An often misunderstood concept is the aircraft maneuvering speed depicted as Va. This limitation is usually taught as the maximum speed which while maneuvering you will ‘stall it before you break it’. As the explanation implies it is really a structural limitation, but since most airplanes don’t have g-meters we must rely on airspeed indications to ensure the limitation is not exceeded. To complicate matters Va changes with the weight of the loaded airplane. Hopefully this article will help you understand the aerodynamic principles behind maneuvering speed and why it is important to us as a pilot.
In straight and level flight the wings produce exactly enough lift to sustain the weight of the airplane. If we bank the airplane the lift is split into two components: vertical and horizontal. As we increase the bank angle the vertical component decreases while the horizontal component increases. The loss of vertical lift will cause the airplane to descend because it is no longer sufficient to suspend the weight of the airplane.
To maintain altitude we must increase the angle of attack to generate enough vertical lift to suspend the weight. This means the wings must produce enough total lift to both maintain altitude AND turn the airplane. As the bank angle increases the amount of total lift required increases exponentially. Of course we also have equal and opposite forces thanks to Mr. Newton. Centrifugal force opposes the horizontal component of lift and the resultant load opposes the total lift. Therefore the resultant load (g-force) also increases exponentially with bank angle, assuming altitude is constant.
This load factor chart depicts the amount of g-units resulting from a given bank angle in level flight. In a 60° bank the aircraft load is 2 g’s. In other words, the wings must produce twice the lift required for straight and level flight. If your loaded airplane weighs 2,200 pounds the wings are producing 4,400 pounds of lift to maintain altitude. In a 80° bank the wings are producing 13,200 pounds of lift! Cessna 172s are certainly sturdy airplanes but I don’t think they designed them quite that durable!
14 CFR Part 23 contains airworthiness standards for normal, utility, aerobatic and commuter categories. In addition to airframe, performance and stability standards the regulations specify load limitations ensuring structural strength. Normal category aircraft are designed to withstand +3.8 to -1.52 g’s, or units of gravity. This does not mean at 3.9 g’s the wings will separate from the fuselage but structural damage may occur. Aircraft designers add a 50% buffer on top of the load limit before structural failure. Structural damage may include cracks, dents, popped rivets, delamination, etc. Obviously care must be taken to not exceed these limits.
Effects of Weight
So you understand why we can’t exceed the load limit but why does Va change with weight? Remember, maneuvering speed is defined as the highest speed at which full deflection of the controls about any one axis are guaranteed not to overstress the airframe. This is a dynamic limitation on the forces imposed on the airframe.
Again Mr. Newton helps us out: Force = Mass x Acceleration or Acceleration = Force / Mass. Mass is the weight of the airplane and acceleration is the imposed load. Take two identical airplanes, one light and one fully loaded. If we apply a force on the controls maneuvering the light airplane the acceleration is much more than the same force applied to the heavy airplane.
But how do we guarantee we will not overstress the airframe? Simple – it will stall first. Remember the heavier airplane will need to fly at a higher angle of attack to produce adequate lift to overcome the weight. That means the heavy airplane will be flying closer to the critical angle of attack and stall more quickly. The light airplane must travel through a greater angle of attack range before stalling providing more time for the airplane to accelerate and exceed the load limit.
It’s a good thing we have that 50% buffer because the airplane is certified NEW. Structural fatigue is cumulative over time so an older plane is not necessarily as strong as when it rolled off the factory floor. Corrosion and metal fatigue can wear down components. Signs of fatigue are checked during annual and 100-hr inspections but many items internal fractures can only be detected using ultra-sonic devices.
Smooth application of the controls will minimize wear and tear. Fly your C172 like a Boeing and it will last you a long time. If you encounter moderate or greater turbulence, SLOW DOWN. As long as you are below Va for your given weight you will stall before damage occurs. That sounds like a bad thing but I would much rather recover from a stall than wonder why cracking sounds are coming from the wing spar!