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Dec 23, 2017, 12:09 PM
Ronaldo Nogueira
ronaldopn's Avatar
Thread OP
Help!

Ideas to fix a snap turn to the right


Hi folks, I'd love to have some ideas on how to fix this issue.
My brand new Skyline Raven 30cc is snapping quite violently to the right when puling up on the elevator. I first noticed the snap when flying KE figure eights: during some tighter KE turns the model rolled inverted violently which scared me a lot since I fly very low.

To confirm the behavior I did a couple of straight and level passes, pulled up and noticed the plane rolling to the right. The harder I pull the harder it rolls.

I edited a video with a few of these turns so you can have an idea.

I use a dual servo setup on the elevators and the linkages are quite rigid with minimum slop. Servos are Goteck HC1621SG running on 7.5V a bit overkill for the application.
I always maximize torque/ resolution by using almost all available servo travel (usually above 110% ATV).

The troubleshooting so far:
- Removed my home made SFGs - no difference
- Checked symmetry of elevator throws at different positions - equal travel through the entire range
- Pilot error (applying right aileron when pulling elevator) - definitely no

My hypothesis: I had to trim the ailerons considerably on the maiden flight to make it fly straight and now they are very off centered (see pictures). IMO this means I have a warp or incidence issue on the wings.
I believe an incidence difference between left/ right wings could be causing the snap because the ailerons are trimmed for zero (or close to zero) AOA (level flight). When up elevator is applied the AOA increases for a brief moment and the incidence effect is now more pronounced.

Changing wing incidence will not be easy because the recess on the fuselage is quite snug on the wing, which gives almost no margin to rotate the wings around the wing tube.

Any other ideas/ suggestions will be really appreciated.
BTW, I know this is a cheap ARF kit, definitely not as straight and true as the top brands but I would love to be able to fix it somehow.

Thanks
Ronaldo

Raven snap (0 min 25 sec)
Last edited by ronaldopn; Dec 25, 2017 at 10:19 AM.
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Dec 23, 2017, 12:14 PM
Ronaldo Nogueira
ronaldopn's Avatar
Thread OP
I forgot to check the behavior with down elevator, this could help understand the issue a bit better, will try that next time.
Dec 23, 2017, 01:05 PM
Registered User
Dennis C's Avatar
I would suspect a warp in the wings

Could be one wing does havea different angle of attack

Most probably a design issue. Airfoil shape and wing loading

Measure wing incidence both wings at the root and tip correct if possible

Dennis

Ps not many designs will do what you are asking for
Dec 23, 2017, 03:28 PM
Ronaldo Nogueira
ronaldopn's Avatar
Thread OP
Thanks Dennis, my suspicion too. I decided to check the gap around the wing root and luckily I was wrong, there is sufficient clearance to change the angle of incidence on the wings. I also noticed that the right wing seems to have a positive incidence in relation to the recess on the fuselage. This is in line with how the ailerons had to be trimmed.
I will slightly offset the guide pin holes on the fuselage to rotate the wing slightly. If it at least helps removing aileron trim I will be happy.
Ronaldo
Last edited by ronaldopn; Dec 25, 2017 at 10:19 AM.
Dec 23, 2017, 04:06 PM
Registered User
Dennis C's Avatar
Measure before you change

Set the fuselage so that the wings are level. Measure incidence at root and tip. Root is the base line. If the tips are different then you have a warp

Don't assume that the fuselage recesses are accurate

An incidence meter is easier

Dennis
Dec 24, 2017, 06:55 AM
Big Bushy with a STING (errrr)
Chris SW's Avatar
Happy Xmas Ronaldo.

I agree with Dennis. Check sack panel for a warp. Then check the incidence. Something is causing the need for aileron trim and I would bet you a Xmas pudding that if you can adjust to get rid of the trim she will fly straight and true.

Chris
Dec 24, 2017, 09:45 AM
Registered User
I'm at a loss as to why you would be using enough elevator to cause a snap playing in KE figure 8's? Wondering if you might have a CG issue? If it's tracking toward the canopy or the gear, that's pointing to a CG issue, especially if you're using enogh elevator to cause it to snap. Plane should track through those figure 8's with very little elevator input. Enough to allow for cross winds should be about it. That's my opinion anyway....

From there, if the snap is in the same direction every time, it would not be hard to understand the wing incidence issue. I get how you could pivot the wing a hair by resetting the locating dowel position, but isn't that wing installed in pockets on each side? Is there enough slop between the wing and pocket to be able to twist the win a little within that pocket? You may be forced to "warp" the wings a little to correct your issue. -Al
Last edited by ahicks; Dec 24, 2017 at 09:52 AM.
Dec 24, 2017, 01:56 PM
Ronaldo Nogueira
ronaldopn's Avatar
Thread OP
Happy Christmas to all

I guess I have a winner
Followed my instinct and added a bit of negative incidence to the right wing last night. I don't have an incidence gauge so I just estimated the adjustment so I could test fly it this morning.
Hopefully this would reduce the amount of aileron required for leveled flight and maybe the snap issue. I removed all aileron trim before the test flight.

The improvement was simply amazing, it took only a few clicks of fine trim (still to the right) to make it fly beautifully straight. I also had to add a few clicks of up elevator which was kind of expected since the average incidence of both wings was now lower.
The snap issue almost disappeared and I readjusted my KE mixes to match the new aileron position and obviously the new airframe behavior.

