PPRuNe Forums - View Single Post - What is higher, Vmcg or Vmca on a B747-400 and why?
Old 10th August 2003 | 21:07
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jtr
 
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Was feeling a bit of an ass pushing for more info, but in light of your last post john, I will ask my bit.

I dont see how on a "normal" multi the certified speeds are going to show a Vmca higher than Vmcg.
When you consider the distance from the wheel turning fulcrum (lets say a central point b/t all wheel sets for arguments sake) and the CG, and make it a % of the total arm distance (rudder to CG) it will result in a force application difference in the order of 5% (guestimate) I find it hard to imagine any of the other factors you mentioned earlier, flap settings, tyres, gear layout, TOW, etc having a combined effect on the two speeds such that it exceeds the advantage obtained from the combination of the CG/wheel pos, and the allowable bank. When I look at where my guestimate of the position of a 22% MACTOW is on the -400 (typical T/O pos.), and compare it to where the turning fulcrum of the tryes is, the distance is not insignificant. Couple this with the H/stab doing all that it can at 120 odd knots, and I cant see how it is going to be any other way. In my head, this translates throught to all smaller jet a/c, though stick a prop on the wing, and it may be a bit more complex.

I realise this is really just academic, as the speeds in reality are probably quite a bit lower than the published ones, but still stand on the a. speed being lower than the g. speed.

Please appreciate I am not trying to bait you into an argument, just trying to clarify in my head what you are saying.

Thanks
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