Buried units

Engines mounted within the wings are the oldest installations for jet aeroplanes discussed so far. The

arrangement was convenient aerodynamically because the aeroplanes being designed in the 1940s employed sections with much larger thickness ratios than are now used.

Failures of engines buried inside wings are more likely to affect the surrounding structure when, for example, turbine-blades are shed. Engine changing can be more of a problem too. The buried installation is perhaps the cleanest of all aerodynamically.

Several engine installations are shown in Fig. 7.11, in which the maritime Comet shown in Fig. 7.11(d) is a project utilizing a number of well-tried components from the original De Havilland Comet family.

Fig. 7.11 Turbojet and turbofan engine installations. 7.5 Interference and symbiosis

The degree of interference between the external and internal aerodynamics of an aeroplane is hard to

calculate with certainty. The combinations of speed, normal acceleration, height and ambient temperature (all of which add up to attitude to the flight path: the angle of attack), and Reynolds number, that result in flow instabilities are almost impossible to predict. The engine installation is one of the most thoroughly explored problems in all wind-tunnel testing, yet it is one of the most thoroughly misrepresented — simply because of the difficulties besetting accurate representation.

The installed drag of an installation is made up of several isolated parts: Installed drag = f (isolated drag) (7-7)

Isolated drag = (nacelle wave drag) + (nacelle skin friction) + (intake drag) + (nozzle drag) (7-8) The essential problem is the evaluation of f. If f = 1.0, then the installed drag is simply the sum of the isolated parts. If the engines can be placed in aerodynamically favorable positions around the airframe, without sacrificing handling performance and structural integrity, then f might be made less than 1.0. If, however, f is

greater than 1.0 the implication is that there is unfavorable interference between airframe and installation aerodynamics.

Although interference is usually unfavorable in its adverse effects upon powerplant performance, the effectiveness of aerofoil surfaces and the authority of controls, that is not always so. In an effort to maintain flexibility of performance at high speed over a wide part of the flight envelope, increasing use is made of symbiosis in powerplant design. Simply by making different parts mutually dependent upon one another a whole composite powerplant can be created which has improved efficiency. Examples seen already are the combination of propeller and gas turbine which together form a turboprop unit, and similarly, the turbofan and the propfan.

In the 1950s, with the danger of the Cold War between the Soviet Union and the West escalating, there was much discussion about symbiosis. An Operational Requirement, OR. 301, called for a rocket-propelled fighter with the ability to reach high-flying targets. The advantage of the rocket was that it did not depend upon atmospheric oxygen, because it carried its own source of O2. The UK aircraft industry produced a wide variety

of proposals, many of which resorted to the combination of rocket and air-breathing turbojet, to extend endurance over that of the pure rocket. The design study in Fig. E.2 is such an example, with an additional weight-penalizing variable-geometry wing, intended to further extend the ability to loiter.

A French example of symbiosis which was also not developed was the experimental French Nord Griffon 02, shown in Fig. 7.12. This aeroplane, which combined an air-breathing turbojet inside a ramjet duct, formed a turbo-ramjet.

Fig. 7.12 Experimental symbiosis in the form of the French turbo-ramjet propelled Nord Griffon 02. In 1957 the aeroplane broke the world record for a closed 100km circuit at 885 knots (1640kph). It was used for

supersonic research, funded by the US Air Force, until 1960.

Although such aircraft might appear to be out of date, they are not. In aviation, as in much else, there is nothing new under the Sun. One needs to know these things. There will be future occasions when the design engineer can then pull out and dust off past records of what has been attempted before, to give him or her a useful slant on what appears at first sight to be a new problem, but which is not.

