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MEGILE
12-07-2004, 05:22 AM
Can anyone give me the forumla to work out True Airspeed, using Altitude and Indicated Airspeed for both Metric and Imperial values?

I know that there is a table on the CD, but I would very much like to know the forumlas.

TY

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BM357_TinMan
12-07-2004, 05:24 AM
I have it at home.

If nobody answers your question by 20:00 CT today, I'll be able to answer then.

Salfordian
12-07-2004, 06:33 AM
TAS=EAS/[sqrt(sigma)] <- can be used for any units

sigma is relative density, (density at altitude / density at sea level), and can be found from any altitude table given in aircraft performance text books. Alternatively if in troposphere (ISA upto 11km), use:
sigma=(Talt/Tsl)^(4.26), where Talt=Tsl-(6.5*altitude(in km))
Tsl=288K ISA

EAS=IAS at low speeds, but above Mach 0.4 compressibility effects give errors on ASI. Therefore gets more complicated, can give you the equations for this, but assuming EAS=IAS is close enough.

darkhorizon11
12-07-2004, 11:51 AM
Yeah ussually the difference between IAS and TAS is negligible below 250 knots and 20,000 feet pressure alt.

In Piper Warrior and most GA aircraft theres a knob you can turn to set the a/s indicator to read TAS. You just have to set the proper PA up against the outside air temp. Flying up here in North Dakota can get extremely cold. Last Friday I flew eastward too Baudette Minnesota. I needed TAS for my planning. At about 7000 feet with an OAT of -9 celcius (which isn't THAT cold), my IAS was about 105 knots and my true was about 110. So yes its relative negligible. In FB numbers thats like 15 or 20 kph right?

Platypus_1.JaVA
12-07-2004, 12:08 PM
There is a more simple formula than the one with the air pressure. Anyways, you cannot know the airpressure in PF.

This simple forumla only works for lower airspeeds (in real life at least) Above a certain airspeed the compressability of the air needs to be accounted for. That's when you use the previous posted formula.

TAS=IAS+(2%IAS for every 1000 feet) So, when you want the TAS, you should add 2% of the IAS value for every 1000 feet ABOVE SEA LEVEL!! TAS and IAS need to be in Knots.

If you use metrical units, 1 knot= 1.856 km/h and 1 foot = 0.32808m 1 Mph= 1.605 km/h

Nice formula but, not all sources use 2%. It is sometimes even 1.6%

Happy bombing http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

Platypus_1.JaVA
12-07-2004, 12:11 PM
This website should also prove usefull

http://www.csgnetwork.com/aviationconverters.html

Look towards the end of the list

MEGILE
12-07-2004, 12:12 PM
I'm learning programming you see... so I wanted to make a small personal app which I could use, for both practise in prgoramming, and for in game usage.

I saw another formula today, which invloves multiplying each thousand of feet by 2% of the IAS to get TAS.
Is this formula more of an estimation, than accurate?

cheers

Platypus_1.JaVA
12-07-2004, 12:15 PM
I found the table included on the game CD a bit useless. I made an excel spreadsheet and printed it out for the He-111. Now I should make another for the B-25 http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

MEGILE
12-07-2004, 12:23 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR> TAS=IAS+(2%IAS for every 1000 feet) So, when you want the TAS, you should add 2% of the IAS value for every 1000 feet ABOVE SEA LEVEL!! TAS and IAS need to be in Knots.

If you use metrical units, 1 knot= 1.856 km/h and 1 foot = 0.32808m 1 Mph= 1.605 km/h <HR></BLOCKQUOTE>


So are you saying that it needs to be calculated in KNOTS first, and then the results are converted into metric?

hehe, I just got confused some more. http://forums.ubi.com/images/smilies/53.gif

Can we try an example. http://forums.ubi.com/images/smilies/16x16_smiley-happy.gif

Lets say,

30,000 Feet and an IAS of 150 MPH.
Would you take 2 percent of 150 MPH, and then times it by 30 to get the difference between IAS and TAS?
And then to convert the MPH into KMH, you can just multiply it by 1.605?

BBB_Hyperion
12-07-2004, 06:17 PM
Hmmm well its a little more complex as most of you posted.

I did code this VB example to show CAS to TAS calculation for ISA conditions.

Hopefully it is correct no waranty http://forums.ubi.com/groupee_common/emoticons/icon_biggrin.gif

http://www.butcherbirds.de/hypesstorage/CAS2TAS.pdf

The testplane flys at 20000 ft and has a CAS of 320 knots outside temp -23?C.

Happy coding or copying .)