Vlof,Vmu and Vr
 

hi!!

my first post in pprune..

I am really having a difficult time in differentiating the three.

Its a basic question but i can't seem to figure out the difference.:ugh:

thanx in advance..cheers

Basic explanation:

Vr - rotation speed; the speed at which you should rotate the aircrat (it should be at least 1,1 * Vmu if I remember correctly)
Vmu - minimum unstick speed; minimum speed at which you can lift the nose wheel off the ground (but the aircraft is still unable to fly though)
Vlof - lif off speed; speed at which main gear leaves the ground after rotation

Vmu < Vr < Vlof

Vmu is not when the nose gear lift off the ground. It is for the main gears and the airplane is able to fly after Vmu.

My opinion is:

Vr= indeed rotation speed. Rotating at this speed you will achieve V2
at screen heigth
Vmu= minimum speed at which lift off is possible without tailstrike
Vlof= speed at actual lift off when rotating at VR.

so that means that a/c lifts off the ground at Vmu but would not be able to reach V2 by screen height?? :confused:

I am quite positive on the meaning of Vr being "speed at which the pilot starts to rotate the aeroplane for take-off" and Vmu as "the minimum speed at which the aircraft can leave the ground with all engines operative and climb with no hazard". Not sure on Vlof though, so holding out for some more authoritative posts

because Vmu is important in calculating other speeds...manufacturers go to great lengths to get the lowest Vmu possible and they put a piece of wood as a tail skid or protector...Vmu is the min speed to get airborne but you are allowed a tail strike

Vr leading to V2 at the 35' screen height is imprtant too, esp with engine out...otherwise you will be faster with all engine running at screen height

for practical matters stick to V1, Vr, V2

but in the back of your mind, y ou have to have an idea about VMU...what if you take off on the wrong runway and need to get UP? or if there is traffic on your runway and you need to get over???

A few strange ideas about Vmu in this thread. It means what it says - as, for example, at YouTube - Airbus 380 Vmu demo - it brings a whole new meaning to Tail

Vmu is like a few other certification animals (Vmca, for instance). The routine line operation has no business being near them.

Vr = rotation speed. Rotation at this speed at 'normal' rate of rotation with one engine inoperative you will lift off at Vlof and achieve V2 at screen height. With all engines operating, Vlof and the speed at screen height will be higher than with one inoperative.

Vr must meet a number of criteria, one of which its its relation to Vmu, the minimum speed at which lift off is possible. Due to the vertical component of thrust, the all-engines-operating Vmu is usually less than the one-engine-inop Vmu. In either case Vmu can be limited by one or more of three criteria:
(a) the stall AoA in ground proximity,
(b) the maximum AoA attainable on the ground due to the airplane geometry with mainwheels on the runway and the tail touching the ground, or
(c) the minimum speed at which it is possible to lift the nosewheel off the runway at forward center of gravity.

regards,
HN39

Thanyou hazelnuts

I have always read the geometrical as the main reason for the VMU. But this is the first time i have read the actual other than geometric reasons.

Thanks HN39, didn't know any of that! Thanks for the video link JT, I had actually never seen a practical application of Vmu...good to know it's only academic/flight testing... wouldn't want to pull a trick like that on the line :ooh:

just to add to JT's video link... I thought this is also somewhat interesting with some "Discovery Channel narrative" added