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my cub flies a lil funny

Old 09-12-2010, 07:27 PM
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opie99
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Default my cub flies a lil funny

i have a great planes cub that i built from a kit, its a 76.5 wingspan with a magnum .91(i know too much but its all i got and i want to put floats on it). this is the first plane i have built. im thinking i might have a warped wing but i dont know because i never have flown with one. the problem is when i have the plane at level flight and all the controls are nuetral it wants to go the the right pretty hard. i know when i built i had thelateral and cg balance spot on. so after the madian i decided to added about 2oz ofweight to the left wing, but i still had to add alot of trim to it and it will sometimes still slightly go to the right. ireally dont want to add any moreweight.it gets a lil frustraiting. another thing i was worried about was that with having an engine that big does the angle at which the engine sits against the firewall have to be increased. when i mounted the engine i didnt add any in since i thoungt the firewall already had it built in. any advice would be great. other than this lil problem it is a blast to fly. i really hope someone can help me

kyle
Old 09-12-2010, 07:58 PM
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Default RE: my cub flies a lil funny



Hello kyle,

You no doubt have given this matter a lot of thoughts before asking for help (ideas).

You already assured yourself that the lateral balance was good before adding 2 oz to the left wing.
I do not think that was a proper solution.

Other things to think of are ___
Warped wings. Can be checked with a good incidence gauge and can ususally be seen just by eyeballing the wings.
A fin that is out of line with the longitudinal axis or is warped. Again it can be eyeballed.
The lateral symmetry of the wings. Is the left wing longer than the right thus producing more lift.
Same as line above for the horizontal stabilizer.
Any nonsymmetrical curvature of the fuselage.
Too much right thrust in mounting the engine.

What else can I think of ???

Let us know if you find something along those lines.

Regards de Zor
Old 09-12-2010, 08:03 PM
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Default RE: my cub flies a lil funny

I would definitely take a tape measure and measure your thrust angle. This may or may not be your problem but at least it will eliminate that as a cause.
Old 09-12-2010, 08:07 PM
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Default RE: my cub flies a lil funny

On the engine, if it pulls hard to the left when the throttle is quickly increased, then you need more right thrust. However, with that big engine it's possible you already have too much right thrust.

Since the Cub is a flat-bottom wing, you can easily check for warps. Sight along the bottom of the wing from the tip to the root. You will easily be able to see if the bottom of the tip rib is in alignment with the root rib. If it is warped, try this. Gently twist the wing about the same amount in the opposite direction and re-shrink the induced wrinkles out of the covering.

If the wing is not warped, look to your controls and surfaces. Make sure the vertical stab is at right angles to the fuselage. This may be hard to measure and you may have to resort to a TLAR (That Looks About Right) system. Be sure your rudder is straight to the fuselage. Check the ailerons to be sure you have them level at neutral. Lastly, be sure the wing attaches at right angles to the fuselage.

Good luck with it.
Old 09-12-2010, 08:46 PM
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Default RE: my cub flies a lil funny


ORIGINAL: opie99

the problem is when i have the plane at level flight and all the controls are nuetral it wants to go the the right pretty hard.
Kyle,

Does the plane rolls or yaws to the right?
Old 09-12-2010, 10:49 PM
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Default RE: my cub flies a lil funny


[quote]ORIGINAL: Lnewqban


ORIGINAL: opie99

the problem is when i have the plane at level flight and all the controls are nuetral it wants to go the the right pretty hard.
Kyle,

Does the plane rolls or yaws to the right?
It may be difficult to tell.

If it yaws it will roll,
If it rolls it will yaw.

Zor

Old 09-13-2010, 12:39 AM
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Default RE: my cub flies a lil funny



it is hard to say but i would have to say it rolls. the wing looks pretty straight to me but im finally going to get some batteries for my wing incidence guage. i know adding the 2oz of weight to the wing wasnt a good idea but i just stuck them to the outside just to see if it would make a difference and i really couldnt tell a difference so im just going to start making my way through it. i did notice one of my ailerons looks to be a lil warped but it doesnt look to be to bad, im going to fix it tomorrow. i also noticed that one side of my elevator is not perfectly mounted in the middle of the horizontal stabilizer, don t really know how to fix that one. other than that everything else looks pretty good. the plans say the thrust line is 2deg down and 2deg to the right if i remember correctly, anyone know a good way to measure to check?

kyle

Old 09-13-2010, 05:51 AM
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Default RE: my cub flies a lil funny


ORIGINAL: opie99

im thinking i might have a warped wing but i dont know because i never have flown with one. the problem is when i have the plane at level flight and all the controls are nuetral it wants to go the the right pretty hard.

kyle
Let start at the begining,, have you tried to adjust the trims to get it to fly straight/level?
Old 09-13-2010, 07:21 AM
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Default RE: my cub flies a lil funny


Hello Kyle,

Let me say first that I am not trying to monopolize this thread. I hope others will contribute to your cause. The best I can do is some thinking and pass along my thoughts.

We have a CG that is moving along at a fair speed and free to rotate in 3 axis due to 4 aerodynamic forces. It may be7 forces if 3 forces are rotary to prevent attitude changes.

