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help new arf cub flying like a kite

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Old 06-15-2014, 09:39 PM
  #26
rcworld2000
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Quote:
Originally Posted by jester_s1 View Post
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.

Wingspan: 70.6 in (1795 mm)
Wing Area: 682 sq in (44.0 sq dm)

length 45 inches
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Old 06-15-2014, 09:42 PM
  #27
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thanks hoss
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Old 06-15-2014, 09:44 PM
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at the 21/4 it took moving battery as far foward as could and 3 1/2 oz of weight. is that alot ?

and i know some mentioned engine size i am running a os .46 la 9the blue) engine it does seem real light
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Old 06-15-2014, 10:26 PM
  #29
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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.

Wow if you were truly balanced on a nine inch chord at three an a quarter inchs those first flights had to interesting to say the least, your lucky you still have the airplane indeed! Of course that is what made the airplane seem so strange.

Lets take just a minute to talk about the so called "stock CG". Of the no name Arfs I think incorrect information is more the norm rather than than an infrequent mistake. And here is the reason: The most expensive thing that goes into the box of a new quality arf is not the airframe or the hardware it is in fact the instruction manual. Bottom line it costs money to develop the manuals and flight details and that is why any given reseller can sell so cheaply just by cheaping out on the manual or depending on a Chinglish pictograph.

OK so lets talk about differential, Yes it can help your cub to fly nicer and as Larry carefully explained above it is not related to an airplane with the much more serious problem of being severely tail heavy. In fact one of the Cub models with a reputation as being one of the best, The Goldberg Kit built Cub has a little secret and that is if you build it according to plan with the plywood aileron control horns which are raked aft 15 degrees from the hingeline you have now induced mechanical aileron differential which is responsible for that reputation.

Of course you can even use electronic differential these days also But do be aware the problem from the start of this thread is your improperly balanced airplane to far aft.

John
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Old 06-15-2014, 10:37 PM
  #30
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Quote:
Originally Posted by rcworld2000 View Post
at the 21/4 it took moving battery as far foward as could and 3 1/2 oz of weight. is that alot ?

and i know some mentioned engine size i am running a os .46 la 9the blue) engine it does seem real light


No 3 1/2 ounces in the nose is not excessive and the .46LA is just fine. If it is now at 25% it will be a completely different airplane and a pleasant surprise.

John
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Old 06-16-2014, 12:31 AM
  #31
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thanks john. i will let you know what happends
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Old 06-16-2014, 02:08 AM
  #32
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Tail heavy
Stoner.
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Old 06-16-2014, 06:59 AM
  #33
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If the chord is 9 inches then 2 1/4 will indeed put you at 25%. You might find that setting a tick nose heavy (I did on my Cub) but it will at least give you a flyable airplane that you can tweak to your liking.
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Old 06-16-2014, 10:00 AM
  #34
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Sounds like a 1/5 Cub. OP didn't state how the wing is attached. If bolted on, good. If rubber banded, use plenty. A plane can be 'pitchy' if not using enough rubber bands, up elevator can cause the wing to unseat momentarily and create a wild ride.
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Old 06-16-2014, 10:25 AM
  #35
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Quote:
Originally Posted by eddieC View Post
Sounds like a 1/5 Cub. OP didn't state how the wing is attached. If bolted on, good. If rubber banded, use plenty. A plane can be 'pitchy' if not using enough rubber bands, up elevator can cause the wing to unseat momentarily and create a wild ride.
bolt on. 2 wing pieces
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Old 06-16-2014, 01:50 PM
  #36
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At 70.6" the Cub in question is about 1/6 th scale and an O.S. 46 LA will fly it well. A 1/5 th Cub will have about an 80"-82" wing span.
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Old 06-16-2014, 04:27 PM
  #37
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I wouldn't worry about differential aileron mixing until the airframe is sorted. Actually, if you really want to learn to fly a Cub, don't use differential at all. Once you loosen up that left thumb, you'll find yourself using rudder ALL the time to finesse the flight path of the model (including your sport models that "don't need" rudder).

And if you think such mastery of the rudder is overkill, just wait til you see what it does for your crosswind landings.
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Old 06-16-2014, 08:26 PM
  #38
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Quote:
Originally Posted by JohnBuckner View Post
No 3 1/2 ounces in the nose is not excessive and the .46LA is just fine. If it is now at 25% it will be a completely different airplane and a pleasant surprise.

John
sorry more like 6 oz of weight John
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Old 06-16-2014, 08:57 PM
  #39
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No need to be sorry and six ounces is still not out of reason but but add the weight in a temporary fashion. The very first thing you need to do is go fly the airplane once more after you balanced truly at the quarter chord (25%].

Now after doing that I am betting two burned out glow plugs aginst a Stale Glazed Donut that you come back talking about a what a nice flying airplane it is. And then we can talk about a few ways to eliminate that little bit of extra weight and without a huge fuss.



