Experimental Verification of Lift and Drag on a Flight Vehicle
It should be clear from this discussion that predicting the behavior of the flow field and the resulting forces and moments for a complete flight vehicle using analytical or computational models is an involved process. The time may not be far off when this can be accomplished by fully three-dimensional computations on supercomputers. However, even if this were possible, there still would be the need to verify the results by actual testing of the vehicle. Consider that in such testing, many environments must be represented. That is, to fully map out the aerodynamic performance, it is necessary to cover a range of speeds, fluid densities, and possibly even atmospheric temperatures that are compatible with design specifications and intended use of the vehicle.
Several methods are used to verify predictions made by application of analytical and computational models. Those most often used are as follows:
• Wind-tunnel testing using scale models or, when possible, the full-sized vehicle
• Full-scale (or scale-model) flight testing
Testing of models or even the full-scale vehicle in a wind tunnel (i. e., for flight or land vehicles) or a water channel or basin (i. e., for watercraft) is a routine part of the design procedure. The tests provide direct verification of the predictions but usually also introduce an additional set of unknowns. For example, was the test carried out with full static and dynamic similarity to the full-scale prototype? That is, in addition to a precise representation of the geometry, are the Re numbers and Mach-number ranges appropriate? Other similarity parameters such as the Froude number also must be matched in some cases (e. g., testing of boats).
To see how important such questions may be, we examine the sensitivity of the airfoil measurements in Chapter 5 to the test Re numbers. It is clear that this type of testing may provide confidence in our predictions, but it does not completely verify them.
The second method—testing with a full-scale prototype of the vehicle in the intended operational environment—is often considered the only truly reliable verification method. However, many uncertainties still must be addressed in interpreting test results. For example, in flight testing, we must ask questions such as: Were the test instruments properly calibrated? Were atmospheric disturbances such as turbulence present that would influence the test data? Was there a patina of squashed insects layering the leading edges? And so on, ad infinitum.