Production of all weapons of war is governed by necessity. The airplane, because it can be produced with comparative speed as compared with the battleship and other heavy weapons, is a typical example of a weapon custom-made to meet circumstances, adverse or otherwise.
Of all American airplane types the heavy bomber and its cousin, the transport plane, have made the swiftest progress of design and performance. Britain has had to carry on with modifications of her 1936 bombers, Germany has had to switch her production to fighter planes to ward off the attacks of Allied bombers. The United States has developed her Fortress and Liberator to the limit of their capacity , and at the same time has developed the super-bomber which would seem to fulfill the dream of those who have supported the theory that the bomber should be an aerial battleship. The idea is not new, of course, when you remember that it prompted the original design of the Fortress in 1935.
General Arnold, the Commanding General of the Army Air Forces, who has been a consistent advocate of bombardment aviation , has on several occasions prognosticated the future of American bombardment with the promise of super-bombers that will make the Liberator and the Fortress seem comparatively small. The General , to whom must go a great part of the credit for building the world's largest air force and stimulating the production of material to equip it, reported as far back as the summer of 1942 that the U.S.A.A.F. were steadily improving the standard models of fighters and bombers, and that new fighters and bombers were on the way with tremendously increased speed, fire power, bomb loads, range and maneuverability, indicating that the latest American aero engines , both air and liquid cooled, had been vastly stepped up in horsepower. General Arnold, who as many know was one of the first three army officers to learn to fly, has never been given to boasting; neither can it be said that his Army Air Forces have ever been guilty of the misuse of air power. The sole mistake that can be laid at the door of the United States was a lack of numbers of fighters in the early stages of the Pacific combat, a fault to which democratic nations who think in terms of peace rather than war are always prone.
In his report to the Secretary of State for War, the General wrote that the No. 1 job of an air force is bombardment, and that the United States must have long-range bombers capable of hitting the enemy before he hit us, that planes had to be built to function under all climatic conditions, from the North Pole to the South Pole, that the United States Army Air Forces believed in daylight operations and the strategic precision bombing of key targets deep in the enemy's territory, adding that from 1935 to 1938, despite limited budgets, the Army Air Forces developed detailed plans for a fighting air arm. General Arnold's report is one of the most important documentations on air power made public during this war, and deserves far more attention than was given to it in the daily press. It is of particular interest because the General not only takes the public into his confidence in revealing how the United States has used its air power, but gives a large share of the credit for the building of the world's largest air force to Americans. This is how he puts it: "In the most direct and personal sense, the building of what is now the world's largest air force involved 130,000,000 people. Our enterprise reached the lives of every American. Industries were mobilized , plants were constructed or converted with dazzling speed, millions of our citizens pulled up stakes and pitched into work at hand with the spirit, resourcefulness, and enthusiasm of our country 's pioneers. The high school graduate put his diploma in a bureau drawer, and enlisted as an aviation cadet. The housewife became a welder, three thousand miles from home. The farmer and his family
walked to church on Sunday to save half a gallon of gas for use over embattled Henderson Field. The skill, devotion, and hope of our people were focused on Army Air Forces." In those few words the General gives a picture of the work behind the scenes in building bombers, of men and women learning new crafts, of piano factories making bomber parts, of girdle manufacturers making parachutes, of pickle plants turning out airplane skis and floats.
The results of this labor are seen in the American airplanes gaining control of the air on every front on which they appear, in welltrained air crews, in constantly populated supply lines, in the mass of spares and the tons of gasoline brought across land or ocean to the fighting front.
The U.S. super-bomber, the B-29, is a result of the Army Air Forces' policy to keep ahead of the enemy by building weapons to match its policy of aggressiveness. The B-29, which is a superFortress , is the United States aircraft industry's super achievement in bomber design and production. This massive giant, heavily armored and armed, has a span of 141 feet, a length of 99 feet and a wing area of 1739 square feet. It is powered by four specially modified Wright Cyclone Duplex engines of 2000 hp. each driving a three-bladed air screw, has a cruising speed of 250 miles per hour at 2 5,000 feet, and can carry a bomb load of 17,500 pounds on a 1000-mile trip, or 6000 pounds on a 3000-mile sortie. Details of its armament are secret. Into it will have been built the experience gained from combats on all fronts. Certainly it will have chin turrets , or forward-firing guns, increased dorsal and ventral fire power, and some models will possibly carry heavy cannon similar to that installed on the B-25. The weight of the new monster, said to be between 100,000 and 120,000 pounds (twice that of the British Lancaster), is carried on the ground on a tricycle undercarriage each unit of which has twin-tired wheels.
