Determining True Airpspeed Using a GPS

Dean_Dolph

Newbie
Joined
Nov 16, 2003
Messages
2,923
Location
Katy, TX.
Aircraft
None but enough kits in the garage to keep me busy the rest of my life!
Total Flight Time
Zilch!
Okay, I didn't know what forum to place this in but decided that 'flight testing' was as good a place as any and better then most!

Others have posted about using a GPS to determine true aispeed and pitot errror and I happened to come across an article (way behind on my reading!) in the 8/09 issue of Kit Planes about this subject. The article provided links to the National Test Pilot School where there are downloads available under the 'Information' tab in the top menu.

The link to the NTPS web site is http://www.ntps.edu/ click on 'Information' and then on 'Downloads.

There are three downloads available; the 1st one is a Word document, the 2nd one is an Excel spreadsheet and the 3rd is a paper by the Aussie that created the spreadsheet.

According to the Kit planes article there are errors in the methods that most of us use to determine the true airspeed. I guess I won't comment on that because I ain't one of 'us' that use a method!

I hope someone will check this info out and comment on its usefulness.
 

StanFoster

Newbie
Joined
Nov 16, 2003
Messages
16,880
Location
Paxton, Il
Aircraft
Helicycle N360SF
Total Flight Time
1250
Dean- If you hold altitude and power settings, fly into the wind and record your groundspeed given by the gps, then fly 180 degrees opposite course, record that new groundspeed, then average the two, you will have your true airspeed. Any slight error of not going exactly into the wind will cause a negligible very minor error. Stan
 

MichaelBurton

Gyro CFI
Joined
Oct 2, 2004
Messages
1,261
Location
Pleasant Grove, Utah. USA
Aircraft
Sparrowhawk, B95, AC680, BE23, Calidus, MTO Sport, Cavalon, Vortex, Dominator, WindRyder, RAF2000
Total Flight Time
6000+ over 3000 hrs in gyro
TAS and windspeed from three (GPS) groundspeeds.

TAS and windspeed from three (GPS) groundspeeds.

Aviation Formulary V1.44
By Ed Williams

Determine your groundspeed on three headings that differ by 120 degrees (eg 40, 160 and 280 degrees), call these v1, v2 and v3

Let

vms = (v1^2 + v2^2 + v3^2)/3
a1= v1^2/vms -1
a2= v2^2/vms -1
a3= v3^2/vms -1
mu= (a1^2 + a2^2 + a3^2)/6

Let bp and bm be the roots of the quadratic b^2 -b + mu =0 ie:

bp= 1/2 +sqrt(1/4-mu)
bm= mu/bp

The TAS and windspeed are then given by sqrt(vms*bp) and sqrt(vms*bm) provided that the TAS exceeds the windspeed. If this is not the case, the roots are exchanged. This is a handy way to check your TAS (and the calibration of your airspeed indicator) using your GPS groundspeed, even though the wind is unknown.
 
Last edited:

Redbaron

Banned
Joined
Apr 20, 2009
Messages
2,679
Location
earth
Aircraft
bee
true airspeed using a gps? where u at birdy? lol airspeed is constantly changing from vertical and horizontal gusts. ground speed doesn't count! turbulence is always there making you rise and fall "in a stable gyro" :eek:
 

ckurz7000

Senior Member
Joined
Nov 7, 2006
Messages
3,438
Location
Vienna
Aircraft
ArrowCopter
Stan and Michael got it right. The way Michael described is the better -- albeit more cumbersome -- one. And you don't have to fly three headings that are exactly 120° apart. The farther apart, though, the more accurate. What I did was to fly 4 headings 90° apart. Out of the 4 measurements I picked 4 sets of data, each comprising of 3 measurements. With just one more heading to fly you get three more data points for each IAS. This gives you a higher confidence and accuracy.

Another issue to consider is, of course, the conversion of TAS to CAS. You can't just use the GPS derived TAS and compare it to your airspeed indicator without taking effects of air density into account. Failing to do so can easily result in 6-7 mph error under unfavorable conditions.

-- Chris.
 
Top