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User avatar
By Rob P
#1825022
Been giving this a little lockdown thought.

Is Vy affected by prop pitch or engine power or does it remain constant?

Obviously an identical airframe with more power or a prop optimised for climb will get to a given height faster.

But does the speed at which to pitch for the maximum rate of climb vary, or stay constant given the same airframe at the same weight?

Rob P
User avatar
By GrahamB
FLYER Club Member  FLYER Club Member
#1825059
Flyin'Dutch' wrote:If the airframe remains the same Vy will not change.

Weight and configuration (flaps) will make a difference.

Actually there’s more to it than that.

Vx is the point on the thrust available v airspeed curve and drag v airspeed curve where the excess of thrust minus drag is greatest.

Vy is similarly the speed at which the excess of power over that required to sustain straight and level is greatest.

So changing the power or thrust curves may well change both points of excess over the basic aerodynamic drag/power required curves.

When I applied an STC to my 150hp Lycoming to increase compression ratio, effectively changing it to a 160hp model, it very definitely increased Vy.
User avatar
By Genghis the Engineer
FLYER Club Member  FLYER Club Member
#1825392
Vy will be the speed, roughly speaking, where you have the greatest excess thrust that you can turn into a climb rate.

Changing prop pitch, will change that speed/thrust curve - there would be no point in doing so if you didn't. So I would anticipate normally Vy to increase with a coarser prop, and reduce with a finer prop.

G
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By Flyin'Dutch'
FLYER Club Member  FLYER Club Member
#1825403
GrahamB wrote:When I applied an STC to my 150hp Lycoming to increase compression ratio, effectively changing it to a 160hp model, it very definitely increased Vy.


Your rate of climb will have increased, no doubt, but the speed at which that occured?

@Genghis the Engineer

I don't think that is correct but happy to be shown wrong; which reminds me, some time ago you stated that an aeroplane making some fuel stops was more efficient than loading up go juice for the same total distance in one go, you offered to get back to us with some CPL notes you'd made.
Did you find those?
Last edited by Flyin'Dutch' on Sun Feb 07, 2021 9:29 pm, edited 1 time in total.
User avatar
By Flyin'Dutch'
FLYER Club Member  FLYER Club Member
#1825410
No.

Vx and Vy are unique to an airframe of a given configuration and weight. They are different figures and vary with different weights and different configurations.

At higher weights those speeds will be higher and the achieved climb rates lower.
With a changed wing configuration i.e. deployment of flaps will lower the speeds but due to increased drag the achieved rates will be lower.
Only if you can reduce drag (as with some gliders) can you achieve a better rate at a higher speed (which in gliders translates in a better L/D ratio)
User avatar
By GrahamB
FLYER Club Member  FLYER Club Member
#1825434
Flyin'Dutch' wrote:
GrahamB wrote:When I applied an STC to my 150hp Lycoming to increase compression ratio, effectively changing it to a 160hp model, it very definitely increased Vy.


Your rate of climb will have increased, no doubt, but the speed at which that occured?

Yes, because I reckon not only did the increase in compression ratio raise the power v airspeed curve, but changed its shape as well, thus shifting the point of excess to the right.
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By Genghis the Engineer
FLYER Club Member  FLYER Club Member
#1825442
Flyin'Dutch' wrote:
GrahamB wrote:When I applied an STC to my 150hp Lycoming to increase compression ratio, effectively changing it to a 160hp model, it very definitely increased Vy.


Your rate of climb will have increased, no doubt, but the speed at which that occured?

@Genghis the Engineer

I don't think that is correct but happy to be shown wrong; which reminds me, some time ago you stated that an aeroplane making some fuel stops was more efficient than loading up go juice for the same total distance in one go, you offered to get back to us with some CPL notes you'd made.
Did you find those?


I don't think that they were from my CPL, but it's a discussion I've had a few times. From one of the RAeS Greener by Design reports I think, although I first saw those figures in the early 90s on my undergraduate degree. I'll have a dig.

G
User avatar
By Flyin'Dutch'
FLYER Club Member  FLYER Club Member
#1825576
Genghis the Engineer wrote:Changing prop pitch, will change that speed/thrust curve - there would be no point in doing so if you didn't. So I would anticipate normally Vy to increase with a coarser prop, and reduce with a finer prop.

G


HI G,

I asked if you were sure that was true.

Not sure it is, for starters as there are no tables suggesting that to get a better rate of climb one has to speed up and coarsen pitch.

It also goes against the notion that there is one Vy and one Vx for a given airframe.
User avatar
By Rob P
#1825578
Maybe now you see why I asked the question?

Trying to establish Vy for the RV I found it very difficult as there are multitudes of answers online and no great consensus.

Rob P
User avatar
By Flyin'Dutch'
FLYER Club Member  FLYER Club Member
#1825581
Rob P wrote:Maybe now you see why I asked the question?

Trying to establish Vy for the RV I found it very difficult as there are multitudes of answers online and no great consensus.

Rob P


Awful problem to have. :)

You will have go and try.
User avatar
By Rob P
#1825586
Well I know how long it took me to reach FL100 last week courtesy of SkyDemon, but that's not very helpful as I wasn't concentrating on holding any specific angle/speed and there would have been a lot of variation.

So my thinking was to establish Vy and try it next time.

Normally I climb at 90kts, but I have no idea why I have fixed on this figure, other than it feels about right.

Edit: Thanks @Flyin'Dutch' So according to that my ad hoc speed is just about in the middle. However, with deeper searching you will find lots of alternative figures offered.

Rob P.