INTRODUCTION

There are many explanations of maneuvering speed but many are misleading or incomplete, and few of them intuitively describe how Va is derived. Va is the airspeed at which severe turbulence or sudden full deflection of flight controls will not break the airplane (e.g. rip the wings off). At and below Va, the airplane will stall before it breaks.

Generally, Va does not impose a speed limit on the airplane because in normal flight, including most standard maneuvers and turbulence, it is not necessary to observe Va. Va becomes important during severe turbulence, extreme maneuvers, or any time the airplane will be subjected to sudden powerful accelerations.

NOTE: there is a common misperception that Va applies to all possible movement of the airplane: roll, pitch and yaw. This is FALSE. One should assume that Va only applies to pitch - to the main wing and elevator. For example, there has been at least one incident I know of where a pilot flying below Va (in a large passenger jet) broke the airplane's rudder by aggressively applying extreme inputs. Many pilots believe that sudden full deflection of all flight controls are safe below Va, but this is a potentially deadly misperception.

WHERE DOES Va COME FROM?

The idea behind Va is that the faster an airplane goes, the more force the wing would sustain in a sudden deflection or gust of turbulence. Why? Because lift is proportional to airspeed. The faster an airplane flies, the more lift the wing generates, so less AOA (angle of attack) is used in straight level flight. As the airplane speeds up, it has to be nosed down to prevent climbing. Now a wing always stalls at the same AOA, regardless of airspeed or orientation. Thus, the less AOA necessary for level flight, the more AOA can be increased before a stall occurs. The G factor or acceleration of the airplane is proportional to the change in AOA. Thus a lower AOA in level flight means there is more "headroom" for AOA to increase before reaching the critical AOA. Thus, a bigger G load can be exerted on the airplane before the wing stalls.

Breaking something requires force. A G is a unitless measurement of acceleration. A G becomes a force only when multiplied by the mass being accelerated. So the absolute strength of a wing is measured in pounds, not in G. Imagine that the strength of a wing is the amount of weight that can be suspended from the wing before it breaks. Thus, a light airplane exerts less force on the wings than a heavier airplane, when pulling the same G load.

EXAMPLE: Va AND WEIGHT

Now let's consider an airplane flying at its maximum gross weight (GW). Suppose the maximum designed G rating for this airplane is 4 G. Suppose the GW is 2400 lbs. This means the maximum force the wing is designed to withstand is 4 * 2400 = 9,600 lbs. The book value for Va for this airplane will be the speed at which, when the elevator is suddenly fully deflected from level flight, the wing stalls just before generating 9,600 lbs. of lift. Normally, this speed will be equal to the clean stall speed of the aircraft (Vs), multiplied by the square root of the designed maximum G load. Suppose for our example airplane, Vs is 50 kts, so Va will be 50 * sqrt(4) = 100 kts. This is the airspeed at which the airplane will stall just as the wing reaches its designed peak load of 9,600 lbs.

Now what happens if the airplane is being flown below its GW of 2400 lbs? How does this affect Va? There are two factors to consider:

These two effects are opposite and tend to cancel each other. In short, the airplane can pull more Gs from level flight, but that higher G load represents the same force on the wings. Thus, it would appear that Va should be unaffected.

For example, consider the airplane being flown at 1800 lbs. weight. When the wings are at their maximum lift of 9,600 lbs., the G force on the airplane is 9600 / 1800 = 5.3 G. So flying at Va at 1800 lbs. the airplane can be subjected to 5.3 G of acceleration before the wings break. This force is the same as what the wings endure at 4 G at 2400 lbs.

AN AIRPLANE IS MORE THAN A PAIR OF WINGS

But what about the rest of the airplane? What about the engine mounts, the battery, the seats, etc.? Suppose our 2,400 lb. GW airplane's engine weighs 250 lbs. At the designed load of 4 G, the engine mounts are withstanding 1,000 lbs. of force. Now if the airplane is flying at 1,800 lbs. (well below its GW), at airspeed Va and the pilot suddenly fully deflects the elevator, we just subjected the engine mounts to a higher G force. If it is linear - 33% less weight means 33% less AoA which means it can pull 33% more Gs - that's 5.3 G which is 5.3 * 250 = 1,325 lbs. of force. If they were only designed for 4 G, that's only 1,000 lbs., which we exceeded so they may break. NOTE: the relationship doesn't have to be linear. The point is, when the airplane is lighter, it can pull more Gs, which puts more stress on components like the engine that always weigh the same.

Thus, Va may be slower when the airplane is lighter. When the airplane is lighter, it can pull more Gs. This won't break the wing, because higher Gs at lighter weight is the same total force on the wing. But it can break the engine, battery box, etc. because - and this is a very important key point - even though the overall airplane is lighter, these individual parts always weigh the same and are being subjected to more Gs.

HOW WEIGHT AFFECTS Va

Why "may be slower" rather than "will be slower"? It depends on where the weak point of the airplane is. Is the rest of the airplane stronger than the wings? The same strength? Or weaker? Va at GW is based on the strength of the wings. If the entire airplane - wings and the rest - is equally strong, meaning it all has the same G load limit, then Va will get slower as the airplane gets lighter. The above engine example explains why - a lighter plane can pull more Gs, which exerts forces on individual components (like the engine) which exceed their designed G load.
HOWEVER... if the rest of the airplane is stronger than the wings, then Va may be just as fast at slower speeds as it is at GW.

We won't discuss the case in which the rest of the airplane is weaker than the wings. Why? Because the airplane's maximum designed G rating applies to the entire airplane. And it's based on the G load the wings can withstand, to produce the book value of Va. Thus the rest of the airplane must be at least as strong as the wings (else the overall G rating would be lower and Va would be slower).

Now what would happen if the airplane is flown above its GW? Many pilots have the intuitive notion that since Va at GW is higher than Va when empty, that Va increases as the plane gets heavier, thus Va would be even higher when flown over GW. The idea is that a heavier plane "penetrates" turbulence better. This is a potentially deadly misperception.

Consider that the maximum load the wing can withstand is a force in lbs., not a unitless G factor. In our example it is 9,600 lbs. Now consider the airplane when loaded 10% above its GW, to 2,640 lbs. When the wing is at its load limit of 9,600 lbs, the G factor is only 9600 / 2640 = 3.6 G. Thus when loaded above max GW, the airplane's wing cannot sustain its designed G load. It will break before reaching that G load.

Now picture this overloaded airplane flying along at Va of 100 kts. We have the opposite case of the lighter airplane above. Because it's heavier it needs more lift, so at the same airspeed it must have a higher AOA. This means there is less AOA "headroom" before the wing stalls. Thus, a sudden deflection of the controls will produce a smaller G load before stalling. That smaller G load is a bigger force due to the higher weight. The two factors cancel each other, producing the same Va.

Now imagine what happens when the pilot mistakenly believes Va is higher since he's heavier. The airplane is flying along at a higher speed, so there is less AOA. Less AOA in level flight means a sudden full deflection of the controls produces a bigger G load, which is a bigger force on the wings, Since that force was at the wing's load limit at the slower speed of 100 kts., the force at this speed MUST exceed the wing's design limit. Whoops - wings break during flight. That is A BAD THING.

CONCLUSIONS