Talk:Speed to fly

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[edit] Aim of StF

Speed to fly is a technique
I would not call Speed to fly a technique
there must be better wording.
maybe calculation or computation or indication given by the ring.

I would call technique the different type of flying thermals, ridge lift, dolphing flight and wave.
These different types require different skills and different calculation.
Moreover when flying between sources of lift, usually thermals, ridge lift and wave
I would say : when flying between thermals. Period!

flying thermals sources of lift, you circle and climb
flying ridge sources of lift, you go straight and stay as low as skills permit
flying wave sources of lift, can circle, go straight do s turns and climb forever
flying thermals street sources of lift, go straight and fly slower in lift, faster in sink

different techniques

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Dansco2903 (talk) 13:02, 27 February 2008 (UTC)


Dansco2903 (talk) 21:02, 26 February 2008 (UTC)

I've rephrased the definition in the opening para, & also pointed out that StF is independent of the wind speed. I hope it's an improvement. Of course the previous version:

"minimize the amount of time spent in the sinking air"

wasn't wrong—it just didn't tell the whole story, & almost suggested that you should fly at VNE between thermals! --NigelG (or Ndsg) | Talk 11:01, 22 February 2007 (UTC)

I now wonder whether the phrase optimizing the airspeed in the sinking air that is found between the areas of rising air is quite correct. Surely STF applies when flying in lift too, eg under a cloud street. Comments? --NigelG (or Ndsg) | Talk 09:00, 1 October 2007 (UTC)
I've gone ahead & modified the statement to cover SfT in both rising & sinking air. --NigelG (or Ndsg) | Talk 16:48, 25 November 2007 (UTC)

"minimize the amount of time spent in the sinking air" is quite correct and does not suggest to fly at VNE
it suggests to get out of there asap,
the reader able to grasp this concept certainly understand first speed vs sink ie polar.

optimizing the airspeed in the sinking air does not suggest to get out of there asap one could even think to fly slow.............???? Dansco2903 (talk) 19:59, 26 February 2008 (UTC)

Thanks for your suggestions. You're right about technique: I've changed that to principle. No: minimize means make as small as possible—it has no other meaning; optimize means choose the "best" in some sense (in this case the speed required to get the best XC average speed). If you really meant "get out of there asap" you would fly at VNE. In fact you clearly mean "get out of there fairly fast", & STF theory tells you exactly how fast! --NigelG (or Ndsg) | Talk 22:30, 26 February 2008 (UTC)

[edit] Theory

What about a shortish section on the theory? The articles in S&G aren't available to the average reader, so a summary here would be useful. Perhaps someone with more graphical skill than myself (take a bow, JMcC!) could upload the classic Polar curve diagram which enables you to read off the StF & the average XC speed simultaneously. From an intuitive point of view, the whole derivation stems from the "steady state" fact that over the flight as a whole the total climb equals the total sink.

Incidentally, the Performance section in Polar curve should mention that the average thermal strength is part of the equation. --NigelG (or Ndsg) | Talk 16:05, 23 February 2007 (UTC)

the Performance section in Polar curve should mention the expected strength of the next thermal.................... Dansco2903 (talk) 20:04, 26 February 2008 (UTC)

[edit] Effect of wind

I removed the sentence "The optimal airspeed is independent of the wind speed, because the fastest average speed achievable through the airmass corresponds to the fastest achievable average groundspeed." I am fairly sure this is wrong, and indeed the link [1] seems to confirm that actually as a rule of thumb you need to add half the windspeed to the speed to fly in a headwind, and remove half in a tailwind. I'm quite new to this subject so please correct this if it's not right. —Preceding unsigned comment added by 80.42.49.106 (talk) 15:32, 14 February 2008 (UTC)

No, it isn't right: the original statement was correct (though you in turn rightly pointed out that it wasn't referenced!). What you're referring to is a one-off glide to a destination—typically your home airfield on a final glide—in which case you need to maximize your distance from a given height by applying your rule of thumb. STF, on the other hand, determines the best speed to fly over a (longish) series of climbs & glides. In this case you're aiming at the highest possible average speed through the airmass: of course this will result in a higher groundspeed with a tailwind than against a headwind, but that has no bearing on your optimal airspeed. STF theory tells you to fly faster in sink, or as the thermals get stronger, not against a headwind. --NigelG (or Ndsg) | Talk 16:50, 14 February 2008 (UTC)

I would still dispute the claim that "the fastest average speed achievable through the airmass corresponds to the fastest achievable average groundspeed". You're effectively saying that STF is only dependent on the strength of lift and the glider's polar curve. But, to take an extreme example, say the STF is indicated as 70kts. If you're flying into a 70kt headwind, your cross-country speed will be zero. How can this give you a higher cross-country speed than flying at 80kts? Despite having a slightly higher sink rate in the latter, your ground speed is at least positive. In fact, the reference I quote, it actually tells you how to adjust the speed to fly based on headwinds / tailwinds (i.e. move the origin of your tangent line left or right along the airspeed axis). The reason it generally isn't taken into account is probably because there is no glider instrument (at least until GPS came in) that can tell you the wind speed and direction. —Preceding unsigned comment added by 80.42.125.4 (talk) 03:11, 25 February 2008 (UTC)

You're still talking about final glides to a destination. If the MacCready STF is 70 kt, then that is the best you can do in a series of climbs & glides. In your example, your best XC speed will indeed be zero: annoying, isn't it?! If you were to fly too fast (eg at your hypothetical speed of 80 kt), then you would make a little headway during each glide, only to see it cancelled & then reversed in the subsequent climb. If, on the other hand, flying at 80 kt did yield a positive average XC speed, then the original STF can't have been optimal in the first place—which is a contradiction. --NigelG (or Ndsg) | Talk 10:25, 25 February 2008 (UTC)
The speed to fly depends on how fast the air is sinking or rising, the head-wind component and the amount of ballast you are carrying. A head wind effectively shifts the origin of the polar curve to the right. In a 10 knot wind the tangent starts from 10 knots along the horizontal axis. If you read off the horizontal speed where the tangent meets the polar, the spedd will be higher than if there was no wind. I contemplated editing this article but I realised that I have enough to do updating the book at present, so I will let you guys fight it out. Incidentally people who do not register can give the appearance that they are unsure of their facts and so wish to remain anonymous. JMcC (talk) 12:57, 25 February 2008 (UTC)
I've now reverted the change by 80.42.125.4. Please see the extended discussion elsewhere. --NigelG (or Ndsg) | Talk 15:06, 26 February 2008 (UTC)
The rule of thumb for applying half the headwind only applies to the final glide, when flying to a point on the ground, rather than the next thermal. It actually is a special application of the speed to fly theory, and the actual speed to fly is determined by shifting the origin of the tangent line along the horizontal V-axis by the headwind or tailwind component. During the rest of the flight, the pilot is optimizing the speed to the next thermal, and since the thermals and the aircraft are assumed to be equally affected by the wind speed in the classical speed to fly theory, the effects cancel, and the wind speed is not relevant. Dhaluza (talk) 01:52, 4 March 2008 (UTC)
Many thanks for your contribution, which explains the point most succinctly. --NigelG (or Ndsg) | Talk 22:31, 4 March 2008 (UTC)