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flutter in aileron and flaps - 8/12/2006 9:37:04 PM   
flaphoto


 

Posts: 68
Joined: 6/17/2006
From: Fenton, MI, USA
Status: offline 		Flew my first flights today. Had two experienced pilots with me. The new bird flew great but had a flutter that only appeared to occur out of a shallow dive, not high speed and not under power. The other pilots said a couple of things about the linkage, but it appears to be slop free. One of the pilots pointed out that the servos did not appear to be locked in, in other words, they had play around neutral center when they should have been rock solid stiff. Ther servos are Hitec HS 512mg. No other problems, flew well and didn't even need to trim it. But, this is my first fixed wing and I could have overlooked something newbe-ish, so veterans, please help me out.
Attached are photos in case that helps.

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       Post #: 1

RE: flutter in aileron and flaps - 8/12/2006 11:26:53 PM   
flaphoto


 

Posts: 68
Joined: 6/17/2006
From: Fenton, MI, USA
Status: offline 		are my servo horns too long? Should I use the metal ones that came with the servo instead of the plasic ones?

Thought of additional detail that may or may not have contributed. I had set up the radio so the flight modes (launch, cruise, land) would only operate the prop in launch position. And, when in launch position, the entire trailing edge was a a few millimeters down (both aileron and flap). I was flying around in this mode (launch with flaps and aileron slightly down) because I didn't want to have to start flipping switches if I suddenly felt I needed to power out of trouble. So, when it fluttered coming out of shallow dive, prop not powered, the aileron and flaps were still down a few millimeters neutral. Could this contribute to the flutter I experienced?

(in reply to flaphoto)
       Post #: 2

RE: flutter in aileron and flaps - 8/12/2006 11:42:01 PM   
da Rock



Posts: 6968
Joined: 10/11/2005
From: western, NC, USA
Status: offline 		Flutter most often comes from slop in the linkages. Way more often than from anything else. I've only run into a few servos that were sloppy around center, but that'd be the very first thing I'd fix and I'd fix it immediately. Are those servos new? If they're used, I'd swap them out in a heartbeat. Matter of fact, I'd quit reading this post right now and go do it. Right now.

There is something that should be mentioned here. Servo's usually draw the least current at center. And they also draw the least when they're under the least aerodynamic load. When your TX tells them to move that few millimeters down, they're no longer sitting at that sloppy center and probably not under the least aero load. Or maybe their gears are in an even sloppier orientation inside. Who knows...........

(in reply to flaphoto)
       Post #: 3

RE: flutter in aileron and flaps - 8/12/2006 11:49:08 PM   
da Rock



Posts: 6968
Joined: 10/11/2005
From: western, NC, USA
Status: offline 		I'd like to wax eloquent about a couple of things in your pictures.

I noticed that you have threaded ends on both ends of the connecting rods. Both clevis are threaded and free to move right?

Neither clevis can unscrew/screw because they're trapped by the horns BUT........ the rod can do whatever it wants, and just might. I landed a big 4Star the other day with the connecting rod on one aileron dangling in the breeze. So was the aileron. The rod had screwed itself deeper into one clevis and unscrewed from the other. The plane's owner had forgotten to tighten the lock nuts on the rod. The other aileron connector was almost out as well. Wonder what it'd flown like with both ailerons unhooked?

If I were you, I'd epoxy one clevis to it's rod. You've got way more than enough adjustment on one end.

Just a suggestion.

(in reply to da Rock)
       Post #: 4

RE: flutter in aileron and flaps - 8/12/2006 11:56:48 PM   
da Rock



Posts: 6968
Joined: 10/11/2005
From: western, NC, USA
Status: offline
quote:

are my servo horns too long? Should I use the metal ones that came with the servo instead of the plasic ones?


Wellllllllllllllllllll

About your horns......

A couple of suggestions:
It is always worthwhile to have the servo arm connection closer to the servo and the connection on the horn on the surface farther from the hingeline. The idea is to give the servo the mechanical advantage. It's also worthwhile to give the surface less mechanical advantage. The way you have your connections, the servo has the least advantage to move a horn that is going to have even more advantage resisting that servo.

I would suggest that the leverages that're present will contribute to the problem you're having. They will also help the aileron flutter beat the servo's gears to death.

