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Pilot's Handbook of Aeronautical Knowledge
Flight Controls

Flight Control Systems

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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

T-Tail
In a T-tail configuration, the elevator is above most of the
effects of downwash from the propeller as well as airflow
around the fuselage and/or wings during normal flight
conditions. Operation of the elevators in this undisturbed air
allows control movements that are consistent throughout most
flight regimes. T-tail designs have become popular on many
light and large aircraft, especially those with aft fuselage mounted
engines because the T-tail configuration removes
the tail from the exhaust blast of the engines. Seaplanes and
amphibians often have T-tails in order to keep the horizontal
surfaces as far from the water as possible. An additional
benefit is reduced vibration and noise inside the aircraft.
At slow speeds, the elevator on a T-tail aircraft must be moved
through a larger number of degrees of travel to raise the nose
a given amount than on a conventional-tail aircraft. This is
because the conventional-tail aircraft has the downwash from
the propeller pushing down on the tail to assist in raising the
nose.

Since controls on aircraft are rigged so that increasing control
forces are required for increased control travel, the forces
required to raise the nose of a T-tail aircraft are greater than
those for a conventional-tail aircraft. Longitudinal stability of
a trimmed aircraft is the same for both types of configuration,
but the pilot must be aware that the required control forces are
greater at slow speeds during takeoffs, landings, or stalls than
for similar size aircraft equipped with conventional tails.
T-tail airplanes also require additional design considerations
to counter the problem of .utter. Since the weight of the
horizontal surfaces is at the top of the vertical stabilizer,
the moment arm created causes high loads on the vertical
stabilizer which can result in flutter. Engineers must
compensate for this by increasing the design stiffness of the
vertical stabilizer, usually resulting in a weight penalty over
conventional tail designs.

When flying at a very high AOA with a low airspeed and
an aft CG, the T-tail aircraft may be susceptible to a deep
stall. In a deep stall, the airflow over the horizontal tail
is blanketed by the disturbed airflow from the wings and
fuselage. In these circumstances, elevator or stabilator control
could be diminished, making it difficult to recover from the
stall. It should be noted that an aft CG is often a contributing
factor in these incidents, since similar recovery problems
are also found with conventional tail aircraft with an aft CG.
[Figure 5-11]

Airplane with a T-tail design
Figure 5-11. Airplane with a T-tail design at a high AOA and an
aft CG.

Since flight at a high AOA with a low airspeed and an aft
CG position can be dangerous, many aircraft have systems
to compensate for this situation. The systems range from
control stops to elevator down springs. An elevator down
spring assists in lowering the nose of the aircraft to prevent a
stall caused by the aft CG position. The stall occurs because
the properly trimmed airplane is flying with the elevator in a
trailing edge down position, forcing the tail up and the nose
down. In this unstable condition, if the aircraft encounters
turbulence and slows down further, the trim tab no longer
positions the elevator in the nose-down position. The elevator
then streamlines, and the nose of the aircraft pitches upward,
possibly resulting in a stall.

The elevator down spring produces a mechanical load on the
elevator, causing it to move toward the nose-down position
if not otherwise balanced. The elevator trim tab balances the
elevator down spring to position the elevator in a trimmed
position. When the trim tab becomes ineffective, the down
spring drives the elevator to a nose-down position. The nose
of the aircraft lowers, speed builds up, and a stall is prevented.
[Figure 5-12]

The elevator must also have sufficient authority to hold the
nose of the aircraft up during the roundout for a landing. In
this case, a forward CG may cause a problem. During the
landing .are, power is usually reduced, which decreases the
airflow over the empennage. This, coupled with the reduced
landing speed, makes the elevator less effective.
As this discussion demonstrates, pilots must understand and
follow proper loading procedures, particularly with regard
to the CG position. More information on aircraft loading, as
well as weight and balance, is included in Chapter 9, Weight
and Balance.

 

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