The drag coefficient equation will apply to any object if we properly match flow conditions. If they are very different, we do not correctly model the physics of the real problem and will predict an incorrect drag. If the Reynolds number of the experiment and flight are close, then we properly model the effects of the viscous forces relative to the inertial forces.
Aircraft aspect ratio calculator skin#
In our discussions on the sources of drag, recall that skin friction drag depends directly on the viscous interaction of the object and the flow. The important matching parameter for viscosity is the Reynolds number that expresses the ratio of inertial forces to viscous forces. It is even more important to match air viscosity effects. So it is completely incorrect to measure a drag coefficient at some low speed (say 200 mph) and apply that drag coefficient at twice the speed of sound (approximately 1,400 mph, Mach = 2.0). At supersonic speeds, shock waves will be present in the flow field and we must be sure to account for the wave drag in the drag coefficient. At higher speeds, it becomes important to match Mach numbers between the two cases. Mach number is the ratio of the velocity to the speed of sound.
For very low speeds (< 200 mph) the compressibility effects are negligible. Otherwise, the prediction will be inaccurate. The drag coefficient contains not only the complex dependencies of object shape and inclination, but also the effects of air viscosity and compressibility. To correctly use the drag coefficient, we must be sure that the viscosity and compressibility effects are the same between our measured case and the predicted case.
Air Viscosity and Compressibility Effects We can predict the drag that will be produced under a different set of velocity, density (altitude), and area conditions using the drag equation. When reporting drag coefficient values, it is important to specify the reference area that is used to determine the coefficient. As pointed out on the drag equation slide, the choice of reference area (wing area, frontal area, surface area, …) will affect the actual numerical value of the drag coefficient that is calculated. Through division we arrive at a value for the drag coefficient. In a controlled environment (wind tunnel) we can set the velocity, density, and area and measure the drag produced. This equation gives us a way to determine a value for the drag coefficient. The drag coefficient then expresses the ratio of the drag force to the force produced by the dynamic pressure times the area. The quantity one half the density times the velocity squared is called the dynamic pressure q.
The drag coefficient Cd is equal to the drag D divided by the quantity: density r times half the velocity V squared times the reference area A. This equation is simply a rearrangement of the drag equation where we solve for the drag coefficient in terms of the other variables. The drag coefficient is a number that aerodynamicists use to model all of the complex dependencies of shape, inclination, and flow conditions on aircraft drag. Home > Beginners Guide to Aeronautics Drag Coefficient
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