Maneuver Speed

[inbalance]

My books say that the Va (maneuver speed) increases with higher weight. Why is that ???
All the books and my instructors agree with that, but non of them could tell me why.

[buttonmonkey]

Isn't it something like,

A heavy aircraft flying at a given speed must fly at a greater AoA than an identical aircraft that is lighter in weight flying at the same speed.

AoA at Va and stalling AoA are proportional for a given weight.

So for the heavier aircraft to maintain the same margin of AoA below stalling AoA (because it's already flying at an increased AoA for a given speed) The AoA must be reduced by increasing speed, in order to restore the margin of AoA below stalling AoA, which results in an increased Va for an increase in weight.

But I could be completely wrong and it could all be down to the lift fairies!

[john_tullamarine]

The principal factor determining Va is the intersection of the limit load factor and the stall line for the design weight .. ie max weight stall speed while pulling limit g. What this means is that the aircraft is somewhat protected against excessively ham-fisted piloting in pitch.

Playing with the algebra shows that

Vs is proportional to sqrt(nW)

or the stall speed varies as the sqrt of the gross weight.

Looking at different weights, W1 and W2, the respective manoeuvring speeds, Va1 and Va2, are related by the equation

Va1/Va2 = sqrt(W1/W2)

As the flight manual Va relates to maximum weight (although some schedule Va varying with weight), it is better to say something along the lines of "Va REDUCES with REDUCING weight".

[healyg]

va is there to ensure that the airframe is not overstressed when the max deflection of any control surface is applied.

As weight increases , the effect of a full deflection of a control surface will take slightly longer due to inertia.

This causes less stress to the airframe, so va can be increased.

[Keith.Williams.]

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.

[john_tullamarine]

Keith and I probably ought to have made the additional observation that the design standard requires consideration of control deflection in each axis, not just pitch.

It is quite possible, if not all that usual, for the Va to be more limiting due to, for instance, directional problems. If my recollection serves me correctly, Va for the early 125s was limited by directional stability considerations for the rudder input test point.

[Volume]

Normally Va is calculated with MTOW and kept contant if weight is reduced.
It´s not the g´s that load your airframe, but the total lift force which bends the wings. At lower weight the wings stand higher g´s and vice versa. The aircraft primary structure is normally designed for MTOW or for maximum zero fuel weight (especially for any airplane having wing tanks). At lower weight it stands higher g´s so it is accepted that your Va now allows for more than the design acceleration and therefor no VA reduction is required.

[john_tullamarine]

One ought not to dismiss the importance of design load factors so easily ... what about all the bits and pieces and other lumps of tin and such which are glued onto the aircraft ? The fixings designs take no account of gross weight and wings .. they are designed for a specified maximum loading acceleration which is fixed.

Whether the Va is scheduled against weight or not is immaterial .. the maximum design load factors still must not be exceeded intentionally.