% mac ?

    thanks for the help john.

    Yes and it is so simple:

    measure the distance (for this particular airplane) from the leading edge to the trailing edge, next divide that distance you just measured by four. That 'figure' will be the quarter chord, 25%mac. All that is now left to do is just measure back from the leading edge that figure or distance and that is your target point the cg. Yes if you like for ease of checking the cg just project this figure inward next to the fuselage.

    The last step is to do whatever you gotta do to make the airplane balance 'level' at that target point, not a little nose down for Mother and Country. if you do the nose down a little thing then that means you have not balanced at the target point but some unknown point forward.

    Now notice I have completely ignored your info on what the instructions say or said, I don,t care. I have run into so many no name wanna be so called instructions some even just 'pictografts. And many dangerously wrong and yes only just my opinion that I feel this is problem for many people.

    If you want the best chance to make your ship a nice flyer then truly balance at the 25%mac or quarter chord and I know (yes my opinion agine) that you will be rewarded with the best ship possible.

    John
    Last edited by JohnBuckner; 06-06-2014 at 12:19 PM.
    \"Keep your controllines tight\"

  9. #9
    hey buzzard the plane was just very hard to cotrol . it wanted to float like a kite. not fly and end order to get it to move foward it needed a lot of power and even then it just seemed out of control my trainer had to do all he could just to get it back around and steady enough to land.

  10. #10
    jester fly like a kite means the plan would nose up and just wanna float and not steer. and as for wind tossing the plane around i thought that maybe was the case but no other planes were having trouble and the were even foamies out that day. only about 8 mile hour wind (ground)

    that is the first time many of the guys at the club said they never saw a plane act that way. the only way i can describe it is it looked like a kite . it just didnt wanna be controlled

  11. #11
    thanks john i will measure that wing and see what i come up with
    Last edited by rcworld2000; 06-06-2014 at 04:03 PM.

  12. #12

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    Two possibilities or a combination of both. You either have a wing or tail incidence problem or the engine thrust line is off. Too many modelers believe that an ARF can be assembled and flown right out of the box. I made this mistake twice with H9 products. The most recent was a H9 Super Cub. For those who are unaware it has built-in positive incidence on the horizontal stabilizer. This was designed for those who don't know how to take-off a tail dragger. It took me buy surprise when the tail popped up after releasing up elevator. In the air with the throttle set at 75% it took some down trim to compensate for the stab placement.

  13. #13
    Quote Originally Posted by flycatch View Post
    Two possibilities or a combination of both. You either have a wing or tail incidence problem or the engine thrust line is off. Too many modelers believe that an ARF can be assembled and flown right out of the box. I made this mistake twice with H9 products. The most recent was a H9 Super Cub. For those who are unaware it has built-in positive incidence on the horizontal stabilizer. This was designed for those who don't know how to take-off a tail dragger. It took me buy surprise when the tail popped up after releasing up elevator. In the air with the throttle set at 75% it took some down trim to compensate for the stab placement.
    ok how do i test for these ???

  14. #14

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    Find another modeler who knows how to use an incidence meter and walk you through the set-up processes.

  15. #15
    hows this one ? AnglePro 4-in-1 Digital Throw/Incidence Meter

    Click image for larger version. 

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  16. #16

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    That is exactly what I use. I also have a Robart non digital but the AnglePro is more precise.

  17. #17
    ravill's Avatar
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    Digital incidence meter on a cub?

    Please.

    To the OP, I think making sure your CG is a little more nose heavy will likely help. You are doing the right thing by going for help.

    As you are new to the hobby, you haven't really got a feel for what "tail heavy" vs "nose heavy" will feel like on the ground. 25% MAC is usually a very safe place for most airplanes.

    The Piper Cub is a proven airframe and should fly fine. Even your "no name" Chinese ARF. Make sure you pick a nice calm day for your first flights. You'll do fine!
    RAVjets Demo Team. All We Do Is Fly.

  18. #18

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    rcworld2000, Listen to what John Buckner told you about using 25% of the MAC to set your C/G. Do it his way, he is right!

  19. #19
    ok my plans were to try johns method first. he has helped me out in almost every post i have had on here.. i will try the 25% of mac this weekend and see what it shows me about the cg and post a update. thanks guys

  20. #20

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    If the plane was constantly raising its nose that sounds like your elevator is just out of trim. Was the pilot ever able to get the plane to fly a pass straight and level? Did he try doing any flight trimming? I have had a couple of planes before where I ran out of trim on the elevator and had to land it out of trim. That's not a problem with the airframe, but just a badly missed initial elevator setup. Of course, that doesn't mean you don't have a CG issue or any number of other things, but you really can't know if there is anything else wrong until you get the plane trimmed for hands off straight and level flight.
    No kid, I said break ground and fly into the wind!

