I think it's a good question. I have often asked myself that. The only conclusion I could come to based on some research is that Va is obviously based on the structural integrity of the aircraft for abrupt control movements also related to severe turbulence. Rough air penetration speeds are related to passenger comfort but more importantly aircraft stability based on the abilities of controlling the aircraft in moderate or greater turbulence. It was probably determined during initial flight tests. I noticed that the CR7 is a much more stable and a comfortable ride when you slow it to .75/280kias whichever is lower when you encounter moderate or greater turbulence. That number is a constant where as Va changes based on weight and altitude. That could be total B.S. but it's a start.

i think it might also have some thing to do with the fact that manuvering speed is only a positve g load where turbulence could subject you to asymetric and negative loads that could exceed the aircraft limits.

Here is what part 25 says:

Design maneuvering speed V A
For V A,the following applies:
(1) V A may not be less than V S√ n where—
(i) V S is a computed stalling speed with flaps retracted at the design weight, normally based on the maximum airplane normal force coefficients, C NA ; and
(ii) n is the limit maneuvering load factor used in design
(2) The value of V A need not exceed the value of V C used in design.


Design speed for maximum gust intensity, V B
For VB, the following apply:
(1) VB may not be less than the speed determined by the intersection of the line representing the maximum positive lift, CNMAX, and the line representing the rough air gust velocity on the gust V-n diagram, or VS1√ ng, whichever is less, where:
(i) ng the positive airplane gust load factor due to gust, at speed VC(in accordance with §23.341), and at the particular weight under consideration; and
(ii) VS1 is the stalling speed with the flaps retracted at the particular weight under consideration.
(2) VB need not be greater than VC.


They both do the same thing but for different reasons.

Sounds like symantics to me..... stall before you break the wings off... maneuvering speed....

Or stall before the turbulence breaks the wings off...Vb...

I look at Vb another way. It is the speed that gives you the max positive or negative Gs and still fly generally straight and level.

This is with respect to not trying to full deflect any one control surface. Va is the speed at which a control surface with stall with full deflection prior to structural damage. There is a difference there.

Side note my airplane manufacture says at any speed if you go full deflection in one directed immediately followed by full deflection in the other direction it will do structural damage! Yikes!

Va deals with more than just popping wings. It deals with control surfaces of the aircraft. A full deflection could stall the tail before the wings. Of course Va is also being rethought by the FAA since there have been instances where full scale deflections still caused damage. Think of the airbus that sheered it's tail off after being caught in the wake of the 757.

Vb also deals with structural integrity of the aircraft I believe. IE those fancy CRJ900 or CRJ 1000s caught in some bumbs. The wing might never stall but that long body could snap or crinkle.... Since you aren't making full scale control movements in turb, where inertia plays a role, it's possible the control surfaces or wings won't stall before structural damage occurs while flying at Va. If I had to take a guess, which I would, I'd think it would apply to heavier aircraft or those with a smaller load factor per sq ft IE MD-80, CRJ-900s, 757etc.

It is important to note that Va only protects you against a full, abrupt, positive elevator control movement.

It does not say anywhere that if you kick the rudder or roll the ailerons fully that you are protected because these control surfaces do not directly increase load factor!

I am a little less educated on Turbulent air penetration speed. however, i believe we are on the right track, you stall before you break. Or maybe they figure out the amount of load (positive, negative, twisting) of each part of the airframe and take the lowest so that in turbulence the aircraft will regain structural integrity

btw, way to exemplify your desire to know the bare minimum about something and encourage others to do the same...kinda relates to other decisions you've made too! Don't chastise someone for wanting to know more about aviation. Just cause you wanna know the bare minimum, doesn't mean we all do.

You would expect them to be similar.

Va is the max speed where full and abrupt control movements will not destroy the airplanes structure. Either the wing will stall first, or the airflow over the control surfaces is insufficient to generate damaging force. DANGER: Va may not, and probably does not, apply to the rudder on transport category aircraft (not sure about non-transport)...see AA 587 for more details.

Vb basically protects the wing from gust loads by ensuring the wing will stall before gust loads generate damaging forces.

I don't know about the rest of you but I'm not interested in determining what constitutes "full and abrupt movement" of control surfaces in a transport category aircraft. And which of those surfaces may be excluded...I need my tail to fly....