TAS & IAS

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decaff_42
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TAS & IAS

Post by decaff_42 » Sun Aug 30, 2015 8:06 pm

In my investigations I have begun to wonder how TAS/IAS is calculated in YSFlight. Does anyone know how this is done? There are many 'rules of thumb' and equations out there, but none of the ones I have tried have worked out.
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Re: TAS & IAS

Post by waspe414 » Mon Aug 31, 2015 4:18 am

I'll take some data points and see if I can reconstruct it.

-

Image

The first differences are surprisingly clean, and show no change in the difference with airspeed (dV is constant for a given altitude):
https://i.imgur.com/721tjKo.png

This means that if the equation for TAS is known, IAS is something simple along the lines of TAS - x*altitude.

As for what the equation for TAS is, it's past my bedtime.

EDIT:
It occurs to me that the equation for TAS does not exist. There are many equations for changes in airspeed, but TAS is just a number.

The equation that fits the data I took is [graph]
TAS = IAS + Altitude/3000 + (Altitude^1.3)/15500 + 1.7^(Altitude/6400) -1

Which is close enough to TAS = IAS + (A^n)/x + b^(A/c)

I'll take some more data the other way around and see if I can get a similarly good fit for more accurate data.

-----

Still taking more data, but I switched the curve fitting criteria to delta-V (TAS-IAS) to a ratio (TAS/IAS), and have a much better fit:
TAS = IAS * (1 + 1.4e-5*ALT + 6e-11*ALT^2 + 6e-15*ALT^3)
Still using old data. I expect the relationship is not a complicated one, given the nature of YS.
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Re: TAS & IAS

Post by waspe414 » Thu Sep 03, 2015 5:22 am

New data is in:
https://i.imgur.com/rPj5uHd.png

The curve fit to that is IAS = TAS * (1 - 1.36e-5*ALT + 1.38e-11*ALT^2 + 2.15e-16*ALT^3)
It fits the available data to within 0.95% (5.8 kts at 1000 KTAS), with a mean error of 0.436%. Each pair of lines is one actual IAS and one predicted IAS at each altitude: https://i.imgur.com/Q5xPSy2.png

It starts to separate from the logical progression around 100,000 feet, and inverts (IAS less than zero) at 124,000ft. Neither of those are within practical flight regimes for most aircraft (124k being beyond the atmospheric limit), so it should be good.
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Re: TAS & IAS

Post by decaff_42 » Thu Sep 03, 2015 11:04 am

That is awesome work! Thank you for your investigations! I am suspicious that aircraft CL and CD may be related to KIAS, so from the calculated in-flight TAS I can now calculate the IAS!
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