Built for high-altitude flying, the operation of the new bomber requires special crew training, put into operation as the result of tests made on the experimental B-19. The opening of bomb bays at extreme altitude involves abrupt changes of pressure and temperature in the fuselage of the bomber, which must be compensated for to prevent discomfort to the crew and reduction of efficiency.
Whether the B-29 will be the last of the big bombers to appear in this war is a matter on which no one would dare to hazard a guess.
The chances are that it will be, basing one's assumption on the life of the Fortress which will be ten years old in 1945, and which has undergone numerous type modifications. The life of an airplane type is short in war, but just as the Fort was several years ahead of its time, so the B-29 can be expected to head the opposition by several years.
1943 saw the introduction into aviation of several new devices which may be featured as regular fitments on the bomber of the immediate future, and may even be included in the modifications made to existing types. One of the most important of these is the perfection of dual-rotating propellers by the Curtiss-Wright Corporation of America and the De Havilland Aircraft Company of England.
The dual-rotating propeller is the answer to many of the airplane designers' most harassing problems, particularly that of obtaining thrust efficiency at extreme altitude. Previously one of the great bugbears to altitude flight was persuading the engine to give full revolutions in the rarefied air which reduced the pressure on the induction pipe, and caused the engine to splutter and cough with a woeful loss of horsepower. This has been overcome by the fitting of the turbosupercharger perfected by the General Electric Company.
The turbosupercharger is a device worked by the exhaust gases of an aircraft engine, acting on an impeller which compresses the air to give it sea-level pressure, thus insuring the same combustion performance as at sea level. The turbosupercharger is the most economical way of getting results, being far lighter than the mechanical supercharger operated by gears on the engine crankshaft. It is the nearest thing to perpetual motion imaginable and actually derives its power from the exhaust gases which are the waste product of the engine. It operates automatically and cuts itself out when atmospheric pressure is equivalent to that of sea level. Planes fitted with turbosuperchargers can operate at 30,000 to 35,000 feet, with their engines giving the same power as at sea level. A nonsupercharged engine loses more than 50 per cent of its power at 25,000 feet owing to the reduction of atmospheric pressure.
The high-flying airplane has another problem, however, and that is propeller slip, which is a reduction of thrust caused by rarefied air, eddies and turbulence. The thrust of a propeller is the distance the airplane should travel according to the pitch of the blades, if there were no slip. Pitch is the angle at which the blades are set to the air.
If a propeller were turned in an element having the consistency of butter it would progress forward the exact distance through which the propeller would advance in one revolution. Slip is always present in atmosphere, and from the beginning of airplane design men have tried to overcome slip.
The Propeller Division of the Curtiss-Wright Corporation kindly provided me with their latest data on propeller performance to assist in my explanation of the dual-rotating propeller which may soon be giving American bombers that little extra plus in propulsion efficiency.
An airplane's propeller corresponds to the wheels and transmission system of your automobile. It absorbs engine power and converts it into motion. The angles (pitch) of the propeller blades are equivalent to the gears of the car. Low gear starts an automobile, low blade angle is used for airplane take-off. Medium pitch is second gear, greatest angle, high gear. The pilot, however, need not worry about changing his gear. His automatic pitch control does that for him.
On a single-engined plane he has a transmission worry though— torque. The rotation of the propeller tends to make the machine turn over laterally. The pilot corrects this with his rudder or fin tabs.
The twin-engined bomber torque is counteracted by the propellers on each engine turning in opposite directions.