The mass involved is small in terms of inertia to the axis (plural ?) of rotations.
It takes a very little torque to rotate the airplane attitude around the 3 axis.

Some of, or a combination of, the observatons you made can very well be the main cause of turning to the right.

Concening right and down thrust, no great accuracy is needed (only an opinion for whatever it is worth).
Some incidence meters claim to measure these angles but, from what I have seen, I could notmake out their procedure explanaton and concluded it was a far remote claim.

What I do follows ___
I place the model on a flat tableand set it up so that the reference line is paallel to the table top.
I have a fair judgment of what 2 degrees look like from having drawn such an angle on a 8 1/2 X 11 inches cardboard. Do not use a protractor; use dimensions and a caliper to draw as accurately as possible. I cut this angle out of the cardboard and place it on top of the engine shaft. It becomes easy to judge if the side of the cardboard triangle that is not contacting the shaft is parallel to the reference line.

For side thrust angle I place a long straight edge on the table and line it up with the fuselage center line (line of symmetry). Then placing the cardboard triangle on the side of the engine shaft and looking vertically down i can judge the angle.

This process might not be highly accurate but I think it is well within the accuracy requirements.

Other people may have a better method and hopefully do a posting.

Enjoy and have fun,

Zor

P.S.: edited to correct two spelling errors.

Old 09-13-2010, 07:38 AM
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Default RE: my cub flies a lil funny

Further to post #9 ___
I started to write post #9 a 06:00AM and went to breakfast and finished writing it after 08;00 AM.
By then there was another post.

My concept of a nice flying model includes the ability to fly straight and level at cruise power setting with control surfaces not deflected and in line with their supporting structures.

In my previous postings I aimed at finding the cause and cure it as contrasted to find a remedy such as trimming.

Just to keep the perspective asI see it.

Zor

edited to change previus to previous.
Old 09-13-2010, 08:46 AM
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Default RE: my cub flies a lil funny

I will bet it is all in the size of the engine and prop. Don't run the engine full bore and I bet it fly's great. Engine size and prop size have a lot of affect on a Cub.
Old 09-13-2010, 09:16 AM
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Default RE: my cub flies a lil funny

I'd be more apt to believe you have a slight warp or twist in one or both wings. Get about 10 or 15 feet behind the plane and then have the plane propped up so you can see the trailing edge of both wings. As you sight at the trailing edge, see if you can see the same amount of wing surface on both sides as you raise or lower you gaze at the trailing edge. As you move your gaze, the lower surfaces should all disappear at the same time. Since this is a flat bottom wing, get 4 straight firm sticks of wood and attach one each near the bottom of each root rib and the other two at the bottome of the outermost full length rib and sight from the side. All sticks should line up perfectly, if one or more is tilted more up or down than the others, you have a warped wing. No need for a fancy incidence meter.
Old 09-13-2010, 07:48 PM
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Default RE: my cub flies a lil funny

ORIGINAL: opie99

it is hard to say but i would have to say it rolls.................... i did notice one of my ailerons looks to be a lil warped but it doesnt look to be to bad, ................... i also noticed that one side of my elevator is not perfectly mounted in the middle of the horizontal stabilizer, don t really know how to fix that one. ......................... the plans say the thrust line is 2deg down and 2deg to the right if i remember correctly, anyone know a good way to measure to check?
kyle,

If the plane is balanced laterally and the wings or stab are not warped or have different angles of incidence (side to side), then the plane may have more lifting area on the left side (this includes wing area and stab area as well).
Not trying to say here that your building skills are poor or anything like that; just that it is something worth to be verified.

As you know, there are two types of balance: one is the static balance (the one you have measured side to side), the second is the dynamic balance (the one created by the airflow over the wing and stab).
There is no practical way to compensate an unbalance with another: both types must be equally in equilibrium for a hands-off trimming.

For example, the wingtip weight that is helping you in level flight, will work against you during inverted flight and during landing.

I suggest you determining how off the stab is, and try correcting it by reducing the excess of area.

Don't worry much about the thrust angles, since they are there to compensate for the several effects of the propeller, which in any event would tend to roll the plane to the left.

Old 09-13-2010, 11:23 PM
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Default RE: my cub flies a lil funny



thanks for all the good imput guys. i was thinking about the thrust line being off and i thought i would make it roll to the left from the torq of the engine, so ive kinda ruled that one out. i know the engine is too big so i cut the max throttle down to 75%(left it a lil high to be on the safe side) and i still only use about half and thats on take off. i was looking at the wings and i would say the ailerons are sittilng at a 30* from neutral and i was thinking that was kinda alot for trim, i was thought that trim was only ment for a couple of clicks and after that is a lil drastic for trim and thats when you should check to make sure everything is neutral and i have already done that. im going to check for wing warp when i get home and let you guys know what i find out.


lnewqban,
are you saying i just might need to add more trim to the rudder? from what ive read the cubs are a lil tricky to fly so it just might be me and need someone that has a lot more experience to fly it.

Old 09-14-2010, 08:51 AM
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Default RE: my cub flies a lil funny


Opie99 and all readers,

The Cub is not tricky to fly.

