FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1963
1963 - 1044.PDF
FLIGHT International, 27 June 1963 1013 to swivelling elbows, as in the well-known Bristol Siddeley Pegasus design. A further method, based on the Rolls-Royce thrust reverser, is the switch-in deflector. The thrust switching device consists of a pair of clamshell doors (or eyelids) which normally form part of the jetpipe inner wall. When deflected thrust is required the eyelids are swung rearwards to blank off the propulsion nozzle, at the same time uncovering an aperture at each side of the jetpipe. Extending from each aperture is a very short duct leading to a circular assembly of deflecting cascade vanes. The two cascade nozzles are mounted on large bearings and can be rotated under control of the pilot. The geometry of this type of deflector will give a deflection angle which is continuously variable from full propulsion through lift to full reverse. The switch-in deflector is easy to instal, as it can simply be bolted to the rear engine flange and normal engine mounting points used. The whole engine and deflector unit would be mounted in a stream lined pod from an underwing pylon. On installations requiring a double-engine pod, single-sided deflectors would be used. In both the straight-through and deflected-thrust positions, the deflector loads are easily taken through the engine casings to the pylon. Three types of deflector providing a fully variable thrust vector are illustrated in Fig 2, and more detailed pictures of the switch-in deflector are given in Figs 3 and 4. There are several important advantages of the switch-in deflector. First, it has excellent performance characteristics. At cruise the internal losses are extremely low, since the deflector vanes are switched out allowing uninterrupted passage of the exhaust gas to a conventional high-efficiency nozzle. The improvement in cruising specific fuel consumption over a deflector sytem having the mixed jet discharged through two rotating elbows as in Fig 2(a) would be about four per cent. In addition, compared with the system illus trated in Fig 2(b) the switch-in deflector takes advantage of the gain in specific fuel consumption obtained from mixing the hot and cold exhaust streams before discharging them through a common nozzle. A feature of using a separate conventional nozzle for cruise and cascade nozzles for take-off and landing is that the cascade area can be chosen to give the optimum engine working point for maximum thrust during the lifting phase, and the plain nozzle area chosen to provide the lowest altitude cruise fuel consumption. This freedom is particularly significant when bleed air is required for flap-blowing during the lifting phase. A switch-in deflector enables the projected frontal area of the pod to be minimized. It may be as much as 15 per cent lower than a four-elbow installation and 25 per cent lower than the twin-elbow scheme. Additionally a low-drag fairing can be deigned around the cascade assembly since the cascade nozzles are not used during cruise, and the advantages following from low pod drag result in a further improvement in cruising fuel consumption. The effect of specific fuel consumption on the aircraft operating cost can amount to a substantial sum during the service life of the fleet. For example, over a fifteen-year period, a four-engined trans port aircraft could be expected to use about 10m Imp gal of fuel. An improvement of four per cent in engine cruising fuel consump tion, due solely to the switch-in deflector, would provide a total saving of 400,000gal of fuel per aircraft. The installation is not complicated by the problem of interference between the front and rear jets that is always present with the four- elbow installation; the latter must either suffer a cosine component at the front nozzles, or the front and rear nozzles must be staggered thus incurring a drag penalty. Furthermore, since the jets do not have to be permanently angled away from the direction of flight with the switch-in deflector, there is no question of a fatigue prob lem on the fuselage due to noise pressure fluctuations from the jets. Secondly there are a number of operational advantages attribut able to the switch-in deflector. Initial take-off is carried out using the conventional nozzles, and the deflectors switched-in just before lift-off. Prior to open:Jig up the engines the rotating cascades can be preset to the required angle and checked in position, thus giving an extra degree of certainty that the correct thrust vector will be available during the critical take-off path. Where runway limitations do not exist, the aircraft will take-off using the propulsive nozzles and in these circumstances the cascades can be preset in the reverse position in case the take-off has to be abandoned. In the combat area where the aircraft is liable to damage by enemy action, no hazardous condition can arise due to damage to the cascades as the aircraft can continue and return to land at its base using the conventional nozzles. Also, the low intensity of infra red radiation—as a result of the low gas temperature resulting from mixing the by-pass and turbine exhaust gases—substantially reduces the risk of unwelcome contact with anti-aircraft missiles fitted with infra-red homing devices. Thirdly, it is worth stressing the advantages the switch-in deflec tor enjoys in terms of life and reliability because the design is based on existing thrust-reverser principles. Hundreds of thou sands of hours of successful operation have led to reverser over haul lives in excess of 4,000 hours. The development work involved in this achievement has led to an accumulation of much special ized knowledge in this field. It is not generally realized that the eyelids, seals, pivot bearings and actuating mechanism have to withstand the temperature and vibration conditions over the whole range of engine operation from idling to take-off, and not just the conditions when reverse is selected. The internal contours of the (concluded at foot of page 1016)
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
