Turbojet engines

The basic gas-generating core of the jet engine is a compressor, linked by a shaft driven by a turbine which runs in the hot gas efflux from the combustion chamber(s) in which an air/fuel mixture is burnt. They are located between the compressor and turbine stages (Fig. 7.4(a)). Air is ducted to the compressor from the air intake(s). For certain types of installation a plenum chamber (a form of collector-box from which the air is then drawn by the compressor) may be added between the intake(s) and the engine. The advantage of a plenum chamber is that a water separator, or filter for sand or ice, may be incorporated to protect the jewel-like rotating mechanism of the engine in extreme operational conditions, at sea, or in the desert, Arctic and Antarctic.

After passing through the compressor, fuel is injected into the airflow, where it is then burnt, before exhausting through the turbine, to accelerate along the jet-pipe, generating thrust. Combustion is never complete. When more fuel is injected into the exhaust jet the remaining air—fuel mixture ignites to increase thrust further by reheat (UK), in the process of afterburning (USA). The modifications needed for reheat are mechanically complex, as we shall see, involving mechanical devices which increase the cross-sectional area of the jet-pipe nozzle(s), to accelerate the flow to supersonic speeds (see Eqn 5-2). One is aware of reheat when selected by a pilot on take-off, by the noise, the long flame from a tailpipe and the bright shock diamonds which form as the flow decelerates back again to locally subsonic conditions within the jet exhaust.

The basic turbojet is a thirsty engine (and much more so when reheat is fitted). In an effort to achieve flexibility of performance much ingenuity has been applied, to convert the energy in the fuel into useful work as efficiently as possible. The commonest example is the combination of a turbojet driving a propeller either outside or inside a duct (as a fan). Thus the turboprop, turbofan and now the propfan have been introduced for different purposes.

The advantage of a propeller is that thrust is produced most economically when a mass of air, M, is given the least acceleration to attain velocity V relative to the engine. The energy to be provided by the fuel varies directly with M and as V2. Thus, on the face of it, the turbo-propeller unit, which moves a large mass of air relatively slowly and quietly, is the most fuel-efficient gas turbine adaptation — an example of symbiosis (see later), in which both gas generator (core engine) and propeller are mutually beneficial. Not only that, but the power/unit weight of a turboprop is superior to that of a piston-propeller engine, on top of which the fuel is the same aviation kerosene as for a turbojet. The disadvantage of a turboprop is that beyond about 300 KTAS and a propeller tip speed around M = 0.6, propeller efficiency falls rapidly, and this marks the limit for economical turboprop operations.

Propeller-driven aircraft operate at relatively low airspeeds where propeller and intake icing problems can be serious, because kinetic heating of the air is insufficient to prevent ice forming.

Apart from the need to provide adequate cooling and protection of the airframe from the effects of hot exhaust gases by efficient ducting, perhaps the most significant considerations when using propellers are disposition of the engines, and arrangement of the undercarriage. Although a large-diameter propeller with a small number of blades is more efficient than the reverse for converting the brake horsepower of an engine into tractive power, propeller size and undercarriage arrangement are mutually dependent. Propeller tips must not be too close to the ground, because of the danger of sucking up debris, or fouling obstacles, which are important considerations in bush and outback operations. Excessively long undercarriage units impose unnecessary weight penalties. Smaller ground clearances are possible with jet aircraft, but they must not be too small because of the dangers of ingesting debris, and they must allow adequate tyre and shock-absorber deflection to cope with a wide range of loading and wind conditions.

Where engines are wing-mounted there must be enough clearance without overlap between propeller tips and adjacent airframe surfaces, to avoid transmission of fatigue-provoking vibrations. In general the propeller disc should not lie in the plane of aircrew members, passengers, fuel and other service systems. In this way ice and other materials shed from the blades only causes airframe damage without personal injury or mechanical failure.

Too much clearance between propeller and fuselage of a multi-engined aircraft increases asymmetric problems (one or more engines out). The asymmetric yawing moment is proportional to the lateral offset of the engine from the centre of gravity. The larger the moment the bigger the required fin and rudder areas, or the higher the approach and landing speeds. Generally speaking, high-winged propeller-driven aircraft need more fin and rudder area to cope with asymmetric moments than low, because wing—body junctions and other sources of friction and turbulence have more severe wake effects upon dynamic pressure recovery at the tail than when wing—body and other junctions are less in line. A number of heavy, high-winged, twin-fin

piston-propeller transport aircraft of the 1940s onwards were forced to grow third fin surfaces for this reason. A propeller is treated theoretically as an 'actuator disc’, a device which actively imparts a change of momentum to the airflow passing through it. The greatest thrust from a propeller is achieved when the airflow relative to the engine is zero. When the aeroplane is at rest in still air the propeller thrust is a maximum. It follows that a pusher propeller, working in the sluggish wake of air slowed by friction and displacement by fuselage surfaces, wing junctions and tail surfaces, produces more thrust than a tractor propeller, located in front of the wings or the nose. The propwash behind a tractor propeller in flight loses momentum as it flows aft over the skin, reducing thrust and increasing drag by friction and displacement. The rate of climb, which depends upon excess power available at a given airspeed, is higher for a given horsepower and propeller efficiency with a rear-mounted engine than one in front.

