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An aeroplane with reciprocating engines is flying at a constant angle of attack, mass and configuration. With increasing altitude the drag...
  • A
    decreases and the CAS decreases too because of the lower air density.
  • B
    increases at constant TAS.
  • C
    remains unchanged but the CAS increases.
  • D
    remains unchanged but the TAS increases.

Refer to figure.
The drag curve is plotted as an EAS, which is the same as CAS, except for the effect of compressibility. At speeds near VMD, compressibility is insignificant, so EAS can be thought of as being the same as IAS or CAS. The drag curve plotted against EAS remains unchanged, with altitude and temperature, for a given angle of attack, mass and configuration.

Also, for a fixed IAS, as the aircraft climbs at higher altitudes, thus less dense air, then the TAS increases progressively.

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