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GS quirk on first leg when manually setting all legs wind


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sjhoare77
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Hi,

Planning a route today on my Android tablet using the latest version of SkyDemon and I noticed the following oddity. I was overriding the auto winds, as I was practising using my whizz wheel, so had set an all legs wind of 300/10. However on the first leg with a track of 324 deg my PLOG (attached) shows a ground speed of 90 kts; whereas the minimum it could be fighting a straight on headwind would be 100 kts. Has anyone seen this before or can indicate where I've screwed up?

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grahamb
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sjhoare77 - 6/10/2020 10:10:46 PM
Hi,

Planning a route today on my Android tablet using the latest version of SkyDemon and I noticed the following oddity. I was overriding the auto winds, as I was practising using my whizz wheel, so had set an all legs wind of 300/10. However on the first leg with a track of 324 deg my PLOG (attached) shows a ground speed of 90 kts; whereas the minimum it could be fighting a straight on headwind would be 100 kts. Has anyone seen this before or can indicate where I've screwed up?

Whilst the PLOG shows the cruise speed TAS, the calculation is actually using TAS based on your aircraft performance tables, which will be lower in the climb. 

Tim Dawson
Tim Dawson
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You haven't screwed anything up (probably), it's just difficult to compare SkyDemon's flight modelling results with that of a whizz wheel. What you're seeing is because you're spending a proportion of that first leg in a climb, and that portion will have a lower groundspeed, so that is bringing the leg average down.
sjhoare77
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Tim Dawson - 6/12/2020 8:35:31 AM
You haven't screwed anything up (probably), it's just difficult to compare SkyDemon's flight modelling results with that of a whizz wheel. What you're seeing is because you're spending a proportion of that first leg in a climb, and that portion will have a lower groundspeed, so that is bringing the leg average down.

Thanks Tim - much appreciate your input on that. I really did think I was going slowly mad in lockdown. Just out of curiosity, is there a way of displaying the climb speed or the average leg speed on the Plog?
Tim Dawson
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The average leg groundspeed is essentially what is shown already - it's a simplistic figure derived from the distance of the leg divided by the total time it'll take you to fly it - but there's no way of switching to show the speed during the climb (which itself will vary as your altitude and the wind changes).
grahamb
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Tim Dawson - 6/15/2020 8:54:36 AM
The average leg groundspeed is essentially what is shown already - it's a simplistic figure derived from the distance of the leg divided by the total time it'll take you to fly it - but there's no way of switching to show the speed during the climb (which itself will vary as your altitude and the wind changes).

Perhaps the TAS shown could be averaged as well, to help comprehension of the lower GS?

Tim Dawson
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The TAS/IAS column could be, I suppose. I am not sure how we would arrive at a number that would be helpful to most people.
PaulSS
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I, personally, wouldn't bother. Maybe put a one sentence explanation in the manual but, apart from that, all that's important is the time and fuel at the next waypoint after takeoff. If SkyDemon calculates that correctly then I think I'd rather look out the window during the climb and not really bother about exactly how the app is calculating it. Fly at the climb speed you used for planning and all will be well.
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