British Jet Engines

by Louis Bruchiss
Releasing a wealth of information on jet engines and jet propelled airplanes, Air News with Air Tech presents in this article the second and final installment on the development and details of British gas turbines. The various parts of the jet engines will be discussed and their application to integrated design with the airplane analyzed. The material is based upon the paper read by Dr Harold Roxbee Cox at the ninth annual Wright Brothers Lecture of the Institute of the Aeronautical Sciences.

To the rabidly nationalist American, jet propulsion is a remarkable something wrapped up in the Stars and Stripes. To some British subjects, the miracle power is best symbolized by a heart-red Union Jack. But in rational engineering circles, jet power for aircraft is the end result of selfless collaboration — first established between British organizations engaged in research and manufacture, then extended to the United States.

The result has been production and improvement of the Whittle jet to a point where the Bell P-59A with GE I-40 turbines has become a reality.

The problems encountered by the British technicians were many-fold. Bearings for the moving parts were always a headache. Most designs used ball bearings, some used roller bearings at the turbine end of the jet engine shaft. Lubrication has been improved with "solid" lubrication as the more common practice compared to the atomized spray with low viscosity oil.

Despite discouraging results, all of the jet engines have been using sheet metal as the major structural portion of the combustion system. Insufficient experience with resistance welding, failures due to fatigue of the metal itself, oxidation through contact with the flame and distortion caused by unequal temperature distribution were all sources of trouble. Success came with more careful attention to the choice of material and finish, accumulated experience in welding.

Fatigue failures have been largely eliminated through removal or reduction of vibration sources, by stiffeners, through redesign of corners and junctions, and by deliberately locating some components to allow for expansion.

The early Whittle engine flame tubes were constant victims of poor heat distribution. In all but annular combustion chambers this problem has been cleared up by elimination of thin metal parts in the flame region and by rigid control of the fuel injection.

As engine power ratings increased, compressor impeller failures began to show up. Due mainly to the resonant vibrations set up by impulses from the diffusers, this problem was eliminated only after a prolonged delay because pilots and test personnel avoided resonant frequencies because they found the noise uncomfortable. Test runs at low speeds gave 200-hour life to impellers whereas under high speed test they gave out after they had been used for five hours.

Development of high temperature steel was of paramount importance in the turbine part of the jet engine. Breakage at the root junctions was eliminated through the use of larger blade radii, modern stress analysis, and study of vibration problems. British turbine blades made from Nimonic 80 alloy have excellent creep and fatigue properties at turbine temperatures and are readily forged at 1100°C. High temperature effects on turbine discs and rims have been lessened by air and oil cooling.

Because combustion is the object of the entire setup, particular attention has been paid to this phase of operation. While there was a background of experience in oil burning, combustion technique in an aircraft gas turbine was something else. Weight and pressure losses, affecting engine performance, both demanded consideration.

Whittle originally built his unit with one large combustion chamber and fuel injection through burners of the low dispersion pencil type. Unsatisfactory combustion recommended vaporizing of fuel prior to injection and final success came with introduction of the ten separate combustion chambers. This arrangement permitted each chamber to be individually tested with a relatively small air supply. A second technical achievement was the development of the Lubbock fuel burner which gave a finely atomized spray of liquid fuel over a wide range without overheating or clogging.

Current combustion practice favors multiple chambers, with downstream atomized injection and air admission utilizing various degrees of swirl. Metropolitan-Vickers employs an annular chamber with upstream pencil injection, while Armstrong-Siddeley has an engine which is based on vaporization and partial mixing with air before burning starts.

Testing of components and complete units posed difficulties which were not overcome immediately. The early small axial compressors required test equipment that was readily available and the larger compressors of the B.10, D.11, and F.2 engines were tested with a stream turbine built for a power station by Metropolitan-Vickers.