Early airplane propellers had two blades, because people thought any additional blades turning in the same air path were wasted weight. Later it was proved that a three-bladed propeller is nearly one and a half times as efficient as a two-bladed one, and a fourbladed a little less than one and a third times as efficient as the three.
The little less efficiency is accounted for because one blade passes through air distributed by the blade in front of it. To fit more blades was not practical; neither could the length of the propeller blades be increased, because the diameter of the propeller is limited by the height of an airplane's landing gear and its ground clearance.
The 2000-hp. engine fitted to modern bombers and fighters demands the highest possible efficiency in propellers. To make the greatest use of this colossal power, 2000-hp. planes are fitted with three- or four-bladed propellers, but there is still slip and a marked loss of thrust at high speed.
In their quest for more thrust, efficiency, and greater speed, engineers thought of utilizing the air in front of the circle of rotation of the four blades. The principle was first demonstrated in the Italian Fiat-Macchi seaplane which topped 400 miles per hour in 1930. This tiny monoplane had two engines each driving a twobladed fixed-pitch propeller in opposite directions. The handicap of fixed pitch, however, outweighed the advantages provided by the variable-pitch three-bladed propeller assemblies then developed.
When the variable-pitch blade system had been perfected, the designers again turned their attention to dual-rotating propellers, and today on both sides of the Atlantic these propellers are ready for action.
United States planes have already been fitted with the CurtissWright six-bladed unit, which actually consists of two hollow threebladed variable-pitch propellers driven in opposite directions by one engine.
The perfection of the new propeller is a saga of American engineering endeavor and persistence. Experiments were wearisome, often heartbreaking. But from each failure much was learned. Most gratifying discovery was that the front propeller set sufficiently ahead of the other to allow for blade angle changing, but did not cause the expected excessive turbulence that might adversely affect the thrust of the rear blades.
The explanation is that the front propeller works in undisturbed air, the rear in the compressed slip stream of the other. Laboratory tests showed this slip stream follows the rotation path of the tips of the front blades and also of the angle of the blades, making pressure in two separate directions. The rear blade, however, rotates against the directions of both these pressures and therefore is turning in the compressed flow of solid air forced back by the front blades.
In early flight tests "bugs" began to manifest themselves. It was found that at take-off speed, the slip stream set up by the angle of the front blades resisted the slip stream created by the rotating propeller tips, which meant that the rear propeller blades had to be set at a sharper angle than those in front. Under such conditions the gain in thrust was so little that the weight of the extra blades seemed hardly worthwhile.
Then someone discovered that at high speeds and high altitudes, where the increase of power was most needed, the two opposing slipstream forces obligingly unite into what amounts to a solid thickly compressed layer of air. This gives the rear propeller such an efficient blade bite that it has to be set at a lesser blade angle than the front.
All the pilot has to do is to climb to the desired altitude. As he opens his throttle the efficiency of his propellers increases with his speed, instead of decreasing as with the normal type of propeller; and at all speeds and altitudes he has no engine torque. That has been canceled out by the two propellers rotating in opposite directions.
He can fly his machine hands and feet off, and throw it about in aerobatics in any direction with equal ease. Ask any of your singleengined pilot friends what this means and watch his eyes glisten.
For the trainee dual-rotating propellers mean an end to the dangerous and humiliating ground loop (when the plane tears round in a circle like a bee in a bowl); for the single-engined dive bomber they mean quicker arrival at the target and more deadly accuracy. To the single-engined torpedo plane pilot they bring an even greater advantage of ease in straight shooting and, of course, greater speed in getaway.
These propellers mean a lot to postwar aviation. By increasing engine efficiency their use will produce higher speeds on lower fuel consumption, and consequently lower fares and freight rates, both vital if we are to make the fullest possible use of air transportation.
You will hear a great deal about these dual-rotating propellers in the future. They are as revolutionary to the% modern airplane as was the three-speed gear to the bicycle, perhaps more so.
A bomber fitted with four 2000-hp. turbosupercharged engines operating dual-rotating propellers would have the most efficient means of propulsion aeronautical science can provide, and be capable of operating at all heights with the greatest economy of fuel and mechanical energy. There is, however, yet another revolutionary means of propulsion which has been brought to perfection after years of experiment.
