Spreadsheet - Emulating CAS/TAS whizz wheel

[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

[selfin]

Re: Spreadsheet - Emulating CAS/TAS whizz wheel

Try this. You need only modify the values in blue. Alt is pressure alt, CAS M/S is CAS in meters per second, ISA CORR is the ISA temp. correction, P is the Pressure at altitude derived from P0 / (T0/T)^g0/LR where g0 is reference acceleration due to gravity, L is lapse rate (0.0065 K / meter) and R is gas constant (287.053 J / K / kg). T0 is obviously sea level temp, T is your P.ALT temp, 1/d is sea level pressure (ISA!) over computed pressure at crz alt: you'll need to alter this internally from 101325 N / m^2 to whatever value you have on the day. A0 is sea level speed of sound in m/s, A is LSS in m/s for crz alt. Tt is computed TAT. Output TAS is in knots. This sheet gives you values commensurate with the dashed line in this graph - ie, don't use for values where CAS exceeds A0.

[Genghis the Engineer]

Re: Spreadsheet - Emulating CAS/TAS whizz wheel

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

[john_tullamarine]

Re: Spreadsheet - Emulating CAS/TAS whizz wheel

one25six,

Good Heavens .. a chap is away for a bit and the world jumps in to fix the problem.

In addition to the information in the two previous posts, I dug out some old FT spreadsheets I used to use and extracted the relevant bits to suit your needs. As I don't have a convenient means to link the sheet here, I will email it to you. Provided that I haven't made any cell coding errors it should replicate the prayer wheel for subsonic/tropospheric calculations. If you have a play with it and the answers from the prayer wheel don't match up, please send me some examples and I will track down my coding error(s) for you.

You will note that selfin's approach is akin to the physicist's view on life (perhaps he/she is one of those good folk), while Genghis' is the traditional engineer/aerodynamicist/FT approach (an instance of which he is). My sheet is much the same as G's suggestion but lets you play with higher subsonic Mach if you want to amuse yourself over a coffee and daydream a bit as to how your bird might go with an old mil surplus jet strapped to it ....

One note .. be prepared for some minor variations between one method and another .. round-off errors, minor variations in constants, etc., play a part in the final numbers coming out of the sausage machine ...

Do let us know how the FT work pans out in due course .. have fun.

PS .. email didn't work out too well as the previous comms have been via PM .. perhaps you can PM/email me with your email contact details. If you have it somewhere already, my apologies .. but we are all still trying to get up to speed with the new, improved PPRuNe ..

[Genghis the Engineer]

Re: Spreadsheet - Emulating CAS/TAS whizz wheel

Genghis' is the traditional engineer/aerodynamicist/FT approach (an instance of which he is).

G

[john_tullamarine]

Re: Spreadsheet - Emulating CAS/TAS whizz wheel

G,

Love it ... definitely a much talented chap ..

J