Any airplane, within the limits of its structure, may be stalled at any airspeed. When a sufficiently high angle of attack is imposed, the smooth flow of air over an airfoil breaks up and separates, producing an abrupt change of flight characteristics and a sudden loss of lift which results in a stall.
A study of this effect has revealed that the airplane's stalling speed increases in proportion to the square root of the load factor. This means that an airplane with a normal unaccelerated stalling speed of 50 knots can be stalled at 100 knots by inducing a load factor of 4 G's. If it were possible for this airplane to withstand a load factor of 9, it could be stalled at a speed of 150 knots. Therefore, a competent pilot should be aware of the following:
1. The danger of inadvertently stalling
the airplane by increasing the load factor, as in a steep turn or spiral;
2. That in intentionally stalling an airplane above its design maneuvering speed, a tremendous load factor is imposed.
Reference to the charts in Figs. 17-48 and 49 will show that by banking the airplane to just beyond 72 degrees in a steep turn produces a load factor of 3, and the stalling speed is increased significantly. If this turn is made in an airplane with a normal unaccelerated stalling speed of 45 knots, the airspeed must be kept above 90 knots to prevent inducing a stall. A similar effect is experienced in a quick pullup, or any maneuver producing load factors above 1 G. This has been the cause of accidents resulting from a sudden, unexpected loss of control, particularly in a steep turn or abrupt application of the back elevator control near the ground.
Since the load factor squares as the stalling speed doubles, it may be realized that tremendous loads may be imposed on structures by stalling an airplane at relatively high airspeeds.
The maximum speed at which an airplane may be stalled safely is now determined for all new designs. This speed is called the "design maneuvering speed," (VA) and is required to be entered in the FAA approved flight manual of all recently designed airplanes. For older general aviation airplanes this speed will be approximately 1.7 times the normal stalling speed. Thus, an older airplane which normally stalls at 60 knots must never be stalled at above 102 knots (60 knots x 1.7 = 102 knots). An airplane with a normal stalling speed of 60 knots will undergo, when stalled at 102 knots, a load factor equal to the square of the increase in speed or 2.89 G's (1.7 X 1.7 = 2.89 G's). (The above figures are an approximation to be considered as a guide and are not the exact answers to any set of problems. The design maneuvering speed should be determined from the particular airplane's operating limitations when provided by the manufacturer.)
Since the leverage in the control system varies with different
airplanes and some types employ "balanced" control surfaces while others
do not, the pressure exerted by the pilot on the controls cannot be accepted
as an index of the load factors produced in different airplanes. In most
cases, load factors can be judged by the experienced pilot from the feel
of seat pressure. They can also be measured by an instrument called an
"accelerometer," but since this instrument is not common in general aviation
training airplanes, the development of the ability to judge load factors
from the feel of their effect on the body is important. A knowledge of
the principles outlined above is essential to the development of this ability
to estimate load factors.
A thorough knowledge of load factors induced by varying degrees of bank, and the significance of design maneuvering speed (VA) will aid in the prevention of two of the most serious types of accidents:
1. Stalls from steep turns or excessive
maneuvering near the ground; and
2. Structural failures during acrobatics or other violent maneuvers resulting from loss of control.