Discussion
Consider the following statements about non-dimensional derivatives.
1. They are independent of dynamic pressure at a given Mach number
2. They are independent of the reference area chosen
3. They still vary with incidence and configuration
Which of the statements given above is or are correct?
Answer: D
The first and third hold. Non-dimensionalising removes the dynamic pressure dependence at fixed Mach number, but the coefficients still change with incidence and with flaps, gear and stores. The second is false: a coefficient is defined against a particular reference area and length, and quoting it against a different reference changes its value. That is why data sheets state their reference geometry, and why mixing two sources without checking references is a reliable way to introduce a large error.
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