This is jet propulsion. Jet propulsion is the giving of forward motion to the plane by means of the discharge of gas, on the same principle as the rocket. Jet propulsion is not new. In 1660 Sir Isaac Newton, the English scientist, produced a model of a vehicle using a steam jet to give it forward motion, but long before, in the dark ages, Hero, the Alexandrian philosopher, was working on the same principle, and developed a turbine with jets of steam turning a sphere. In 1910 a French inventor published drawings showing how jet propulsion could be employed on the supercharger principle, but it was many years before aeronautical engineers began to give serious thought to the possibility of using jet propulsion for aircraft. The Caproni Company of Italy was the first to make public news of successful flights, although Germany and England were known to be experimenting with jet propulsion. In 1941 a Caproni jet-propelled airplane made its first flight averaging 130 miles per hour from Milan to Rome, with one stop en route. This machine was fitted with a normal radial aero engine driving a compressor which sucked air in at the nose of the plane. The compressed air, first heated by the engine , then by mixing with the exhaust gases, is given further expansion by the injection of liquid fuel at the point of discharge in the rear of the plane.
The result of this is to produce the effect of a rocket explosion, and thus provide forward motion. Rockets said to be invented by the Chinese were first used in Europe by Sir Sidney Smith in 1806.
He bombarded the French port of Boulogne with rockets, but they seemed to be inaccurate and did little damage. The Germans used rocket force on a boat in 1930, and then began to employ them in rocket planes. The use of rockets in planes led to the employment of liquid fuel, and this encouraged experiments in jet propulsion using mechanical compressors to mix atmosphere and liquid fuel to obtain high explosive force. The National Advisory Committee for Aeronautics held an investigation into jet propulsion and reported that when perfected the system should be 80 per cent more efficient than any other power system in use. A joint announcement by the United States and British air forces, stating that a new type of jet-propelled plane based on designs perfected by Captain Frank Whittle of the R.A.F. was being manufactured by the Bell Aircraft Corporation, centered interest on the new type of propulsion. The new machine flew for the first time in May, 1941, and has since made hundreds of successful flights, many at high altitudes and extreme speeds. But warned a British official spokesman: "Dramatic as the advent of the new weapon may be, its immediate use in operation should not be expected . Plans are being made for the production of training models, but there is bound to be a considerable time lag between going into production with the new plane and getting it into service. Furthermore , the technique of flying a jet-propelled plane is quite different from that of flying a screw-propelled aircraft. Even when training models are delivered more time will be needed to train the pilots." This writer's hazard is that it will be many years before jet propulsion is featured in bombers. It has certain advantageous features for immediate use in interceptors, namely, weight saving for power ration, and excessive speed and altitude.
Rocket propulsion may be employed to launch heavily loaded bombers at take-off. The Germans already use this principle to get their D0-217 and He-177 off the ground, but the advent of the dualrotating propeller and the perfection of the 2000-hp. unit would seem sufficient. Even more powerful engines may follow the present type. The British already have a 2400-hp. liquid-cooled aero engine smaller and slimmer than anything yet produced, which may replace the Merlin for their heavies and point the way to more powerful engines for our own.
One final thought in considering the bomber as an airplane. Every progressive step in its design, every increase in its design, is of immense importance to commercial aviation and a contribution to the postwar world. While the fighter is a freak airplane bred for speed and destruction, the bomber is the unit of weight carrying.
Our present bomber types stripped of their armament will surprise the world by their utility and durability. While the fighter planes go on the scrap heaps, the bombers will carry on as work-horses of commerce, and from them will come the super-efficient transport planes of the future, to which we confidently look forward. To the bomber we also look for the preservation of world peace, by keeping the world's potential trouble spots within a matter of hours from corrective armed power, if ever occasion arises for its use.
Here is a salute to the bombers, large and small, to the men who fly them, to the men who fight in them, to the men and women who manufacture them, and keep them flying. Of them it may truthfully be said, they make a major contribution to the future world.
Of them it may well be said in the words of the prophet: "And after these things I saw another angel come down from heaven having great power . . ."