is right and you seem to have got it backwards : I've flown five generations of flight instruments, represented by :
- the DC-4
- the Nord 262
- the 747
- the 737
- the Tristar
- the A-320 ,
and their ASIs present the same phenomenon : For a given IAS, the higher you go, the greater the TAS
The compressibility effect is quite negligible at lower Mach numbers and goes up to around 15% of the speed you'd read on the gauge.
The passage from IAS to TAS
is as follows :
-->+ Ki[/] -->CAS--> x [b]Kc
-->x 1/d -->TAS
Ki is the instrument correction - we used to use correction charts for navigation purposes. On the DC-4 / C-54, we still used this correction graphs.
is the corrected AS
Kc is the compressibility correction and Kc = 1/.2 x M square ( approximated St Venant's law )
d is the air density at a given altitude.
is the Equivalent AS
, only used for aerodynamics purposes
As all these factors add or increase the progression, IAS
becomes smaller than TAS
the higher one flies.
[Edited 2007-06-15 20:10:40]