posted on Sep, 12 2015 @ 05:48 AM
originally posted by: RogueWave
a reply to: 727Sky
At 37,000+ feet I may be only indicating 200 knots or even 220 but my true airspeed can be 440 or 460 knots across the ground under no wind.
Do you mean this difference is explained by backwind?
No. Wind has no effect on True airspeed. A discussion of airspeed begins with Indicated Airspeed, which is merely a reflection of the amount of air
entering the pitot-static system through the opening in the pitot tube, which is normally pointed more or less straight ahead with reference to the
longitudinal axis of the aircraft. Sometimes the position of the pitot tube can affect the amount of air going in because of, for example, venturi
effect near the pitot location, so the aircraft manual gives a correction to arrive at Calibrated Airspeed. Since the airspeed indicator measures the
mass of air going in the pitot, anything that affects that mass, like density,must be corrected for. So, you must correct for pressure and
temperature. In practice, temp and pressure are most closely related to altitude, so pilots use either a preprogrammed calculator or a slide rule sort
of thing called an E6B or C-12, where you put in indicated or calibrated airspeed and pressure altitude and sometimes temperature, and it gives you
the True Airspeed. In modern jets the aircraft flight computer figures it for you. It gets more complicated when you get into the transonic and
supersonic regions where you have to figure in compressibility and the like.