Vyse and Vxse being equal

[BigGrecian]

A question for the flight test engineers etc - (or Genghis the Engineer.)

Why do many light twins i.e the BE76 publish the same speed i.e 85kts for both Vyse and Vxse.

We know that Vy is an excess of power, Vx is an excess of thrust but in reality are these both equal when one engine is inoperative or are they arbitary and issued by the manufacturer to ensure safe flight and to do with helping to maintaing directional control?

Any thoughts would be appreciated - although some science is what we're really after....

[Genghis the Engineer]

Well I was an FTE once, I'm not sure I know what I am now. Boffin seems to do it.


I'm about 60 miles from my Duchess POH, but I have had this issue on an SEP I was (part of the team) certifying. Vx was below Vy (as ever); however both presented us with controllability issues at those speeds with full power.

So, we incerased both to a co-incident "best climb" speed that complied with the design code, but also gave what we judged was an acceptable degree of controllability.

Yes, I'm guessing - but taking a quick look through FAR-23 there is certainly provision for this to be done if the certification team chose to. My guess is that there was some controllability issue, but adequate power - so a single "best climb single engine" speed was selected, which met the minimum requirements, but also gave good controllability, and declared as both Vxse and Vyse - since that's what FAR-23 says they had to provide in the manual.

G

['India-Mike]

I'm wary of dipping my toe in here as despite having a current MEP class rating I'm very inexperienced on the class. But I can always be distracted by a good technical problem like this.

The PA34 which is the sum total of my light twin experience has Vxse=78kias and Vyse=89kias, so there's a bit of a split on that one.

However in general Vxse>Vx, and Vyse<Vy due to the relative shapes of the thrust and power available curves vs speed, when displaced downwards to reflect the loss of an engine. The thrust and power required of course will shift up very slightly. The net result is that Vxse and Vyse tend to be closer together than Vx and Vy. See Robson's book 'Multi-engine piston' pp81-82.

In summary the Vxse and Vyse speeds tend towards one another - specific aircraft may therefore have the two figures closer than the PA34 ones I quote above.

We really need someone like madfltscientist on this one.

[FlyingForFun]

Very good question. I've always explained it away by saying that the climb performance is so poor that really the only speed at which you'll be able to climb at all is 85kt.... but I've always had this little niggle that my explaination doesn't completely make sense.

To answer Genghis' suggestion - the BE76 most definitely can be flown at speeds lower than 85kt on one engine. Vmca is 65kt, and I do a Critical Speed demonstration with all my students which shows that we loose control typically at around 70kt. What's more, I often find myself berating students for getting too slow on EFATO drills - and while many of them do loose directional control when they're too slow, some of them somehow manage to keep it going the right way!

The full set of numbers, by the way is:

Vmca = 65kt
Vx = 71kt
Vy = 85kt
Vxse = 85kt
Vyse = 85kt

Vs1 is 70kt, but that's with power off. With power on, even on just one engine, actual stall speed is significantly lower.

I'd agree with India Mike's theory, but it doesn't match the POH numbers for the BE76. Genghis' theory explains perfectly why this might not be the case, but my practical experiences don't match Genghis' theory.

Interesting......!

FFF
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[FlyingForFun]

Hmm... ok, I've not been on this site regularly for quite a few years. But since when did it change so that if I type an acronym like "E F A T O" it automatically expands it for me? I'd have thought anyone reading a flight testing forum wouldn't need that one explaining!!!

FFF
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[Genghis the Engineer]

There's a function on the website that has certain words displayed as certain other words. Probably the best use of this is that laser doesn't get displayed in a way that Google ads picking it up and displaying adverts for lasers (there was some concern about somebody shining them at aircraft and using the defence in court that the device was advertised via a pilots website!).

Anyhow, maybe it's time to look at FAR-23....


Sec. 23.67 Climb: One engine inoperative.

(a) For normal, utility, and acrobatic category reciprocating engine-
powered airplanes of 6,000 pounds or less maximum weight, the following
apply:
(1) Except for those airplanes that meet the requirements prescribed in
Sec. 23.562(d), each airplane with a VSO of more than 61 knots must be able to maintain a steady climb gradient of at least 1.5 percent at a pressure altitude of 5,000 feet with the--

(i) Critical engine inoperative and its propeller in the minimum drag
position;
(ii) Remaining engine(s) at not more than maximum continuous power;
(iii) Landing gear retracted;
(iv) Wing flaps retracted; and
(v) Climb speed not less than 1.2 VS1.
(2) For each airplane that meets the requirements prescribed in Sec.
23.562(d), or that has a VSO of 61 knots or less, the steady gradient of
climb or descent at a pressure altitude of 5,000 feet must be determined with the--
(i) Critical engine inoperative and its propeller in the minimum drag
position;
(ii) Remaining engine(s) at not more than maximum continuous power;
(iii) Landing gear retracted;
(iv) Wing flaps retracted; and
(v) Climb speed not less than 1.2VS1.

(b) For normal, utility, and acrobatic category reciprocating engine-
powered airplanes of more than 6,000 pounds maximum weight, and turbine engine-powered airplanes in the normal, utility, and acrobatic category--
(1) The steady gradient of climb at an altitude of 400 feet above the
takeoff must be measurably positive with the--
(i) Critical engine inoperative and its propeller in the minimum drag
position;
(ii) Remaining engine(s) at takeoff power;
(iii) Landing gear retracted;
(iv) Wing flaps in the takeoff position(s); and
(v) Climb speed equal to that achieved at 50 feet in the demonstration of
Sec. 23.53.
(2) The steady gradient of climb must not be less than 0.75 percent at an
altitude of 1,500 feet above the takeoff surface, or landing surface, as
appropriate, with the--
(i) Critical engine inoperative and its propeller in the minimum drag
position;
(ii) Remaining engine(s) at not more than maximum continuous power;
(iii) Landing gear retracted;
(iv) Wing flaps retracted; and
(v) Climb speed not less than 1.2 VS1.

<stuff about commuter category and four engined aeroplanes deleted for brevity>



This actually seems to explain it fairly simply. Vxse and Vyse are both required to be at-least 1.2Vs1

70kn Vs1 x 1.2 = 84; I'm guessing that the 1 knot difference is just down to PECs, and that the actual values of Vxse and Vyse are equal to, or less than, 1.2Vs1.

G

[FlyingForFun]

Aha! That does make sense!

FFF
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['India-Mike]

Yep, the PA34 Vxse is consistent with 1.2 x Vs1 - to about 1 knot anyway.