Temporary Fashion

John
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Old 06-17-2014, 05:30 AM
  #40
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Let an instructor test fly the aircraft and diagnose
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Old 06-17-2014, 09:33 AM
  #41
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Ok john. I will probably not get out this week. Rain and high winds all week . Hopefully next week. I will keep u posted
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Old 06-26-2014, 05:33 AM
  #42
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Hi!
Did you check for any wash-.in or wash-out in the wing?
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Old 07-03-2014, 06:57 PM
  #43
rcworld2000
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ok john that cub i have been having troubles withalso its a .40 with struts??????? and after moving the cg it was better not great but better. i think i need more weight in the nose.. it seems really LAZY on turns. also on lift off it pulls left untill you back off motor ???
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Old 07-03-2014, 08:54 PM
  #44
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Cubs aren't trainers. They don't automatically fly straight. It's normal for a Cub to pull left on takeoff and also whenever you pull hard on the elevator. That's a consequence of the short tail, low airspeed, and largish prop that Cubs have. I don't know what lazy in the turns means, but Cubs aren't fast at anything. There is a certain pleasure to flying them that you can't get from any sport plane, but speedy aerobats they are not.
Do you think you have your CG problem sorted out? You should be getting smooth landing approaches with at least a little bit of elevator control all the way to the ground. A good CG on a Cub lets you 3 point the landing if you want to. After that, it's a matter of getting the aileron differential right and getting the throws set to your liking. A well trimmed Cub is a slow and lazy flyer on calm day, very relaxing but requiring a lot of subtle skill to keep it flying where you want it.
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Old 07-04-2014, 06:47 AM
  #45
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Quote:
Originally Posted by jester_s1 View Post
Cubs aren't trainers. They don't automatically fly straight. It's normal for a Cub to pull left on takeoff and also whenever you pull hard on the elevator. That's a consequence of the short tail, low airspeed, and largish prop that Cubs have. I don't know what lazy in the turns means, but Cubs aren't fast at anything. There is a certain pleasure to flying them that you can't get from any sport plane, but speedy aerobats they are not.
Do you think you have your CG problem sorted out? You should be getting smooth landing approaches with at least a little bit of elevator control all the way to the ground. A good CG on a Cub lets you 3 point the landing if you want to. After that, it's a matter of getting the aileron differential right and getting the throws set to your liking. A well trimmed Cub is a slow and lazy flyer on calm day, very relaxing but requiring a lot of subtle skill to keep it flying where you want it.
thanks jester. yes think cg is better and as for lazy turns it just seems to wanna turn good one turn and vey slow others. its a 40 size cub with a os 46 what size prop do you suggest.
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Old 07-04-2014, 06:59 AM
  #46
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OK RCWorld2000 Excellent. If you have balanced the airplane at the quarter chord (25%) and it sits level when checked at that point then there is no need to add any weight or change the balance any further.


What you are experiencing with the cub now are two separate phenomenon.


First the tendency of the airplane to Yaw to the left at takeoff rotation and in a climb. All propeller driven aircraft do indeed do this and the only exceptions are counter rotating as well as in line twins. It is called (P) factor or propeller factor. What happens is at the point of rotation on take off and in a steady nose high climb the relative wind or angle the airflows into the propeller is at an angle from below. This will cause the descending propeller blade to be at a higher angle of attack on the right side (in the case of conventional engine rotation) than the angle of attack of the ascending blade on the other side.

This dissymmetry of thrust will always cause the airplane to yaw (or swerve) left at the point of rotation for takeoff and will always be present during a climb at a high deck angle.

Now the second of the two phenomenon and the reason for the "Lazy Turns" is called adverse Yaw and that is what you are seeing. Its that tendency for the nose to want to yaw in the opposite direction that you have banked the airplane with the airplane wallowing through the turn. What is happening is the descending aileron is producing more drag than the aileron that goes up. The nose will always want to yaw in the opposite direction than you have applied the ailerons.

There are a number of ways to correct for this and the first is obviously to use the rudder and in basic full scale instructor will always preach rudder and aileron together (with the exception of some manuvers). It can be minimized by design and differential of aileron controls either electronic or mechanical.

That's it in a nut shell and most all cubs model or full scale have both of these characteristics in abundance and why at least in the RC world cubs are not the greatest of trainers. Beleve it or not manually correcting either of these two is more difficult with the RC model than the full scale airplane.

John
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Old 07-04-2014, 07:19 AM
  #47
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ok great thanks john
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Old 07-04-2014, 11:41 AM
  #48
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As others have said Cubs aren't trainers. If you have not mastered rudder control this is not the plane for you. I started with a clip wing cub then went to a H9 1/4 scale Cub followed by a H9 1/4 scale Super Cub. Of the three Cubs I liked the clip wing the best. You don't need differential aileron if you know how to coordinate your turns with rudder. A real Cub is a stick and rudder airplane and the same thing applies to a model of this airframe. I got my second Cub off a fellow who said it flew like crap. What he really meant to say was he lack the skill to fly it. If you determined to stay with your Cub than seek the assistance of another modeler with the same airframe who can walk you through the learning process.
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Old 07-04-2014, 12:46 PM
  #49
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ok thanks for info. i just picked up a pete n poke . will i fly like a cub ?
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Old 07-04-2014, 08:50 PM
  #50
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For Cubs, you need pull, not speed. If the OS .46 you are using is a .46 AX, an 11x5 should be good. If it's a .46 LA, you may need to go smaller to be able to let the engine unwind enough to make good power.
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