Combustion chamber design, however, demanded special facilities and apparatus. In the case of the small chambers as used in multi-chambered setups, each individual unit can be tested with existing air compressors delivering sufficient quantities of air at approximately atmospheric pressure of 15 lb per sq in. For the large single annular chambers of the F.2 type of engine, far greater power is necessary. As an expedient, compressors were used which had been built for work on a new tunnel under the Thames River. Power Jets, Limited, have been employing two compressors of 600 hp each, providing 3 lb of air per second at 40 lb per sq in. Meanwhile, Rolls Royce has been using a 6000-hp power station belonging to the Northampton Electric Supply Company, to test de Havilland H.1, Metropolitan-Vickers F.2 and Armstrong-Siddeley ASX engines.

Power Jets has a special 6000-hp plant which can turn at approximately 18,000 rpm. Because of this high rpm, the turbine of the power plant is exceptionally small. The maximum diameter over the blades is 13 inches and the drum on which the blades are mounted is only 8 inches in diameter. A marine type boiler supplies steam to this test turbine, and the plant can actually test a compressor, a turbine, a combustion system or supply air for any special tests.

Of particular value to British gas turbine testing has been the flying test stand. The first flying test stand, a Wellington bomber modified to take a W2B engine in the tail, flew with its gas turbine in August, 1942. This craft, with a similar second one, is now being used by Rolls Royce for Whittle engine development work. Power Jets is using a third Wellington for the same purpose.

A Lancaster bomber is another aircraft from which the tail turret has been removed to allow mounting of a Metropolitan-Vickers F.2 at the rear end of the fuselage. A large intake was mounted on top of the fuselage without interfering with the tail control group unit. In another Lancaster owned by Power Jets, the test mounting is installed in the bomb bay space. Flying test stands appear to offer the greatest possibility of engine research under actual flight conditions, having the advantages of a flight staff observing results with all necessary instrumentation. The only disadvantage is the low flight speed attainable in bomber aircraft.

A most significant development associated with the aircraft gas turbine is the possibility of more closely coordinating airplane structure with the engine or engines. With the conventional reciprocating engine, airplane and power plant have been and are considered two separate units. This is not practical with gas turbines. Here the components, compressor, combustion chamber, turbine and exhaust nozzle are separate entities with varying shapes and predetermined locations.

It is true, nevertheless, that the difference between centrifugal and axial flow compressors will establish to some extent their placement on aircraft. Whether the true jet turbine, the combination propeller-gas turbine, or the ducted fan turbine is indicated in any particular application will depend entirely upon the mission to be performed by the aircraft. It is generally accepted that for the highest sustained subsonic and near-sonic speeds the true jet engine is the best. Where economy of operation is important the jet propulsion engine appears inferior to propeller and ducted-fan engine. In any aircraft, if the payload is to be at a maximum, then the powerplant plus the fuel load must be at a minimum.

In addition to the factor of powerplant weight, fuel load and speed there are considerations of initial cost, maintenance, operational altitudes, and ultimate purpose of the plane. All of these things must be weighed and balanced on the basis of each type of propulsion.

Two advantages of the jet engine stand out here. It can use the cheapest quality of fuel, and it may develop that for initial cost and lightness, the jet engine will be adopted as the most practical for all aircraft. Among the changes forced upon aircraft designers by the advent of the gas turbine are the large holes required in front of the plane for air intake; the large holes in the rear for exhaust emission; the elevation of the tail surfaces to clear the exhaust jets. For jet powered aircraft operating out of restricted fields such as carrier decks, for example, another design change is indicated.

Ordinary landing approaches are made with the engine throttled back. With the jet engine, if the landing is balked and throttle opened up again, about five seconds elapse to attain full throttle from idling rpm, which would be extremely hazardous. A special device is used for these cases known as a thrust spoiler. This is a pair of swing doors, which, when fully opened, leave the jet unrestricted, but when closed, deflect the thrust in such a manner that thrust is negative. Intermediate positions of the doors are available for variable thrust at constant rpm. The doors can be swung open from closed position in a split second and the maximum braking thrust obtained so far is 12 per cent of the full unimpeded jet.

Another change allowed by jet engines is shortening of the landing gear. Still another is the possibility of fully absorbing the engines within the wings. This is already being done in many designs.

