Following the first encroachments on the sonic barrier, the exploits of aircraft such as the Douglas Skyrocket strained the public imagination to the breaking point. If two thousand miles per hour were possible this year, would not four thousand be reached next, and so on ad infinitum? The citizen’s mind is now filled with dreams of automatically controlled nuclear aircraft, not to mention jet-borne vacations—New York to Paris in an hour or less.
These projections are understandable. Speed is apparently becoming the foremost criterion of air efficiency, and the “leading edge” of aircraft design depicts craft shaped like shells with skins of glass or cermet construction underlaid with elaborate cooling systems to push them further through the thermal barrier.
Naval aviation in the United States has made commendable strides since the close of World War II. For a time, the Navy made the quest for the world’s speed record seem like a private tussle with the U. S. Air Force. Developments in seadrome-based fighters, vertical take-off aircraft, and heavier and faster planes to take off from and land on carriers have caught up the civilian and the military man alike and carried them in the wake of bigger, faster, and more devastating aircraft.
Silently accompanying the display of speed engineering now in progress, however, lies the cold military truth that equipment designed for a specific task must accomplish that mission superbly, with only secondary consideration for secondary purposes. To hunt and kill a submarine maneuvering at 12 knots, one would not choose an aircraft cruising at 1,200 knots. The aircraft designed for speed is not at its best in slow flight. The logical choice is a craft which can emulate the submarine and pinpoint its movements. Hence, it is reasonable to assume that the patrol bombers of the future will have much in common with the lumbering PBY. The goal of flying faster, higher, and farther may well be left to the fighter and high-altitude bomber. Patrol craft, observation planes, low-level bombers, and transports are dependent on maneuverability, endurance, pay load, long range, and economy of operation— the very features which are so often shuffled to the background today.
The propeller-driven plane, which presents so many difficulties in adaptation to flight beyond the sonic barrier, has proved the ideal power plant for patrol, reconnaissance, and transport work in the past, and will continue to hold sway in these fields for a number of years. Even in the race for speed, the propeller is still present. Turbo-prop combinations have met with great success and may prove the ideal balance between speed and economy.
At the mention of propeller driven aircraft, an air-cooled radial gasoline engine comes to mind, the engine which has pushed or pulled naval aviation through the air for the past thirty years. The gasoline engine is a remarkable device, but no more so than the compression-ignition, or diesel, engine which few people associate with aircraft. Since the end of World War II, the development of the diesel engine for aircraft use has taken such strides that the United States can no longer overlook the fertile possibilities of diesel aviation.
To make any comparison between the compression- and spark-ignition engines, one should begin with the inherent advantages of each type to the desired application, and not with the progress of gasoline-powered flight and the lack of progress in aircraft diesels. Because it must withstand compression and combustion pressures from 800 to 3,000 pounds per spare inch, the diesel is a heavier engine than the gasoline power plant. 05- setting this factor is the greater thermal efficiency possible with the diesel—38% for the gasoline engine and 45% with diesel power. Diesel fuel is cheaper, and the engine is more adaptable to di5erent grades and types of fuel, but its complex fuel injection and compressor systems are noted for poor performance when the engine is subjected to varying altitudes. Once started, the diesel shows greater endurance and is less subject to accidental fires, but it is inherently hard to start. A compression-ignition engine will continue to operate after much greater structural damage but cannot match the spark- ignition engine in the high crankshaft speeds necessary for very rapid acceleration.
It was previously mentioned that there were four or five types of military aircraft now dependent on the propeller for motive force. Of these, the patrol bomber and the reconnaissance aircraft are what may be termed long-range propeller fields. It is doubtful that any other power plant will replace the internal combustion engine in these craft as long as warfare remains within the present bounds of our imagination. Low altitude bombers and freight and personnel transport planes may be termed short-range propeller fields. A drastic advance in jet, rocket, or nuclear propulsion may make propeller craft of these types obsolescent, but this is not likely in the immediate future.
It would be worthwhile to examine the primary missions of the types mentioned in order to understand the advantages which would be presented if aircraft diesel engines were made practical for these aircraft. The patrol bomber must successfully execute search, long range, convoy, and scouting patrols, antisubmarine warfare, bombing and torpedoing, transport, and rescue work. The keynote in patrol plane operation is range, endurance, payload, and durability. Scout, observation, and associated aircraft must accomplish missions of scouting and search, antisubmarine patrol, observation of gunfire, attack on light forces, and occasional photographic missions. These craft must depend on variable range, maneuverability, adapt-ability to weather and fuel conditions, and smooth operation over a wide speed range. Transport aircraft must render full tactical and logistic support, whether paratroops or peanuts are needed. Safety and endurance are primary consideration; economy of operation and payload are also of prime importance. Low-altitude bombers differ little from World War II planes of this type. Their primary requisites are still moderate speed, good range, and ability to absorb punishment without failure. With the present plans for carrier complements of these aircraft, ease of control and maneuverability should be given adequate design consideration.
