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  #41  
Old 10-22-2021, 06:53 PM
BobTurner BobTurner is offline
 
Join Date: Dec 2011
Location: Livermore, CA
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Quote:
Originally Posted by gmcjetpilot View Post
Tell me your method. The math is simple. The three legs and GPS SPEED is averaged. (V1 + V2 + V3)/3 = Average TAS Speed..
Just a calculated example: I used TAS=150 knots, wind = 30 knots from 180, fly GPS ground tracks of 030, 150, 270. If everything is perfect the gps ground speeds will be 175.2, 123.3, 147.0 knots, respectively. Note the average is 148.5, not 150. It just isn't that simple. Is 1.5 knots important? Probably only for bragging rights.
Curiously, for the same numbers as above, but this time just flying N then south and averaging gps ground speeds, of course you get 150 knots as you should. But what if you're wrong about the wind direction? If you're off by 20 degrees it only makes an error of 0.4 knots. Off by 40 deg, error just 1.3 knots. This is why I advocate the "into the wind, with the wind, average GPS ground speed" method. It's the quickest, and that, I think, minimizes other uncontrollable errors like the winds changing with time or physical location, etc.
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  #42  
Old 10-23-2021, 06:44 AM
BoydBirchler BoydBirchler is offline
 
Join Date: Jul 2020
Location: Indianapolis
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Bob,
You are wrong! The runs are not "ground tracks" they should be headings: 90 or 120 degrees apart. This difference lets the ground track drift with the air mass, effectively canceling out the effects of the wind.
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  #43  
Old 10-23-2021, 01:31 PM
BobTurner BobTurner is offline
 
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Quote:
Originally Posted by BoydBirchler View Post
Bob,
You are wrong! The runs are not "ground tracks" they should be headings: 90 or 120 degrees apart. This difference lets the ground track drift with the air mass, effectively canceling out the effects of the wind.
I’m sorry, Boyd, but I’m not wrong. You can easily verify this yourself without doing any calculations, for the case where the wind is out of the south, and you fly N, S, E, and W headings. The average gps ground speed of the N and S heading runs will equal true airspeed, as previously discussed. For the E and W runs, you will drift north. But the gps sees this, as it measures ground track and ground speed. For both legs, the gps ground speed will be the square root of the sum of true speed squared plus wind speed squared. e.g., the average of 4 legs will be higher than true airspeed if there’s any wind. It doesn’t magically cancel out.
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  #44  
Old 10-23-2021, 02:46 PM
GalinHdz's Avatar
GalinHdz GalinHdz is offline
 
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Quote:
Originally Posted by BobTurner View Post
I’m sorry, Boyd, but I’m not wrong. You can easily verify this yourself without doing any calculations, for the case where the wind is out of the south, and you fly N, S, E, and W headings. The average gps ground speed of the N and S heading runs will equal true airspeed, as previously discussed. For the E and W runs, you will drift north. But the gps sees this, as it measures ground track and ground speed. For both legs, the gps ground speed will be the square root of the sum of true speed squared plus wind speed squared. e.g., the average of 4 legs will be higher than true airspeed if there’s any wind. It doesn’t magically cancel out.
^^^^^ THIS ^^^^^ is correct. You must use ground track and ground speed (GPS derived is the easiest way) for the calculations.

FWIW every spreadsheet I have ever used, including the ones available from the National Test Pilot School, specifically state this.

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Last edited by GalinHdz : 10-23-2021 at 04:08 PM.
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  #45  
Old 10-23-2021, 07:43 PM
BobTurner BobTurner is offline
 
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Actually, you have 3 unknowns (TAS, wind speed, wind direction) so taking data from 3 different legs is needed to get an unambiguous answer. In theory you can write equations for headings or ground track (slightly different equations) but most think gps track is more accurate than a magnetic heading.

Last edited by BobTurner : Yesterday at 12:20 AM. Reason: Changed bad grammar
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  #46  
Old 10-23-2021, 08:30 PM
Cumulo Cumulo is offline
 
Join Date: Apr 2015
Location: KHMT
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Bob, GPS direction is a track, right? I think that is what you meant.

I have used the method you describe for decades. Simple. As a practical matter, other factors will determine accuracy rather than the smidgen of wind direction error.
ron

Last edited by Cumulo : 10-23-2021 at 08:34 PM.
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  #47  
Old Yesterday, 12:31 AM
BobTurner BobTurner is offline
 
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Thanks, fixed bad grammar.
I agree: I favor the ‘with the wind, against the wind’ method because I think it minimizes the chances of other errors (up/downdrafts, changing winds, etc) creeping in.
BTW, doing 4 legs results in the TAS being ‘over-determined’. A good program will use 3 legs at a time, and calculate 4 answers. If there’s a significant spread in those answers, it suggests something went wrong (wind changed, etc). And if they’re all close, it gives you a warm and fuzzy feeling!
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