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Pilot's Handbook of Aeronautical Knowledge
Aerodynamics of Flight
Load Factors

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Pilot's Handbook of Aeronautical Knowledge

Preface

Acknowledgements

Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making

Appendix

Glossary

Index

Load factor changes stall speed.
Figure 4-46. Load factor changes stall speed.

For a coordinated, constant altitude turn, the approximate
maximum bank for the average general aviation aircraft is 60°.
This bank and its resultant necessary power setting reach the
limit of this type of aircraft. An additional 10° bank increases
the load factor by approximately 1 G, bringing it close to the
yield point established for these aircraft. [Figure 4-46]

Load Factors and Stalling Speeds
Any aircraft, within the limits of its structure, may be stalled
at any airspeed. When a sufficiently high AOA 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 aircraft's
stalling speed increases in proportion to the square root of
the load factor. This means that an aircraft with a normal
unaccelerated stalling speed of 50 knots can be stalled at 100
knots by inducing a load factor of 4 Gs. If it were possible
for this aircraft to withstand a load factor of nine, it could be
stalled at a speed of 150 knots. A pilot should be aware:

• Of the danger of inadvertently stalling the aircraft by
increasing the load factor, as in a steep turn or spiral;
• When intentionally stalling an aircraft above its
design maneuvering speed, a tremendous load factor
is imposed.

Figures 4-45 and 4-46 show that banking an aircraft greater
than 72° in a steep turn produces a load factor of 3, and the
stalling speed is increased significantly. If this turn is made
in an aircraft with a normal unaccelerated stalling speed of
45 knots, the airspeed must be kept greater than 75 knots to
prevent inducing a stall. A similar effect is experienced in a
quick pull up, or any maneuver producing load factors above
1 G. This sudden, unexpected loss of control, particularly in
a steep turn or abrupt application of the back elevator control
near the ground, has caused many accidents.

Since the load factor is squared as the stalling speed doubles,
tremendous loads may be imposed on structures by stalling
an aircraft at relatively high airspeeds.

The maximum speed at which an aircraft may be stalled safely
is now determined for all new designs. This speed is called
the "design maneuvering speed" (VA) and must be entered in
the FAA-approved Airplane Flight Manual/Pilot's Operating
Handbook (AFM/POH) of all recently designed aircraft. For
older general aviation aircraft, this speed is approximately 1.7
times the normal stalling speed. Thus, an older aircraft which
normally stalls at 60 knots must never be stalled at above 102
knots (60 knots x 1.7 = 102 knots). An aircraft with a normal
stalling speed of 60 knots stalled at 102 knots undergoes a load
factor equal to the square of the increase in speed, or 2.89 Gs
(1.7 x 1.7 = 2.89 Gs). (The above figures are approximations
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 aircraft's operating limitations
provided by the manufacturer.)

Since the leverage in the control system varies with different
aircraft (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 aircraft. In most cases, load factors can be judged
by the experienced pilot from the feel of seat pressure. Load
factors can also be measured by an instrument called an
"accelerometer," but this instrument is not common in general
aviation training aircraft. The development of the ability to
judge load factors from the feel of their effect on the body is
important. A knowledge of these principles is essential to the
development of the ability to estimate load factors.

Since the leverage in the control system varies with different
aircraft (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 aircraft. In most cases, load factors can be judged
by the experienced pilot from the feel of seat pressure. Load
factors can also be measured by an instrument called an
"accelerometer," but this instrument is not common in general
aviation training aircraft. The development of the ability to
judge load factors from the feel of their effect on the body is
important. A knowledge of these principles is essential to the
development of the ability to estimate load factors.

A thorough knowledge of load factors induced by varying
degrees of bank and the VA aids in the prevention of two of
the most serious types of accidents:
1. Stalls from steep turns or excessive maneuvering near
the ground.
2. Structural failures during acrobatics or other violent
maneuvers resulting from loss of control.

Operating at or below
design maneuvering speed does not provide structural
protection against multiple full control inputs in one axis or full
control inputs in more than one axis at the same time.

 

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