Understanding Wing Technology

Doug

Doug, I do not think, by far I prefer to read Tom Speer's comments, and advise you to do so, religiously!!

And If you have not done it yet, you will find here on BoatDesign a thread about "sail theory" with great academic comments, most of them by Tom Speer, but also Mark Drela and Leo Lazauskas,,and other ..

You will find all the relevant references including A.O. Smith, Munk and others .. which are the fondations for hight lift and other workpapers for low Reynolds, drag analysis, all the conceptual toolbox is there.

You will find also on Sailing Anarchy a great thread "Fred is in so much trouble".

These 2 threads are nearly a full time job for a few days, but so much insightfull

I took time to mention these references, mostly to alleviate the holy duty of our prefered Fluid mechanic gurus who kindly disseminate some light in our brains with such a patience, that sometime I feel a bit guilty to see them taking time to repeat the same academic explanation for another wing-newbie like me.

Thank Mr Speer for your comment and happy new year.

Regards

Erwan

 

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Oh I do as well as a cursory reading of this thread would illustrate....

 

Wing pressure distribution

Mr Speer,

"The peak local speed on the wing main element can be lower..."

does it mean the pressure distribution on the lee side of the main element is more "square" or rectangular than on similar chord-lengh of the soft sail ?

Regards

Erwan

 

Exactly. The interference between the flap and the main element results in a lot of aft loading on the main element. In many ways, that is a principal objective of a slotted flap section.

 

Here is an example:

 

Understanding Wing Technology----Moth Wing

See here, post #1243 for an excellent video on the Moth wing and control system: http://www.boatdesign.net/forums/sailboats/moth-foils-30-7-knots-35-3-mph-11209-83.html#post433018

 

Rectangular lift distribution and transition location ?

Thank you Doug for the Moth links, interesting comment from smart guys.

Thank you Mr Speer for you patient pedagogy, and your last comments spars a question I have been hesitating to post because it is probably a bit "Not Relevant" but I'd prefer to be sure.

The assumption is the Reynolds regime is < 500 000:

In these condition, considering the influence of the flap on the lift distribution and on V/V0, can we conclude that:

1-because the flap flattens the lift distribution, decreasing LE velocities, the velocity gradient in the BL is smaller, therefore everything else equal, the transition point is likely to move aft ?
(When compared to the same wing section, same AoA, but without the flap effect on the 1st elet.).

2- And if the former question is relevant, so it sparks another question:
What happens for the laminar bubble which is likely to occur at this Reynolds regime ?

If these question are not relevant, don't waste your time writing long answer, just post : N/R, I will go back to do some FD revisions.

Best regards to all

Erwan

 

Your questions are very relevant. Here are the transition points that go with the figure above:

Note that the chordwise distances are relative to the leading edge of the element, so where you see transition at 50% chord on the flap, that is at the flap trailing edge (fully laminar for that surface).

At the lowest angle of attack shown, transition on the main element is at 20% of total chord, or 40% of the main element chord, coincidentally the same for both surfaces. As the angle of attack increases, the pressure gradient on the leeward side becomes progressively more adverse and transition moves forward. On the windward side, transition moves aft for the same reason. The pressures on the flap are not as significantly affected by angle of attack, so transition on the leeward side of the flap stays fixed in position just behind the flap leading edge.

Transition in the Re<500,000 regime is via laminar separation & turbulent reattachment, forming a laminar separation bubble. As the leading edge pressure peak forms, the adverse pressure gradient on the back side of the peak results in laminar separation and the transition point moves forward with angle of attack.

The design of a multi-element section should use the same design philosophy as for a single element section, shaping the pressure distribution so the transition point moves smoothly forward as the angle of attack increases. A flat rooftop pressure distribution that carries its rooftop right to the leading edge will form a very sharp pressure peak when operated above the design lift, and this risks laminar separation without reattachment, resulting in an abrupt leading edge stall. When you shape the pressure distribution so the transition point moves smoothly, you are avoiding this problem.

 

Understanding Wing Technology-AC45

Pictures of the first AC45 wing: http://www.boatdesign.net/forums/multihulls/34th-americas-cup-multihulls-34612-4.html#post435452

 

Did you miss the web cam of Core finishing the wing

 

I placed a question in another thread about building a wing for a model boat:

http://www.boatdesign.net/forums/materials/layup-polystyrene-36422.html

Maybe some of you could help with it. Thanks!

 

Very interesting-thanks to "amati" on SA(that would be Paul Scott on boatdesign):

http://www.pa.uky.edu/~rplebeau/cfd/Morphing_wing.html

 

Understanding Wing Technology-covering

If you're building a wing and want to cover it this is what you can use( from post 31 of this thread):


The film used is 50 micron heat shrink, the same used for sealing CD's in stores.

Steve Clark: The stuff we have been using since 1995 is DuPont Clysar. This is a packaging shrink film that you find around Cd's and Video cassettes. It is pretty cheap and readily available off the Internet. We have also used 3M storm window kits when in a pinch. These come with their ow supply of double sided tape, but because the pieces are not all that big you have top count on overlaps at the ribs to cover a big wing. But, you should know that the Stars and Stripes 88 wing was covered with this stuff. We have never been able to determine that one film was any better than another film. So until we know that, I would say any heat shrink will work.

 

Understanding Wing Technology-Steve Clarks Open Wing Study Plans

Great-and generous-effort from Steve Clark:

 

A first step for XFOIL inverse design

Sorry Dough for posting episodically, But it is difficult to post relevant comments or questions which will not downgrade the high level of these topics.

Thank you Mr Speer for your explanations on transition. They have prompted me to re-read the well known workpapers on high lift.

I guess, managing transition with an inverse design method for a wing section, means prescribing a pressure distribution which rooftop exhibits a little positive slope (the favorable gradient) instead of being horizontal ?
(the first step for XFOIL inverse design)

A.O.Smith workpaper does not address transition in depth, instead it is more focused on separation.

But when looking at the Liebeck wing sections with laminar rooftop, I became a bit confused with regards to transition:

According to the hight Reynolds numbers, I don't understand how natural transition has not occured well before the start of the pressure recovery.

In the case of turbulent rooftop, I guess there are 2 transition points:
1-The first from laminar to turbulent BL, after the leading edge (not mentionned)
2- the second transition point, (named "Michel transition point) as mentionned on the grahics, which seems to be at the start of the recovery, and which possibly triggers the recovery ??

The only assumption I can have is that it is a turbulent separation followed by a turbulent reattachment ? did I guess well ?

Also, for these Liebeck wing sections I wonder if this inflexion on the leeward side is not the "bed" for a turbulent bubble, and allow a kind of "Off the surface recovery" on the chord distance corresponding to this bubble lenght ?

Beside transition issue, A O Smith mentionned:

"From general airfoil theory we know that at the trailing edge (V/Vo)2=0.8

When looking at the velocity distribution of your combination (S 901F + NACA 012) I observe that the (V/Vo)=1 at the TE and therefore if squared it is still equal to 1.

Can we concluded, reading your graphic that the dumping velocity is increased by 25%, and the lift as well ?

Thanks and best regards

Erwan