Field International Ltd… Design, Precision Engineers & GSE Specialists

Field House, 18-20 Nuffield Ind Est

Poole, Dorset, BH17 0RB.

Tel:+44 (0)1202 676331

Fax:+44 (0)1202 684043

email: sales@fieldinternational.com

website: www.fieldinternational.com

Ground support equipment

Ground support equipment turnkey Design, Manufacture & Installation including Aircraft Jigs & Fixtures,ground support equipment component Manufacture, aero engine tooling, wing repair tooling, Undercarriage handling trolleys, wheel recovery jacks, Oil test rigs, engine covers and engine cowling bungs, rubber components and machined parts and brazed components.

BVQI, NATO & ISO Accreditations

Aero engine tooling

The challenge for a manufacturer in this sector is the types and variations of aero engines and the locations on an airframe where engines can be installed.

A wide range of engine types are used in aircraft. Each type has particular advantages and drawbacks, and its performance in terms of power output or thrust and fuel consumption is critically important in most cases. For a particular type of aircraft, there is often a single engine type that is most suitable, so the choice is primarily concerned with specifying the manufacturer and model.

Also of importance are the location and installation of the engine or engines on the aircraft. The choice of installation is influenced by the type of aircraft, the type of engine and the way in which the aircraft will be used in service.

Aircraft power plants fall into five main types: turbo jet, turbo-fan, turbo-prop, prop-fan and piston engine

The first four on the list are variants of the gas-turbine engine, which is capable of delivering high power in an extremely compact size and with low weight. Each variant is most suited to a particular aircraft flight speed. The operating efficiency (loosely defined as power absorbed divided by the rate of fuel burn) is maximised when the velocity of the air expelled from the jet, fan or propeller is close to the speed of the aircraft. This means that an aircraft such as an airliner that flies at sub-sonic speeds cannot be efficiently powered by an engine such as a turbo jet, which has a high exhaust velocity.

Turbo-jet engines produce all of their thrust as a high-speed gas stream of relatively small diameter, because all of the exhaust gases have passed through the combustion process. This is ideally suited to very high-speed aircraft. It may be the only viable power plant.

Turbo-fan engines are based around the pure turbo-jet, but differ in that some of the exhaust gases are made up of air that has by-passed the engine core or gas generator (hence the term bypass engine) and passed only through a fan. So the thrust is produced by an air stream of larger diameter than the turbo jet, but at a lower velocity. The turbo-fan engine is suited to aircraft that fly in a range of speeds from around Mach 2 down to around 350 km/hour, depending on the by-pass ratio. The by-pass ratio is the ratio of the air that passes only through the fan to that passing through the core of the engine, providing the oxygen required to burn the fuel. The higher the by-pass ratio (i.e. the higher the proportion of air passing only through the fan), the lower the aircraft speed at which the engine will be efficient, but the larger the engine's diameter for a given thrust.

At lower speeds still, even the turbo-fan becomes less efficient, and the propeller comes into its own. The turbo-prop engine still takes advantage of the high power capability and low weight of the gas turbine engine, but uses the power produced to turn a propeller via a gearbox. The small diameter of the engine itself in comparison with a piston engine of the same power output is a distinct advantage, allowing streamlined engine installations.

The prop-fan engine is a very new development, and seeks to gain the advantages of the turbo-fan and turbo-prop engines. The fan duct, which makes up the outer wall of the turbo-fan, is omitted in this design, and the exposed fan combines features of both a fan and a multi-blade propeller. The fan may run at high speed, driven directly from the turbine, or may run more slowly, via a gearbox. The fan is normally two-stage (two sets of blades one behind the other), and contra-rotating. These engines are still under development, and technical problems are progressively being overcome.

For the smallest aircraft, a gas turbine engine is too expensive, both to buy and to maintain, so piston engines are the most common choice. They are cheap, simple and reliable, require little in the way of specialist servicing and provide adequate power for most light aircraft.

Once the type of engine has been selected, the location of the engine or engines in the airframe is of prime concern. If only one piston engine or turboprop engine is fitted it will normally be in the nose. If a single turbo-jet or turbo-fan engine is used it will also normally be in the fuselage, but further aft, near the centre of gravity. If there are two or more engines there is a variety of positions to be considered.

With turbo-prop and piston engines, the diameter of the propeller places severe limitations on their location; twin- and four-engined turbo-prop aircraft will almost inevitably require the engines to be wing-mounted. Turbo-fan, turbojet and prop-fan engines allow greater flexibility in their location.

Most engines have similar mounting points provided regardless of where they are to be located. The most common arrangement is for the major thrust and weight loads to be borne at two primary locations at the top of the engine, with stabilising links to prevent rolling, or at both sides and the rear. The way in which these points are connected to the aircraft structure, however, varies widely depending on the type of installation.

Military aircraft engines.

Combat aircraft use mainly external installations for the weapon systems, and the fuselage is not required to carry an internal payload, so it is an ideal location for the engines. This gives the aircraft smooth exterior lines, for low drag, and lends the engines some protection from battle damage. The thrust must be large in relation to the aircraft weight, so the engine are relatively large, and take up the major portion of the fuselage volume. Intakes must be designed to supply large volumes of air to the engines, and the shape of these can be extremely critical to provide the best possible performance over a wide range of speed, angle of attack and altitude. In twin engine installations, the engines are close together, reducing the adverse yaw effects if an engine fails in flight.

One of the most remarkable aircraft with an internal engine is the British Aerospace Harrier. Using vectoring nozzles for both the by-pass air and the engine exhaust, coupled with an engine that can produce more thrust than the weight of the aircraft, the Harrier can land and take off vertically when lightly loaded. Even with a heavy weapon load, the take-off run is substantially reduced, and vectoring the nozzles in forward flight (called VIFFing) allows a superior turn performance over most normal fighters. The secret of the Harrier's success are the the Rolls Royce Pegasus engines.

Most light aircraft, and some small turbo-prop aircraft, have a single engine mounted in the nose. The engine is supported by a welded framework cantilevered from a firewall, or bulkhead, immediately forward of the cabin. The firewall, as its name suggests, separates the hot engine from the rest of the structure. The engine mounting attaches directly to the flat firewall, so a different engine can often be fitted simply by replacing the engine mounting framework, provided the weight is not substantially different. This allows a manufacturer to update the design to incorporate a more powerful or more economical engine, perhaps giving an ageing design a new lease of life.

Most transport aircraft have externally mounted engines, leaving the fuselage interior volume clear for carrying passengers or freight. The designer has a choice of locations for turbo-fans and prop-fans, as already stated, but turboprops are essentially limited to wing locations. Engines may be mounted on the rear fuselage, or they may be mounted on the wings, generally in under-slung pods. Each location has particular advantages and disadvantages, and the designer has to balance these when selecting the location.

Twin- or multi-engined propeller-driven aircraft must have their engines spaced out along the wing to provide clearance between the propeller tips and the fuselage. The closer the tips are to the fuselage, the more noise is generated inside the fuselage, and the further away they are, the more the aircraft yaws if an engine fails. The radius of the propeller also creates ground-clearance difficulties with low-wing arrangements in some cases, and high-wing turboprops are common in the medium-size category. The high-wing arrangement allows the aircraft to sit close to the ground, which makes loading easier, especially when the aircraft has a rear loading door. It can cause problems with undercarriage design, though, and the choice is usually between long, spindly main units that retract into the engine nacelles and short, stocky units that retract into the fuselage. To reduce the effects of intrusion into the load space, the fuselage may have bulges into which the undercarriages can be retracted, giving the undercarriage a wider track, improving stability on the ground.