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View Full Version : Vmca vs Flap again


HEALY
23rd Oct 2007, 02:39
Hi

It seems that increase in flap can on some aircraft increase the Vmca. I see this as being for the following reason.

1) Increase Flap = increase lift on live engine due to slipstream and subsequent roll and yaw to dead side.
2) Aileron on dead side defected down to compensate which leads to higher angle of attack and higher drag which requires more rudder to compensate which is now no longer available.

However certification requires the Vmca speed to be no more than 1.2Vs. This would mean a lower Vmca speed with associated lower stall speed with flap increase.

So, is there a more definitive answer or a case of individual aircraft having different chararcteristics.

Cheers

Nathan Parker
23rd Oct 2007, 04:44
It seems that increase in flap can on some aircraft increase the Vmca. I see this as being for the following reason.Can you cite some examples, along with data? The reverse is more commonly the case.

Increase Flap = increase lift on live engine due to slipstream and subsequent roll and yaw to dead side.The accelerated slipstream can indeed increase lift on that wing, which would increase rolling tendencies. However, the drag would tend to decrease Vmc, because it would oppose the yaw due to the asymmetrical thrust.

If it's a aileron-limited Vmc, rather than rudder-limited, the increased rolling moment could well increase Vmc.

Aileron on dead side defected down to compensate which leads to higher angle of attack and higher drag which requires more rudder to compensate which is now no longer available.Possible, but my guess would be that the deflected flap in the slipstream would more than offset this effect.

However certification requires the Vmca speed to be no more than 1.2Vs. This would mean a lower Vmca speed with associated lower stall speed with flap increase.It's not clear to me that this requirement applies to all configurations.