ITEMS OF AIRPLANE. PERFORMANCE

The various items of airplane performance result from the combination of airplane and powerplant characteristics. The aerodynamic characteristics of the airplane generally define the power and thrust requirements at various conditions of flight while the powerplant characteristics generally define the power and thrust available at various conditions of flight. The matching of the aerodynamic configura­tion with the powerplant will be accomplished to provide maximum performance at the speci­fic design condition, e. g., range, endurance, climb, etc.

STRAIGHT AND LEVEL FLIGHT

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. In certain conditions of airplane performance it is con­venient to consider the airplane requirements by the thrust required (or drag) while in other cases it is more applicable to consider the power required. Generally, the jet airplane will require consideration of the thrust required and the propeller airplane will require consid­eration of the power required. Hence, the airplane in steady level flight will require lift equal to weight and thrust available equal to thrust required (drag) or power available equal to power required.

The variation of power required and thrust required with velocity is illustrated in figure

2.20. Each specific curve of power or thrust required is valid for a particular aerodynamic configuration at a given weight and altitude. These curves define the power or thrust re­quired to achieve equilibrium, lift-equal – weight, constant altitude flight at various airspeeds. As shown by the curves of figure

2.20, if it is desired to operate the airplane at the airspeed corresponding to point A, the power or thrust required curves define a par­ticular value of thrust or power that must be made available from the powerplant to achieve equilibrium. Some different airspeed such as that corresponding to point В changes the value of thrust or power required to achieve equilibrium. Of course, the change of air­speed to point В also would require a change in angle of attack to maintain a constant lift equal to the airplane weight. Similarly, to establish airspeed and achieve equilibrium at point C will require a particular angle of attack and powerplant thrust or power. In this case, flight at point C would be in the vicinity of the minimum flying speed and a major portion of the thrust or power required would be due to induced 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 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.