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Pilot's Handbook of Aeronautical Knowledge
Aircraft Performance

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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




While the parasite drag predominates at high speed, induced
drag predominates at low speed. [Figure 10-5] For example,
if an aircraft in a steady flight condition at 100 knots is then
accelerated to 200 knots, the parasite drag becomes four
times as great, but the power required to overcome that
drag is eight times the original value. Conversely, when the
aircraft is operated in steady, level flight at twice as great a
speed, the induced drag is one-fourth the original value, and
the power required to overcome that drag is only one-half
the original value.

When an aircraft is in steady, level flight, the condition of
equilibrium must prevail. The unaccelerated condition of flight
is achieved with the aircraft trimmed for lift equal to weight
and the powerplant set for a thrust to equal the aircraft drag.
The maximum level flight speed for the aircraft will be obtained
when the power or thrust required equals the maximum power
or thrust available from the powerplant. [Figure 10-6] The
minimum level flight airspeed is not usually defined by thrust
or power requirement since conditions of stall or stability and
control problems generally predominate.

Power versus speed.
Figure 10-6. Power versus speed.

Climb Performance
Climb performance is a result of using the aircraft's potential
energy provided by one, or a combination of two factors. The
first is the use of excess power above that required for level
flight. An aircraft equipped with an engine capable of 200
horsepower (at a given altitude) but using 130 horsepower
to sustain level flight (at a given airspeed) has 70 excess
horsepower available for climbing. A second factor is that
the aircraft can trade off its kinetic energy and increase its
potential energy by reducing its airspeed. The reduction in
airspeed will increase the aircraft's potential energy thereby
also making the aircraft climb. Both terms, power and thrust
are often used in aircraft performance however, they should
not be confused.

Although the terms "power" and "thrust" are sometimes
used interchangeably, erroneously implying that they are
synonymous, it is important to distinguish between the two
when discussing climb performance. Work is the product of
a force moving through a distance and is usually independent
of time. Work is measured by several standards; the most
common unit is called a foot-pound. If a one pound mass
is raised one foot, a work unit of one foot-pound has been
performed. The common unit of mechanical power is
horsepower; one horsepower is work equivalent to lifting
33,000 pounds a vertical distance of one foot in one minute.

The term power implies work rate or units of work per unit
of time, and as such is a function of the speed at which the
force is developed. Thrust, also a function of work, means
the force that imparts a change in the velocity of a mass. This
force is measured in pounds but has no element of time or
rate. It can be said then, that during a steady climb, the rate
of climb is a function of excess thrust.

This relationship means that, for a given weight of an aircraft,
the angle of climb depends on the difference between thrust
and drag, or the excess power. [Figure 10-7] Of course, when
the excess thrust is zero, the inclination of the flightpath is
zero, and the aircraft will be in steady, level flight When the
thrust is greater than the drag, the excess thrust will allow
a climb angle depending on the value of excess thrust. On
the other hand, when the thrust is less than the drag, the
deficiency of thrust will allow an angle of descent.

Thrust versus climb angle.
Figure 10-7. Thrust versus climb angle.

The most immediate interest in the climb angle performance
involves obstacle clearance. The most obvious purpose for
which it might be used is to clear obstacles when climbing
out of short or confined airports.