Vx climb performance chart?

[Ulysse]

My POH indicates a Vx of 79kias and a Vy of 96kias

There is a chart giving climb performance at 96kias at different weight/temp/alt but nothing regarding climb performance at Vx.

Is this just my POH or does Mooney not provide this chart for their aircafts ?

[kortopates]

Interesting question but we only use Vx to clear the 50’ or so proverbial obstacle - never to climb to altitude. The actual angle is very dependent on wind which is if it’s not calm is changing significantly between the surface and 50’.
what is useful though is the airspeed we need to get above an obstacle right off the runway but then we’ll transition to Vy or faster once clear.


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[carusoam]

Climbing at Vx has two issues…

1) change of wind, in a shear… can be challenging/disastrous… extending the time at Vx increases the exposure…

2) CHT control is really tough… Where climbing at Vy comes with high CHTs as it is…

 

Many people select 120ias for good climb performance and temp control…

So… if you can’t find a climb chart using Vx… these are probably the reasons…

PP thoughts only,

-a-

[MikeOH]

My thoughts are that Vx is nearly a constant speed that does not depend on temp, or altitude. Kind of like best glide speed. So, as you climb Vy decreases but Vx stays about the same and they converge at the aircraft's absolute ceiling.  Vx, I think, represents the angle-of-attack where the wing is most efficient while under power.

My $0.02

[rbp]

I found this article very helpful: https://www.boldmethod.com/learn-to-fly/performance/vx-vy/

 

[Ulysse]

2 hours ago, kortopates said:

we only use Vx to clear the 50’

The reason I would like to use Vx a little longer is that my home airport is surrounded by buildings. So in case of engine failure during initial climb, there is no way you can land straight ahead.

There is a less dense area on the left side of the DER, so I was trying to compute the height I would need to turn 90° at best glide speed and still make it there. 

If I climb at Vy, I might be too far away.

17 minutes ago, rbp said:

I found this article very helpful: https://www.boldmethod.com/learn-to-fly/performance/vx-vy/

Thank you

Edited April 30, 2022 by Ulysse
Coquille

[MikeOH]

From a 1968 M20F POH:

 

[PT20J]

Vy is the speed where there is the maximum excess power available (any speed involving a rate is related to power since power is the rate of expending energy).

Vx is the speed where there is maximum excess thrust available. Thrust is maximum at zero airspeed and decreases as speed increases. At low speeds, the thrust required (drag) is dominated by induced drag.

The higher the aircraft thrust to weight ratio, the lower Vx. (Jet fighters can climb vertically at zero angle of attack). So, in trainers, it makes sense to practice climbs at Vx. In high performance airplanes, Vx can be close to the power off stall speed and it is wise to weigh the altitude gain against the possibility of a low altitude stall if the engine quits. Best be primed to abruptly lower the nose.

Skip

[PeteMc]

4 hours ago, Ulysse said:

There is a chart giving climb performance at 96kias at different weight/temp/alt but nothing regarding climb performance at Vx.

My K (231) POH has a Speed At 50 Ft. IAS chart for different weights on the page with the graphs for Takeoff Distance Over A 50 Ft. Obstacle.

There is no mention of V speed or if  clean or dirty, but I've always believed that is Mooney's recommended climb speed at weight until you get above the obstacle.  Then you go to Vy or whatever climb speed you want. 

 

[A64Pilot]

I don’t think and have posted this before and been argued with in the impossible turn thread that climbing at Vx with the idea that’s it’s safer is incorrect.

My opinion is that if the engine suddenly quits what you need to return is energy, yes climbing at Vx will give you the most altitude over distance covered but your at a low airspeed so therefore low energy, speed and altitude are after all interchangeable.

Then add in that pretty much anyone if climbing at Vx has a sudden engine failure will most likely find themselves in a “mush” that is very close to stall with a high rate of descent, to break this mush you have to push the nose hard over and swap altitude for airspeed, you’ll end up far closer to the ground than the guy who was climbing at Vy as they have more time to react due to higher airspeed, plus most are adverse putting the nose at the ground and building airspeed when the ground is rushing up at them.

