VX and VY

[michelda]

https://cimg7.ibsrv.net/gimg/pprune.org-vbulletin/2000x687/f41a5091_1dc2_4649_a1e9_0d783e941ab8_2567ff37cf25632f1ccfcda be42037dde4981cd6.jpeg
Hello everybody,

somebody knows how it can happen?
omitting type mistake, I think it could be only a pitot position problem

[Banana Joe]

https://www.youtube.com/watch?v=0z27FAk1qtE
https://www.boldmethod.com/learn-to-fly/performance/vx-vy/

It's got do with the relationship between IAS and TAS.

[michelda]

Thanks Banana Joe,

but I missed the point.
How can Vx be larger than Vy?

[mustafagander]

Easy Michelda, available power in an atmo engine declines as altitude increases and ROC depends on excess thrust from props or jetpipe. Vx is an aerodynamics thing related to drag. It's all about getting away from the bricks and time is immaterial.

[oggers]

Good question Michelda. It is not because of the relationship between TAS and IAS. It is probably just a mistake.

Whether you talk in TAS or IAS the Vx and Vy converge on the same value at the absolute ceiling. It is impossible for the Vx to increase above Vy. Your question implies you know this already, and your suspicion is correct. If there is a rational explanation for this not being a mistake I would be very pleased to read it. Definitely, nobody has managed one so far.

[Pugilistic Animus]

Like others have shared it is a function of TAS
Vx is the greatest difference in power required and power available. Whereas Vy is the greatest difference between thrust horsepower required and thrust horsepower available where thrust horsepower = Thrust* Velocity (TAS)
​

[michelda]

Hi Oggers,

you had the point and my only answer is the error of the airspeed indicator due to high AOA. The weak point of this theory is the big difference of position error in a very small range of speed.

Last time I studied aerodynamic and instrument was around 30 years ago so I’m not really reliable 😜

[oggers]

Like others have shared it is a function of TAS
Vx is the greatest difference in power required and power available. Whereas Vy is the greatest difference between thrust horsepower required and thrust horsepower available where thrust horsepower = Thrust* Velocity (TAS)
​


It is not a function of TAS. The question is why in that particular AFM the Vx at 10,000' is quoted as higher than the Vy at 10,000'. Vx cannot be faster than Vy at any given weight and altitude, it is impossible and it doesn't matter whether you are tallking CAS or TAS. The OP has asked if it could be attributed to position error, in other words IAS vs CAS. I say no but would be happy to read a cogent explanation that addresses the question actually asked.

[Beakor]

Below from the B757/767 FCTM:

“Maximum Rate Climb
A maximum rate climb provides both high climb rates and minimum time to cruise altitude. Maximum rate climb can be approximated by using the following:
757-200, 757-300
• flaps up maneuver speed + 50 knots until intercepting 0.76M
767-200, 767-300, 767-400
• flaps up maneuver speed + 50 knots until intercepting 0.78M
Note: The FMC does not provide maximum rate climb speeds.

Maximum Angle Climb
The FMC provides maximum angle climb speeds. Maximum angle climb speed is normally used for obstacle clearance, minimum crossing altitude or to reach a specified altitude in a minimum distance. It varies with gross weight and provides approximately the same climb gradient as flaps up maneuver speed.”

So for a 90t jet, Max rate climb speed would be approx 280kts, max angle climb speed approx 230kts.

Max rate is used, for example, to climb to FL280 in the shortest time possible. Max angle to, for example, reach FL280 in the shortest forward distance, say by a specific waypoint.

[Pugilistic Animus]

That's the definition of Vx and Vy. Where Vy is defined as the greatest difference between thrust horsepower required and thrust horsepower available and Vx is greatest difference between power required and power available. Thrust Horsepower= Thrust x Velocity. Where V is equal to TAS. That being said I am not sure of the cause of the Vx Vy switch but if it's in the AFM POH then it's authoritative. Check out HHH Jr. Aerodynamics for Naval Aviators for more information.

