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I got involved a week ago on another forum with a guy who said that the best glide speed was Vmd. I KNOW its not that, for a bit I thought it was where the tangent through the origin touched the drag curve then I went all dizzy and realised I couldn’t even remember where my CFS notes were.

If somebody here can put the lights on, dust the place and present a paper on the subject I sure would be grateful.

JF

Hi John,

I think I know where my old notes are but I'm not getting into the attic at this time of the night!

Yep, from memory I think it's the tangent you describe and for the normal drag/IAS curve I think it was 1.32Vmd that gave you best glide in still air. Offset the y-axis on the x-axis for a head- or tail- wind and keep drawing tangents from the new origins and you get the rule of thumb for gliding more slowly on a tail wind and faster into a head wind.

It all seems a long while ago, however! I've got the mathematical stuff on this somewhere but, er, later...

Regards, Ian

[This message has been edited by Ian Hamilton (edited 10 November 2000).]

By a staggering co-incidence I'm reading a book on the subject at the moment. It goes by the name of "Aircraft Performance - theory and practice" by a chap called Martin Eselby. The publishers sent me a copy for my comments, which so far are fairly favourable, although I can't seem to digest this stuff as fast as I once did.

Consulting it (which to my relief roughly co-incides with my hazy memory), best glide (i.e. best endurance) co-incides with Vmd. However, best range in the glide is a little faster. Specifically it's the speed at which Cd / Cl^(3/2) is a minimum, which comes out at Vmd x 1.316. (1.316 = 3^0.25).

But this relationship assumes still air, and is the speed at which the tangent from the origin meets the drag curve. For a particular case, you should shift the origin left or right a bit to account for the headwind or tailwind component to give the airspeed for best range on the ground.

Ouch, that's too much thinking for a Saturday morning.

G

Thanks chaps. Very helpful.

Only residual snag is that I have received this very morning (in the funny paper mail thingy) a nice algebraic proof that the best RANGE speed is Vimd. So I now have a new problem i.e to find the mistake in that!

(I also have to bear in mind that Wason does not make errors......)

While on the subject of duff gen – in particular P of F – there is a tremendous fund of it on various prune forums. Just goes to show that you don’t have to know how an aeroplane works in order to use one

(bit like me using my watch)

JF


[This message has been edited by John Farley (edited 11 November 2000).]

John,


Is your topic concerned with 'gliders' as in high aspect ratio, no engine devices or does the discussion include powered aircraft?


Not suggesting that I have much to offer, CFS never caught up with me, but I do recollect that 'range speed' is influenced by engine efficiency requirements.

[This message has been edited by lightningmate (edited 11 November 2000).]

Hi L

I was thinking of any heavier than air fixed wing device that had no thrust

I should also have said in my first post that the best gliding speed that I was trying to work out was the one that gave you the most range for the height you had (usually the pilots problem) as opposed to the one that gave you the longest time before you got to the ground.

I fully accept that if you have thrust available from a donk then the best "range" speed will depend on both the airframe and the engine characteristics and as such is a much more complex issue than the simple gliding for range thing.

JF

John,

Thanks for the clarification.

I remember long, drawn out ground school 'arguments' over these questions. You are x miles over the sea etc, you have a y knot headwind your engine fails, what is the best gliding speed to reach land and - do you jettison your external stores?

Beats me how you work all that out, and cope with the engine out emergency problems to boot.

Thankfully, it never happened to me, or at least it never happened with only a single engine. I remember how the Hunter Avon always sounded much rougher crossing the coast outbound, particularly at night!

What! A Hunter pilot flying at night?