PPRuNe Forums - View Single Post - IAS /MACH and TAS with increasing altitude
Old 16th Oct 2012, 17:46
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Jetpipe.
 
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z.khalid,

The subject has been brought up several times, it's much easier if you just search the forum!

The basic LCC formula is

LCC=√γRT

Where γ=1.4 the adiabatic index (think of it as a constant), R=286.9 J·mol−1·K−1 specific gas constant for dry air, T= temperature (kelvin).
This is why LCC is only dependent on T change as both γ and R are constant. (Note that the outcome will be in m/sec!)

I can remember that IAS decreasing with unceasing altitude, however I cannot remember why.
Are you sure you remember correct? How about climbing with constant EAS?

Briefly, at low speeds EAS≈IAS. As speed is increased so is compr. error making EAS<IAS because IAS=EAS+compr.error

For ex. Climbing with constant IAS: (Assuming CAS=IAS)
As altitude is increased, the density drops. Because of IAS being a measure of pressure on the wings (pitot-tube), as altitude is increased the air gets thiner. To counteract that, the aircraft must fly faster to keep a constant Lift. Thats why TAS increases. The increasing TAS though, compresses the air in the pitot more and more, causing an increasing error in the IAS reading. So, EAS + Compr.Error = IAS. Remember, since we are keeping constant IAS and the Compr.Error increases, the EAS must decrease.

Now if you are climbing with constant EAS all other speeds will be increasing!
EAS<IAS<TAS<M

IAS & TAS for the same reason we discussed above and M because of LSS decreasing and TAS increasing as previous posters mentioned!
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