True Speed Calculation

[Hank]

My search-fu has let me down just now. I know GPS speed calculations have been discussed ad naseum, and I think I remember how to do it flying three headings 120 degrees apart. But just like I can't find the 'degree' sign on this new mini, I can't find the threads discussing three-course and all four cardinal direction speed calculations.

Anyway, I was out today on my post-annual flight, playing in the practice area, and noticed that my last two turns were aligned well with N and W, so I just added E and S and came up with the following:

M20-C, guppy mouth closure, 201 windshield and Hartzell 3-blade prop.

Altimeter setting = 30.23; 3500 msl; 23", 2300, 50 ROP; fuel flow not available.

N = 141 kts

S = 126 kts

E = 149 kts

W = 111 kts

This averages to 132 knots, which is a little low, but it's also low altitude. These were all uncorrected DG headings, too, which may invalidate the simple average or require a repeat.

I do have to say, though, the new shock discs sure do taxi and land nicely! For general performance data, solo with 40 gallons, I turned crosswind at 400-500 agl to miss Class D airspace, and was at 2000 agl when I reached the departure end of the 3000' runway on downwind. Love it, love it! Didn't realize how much I'd been missing this while she was in the shop!

[Awful_Charlie]

The one you want can be found on Ed Williams site, in particular http://williams.best.vwh.net/avform.htm#3gs

 

The three headings (NOT track!) is better than the averaging of four cardinal headings, which if you think about it can give you a higher than true value

[kmyfm20s]

The one you want can be found on Ed Williams site, in particular http://williams.best.vwh.net/avform.htm#3gs

 

The three headings (NOT track!) is better than the averaging of four cardinal headings, which if you think about it can give you a higher than true value

Wow, that was heavy stuff!

[Awful_Charlie]

I guess that formula could be extended to cope with four speeds on headings 90 degrees apart, but would prefer a maths man (come on Erik!) validates this:

 

I'm thinking it should become:

     vms = (v1^2 + v2^2 + v3^2 + v3^2)/4
     a1= v1^2/vms -1
     a2= v2^2/vms -1
     a3= v3^2/vms -1
     a4= v4^2/vms -1
     mu= (a1^2 + a2^2 + a3^2 + a4^2)/8

So with your figures above, that's 130.9 KTAS and 20.4 knots of wind

[N601RX]

Here is a document that I had book marked a long time ago. There are also a couple of online calculators if you don't want to have to get the calculator out. Just google online true airspeed caculator.

http://contrails.free.fr/temp/TAS_FNL4.pdf