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Pilot's Handbook of Aeronautical Knowledge
Aerodynamics of Flight
Forces Acting on the Aircraft

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Pilot's Handbook of Aeronautical Knowledge



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




Angle of attack at various speeds.
Figure 4-3. Angle of attack at various speeds.

Likewise, if the engine power is increased, thrust becomes
greater than drag and the airspeed increases. As long as
the thrust continues to be greater than the drag, the aircraft
continues to accelerate. When drag equals thrust, the aircraft
flies at a constant airspeed.

Straight-and-level flight may be sustained at a wide range
of speeds. The pilot coordinates angle of attack (AOA)—the
acute angle between the chord line of the airfoil and the
direction of the relative wind—and thrust in all speed regimes
if the aircraft is to be held in level flight. Roughly, these
regimes can be grouped in three categories: low-speed flight,
cruising flight, and high-speed flight.

When the airspeed is low, the AOA must be relatively high
if the balance between lift and weight is to be maintained.
[Figure 4-3] If thrust decreases and airspeed decreases, lift
becomes less than weight and the aircraft starts to descend.
To maintain level flight, the pilot can increase the AOA
an amount which will generate a lift force again equal to
the weight of the aircraft. While the aircraft will be flying
more slowly, it will still maintain level flight if the pilot has
properly coordinated thrust and AOA.

Straight-and-level flight in the slow-speed regime provides
some interesting conditions relative to the equilibrium of forces because with the aircraft in a nose-high attitude, there is a
vertical component of thrust that helps support it. For one thing,

wing loading tends to be less than would be expected. Most
pilots are aware that an airplane will stall, other conditions
being equal, at a slower speed with the power on than with the
power off. (Induced airflow over the wings from the propeller
also contributes to this.) However, if analysis is restricted to
the four forces as they are usually defined during slow-speed
flight the thrust is equal to drag, and lift is equal to weight.

During straight-and-level flight when thrust is increased and
the airspeed increases, the AOA must be decreased. That is,
if changes have been coordinated, the aircraft will remain in
level flight, but at a higher speed when the proper relationship
between thrust and AOA is established.

If the AOA were not coordinated (decreased) with an
increase of thrust, the aircraft would climb. But decreasing
the AOA modifies the lift, keeping it equal to the weight,
and the aircraft remains in level flight Level flight at even
slightly negative AOA is possible at very high speed. It is
evident then, that level flight can be performed with any
AOA between stalling angle and the relatively small negative
angles found at high speed.

Some aircraft have the ability to change the direction of the
thrust rather than changing the AOA. This is accomplished
either by pivoting the engines or by vectoring the exhaust
gases. [Figure 4-4]

Some aircraft have the ability to change the direction of thrust.
Figure 4-4. Some aircraft have the ability to change the direction of thrust.