And if your rocket starts to stray from a vertical path, the model will cant much further over before the AOA is high enough to force a larger Coefficient of Lift. So if your rocket is flying slow, and has very small fins, the Reynolds number might be so low that the fin will be very ineffective (because the Coefficient of Lift will be smaller). Therefore, it will be more efficient at creating a restoring force to correct the path of a rocket. You can see from the figure below, that the higher the Reynolds Number, the higher the fins Coefficient of Lift. The Reynolds Number is often used to determine the Coefficient of Lift of the fin at various angle of attacks (AOA). The last two factors are also used with other parameters to determine the Reynolds Number for the rocket. The profile drag force is determined by a number of factors, including the surface finish on the fin, airfoil used, area of the fin, the length of the fin chord, and the speed at which the rocket travels. It is a combination of friction drag and pressure drag. Profile drag on the other hand, is always present. Therefore, it is highly likely that your rocket will have the same induced drag forces no matter what shape fin you use - because typically a model flies straight and true and the induced drag in the rocket is very, very small. Hence, the induced drag on the rocket may be near zero. So if the rocket is flying along nice-and-stable, the fins don't have to create any lift forces to straighten out the flight path of the rocket. Induced drag only occurs when the fin creates lift. There are two types of drag on a rocket induced drag, and profile drag. The reason is buried in the very technical subject about something called the fin's "Reynolds Number." I'll try to describe this without getting too technical, because I want even young modelers to understand this (I've seen too many science fair projects with the subject being 'optimum fin shapes' - which you won't find in my book: 69 Simple Science Fair Projects with Model Rockets: Aeronautics).
AIRFOIL SHAPE ROCKET FINS FULL SIZE
While that may be true for full size airplanes, it may not be necessarily true for small model rockets. Many people have been told that the elliptical fin shape has the lowest induced drag. What I'm about to tell you about this may shock you. I'm often asked the question of which fin shape is best for small competition rockets.
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