Flutter

[polarbear 1]

Recently, I have noticed a couple of Icarus X-braced canopies (both FX and VX) do something a bit odd. Every once in a while during the dive to landing, the aft section of the canopy between the inboard brake lines will begin to flap around, making a great deal of noise (and probably drag). I imagine this is probably aerodynamic flutter; is this right? Has anybody else seen this?

"Holy s*** that was f***in' cold!"

[freeflir29 0]

Get video!!!!!

[quade 3]

Are you flying the canopies or watching the canopies?

Does it look like the canopies are being front risered at the time?

quade -
The World's Most Boring Skydiver

[ramon 0]

I saw a video where Hook did a high altitude hop n pop on an FX 70 and yes he was doing riser maneuvers front and maybe rear. (His brake lines were probably quite loose so he was not deflecting his tail during front riser maneuvers) and he was wearing a belly cam. He did many riser turns and dives and in several instances was travelling extremely fast through thinner layers of atmosphere.

His tail fluttered a little bit in video..during some of the high speed flying. I am thinking possibly turbulence induced by the low pressure zone behind the wing.

These wings are not perfect and they are flexible.

???Quade??

"Revolution is an abrupt change in the form of misgovernment.", Ambrose Bierce.

[quade 3]

I'm thinkin' that anytime you do a front riser maneuver, all bets are off for the exact aerodynamic qualities of the canopy. Essentially, at that point you're reshaping the wing by various degrees and wacky things can happen depending on how much you're pulling and where you're pulling it.

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???Quade??

Uh, yes? Was there a question about my name?

quade -
The World's Most Boring Skydiver

[polarbear 1]

I'm watching them. Ain't a good enough pilot to get under one of those, yet. Maybe some day.

Yes, they are being front risered, and this seems to happen towards the end of the dive (when the speed is real high). I would definitely agree that when a front riser is pulled down all sorts of funky shit can happen with the distorted airfoil, especially with the higher speeds. I just don't see it happen on other canopies.

"Holy s*** that was f***in' cold!"

[freeflir29 0]

Hmmm...I wonder if they don't need vortex generators like an airplane to control that "funky" air.....*shrug*

[OliShann 0]

In my own humble opinion, what you are describing is not flutter. It is just that under these conditions this part of the wing is a bit loose and subject to strong vortices, therefore inducing the observed vibration.

Flutter is an aerodynamically induced vibration of a wing, tail, or control surface that can result in total structural failure in a matter of seconds, because the vibration is DIVERGENT.

What you observed is not divergent, and the fact that the vibration applies to a non rigid structure of little weight brings up the following question :

Can we talk of flutter at all in the case of a parachute ?

Bye.

[quade 3]

Yes, you are correct in your terminology, but don't expect the same understanding of terms from most skydivers.

quade -
The World's Most Boring Skydiver

[polarbear 1]

You can expect the understanding from me.

That is a good question...a flying parachute would have interesting mass and stiffness properties. I'm not sure what to expect in terms of vibration.

It is interesting to see such a strong reaction to shedding of vortices.

"Holy s*** that was f***in' cold!"

[freeflir29 0]

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It is interesting to see such a strong reaction to shedding of vortices

They can tear wings apart when severe enough. Ever seen the film of a Lear 23 breaking the sound barrier? It's not pretty. Certainly this is on a much smaller scale and I think it has to do with the fact that parachutes have now become efficient enough to produce this sort of problem. Are we going to see little vortex generators on HP parachutes of the future?

[OliShann 0]

One more thing :

The vibration will be divergent when its frenquency hits the natural frequency of the structure.

Therefore our problem reduces to : does the trailing edge of our parachutes have a natural frequency ?

At first view, if it has any it is when inflated, ie flying. Hence the whole thing strongly depends on the speed of the wing. But then again it is non rigid and of so little weight that I doubt we can get a solution for that.

Anyone knows anyone designing canopies, who could light us a bit ?

PS : I may be wrong but I don't think vortex generators have anything to do with flutter. They are designed to insure good energy feeding to control surfaces. The only way to prevent flutter is to change the natural frequency of the structure under study, generally by adding some weight to it.

[polarbear 1]

Well, the parachute and air do have some mass, and the air will have some stiffness. It will be highly nonlinear. I would expect the damping to also be quite strong. I don't think it will "vibrate" in the normal sense.

The shedding of vortices makes the most sense, I think. The movement take4s place in the aft section between the inboard brake lines, where there are no suspension lines. In this section I would expect a low stiffness (i.e. the parachute should deflect large amounts in reaction to a shed vortex).

"Holy s*** that was f***in' cold!"

[OliShann 0]

Yep... sounds right.

I love this topic ;-)

[flyhi 24]

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I may be wrong but I don't think vortex generators have anything to do with flutter. They are designed to insure good energy feeding to control surfaces.

I thought vortex generators (turbulators?) were used to ensure the airflow didn't detach from the surface. Not sure if that is a problem at the speeds we go, although I know a few pro ball players used finger nail files to create "vortex generators" on baseballs traveling at around 90 mph.

Shit happens. And it usually happens because of physics.

[freeflir29 0]

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I may be wrong but I don't think vortex generators have anything to do with flutter

I'm not positive that they would....but I know that they are used to ensure a smoother flow of air over the wing. This increases lift and wing efficiency. With a smoother airflow I would think that might also reduce the forces that cause flutter. I could very well be wrong though. I'm a "couple" credits short of my Aeronautical Engineering Degree.

[OliShann 0]

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I thought vortex generators (turbulators?) were used to ensure the airflow didn't detach from the surface.

Exactly, and by not detaching from the surface, the flow keeps on going over the control surfaces which in turn keep on being effective.

Indeed it is not a problem for us.

[OliShann 0]

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With a smoother airflow I would think that might also reduce the forces that cause flutter.

I wouldn't say so :( ...but still am curious to hear a specialist about that.

[JGarcia 0]

I believe this is the same idea behind the "front air deflectors" option on the Skyflyer Bird-Man suits. According to them, they "[force] the airflow to stay longer on the wing, creating lift, and reducing negative lift that occurs on some occasions at the wing tip area."
Jairo

[roq 0]

I think the problem is the neutral of attack angle in outside cells when in full flight or with front risers input, by long brake lines and a bit more incidence angle in outside cells, for to make less drag in HP canopys. When it happen the outside cells as free and they tremble because the induction generated for the induced vortexes in outside for inside and bottom for top of the canopy.

Roq