|Reference to the chart in figure 1-35 will show that banking
an airplane just over 75° in a steep turn increases the stalling speed
by 100 percent. If the normal unaccelerated stalling speed is 45 knots,
the pilot must keep the airspeed above 90 knots in a 75° bank to prevent
sudden entry into a violent power stall. This same effect will take place
in a quick pullup from a dive or maneuver producing load factors above
1 G. Accidents have resulted from sudden, unexpected loss of control, particularly
in a steep turn near the ground.
Reference to the chart in figure 1-35 will show that banking an airplane just over 75° in a steep turn increases the stalling speed by 100 percent. If the normal unaccelerated stalling speed is 45 knots, the pilot must keep the airspeed above 90 knots in a 75° bank to prevent sudden entry into a violent power stall. This same effect will take place in a quick pullup from a dive or maneuver producing load factors above 1 G. Accidents have resulted from sudden, unexpected loss of control, particularly in a steep turn near the ground.
Design maneuvering speed can be found in the Pilot’s Operating Handbook
or on a placard within the cockpit. It can also be determined by multiplying
the normal unaccelerated stall speed by the square root of the limit load
factor. A rule of thumb that can be used to determine the maneuvering speed
is approximately 1.7 times the normal stalling speed.
Thus, an airplane which normally stalls at 35 knots should never be stalled when the airspeed is above 60 knots (35 knots x 1.7 = 59.5 knots).
A knowledge of this must be applied from two points of view by the competent pilot: the danger of inadvertently stalling the airplane by increasing the load factor such as in a steep turn or spiral; and that intentionally stalling an airplane above its design maneuvering speed imposes a tremendous load factor on the structure.
Effect of Speed on Load Factor
The amount of excess load that can be imposed on the wing depends on how fast the airplane is flying. At slow speeds, the maximum available lifting force of the wing is only slightly greater than the amount necessary to support the weight of the airplane. Consequently, the load factor should not become excessive even if the controls are moved abruptly or the airplane encounters severe gusts, as previously stated. The reason for this is that the airplane will stall before the load can become excessive. However, at high speeds, the lifting capacity of the wing is so great that a sudden movement of the elevator controls or a strong gust may increase the load factor beyond safe limits. Because of this relationship between speed and safety, certain “maximum” speeds have been established. Each airplane is restricted in the speed at which it can safely execute maneuvers, withstand abrupt application of the controls, or fly in rough air. This speed is referred to as the design maneuvering speed, which was discussed previously.
Summarizing, at speeds below design maneuvering speed, the airplane should stall before the load factor can become excessive. At speeds above maneuvering speed, the limit load factor for which an airplane is stressed can be exceeded by abrupt or excessive application of the controls or by strong turbulence.
Effect of Flight Maneuvers on Load Factor
Load factors apply to all flight maneuvers. In straight-and-level unaccelerated flight, a load factor of 1G is always present, but certain maneuvers are known to involve relatively high load factors.
• Turns—As previously discussed, increased load factors are a characteristic of all banked turns. Load factors become significant both to flight performance and to the load on wing structure as the bank increases beyond approximately 45°.
• Stalls—The normal stall entered from straight-and-level flight, or an unaccelerated straight climb, should not produce added load factors beyond the 1G of straight-and-level flight. As the stall occurs, however, this load factor may be reduced toward zero, the factor at which nothing seems to have weight, and the pilot has the feeling of “floating free in space.” In the event recovery is made by abruptly moving the elevator control forward, a negative load is created which raises the pilot from the seat. This is a negative wing load and usually is so small that there is little effect on the airplane structure. The pilot should be cautioned, however, to avoid sudden and forceful control movements because of the possibility of exceeding the structural load limits.
During the pullup following stall recovery, however, significant load factors are often encountered. These may be increased by excessively steep diving, high airspeed, and abrupt pullups to level flight. One usually leads to the other, thus increasing the resultant load factor. The abrupt pullup at a high diving speed may easily produce critical loads on structures, and may produce recurrent or secondary stalls by building up the load factor to the point that the speed of the airplane reaches the stalling airspeed during the pullup.
• Advanced Maneuvers—Spins, chandelles, lazy eights, and snap maneuvers will not be covered in this handbook. However, before attempting these maneuvers, pilots should be familiar with the airplane being flown, and know whether or not these maneuvers can be safely performed.