On August 27, 1940, when the world was watching the rising fury of the air battle for Britain, Col Mario de Bernardi, the famous Schneider Trophy pilot, made an epic flight over Taliedo aerodrome in Italy. This flight lasted only 10 minutes but the ship was a radical new design that may revolutionize high speed aircraft. The job that de Bernardi piloted that day was the jet-propelled Caproni-Campini CC-1.
Not much is known about the CC-1 except that it was an 8,000 pound single-seater that took off by means of a conventional propeller, then switched to jet propulsion. Moreover, it was said to feature a pressure cockpit. Observers tended to laugh off the "flying squirt" as the British so aptly christened it as just another of those crackpot Italian ideas. But it begins to look as if the Italians really have something. On December 1, 1941, de Bernardi flew the CC-2, a two-place jet ship, between Taliedo and Guidonia (the Italian equivalent of our NACA), covering the 168 miles at an average speed of 130 mph. For good measure de Bernardi toted along a passenger and a mail bag, according to reports. The really amazing part, though, was that the CC-2 is propellerless! This 11,000-pound low-wing monoplane apparently can take off without a propeller. In six months the Italians had accomplished a trick that so far as frustrated the best brains in aviation. Our own NACA reported in 1923 that jet takeoffs would require a jet heated to 2,500° Fahrenheit moving at a speed of mile per second. Picture that around he local airport!
In the light of recent German developments along these same lines, the Caproni-Campini is getting the serious attention oi warplane designers of every nation. Junkers is working on a jet- propelled airplane of which more later and, like as not, persistent reports of the German use of rocket power for assisting takeoffs of heavily loaded ships or for temporarily boosting fighter speeds in an emergency may actually refer to jet propulsion, since considerable heat is added to the air jet. The general appearance and sound of the jet ships could cause the uninitiated to confuse them with rocket ships. Ernst Udet, who was reported killed while testing a secret "weapon," is supposed to have perished flying a rocket (or jet) airplane. In de Bernardi's own words, the jet craft are out of the dream class and are now a practical proposition. Clearly, the flying squirts are of military importance.
Engineers say the existing engine-prop setup is approaching its maximum speeds; some entirely new means of propulsion must be developed in the near future. As speeds increase jet propulsion compares more and more favorably with propellers which lose efficiency progressively, until at about 800 mph the efficiency of jets actually surpasses that of propellers. Even on current 400 mph fighters the insignificant jets of the exhaust stacks add another six miles or so to the maximum speed. Supersonic speedsters are a possibility with jets. Here it is interesting to recall Italian interest in supersonic wind tunnels at Guidonia. But while supersonic airplanes may be just around the corner, Junkers is said to be interested in speeds just under the speed of sound. In Junkers opinion the Italians are going off the deep end in a strange new field where speeds are so great that the basic data is lacking for the streamlining or proper shaping of aircraft. The only applicable information is that of ballistics or the flight of projectiles.
In theory jet propulsion is simple. It is the oldest and simplest method of converting heat into mechanical energy. The principle of the legendary aerophile of Hero's was a hollow sphere rotated on an axle by steam jets set tangent to the circumference of the sphere. All jet-propelled schemes now under consideration involve compressing air, heating it, then expanding it rapidly and releasing it rearward through a jet opening or orifice.
On the Caproni-Campini jobs the air is taken in through a large circular opening n the nose, run through a motor-driven compressor (which heats it automatically), mixed with the hot exhaust gases for further rapid expansion, then expelled through a moveable tail jet that works as both a throttle and an elevator control. Contemporary experimenters have mixed the fuel with the compressed air as it rushed through the jet tunnel and burned the mixture for increased thrust. Powdered coal, paraffin, gasoline and many other fuels have been used successfully. The normal air jet alone would not develop sufficient thrust and therefore the jet must be augmented by heating and compressing the air for further rapid expansion and by careful design and shaping of the jet tunnel.
