Angle of Attack, Weight and Maneuvering Speed
An aircraft’s design maneuvering speed (Va) is the speed at which the airplane will stall before exceeding its maximum load limit or when the flight controls are suddenly and fully deflected in flight.
Airplanes are certified in one of three categories: normal, utility and aerobatic. The stress limits for each category, are: +3.8Gs and -1.52Gs for normal category airplane; +4.4Gs and -1.76Gs for utility category airplane; +6Gs and -3Gs for the aerobatic category airplane.
To understand how maneuvering speed can prevent us from exceeding these limits you begin by understanding you experience “one” times the force of gravity or 1G while flying as long as you are straight and level and at a constant speed; however, the G-force will increase when the airplane turns or the angle of attack suddenly increases, as it does in turbulence.
The G-force can be calculated by dividing lift by weight, or Lift/Weight = G-force.
When flying straight and level, the airplane’s lift is equal to its weight which equates to a G-force of one, or 1G; however, if the angle of attack suddenly increased (by pulling back on the yoke or encountering a gust of wind, for example), the wings would produce more lift and the G-force would increase proportionally to the sudden increase in lift.
If the lift doubles, triples or quadruples, the G-force doubles, triples or quadruples accordingly. Also, there is a direct, almost one-to-one relationship between lift and angle of attack.
For instance, at a constant airspeed, in a 1G condition, a sudden doubling of the angle of attack doubles the lift produced by the wing thereby doubling the G-force. Ergo, tripling or quadrupling the angle of attack triples or quadruples the lift and therefore the G-force.
Assume an airplane is cruising at a speed of 140 knots. The airplane is experiencing 1G at an angle of attack of 3 degrees required for level flight. It suddenly experiences turbulence that doubles its angle of attack to 6 degrees; thereby, doubling the lift and doubling the load factor to 2Gs. A sudden increase in angle of attack to 9 degrees triples the lift and the G-force on the airplane would jump to 3Gs.
Proportionally, an increase to 12, 15, 18 degrees increases lift and G-force to 4Gs, 5Gs and 6Gs, respectively.
The maximum G-force is reached at 18 degrees because any further increase would result in a stall; thus, eliminating the load altogether.
The critical angle of attack on general aviation airplanes is typically between 15 and 20 degrees. We will use 18 degrees to complete the example of the airplane flying straight and level at 140 knots. To maintain straight and level flight at 140 knots the airplane requires an angle of attack of 3 degrees. At that speed the aircraft would experience a load factor of 6Gs before it stalled. In other words, at 140 knots the critical angle of attack of 18 degrees is six times the angle of attack required for level flight (3 degrees). Therefore, at 140 knots this airplane is capable of experiencing 6Gs before the wings stall. If the airplane in this example was certified in the normal category, it might experience structural damage at this speed in strong turbulence.
If we slow this same airplane down to 110 knots, then a slightly higher angle of attack of 4.5 degrees would be required to maintain level flight. If that same turbulence doubles the original angle of attack, the lift doubles and we now feel 2Gs. A sudden tripling of the angle of attack to 13.5 degrees, triples the lift and we experience 3Gs.
And finally, quadrupling the original angle of attack to 18 degrees produces four times as much lift and we experience 4Gs. It’s not possible to pull more than 4Gs in this example because 18 degrees is the critical angle of attack so any additional angle of attack will stall the airplane.
Consequently, 110 knots is the correct maneuvering speed for this airplane having a maximum load factor of 4Gs.
Whether turbulence causes an airplane to experience more than 4Gs depends on the angle of attack in its 1G condition. Clearly, it’s easier to double, triple or quadruple the angle of attack over its starting value when the airplane is flying faster because the angle of attack required for straight and level is lower.
Weight Change and Maneuvering Speed
Maneuvering speed is based on the airplane being at gross weight. If the airplane is below gross weight, the maneuvering speed decreases because at a reduced weight, the airplane requires less lift for straight and level flight; thus, a lower angle of attack.
In other words, an airplane at 2,500 pounds may require a 4.5 degree angle of attack at 110 knots to remain in level flight; whereas, decreasing the weight to 1,800 pounds may require an angle of attack of only 3 degrees to remain in level flight.
With a speed of 110 knots, at this lower weight, a sudden and very strong gust could increase the angle of attack from 3 to 18 degrees. From our previous example, this produces six times the original lift for a force of 6Gs which is beyond the limit for a normal category airplane.
At lighter weights, what can we do to keep from exceeding our example load limit of 4Gs?
The answer is to slow down. At a slower speed (95 knots for example) a larger angle of attack (let’s say 4.5 degrees) is necessary for level cruise flight at this lower weight.
At this speed, we can increase the angle of attack four times before the airplane reaches the critical angle of attack and stalls; therefore, 95 knots becomes our new maneuvering speed.
Most of the newer Pilot’s Operating Handbooks publish two or three different maneuvering speeds for different weight conditions. If yours does not, compute a new one by reducing maneuvering speed by 1% for every 2% your airplane is below gross weight.
Happy Flying and remember to slow down!