HELP!!! I'm a 650 hour flight instructor, preparing a briefing about large aircraft performance for my upgrade...
Can anyone give me good definitions and explanations for Vmu, Vr and Vlo? the books I have available are very vague about it... Any other interesting info about performance and the factors which affect it will be much appreciated
Vmu - The miniumum demonstrated unstick speed at which it is possible to get airbourne on all engines and to climb out without hazard.
Vr - the (pre-planned) speed at which the pilot initiates rotation, to achieve V2 by the screen height even with an engine failure. The monitoring pilot calls "Rotate" at said speed and the Handling Pilot pitches up towards whatever the target pitch is.
Vlo - The speed at which the main gear lifts off. Will be higher than Vr as the aircraft will continue to accelerate between the nose gear and the main gear lifting. Not a speed used for planning purposes though.
There are lots of factors which affect and lots more interesting info about performance, but for that I'd have to write a book, not merely a prune post!
Can anyone tell me what Vef is? Also - I know the definition of Vmu, but what is the significance of knowing what your Vmu is? It is not published in the manuals as far as I know... in what kind of situation would you need to get airborne at Vmu (something on the runway?) Just wondering...
Vr Speed. Vr is defined as rotation speed and is applicable to transport category airplanes certified under SR 422A and later rules and commuter category airplanes. Vr is determined so that V2 speed is reached before the aircraft reaches 35 feet above the runway surface. Vr may not be less than Vmu or 1.05 Vmca.
Vef Speed. Vef is the airspeed at which the critical engine is assumed to fail. Vef is selected by the aircraft manufacturer for purposes of certification testing, primarily to establish the range of speed from which V1 may be selected. Vef may not be less than Vmcg.
Vmu Speed. Vmu is defined as minimum unstick speed. Vmu is the minimum speed demonstrated for each combination of weight, thrust, and configuration at which a safe takeoff has been demonstrated.
what is the significance of knowing what your Vmu is?
There is no significance in knowing it as an operating pilot. Nor is there any significance in the pilot knowing what Vef, Vloff, Vmcg, Vmca are, because they are NOT operational speeds. They are of extreme importance to the Performance Engineers in deriving the day to day practical speeds, such as V1, Vr, and V2, after applying suitable margins and tolerances.
As for Venr, perhaps appropriately labeled as Vcl, Vfto, V4 - The final Takeoff speed following engine failure with the aircraft in the Clean configuration, and the remaining engine/s set at Maximum Continuous Thrust / Power.
I donít feel the actual value of Vmu has much (if any) significance. What matters is that the tests prove that with an early rotation and the tail subsequently held on the ground an unstuck is still possible.In other words you cannot stall the wing during the takeoff run. In years gone by it was possible to stall some wings by early rotation, the original Comet being an example
After reading John Farley's post, immediately following mine, I should have added "and procedures" to "deriving the day to day practical speeds". Knowledge of speeds such as Vmu allows the procedural designers to intelligently create such procedures as might be necessary in the event of windshear etc. when early rotation may be essential.