dc10
Posts: 122
Joined: Sun Apr 01, 2007 11:53 pm

### VMU

Hi, I saw yesterday a reportage on TV about test flights, and I saw a very impressive take-off of a test plane A330 at VMU:
1/ what is exactly the VMU? I guess it's the minimum speed you have to reach for take-off, but is it calculated without pax/freight?
2/ Is the VMU dependand of the weight of the aircraft?
3/ If yes: If there is pax/freight, what's the difference between VMU and Vr? Vr is the "rotation speed" (ie) the speed you have to reach to rotate and take-off, so it's "the smallest" speed you can take-off and so Vr=VMU?
4/ If no: is there any use to know this speed for an air carrier pilot?
Thanks!
DC10

VC-10
Posts: 3546
Joined: Tue Oct 26, 1999 11:34 am

### RE: VMU

I Think you are getting your Vr & V2 speeds confused.

Vmu is the minimum demonstrated unstick speed. The min speed at which it is possible to leave the ground, all engines, and climb out without undue hazard.

Vr is the speed at which the pilot starts to rotate the a/c for T.O.

V1 Decision speed in the event of an engine failure on T.O.

V2 T.O. safety speed. The lowest speed at which the a/c complies wuth the handling criteria associated with the climb after T.O. following an engine failure.

The Vmu test came about after a number of accidents that occured in the early days of the Comet. In these cases the a/c rotated but failed to T.O.

It was reported that the a/c acceleration did not build up normally due to the progressive nose up attitude on T.O. This phenomena was a new one experienced with the low drag of the jet airliners. Without the extra lift generated by propwash across the wings, the wing was stalling on rotation. As a result every a/c has to carry out the Vmu test to ensure it won't ground stall.

Once again the Comet led the way.

aaron atp
Posts: 517
Joined: Sun Mar 19, 2000 1:17 pm

### Minimum Unstick

In operation, Vmu is normally only used when one needs to lift off the surface without continuing the climb immediately. Soft field takeoffs are done at Vmu to get the wheels out of the grass (or off of any other surface) so the aircraft can continue to accelerate. It may also be done when leaving hard runways that are in such poor condition that the gear could be damaged by rolling on the surface at high speeds -because of potholes et al.

aaron

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