It is a function of stall speed at your current weight (one G) and the design load capability of your aircraft. Therefore, Maneuver Speed varies....it is not one hard and fast number.
PantLoad,
That isn't really correct. It is only correct when Va = Vo. Va ONLY refers to control surface's and their supporting structure's integrity.
One more caveat....maneuver speed does not protect you from abrupt full and opposite control input. So, while it may protect you from loads from extreme turbulence, it does not protect you from rapid, full, and opposite control inputs.
That is true that you can overstress the
control surface or its supporting structure if you apply full control deflection of more than one control surface or if you oscillate the control surface. But Va isn't a turbulence speed.
Vb, Vc and Vd are all related to turbulence. If
Vb wasn't published for your airplane, you should fly at the Vo speed for your weight and configuration. If Vo isn't published you can easily calculate it by using the formula: Vo=Vs*sqrt(n), ensuring the correct stall speed for your weight is being used.
Steve Pomroy has taken the time to write a few good articles on these subjects:
The Flight Writer: Va: Not a Bad Speed, Just Misunderstood (Part 1)
The Flight Writer: Va: Not a Bad Speed, Just Misunderstood (Part 2)
The Flight Writer: Tubulence Penetration