![]() 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. Based on these, the implementation and the numerical investigation of canted fins to improve the efficiency of the sounding rocket with lesser resources used for testing was the way forward.I'm often asked the question of which fin shape is best for small competition rockets. where they has used a movable canard surface. A good understanding of aerodynamic loads was found in Dongyang et al. The importance of Reynolds averaged Navier-Stokes was understood from Jan Bartl et al. has shown how to tabulate and analyze the CFD results. have presented methods for estimating mathematical models useful for stability and control analysis which can be applied to CFD simulations and wind tunnel experiments. The results obtained were tabulated and graphically represented, and the trends of aerodynamic coefficients like \(C_\) ratio of different fin geometries, and the results can be used for designing aerodynamically efficient fins for roll control. The simulations were performed with the help of \(k-\epsilon \) standard turbulence model. In this paper, three-dimensional, incompressible simulations were performed on different models of sounding rockets using commercial CFD package fluent. Therefore, computational fluid dynamics (CFD) is more efficient than extensive wind tunnel tests. It is crucial to maintain the similarity parameters while testing a scaled-down model in a wind tunnel. Inducing a rolling moment also leads to an increase in the rocket motor’s power consumption due to the rise in drag, so inducing an optimal rolling moment with a minimal increase in drag is a crucial design criterion. Fins with cant angles are generally used to provide a rolling moment in sounding rockets and missiles to minimize the instability. ![]() Missiles and sounding rockets usually deviate from the trajectory due to unstable roll. ![]()
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