Imbalance,
VA is the speed at which an aircraft stalls just as it attains its limiting load factor. In the case of aircraft certificated under JAR 25 that is 2.5g. This means that VA is the 2.5g stalling speed.
Load factor is equal to lift divided by weight, so an aircraft pulling 2.5 g is generating lift equal to 2.5 times its weight. So if the weight is increased, so will the amount of lift required at VA. CL Max is fixed (unless we change configuartion) so to achieve this extra lift the aircraft must fly faster. This means that VA increases and decreases with weight in exactly the same way as the 1g stalling speed.
That is to say, VA at the new weight is equal to the VA at the old weight multiplied by the square root of (the new weight over the old weight).
JT is of course correct in stating that VA refers to the max weight condition, but this fact is usually ignored in JAR ATPL exams.
Last edited by Keith.Williams.; 10th October 2002 at 20:03.