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 Florian2 November 14, 2011 17:14

Simple Question - or not?

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?

 raj.cfd November 14, 2011 18:12

The airfoil needs to be really thin to obtain maximum lift.

 Florian2 November 15, 2011 05:15

Hi Raj,

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...

 truffaldino November 15, 2011 07:25

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

Truffaldino

 swetkyz November 16, 2011 13:52

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.

 swetkyz November 16, 2011 13:54

P.S. XFOIL, mentioned by Truffaldino, is a great tool!

 truffaldino November 17, 2011 06:32

Quote:
 Originally Posted by swetkyz (Post 332388) 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.

 Florian2 November 17, 2011 15:37

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!!!

 swetkyz November 17, 2011 17:29

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.

 truffaldino November 17, 2011 17:29

Quote:
 Originally Posted by Florian2 (Post 332560) 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

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.

 truffaldino November 17, 2011 17:48

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: