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By Rob P
#1825692
We use 85 kt.

Rob P
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By Rob P
#1825730
Really?

Other sources suggest best glide will be roughly halfway between Vx and Vy, which introduces a nice element of circularity into the discussion.

Rob P
By TopCat
FLYER Club Member  FLYER Club Member
#1825733
Flyin'Dutch' wrote:Best glide = Vy if Best glide is furthest glide.

No. It will be close to it, probably, but they're not the same.

Given that Vy is the speed where the excess of power available vs power required is a maximum, it is dependent on power.

Best glide is the speed at which L/D is maximum, which isn't.

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Edit: added 'is a maximum' above. Now a whole sentence, sorry!
Last edited by TopCat on Tue Feb 09, 2021 3:18 pm, edited 1 time in total.
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By Flyin'Dutch'
FLYER Club Member  FLYER Club Member
#1825746
TopCat wrote:
Flyin'Dutch' wrote:Best glide = Vy if Best glide is furthest glide.

No. It will be close to it, probably, but they're not the same.

Given that Vy is the speed where the excess of power available vs power required is a maximum, it is dependent on power.

Best glide is the speed at which L/D is maximum, which isn't.

__
Edit: added 'is a maximum' above. Now a whole sentence, sorry!


TC so you think that Vy is dependent on power?
By TopCat
FLYER Club Member  FLYER Club Member
#1825768
Flyin'Dutch' wrote:
TopCat wrote:
Flyin'Dutch' wrote:Best glide = Vy if Best glide is furthest glide.

No. It will be close to it, probably, but they're not the same.

Given that Vy is the speed where the excess of power available vs power required is a maximum, it is dependent on power.

Best glide is the speed at which L/D is maximum, which isn't.

__
Edit: added 'is a maximum' above. Now a whole sentence, sorry!


TC so you think that Vy is dependent on power?

Well it's a good question.

I suppose if you took the engine out, replaced it with one of a different power but the same weight, and the power available curve was exactly the same shape as the one before (just higher than, and parallel to it at all points), it wouldn't be, as the airspeed at max(available - required) would be in the same place as before.

So I guess I should have said that it's dependent on that power excess, not just the power on its own.

But that doesn't change the fact that max(L/D) doesn't happen at Vy, except by accident.
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By Flyin'Dutch'
FLYER Club Member  FLYER Club Member
#1825778
TopCat wrote:But that doesn't change the fact that max(L/D) doesn't happen at Vy, except by accident.


Physics rarely has anything to do with 'by accident'

I am sure that Vy is not exactly the same as best glide as a power off (of idling engine) airframe is a different airframe than that with a pulling engine, but it will be close.
By TopCat
FLYER Club Member  FLYER Club Member
#1825851
Flyin'Dutch' wrote:
TopCat wrote:But that doesn't change the fact that max(L/D) doesn't happen at Vy, except by accident.


Physics rarely has anything to do with 'by accident'

Well of course, by 'by accident' I was neither saying nor implying that physics is non-deterministic. "By accident" in this context means 'as a deterministic consequence of at least partially different causes'.

I am sure that Vy is not exactly the same as best glide as a power off (of idling engine) airframe is a different airframe than that with a pulling engine, but it will be close.

No one is disputing that it will be close. What is neither obvious, nor shown in anything I've read on the subject, is a relationship between Vy (the airspeed for max(power available - power required)), and the airspeed where L/D happens to be a maximum in the glide, which of course has nothing to do with the engine.

Flyin'Dutch' wrote:Best glide = Vy if Best glide is furthest glide.

... but what is clear, is that this is not so.

Any more detail than this is above my pay grade, so hopefully someone with real knowledge of aeronautics will be along. It would be nice to know in detail why the numbers are close.
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By Paul_Sengupta
FLYER Club Member  FLYER Club Member
#1825878
Engine power goes up as RPM goes up, and with a fixed pitch prop the RPM will go up as speed goes up. I'd have thought the pitch of the prop would affect the power curve. This is the reason why, apparently, for a fixed pitch prop at least, Vy is a fair bit higher than max range glide.

https://aviation.stackexchange.com/questions/73997/how-can-best-glide-speed-be-lower-than-best-rate-of-climb-speed

"Nevertheless mentioned Cessna converts fuel to mechanical energy through not-so-magical fixed pitch propeller which pushes some amount of air around. Such propeller would be typically optimized for highest efficiency at the cruise speed, so with decreasing speed the amount of available mechanical energy decreases. Which results in changed shape of power curve and maximum point moves towards higher speeds compared to the "unpowered" power curve.

This shift can be high enough that Vy ends up higher than Vg.

The Vy is always higher than the speed for the best climb angle, because these two speeds are found on the same (max power) power curve. For the same reason best glide speed will be always higher than speed for minimum sink. But speeds for best rate of climb and for the least sink does not need to be the same because of effects of real propeller."
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By GrahamB
FLYER Club Member  FLYER Club Member
#1825893
Genghis the Engineer wrote:Vy will be the speed, roughly speaking, where you have the greatest excess thrust that you can turn into climb rate.

Sorry G but that is wrong.

Vy is the speed where you have the greatest excess power; Vx is the speed where you have the greatest excess thrust.
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By Genghis the Engineer
FLYER Club Member  FLYER Club Member
#1825902
"greatest excess thrust you can turn into climb rate" was what I said, and a prefixed that with "roughly speaking".

But yes, excess power is to be fair, a better term.

Your definition of Vx however is wrong - Vx is all about climb angle, not thrust.

G
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By GrahamB
FLYER Club Member  FLYER Club Member
#1825903
Genghis the Engineer wrote:Your definition of Vx however is wrong - Vx is all about climb angle, not thrust

Of course, it’s defined as the speed that gives best angle of climb, but that’s where excess thrust is at its greatest.