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Pilot's Handbook of Aeronautical Knowledge
Aerodynamics of Flight
Stalls

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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

Low speed is not necessary to produce a stall. The wing can
be brought into an excessive AOA at any speed. For example,
an aircraft is in a dive with an airspeed of 100 knots when the
pilot pulls back sharply on the elevator control. [Figure 4-32]
Gravity and centrifugal force prevent an immediate alteration
of the flightpath, but the aircraft's AOA changes abruptly
from quite low to very high. Since the flightpath of the aircraft
in relation to the oncoming air determines the direction of the
relative wind, the AOA is suddenly increased, and the aircraft
would reach the stalling angle at a speed much greater than
the normal stall speed.

The stalling speed of an aircraft is also higher in a level turn
than in straight-and-level flight [Figure 4-33] Centrifugal
force is added to the aircraft's weight and the wing must
produce sufficient additional lift to counterbalance the load
imposed by the combination of centrifugal force and weight.
In a turn, the necessary additional lift is acquired by applying
back pressure to the elevator control. This increases the wing's
AOA, and results in increased lift. The AOA must increase
as the bank angle increases to counteract the increasing load
caused by centrifugal force. If at any time during a turn the
AOA becomes excessive, the aircraft stalls.

Increase in stall speed and load factor.
Figure 4-33. Increase in stall speed and load factor.

At this point, the action of the aircraft during a stall should
be examined. To balance the aircraft aerodynamically, the CL
is normally located aft of the CG. Although this makes the
aircraft inherently nose-heavy, downwash on the horizontal
stabilizer counteracts this condition. At the point of stall, when
the upward force of the wing's lift and the downward tail
force cease, an unbalanced condition exists. This allows the
aircraft to pitch down abruptly, rotating about its CG. During
this nose-down attitude, the AOA decreases and the airspeed
again increases. The smooth flow of air over the wing begins
again, lift returns, and the aircraft is again flying Considerable
altitude may be lost before this cycle is complete.

Airfoil shape and degradation of that shape must also be
considered in a discussion of stalls. For example, if ice,
snow, and frost are allowed to accumulate on the surface of
an aircraft, the smooth airflow over the wing is disrupted.
This causes the boundary layer to separate at an AOA lower
than that of the critical angle. Lift is greatly reduced, altering
expected aircraft performance. If ice is allowed to accumulate
on the aircraft during flight [Figure 4-34], the weight of
the aircraft is increased while the ability to generate lift is
decreased. As little as 0.8 millimeter of ice on the upper
wing surface increases drag and reduces aircraft lift by 25
percent.

Inflight ice formation.
Figure 4-34. In-flight ice formation.

Pilots can encounter icing in any season, anywhere in the
country, at altitudes of up to 18,000 feet and sometimes
higher. Small aircraft, including commuter planes, are most
vulnerable because they fly at lower altitudes where ice is
more prevalent. They also lack mechanisms common on jet
aircraft that prevent ice buildup by heating the front edges
of wings.

Icing can occur in clouds any time the temperature drops
below freezing and super-cooled droplets build up on an
aircraft and freeze. (Super-cooled droplets are still liquid
even though the temperature is below 32 ┬░Fahrenheit (F),
or 0 ┬░Celsius (C).

 

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