Equation to determine True Airspeed

Gene Whitt

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Mar 21, 2005, 10:56:34 PM3/21/05

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Help,
Does anyone know the math needed to determine true air speed.
I have my WWII E6-B and an electronic E6-b but there must be
an algebraic way to make a graphic representation.

I'm in touch with an Air Force Captain who asked this qusestion of me. I
would like to find an answer for him.

Gene Whitt

Mike 'Flyin'8'

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Mar 21, 2005, 11:08:30 PM3/21/05

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I found this PDF via google.... Yikes...

http://www.nar-associates.com/technical-flying/airspeed/airspeed_wide.pdf

On Tue, 22 Mar 2005 03:56:34 GMT, "Gene Whitt" <gwh...@ix.netcom.com>
wrote:

Mike Alexander
PP-ASEL
Temecula, CA
See my online aerial photo album at
http://flying.4alexanders.com

BTIZ

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Mar 21, 2005, 11:08:53 PM3/21/05

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gene... page 42 of Kershner's Advance Pilot Flight Manual.. has all the
formulas.. using "density ratio" between air density at some altitude (for
which you are looking for TAS) and sea level..

essentially, TAS equals CAS divided by the square root of the density
ratio.. formulas are not easy to type on usenet

shall I scan the page and send directly?

BT
retired AF Master Navigator

"Gene Whitt" <gwh...@ix.netcom.com> wrote in message
news:SHM%d.810$gI5...@newsread1.news.pas.earthlink.net...

tony roberts

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Mar 21, 2005, 11:09:37 PM3/21/05

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Hi Gene

Try this:
http://www.mathworks.com/access/helpdesk/help/toolbox/aeroblks/aero_s17.h
tml

or this:
http://www.reacomp.com/true_airspeed/

HTH

Tony

Indiacha...@hotmail.com
Tony Roberts
PP-ASEL
--
VFR OTT
Night
Cessna 172H C-GICE

In article <SHM%d.810$gI5...@newsread1.news.pas.earthlink.net>,

BTIZ

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Mar 21, 2005, 11:30:51 PM3/21/05

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I noticed that most of the formulas get back to the "density ratio" between
the altitude where TAS is being computed and sea level.. the "reacomp.com"
site below only converts various CAS or IAS readings on set headings into a
computed TAS based on negating the effects of wind or density ratio on the
CAS indication.

BT

"tony roberts" <nos...@nowhere.ca> wrote in message
news:nospam-A233C0.20120221032005@shawnews...

Barry

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Mar 22, 2005, 12:26:17 AM3/22/05

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> Does anyone know the math needed to determine true air speed.
> I have my WWII E6-B and an electronic E6-b but there must be
> an algebraic way to make a graphic representation.

A useful rule of thumb is that true airspeed increases relative to calibrated
airspeed by about 1% for each 600 ft increase in density altitude. This isn't
exact, but works pretty well for typical light airplane performance numbers.

RST Engineering

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Mar 22, 2005, 3:52:42 PM3/22/05

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If I could enter equations in the ng format, I could do this in one step,
but here it is in a bunch of steps. Combine them as you see fit:

C = Degrees Celsius

A = Pressure Altitude

D = A / (63691.776-(0.2191 * A))

Q = 10^D

TAS = IAS * sqrt((273.16 + C)/(288/Q))

(reference: Axioms of Flight by James Embree, ISBN 0-9601062-7-8)

Jim

"Gene Whitt" <gwh...@ix.netcom.com> wrote in message
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EBrown

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Mar 23, 2005, 11:22:26 AM3/23/05

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Does that square root symbol encompass both the numerator and the
denominator in that equation? (Hard to tell in NG post)

CryptWolf

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Mar 23, 2005, 7:46:31 PM3/23/05

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"Gene Whitt" <gwh...@ix.netcom.com> wrote in message
>news:SHM%d.810$gI5...@newsread1.news.pas.earthlink.net...

The best way to do it is to calculate the various numbers directly
using the standard atmosphere. The problem here is that it is limited
to about 36,000 feet where the constants change for the model.
This is good enough for general aviation use and very few prop
planes can go this high anyway.

IAS = indicated air speed (knots or MPH)
TAS = true air speed (same units as IAS)
ALT = altitude (feet)
TEMP = temp (Celsius)

Note that the formula below is one line in my spreadsheet.
It was split to prevent strange line wraps in the news readers.
With almost no work, you could drop this into a BASIC program
and have it work also.

TAS = IAS/(1-6.8755856*10^-6*(ALT+((273.15+(15-0.0019812*ALT))/0.0019812)
* (1-((273.15+(15-0.0019812*ALT))/(273.15+TEMP))^0.234969)))^2.12794

Anaconda

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Mar 23, 2005, 7:33:26 PM3/23/05

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Yes. If you look at the formula again, you'll note the double brackets after
the sqrt. Easy to miss.

Richard Thomas

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Mar 25, 2005, 12:12:17 AM3/25/05

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On Tue, 22 Mar 2005 12:52:42 -0800, "RST Engineering"
<j...@rstengineering.com> wrote:

>If I could enter equations in the ng format, I could do this in one step,
>but here it is in a bunch of steps. Combine them as you see fit:
>
>C = Degrees Celsius
>
>A = Pressure Altitude
>
>D = A / (63691.776-(0.2191 * A))
>
>Q = 10^D
>
>TAS = IAS * sqrt((273.16 + C)/(288/Q))
>
>(reference: Axioms of Flight by James Embree, ISBN 0-9601062-7-8)
>
>Jim

Does this mean we can simplify the above to the following?

TAS = IAS * sqrt ( T / ( 288 / Q ))

Where:

T = Temperature in Kelvin
Q = 10^D
D = Pressure Altitude / (63691.776 - (0.2191 * Pressure Altitude))

How about Compressibility Errors above 300 Kts TAS? Or does this
formula take this into consideration?

I presume the last bit where it refers to 288 is actually the
temperature in Kelvin at sea level under ISA conditiosn? Ie, 273 K +
15 C.

I'll stick to the CRP-5 Whizz Wheel I think for the JAA ATPL exams.
The above formula would probably be too acurate for the multiple
choice answers. :-)

Best wishes,

Richard Thomas
FAA CP-ASEL AMEL IA
Studying for the JAA ATPL Written Exams