Try it, climb to several thousand ft altitude to give yourself more room than you think you could possibly need to recover, then trim for a Vx climb at full power, then pull the throttle to idle rapidly.

If you don’t practice this, it’s unlikely you’ll pull it off successfully, but with practice you may. The first time you try a maneuver shouldn’t be when your trying to use it to save your life.

It’s why I believe upset training is so valuable, or as we called it in the Military, unusual attitude training.

 

Edited April 30, 2022 by A64Pilot

[jaylw314]

IIRC, practically, Vy decreases with altitude and Vx increases with altitude.  Usually, but depending on the aircraft, Vx increases faster than Vy decreases.

[1980Mooney]

4 minutes ago, jaylw314 said:

IIRC, practically, Vy decreases with altitude and Vx increases with altitude.  Usually, but depending on the aircraft, Vx increases faster than Vy decreases.

@MikeOH posted his M20F Vx table above, It shows 94 mph as constant Vx from sea level all the way to 16,000 ft.  - no change.  And yes Vy decreases.

 

[A64Pilot]

I believe what he said is correct, maybe he got it backward, I think Vx changes less, Vx and Vy change with altitude, and in fact converge of course at absolute altitude, because that’s as high as you can climb,

http://cospilot.com/documents/Why Vx and Vy Change with Altitude.pdf

Flight at absolute altitude is possible only at one speed of course

Edited April 30, 2022 by A64Pilot

[PT20J]

Yes, Vx increases with altitude whereas Vy decreases and Vy decreases faster than Vx increases. But, it’s not critical because the spread is pretty small at altitude. If I need max performance climb at high altitude - mountain flying for example - I just use a speed halfway between sea level Vx and Vy. 

[Ulysse]

3 hours ago, A64Pilot said:

very close to stall with a high rate of descent, to break this mush you have to push the nose hard

Yes, I understand that the startle time added to the time to go from 79 Vx climb speed to best glide around 90 (and probably more to make the turn) does not leave much room if the engine out happens at 500ft. You are right that speed and height are interchangeable from an energy point of view, but speed also means more distance, so I would need more than a 90deg turn to get to the open space. Almost the impossible turn!

[jaylw314]

2 hours ago, 1980Mooney said:

@MikeOH posted his M20F Vx table above, It shows 94 mph as constant Vx from sea level all the way to 16,000 ft.  - no change.  And yes Vy decreases.

 

LOL, yes, I started second guessing myself when I looked at that.  I suspect the aircraft plays a big role and it is not a general rule

[MikeOH]

34 minutes ago, jaylw314 said:

LOL, yes, I started second guessing myself when I looked at that.  I suspect the aircraft plays a big role and it is not a general rule

I suspect you are right, dependent on the particular aircraft.  As @PT20J points out, a jet with thrust greater than weight is a whole 'nother animal!  But, for small SEL pistons, I don't think Vx varies much.

[carusoam]

There is an aged discussion around here…

Recovering from an engine out during the initial climb…

“Getting light in the seat” are the memorable key words…

And having the nose down un-weights the wing, to help with stall prevention at less than 1g…

and praying to several gods of your choice may be helpful…

 

Also work on brief engine monitor checks as power comes up… there may be key things to look for beyond MP and rpm…

my big fear of using a short runway often… buildings/houses off the far end… 

 

PP thoughts only, not a CFI…

-a-

[PT20J]

53 minutes ago, MikeOH said:

I suspect you are right, dependent on the particular aircraft.  As @PT20J points out, a jet with thrust greater than weight is a whole 'nother animal!  But, for small SEL pistons, I don't think Vx varies much.

My M20J POH says Vx increases approximately 1 knot per 5000'.

[MikeOH]

1 minute ago, PT20J said:

My M20J POH says Vx increases approximately 1 knot per 5000'.

So, 3 knots delta to 15,000 feet.  As I said, I don't think Vx varies much