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Old 12-09-2017, 05:12 PM   #21
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The mistake you apparently made is that you assumed that the hypotenuse (longest side) of that right triangle corresponds to the ground speed.
Well, almost. I assumed that the hypotenuse was the distance you'd have to fly through the air to fly 100 kt on a north track, so the ratio of ground speed:true airspeed would be 100 divided by the hypotenuse. But that's actually my northerly ground speed component if I left the plane on a north heading, and just let it drift sideways, which isn't quite the same thing.


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Old 12-09-2017, 05:25 PM   #22
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Well, almost. I assumed that the hypotenuse was the distance you'd have to fly through the air to fly 100 kt on a north track, so the ratio of ground speed:true airspeed would be 100 divided by the hypotenuse. But that's actually my northerly ground speed component if I left the plane on a north heading, and just let it drift sideways, which isn't quite the same thing.


Actually you have the nose pointed to heading vector You you still flying the straight north as the wind will keep you on north track. So you actually arenít flying the hypotenuse.


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Old 12-09-2017, 05:43 PM   #23
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Actually you have the nose pointed to heading vector You you still flying the straight north as the wind will keep you on north track. So you actually arenít flying the hypotenuse.
Yeah, I'm sleepy still this morning, and realised that didn't make sense as soon as I posted it. So why [i]isn't[i] the hypotenuse the distance I have to fly through the air to make 100 nm on a northerly track?
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Old 12-09-2017, 07:33 PM   #24
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Yeah, I'm sleepy still this morning, and realised that didn't make sense as soon as I posted it. So why [i]isn't[i] the hypotenuse the distance I have to fly through the air to make 100 nm on a northerly track?
The first thing you have to consider is whether the direction you are flying is referenced to track or heading. If the wind is from 270 degrees you are flying a HEADING of 360 degrees then the hypotenuse of the wind triangle represents your ground speed (as you drift to the east). However if you are flying a TRACK of 360 degrees (as shown in the diagram I included in my last post), the hypotenuse represents the TAS on your heading, which is west of 360 degrees. The phrase "distance I have to fly through the air" is a bit ambiguous, since while you are flying that distance the air is moving (i.e. wind) over the ground. But referenced to the body of air, if your TAS is 100 kts then the distance you have to fly at a given ground speed to cover 100 nm over the ground is 100 X (100/GS). This of course assumes that GS is a positive number .

You can, of course, do the wind triangle where the three sides represent distances rather than speeds, but for the object of the task we are discussing (calculating TAS based on measurements of GS on flown tracks or headings) it is less useful.

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Old 12-09-2017, 08:07 PM   #25
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The first thing you have to consider is whether the direction you are flying is referenced to track or heading.
As I mentioned at the start, I've consistently been using track (but assuming that the hypotenuse was the heading I'd have to fly to make good that track).
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Old Yesterday, 01:50 AM   #26
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As I mentioned at the start, I've consistently been using track (but assuming that the hypotenuse was the heading I'd have to fly to make good that track).
The heading required to make good a particular track IS the hypotenuse of the wind triangle when the wind triangle is a right triangle (i.e. when the wind direction is perpendicular to the track), but the length of that hypotenuse is the TAS. The length of the leg that falls on the track is the ground speed. See the diagram.


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