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Old   November 14, 2011, 17:14
Default Simple Question - or not?
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Florian Ernst
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Hi,

I am new to cfd and my question to the members of this Forum seems to be pretty simple:
What shape has an airfoil with maximum lift when drag is abolutly not relevant?
I am searching for this shape for a long time and even thougt about building a small windchannel to get an answer.
Perhaps the answer is extremly simple, but perhaps it is an extremly complicated airfoil consisting of many pieces (e.g. a grid of some kind...)

Let me get more specific:
The air foil has to have max. lift.
The angle of incidence does not matter, air speed does not matter (within "normal" ranges like 5 m/s up to 300 m/s).
So is there any Software out there that could help me? Is there a simple answer?
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Old   November 14, 2011, 18:12
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The airfoil needs to be really thin to obtain maximum lift.
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Old   November 15, 2011, 05:15
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Hi Raj,
thanks for your fast answer!

So a thin airfoil (probably at a high angle) gives maximum lift?
How did you find out that fast?

I always thought the answer would be much more tricky...
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Old   November 15, 2011, 07:25
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Thin airfoil is not necessary gives high lift: it could stall at small aoa. The answer is really tricky. I would rather said: strongly cambered airfoil of an optimal thikness (depending on range of Re number) to delay stall.

There is a technique developed by Eppler and Seilig for a rough estimate of airfoil shape with desired characteristics and there is an excellent free software called "x-foil" by Mark Drela from mit (or its version with good GUI called XFLR-5) that has direct and inverse airfoil design tools.

You can see e.g.

http://smartsonde.org/Project/Vehicl...A-2137-802.pdf

http://www.desktop.aero/appliedaero/...lsections.html

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Old   November 16, 2011, 13:52
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I might suggest perusing "Theory of wing sections: including a summary of airfoil data" by Abbott and Doenhoff. This should help you to understand how camber and relative thickness play a role in airfoil characteristics.

To give you an idea of what is possible, it is said that the 737 wing has a maximum lift coefficient of about 3.2--and very low drag at low angles of attack.
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Old   November 16, 2011, 13:54
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P.S. XFOIL, mentioned by Truffaldino, is a great tool!
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Old   November 17, 2011, 06:32
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Quote:
Originally Posted by swetkyz View Post

To give you an idea of what is possible, it is said that the 737 wing has a maximum lift coefficient of about 3.2--and very low drag at low angles of attack.
Yes, with junkers flaps and slats we can reach such Cl's. Unfortunately x-foil analyzes only single-piece airfoil. There is another software by M.Drela (i do not remember the name) which is a multiple foil analog of x-foil, but it is not free (can be sent by M.Drela on request).

With blown/ or bl suction foils one can reach even twice of this.
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Old   November 17, 2011, 15:37
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Hi,

thanks for all answers so far.
@swetkyz: 3.2 sounds a little small; so what would the lift coefficient be when the angle is at its maximum?
@truffaldino: I tried xfoil allready - but as you said it is only possible to calculate single foils.
What you mean with "blown/ or bl suction"? Is this the technique of sucking in air on top of the wing?

I think multi-foil wings will be my choice; probably the give max lift (when calculated properly).

Again THANKS A LOT for your help!!!
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Old   November 17, 2011, 17:29
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You may find "Simulation and Optimization of Flow Control Strategies for Novel High-Lift Configurations" interesting if you are willing to work with slats and flow control. This paper is by Meunier, and it was published in the AIAA Journal in 2009.
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Old   November 17, 2011, 17:29
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Quote:
Originally Posted by Florian2 View Post
Hi,

What you mean with "blown/ or bl suction"? Is this the technique of sucking in air on top of the wing?

Again THANKS A LOT for your help!!!
Yes, this is a technique of sucking the boundary layer on the top to delay separation. Some people say that blowing is even more effective.

You can take a look at a book by Bryan Thwaites on incompressible aerodynamics. He has examples of wing with bl suction up to 30% thick with Cl up to 6 or 9.

As for max Cl without suction devices, Antonov-70 has max Cl=7.2 with slats and flaps only: you can look

http://www.youtube.com/watch?v=PdqMHJJDiFM.


As for software: there is a fast software MSES/MISES by Mark Drela (this is a multiple airfoil x-foil). But it you have to ask M.Drela for acopy.

You could also try Fluent, of course, if you are at large reynolds numbers and flow is almost fully turbulent: Just use S-A turbulence models with multiple airfoils.
In general, when you are at low Reynolds numbers, PDE based CFD software like Fluent cannot handle transition and separation bubbles (in spite of a lot of publicity of new sst transitional models) without a huge user input and comparison with experiments.
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Old   November 17, 2011, 17:48
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I have also found a nice video on youtube on bl succion airfoil used by J.Y. Cousteau as a sail which has 5 to 6 times max Cl of an ordinary sail:

http://www.youtube.com/watch?v=X5E0HDBM4xA
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