V1 is the same as Refusal Speed...which DOES depend on actual runway length.
It's the maximum speed that you can accelerate to, lose an engine, and stop in the remaining runway distance.
Hacker, in big jets V1 can be based on a lot more than just the Refusal speed, and it usually varies from jet to jet.
I'll give an example. In the KC-135 S1 (decision speed, or go-no go speed... same as V1) was the largest of Vmcg, Vcefs, Vnefs unless Vrotate was less than the largest of those three, in which case it was Vrotate. In no case could the S1 be larger than Vrefusal or Vmax brake, or you would be in a take-off No go scenario. (Basically you aren't allowed to take off).
So, some definitions:
Vmcg or V "
min
control on the
ground"- the minimum speed at which you could lose the downwind, outside engine (worst case) and still maintain directional control within particular limits (which I can no longer remember) and without pulling power on any other engines or using differential braking. So basically, the min speed at which you can continue the take-off and maintain directional control of the airplane if you lose the worst case engine. Vmcg was a controllability number, rather than a performance number. There was also a Vmc
a- min control
airborne, but it was guaranteed to be less than Vrotate, so we didn't worry about it.
Vcef or "
critical
engine
failure
speed"- The speed at which if you accelerated to it on all engines operating, and then lost an engine it would take the exact same distance to continue the take-off as it would to stop the airplane. That distance was defined as the critical field length. Vcefs is a performance number, and it basically says that before you reach that speed, stopping takes less distance than going. After that speed going takes less distance than stopping, and AT Vcefs, they're the same.
Vnef or V "
non-
critical
engine
failure
speed"- Some civillian airplanes define a non-critical engine failure speed that is based off of the engine that is NOT the worst case failing... but that is not what a KC-135 Vnef meant. In the 135, Vnef was basically an adjusted refusal speed. You took the actual runway available and subtracted 2000' and with this adjusted runway you computed a refusal speed. This adjusted refusal speed was our Vnef, and basically if Vnef was greater than Vcef you had a "non-critical takeoff." If Vcef was bigger, you had a critical takeoff. In a critical takeoff, we were not allowed to reduce the thrust, unless we needed to for controllability. It is a stop speed, but it allowed us to keep a bad airplane on the ground longer than using Vcef or Vmcg for decision speed, and still retain a 2000' margin of safety on stopping. We would only use these numbers if the take-off were a non-critical scenario, or in other words, if the take-off were easy to make on the available runway.
Vrot - Rotate speed. self explanatory
Vrefusal- Refusal speed. The fastest speed you could accelerate to and still stop in the remaining runway available. It's a stop speed.
Vmax brake- The fastest speed you could accelerate to and still stop the airplane before the brakes exploded. Interestingly enough, sometimes this number was smaller than Vrefusal, and if it was it meant that your Vrefusal number was bogus. It's also a stop speed.
So the philosophy was basically that you had to be faster than your fastest minimum "Go" speed, and still slower than your slowest maximum "Stop" speed to have a safe take-off.
Of course, we had a mission computer that would compute it all, or it would take all day to do it by hand. The guys who flew tankers before the computer would do it, though.
