V2 Vs ratio
 

Hi folks,

I´m a bit confused. Can some one explain me why in airbus V2 / Vs ratio used for takeoff can range from V2 being 1.2 Vs to 1.35 Vs?

Actually, there are probably some Airbuses where it's lower too, since they'll be using "Vsr" and you're allowed as low as 1.13Vsr now.

The reason it will be varying will be one of:

(1) Some other requirement is overriding the V2min=1.2Vs requirement - such as V2min=1.1Vmca. At light weights you might be forced to respect that limit instead, which would give a V2 or greater than 1.2Vs. There are a number of different speed scheduling regulations, one of which is the 1.2vs requirement. ANY of them might apply.

(2) Choice. Higher V2 will usually give improved obstacle clearance or WAT limits, but at the expense of runway distance. They may simply have elected for a higher V2.

My money is on (1) if these are "standard" V2 numbers.

MFS, do you mean that there are some aircraft where 1.13 Vsr is less than 1.2 Vso?

Yes.

The idea behind choosing 1.13 for a Vsr aircraft was supposed to be that the resulting speeds would be equivalently safe to the old 1.2Vs rule. But inevitably there will be aircraft using Vsr which end up with slightly lower V2 speeds as a result.

I was under the impression that the whole Vsr thing was to avoid falsely low stall speeds. The Vsr thing doesn't rely on test data that could show lower than actual stall speed. So I would be surprised if any 1.13Vsr speeds are lower than the equivalent 1.2 Vs.

Agreed, the motivation to move to the "1-g" stall speed definition was to avoid the variability and nonsense that was inherent in the "min speed" Vs methodology. Whereby a skilled TP could always shade the manoeuvre to generate a speed that was, shall we say, on the low side.

BUT, Vsmin and the 1.2Vs rule "worked". V2 wasn't causing any safety problems. So there's no way industry would have accepted simply changing Vs to Vsr and bumping up all the speeds - because Vsr is ALWAYS higher then Vsmin, since 'g' is less than 1.0 in a Vsmin stall. So, they went with equivalent safety - and picked a V2/Vsr ratio which on average would give an equivalently safe V2 speed. Basically, the same aircraft tested "both ways" would on average give a Vsr about 6% higher than Vsmin. So, knock about 6% off 1.2 and you get 1.13, hence the 1.13 Vsr ratio.

That, however, is an average. There will be specific aircraft where it works the other way, inevitably. One example would be an aircraft where the stall is completely defined by stick pusher activation. In such a case the slowdown to the stall will be pretty much at 1.0'g' - there's no aerodynamic pre-stall buffet which might be associated with a slight drop in 'g' to 0.98 or so. As soon as the pusher fires, the pilot's going to recover. That is pretty much going to be Vsmin right there. It's also going to be the Vs1g speed as well. If nothing else applied then you'd have Vs1g=Vsr=Vsmin=Vs. Which would put the Vsr-derived V2 some 6% lower than the Vs-derived one.

To account for that, at least some Vsr/Vs1g aircraft with pusher-defined stalls apply a 2% penalty to the Vs1g to derive Vsr. That still gives a 4% margin, where the Vsr-derived V2 can be lower.

Thanks MFS,
It's always a pleasure to read your posts.