In its working principle jet propulsion is similar to rocket propulsion. The difference is that the jet ship takes its combustible oxygen from the surrounding air and the rocket ship carries its oxygen (for that matter the entire weight of all materials jetted away for thrust) in its fuel. Thus, while the flying squirts are limited necessarily to the stratosphere at best, the rocket ship is independent of altitude or air. The rocket principle works in a vacuum and, in the vacuum of space, rocket ships would accelerate indefinitely. For our purposes the fact that the jet principle actually has propelled aircraft is sufficient proof of the possibilities of rocket power, though for rocket ships a fuel is needed that will burn for sufficient length of time while developing the required thrust. The jet jobs are truly the stepping stone to the rocket planes of the future.
What makes a jet-propelled ship move? Probably the simplest explanation is to consider a water-filled cube, measuring, say, a foot square. Physics tell us that if a certain pressure, ten pounds for instance, is exerted on any square inch of the surface of that cube a similar pressure will be exerted from within on every square inch of the surface of the cube the principle of the hydraulic press. Now suppose a one inch square hole is cut out of that surface. Then, on the opposite side of the cube, the square inch directly across from this hole would bear an unbalanced pressure of 10 pounds tending to move the cube in the direction of the pressure. Inflate a toy balloon and release it without first having fastened the neck and you'll have a parlor demonstration of jet propulsion. Radiator tunnels on liquid-cooled engines can function like jets and can cause a slight additional thrust. The speed of the airplane rams the air under pressure through the radiator where it is heated and expanded rapidly in passing all the elements of jet propulsion!
Although the Italians use a motor-driven compressor and Junkers is using a multi-bank two-stroke radial in their experiments, the ideal would be a rotary compressor and a gas turbine on a common shaft. Ask an engineer how he'd design an engine for jet propulsion only and you'll get a novel line of thought. First, he'd dispense with the crankshaft, since there is no propeller. Without a crankshaft there would be no need for a piston. So, why not eliminate the exhaust valve and make firing continuous so that a steady jet of burned and expanding gases would be blown out the open cylinder bottom? All that remains after this is to alter the shape of the cylinder as a combustion chamber and narrow the bottom opening into a properly shaped jet for efficiency. In the case of the rocket motor in which the fuel would be carried in the cylinder or combustion chamber, even the intake valve would be eliminated. Jet development actually follows this line of thought. In 1908 Lorin, in France, placed a conventional six-cylinder engine flat on its side and installed six long tapering tubes in the cylinder heads so that the explosion within the cylinders blasted right out through the open tubes to create a crude form of jet. Current jet ideas have bee evolved as outlined above and conceive batteries of single combustion chambers arranged in circles to fire through on jet tube.
European engineers picture the idea jet-propelled ship as having a pressurized egg-shaped cabin nose section for the crew. This "egg" would fit into the open end of the fuselage proper so that a circular opening would ring the fuselage at the junction of the front and rear sections. The high-pressure boundary layer of air would rush through this opening to the compressor amidships. This compressor, incidentally, could be made to heat and pressurize the cabin. Jet adherents say that ultimately such power plants will prove relatively cheap, small and of a convenient shape for small fuselage cross-section area and that a more powerful thrust for weight ratio will result in speeds beyond present engines. Lack of a propeller obviates a long landing gear for prop clearance.
The schemes discussed thus far obviously involve using most of the internal fuselage space for the compressor-power-plant and the jet tunnel. However, some designers are looking beyond this. N Schurter of Zurich, Switzerland, envisions a conduit along the leading edge of the wing with rearward pointed jets from the semi-chord point on the bottom of the wing. Schurter's design would serve as a deicer since the air within the conduit would be at an elevated temperature. F Whittle, in England, sees blower-type rotary compressors, the rotor bearing and the compressor being liquid cooled. Other designers would install the motor-driven compressors in nacelles or within wings in orthodox multi-engined arrangements.