The situation is that many have the kind of problems that Opie is experiencing.
Something is not correct with the geometry of the airplane. It is a matter of findng what the cause is.

It is usually easy to find when we have access to the airplane but only things to look at can be mentioned in a forum. It is also necessary to look at things in specific ways. How to look at certain things is not always easy to explain by writing text.

How to look for main wings warp has been explained nicely but how well to look depends on whoever is doing the looking.

How to check or mesure side thrust and downthrust does not need great accuracy but a closeby figure is needed. We have to assume here that the figures given are appropriate.

Any trim should not exceed 3 to 5 degrees. If 30 degrees are needed to keep the wings level, there sure is a geometrical problem; either warp, incidence angle or area distributon. It can also be airfoil. As an example, if the covering on the top of the wings is not fastened properly it can lift in flight and change the airfoil. It can happen on one wing more than the other creating a serious lift unbalance.

What else can I say trying to help a fellow ? A good understanding of the forces acting on an airplane inflight both static conditonson the ground and dynamic conditons in flight are essential to help find the cause and correct it.

Wishing good luck in finding and correcting the problem(s).

Zor

Old 09-14-2010, 11:59 AM
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Default RE: my cub flies a lil funny

ORIGINAL: opie99

lnewqban,
are you saying i just might need to add more trim to the rudder? from what ive read the cubs are a lil tricky to fly so it just might be me and need someone that has a lot more experience to fly it.

Nope.
The rudder should be OK, since you have not mentioned an evident yaw or crabbing problem.

Finding an experienced pilot to help you identifying the problem would be great.
I agree with Zor's post above, these problems are difficult to be helped from a distance.

Please read this post:

http://www.rcuniverse.com/forum/fb.asp?m=9496357
Old 09-14-2010, 02:22 PM
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Default RE: my cub flies a lil funny

zor and lnewqban, you guys have been alot of help and i know what your saying as far as trying to help from a distance and i thank you for trying. it isnt any easier trying to explain my problem either. lnew that post you postedhas alot of good information and everybody should read that. im heading home now and im going to check a few things and see what i can come up with. check back in later and let you know what i find out
Old 09-14-2010, 04:54 PM
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Default RE: my cub flies a lil funny

im pretty sure i found the problem and i think both sides of my wings have a lil warp to them so now i get to have some fun and try to fix it. i should have known this whole time but i didnt put two and two together and just thought there was something wrong with the ailerons. well the lesson was learned since ive never seen it icant feel to imbarresed by this. now i know what to look for and how to look for it.
Old 09-14-2010, 05:58 PM
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Default RE: my cub flies a lil funny

Takes a big man to admit his mistakes,, You'll learn,, we were all newbies at one point too

good luck
Old 09-14-2010, 11:11 PM
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Default RE: my cub flies a lil funny

thanks, some of the best lessons learned are from mistakes. im a machanic by trade so im a firm believer of that. when it comes to things like this i dont care how much you read, you dont know untill you see it in person. i think my wings warped when i covered it because it looks like they are both warped at the tips. it looks like the leftside wing tip is angled up and the right side looks to be angled a lil down. i think once i fix the wings this thing is going to fly like a dream. thanks guy
Old 09-15-2010, 07:05 AM
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Default RE: my cub flies a lil funny


ORIGINAL: opie99

thanks, some of the best lessons learned are from mistakes. im a machanic by trade so im a firm believer of that. when it comes to things like this i dont care how much you read, you dont know untill you see it in person. i think my wings warped when i covered it because it looks like they are both warped at the tips. it looks like the leftside wing tip is angled up and the right side looks to be angled a lil down. i think once i fix the wings this thing is going to fly like a dream. thanks guy
Thanks opie99 on behalf of all that benefit from your report.

It is rewarding as well to all those who tried to help because it shows their efforts were not in vain.

Let wing warped up meaning some "washin" (higher angle of attack) and the right wing warped down meaning some "washout" (lower angle of attack).

Results are a roll to the right.when the ailerons are neutral.

I assume by guessing that the covering is a heat shrinkable film that logically needs more heat at the wing tips to stretch the film over more compounded curves thus explaining the tip warps.

Reheating the wing tips while twisting the wing tips in slightly opposing direction should remove the warp. Go at it gently till no warp is discernable.

Again thanks for your report that is more beneficial to all then the reasoning and speculations helpers could do.

Best flying enjoyment to you,

Zor.

P.S.: Too many times the recipient of help does not have the decency to come back and let us know if our time spent to assist was well worth the effort..

Old 09-15-2010, 09:51 AM
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Default RE: my cub flies a lil funny


ORIGINAL: Zor
I assume by guessing that the covering is a heat shrinkable film that logically needs more heat at the wing tips to stretch the film over more compounded curves thus explaining the tip warps.

Reheating the wing tips while twisting the wing tips in slightly opposing direction should remove the warp. Go at it gently till no warp is discernable.
Just to add onto Zor's advise,

If the warp is built into the wing, not just the covering, and being held into place by the spar forward sheeting, then just twisting and re-heating might not do the trick 100%,, sure worth a try before preforming surgery though.

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