When I got home, since there was still a bit of aileron trim and no margin for adjustment on the right wing (almost touching the fuselage) I decided to do the final adjustment on the left wing. I'll have to test fly again to confirm but I expect the plane to require virtually no aileron trim.

The pictures will help visualizing what was done with the plane.

Ronaldo
Dec 24, 2017, 03:53 PM
Ronaldo Nogueira
ronaldopn's Avatar
Thread OP
Here is one of the flights from this morning after right wing incidence adjustment. I'm using an SJ5000X Elite which is an amazing camera but like all action cameras has a very wide FOV so I recommend full screen mode.

Sorry, not one of my best flights, still playing with mixes and quite nervous to do KE turns very low but already happy with the results of the wing incidence adjustment.
Ronaldo
Skyline Raven 30cc - wing incidence adjustment (5 min 11 sec)
Last edited by ronaldopn; Dec 24, 2017 at 04:00 PM.
Dec 24, 2017, 04:05 PM
Ronaldo Nogueira
ronaldopn's Avatar
Thread OP
Quote:
Originally Posted by Chris SW
Happy Xmas Ronaldo.

I agree with Dennis. Check sack panel for a warp. Then check the incidence. Something is causing the need for aileron trim and I would bet you a Xmas pudding that if you can adjust to get rid of the trim she will fly straight and true.

Chris
Hello my friend, merry Christmas!
Since I don't have an incidence gauge I was not able to check for warps so I went straight to the only thing I could do for the moment which was adjust the incidence. I chose right wing as it was the one which looked farther from centered position in relation to the recess on the fuselage (although I agree with Dennis you cannot rely completely on the precision of this area).
Even if I could detect a warp I don't think I would be able to fix it as the wing is extremely stiff. I would never be able to take the warp out with a heat gun.
Ronaldo
Last edited by ronaldopn; Dec 25, 2017 at 10:22 AM.
Dec 24, 2017, 04:18 PM
Ronaldo Nogueira
ronaldopn's Avatar
Thread OP
Quote:
Originally Posted by ahicks
I'm at a loss as to why you would be using enough elevator to cause a snap playing in KE figure 8's? Wondering if you might have a CG issue? If it's tracking toward the canopy or the gear, that's pointing to a CG issue, especially if you're using enogh elevator to cause it to snap. Plane should track through those figure 8's with very little elevator input. Enough to allow for cross winds should be about it. That's my opinion anyway....

From there, if the snap is in the same direction every time, it would not be hard to understand the wing incidence issue. I get how you could pivot the wing a hair by resetting the locating dowel position, but isn't that wing installed in pockets on each side? Is there enough slop between the wing and pocket to be able to twist the win a little within that pocket? You may be forced to "warp" the wings a little to correct your issue. -Al
Merry Christmas Al
CG looks pretty close to neutral, plane stays in the 45 degree upline forever. It does have some aileron/ elevator coupling and tracks towards the LG without mixes (even with neutral CG).
Yes, snaps were always to the same direction and luckily there was enough clearance with the fuselage to adjust incidence (see post #4).
The wing is extremely stiff, i doubt I could take any warp out with a heat gun.
Ronaldo
Dec 24, 2017, 04:42 PM
Registered User
Merry Christmas!

I can identify with lack of confidence with a new plane. I have an Edge with just a couple of flights on it over the last couple of days (a twin to the one you put in a few weeks back). Yes close, but do I trust it? HECK NO! The only time I'm below 10' is when landing it at this point!

Though an admitted rookie, my thought is that the 45 degree inverted up line test is just a guide regarding CG - to see if/confirm you're close? The real test is whether or not you elevator is trimmed absolutely flat for level flight. Final CG is adjusted to allow that. Elevator up in back indicates a forward CG. Down in back indicates heavy in the rear. The point is there is no way on earth that plane is going to track straight in KE if the elevator isn't absolutely flat. That's my take anyway.

Glad to hear you've cured the aileron trim. Being that for off is pretty unusual with these kits in my experience.

-Al
Dec 25, 2017, 10:12 AM
Ronaldo Nogueira
ronaldopn's Avatar
Thread OP
This morning I went quickly to the field to test the adjustments made on the left wing (yes, I'm insanely addicted ). The aileron trim was completely removed and plane now pulls to a perfectly straight upline!

The incidence adjustment also reduced the elevator mix required on KE.

The Raven is as perfect as I would like for now. It is not a 3D plane according to modern standards (needs larger control surfaces and ailerons which would extend to the wing root) but I'm really enjoying its flying characteristics and I'm sure I'm only scratching its flight envelope so I have tons to learn from it.
Ronaldo
Last edited by ronaldopn; Dec 25, 2017 at 06:40 PM.
Dec 25, 2017, 10:22 AM
Big Bushy with a STING (errrr)
Chris SW's Avatar
Great result Ronaldo.

Donít you just love it when things go as planned.
Dec 25, 2017, 02:56 PM
Registered User
Dennis C's Avatar
You can still check incidence and warping

on a level surface block the tail up until the wing is close to level measure at the root trailing edge and at the center of the leading edge, adjust so that they are both the same measurement. then check the other wing, that will tell you if there is a difference in incidence

check the measurements at the wing tip and that will tell you if you have a warp.

future reference and glad you got it sorted.

Dennis


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