However, a rear-mounted pusher-propeller is noisier than one in front, because the blades encounter chopped up and lumpy air, flowing aft in the wake with different velocities, causing vibrations and sound waves of different frequencies from the propeller blades.

Propeller ‘P’ effects (or ‘P’ factor)

Propellers are more complicated than they at first appear, generating what are called ‘P’-effects. These adversely affect flying qualities and pilot handling:

(1) Asymmetric blade effect: when the propeller shaft is inclined to the relative airflow the blades over one half of the disc are advancing with larger angles of attack than those over the other half. The result is uneven lift across the diameter of the propeller, which produces a pitching or yawing couple.

(2) Rigidity: in that the propeller, like a flywheel, tries to remain fixed in space.

(3) Precession: again like a flywheel, if a couple is applied to tilt the plane of rotation, the resulting motion is precessed 900 onwards in the direction of rotation.

(4) Pitching moment: is introduced when a propeller which is mounted ahead of or behind the centre of gravity, is inclined to the flight path. When tilted up or down it generates components of thrust in the pitching plane. When yawed to one side or the other, thrust components affect the aircraft directionally.

(5) Propwash: moves as a spiraling helix, like a corkscrew, in the same direction of rotation as the

propeller. The wash strikes the body, wings and tail surfaces at different angles of attack, affecting yaw especially. For example, a right-hand tractor propeller (rotating right as viewed by the pilot) causes propwash from the left at the tail, inducing a fin side force to the right, which tends to yaw the nose to the left. The pilot counters this by applying a touch of right rudder.

(6) Torque: is the opposite reaction to the engine turning the propeller, which tries to stand still while turning the aeroplane around it; thus a right-hand propeller would cause the aeroplane to attempt to rotate to the left. Counter-rotating propellers (contra-props) are needed to cope with torque effects which are too powerful for the authority of the flying controls (Fig. 7.3(a)). Opposite-handing of the propellers of multi-engined aircraft is another, slightly cheaper, way of countering torque effects (Fig. 7.3(b) and (c)).

7.2.3 Turbofan

For higher airspeeds with improved fuel efficiency the turbofan engine is now widely used and examples are shown in Fig. 7.4(b)—(d). It is also called a fan-jet or a bypass engine; it has a turbojet core, the shafting of which drives a larger-diameter fan within a duct. A mass of larger-diameter air is accelerated rearwards by the fan, to bypass the main engine core. A turbofan is described as having a given bypass ratio, defined as the ratio of the total mass swallowed to that passing through the combustion chambers. Thus, a turbofan engine in the 40 000 lb static thrust class might have a total mass flow at sea level of 1300 lb/sec and a bypass ratio of 8:1, achieved by burning 165 lb/sec of air in the combustion chambers.

The advantage of the turbofan is that it has a propulsive efficiency comparable with the turboprop, exceeding that of the pure jet engine. A medium bypass ratio turbofan passes all of the air through a low-pressure compressor before a percentage is ducted through the high pressure compressor, the

combustion chambers and turbines. The remainder is mixed downstream with the exhaust, after it has passed through the turbines. As the turbofan handles a greater mass flow than the turbojet, it is possible to reduce the thrust specific fuel consumption and fuel carried for a given mission. One manufacturer has claimed typical sfc around 15% lower than that of an equivalent turbojet, resulting in an aircraft 20 - 30% lighter, burning 30 - 40% less fuel for a given range and cruising speed.

Fig. 7.3 Torque counteraction using opposite-handed propellers.

Fig. 7.4 (b)—(d) Turbofan variants.

Fig. 7.4 (e) The advanced propfan: high performance, good specific fuel consumption, supercritical rotor blades - noise is a problem.

A turbofan with a high bypass ratio has a short, large-diameter cowl. The air is not mixed with the exhaust downstream of the turbines. So, handling a large quantity of unburnt air, it has a considerable scope for reheat. The percentage increase in thrust due to reheat is much higher than a turbojet, because the bypass air is still oxygen rich. At the tropopause, 36000ft and at M = 2.0, the basic turbojet thrust is about 1/3rd of the sea level static thrust. However, a turbofan with a bypass ratio of 1.0 would, with reheat, increase the thrust

fourfold, to 1.33 times the sea level static thrust. Thus, while the turbofan can improve subsonic cruise performance by using even higher bypass ratios, it retains a 'supersonic dash’ capability.

7.2.4 Propfan

Depending upon the price of fuel in the future, the advanced ducted turbofan engine, with its low specific fuel consumption, has a competitor in the experimental propfan engine (Fig. 7.4(e)). The diagram shows this to have a gas-generating turbojet core, with the exhaust passing through a separate turbine, which in turn drives counter-rotating open rotors. The blades, being scimitar-shaped, are aerodynamically equivalent to swept wing surfaces, to handle cruise Mach numbers higher than those for which the straight blades of a conventional turboprop are designed.

propulsive efficiency and saves the weight of a duct, it has a heavy gearbox and rotor drive. The rotors are larger in diameter than the ducting of a turbofan, which tends to constrain their mounting to pylons protruding from the rear fuselage. Whereas the ducting around a turbofan usefully absorbs some of the more offensive noise frequencies, reducing vibrations, the rotor tip speeds of the propfan are high, and special sound insulation is needed for passengers, bringing attendant penalties in weight.