[Machdiamond]

This is the Husky A-1B AFM and owners have already wondered about this apparent error, for example in their owner forum:

Crosswind Limits - Some thoughts | Flyhusky.com (http://flyhusky.com/forum/index.php?threads/crosswind-limits-some-thoughts.158/)
See post #14
"One last point I would caution the readers about the POH since you make an excellent point that these numbers are suspect.
[...]
I have asked this question of Vx/Vy difference and the apparent contempt for physics in the newer manuals before. The silence is deafening."

I agree with oggers that it is not physically possible and an AFM typo is the only explanation I can think off.

[fitliker]

VX and Vy are the same at service ceiling :)

[Machdiamond]

A bit further research on the Husky A-1 with published VY that is within one knot of the A-1B, and VX should be very similar. https://www.aviator.at/pics/husky/Checklist_Aviat_Husky_A-1.pdf

However the A-1 published VX numbers are 58 MPH at sea level and 60.5 MPH at 10000 ft. The engine powering the A-1B is generally the same O-360-A1P as the A-1A so the climb speeds should be generally the same also.

Thus the most likely explanation is that it is indeed a typo in the A-1B AFM and the correct numbers are probably 57 MPH at sea level and 60 MPH at 10000 ft, instead of 67 and 70.

[vilas]

Note: The FMC does not provide maximum rate climb speeds.
beakor
That is true but if you put CI 0 FMC will try to reduce the time in climb phase by selecting the best ROC speed.

[scifi]

Let me just correct a few incorrect posts....

Vx and Vy have nothing to do with Engine Power. You have Vx and Vy in gliders.

It's not service altitude, which occurs at +100fpm, it is Absolute Altitude (at 0fpm.)

Many Thanks..

[Machdiamond]

Let me just correct a few incorrect posts....
Vx and Vy have nothing to do with Engine Power. You have Vx and Vy in gliders.


Engine power has a major aerodynamic effect on the wing, lift to drag ratio, and Vx and Vy.

One step further, what happens to Vx when you have enough power to climb vertically?

[tttoon]

Maybe 10,000ft is above the absolute ceiling, and the Vx and Vy values are of an airplane in descent after a zoom climb?

I'll get my hat!

[janrein]

A plausible explanation for the error is that all the numbers are correct but stated in the wrong place, i.e. that the Vx and Vy values at altitude had ended up swapped in the AFM. It would be interesting to know what the absolute ceiling is as this is where the increasing Vx and the decreasing Vy would cross.

?

jr

[Machdiamond]

Flight manual of A-1B says absolute ceiling is 16000ft (by extrapolating the climb curves at ISA temperatures from 6000 ft to 10000ft).

Maybe my prior message was not clearly expressed: the A-1A is basically the same aircraft as the A-1B and has all similar performance data except, suspiciously, Vx which is 58MPH at sea level and 60MPH at 10000 ft, likely the correct values for the A-1B give or take a knot.

Another potential explanation besides a typo is that Aviat does not like recommending climbing at that speed and decided to pad some margin on VX, because it is so close to stall speed on that aircraft. An engine failure in a climb at the real VX would result in a nearly instantaneous stall.

I have myself added such margin above the real VX for that particular reason when publishing AFM data of a (quite different) single engine aircraft. If you keep that margin constant, VX ends up crossing VY at some altitude then, apparently violating the laws of physics. That is because it is not the real VX but rather a recommended best angle of climb speed that adds some safety margin above the stall speed. These margins are normally not disclosed by the manufacturers, even upon specific requests.

This is my best guess, I am not familiar with the Husky.

[jimmyg]

We are looking at a propeller aircraft and we must equate for two different animals. Power = Force X Velocity (Thrust X TAS)

For the Jet aircraft, all performance considerations come back to Thrust, except in the ABSTRACT sense in evaluating Rate of Climb by comparing Thrust X TAS at varying speeds. Because (for the jet) Power increases with increasing speed, best Rate of Climb speed is very high, often in EXCESS of the cruise speed to follow the climb. The opposite is true for the propeller aircraft.

