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.
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 overcoming 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 cruise flying speed, with the angle of attack nearly zero, induced drag is minimal.
After attaining cruise 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 that is involved at speeds beyond 260 knots.
To sum up these changes, as the speed increases from stalling speed to VNE, the induced drag decreases and parasite drag increases.
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 thrusts required equal the maximum power or thrust available from the powerplant. 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.
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