Yes thanks Keith for the wise words on that.
Out of interest where did you find this question? I assume it was some QB of some kind.
Mutt - you still want me to "nail" me on that weight issue!
I do believe from what I read, Vmcg does not change with weight, Vmcg can only change because of the density - pressure, temperature etc.
Low pressure altitude, high density, low temperature etc. will make Vmcg higher, however V1 is effected by weight.
I have some book somewhere, which states that fact, can't get the exact quote now, but it was something like if you are Field Limited, V1 will decrease with increased mass (of course there will be a limit, where you will have to start reducing more mass, to get a better V1), and if you not field limited V1 will increase with weight.
This does make logical sense, as an example your MTOW is 60T, if you are field limited and can only take off with 57T, what will you do if during your calculations you find out that your V1 is below Vmcg?
You have to find some way to increase V1, because Vmcg will not change unless the weather changes or you use reduced thrust, which might put you within limits.
However to use reduced thrust, you might also need to reduce the mass, but disregard this for one moment, just on the V1 issue. To change V1, you need it to reach a higher V1 within a shorter distance, the aircraft must accelerate faster, and reach that point earlier, if it has less mass. Now as it is faster, it will also require a longer stop distance, you will have this because it reached V1 faster and earlier as you took some weight of it.
However to be able to stop within these constraints, you might need to reduce the mass more, as increasing V1 GO to Vmcg is not enough, if V1 STOP is to high, for it to be a balanced V1, the mass must be reduced to make sure you can stop or go at your V1, and the main thing to give you this would be reduction of mass.
If you are not field limited, it does not matter, you increase your acceleration distance, you can increase your V1, as long as you can Go or Stop, and if not field limited that will not be an issue, in that case V1 will be increased with mass, as you will need a higher Vr with increased mass to take off. And no field limit, of course you want to be able to make that V1 has high as possible, to give you best options in case of an engine failure!
V1 must be less or equal to Vr. Ideally you would like that V1 was equal to Vr, as in the worst case you could still decide to stop, however for this to happen your runway can't be field limited due to your TOW. If it is, the main way to adapt is reduce the weight!
Last edited by truckflyer; 4th June 2012 at 22:50.