In both the long- and short-range propeller fields, it is evident that the aircraft diesel is an ideal power plant if four major difficulties could be surmounted:
1. High specific weight, the ratio of engine weight to engine brake horsepower.
2. Unreliability of fuel injection and compressor systems.
3. Difficulty in starting, and length of warm-up period.
4. Difficulty of sufficient and uniform engine cooling.
These hurdles are complicated by the fact that the two-cycle diesel is much better adapted to aircraft than the bulkier four-cycle engine. Two-cycle operation adds to fuel injection and cooling problems since the number of power strokes and number of injections per cylinder are doubled at equal crankshaft speed. The solution to these problems is nearer than it has ever been. It is now so near that the United States and the Navy in particular will find themselves at a disadvantage if the promise of aircraft diesel power is not diligently pursued.
The use of diesel power plants for aircraft is not a recent innovation. The diesel’s promise of higher thermal efficiency and remarkable durability under conditions of marginal maintenance set off sporadic flurries among aircraft designers as early as 1913. In Germany Dr. Hugo Junkers developed a two cycle, four cylinder, opposed piston engine designated the MO-3. It was a successful aircraft engine in all respects except that it was never flown in a successful aircraft. The MO-3 was a remarkable power unit, developing 350 HP at a time when the most powerful gasoline engines were producing 300-325 HP. Interest in the early aircraft diesels had an economic, rather than an engineering impetus. The best gasoline engines of the day had a thermal efficiency of 25%, while the diesel could reach 40%. The cost of gasoline was more than three times that of diesel fuel. Today diesel fuel is still only half the price of gasoline, but the efficiency figures have moved much closer together. This trend was noticeable in the early thirties, when aviation writers were inclined to dismiss the earlier attempts at practical aircraft diesels with a wave of the pen, opining that the high specific weight and complexity of the diesel fuel system effectively barred it from aircraft application. Junkers followed the MO-3 with an in-line five cylinder, liquid cooled, two cycle engine in 1935, and from then until World War II led the European air diesel field by periodically refining and enlarging this basic engine. The most powerful of the series was the Jumo 206, which developed 1,200 HP at 3,000 RPM—the fastest crankshaft speed ever attained in an aircraft diesel. This plant had a specific weight of 1.4 lbs./HP which was comparable to good gasoline engines of the day, and certainly an accomplishment for a water-cooled diesel. Junkers workhorse, the Jumo 205, flew as many air miles as many popular American gasoline engines from 1935 to 1943. This model was a six cylinder, in-line plant taking off at 700 HP with a specific weight of 1.72. In its nine year active life, it served as standard equipment on six types of multi-engined transport planes, four flying boats, two heavy bombers, and a dive bomber.
The diesel development program in Germany was divided between heavy transport craft and lighter-than-air ships. Daimler- Benz produced a six-cylinder, in-line engine for the largest of Germany’s dirigibles. Designated the DB-602, the engine had a specific weight of 3.3 lbs./HP. Four of these bulky plants powered the Hindenburg, and four more drove the Graf Zeppelin.
While Germany was concentrating on inline engines, Great Britain and the United States turned their interest to radial development. The American Packard diesels of 1928-31 seemed destined for a brilliant future, but were abandoned by the company after the death of the designer and chief proponent, Capt. Lionel Woolson. In May of 1931 Walter E. Lees and Frederick A. Brossy flew a diesel-powered Bellanca “Pacemaker” 84 hours and 32 minutes without refueling. The engine was a Packard, and the mark stands as an endurance record for both gasoline- and diesel-powered aircraft. Although they were working with a four cycle engine, Packard engineers did the first credit- .able job of air-cooling the aircraft diesel. The nine cylinder radial engine was reputedly free of cylinder head hot-spots which had plagued earlier air-cooled efforts. In fact, the Packard engine ran considerably cooler than many radial diesels developed after it. The engine was evidently highly regarded at the time of its designer’s demise, for the company received the Collier Trophy in 1931 for its development.
No sooner had the Packard engine left the scene than a smaller unit of similar design, the Guiberson A-980, appeared to take its place. This 185 HP, nine cylinder, radial diesel was the prototype for a series of Guiberson models producing up to 310 HP. This has been America’s most versatile aircraft diesel, holding a Navy test contract for some time and appearing as a replacement engine in light Stinson and Waco private aircraft.
