Foil Drag

[Panos_na]

I have the book ''Theory of wing sections'', which includes many diagrams for NACA foils.

These diagrams are a result of experiments and are very helpful.

I have a question.

The Cd (drag coefficient) includes both form and friction drag, or only the form drag?

[Guest625101138]

Originally Posted by Panos_na

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The Cd (drag coefficient) includes both form and friction drag, or only the form drag?

Cd includes viscous drag and drag from lift. It does not normally include induced drag or, if you are looking at water near surface, wave drag. For a water foil there is also strut drag.

There is a nice little Java applet called JavaFoil that will give you the polar curves and a whole lot of other data for foils. You can set different aspect ratios in the options view so it will show the impact of aspect ratio i.e. induced drag.

There are formula for wave drag but I do not know where I got them from. At about two chords deep you can probably neglect wave drag but then the deeper you go the more strut drag so it is a compromise.

If you have an application in mind I can probably give you a starting point.

Rick W.

[tspeer]

Originally Posted by Panos_na

...The Cd (drag coefficient) includes both form and friction drag, or only the form drag?

Both form and friction. But not induced drag - that has to be added, depending on your span and planform shape.

[Panos_na]

Thank you very very much!!!

[Petros]

Actually the "drag polars" in the back of the book include induced drag as well as form and skin friction drag. but you have to determine what Cl (or angle of attack) it will be operating at, that is why the Cd goes up with increasing lift. Then you have to adjust the 2-D drag for your planform shape and Rn number you will be operating in to get 3-D drag estimate. It is covered in the chapters of the book if I recall. Exact drag predictions are not usually possible, but you can get good comparisons, and you can get close enough to make it work.

I have "Theory of Wing Sections" and I have used it many times in my own design studies. I used to work in aerodynamics in the aircraft business, if you need any specific help I will be happy to see if I can assist you if you need it.