Extent of laminar flow possible with flexible wing skins

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danmoser

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It seems the vast majority - if not all - successful laminar flow airfoil and fuselage surfaces are of rigid construction.
But is it possible to achieve significant laminar flow with flexible materials?
I'm thinking of stretched laminated sailcloth, doped fabric, thin composite sheets without supporting foam core underneath, etc.
Let's assume the roughness and contour accuracy of the flexible skin is adequate for laminar flow .. how much vibration and rippling induced by the passing airflow will cause a turbulent transition?
Higher tension should reduce displacement amplitude, but also increase frequency .. where's the critical transition point?

I have been searching for data on this topic, but to no avail .. this now closed old HBA thread came about the closest: https://www.homebuiltairplanes.com/forums/light-stuff-area/9165-triple-surface-flex-wing-trikes-hangliders-homebuilt.html#post89664

In that thread, Mr. Skeates talked of laminar wind surfer and sailboat sails.. is that true, or marketing hype?
 

BBerson

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I've tried for years to come up with a nice flexible but stiff leading edge material. No successs.
The problem is that any stiff sheet is heavy. A small and stiff sheet of aluminum can form the nose radius but it leaves an edge that trips transition to turbulent. Very difficult to make a smooth transition.
 

WonderousMountain

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From what I've read, 1/3 of the upper surface and 2/3 of the lower surface is reasonably achievable on a moderately cambered profile.
However, turbulent drag is likely much lower than most fabric skins are achieving.
 

autoreply

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Skeates... well...

The only data I have is that the sag in earlier glass gliders (Nimbus 2) with a lot of water caused enough disruption to seriously hamper laminar flow.

Seems ideal for a roof-top wind tunnel?
 

danmoser

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I've tried for years to come up with a nice flexible but stiff leading edge material. No successs.
The problem is that any stiff sheet is heavy. A small and stiff sheet of aluminum can form the nose radius but it leaves an edge that trips transition to turbulent. Very difficult to make a smooth transition.
Modern flexwing HGs & trike wings use a Mylar or composite LE stiffener inserted into a sewn pocket.. problem is, the insert's natural shape is a flat sheet, so there is usually a curvature discontinuity, especially on the bottom surface (I'll take a picture of that to illustrate within a few days).

This curvature discontinuity is bound to cause a turbulent transition, as the stitching on the sides of the LE insert pocket will as well.
But assuming these discontinuities can be fixed by increasing the pocket width and molding the stiffener insert to the proper contour, I'm still wondering what the prospects are for attaining laminar flow over flexible surfaces... IOW, can laminar flow be maintained on a surface that is mildly bouncing up & down like a loose snare drum head?:ponder:
 

Himat

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It all depends on how you think about laminar flow.
And what is considered flexible?
The pictured aircraft might be considered to be flexible and have laminar flow, or?
IndoorModel.jpg
(Picture from the net.)


Anyway, your question just reminded me that there is a lot I don’t know about fluid flow and where the knowledge is not easy attainable.
 

Aerowerx

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I'm not sure this is related to what you are thinking, but...

I read many years ago that Dolphins, besides the streamlined shape, have a flexible skin. This allegedly lowers the drag compared to a stiff skin.

Less drag---less turbulence---less noise. The Navy was trying to duplicate the flexible skin of a Dolphin on their submarines, which would make them quieter and harder to detect.
 

danmoser

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I'm not sure this is related to what you are thinking, but...

I read many years ago that Dolphins, besides the streamlined shape, have a flexible skin. This allegedly lowers the drag compared to a stiff skin.

Less drag---less turbulence---less noise. The Navy was trying to duplicate the flexible skin of a Dolphin on their submarines, which would make them quieter and harder to detect.
.. seems to me a dolphin skin is very different from an aircraft covering, in that it is fully supported by dolphin flesh underneath, and tightened with muscles, blood pressure, etc. .. plus the difference between water & air is enormous.. totally different Reynolds number range.

Perhaps the Navy was trying to duplicate the quiet propulsion of a flexible flipper .. and/or the dolphin's skin texture (?)

How would a flexible skin result in drag reduction?
 

Himat

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.. seems to me a dolphin skin is very different from an aircraft covering, in that it is fully supported by dolphin flesh underneath, and tightened with muscles, blood pressure, etc. .. plus the difference between water & air is enormous.. totally different Reynolds number range.

Perhaps the Navy was trying to duplicate the quiet propulsion of a flexible flipper .. and/or the dolphin's skin texture (?)

How would a flexible skin result in drag reduction?

I do think the main goal of replicating the dolphin skin was noise and drag reduction. No propulsion as far as I remember.
The drag reduction I think was due to the flexible "skin" averaging out the pressure on the surface of the body. But that is what I remember, probably not exactly what I did read:).
 

delta

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Create a negative internal pressure within an unpainted polyester covering (with porosity like Koveral), flow separation might be delayed and have the dimpled advantage of a golf ball. :ponder:
 

danmoser

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Create a negative internal pressure within an unpainted polyester covering (with porosity like Koveral), flow separation might be delayed and have the dimpled advantage of a golf ball. :ponder:
That's inducing turbulence to maintain flow attachment, and it works if done properly.
But it's almost the opposite of what I'm asking about.

I was hoping to find out how far laminar flow could be extended over a flexible skin surface before transitioning turbulent, so as to reduce skin friction drag at higher speeds.
 

Aerowerx

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Probably yes, if that heavy.
I found the picture, but not the spesifications on that plane on the net.
I never built one, but read about them back when I was a teenager and building rubber powered balsa planes (Guillow). In their competitions, they actually used a penny to check the weight. Of course, pennies weighed more back then than they do now.

