At times like this, it's often best to refer to the rulebook: -
V2, Take-off safety speed' means a referenced airspeed obtained after lift-off at which the required one-engine-inoperative climb performance can be achieved. (From JAR-1, definitions)
The rotation speed, VR, in terms of calibrated airspeed, must be selected by the applicant and must not be less than the greatest of the following:
(i) V1; or
(ii) 1.05 VMC determined under JAR 23.149 (b); or
(iii) 1.10 VS1; or
(iv) The speed that allows attaining the initial climb-out speed, V2, before reaching a height of 35 ft above the take-off surface in accordance with JAR 23.57 (c)(2).
(From JAR-23, the usual light aircraft standard)
The take-off safety speed, V2, in terms of calibrated airspeed, must be selected by the applicant so as to allow the gradient of climb required in JAR 23.67 (c)(1) and (c)(2) but must not be less than 1.10 VMC or less than 1.20 VS1.
(Also from JAR-23, 23.67 is very complicated but depending on conditions the minimum climb gradient is generally between 1 and 2%)
And if you thought that JAR-23 was complicated, try the following from JAR-25, the airliner standard...
) V2MIN, in terms of calibrated airspeed, may not be less than--
(1) 1.13 VSR for--
(i) Two-engined and three-engined turbo-propeller powered aeroplanes; and
(ii) Turbojet powered aeroplanes without provisions for obtaining a significant reduction in the one-engine-inoperative power-on stall speed;]
[(2) 1.08 VSR for--
(i) Turbo-propeller powered aeroplanes with more than three engines; and
(ii) Turbojet powered aeroplanes with provisions for obtaining a significant reduction in the one-engine-inoperative power-on stall speed: and
(3) 1.10 times VMC established under JAR 25.149.]
(c) V2 in terms of calibrated airspeed, must be selected by the applicant to provide at least the gradient of climb required by JAR 25.121(b) but may not be less than--
[(1) V2MIN;
(2) VR plus the speed increment attained (in accordance with JAR 25.111(c)(2)) before reaching a height of 35 ft above the take-off surface; and
(3) A speed that provides the manoeuvring capability specified in JAR 15.143(g).]
Commenting upon what other people have said before. V2 is the speed that should be established by a given height, which varies according to conditions (although 35ft seems normal). This is a separate (but inevitably related) issue to the take-off distance, which is the distance to reach the screen height, which is (in my experience) usually 15m (or 50ft) using the recommended take-off speeds and methods.
Changes in screen height are unlikely to be due to ground effect (which is generally considered to become negligible at about 2 wingspans), and would probably be associated with a matched increasingly stringent set of requirements for airport approval.
G
[This message has been edited by Genghis the Engineer (edited 16 January 2001).]