| Home | Privacy | Contact |

Pilot's Handbook of Aeronautical Knowledge
Aerodynamics of Flight
Stalls

| First | Previous | Next | Last |

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

As forward pressure is applied to the control yoke to initiate
the descent, the AOA is decreased momentarily. Initially,
the momentum of the aircraft causes the aircraft to briefly
continue along the same flightpath. For this instant, the AOA
decreases causing the total lift to decrease. With weight now
being greater than lift, the aircraft begins to descend. At the
same time, the flightpath goes from level to a descending
flightpath. Do not confuse a reduction in lift with the
inability to generate sufficient lift to maintain level flight
The flightpath is being manipulated with available thrust in
reserve and with the elevator.

To descend at the same airspeed as used in straight-and level
flight, the power must be reduced as the descent is
entered. The component of weight acting forward along the
flightpath increases as the angle of rate of descent increases
and, conversely, decreases as the angle of rate of descent
decreases. The component of weight acting forward along the
flightpath increases as the angle of rate of descent increases
and, conversely, decreases as the angle of rate of descent
decreases.

Stalls

An aircraft stall results from a rapid decrease in lift caused by
the separation of airflow from the wing's surface brought on
by exceeding the critical AOA. A stall can occur at any pitch
attitude or airspeed. Stalls are one of the most misunderstood
areas of aerodynamics because pilots often believe an airfoil
stops producing lift when it stalls. In a stall, the wing does
not totally stop producing lift. Rather, it can not generate
adequate lift to sustain level flight.

Since the CL increases with an increase in AOA, at some
point the CL peaks and then begins to drop off. This peak is
called the CL-MAX. The amount of lift the wing produces drops
dramatically after exceeding the CL-MAX or critical AOA, but
as stated above, it does not completely stop producing lift.

In most straight-wing aircraft, the wing is designed to stall
the wing root first The wing root reaches its critical AOA
first making the stall progress outward toward the wingtip.
By having the wing root stall first, aileron effectiveness is
maintained at the wingtips, maintaining controllability of
the aircraft. Various design methods are used to achieve
the stalling of the wing root first In one design, the wing is
"twisted" to a higher AOA at the wing root. Installing stall
strips on the first 20–25 percent of the wing's leading edge
is another method to introduce a stall prematurely.

The wing never completely stops producing lift in a stalled
condition. If it did, the aircraft would fall to the Earth. Most
training aircraft are designed for the nose of the aircraft to
drop during a stall, reducing the AOA and "unstalling" the
wing. The "nose-down" tendency is due to the CL being aft
of the CG. The CG range is very important when it comes
to stall recovery characteristics. If an aircraft is allowed to
be operated outside of the CG, the pilot may have difficulty
recovering from a stall. The most critical CG violation would
occur when operating with a CG which exceeds the rear limit.
In this situation, a pilot may not be able to generate sufficient
force with the elevator to counteract the excess weight aft of
the CG. Without the ability to decrease the AOA, the aircraft
continues in a stalled condition until it contacts the ground.

The stalling speed of a particular aircraft is not a fixed value
for all flight situations, but a given aircraft always stalls at
the same AOA regardless of airspeed, weight, load factor,
or density altitude. Each aircraft has a particular AOA where
the airflow separates from the upper surface of the wing and
the stall occurs. This critical AOA varies from 16° to 20°
depending on the aircraft's design. But each aircraft has only
one specific AOA where the stall occurs.

There are three flight situations in which the critical AOA
can be exceeded: low speed, high speed, and turning.

The aircraft can be stalled in straight-and-level flight by flying
too slowly. As the airspeed decreases, the AOA must be
increased to retain the lift required for maintaining altitude.
The lower the airspeed becomes, the more the AOA must
be increased. Eventually, an AOA is reached which results
in the wing not producing enough lift to support the aircraft
which starts settling. If the airspeed is reduced further, the
aircraft stalls, since the AOA has exceeded the critical angle
and the airflow over the wing is disrupted.

Forces exerted when pulling out of a dive.
Figure 4-32. Forces exerted when pulling out of a dive.

 

4-22