  21. #21

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    I'm no expert but I do maiden planes for people at the field and while "flying like a kite" is hard to understand I may know what you mean (sort of). Anyway, I was maidening a plane for a guy and it had incredible pitch sensitivity but only in up, it was very sluggish down. The plane just wanted to climb and climb and needed almost full down trim for straight and level. After we got it down, I noticed the the top of the windshield was sticking over the seat for the front of the wing. this meant the front of the wing was not sitting on the wing saddle but rather the top of the windshield increasing the incidence by a ton. we fixed it and the plane tirmmed out and flew perfectly. That was almost as tough as a maiden where there was a gyro in the plane and the owner did not tell me. It was reversed for all directions. My hands were shaking after the landing.

  22. #22
    Quote Originally Posted by JohnBuckner View Post
    Yes and it is so simple:

    measure the distance (for this particular airplane) from the leading edge to the trailing edge, next divide that distance you just measured by four. That 'figure' will be the quarter chord, 25%mac. All that is now left to do is just measure back from the leading edge that figure or distance and that is your target point the cg. Yes if you like for ease of checking the cg just project this figure inward next to the fuselage.

    The last step is to do whatever you gotta do to make the airplane balance 'level' at that target point, not a little nose down for Mother and Country. if you do the nose down a little thing then that means you have not balanced at the target point but some unknown point forward.

    Now notice I have completely ignored your info on what the instructions say or said, I don,t care. I have run into so many no name wanna be so called instructions some even just 'pictografts. And many dangerously wrong and yes only just my opinion that I feel this is problem for many people.

    If you want the best chance to make your ship a nice flyer then truly balance at the 25%mac or quarter chord and I know (yes my opinion agine) that you will be rewarded with the best ship possible.

    John
    ok John I finally had some to get my cub on the bench and the wing measures 9 inches so 25% is 2 1/4
    The stock point was 3 1/4 so with the new point the plane was very tail heavy! It took a fair amount of weights added to get the plane back level.

    My trainer was against me changing cg point he thinks the plane needs aileron differential which I am also going to add.

    wish me luck.

  23. #23

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    That's a huge CG change. Are you sure about that measurement? It's pretty rare for a Cub to need significant nose weight assuming you're running an appropriate engine. I'd double check that measurement and go ahead and pull up a CG calculator to make sure you have the location right.

    What method did you use to do the measurement? The right way is with a square of some kind that you can butt up against the leading edge and know that you're getting a precise reading at the trailing edge. If you'll give me the overall length of the plane and the wing measurements, I'll compare it to my Cub which flies nicely.
    No kid, I said break ground and fly into the wind!

  24. #24

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    rcworld, What you refer to as the "stock" C/G point is just plain incorrect. It is "misinformation" printed in your assembly instructions and should be ignored. Somehow it got printed as 3.25" when it should have said 2.25". This happens in ARF instructions more often than it should. Always measure the MAC and caculate the C/G yourself. The general rule of thumb is to balance a model between 25-33% of the MAC, never more than 33%. At 3.25" with a MAC of 9", you were at 36% and damm lucky to get the plane back down in one piece. John's advice to set it @ 25% for this model is correct. Aileron differential is meant to help reduce "adverse yaw", a trait of Cubs. This will not fix a tail heavy plane. You must have the C/G correct. Aileron differential is at your option. To fly a Cub correctly (to overcome adverse yaw) you should learn to add rudder at the same time you bank it with ailerons to make a turn. A lot of flyers today with computer radios program in rudder mix with the ailerons. I am "old school" and add it in with my thumbs as I fly. Either way, some rudder should be used when turning a Cub, even if it has aileron differential. Good luck

  25. #25
    Hossfly's Avatar
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    Quote Originally Posted by rcworld2000 View Post
    ok John I finally had some to get my cub on the bench and the wing measures 9 inches so 25% is 2 1/4
    The stock point was 3 1/4 so with the new point the plane was very tail heavy! It took a fair amount of weights added to get the plane back level.

    My trainer was against me changing cg point he thinks the plane needs aileron differential which I am also going to add.

    wish me luck.
    Well, IMO, rcworld2000, you should FIRE your Trainer. In any situation of the normal airplane configuration, that is, a prop up front or a jet engine just aft of the wing center, and will be flying within the realm of convergent-airflow (sub-sonic) 25% of CG is an excellent starting place to obtain great flying stability. Of course once you get use to the machine, then one can generally be safe by making some changes and just seeing how far you can go with those changes. Your airplane -- treat it as you wish. Now as a student and you have denied one of the best-ever RC pilots, that is telling you the basic aeronautical truths, well keep your glue and stuff handy. You will need a lot of it.
    Differential aileron is absolutely a great thing - I would never be without it in sport, especially Scale models - but, big daddy, it ain't gonna solve your CG problem. I have a 100" gasburner Eindecker. I fly it at Big Bird and sport-scale events. The plans called for a 33-35% CG point. I knew that was wrong but I went with the .33. My first flight looked like I was trying to do 3G. I managed to recover and added lead to get 28%. Still bad flier. Then I added more lead. 25% cg. FLIES SO SMOOTH. Take-Offs and landings are sooo easy. Been that way for about 4-5 years now. OK, Captain rcw..., it's all yours, I am Ba-i-l-in--g... O-u---t..t!
    Last edited by Hossfly; 06-15-2014 at 10:25 PM. Reason: Add help
    Horrace Cain AMA L-93

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