Originally Posted by one25six
Thanks to John Tullamarine who is already looking at this for me, but I thought I would put it out to the wider audience for help.
I am doing some engine tests on my plane after tweaking some things, and would like a quick way of crunching a whole series numbers to work out my TAS without using the Jepp Whizz wheel each time.
Already I have done a crude worksheet using interpolation figures but they are rough - here is what I want to put in:
1.CAS
2.MSL OAT or OAT at Indicated Altitude (either is good - apply STD ISA deviation where neccessary)
3.QnH
The only result I want is TAS.
At my ferocious speeds there is no further consideration of TAT neccessary!
Has anybody already got a spreadhseet for this? I am having trouble applying the atmospheric equation and density heights to the wonders of Excel.
Thanks,
125.6
(1) Multiply (QNH-1013.25) by 27ft, add that to your altitude, and you have the standard pressure altitude, sHp
(2) Now calculate the ISA relative air density (sigma_prime), which is given by (1-(sHd/145442))^4.255876
(3) Now adjust for temperature variation (from ISA), and calculate sigma, which is sigma_prime * (273.15+OAT)/(288.15-( (h/1000)/1.98))
(4) TAS = CAS / (sigma ^ 0.5).
This is slightly approximate, but would be good enough for virtually all flight test data analysis so long as it's at a temperate latitude (somewhere around 30-60°N or S) not done in a rising or descending air-mass, Mach number stays below about 0.6, and you stay below the Tropopause, which will be around 36,000ft.
G