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BMM389EC
31st Jul 2001, 03:18
Two questions:
1. What effect does weight have on Vmca? From what I can gather none, but then why does the definition say at maximum weight etc, etc?
2. With improved climb, why is Vr sometimes higher than V2?

Looking for answers for an enquiring mind!!!

john_tullamarine
31st Jul 2001, 11:04
Vmca normally is determined for whatever turns out to be the critical case ... things like high side thrust engines, low altitude, etc.

As an aside, one of the oft forgotten aspects is that, as the GW reduces, V2 reduces (following the reducing Vs1) until the Vmca requirement becomes limiting (especially with a four engined aeroplane). As a result, one ought to be very careful to control roll during an early climb failure, lest the bank angle variation in Vmca present one with a period of excitement and a very real risk of losing the aeroplane.

Vr less than V2 ? ... could you be mixing the min V2 with the overspeed V2 numbers while at the same time just considering the overspeed case Vr ? If this is the case, and I can only speculate, V2, for the overspeed case, is scheduled above the min V2 value precisely for the reason that, at the (slightly) higher speed, climb performance materially improves. Perhaps you might expand on your initial comment ?

E. MORSE
31st Jul 2001, 12:31
The improved climb procedure is a technique to increase the normal V2 speed towards the optimum gradient speed.

The resulting gradient improvement can then be traded for higher weight , or sometimes better obstacle clearance.

With an increase in V2 , Vr has to be increased as well.
(Obviously it will require additional runway)

It sounds strange however to have a Vr higher then V2.
Allthough there seems to be no requirement that V2 must be higher then Vr !

What airplane are you talking about anyway ?


Cheers
;)

BMM389EC
1st Aug 2001, 01:32
Checked up the figures today for the Vr as mentioned earlier. It only appears to be on a JT8D-9 powered 737-200 that the Vr is higher than the V2 by about 2kts on an improved climb. For the same tempratures, etc on a JT8D-15 it is not the case. So I assume the lower thrust output on the -9 has something to do with it?

john_tullamarine
1st Aug 2001, 02:51
Something's wrong, somewhere. In essence, Vr is linked to V2 by the time/speed increase necessary to do the rotation flare in the OEI case. With the 732, target pitch is 12-13 deg which takes around 4-5 seconds with a split ranging from 6-7 kt at low weights to a couple of knots at the high weight end.

By any reasonable definition, Vr must be not greater than V2.

I suspect that your data - or the overspeed correction procedure - has an error somewhere.

Do give me a call via email - I would love to follow this through for my own reasons.

747FOCAL
1st Aug 2001, 03:15
Read the FARs. V2 may not be less than Vr although they can be equal. Just like Vr can not be less than V1.