niche

My Feedback: (3)

Hello
I just flied foir the first time a sailplane with a Fuse on Carbon fiber /glass fiber.
Wings are 100" on foam/wood.
Once wings are pluggled-in span is almost 104".
I noticed when the plane reaches certain speed, it starts shaking and trembling like having ressonance on the wing tips, I get scared and applied breaks so it stops.
The plugg-in system I set is with 1/4" iron rod (2) put in 9/32" Brass tubing, each rod penetrates about 10" on each half wing.
I used to have these wings on a differente fuse (modified riser) and worked OK, in that set the wings spanned shorter since they placed above fuse with rubbers on it, in the new set each half is by fuse's side.

I am very scared with this problem, just a month ago, I downed the sailplane for speeding up and when pulling for recovering, one half wing just exploded and you wonder what happened.
On this new set I continously slow down with butterfly mix.

If I lengthen the plugg-in rods could the problem stop?
Thanks in advance for any help
Regards

BMatthews

Lengthening the rods won't help. The real problem is that the model is probably flying faster than it used to when the wings were on that other fuselage. What you are seeing is flutter in the wings by the sound of it. It's not just control surfaces that can flutter, whole wings can as well if they are flexible enough. In your case it may be the wings are too flexible or it may be because the rod is too flexible. 1/4 rod for a 100 inch span is only suitable for slow flying floater type gliders. As soon as you start flying faster then the rod needs to be larger and the wings stiffer. For your case the rod should be at least 5/16 and preferrably 3/8 diameter.

The simple answer if you do not want to rebuild the wing is to fly slower. All full sized aircraft have a speed that they must not exceed or parts come off. And some models do as well.

BarracudaHockey

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Vne Velocity Never Exceed [8D]

niche

My Feedback: (3)

Thanks all for the help
It sounds reasonable to increase diameter of the rods.
Since I have 2 rods(one behind CG), what I will do is to enlarge diamater for the nosemost rod to 5/16" (maybe 3/8" )so I can take some lead out from the nose and keep the same weight.
I am positive it's going to work..
Regards
Omar

isoar

My Feedback: (1)

Omar,

With all due respect to the others giving advise, I don't think stronger wing rods will help at all. Stronger wing rods may keep the wing center joint from flexing in a high G-force manuver like a sharpe pull up or a strong winch launch. It likely will not do anything to stop the high speed flutter you get when flying in a strait line.

You mentioned you use "butterfly" to slow the plane down. This must mean you have flaps and ailorons. I strongly suspect that the "shaking and trembling like having ressonance on the wing tips" (or flutter as we call it in the USA) is due to your aileron linkage being lose. If you can grasp your aileron and gently move it up and down and feel lost motion or slop, then this should be corrected. It should not be able to move up or down with gental presure more than about 1/16 inch in each direction maximum.

If you do find that you have sloppy control linkage, check the aileron hinge, the aileron control horn mounting, the push rod stiffness, gear slop in the servo, solid mounting of the servo, and a tght fit of your chosen couplers to the holes in the servo arms and control horns. The aileron control horn should not be short or this will amplify any slop elswhere in the system. I make mine about 3/4 inch long as measured from the hinge point.

This wing itself should be very resistant to flutter being wood skins over a foam core. Open construction wings flutter more easily.

I hope this helps!

fainjon

With all do respect isoar or eyesore. Have you ever seen wind tunnel testing of wings with fixed control surfaces. The wings in general are exceeding there designed flight envelope. Sorry i just couldnt help my self.

BMatthews

Most often we do see (or atually HEAR) flutter in the controls. It's a much more common effect. But the short period of oscialltion thanks to the light weight of the controls themselves puts the frequency up in the sound range we here as a BUZZZZZZ rather than something we can see from the ground.

Back in the old Windsong days with balsa sheeted wings with no spars there was this one guy trying to make it back to the field at high speed. His WINGS definetly did flutter. The tips were flipping up and down about 4 to 8 times a second. He managed to pull up and slow down before it twisted them off. I suspect this can happen quite easily with long high aspect ratio wings with soft, light balsa sheeting.

From niche's description it sounds like what he has is the second description more than the first from above. Niche, can you confirm?

However I tend to agree with isoar that the wing rods are unlikely the point of the problem even if they are too thin to properley handle the wing. In the end I think niche is just going to have to learn to fly this older and lighter set of wings at a slower speed or consider adding something to make them stronger. I suspect that what is truly needed is to strip off the old covering and add a layer of 1/2 oz glass cloth with the weave laid diagonally and set it down with epoxy resin or possibly polyurethane varnish and then sanding sealer and stuff. To keep the weight down the resin or varnish could have some coloring added to make it so the resin is also the finish. Obviously this would add some extra weight over the covering but Monokote and similar is not light so the weight gain may not be that much. But it would sure add a lot of warp resisting stiffness to the wings.

isoar

My Feedback: (1)

Not in a wind tunnel, but I've seen many cases of both control surface flutter and true wing flutter in person on model sailplanes. Which type of flutter is he seeing/hearing on his model - I think it's hard to say from the discription. The reason I guessed it was control surface flutter is that a wing of only 100" span and fully sheeted with wood over foam should be quite stiff. A wing of this construction should be good for anything short of long duration vertical dives or DS. I stand by my guess.

isoar

My Feedback: (1)

Yo Niche,

Don't get us all curious and then leave us hanging. Have you found out anything yet? (perhaps you are doing mods now)

Please inform us of your results.

Thanks!