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       Post #: 5

RE: flutter in aileron and flaps - 8/13/2006 12:05:43 AM   
da Rock



Posts: 6968
Joined: 10/11/2005
From: western, NC, USA
Status: offline 		It looks like you've made excellent choices of hardware. The aileron horns look especially good. They'd be even better if they were longer, but what the heck.

You ask about using metal servo arms. No real advantage since that application really doesn't need that much strength.

quote:

and flaps were still down a few millimeters neutral. Could this contribute to the flutter I experienced?

Actually, yes, but maybe in a surprising way. Every aerodynamic structure will have a specific flutter speed, even a bowling ball. The "airfoil" that resulted from the deflection could have been in "flutter speed range" at the speed you saw the flutter, whereas a "straight" orientation could have been outside that speed.

BTW. Every surface has a flutter speed. But sloppy linkages introduce new and wider flutter speed ranges for those surfaces.

(in reply to da Rock)
       Post #: 6

RE: flutter in aileron and flaps - 8/13/2006 12:23:16 AM   
flaphoto


 

Posts: 68
Joined: 6/17/2006
From: Fenton, MI, USA
Status: offline 		in answer to your question, all the servos are brand new. If I understand you correctly, I should move the linkage on the servo arm inward so the servo arm is effectively shorter? I don't think there is much I can do with my horn on the control surface as I cut them down from their original length....I could always get new ones. Someone I just spoke to on the phone suggested I might be seeing so flexing of the servo mounting as the wing is of the molded variety. I had read that before, but it seems very stiff and has some sore of mesh prebuilt in it to which I epoxied a servo mount frame. I think I could try to remove the 2-3 millimeter of down aileron and flap I was flying around with. Maybe if the wing controls were more neutral the moderate dives that were resulting in flutter would not occur.

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       Post #: 7

RE: flutter in aileron and flaps - 8/13/2006 12:54:12 AM   
Tall Paul



Posts: 4693
Joined: 6/23/2002
From: Palmdale, CA, USA
Status: offline 		The basic hardware looks to be up to the task...
Get rid of the dual clevises.
You only need adjustment at one end.
The connection to the surface. What is that? How securely is it installed?
I've had something like that flutter the wing so badly the slip-on nose cone on the glider came off in flight!
You mention it's a moldie.. the whole servo as mentioned could be moving with the skin.
Pack some stuff around the servo.

_____________________________

Sparky Paul
http://www.angelfire.com/indie/aerostuff

(in reply to flaphoto)
       Post #: 8

RE: flutter in aileron and flaps - 8/13/2006 2:03:41 AM   
flaphoto


 

Posts: 68
Joined: 6/17/2006
From: Fenton, MI, USA
Status: offline 		Tall Paul,
As to getting rid of the dual clevises, can I get away with just soldering one end so it can't rotate? If not, how do I get rid of a clevis...
The connection to the control surface was provided by ZB from Esprit Models. I don't know the proper terminology (combination of being a complete fixed wing novice and no instructions) but ZB supplied something that looked like an aluminium cylinder (almost like a big bicycle spoke nipple). It was too long to install as one piece in the control surface, so I cut in half and used half for the aileron, half for the flap. If you look closely at the photo, you can see one side (aileron) has a flange and the other had none. They were installed by dremeling out a hole and then I made the inside of the hole even wider than the hole in the surface. I filled the hole with JB weld and inserted the fitting. They are all very solid with no play.

Regarding some 'stuff' around the servo...that is another term I am not familier with...some sort of epoxy? Would a very thin piece of ply accompish the same thing?

Could the servos be under powered and not holding? The UBEC I got puts out 5.03 volts, but the hitec digital servos are rated up to 6 v.

(in reply to Tall Paul)
       Post #: 9

RE: flutter in aileron and flaps - 8/13/2006 2:19:30 AM   
Tall Paul



Posts: 4693
Joined: 6/23/2002
From: Palmdale, CA, USA
Status: offline 		I use a Z-bend or a plain 90 degree bend with a wheel keeper, at the servo end on most of my planes.
The yellow is my Stick-It V.. and the other my Somethin' Extra.
Neither flutter, and the pushrods can't rotate.
With your surface connections JBWelded in place, that should be good enough.
The "stuff" around the servo would be balsa.
Or if nothing else works, pull the servos, mount them on a large piece of plywood, and then reinstall in the wings.


