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CAS was Sport pilot

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professor71(at)hotmail.co
Guest

Posted: Thu Mar 16, 2006 8:01 pm Post subject: CAS was Sport pilot

Since the subject of CAS was brought up, I was wondering how to determine 

this. I know you use true airspeed, density altitude, and temp. Is TAS the 

same as the average groundspeed in 3 different directions with a GPS? My 

601HD IAS is normally around 110 to 115 with GPS showing 98-104.  So does 

this mean my IAS is reading high so CAS would be actually lower?

John

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bryanmmartin

Joined: 10 Jan 2006
Posts: 1018

Posted: Fri Mar 17, 2006 1:26 pm Post subject: CAS was Sport pilot

It's not the same as the average groundspeed from three different runs, it's

a vector calculation. The following website contains a link to an Excel file

that will handle the calculations for you.



http://together.net/~billvt/True%20Air%20Speed/true_airspeed_calculator.htm

You just have to fly three GPS tracks and record the track and ground speed

and the file will calculate the true airspeed. Also record the OAT and

pressure altitude for your test runs so you can convert TAS to CAS with an

E6B. This is a backwards calculation in that normally you use the E6B to

find TAS from CAS.



If your GPS determined TAS is reading lower than your IAS that usually means

that your airspeed indicator is reading high. This is common with CH 601s

that have the static source in in the cockpit, since the cockpit is at a

lower pressure than ambient in flight. At very low speeds the ASI will read

much lower than CAS because the air is hitting the pitot tube at an angle

rather than straight on. To get an IAS that accurately reflects CAS, you

need a proper static source and the pitot tube must be aligned directly into

the airstream in relatively undisturbed air.



-- 

Bryan Martin

N61BM, CH 601 XL, Stratus Subaru.

 	  Quote: 	
		   

 

 Since the subject of CAS was brought up, I was wondering how to determine

 this. I know you use true airspeed, density altitude, and temp. Is TAS the

 same as the average groundspeed in 3 different directions with a GPS? My

 601HD IAS is normally around 110 to 115 with GPS showing 98-104.  So does

 this mean my IAS is reading high so CAS would be actually lower?

 John

- The Matronics Zenith-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?Zenith-List

_________________
--
Bryan Martin
N61BM, CH 601 XL, Stratus Subaru.
do not archive.

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