Equivalent Airspeed (EAS)

Funny story. For years on Rise: TVP we had operated with a constant density value for the surface and then modulated it by a density factor which used the scale height formula.

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The resultant density was then used for the lift and drag equations but also for the Indicated Airspeed (IAS) readout.  The formula is actually for Equivalent Airspeed (EAS) but they are very closely related at sub-sonic velocities.  EAS and Calibrated Airspeed (CAS) are also very close.

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At the surface the density factor is 1.0 which means the modulation from the scale height formula had no effect on the IAS versus the True Airspeed (TAS) value.  As you climbed out of Vieneo would would see an expected decrease in IAS as your TAS was allowed to increase.

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However, if we happened to flight test at 50,000 feet (the basis of the old cruise calculations) we would have noticed that we could never attain our Vc speed.  And since we are starting at 35 km above the surface of Venus as a “soft deck” it was immediately apparent that something was wrong with the IAS. Turns out we weren’t taking the square-root of the density factor! Now all the cruise numbers in the Dropship spreadsheet are making sense.

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