PPRuNe Forums - B787 Vmu

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- - B787 Vmu (https://www.pprune.org/tech-log/441815-b787-vmu.html)

This is a link to the B787 VMU test (only a short excerpt shown...) and has the following text attached to it.

Quote:

In the velocity minimum unstick test or VMU, pilots carefully drag the 787's tail on the runway to determine the lowest speed needed to leave the ground.
This test establishes the lowest speed the airplane can leave the ground and requires putting the tail on the runway. “You don’t want to hold the tail on the ground because that’s actually a big brake. It will slow the airplane down and you’ll never get up to take-off speed but then again you can’t let the tail come up because then you’re missing your data. So the idea is to get the tail down quickly, smoothly and just hold it on the ground, just, just hold it on the ground and then hold that altitude and then you have to fly away.”

It seems that the tail was held on the ground very very briefly compared to other VMU tests for other types. Is the "big brake" explanation really the issue or was there a worry of the effects of a tailstrike on the composite structure?

Quote:

Originally Posted by 320Driver

Is the "big brake" explanation really the issue or was there a worry of the effects of a tailstrike on the composite structure?

I don't think drag of the tail skid is the issue, nor is "worry of the effects of a tail strike". The test is successful if the geometry-limited attitude is attained at or before lift-off. To avoid tailstrike and over-rotation one normally wants to have the tail on the runway well before reaching Vmu, but that requires sufficient elevator effectiveness at airspeeds well below Vmu. On some airplane types, the minimum speed at which the nosewheel can be lifted off the runway determines Vmu, rather than the maximum body angle.

regards,
HN39

Usual typo....

Quote:

So the idea is to get the tail down quickly, smoothly and just hold it on the ground, just, just hold it on the ground and then hold that altitude and then you have to fly away.

The typo is of course "altitude" for "attitude".
A bit like the eternal confusion between AoA and pitch attitude....

But... 320DRIVER, thanks for the links!
Interesting to see that the 1:1 Revell kit is coming along.

Oh, and,... HN39, I agree.

CJ