![]() Why is this? Because the thickness as a percent of the chord length increases unless the fin thickness get progressively thinner toward the tip of the fin. To compensate for this, you'll have to increase the size of the fin, which defeats the purpose of trying to make the model as small as possible to help reduce both weight and profile drag.Ĭhanging the Airfoil on the fin affects performance too!Īnother problem associated with tapered fin shapes is that the airfoil shape typically changes too. So, because the Reynolds Number at the tip is lower, the tip is less effective at creating lift to restore the rocket to vertical if it should be disturbed. Of tapering, the Reynolds Number is even further reduced - remember that Reynolds Number is a function of the chord length of the fin. On elliptical fins, and on other shapes where the tip is reduced because The most efficient part of the fin is at the tips where the airflow is nice and smooth because it is outside the turbulence caused by air flowing over the nose of the rocket. We will now see that the wrong shape can make the situation even worse. If you look around for data, you will find that the Coefficient of Lift is determined by the airfoil of the fin, not its shape. This makes it highly desirable to have a fin that has a high Coefficient of Lift, so the model quickly restores to the correct flight path when the AOA is still small. This will then start to bring the rocket back to vertical, but now the induced drag really starts to increase as does profile drag because the side of the rocket is exposed to the airflow. 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). 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. Technical Publication #16 What Type of Fin Shape is Best? ![]() This is the transcript taken out of Apogee technical publications #16 which can be viewed here :
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