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By Genghis the Engineer
FLYER Club Member  FLYER Club Member
#1825905
GrahamB wrote:
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.

The point where excess thrust is at its greatest is actually the point of greatest acceleration in level flight - a condition of basically no interest to anybody but fighter pilots.

G
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By GrahamB
FLYER Club Member  FLYER Club Member
#1825907
Genghis the Engineer wrote:
GrahamB wrote:
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.

The point where excess thrust is at its greatest is actually the point of greatest acceleration in level flight - a condition of basically no interest to anybody but fighter pilots.

G

A nice coincidence then. :D
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By lobstaboy
#1825959
GrahamB wrote:
Genghis the Engineer wrote:
GrahamB wrote:Of course, it’s defined as the speed that gives best angle of climb, but that’s where excess thrust is at its greatest.

The point where excess thrust is at its greatest is actually the point of greatest acceleration in level flight - a condition of basically no interest to anybody but fighter pilots.

G

A nice coincidence then. :D


It's not a coincidence. It's physics - it has to be the same.
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By Genghis the Engineer
FLYER Club Member  FLYER Club Member
#1825965
I just looked it up.

Hale's "Aircraft Performance, selection and design", my favourite aircraft performance and sizing book from when I was an undergraduate, shows a bunch of maths and how to work it out, showing it is a bitch to work out.

Wikipedia says it's at the point of max excess thrust.

Antonio Filipone's book "Flight Performance of Fixed and Rotary Wing Aircraft" which is considerably more thorough than either shows even more maths, and a condition for maximum angle of climb for a piston-prop aeroplane with six terms in the equation.


Then I stopped and sketched it out.

Yeah, you chaps are right, I'm over-complicating it.

G
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By Flyin'Dutch'
FLYER Club Member  FLYER Club Member
#1825998
Genghis the Engineer wrote:Antonio Filipone's book "Flight Performance of Fixed and Rotary Wing Aircraft" which is considerably more thorough than either shows even more maths, and a condition for maximum angle of climb for a piston-prop aeroplane with six terms in the equation.


Ignoring for a minute that this would be max angle rather than Vy - out of interest what are those 6 terms?
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By Genghis the Engineer
FLYER Club Member  FLYER Club Member
#1826038
Flyin'Dutch' wrote:
Genghis the Engineer wrote:Antonio Filipone's book "Flight Performance of Fixed and Rotary Wing Aircraft" which is considerably more thorough than either shows even more maths, and a condition for maximum angle of climb for a piston-prop aeroplane with six terms in the equation.


Ignoring for a minute that this would be max angle rather than Vy - out of interest what are those 6 terms?


Air density, engine power, weight, wing area, propulsive efficiency, engine power. and the Oswald span efficiency variable. (plus speed and angle)

G
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By lobstaboy
#1826085
Genghis the Engineer wrote:
Flyin'Dutch' wrote:
Genghis the Engineer wrote:Antonio Filipone's book "Flight Performance of Fixed and Rotary Wing Aircraft" which is considerably more thorough than either shows even more maths, and a condition for maximum angle of climb for a piston-prop aeroplane with six terms in the equation.


Ignoring for a minute that this would be max angle rather than Vy - out of interest what are those 6 terms?


Air density, engine power, weight, wing area, propulsive efficiency, engine power. and the Oswald span efficiency variable. (plus speed and angle)

G


Engine power twice - must only apply to twins then?
:)
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By AndyR
FLYER Club Member  FLYER Club Member
#1826178
Rob P wrote:We use 85 kt.

Rob P


Sorry Rob. Got distracted and then haven’t had time to visit since.

It’s kinda been argued already, but generally Vy is pretty close to best glide for most airframes and most mere mortals.
There is a difference, there are exceptions, if you’re after an exact number then an aeronautical engineer is probably better. But in the real world and in practice you won’t be too far out.

Interesting discussion though :thumleft:
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By Rob P
#1826192
Hmmmm. 85kt seems very low. I guess some experimentation is needed if I ever get to fly again.

Rob P
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By GrahamB
FLYER Club Member  FLYER Club Member
#1826197
It seems to me that using best glide speed as an approximation for Vy is a pretty hit and miss approach:
Code: Select all              C152        Vy 67Kt IAS         Best glide 60Kt IAS
              C172        Vy 76Kt IAS         Best glide 65Kt IAS
              AA5B        Vy 90Kt IAS         Best glide 72Kt IAS
              TB10        Vy 78Kt IAS         Best glide 86Kt IAS
              WarriorII   Vy 79Kt IAS         Best glide 73Kt IAS
              Dakota      Vy 85Kt IAS         Best glide 85Kt IAS
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By TopCat
FLYER Club Member  FLYER Club Member
#1826341
GrahamB wrote:It seems to me that using best glide speed as an approximation for Vy is a pretty hit and miss approach:
Code: Select all              AA5B        Vy 90Kt IAS         Best glide 72Kt IAS
 


And for comparison between the AA5B and AA5A:

Code: Select all              AA5A        Vy 79Kt IAS         Best glide 72Kt IAS
 

The Cheetah and Tiger airframes are virtually identical, but the Tiger (AA5B) has a more powerful engine (and a beefier spar).

So it's not surprising that their best glides are the same, but the Vy values significantly different.
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