Quest for Performance: The Evolution of Modern Aircraft
Chapter 10: Technology of the Jet Airplane
Jet Propulsion
[223] The speed at which a conventional propeller-driven aircraft may fly efficiently is fundamentally limited by the loss in propeller efficiency that occurs as the tip speed approaches a Mach number of 1.0. (See chapter 5.) One of the important advantages of jet-type propulsion systems is that they overcome this fundamental limitation. The air intake and internal flow systems for jet engines are designed in such a way as to limit the velocity of the air at the first stage of compressor blading so that severe adverse Mach number effects are not encountered.
Another advantage found in jet propulsion systems is the small weight per unit power and the tremendous amount of power that can be packaged in a single unit. An interesting illustration of the power [224] and weight features of jet propulsion can be obtained from the following comparison of some of the characteristics of the modern Boeing 747 Jetliner and the earlier propeller-driven Lockheed 1049G Super Constellation. The Super Constellation is typical of the final generation of high- performance, piston-engine transports and is described in chapter 6. The total power, the power per engine, and the ratio of power to dry engine weight of the two aircraft are compared in the following tabulation:
Characteristic Lockheed 1049G Boeing 747

Weight, W, lb

112 000

700 000

Speed, V, mph



Altitude, ft

23 000

35 000

Lift-drag ratio, L/D



Number of engines



Total cruise power, hp

6 585

59 934

Power per engine, hp

1 646

14 984

Dry engine weight, lb

3 675

8 600

Power to weight ratio



In this case, the power is defined as the total amount of power usefully employed in propelling the aircraft at the assumed used conditions of weight, speed, and lift-drag ratio. The power in the tabulation may be thought of as being proportional to the total number of British thermal units supplied to the engine per unit time multiplied by the overall efficiency with which this energy is converted to useful work; that is,
formula to determine engine power output
where P is in horsepower and V is in feet per second. The given values of W, V, and L/D employed in the equation are only estimates that may not be entirely consistent but are thought to be sufficiently accurate for the present purpose.
The Boeing 747 cruising at 530 miles per hour at a weight of 700 000 pounds is seen to require 59 934 horsepower; the corresponding values for the Super Constellation are 330 miles per hour, 112 000 pounds, and 6585 horsepower. The power per engine for the two aircraft is seen to be 14 984 horsepower and 1646 horsepower for the 747 and Super Constellation, respectively. The Wright 3350 turbocompound engines that powered the Lockheed aircraft are among the most powerful reciprocating engines ever developed for aircraft use. These engines developed a maximum of 3250 horsepower at takeoff; the value given in the table is for a normal cruise power setting. The enormous amount of power generated by the Pratt & Whitney turbofan engines of the 747 as compared with the reciprocating engines that propelled the Super Constellation is obvious. The values of power-to-weight ratio for the two types of propulsion systems are also of great interest. The weights used in this ratio are the dry, uninstalled engine weights as given in reference 205. The turbine engines in the Boeing aircraft develop nearly four times as much power for each pound of engine weight as do the reciprocating engines that power the Lockheed aircraft.
The jet propulsion system avoids the compressibility problem that limits the speed at which the propeller may be efficiently employed, is light in weight for a given amount of power as compared with a reciprocating engine, and can be successfully produced in single units capable of generating very large amounts of power jet propulsion systems also require much less maintenance than do reciprocating engines and may be operated for many thousands of hours without major overhaul. Engine failures are also relatively rare with jet propulsion systems.