speedbird092
Posts: 164
Joined: Thu Apr 06, 2000 9:04 am

IAS To TAS

Sat Jul 28, 2001 2:06 am

Hey!

Just a quick question, how can I calculate my true airspeed based on IAS, winds, etc

Thanks
Speedbird092
 
User avatar
derekf
Posts: 886
Joined: Tue Feb 20, 2001 4:05 am

RE: IAS To TAS

Sat Jul 28, 2001 3:05 am

You need to know several things like altitude, temperature and aircraft pressure errors. Basically TAS = EAS divided by the square root of atmospheric density ratios (hence using temp and altitude). EAS is derived from IAS by knowing the aircraft pressure erros to arrive at CAS the CAS to EAS by the scale altitude law or compressibility correction. If you need any more info let me know.

Hope this helps!

DerekF
Whatever.......
 
Ralgha
Posts: 1589
Joined: Tue Nov 09, 1999 6:20 pm

RE: IAS To TAS

Sat Jul 28, 2001 5:48 am

Or you could just use a flight compter  Big thumbs up. Also, many airplanes have a TAS ring on their airspeed indicator that you can adjust for pressure altitude and temperature, you can then read your TAS right off the airspeed indicator.

 Big thumbs up
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XFSUgimpLB41X
Posts: 3960
Joined: Fri Aug 25, 2000 1:18 am

RE: IAS To TAS

Sat Jul 28, 2001 5:52 am

Isn't it density altitude and not pressure altitude?
Chicks dig winglets.
 
N400QX
Posts: 1981
Joined: Sun May 06, 2001 9:51 am

RE: IAS To TAS

Sat Jul 28, 2001 6:23 am

Here's the equation I've got on my desk:

TAS= [(IAS x 2%) • (ALT/1000)] + IAS

So an example--
IAS= 300 at FL200, 300 • 0.02 • 20 + 300= 240TAS

If I'm wrong, please correct me by all means.
 
bio15
Posts: 1048
Joined: Fri Mar 23, 2001 8:10 am

RE: IAS To TAS

Tue Jul 31, 2001 1:46 am

N400QX, at higher altitudes TAS is greater than IAS. But you got some of the math:

IAS decreases by 2% every 1000ft you climb.

IAS 280 at FL330
---> [280 x (33 x 0.02)] + 280 = TAS 464.8

Winds affect ground speed mostly.

-bio
 
Guest

RE: IAS To TAS

Tue Jul 31, 2001 9:38 am

Get a E-6B or other circular slide rule and read the book. YUCK
 
DG_pilot
Posts: 810
Joined: Tue Sep 07, 1999 10:21 am

RE: IAS To TAS

Tue Jul 31, 2001 2:34 pm

(2% for every 1000 feet) X CAS, then plus CAS = TAS

==EXAMPLE==
CAS: 90 kts
Altitude: 6000 ft

(6 x .02) x 90= 11
11 + 90= 101 kts TAS

And XFSUgimpLB41X, it is pressure altitude I'm pretty sure...
 
sabenapilot
Posts: 2442
Joined: Sat Feb 26, 2000 6:18 pm

RE: IAS To TAS

Tue Jul 31, 2001 11:02 pm

Here's the one and only simple thumbrule to convert IAS into TAS.
(sadly only valid above FL100)

TAS = IAS + half of your flight level

To prove how accurate it is, I've used the same example as Bio15 so you can compare the results:
IAS = 280kts.
FL330
TAS = 280 + 165 = 445

 
Jetpilot500
Posts: 78
Joined: Wed Nov 29, 2000 1:54 am

RE: IAS To TAS

Wed Aug 01, 2001 12:41 am

You are all providing interesting rules of thumb, but there are a lot of factors involved to come up with an accurate answer for all speeds and altitudes. As someone else mentioned, use an E6B. Here is the correct method found on this website of aviation formulas:

http://www.best.com/~williams/avform.html


Mach numbers, true vs calibrated airspeeds etc.