I don't know what they use now, but I recall that some of them were dipped in some kind of solution, something like making soap bubbles, and then dried to a film to form the wing and tail surfaces. That is another possibility, but don't think it would translate very well to a full size aircraft. Now that I think about it, I seem to recall seeing an original 'Penny Weight' at the Smithsonian Air & Space Museum.
 

delta

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That's inducing turbulence to maintain flow attachment, and it works if done properly.
But it's almost the opposite of what I'm asking about.

I was hoping to find out how far laminar flow could be extended over a flexible skin surface before transitioning turbulent, so as to reduce skin friction drag at higher speeds.
I'm not sure how opposite enters the picture when we're all after the same thing. If a negative pressure was maintained in a flexible wing skin situation, the skin itself would be less flexible and conceivably delay flow separation. With porosity the boundary layer could be virtually sucked into the wing. Utilizing a venturi type system it would work better the faster you go. I found this awhile ago and may be applicable.
Micro and Nano Flows for Engineering: Aerodynamic drag reduction
 

Aircar

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Look up 'compliant surfaces for drag reduction' (or similar) -there is a lot of work published on trying to mimic the skin and underlying blubber resilience of seals, whales, dolphins etc and tests done in flight and on wind tunnel walls (and submarine hulls). It can work as shown by tests -- the Sinha 'de turbulator' concept (controversial) seems to be something along these lines --ie reduction of turbulent skin friction . If you could have a skin that moved aft at the same speed (or greater) than the forward speed then skin friction would be zero --like a conveyor belt moving forward at the same speed as the belt was moving aft (or imagine a recirculating 'klein bottle' with stretchy rubber skin . The concept of a skin that effectively moved aft with a small incipient vortex is similar --like 'spinning the tyres' rather than gaining traction on the body and so pulling it back . It is an old idea yet to be realized in practice.
 

Dana

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I suspect the difficulty in maintaining an accurate shape and sufficiently smooth surface finish on a flexible covering would negate any advantages over metal or composites.

Re the "pennyplanes"... that was a class of indoor models introduced, IIRC, during the 1970s. Intended as a "beginner" class in indoor model competition, they were intended to be heavier (at least as heavy as a penny) and thus more durable (and easier to build) than the existing classes. They were covered with "microfilm", where a drop or two of the plastic solution was put on a bathtub full of water, allowed to spread then lifted off with a wire hoop and placed over the structure. Flying at a slow walking speed, with the prop turning so slowly you could count the revolutions, they almost certainly had laminar flow due to the absurdly low Reynolds number. The open class models were larger and even lighter, sometimes flying for over an hour (remember, this is a rubber band powered model with no control!) Quite a fascinating extreme aerodynamic niche, a technogeek hobby I never got involved in despite working at NAS Lakehurst where the huge blimp hangars were used for the contests.

Dana

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Synergy

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At very low Reynolds numbers laminar flow is the rule, not the exception. Insect wings employ extreme discontinuity in their profile specifically to create turbulence and break laminar suction. (google dragonfly wing airfoil images to see more like this one)

incidence_0_Cp-ee523.jpg
On the OP question, a flexwing hang glider the size of a bat is about right to match to the physics of a flexible membrane. For man-carrying Reynolds numbers, flexible surfaces really make it hard to obtain laminar flow beyond quarter chord. On the other hand, there is typically no benefit to targeting more than that, and lots of downside.

The right semi-laminar airfoils are the best idea in my book, but they do require, as discussed by others, a very tricky means of stiffening, not just the forward portions of the airfoil, but ideally the whole top surface since the result of sectional L/D increase is an improvement at lower AoA, resulting in luffing for flexible foils.

By implication, wing changes are required to respond to new optimums.

This illustrates that it's hard to change one variable and get anything but mediocre results and a headache. MDO software can shed light, but it's garbage-in, garbage-out too. Best to REALLY think about the total goal being pursued.

Dan, I would LOVE to collaborate with you on a Swift killer and or a toy that would make Jetman look caveman. Just looking for the right people to play with...
 

BBerson

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Getting laminar flow to the quarter chord would be fantastic, but difficult. I don't see any way to get laminar past the leading edge tube.
The tube obviously trips and destroys the laminar flow since any transition that can be felt by hand (.003" in 2" or something) will trip laminar flow.
I suppose the nose rib could extend forward ahead of the leading edge tube, so that the fabric doesn't actually touch the tube. Then the shrunk fabric might be laminar except at each rib. Would look very bad.
BB
 

danmoser

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I'm not sure how opposite enters the picture when we're all after the same thing. If a negative pressure was maintained in a flexible wing skin situation, the skin itself would be less flexible and conceivably delay flow separation. With porosity the boundary layer could be virtually sucked into the wing. Utilizing a venturi type system it would work better the faster you go. I found this awhile ago and may be applicable.
Micro and Nano Flows for Engineering: Aerodynamic drag reduction
True, delta.. I didn't mean to suggest that you weren't also trying to achieve lower drag with your scheme .. just that it is a different approach .. "opposite" was a lousy word choice on my part.

For now, I'm just investigating the reduction of skin friction drag on a flexible skin via better contouring and minimal stiffening features .. preferably not having to rely on mechanical devices like suction systems pulling air through zillions of tiny holes.
The advanced active drag reduction systems that feature great multitudes of tiny sensors & mechanisms, like in the link you posted, are pretty interesting .. but that seems like a difficult, expensive route to go with uncertain prospects for success .. leave it to university researchers.. they like stuff like that. ;)
 
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