The Guiberson was the first medium- powered engine to operate successfully with air cooling, but its most important contribution was a dependable fuel injection system. Even the smooth-running Jumos had had some difficulty with this factor, but the Guiberson variable-stroke pump brought a flurry of refinements to other diesel fuel injection systems.
One of the most significant pre-war trials emerged from Great Britain. The Bristol Aeroplane Co. began work on an aircraft diesel, designated the Phoenix, in 1937. The design of this engine was borrowed from Bristol’s contemporary gasoline model, the Jupiter VIII-F, and the sea level performance of the two models was almost identical. These engines were tested simultaneously in Westland observation aircraft; the diesel showed 50% less relative power loss at 20,000 ft., 15% greater rate of climb, and 35% reduction in fuel consumption. The Phoenix was an unsupercharged four cycle radial engine. It uncovered one important fact concerning aircraft diesels. The inherent low RPM of the engine could produce a maximum speed of only 110 knots at sea level in the Westland airplane, compared to 130 knots for the unsupercharged Jupiter. At 15,000 feet, however, the maximum speed of the diesel craft was 120 knots while the gasoline engine had fallen off to 105. At that time, diesel superchargers were as efficient as gasoline blowers. In the turbosuperchargers of today, the diesel would hold a slight advantage because of the lower exhaust gas temperatures. The superiority of the diesel as a medium-altitude engine was proved in flight by these tests, and has since been corroborated in several instances. Despite the favorable indications, Bristol was reluctant to press the issue, and their diesel development program was of a secondary nature in the years before World War II.
England equipped two major dirigibles, the R-100 and the R-101, with diesel power in the early thirties. The engines were inline, steam-cooled, four cycle power plants built by William Beardmore and Co. Although they performed very well in flight, their high specific weight (7.8 Ib./HP) prevented their being adapted for use in lighter- than-air craft other than the heaviest dirigibles, or in any heavier-than-air craft. Napier and Sons contributed another British prewar entry, actually a tribute to the Junkers diesels. The Napier “Culverin” and “Cutlass” were licensed from Junkers in 1935, but never reached full production after several disappointing tests. Attempts to make direct diesel conversions of several Rolls-Royce “Condor” gasoline units ended on a rather dismal note when these engines developed only half the power of the original design.
France showed a particular interest in both two and four cycle radial diesels, producing eleven domestic models from 1930 to 1938, but none of these were ever thoroughly test flown. Despite their lack of practical accomplishment, the French did reduce the problems of cooling the radial two cycle engine. The Salmson SH-18, built under Czech license, was a combination air- and water-cooled eighteen cylinder affair, producing 600 HP at 1,700 RPM with a specific weight of 1.9 lb./HP. Incorporation of air- cooling was made possible by a unique auxiliary blower and loop scavenging system. This principle was furthered in a fully air-cooled two cycle engine built in 1937 by Botali et Cie. The fact that the engine never left the test stand was due largely to injection difficulties, and not to lack of proper cooling.
In the years preceding World War II, Germany stood alone in making diesel- powered flight economically feasible on a large scale. A long series of Junkers airliners employed the Jumo engines from 1928 through the war years. Germany’s most successful heavy bombers were adapted from the JU-86 airliner and were diesel-powered. Blohm and Voss and Dornier flying boats, both commercial and military, used the Jumo 205. Junkers and Focke-Wulfe dive bombers spearheaded the early blitzkreigs on dieselized wings.
The advent of jet and rocket propulsion has understandably lulled aviation diesel development in the United States since the war, but the inherent advantages of compression ignition have been hotly pursued abroad. In West Germany, MAN has developed a basic engine capable of adaptation to truck, boat, or light aircraft use. The smaller model produces approximately 150 HP at only 1.5 lb./HP, consumes only 0.30 lb./HP-hour of straight aviation diesel oil, and will operate on almost any modern fuel, including 80-octane gasoline and aviation kerosene. The basic MAN type is an aircooled two cycle engine and promises to be the first wholly successful model of its kind. Cooling is effected by a turbocooler blower operated in tandem with the tubo-super-charger. The efficiency of the unit is remarkable; MAN seems to have no cooling problems in the engine. Engine temperature is also controlled by a radically new combustion progression in the cylinder. The combustion chamber is semi-spherical, and fuel injection is a two-stage process. The initial spray merely wets the walls of the chamber, followed by the ignition spray in the form of the conventional “cloud” of oil filling the chamber. The system produces a controlled combustion rate, but does not utilize an auxiliary injection system. Instead, the “wall-wetting” fuel is introduced as a cloud and thrown onto the cylinder walls by swirling air from the scavenging cycle. Near the end of the injection period, scavenging ceases and the resulting turbulence of gases in the chamber maintain the cloud of fuel. There is no evidence that larger models of this engine, adapted for aircraft, should not produce over 2,000 HP at reduced specific weight and specific fuel consumption. The advanced design of the scavenging system brings the two cycle diesel into direct competition in efficiency with gasoline engines.
