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I am using this Atmospheric Properties Calculator to find the condition a grid fin would encounter when it has a altitude of 86km, and velocity of 7996 km/h. Would this be the total surface area of the part of the grid fin that is encountering the air for the friction coefficient (AKA the underside of the fin)?

And what does it mean by "Temperature Increment"?

Many thanks!

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A reference "length" can't be a surface area, the units are wrong. I suggest using the major dimension of the fin. Example: For an airfoil, it's the chord length.

The introduction to the web page says " An optional temperature increment can be added or subtracted to model a hot or cold day. For example, enter "10" to see the effect of a 10° increase above the standard temperature at the current altitude. " So you can leave it zero.

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    $\begingroup$ To expand on this, a reference length (or quantity) can be literally anything you find convenient for the problem at hand. All that happens is that the specific values of various dimensionless parameters will change. Certain classes of fluid dynamic problems are studied enough that some specific reference values have become customary, e.g., the chord of a wing, the diameter of a cylinder, the planform area of a wing, etc. Choosing something else isn't really wrong, but various flow characteristics will happen at different dimensionless parameters from what you'd expect. $\endgroup$ – Tristan Nov 27 '17 at 15:55

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