|The unit used to measure acceleration is the G, an abbreviation for gravity. Everything tries to accelerate toward the center of the earth. When you stand on the surface, that acceleration force is resisted by the force of the ground against the soles of your feet. You feel this as weight, even though you are not accelerating. The force of lift resists the force of gravity to keep an airplane in level flight, and the wings are bearing the weight of the airplane. In both cases, you experience an acceleration force of 1 times the force of gravity, or 1G. Even though we cannot change the force of gravity, whenever the airplane is changing speed or direction the magnitude of the acceleration can be measured in Gs. While weight always acts straight down, acceleration forces, or G-forces, can be created in any direction. For example, in a 60° banked turn, the acceleration due to gravity added to the acceleration necessary to change the flight path results in a 2G force toward the bottom of the airplane. In this situation, the wings will have to generate twice a much lift as they would with the wings level in order to maintain the same altitude. The load factor is defined as the load the wings are supporting divided by the total weight of the airplane. With this in mind, you can understand why airplane structures are designed to withstand greater loads than normal flight is likely to produce. When an airplane is certified in the normal category, for example, each part of the structure is designed to do its job at 3.8 positive Gs and 1.52 negative Gs. Acrobatic-category airplanes are designed for higher load factors, and airliners for lower load factors.
A heavily loaded airplane has a higher stall speed than the same airplane with a light load. This is because the heavily loaded airplane must use a higher angle of attack to generate the required lift at any given speed than when lightly loaded. Thus, it is closer to its stalling angle of attack, and will encounter Clmax at a higher airspeed. This is important to remember when flying a heavily loaded airplane at low airspeed, since a bump, gust, or abrupt control movement could result in a stall. This property also means that at low speeds and/or high weights, abrupt use of the controls results in a stall instead of a structural overload. In this case, the stall acts as a safety valve, reducing the load factor prior to airframe damage.
At high speeds, the controls are more effective, and abrupt control movements can increase G loads so quickly that the structure could be damaged before the stalling angle of attack is reached. The maximum speed at which full or abrupt use of the controls will result in a stall rather than structural damage is the maneuvering speed (Va). The maneuvering speed will vary with the airplane's total weight, since a lightly loaded airplane is easier to accelerate, and it will also have a larger margin between the angle of attack necessary for level flight and the stalling angle of attack. It takes less force, whether from an abrupt control movement or from a gust or bump, to create a high-G situation, so the maneuvering speed is reduced. The maneuvering speed depicted on a cockpit placard is calculated for the maximum weight of the airplane, but Va for other weights may be found in some Pilot Operating Handbooks.