All of the principal items of flight performance involve steady state flight conditions and equilibrium of the airplane. For the airplane to remain in steady level flight, equilibrium must be obtained by a lift equal to the airplane weight and a powerplant thrust equal to the airplane drag. Thus, the airplane drag defines the thrust required to maintain steady level flight.
All parts of the airplane that are exposed to the air contribute to the drag, though only the wings provide lift of any significance. For this reason, and certain others related to it, the total drag may be divided into two parts, the wing drag (induced) and the drag of everything but the wings (parasite).
The total power required for flight then can be considered
as the sum of induced and parasite effects; that is, the total drag of
the airplane. Parasite drag is the sum of pressure and friction drag which
is due to the airplane's basic configuration and, as defined, is independent
of lift. Induced drag is the undesirable but unavoidable consequence of
the development of lift.
|While the parasite drag predominates at high speed, induced drag predominates
at low speed (Fig. 17-18). For example, if an airplane 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 airplane 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.
The wing or induced drag changes with speed in a very different way, because of the changes in the angle of attack. Near the stalling speed the wing is inclined to the relative wind at nearly the stalling angle, and its drag is very strong. But at ordinary flying speeds, with the angle of attack nearly zero, the wing cuts through the air almost like a knife, and the drag is minimal. After attaining a certain high speed, the angle of attack changes very little with any further increase in speed and the drag of the wing increases in direct proportion to any further increase in speed. This does not consider the factor of compressibility drag which is involved at speeds beyond the top speed of most general aviation airplanes.
To sum up these changes, for a typical, moderately powered airplane: As the speed increases from stalling speed to top speed, the induced drag decreases and parasite drag increases. As a result the total drag decreases for the first part of the range and then increases again.
When the airplane is in steady, level flight, the condition
of equilibrium must prevail. The unaccelerated condition of flight is achieved
with the airplane trimmed for lift equal to weight and the powerplant set
for a thrust to equal the airplane drag.
The maximum level flight speed for the airplane will be obtained when the power or thrust required equals the maximum power or thrust available from the power plant (Fig. 17-54). 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.