Control surface effectiveness?
 

Is there a formula or table that can be used to calculate the effect of a control surface?

For example, if I have a trailing edge surface 6 inches long, 2 inches wide and deployed 40 degrees at 20 mph, what is the extra lift (and ideally, drag) generated?

Any advice for this will be very welcome.

Thanks.

Richard

RE: Control surface effectiveness?
 

rgno1,

I think by the lack of replies you can see this is a tough question. For control surfaces or flaps, most model designers figure what they need by simply comparing to other existing aircraft. Basically what these do is change the airfoil. So the new airfoil section shape with surface deployed should be tested and measured in a wind tunnel just like for any other airfoil. Perhaps one of the computer airfoil design simulators can do this? There may be someone out there with some empirical thumb rule formulas for this. But so often those types of things aren't good enough to produce meaningful results. For models, build it and try it is usually easiest way to evaluate.

Hope that helps.

Multiflyer

RE: Control surface effectiveness?
 

The effectiveness of the surface is based on the airfoil section it is connected to. Ailerons, flaps rudders and elevators only act to alter the camber of the airfoil and it is only by looking at how the "new" airfoil works that you can see what the end result is. So there's no way to really say what the effect will be without running some airfoil analysis on the whole wing section. Dr Micheal Selig did this with some of his sections in his wind tunnel research. Similarly you can do it virtually by downloading and purchasing the full license for Profili 2 that comes with Xfoil. Xfoil is an airfoil analysis program that generates lift and drag curves for airfoil coordinates that you can input.

Actually Xfoil is available to download and run for free but it's a complex command line input. Profili 2 provides a nice user interface.

Once you know how the flapped airfoil performs in section you can figure out how much extra lift the portion of the wing with that surface has when the surface deflects.

RE: Control surface effectiveness?
 

Thanks for the replies and explanations. I did not realise this was so complex! I'll try Profili

RGN01

RE: Control surface effectiveness?
 

I have seen basic calculators on the web that help you make estimates. Check the sticky threads above. The publishers of Model Airplane News have a book called the "Basics of R/C Model Aircraft Design" which has 4 or 5 chapters explaining the details of finding your answer. The book has a lot of engineering formulas, but just reading the explanations are informative. Just to indicate how complicated an accurate answer is ... turbulent airflow from fuselage or downwash from another surface can ruin any paper calculations. Since I have very limited skills with mathematical formulas, I prefer to experiment with a model. I built extra large surfaces and cut them down to as flight testing indicated.