Scale airfoils

[Lip84]

If you scale down an airfoil will it still function properly?? It would be identical to the full sized real airfoil just like 1/4 ro 1/8th the size.

[slipstick]

It will function, but not exactly like it does at full size. Unfortunately you can't scale down the air molecules that are making it work.

But there are still thousands of scale models which actually DO use the full size airfoil.

Steve

[smokejohnson]

Here is some good reading on airfoils.
http://www.airfieldmodels.com/information_source/math_and_science_of_model_aircraft/rc_aircraft_design/plotting_airfoils/about_airfoils.htm

[Lip84]

That website has some absolutely fabulous things to say. I will rely heavily on it for some upcoming scratch projects I have planned.

Thanks Steve!

[adamdb]

Typically (and there are exceptions) full scale airfoils don't work for models due to differences in the Reynolds numbers involved. Reynolds numbers are a measurement of how a fluid flows past an object. Full-scale aircraft operate in Reynolds numbers in the thousands, our models typically a few hundred or less. Look it up on Google and be prepared to spend a while reading. :)

Find a similar airfoil that is known to work well at low Reynolds numbers (i.e. model aircraft) and use it instead of trying to make a full scale airfoil work.
Adam

[fdix]

Full-scale aircraft operate in Reynolds numbers in the thousands, our models typically a few hundred or less.

Model aircraft are generally in the hundred thousands, full scale aircraft well into the millions

A good approximation for air

Re = v * l * 70000

Where v is speed in m/s and l is chord length in m

For example v = 15 m/s, l = 0,2m would give Re = 210000

[adamdb]

That'll larn me to try and talk about something technical from memory.

Thanks for the clarification!

[Lip84]

Good info guys. I think I know where to start for designing the airfoil for a custom built. One more question, and I am sure I will probly be able to figure this out from google but....The plane I am building is a racing plane from the 1930's (the one in my avatar). Any tips on potential airfoils or what I should go for? I know that it needs to be fairly thin and the scale wingspan is fairly narrow. The original wingspan is only 16 feet while the length is 21.5 feet.

Thanks again,
Lip84

[fdix]

That'll larn me to try and talk about something technical from memory.

Thanks for the clarification!

;-)

Lip84

that depends on a lot of factors... What scale are you building? What wing area do you have? How fast do you want this thing to go? What AUW? And so on...
A good all round airfoil would be a MH32 once used on F5D pylon racers but also on F3B or F3J gliders, very good all round airfoil.
For Re > 150k it works fine, responds well to flaps also.