A fair point. Va is calculated at MTOW.
At a lower weight, the aircraft will stall at a lower speed, and thus the normal acceleration limit will be reached at a lower speed. But, the normal acceleration limit is also calculated at MTOW and if you work through the theory, you could increase the positive g limit as you reduce weight. Please forgive my pathetic attempts to do equations in PPruneScriptTM but...
Normally Va = VsMTOW x N1MTOW ^½
But, working with total loading on the wing, you can assume that N1actual=N1MTOW (MTOW/W)
And also, scaling stall speed with weight, Vs1actual = Vs1MTOW x (W/MTOW)^½
Thus...
Va = Vs1MTOW x (W/MTOW)^½ x (MTOW/W)^½ x N1^½
Which cancels out to Va = VsMTOW x N1MTOW ^½
Or in other words, Va can be considered constant regardless of aircraft weight, since the structural strength of the aircraft is actually based upon N1 at MTOW, and not just N1 in isolation.
Clearly there are going to be exceptions to this, but only in more complex types, particularly military aircraft with variable wingborne stores, since in those cases the load distribution over the wing is not constant. But even in those cases, unless Va is actually reduced by this, you'd have to leave it alone because of the effect upon stressing of the primary flying controls - this I am pretty certain is universal to all military and civil airworthiness codes.
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Last edited by Genghis the Engineer; 17th Apr 2003 at 22:19.