Originally Posted by
Old Smokey
Great link BOAC, sorry if I can't resist adding a little more......
V2min is arbitrarily pegged at 1.2 X Vs for older aircraft, and 1.13 X Vs for some more modern aircraft. This provides for a good compromise between satisfactory Field Performance and Climb Gradient Performance. This speed inevitably falls below the best Gradient speed, which is very close to Vmd (for jet aircraft), and thus V2min provides for less than optimum climb gradient performance. (Vmd is usually in the range 1.35 to 1.40 X Vs).
Following this, IF there is sufficient Runway available to achieve a V2 between V2min and up to Vmd, improved Climb Gradient Performance results. Of course, the aircraft must have been certified for this second performance schedule.
It is also because of this, that AFMs advise that if engine failure occurs above V2 (typically V2+10 for 1.2 Vs aircraft, and V2+15 for 1.13 Vs aircraft), then maintain that increased speed, or reduce to it if above. It is note-worthy that the same AFMs don't caution against using the 'additive' speed if already using the Increased speeds V2 schedule - THEY SHOULD!
It is important to emphasise that if engine failure occurs at V2min or at less than V2 plus the approved additive, that existing speed should be maintained, and no attempt made to increase to the +10 or +15 speed. To do so is a potential CFIT in the making, to achieve the acceleration to the higher speed, it would be necessary to 'duck under' the required flight path, which may be obstacle limited.
May be, but it is not always. The airplanes seem to be restricted to WAT limit weights even if there are no obstacles ahead...
Originally Posted by
Old Smokey
inner, you asked "does the increased V2 increases our thrust or decreases or drag??". As described above, drag decreases. Thrust will actually be less, due to the effects of the thrust equation, and V2 being well below that when Ram recovery begins to take place (usually around about M0.5), and this is why the best gradient speed is SLIGHTLY below VMD (but not so much that you'd notice).
Regards,
Old Smokey
And what is the best V2? With zero operating engines, obviously the best gradient would be at Vmd... best gradient speed with all engines operative is at somewhat below Vmd (because thrust derates), and thus, the best gradient with one engine inoperative would be at a speed slightly faster than the best gradient with all engines, but slower than Vmd...