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XyZspineZyX
09-08-2003, 10:09 AM
I've noticed quite a few folks making comments about one airplane or another's maximum airspeed being off one way or the other - usually by a relatively small margin. I'm wondering if the difference isn't that of True vs Indicated airspeeds. I'm pretty sure that any specs you read regarding the performance of an aircraft will be in True Airspeed, and so when you are trying to compare the performance of your particular ride, make sure that you are in the "no cockpit" mode in order to get True a/s. The clock in the cockhouse is giving you Indicated a/s.

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XyZspineZyX
09-08-2003, 10:09 AM
I've noticed quite a few folks making comments about one airplane or another's maximum airspeed being off one way or the other - usually by a relatively small margin. I'm wondering if the difference isn't that of True vs Indicated airspeeds. I'm pretty sure that any specs you read regarding the performance of an aircraft will be in True Airspeed, and so when you are trying to compare the performance of your particular ride, make sure that you are in the "no cockpit" mode in order to get True a/s. The clock in the cockhouse is giving you Indicated a/s.

"Those people in aviation, who think they know it all, are particularly annoying to those of us who really do" /i/smilies/16x16_smiley-happy.gif

XyZspineZyX
09-08-2003, 10:25 AM
Airspeed is meausured by an instrument on the outside of the a/c called a pitot tube.
Basically, the amount of air molecules going down the tube is displayed on your airspeed indicator.Therefore the faster you go, the more molecules are rammed down the tube.
At a higher altitude, the air is less dense.At 30,000 ft there may be half as many air molecules. So if you are indicating 250 kts at sea level, your IAS and TAS will be the same. If you suddenly 'warped' the a/c to 30,000 ft it's indicated speed might read only 125 kts ,because of the less dense air, although it's speed through air might be roughly the same. In theory to get the same IAS at that altitude, you would have to fly the a/c twice as fast !!
In short, the higher you go from sea level the greater the difference between IAS and TAS.

Concorde, flying at mach 2 at 60,000 ft where the air is very thin, might only have an indicated speed of something ridiculous like 125 kts !! that's why at altitude you use mach meters to measure speed.

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Message Edited on 09/08/03â 09:31AM by BlackAdder_

Message Edited on 09/08/0309:35AM by BlackAdder_

XyZspineZyX
09-08-2003, 12:28 PM
Except in the 262.

Above 400kph, the ASI should indicate the same speed as in the 'box' since it had an altitude compensated ASI. ie. TAS = IAS

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