Nearly all of these goals have already been achieved in several versions of British jet aircraft and special missiles. Their first jet plane, the Gloster, had the engine mounted in the fuselage with a large air intake opening in the nose, similar to the early Italian Caproni-Campini of 1938. The British subsequently dropped the single-engine policy and went to twin-jet designs. On three versions of twin-engined Glosters, one had two Metropolitan-Vickers F.2 jet units in underslung nacelles on the wings. The other two had midwing nacelle installations with either Rover W.2B or de Havilland Halford H.1 engines.

Great Britain culminated her military jet activities in the Vampire single-engine fighter built by de Havilland. The plane has a bat-like appearance with the two air intake ducts flowing out from the fuselage to the wings. The engine is installed in the stubby fuselage, with the exhaust jet nozzle not more than a couple of feet from the trailing edge wing root. A twin-boomed ship of the flying wing type, it is powered by the Goblin engine. Weighing only 1,500 pounds with jet pipe and all accessories, the engine develops about 3,000 pounds static thrust. At a speed of 540 miles per hour, a conventional reciprocating engine would have to develop 5,000 brake horsepower to equal the thrust of the Goblin jet. From an operational standpoint, these jet engines have a tremendous advantage over piston types. Since they are designed to withstand high thermal temperatures, and have only a few bearings, they can be started and run up to full speed within a minute or two.

Controls are much simpler, with a single lever replacing the conventional throttle, boost and propeller controls. There are a total of six engine instruments, oil pressure and temperature, turbine rpm, burner pressure, jet-pipe temperature and rear-bearing temperature. There are, of course, the various fuel content gauges, blind flying equipment and accessory switches and buttons. Chief feature of jet engines is their high combustion temperatures. Consequently, a number of thermocouple indicators and gauges are necessary to watch the action at all times.

Nothing has been done, or revealed by Britain, on jet-powered bombers or commercial planes. The Germans had a jet bomber with four engines and were contemplating even more extensive use of jet units. It seems that until the National Physical Laboratory, British counterpart of our National Advisory Committee for Aeronautics, has had time to study the aerodynamic problems involved in flying large ships with jet engines under commercial conditions, the military philosophy will predominate.

While a Royal Aeronautical Society lecture predicted the jet engine as long ago as 1886, the operating difficulties or advantages of jet propulsion have only recently been explored. For instance, there is no slipstream past the elevators at low speeds, so that ground handling becomes somewhat sluggish. For this reason, tricycle landing gear is mandatory for jet aircraft. A divergence from conventional design is necessitated by the jet exhaust which decrees that tail surfaces of the aircraft be kept away from the hot blast.

Aircraft designers have fewer headaches with British jets than with piston engines. They can position the jet units almost at will in the structure. Streamlining, essential as ever, becomes easier. There are no engine oil coolers necessary, no complicated superchargers or other equipment to be carried. Placement of cockpits and cabins, passenger and freight space, will no longer be dependent upon the engine setup.

Great Britain has a problem in her commercial lifelines to various parts of the Empire. There will be both short and long flight routes. Whether she will use all jet ships, gas turbine-driven propeller craft or other combinations is a question which aeronautical engineers and global economic conditions will decide in the next few years.

A question that is current in the minds of many aeronautical designers and aircraft operators is whether there is a case for the very large airplane. If the gas turbine signifies high speed in the future, meaning a journey of only a few hours across the Atlantic, for example, there will be no need for sleeper accommodations or meals, and a smaller plane, flying on more frequent schedules, will suffice. This may be compared to bus service. Buses are not built to carry hundreds of passengers for obvious reasons. Large aircraft would require special fields and facilities not otherwise necessary.

This article was originally published in the April, 1946, issue of Air News with Air Tech magazine, vol 10, no 4, pp 69-71.
The original article includes 14 photos of jet engines and jet-propelled airplanes.
Photos credited to Institute of Aeronautical Sciences, Charles Brown, International, deHavilland, British Air Ministry.

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