V2

[+TSRA]

ozziekiwi,

Here are the explanations I obtained direct from Beechcraft and Bombardier when developing ground schools for the 1900D and DH8 (Classic and Q400). Couple years old now, but like weather, I don't think aircraft performance changes much:

Vcef is the highest speed at which the aircraft can be accelerated, experience a loss of the critical engine, and continue or stop in the computed minimum field length. Vcef must be demonstrated to be at or above Vmcg.

Because most pilots are not test pilots, most regulators require that 1 second of acceleration be added to Vcef. Vcef + 1 second = V1.

V1, therefore, is both the maximum speed at which the pilot must take the first action to stop the aircraft within the accelerate-stop distance (ASDR) and is also the minimum speed, following a failure of the critical engine at Vcef, at which the pilot may continue the take-off to achieve a height of 35' AGL by the end of the Take-Off Distance Required (TODR).

Vr is the speed at which rotation begins. Although some books define it as when the nose wheel leaves the ground, it is not. It is the speed at which rotation may begin. Performance data for first segment climb begins not at Vr but at a speed known as Vlof - Lift Off Speed.

Vr must be demonstrated during flight testing be be not less than 5% above Vmca, to result in at least the minimum Vlof demonstrated during flight testing, and must permit acceleration to V2 prior to reaching 35' above the runway. Furthermore, manufacturers must demonstrate that Vr will not result in an increase in TODR if rotation is begun 5 knots lower than published Vr during one-engine acceleration or 10 knots lower than established Vr during all-engine acceleration.

Vlof, on the other hand, must be at least 5% above the one-engine inoperative minimum unstick speed (Vmu-oei) and 10% above the all-engine operative minimum unstick speed (Vmu-aeo). The minimum unstick speed is literally the minimum speed at which the aircraft will first become airborne - those cool pics of tails being dragged down a runway and the aircraft becoming airborne at stupid low speeds.

V2, while defined as the "take-off safety speed" is actually a factor of the stall speed, specifically it is:
- 1.2 x Vs for two and three engine aircraft.
- 1.15 x Vs for four engine aircraft.

So while V2 is does ensure the aircraft can be controlled, it is more closely linked to achieving a speed as close to the maximum lift/drag coefficient to enable maximum first segment climb performance, thus ensuring close-in obstacle clearance.

Let me explain why V2 is not specifically linked to aircraft control - All "V" speeds must be demonstrated to be at or above Vmcg/Vmca (where applicable). Where they are shown to be below Vmcg or Vmca, the applicable speed is arbitrarily raised to the next highest speed (normally). So while all V-speeds do ensure the aircraft can be controlled, only Vmcg/Vmca specifically deal with it. All the others deal with other performance factors.

Hope this helps to generate some technical discussion!