Symbiosis results in larger-diameter powerplant units. Careful design is needed to balance gains against losses. Reliability is at a premium. Figure 7.13 shows a previous Rolls-Royce proposal for an advanced surface-burning aircraft, able to change its overall propulsive and aerodynamic shape to suit conditions. Such an aircraft was intended to climb at a constant EAS around 400k, until it reached M = 5 around 100,000ft. After changing shape it was expected to reach a hypersonic cruising speed of M = 15, around 200,000 ft. Between subsonic and M = 5 flight the variable geometry would allow use of the main internal engines, with a low aspect ratio integrated engine-box and wing providing lift. Beyond M = 5 the geometry would transform to a double wedge with external surface-burning of the fuel on the trailing

underside. The rise in pressure with combustion would generate lift and thrust components. One hesitates to speculate too confidently on what would happen to lift, drag, pitching moments and controllability around M = 5, when changing over from one aerodynamic and propulsive shape to another. Clearly, the aircraft would have a high-order automatic control system.

Fig. 7.13 Rolls-Royce proposal for a surface-burning hypersonic aeroplane. 7.6 Powered lift

Powered lift, the augmentation of aerodynamic lift by a thrust component, is used to achieve vertical take-off and landing (VTOL), or at least short take-off and landing (STOL), and thus make aircraft as independent of prepared airfields as possible. The penalties incurred by carrying special lifting engines or devices for vectoring thrust and additional fuel for low-speed flight are compensated for in certain cases by smaller wing surfaces and lower structure weights. To achieve maximum economy with a VTOL aircraft one must equate power required for take-off and landing with that required in cruising flight. This is hard to do with anything other than highly supersonic aircraft with low cruising lift/drag.

There are some 15 ways of using power to generate lift, and these are shown in Fig. 7.14, after the original diagram by Campbell of NASA.

Fig. 7.14 The VTOL family of aircraft

All are governed by the same principles as for propulsive thrust: namely that the lifting thrust is the product of mass flow and Jet (or slipstream) velocity, while the power required is a function of the square of the jet (or slipstream) velocity: 2 j aV m 2 1 P= (7-9)

for a jet, and m_aW2 2

1

P= (7-10)

for a propeller.

Just as high aspect ratio is efficient for long-range and high altitude, the most efficient way of

generating powered lift is by a large rotor moving a mass of air at low velocity. However, rotor-craft are limited to relatively low airspeeds (unless the rotor can be retracted or housed in forward flight) and they are not compact. A jet engine is the least efficient of all, as may be deduced from the power equations when typical values of 50ft/sec and 2000ft/sec (15 m/s and 600m/s) are inserted for the rotor and jet, respectively. Jet lift results in compact installations, helped by the modern ability to build very small, light, units generating high thrust/weight. The ducted fan can be regarded as a compromise between the rotor and jet. The ducted fan is smaller in diameter than a propeller or rotor, but tip-losses are reduced by enclosing the fan within an annular duct, possibly fitted with cascades. The total lift is that developed by the fan and an increment contributed by the relative airflow over the ducting.

Disc loading is a term describing the amount of weight, or lift, borne per unit area of rotor supporting it. If the area of the compressor face is taken as the rotor area, in the case of a jet engine, then a

three-dimensional plot of lift generated/unit horsepower can be made against disc loading for the family of aircraft already illustrated. This is done in Fig. 7.15.

Fig. 7.15 (a) and (b) Powered lift spectrum. The Lockheed Martin aeroplane in (b) shows an arrangement of forward fan plus vectoring nozzle aft, for main lift and control in pitch, with lateral nozzles for roll control, to meet a short take-off and vertical landing Joint Strike Fighter (STOVL JSF) shipboard requirement.

There are 3 ways of employing jet lift. The first is to use vectored thrust from the main propulsion engine(s) (see Section 7.6.1). The second is to use independent, dedicated lift engines which are shut down in forward flight. The third is a hybrid: the provision of vertical lift engines forward of the CG, to augment and balance the effect of deflected thrust of the main engine(s) behind it.