The MAN engine is only a prototype. It was not originally designed for aircraft use and has not been thoroughly tested for this purpose. It will probably not find its greatest employment as an aircraft engine, but it does bring out two significant points. Of greatest importance, it now appears feasible to adapt air-cooling to the two cycle diesel and still retain thermal efficiency. Secondarily, the barrier of slow and unreliable starting and long warm-up has been passed by use of an improved flame starter and an enlarged blower capacity. The developments of MAN in Germany are still open to speculation, but speculation of a very positive nature.
The German developments are supplemented by more tangible evidence of success in England. Napier, the firm which undertook the production of Junkers engines before the war, has produced a twelve cylinder, two cycle, liquid cooled diesel engine compounded with a gas turbine and designated the “Nomad.” In the general field of compound engines, the diesel is basically more satisfactory than the gasoline engine for the following reasons:
1. Cylinder pressures in the diesel can be controlled by the rate of fuel input.
2. Exhaust back pressure becomes a help rather than a hindrance when high degrees of supercharging are employed.
3. In the absence of detonation and preignition troubles the compression ration and degree of supercharging may be varied to suit design requirements.
4. The excess air over that used for combustion which passes through the engine may be used for cooling. The only limit to the amount of excess air which can be introduced is the critical size and weight of the compressor which moves the air.
The Nomad is unique in that it is the first aircraft diesel to be so compounded, but this is not the major point of interest. Although the Nomad at 1.03 lb./HP is 25% heavier than gasoline engines of the same power, the fuel consumption of this diesel is 40% lower. Exhaustive comparative tests for endurance and reliability have shown the Nomad to be the equal of the most modern gasoline compounds in such former diesel downfalls as cylinder head hot-spots, fuel injection failure, and compressor difficulties from rapid changes in altitude. The engine’s statistics are a refreshing improvement in aircraft diesel development. It produces 4,000 brake horsepower at 2,050 RPM, with a compression ratio of 27:1. The specific weight of the complete engine is 1.1 lb./HP. The gas turbine drives the blower and also augments engine power through an infinitely variable gear through a quill shaft to the crankshaft. At full power settings, the turbine produces 2,250 HP, of which 1,840 are absorbed by the compressor. Through the turbine coupling, the diesel may drive the compressor when maximum cruising economy is desired. The gas turbine will run on any fuel compatible to the diesel with the exception of gasoline over 100-octane, which produces exhaust gases too hot for proper turbine operation. Napier has successfully introduced water injection into the two cycle diesel, and this system gives the Nomad an additional 400 HP for emergency use.
It should be emphasized that the Napier Nomad, a liquid-cooled engine, is designed primarily for use on air freighters. It would not be readily adaptable for use on Navy patrol craft, although it shows definite possibilities in the field of military transport. Most significant is the added reliability shown by the Nomad at a definite reduction in specific weight.
One may begin to draw together the threads of diesel aircraft development into a picture prophetic for naval aviation. The high-powered, air-cooled, two cycle diesel is no longer the subject of ridicule, and is much closer to realization than our more spectacular goals of Mars or Venus. The medium-powered two cycle plant is a reality; its natural advantages to observation and scout craft are undeniable. These diesels can reduce fuel consumption 30% or increase range the same amount. Endurance is at a peak, elaborate weather guards are unnecessary, engine operation is smoother, and climatic conditions have little effect on fuel or engine.
Patrol aircraft and low-altitude bombers could benefit greatly by the perfection of the high-powered two cycle diesel. Increased range and payload are the primary considerations, but also of importance are reduction of fire and explosion hazards, greater safe periods between engine check and overhaul, and continued operation of the engine after one or more cylinders have failed. In the transport of supplies and personnel aircraft diesels offer drastic reduction in fuel costs and improved safety on long flights, in addition to advantages previously mentioned.
One cannot assume that the aircraft diesel for naval use has fallen into our laps. This is definitely a long range development program. There are still major cooling problems in high-powered two cycle engines. The reliability of high capacity, multi-stage blowers leaves something to be desired. A good aircraft diesel, alone or compounded, offers too much to be lightly dismissed, however. There is clear evidence that the aircraft diesel can strengthen a great part of naval aviation, and thereby strengthen the whole. The challenge is fully as real as that of greater speed and is worthy of due consideration.