AIRPLANE PERFORMANCE

The performance of an aircraft is. the most important feature which defines its suitability for specific missions. The principal items of airplane performance deserve detailed consid­eration in order to better understand and appreciate the capabilities of each airplane. Knowledge of the various items of airplane performance will provide the Naval Aviator with a more complete appreciation of the
operating limitations and insight to obtain the design performance of his aircraft. The performance section of the flight handbook provides the specific information regarding the capabilities and limitations of each airplane. Every Naval Aviator must rely upon these handbook data as the guide to safe and effec­tive operation of his aircraft.

REQUIRED THRUST AND POWER

DEFINITIONS

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.

The total drag of the airplane is the sum of the parasite and induced drags. Parasite drag is the sum of pressure and friction drag which is due to the basic configuration and, as de­fined, is independent of lift. Induced drag is the undesirable but unavoidable consequence of the development of lift. In the process of creating lift by the deflection of an airstream, the actual lift is inclined and a component of lift is incurred parallel to the flight path direc­tion. This component of lift combines with any change in pressure and friction drag due to change in lift to form the induced drag. While the parasite drag predominates at high speed, induced drag predominates at low speed. Figure 2.1 illustrates the variation with speed of the induced, parasite, and total drag for a specific airplane configuration in steady level flight.

The power required for flight depends on the thrust required and the flight velocity. By definition, the propulsive horsepower required is related to thrust required and flight velocity by the following equation:

TrV

325

knots requires one horsepower of propulsive power. However, each pound of drag at 650 knots requires two horsepower while each pound of drag at 162.5 knots requires one-half horsepower. The term “power” implies work rate and, as such, will be a function of the speed at which a particular force is developed.

Distinction between thrust required and power required is necessary for several reasons. For the items of performance such as range and endurance, it is necessary to relate powerplant fuel flow with the propulsive requirement for steady level flight. Some powerpiants incur fuel flow rate according to output thrust while other powerpiants incur fuel flow rate depend­ing on output power. For example, the turbo­jet engine is principally, a thrust producing machine and fuel flow is most directly related to thrust output. The reciprocating engine is principally a power producing machine and fuel flow is most directiv related to power output. For these reasons the variation of thrust required will be of greatest interest in the performance of the turbojet powered air­plane while the variation of power required will be of greatest interest in the performance of the propeller powered airplane. Also, dis­tinction between power and thrust required is necessary in the study of climb performance. During a steady climb, the rate of climb will depend on excess power while the angle of climb is a function of excess thrust.

The total power required for flight can be considered as the sum of induced and parasite effects similar to the total drag of the airplane. The induced power required is a function of the induced drag and velocity.

Thus, induced power required will vary with lift, aspect ratio, altitude, etc., in the same manner as the induced drag. The only differ­ence will be the variation with speed. If all other factors remain constant, the induced power required varies inversely with velocity while induced drag varies inversely with the square of the velocity.

^h = Vi Prh V2

where

Ргіх = induced power required corresponding to some original speed, Vx Pfi2= induced power required corresponding to some different speed, V2

For example, if an airplane in steady level flight is operated at twice as great a speed, the in­duced drag is one-fourth the original value but the induced power required is one-half the original value.

The parasite power required is a function of the parasite drag and velocity.

where

Prv = parasite power required, h. p.

Dp = parafcitfe drag, lbs.

K=true airspeed, knots

Thus, parasite power required will vary with altitude and equivalent parasite area ( /) in the same manner as the parasite drag. However, the variation with speed will be different. If all other factors are constant, the parasite drag varies as the square of velocity but parasite power varies as the cube of velocity.

PrP2_(V2 V PrPl VJ

where

Pfpi = parasite power required corresponding to some original speed, Vx

Pfp2— parasite power required corresponding to some different speed, V2

For example, if an airplane in steady flight is operated at twice as great a speed, the parasite drag is four times as great but the parasite power required is eight times the original value.

Figure 2.1 presents the thrust required and power required for a specific airplane configu­ration and altitude. The curves of figure 2.1 are applicable for the following airplane data: gross weight, 15,000 lbs. span, b = 40 ft.

equivalent parasite area, f—1.1 sq. ft. airplane efficiency factor, e— .827 sea level altitude, <r= 1.000 compressibility corrections neglected

The curve of drag or thrust required versus velocity shows the variation of induced, para­site, and total drag. Induced drag predomi­nates at low speeds. When the airplane is operated at maximum lift-drag ratio, the total drag is at a minimum and the induced and parasite drags are equal. For the specific airplane of figure 2.1, (L/D)moi and minimum total drag are obtained at a speed of 160 knots.

The curve of power required versus velocity shows the variation of induced, parasite, and total power required. As before, induced power required predominates at low speeds and parasite power required predominates at high speeds and the induced and parasite power are equal at (L/D)mai. However, the condition of (L/D)m<u. defines only the point of minimum drag and does not define the point of minimum ■power required. Ordinarily, the point of mini­mum power required will occur at a speed which is 76 percent of the speed for minimum drag and, in the case of the airplane configura­tion of figure 2.1, the speed for minimum power required would be 122 knots. The total drag at the speed for minimum power required is 15 percent higher than the drag at (L/D’)max but the minimum power required is 12 percent lower than the power required at (,L/D’)mex.

Induced drag predominates at speeds below the point of minimum total drag. When the airplane is operated at the condition of mini­mum power required, the total drag is 75 percent induced drag and 25 percent parasite drag. Thus, the induced drag is three times as great as the parasite drag when at minimum power required.