Primarily for general aviation discussion, but other aviation topics are also welcome.
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By carlmeek
FLYER Club Member  FLYER Club Member
#1274197
On my trip to guernsey at the weekend, I coasted out over the Isle of Wight, east of the airway, to join onto the French mainland before routing to Guernsey. I was at 7800ft, and counted the time in the "dead zone" when I was out of glide range according to sky demon. It times out at 11 mins.

Question is: what factors affect the accuracy, and how is it calculated? I had a 35kt tail wind, and I suspect my aircraft glides pretty well wheels up, and I don't recall telling the software any glide ratio stats.

If you were to suffer an engine failure midway-ish, how would you determine whether to glide on or turn back?
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By weirdfish
#1274200
You can edit the glide speed and ratio in the aircraft settings of SkyDemon so that would suggest that any calculation is carried out with those settings in mind. I also suspect the aircrafts position within the glide circle is dependent on winds if that information has been available to SD and thus giving a graphical indication of what your best choice is. I'm not sure on that second point though.
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By Peter Gristwood
FLYER Club Member  FLYER Club Member
#1274201
I'm always impressed how it takes into account terrain as well. Flying back from Wales towards Lynmouth the other day, the glidesafe was not a circle but bent to accommodate the cliffs.
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By DarrenL
#1274202
I had that feature pointed out to me by one of the chaps on the stand at The Flying Show. I asked the same questions about best glide speed, rate of descent and wind effects but the guy couldn't give me a definite answer.

I'll keep an eye on this thread as I'm curious to find out - it shan't stop me subscribing to SkyDemon this year either way though :)
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By carlmeek
FLYER Club Member  FLYER Club Member
#1274206
It clearly does terrain but I didn't notice any wind
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By Rob Hart
#1274207
SkyDemon does look at winds aloft data when drawing the glide-safe ring, but bear in mind that the wind SkyDemon knows about is forecast rather than actual.
By riverrock
FLYER Club Member  FLYER Club Member
#1274208
See http://www.skydemon.aero/inflight/docum ... rnings.htm
My bold:
GlideSafe Technology

When flying over water or mountainous terrain, or even just for your own peace of mind, you can switch on SkyDemon's GlideSafe feature. Open up the main menu on your Mobile MD then choose Settings and Preferences. In the Navigation tab you will see a drop-down menu for GlideSafe which can be turned on all the time, or only over water if you prefer. On your iPad, iPhone and Android device this can be found in the Setup menu under Navigation Options. You will see that a turquoise ring appears on the map around your aircraft symbol depicting the maximum radius where it is safe for you to glide and land if you were to encounter an engine problem, for example.

GlideSafe takes altitude, wind speed and direction, as well as your configured aircraft glide ratio into account when depicting the glide safety radius around your aircraft, so a strong headwind would minimise your glide radius ahead, and a tailwind would have the opposite effect.
By riverrock
FLYER Club Member  FLYER Club Member
#1274209
However I suspect it doesn't include turn back time, if the closest land is behind you. I suppose you could always test is out some day - but I'm not volunteering to be the guinea pig, especially if an engine off test is required...
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By Ridders
FLYER Club Member  FLYER Club Member
#1274214
I've edited the glide ratio on my copy from the default to that which is shown in the poh for the pup.
I would suggest the glide turn back at 4000ft would be a very small percentage of the glide time to deck.
By Paultheparaglider
#1274325
carlmeek wrote:If you were to suffer an engine failure midway-ish, how would you determine whether to glide on or turn back?


You should know your best glide airspeed. As a general rule of thumb, you should slow down to best glide with a tailwind. If you have a strong headwind, it can help to speed up a little, but unless you know the polar curve for your aircraft (the way glide changes with speed), you are probably better off sticking to best glide speed.

As a simple example, if your best glide is at 70 kts, a 35 kt tailwind will take you 3 times further than turning in to wind. The slower the best glide speed, the bigger the effect of any given wind speed. So, assuming 70kts is about right for your best glide, with the 35 kt tailwind you mentioned, if you are a little past the quarter way mark you would be better to carry on.

Google polar curve and you can learn more.
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By Ridders
FLYER Club Member  FLYER Club Member
#1274326
Rob Hart wrote:SkyDemon does look at winds aloft data when drawing the glide-safe ring, but bear in mind that the wind SkyDemon knows about is forecast rather than actual.

I've just noticed what you said Rob.

I wonder why it's not based on actual wind rather than forecasted.
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By defcribed
#1274327
Because without having airspeed data and compass/mag-derived heading data it cannot compute the actual wind by comparing against GPS data?
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By JonathanB
FLYER Club Member  FLYER Club Member
#1274328
Ridders wrote:I wonder why it's not based on actual wind rather than forecasted.


How would it work that out if it doesn't know your actual TAS?
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By Ridders
FLYER Club Member  FLYER Club Member
#1274331
your of course right.

I am thinking back to using my 196 where I could manually input so it could calculate the actual wind.
By Lefty
FLYER Club Member  FLYER Club Member
#1274354
It would be easy enough to test.
First make sure have it set up for your a/c's glide speed & descent rate.
Second, fly towards your home base runway, trim to best glide speed, then as soon as the glide range circle touches the runway, cut the power and glide to the runway.