Background

Gliders were flying long before the Wright brothers first flew, but an engine could not be installed even when automobile piston engines became available – they were too heavy for gliders. The Wright brothers made their own lightweight gaso­line engine with the help of Glenn H. Curtiss. Until World War II, aircraft were designed around available engines. Aircraft sizing was a problem – it was not opti­mized for the mission role but rather based on the number and/or the size of the engine installed.

During the late 1930s, Frank Whittle in the United Kingdom (who died in Eng­land in 1996) and Hans von Ohain in Germany (who died in the United States in 1998) were working independently and simultaneously on reaction-type engines using vane-blade-type precompression before combustion. Their efforts resulted in today’s gas turbine engines; however, at the time, it was difficult for Whittle to con­vince his peers. By the end of World War II, gas-turbine-powered jet aircraft were in operation.

Post-World War II research led to the rapid advancement of gas turbine devel­opment such that from a core gas-generator module, a family of engines can be designed using a modular concept (Figure 10.1); this allowed engine designers to offer engines as specified by aircraft designers. Similar laws in thermodynamic – design parameters permitted power plants to be scaled (i. e., rubberized) to the requisite size around the core gas-generator module to meet the demands of the
mission requirements. The size and characteristics of an engine are determined by matching them with the aircraft mission. It is now possible for both the aircraft and the engine to be sized to the mission role, thereby improving operational economics. Modular engine design also favors low downtime for maintenance.

The potential energy locked in fuel is released through combustion. In gas – turbine technology, the high energy of the combustion product can be used in two ways: (1) converted to an increase in the kinetic energy of the exhaust to produce the reactionary thrust (i. e., turbojet and turbofan); or (2) further extracted through an additional turbine to drive a propeller (i. e., turboprop) to generate thrust.

Initially, reactionary-type engines were simple straight-through airflow turbo­jets (see Figure 10.4). Subsequently, turbojet development improved with the addi­tion of a fan (i. e., long compressor blades that are visible from the outside) in front of the compressor; this is called a turbofan. The intake airmass flow is split into two streams (see Figure 10.5): the core airmass flow passes through the engine as primary flow and is made to burn; the secondary flow through the fan is bypassed (hence, also called the bypass engine) around the engine and remains as cold flow. For this reason, the primary flow is known as hot flow and the secondary bypassed flow is known as cold flow.

Significant general progress has been made in the aircraft power plant design. Engine technology is substantially more complex than aircraft technology. A gas- turbine operating environment demands more aerodynamic considerations than an aircraft. Stringent design considerations must accommodate very high stress lev­els on an engine at elevated temperatures, yet it must be as lightweight as possi­ble. The manufacture of gas turbine parts is also a difficult task – a tough material must be machined in a complex 3D shape to a tight tolerance level. These consid­erations make gas turbine design a complex technology and requires an involved microprocessor-based management.

Gas turbine engines have a wide range of applications, from land-based, large prime movers for power generation and ships (both civil and military) to weight – critical airborne applications. The theory behind all the types has a common base; however, the hardware design differs, driven by the application requirements and technology level adopted. For example, land-based engines are not weight-critical and do not need to stand alone; therefore, they are less constrained in design. Surface-based gas turbines must run economically for days and/or months, generat­ing significant power compared to standalone, lightweight aircraft engines that run for hours on varying power, altitude, g-load, and airflow demands. Even the largest aircraft gas turbine engines are small compared to land-based engines.

The success of a new gas turbine design is achieved by fully understanding and appreciating previous designs. Progress is made in increments by incorporating proven, newer technologies that emerge in the interim. Gas turbine development has a long gestation period compared to aircraft and it depends on previous designs. Typically, a technology demonstrator leads the way in introducing a new design.

Gas turbine designs have advanced to incorporate sophisticated micro­processor-based control systems with automation, which are called full authority digital electronic control (FADEC) and work in conjunction with the FBW con­trol of aircraft. CAD, CAM, CFD, and FEM are now the standard tools for engine design.

Liquid-cooled aircraft piston engines of more than 3,000 HP have been built. However, except for a few types, they are no longer in production because they are too heavy for the power they generate; in their place, gas turbines predominate. Gas turbine engines have a better thrust-to-weight ratio. Two successful pistons were the World War II types: the Rolls Royce (RR) Merlin and the Griffon, which pro­duced 1,000 to 1,500 HP and weighed approximately 1,500 lb dry. Also, AVGAS is considerably more expensive than aviation turbine fuel (i. e., kerosene) (AVTUR). Today, the biggest piston engine in production is approximately 500 HP. Recently, diesel-fuel piston engines (i. e., less than 250 HP) have entered the general-aviation market. In the homebuilt market, motor gasoline (MOGAS)-powered engines have been used and are approved by the certifying agencies.

Chart 10.1 classifies all types of aircraft engines in current use; this book is con­cerned only with the air-breathing types.

The application domains of the types addressed in this book are shown in Figure 10.2. High BPR turbofans are intended for high-subsonic speeds. At super­sonic speeds, the BPR is less than 3. Typically, turboprop-powered aircraft speeds are at and below Mach 0.5. Piston-engine-powered aircraft are at the lowest end of the speed range.

Typical levels of specific thrust (F/ma, lb/lb/s) and specific fuel consumption (sfc, lb/hr/lb) of various types of gas turbine engines are shown in Figure 10.3.

Table 10.1 lists various efficiencies of the different classes of aircraft engines. Table 10.2 shows the progress made in the last half-century, indicating the advances made in engine-weight savings. Since the 1970s, compliance regarding engine-noise levels has been a requirement of the certifying agencies. Pollution levels due to noise and emissions are steadily decreasing (see Chapter 14).

If required (or preferred), the internal contours of the intake and exhaust of a civil aircraft nacelle pod are designed by engine designers in consultation with airframe designers. Shaping of the nacelle’s external contour is the responsibility of aircraft designers. Military aircraft intakes and exhausts have higher degrees of

Thrust/weight ratio

1950s (J69 class)

2.8-3.2

1960s (jT8D, JT3D class)

3.2-3.6

1970s (j79 class)

4.5-5.0

1980s (tF34 class)

6.0-6.5

1990s (F100, F404 class)

0.5-8.0

Current

8.0-9.0

Table 10.2. Progress in jet engines

complexity and are design-specific. Military aircraft intake and exhaust ducts are developed by aircraft designers in consultation with engine designers.

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