PPRuNe Forums - VX and VY

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- - VX and VY (https://www.pprune.org/tech-log/632140-vx-vy.html)

Quote:

Originally Posted by Machdiamond (Post 10772845)

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:

Quote:

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.

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....c39d84a41b.jpg
https://cimg6.ibsrv.net/gimg/pprune....5e55bc4850.jpg

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

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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.