The best Angle of Climb occurs at a speed where there is Maximum Excess Thrust. This speed is, for PRACTICAL purposes, Vmd. Best Angle speed is somewhat less than the normal En-Route Climb speed.

The best Rate of Climb occurs at a speed where there is Maximum Excess Power. As jet engines directly produce Thrust, not Power, it is necessary to consider Thrust multiplied by speed (Power = Force X Velocity). Thus, for a given Thrust setting, Power increases as TAS increases. Thrust actually ‘dips’ as speed increases, but then there is significant Ram recovery at higher Mach Numbers, thus further increasing Power at higher speeds.

[oggers]

Flight manual of A-1B says absolute ceiling is 16000ft (by extrapolating the climb curves at ISA temperatures from 6000 ft to 10000ft)....Another potential explanation besides a typo is that Aviat does not like recommending climbing at that speed and decided to pad some margin on VX, because it is so close to stall speed on that aircraft.

And yet the slower Vy remains so that doesn't make any sense. I think the most plausible explanation is the other one you gave ie:

the A-1A is basically the same aircraft as the A-1B and has all similar performance data except, suspiciously, Vx which is 58MPH at sea level and 60MPH at 10000 ft, likely the correct values for the A-1B give or take a knot.

Also, the AFM says that in the case of alternate static "No airspeed correction necessary". Although it is not definitive, this suggests that the position error is not a significant factor.

Edited to say: under 'Balked Landing' in the AFM the sea level Vx is quoted to be 74 mph (!). Also, according to their 'Normal Procedures' the climb speed to be established for max performance take-off is 65 mph whereas in the performance section they quote 70 mph for the max performance take-off. All in all I think it is fair to say that Aviat (and the FAA) have made a right old mess of getting the performance data into the manual.

[RandomPerson8008]

I do think the built in margin over stall for the published Vx is most likely. Perhaps in Aviat's opinion, Vy doesn't get close enough to stall speed until you're high enough to be well clear of terrain in most cases to bother applying the same correction.

Here is a an aerodynamics project we worked on during University for the Van's RV-7. Sea Level, flaps up, 1800 lb (MGTOW), ISA standard conditions:

Published Vx = 66 KTAS.
Calculated Vx = 58 KTAS

Vs = 53 KTAS.

Compare that to Vy, which we calculated at 84 KTAS and was published as 84 KTAS. The inflation to published Vx was undoubtedly Van's making a decision to prioritize stall margin over climb performance. If an experimental manufacturer/builder does this, it's almost certain a certified manufacturer would, given the liability differences.

https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/1547x848/vx1_f27338277793ebbedef49667d00c69c39d84a41b.jpg
https://cimg6.ibsrv.net/gimg/pprune.org-vbulletin/1429x854/vx2_c466e463c5ffea35d3edbcf1dbdf345e55bc4850.jpg

[scifi]

Glad to see Uni Students are still working to 5th places of decimals, you wouldn't want to ever divide by zero would you...

[oggers]

I do think the built in margin over stall for the published Vx is most likely. Perhaps in Aviat's opinion, Vy doesn't get close enough to stall speed until you're high enough to be well clear of terrain in most cases to bother applying the same correction.

I really don't think that makes sense. It is not uncommon for Husky owners to operate into bush strips at density altitudes of 10,000 plus and the company is based in the Rockies so I doubt they would assume 10,000' is going to be well clear of terrain. Anyway, the clean stall speed is 58 mph whilst the SL Vx is quoted as 67 rising to 70 at 10,000' ie the margin gets greater with altitude, which is the opposite of the above rationale.

If it was about choosing a Vx to increase the stall margin - which is hypothetically plausible - it would defeat the object to retain a Vy for 10,000' which was slower than a Vx that had been chosen to be a safe minimum. Quite apart from the absurdity of recommending a Vx which actually results in a lesser climb gradient than the corresponding Vy, particulalry in an aircraft we know is often operated in mountainous terrain at those altitudes. Considering that there are at least 3 anomalies existing in this AFM around Vx, it is obvious that they have simply made a mistake. These things happen.