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Pilot's Handbook of Aeronautical Knowledge
Aerodynamics of Flight
High Speed Flight

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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

Wingtip stall.
Figure 4-61. Wingtip stall.

The stall situation can be aggravated by a T-tail configuration,
which affords little or no pre-stall warning in the form of
tail control surface buffet. [Figure 4-62] The T-tail, being
above the wing wake remains effective even after the wing
has begun to stall, allowing the pilot to inadvertently drive
the wing into a deeper stall at a much greater AOA. If the
horizontal tail surfaces then become buried in the wing's
wake, the elevator may lose all effectiveness, making it
impossible to reduce pitch attitude and break the stall. In
the pre-stall and immediate post-stall regimes, the lift/drag
qualities of a swept wing aircraft (specifically the enormous
increase in drag at low speeds) can cause an increasingly
descending flightpath with no change in pitch attitude, further
increasing the AOA. In this situation, without reliable AOA
information, a nose-down pitch attitude with an increasing
airspeed is no guarantee that recovery has been effected,
and up-elevator movement at this stage may merely keep
the aircraft stalled.

T-tail stall.
Figure 4-62. T-tail stall.

It is a characteristic of T-tail aircraft to pitch up viciously
when stalled in extreme nose-high attitudes, making
recovery difficult or violent. The stick pusher inhibits this
type of stall. At approximately one knot above stall speed,
pre-programmed stick forces automatically move the stick
forward, preventing the stall from developing. A G-limiter
may also be incorporated into the system to prevent the pitch
down generated by the stick pusher from imposing excessive
loads on the aircraft. A "stick shaker," on the other hand
provides stall warning when the airspeed is five to seven
percent above stall speed.

Mach Buffet Boundaries
Mach buffet is a function of the speed of the airflow over the
wing—not necessarily the speed of the aircraft. Any time that
too great a lift demand is made on the wing, whether from too
fast an airspeed or from too high an AOA near the MMO, the
"high-speed" buffet occurs. There are also occasions when
the buffet can be experienced at much lower speeds known
as the "low-speed Mach buffet."

An aircraft flown at a speed too slow for its weight and
altitude necessitating a high AOA is the most likely situation
to cause a low-speed Mach buffet. This very high AOA has
the effect of increasing airflow velocity over the upper surface
of the wing until the same effects of the shock waves and
buffet occur as in the high-speed buffet situation. The AOA of
the wing has the greatest effect on inducing the Mach buffet at
either the high-speed or low-speed boundaries for the aircraft.
The conditions that increase the AOA, the speed of the airflow
over the wing, and chances of Mach buffet are:

• High altitudes—the higher an aircraft flies, the thinner
the air and the greater the AOA required to produce
the lift needed to maintain level flight.
• Heavy weights—the heavier the aircraft, the greater
the lift required of the wing, and all other things being
equal, the greater the AOA.
• G loading—an increase in the G loading on the aircraft
has the same effect as increasing the weight of the
aircraft. Whether the increase in G forces is caused
by turns, rough control usage, or turbulence, the effect
of increasing the wing's AOA is the same.

 

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