Lift engines which make no major contribution to propulsion represent a considerable weight penalty if used in a relatively small strike aeroplane. Even so, the hybrid third engine arrangement of two separate vertical lift engines forward of the thrust-vectoring main engine was incorporated in the Soviet Yakovlev Yak-38, evolved from the Yak-36 for naval use, the prototype of which flew in 1971. Incidentally, early operational losses of the Yak-38 were similar in magnitude to losses of the AV-8B/Harrier II of the US Marine Corps.

While the Yak-38 was not supersonic, a successor, the V/STOL Yak-141, a supersonic naval fighter, reported to be capable of M = 1.8, was revealed in 1988. It featured a broadly similar engine arrangement. Some indication of the technical complexity involved is that the aeroplane has full-authority digital triplex fly-by-wire control systems, with integrated engine controls and vectoring reheat.

All jet-lift engine installations involve risk. Engine surge or failure, hot gas or debris ingestion can be equally catastrophic. Crew escape by means of ejection seats is vital. In the case of an engine arrangement like that favored by Yakovlev:

(a) Balance of the aircraft at the CG in the hover while using separate thrust sources demands highly reliable combustion and fuel feed, especially with a vectoring nozzle incorporating reheat. Surge or failure of any unit is usually catastrophic for the aircraft. The pilot/crew need an ejection seat system which can be fired automatically. That in turn involves an arming program, linked to the hover mode and incorporating accurate sensors.

(b) Nozzle design is critical. Operation in the vectored mode involves deflection down-wards through an angle in excess of 900, when thrust is needed to bring the aircraft to a halt and then to maneuver backwards. For normal flight the nozzle must produce reheat thrust just as reliably at supersonic speeds.

(c) Loss of thrust caused by ingestion of recirculating high-temperature exhaust gases must be minimal. This involves the incorporation of deflector plates, both fixed and retractable, to cope with large volumes of hot exhaust gases in a relatively confined space adjacent to intakes. Crosswind effects must also be taken into account in their design.

(d) Wear and tear (erosion) of concrete and other surfaces by heat and jet efflux cause the surfaces to break up, with the risk of pieces then being ingested. It is less of a problem on the steel deck of an aircraft carrier.

(e) Lift engines mounted forward of the CG, and main engine nozzle(s) near mainwheel units, can expose undercarriage legs and tyres to temperatures of 10000C or more, depending upon ambient and operational conditions.

Although it might appear easier to install separate, dedicated vertical lift engines in a large and heavy V/STOL transport aircraft, by keeping the engine pods away from tyres and important areas of airframe

structure, there are still problems. The strategic freighter project in Fig. C.9 could also suffer ingestion of debris by the main propulsion engines, from vertical lift units when operating from raw surfaces, and in crosswinds.

Further, loss of one or more lift engines could make life trying. One can hardly give the crew ejection seats while providing none for the passengers.

Even so the weight penalty of using dedicated lift engines for large and heavy V/STOL aeroplanes, is possibly less severe than for smaller aircraft. In Section 12.1.3 it is shown that total engine weight varies theoretically as

n 1

, where n is the number of engines used to generate the required thrust. Thus, indications are that for jet lift the relative weight penalty for a large aeroplane is less than for one that is small. The required number of small lift engines in slender pods can be fitted to hard points under the wings. The pods need not be much larger than overload fuel tanks.

A criticism of powered lift has been that in hovering flight high power and fuel consumption are wasted on going nowhere. The thrust/weight required for lift, control and maneuvering in the hover is around 1.4, while the lift/drag in cruising flight may be around 10 (i.e. the equivalent cruising thrust/weight = 0.1). Such an aeroplane uses 14 times as much fuel hovering as cruising for the same interval of time. Every minute spent hovering reduces the range of an aircraft cruising at M = 1.2 by something like 11 nm. It follows that there is a close correlation between hovering time, range and disposable load. Considerable fuel savings are effected by even a short take-off run. That knowledge led to the introduction of the ski-jump, the raised forward end of a portion of the flight-deck of aircraft carriers operating VSTOL aeroplanes.