Mach Number (M) = TAS/CS
CS = sound speed= 38.967854*sqrt(T+273.15) where T is the OAT in celsius.
TAS is true airspeed in knots.
Because of compressibility, the measured IAT (indicated air temperature) is higher than the actual true OAT. Approximately:

IAT=OAT+K*TAS^2/7592
The recovery factor K, depends on installation, and is usually in the range 0.95 to 1.0, but can be as low as 0.7. Temperatures are Celsius, TAS in knots.

Also:

OAT = (IAT + 273.15) / (1 + 0.2*K*M^2) - 273.15
The airspeed indicator measures the differential pressure, DP, between the pitot tube and the static port, the resulting indicated airspeed (IAS), when corrected for calibration and installation error is called "calibrated airspeed" (CAS).

For low-speed (M<0.3) airplanes the true airspeed can be obtained from CAS and the density altitude, DA.

TAS = CAS*(rho_0/rho)^0.5=CAS/(1-6.8755856*10^-6 * DA)^2.127940 (DA<36,089.24ft)
Roughly, TAS increases by 1.5% per 1000ft.

When compressibility is taken into account, the calculation of the TAS is more elaborate:

DP=P_0*((1+0.2*(IAS/CS_0)^2)^3.5 -1)
M=(5*( (DP/P+1)^(2/7) -1) )^0.5
TAS= M*CS
P_0 is is (standard) sea-level pressure, CS_0 is the speed of sound at sea-level, CS is the speed of sound at altitude, and P is the pressure at altitude.

These are given by earlier formulae:

P_0= 29.92126 "Hg = 1013.25 mB = 2116.2166 lbs/ft^2
P= P_0*(1-6.8755856*10^-6*PA)^5.2558797, pressure altitude, PA<36,089.24ft
CS= 38.967854*sqrt(T+273.15) where T is the (static/true) OAT in Celsius.
CS_0=38.967854*sqrt(15+273.15)=661.4786 knots

[Example: CAS=250 knots, PA=10000ft, IAT=2C, recovery factor=0.8
DP=29.92126*((1+0.2*(250/661.4786)^2)^3.5 -1)= 3.1001 "
P=29.92126*(1-6.8755856*10^-6 *10000)^5.2558797= 20.577 "
M= (5*( (3.1001/20.577 +1)^(2/7) -1) )^0.5= 0.4523 Mach
OAT=(2+273.15)/(1 + 0.2*0.8*0.4523^2) - 273.15= -6.72C
CS= 38.967854*sqrt(-6.7+273.15)=636.08 knots
TAS=636.08*0.4523=287.7 knots]
In the reverse direction, given Mach number M and pressure altitude PA, we can find the IAS with:

x=(1-6.8755856e-6*PA)^5.2558797
ias=661.4786*(5*((1 + x*((1 + M^2/5)^3.5 - 1))^(2/7.) - 1))^0.5


Have fun trying to figure this out!
JetPilot500
 
sabenapilot
Posts: 2442
Joined: Sat Feb 26, 2000 6:18 pm

RE: IAS To TAS

Wed Aug 01, 2001 3:02 am

All are correct (although I haven't really spend time checking them over...)
However, the question was:
how can I quickly get an idea of my TAS based on IAS?
I don't think any of these formulas are helping you any further.

--------------------

BTW, since you talked about it:
here's a quick formula to find OAT from IAT:
OAT = IAT - 20 times the speed in mach
e.g.:
indicated temp = -25°C
M = .70
OAT = -25 - 14 = -39°C

---------------------

And another very usefull notion.
Machnumber equals distance travelled per minute.
e.g.:
At M.70 you travel about 7NM/minute.

Ok, both might be off somewhat at extreme winds, speeds altitudes or temperatures, but they are more then accurate enough for flight follow-up and are often used in the cockpit of planes without FMS, like the B737-200. (I started on that one at Sabena...)
 
N400QX
Posts: 1981
Joined: Sun May 06, 2001 9:51 am

RE: IAS To TAS

Wed Aug 01, 2001 4:45 pm

OK... I understand now.

Bio-- the reason the TAS in my math shows 240 is because of a typo... I believe I meant to put in 340. oops  Big grin

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