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_____________________________

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       Post #: 10

RE: flutter in aileron and flaps - 8/13/2006 2:39:58 AM   
da Rock



Posts: 6968
Joined: 10/11/2005
From: western, NC, USA
Status: offline
quote:

If I understand you correctly, I should move the linkage on the servo arm inward so the servo arm is effectively shorter?


Yes, but that's not even half of it.

You always want the connection to the servo arm "shorter" and the connection to the surface's horn "longer". And you always want your computer radio to have 100% throw on every servo, no more, no less. That's the most efficient, effective situation for the servo. If the connection on the servo arm is as close in as possible, you'll get whatever power the servo can deliver, and it'll deliver it with the most accuracy. If you have that close connection, the next problem is simply to set the connection on the surface's horn so that full movement of your TX stick gives the recommended surface deflection.

With your existing components you will probably have to swap out that aileron horn to be able to shorten the servo arm connection because that short aileron horn is going TO MOVE THAT AILERON a lot being that short. And if you shorten the servo arm connection, the pushrod movement might not be long enough. There is no rule that says you have to use only the stuff that came in the kit/ARF box.

As you've set up the connections, you're using a fairly weak servo and then putting it at every disadvantage.

If it was my airplane, I'd replace the aileron horns with longer ones. What do they cost? a couple of dollars? Then I'd reconnect the pushrods on the servo arm closer to the servo. Then find out where to make the connection on the aileron horn to get adequate deflection.

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       Post #: 11

RE: flutter in aileron and flaps - 8/13/2006 2:44:16 AM   
da Rock



Posts: 6968
Joined: 10/11/2005
From: western, NC, USA
Status: offline
quote:

As to getting rid of the dual clevises, can I get away with just soldering one end so it can't rotate? If not, how do I get rid of a clevis...


You don't have to get rid of dual clevises. I always use clevises on both ends on purpose. The force transfer between a horn and clevis is about as perfect and efficient as it gets. I either solder or epoxy the fixed one. Both solder and epoxy work great. And clevises work excellently well.

(in reply to da Rock)
       Post #: 12

RE: flutter in aileron and flaps - 8/13/2006 2:50:43 AM   
da Rock



Posts: 6968
Joined: 10/11/2005
From: western, NC, USA
Status: offline
quote:

Regarding some 'stuff' around the servo...


Actually, the suggestion is basically that if the servo isn't securely mounted, it's sensible to do something to secure them better.

It's really simple to see if you have that problem. Grab the aileron and hold it so it can't deflect. Move your aileron stick on the TX. This will not only show you if the servo is solid, but it'll also let you feel if there is any slop and where it is. You can feel or see if the clevises are loose on the rod, in the horns, or if the servo gears are not tight or whatever...........

If all the connections and gears are tight, and the servo flexes, check out why. Are they taped down? tape giving? Or is what they're attached to flexing? If so......... stiffen the structure around the servo..... etc etc

edit..... oh wait, I just looked at your pictures again and noticed the servos are screwed down. whatever.........

< Message edited by darock -- 8/13/2006 3:04:12 AM >

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       Post #: 13

RE: flutter in aileron and flaps - 8/13/2006 2:55:07 AM   
da Rock



Posts: 6968
Joined: 10/11/2005
From: western, NC, USA
Status: offline 		BTW, we should have suggest to you that you do the hands-on test first thing. It's what we would have done first thing.... chuckle.....

And how strong are those servos?

quote:

I think I could try to remove the 2-3 millimeter of down aileron and flap I was flying around with. Maybe if the wing controls were more neutral the moderate dives that were resulting in flutter would not occur.


Not a good idea.
The structure has some weakness and has only hinted what that can do. It has shown you there is a problem. There is a problem. You've been given a second chance. If you're trying simply to save yourself some work, it's not going to be much work at all. And shouldn't cost much either. Depending on what you find out with that hands-on test.

< Message edited by darock -- 8/13/2006 3:01:38 AM >

(in reply to da Rock)
       Post #: 14

RE: flutter in aileron and flaps - 8/13/2006 3:21:23 AM