The US Army Air Corps became interested in jet propulsion as long back as 1923 and even before that in 1909 that incredible inventor, Lake, had worked with the jet idea. In 1923 the US Bureau of Standards, on behalf of the Engineering Division of the Air Service, submitted to the NACA the findings of one Edgar Buckingham who had been doing research on the subject of jet propulsion. Buckingham found jets inferior to propellers (jets and propellers are only different ways of obtaining a propulsive reaction from an accelerated stream of air) because the jet combined small mass with high velocity, whereas the ideal is large mass at a lower velocity. Since the required kinetic energy input increases as the square as velocity goes up it is apparent that low velocities require less energy input in proportion. Jets are most efficient when their speed jibes with that of the ship itself. When Buckingham made his tests the highest speeds then conceivable were in the neighborhood of 250 mph and it was on that basis that he made his computations. At that speed he figured the flying squirt would require four times as much fuel per thrust horsepower as the conventional engine and airscrew.
Though Buckingham knew that the picture would improve steadily as speeds climbed toward the speed of sound fuel consumption and the weight of the power plant per thrust horsepower would be less at such speeds he could not have been expected to know that less than 20 years later, designers would be talking in matter-of-fact tones of supersonic airplanes. With technical developments of his day as a guide it was Buckingham who stated that a jet temperature of 2,500° and a jet speed of a mile per second would be required for takeoff. By stepping up the volume of air expelled the Italians have cut those temperatures and speeds enormously, at least that is indicated by jet takeoffs.
Yet other countries persisted to perfect the jet and in 1933 the NACA reopened the question and supplied funds to the Bureau of Standards for further tests and research. This time G B Schubauer substantiated Buckingham in the main but at the same time indicated what had to be done to make the jet a workable proposition. He said that if the thrust of the jet could be augmented without requiring greater horsepower, then the jet could compete with the propeller. That ancient and long tantalizing dream of the simple let for motive power could be realized only if the jet could be further augmented to step up its thrust to practical quantities. Engineers knew generally what had to be done but the Italians with the Caproni-Campini were the first to ring the bell.
Crux of the jet propulsion problem is the necessity of reducing the disturbance between the rapidly moving expelled jet and the surrounding air. The resulting friction tends to hold down the speed of the jet, building up back pressure in the expansion tunnel just as a muffler creates back pressure in conventional engines. Schubauer calculated that at 10 mph only eight per cent of the kinetic energy of the jet was being converted into thrust.
Most effective recent developments pass the jet of air through a Venturi tube or a series of Venturis arranged in such a way that the mouth of each Venturi fits over the open rear end of the Venturi in front By making each Venturi larger than the one before the effect is the same as having one tapering tube with opening running ringlike around the tube at regular stations only the Venturi shape impart added efficiency. In these setups the air outside of the Venturis is sucked through the openings and is entrained with the air jet to increase its mass Added mass means added efficiency. An other interesting factor is that the jet can be made to improve the aerodynamic effect.
In a sense the fuel for the jet ship is really the air itself. It's a fuel available anywhere and it doesn't raise the wing loading a bit. The small amount of the usual fuel necessary to drive the compressor engine or engines means terrific range once the jet jobs are perfected. Intercontinental raids by swift engineless bombers engineless as we now know them may become fact instead of fancy.
Writing in the authoritative English aero journal, Flight, G Geoffrey Smith summarized advantages of the jet-propelled ships. To cite the more important ones, in effect: fuel might be paraffin, powdered coal, diesel oil, tar oils, gasoline or almost any other fuel giving high heat; direct application of power without transmission or conversion. The compressor system can be made to heat and pressurize the cabin; rotary engines would be smaller and lighter than reciprocating engines and would have a superior power-to-weight ratio; planes would be of "lower build" since propeller clearance would be unnecessary. They would have better form drag, nose free for pilot and armament, improved maneuverability because the jet orifice can be swiveled for powerful control effect.
All these features would give freer rein to designing those dream ships of the future. Visitors to the Paris air show in 1938 saw an interesting model depicting the French conception of a jet-propelled airplane of the near future. The project was for a midwing monoplane of 14,000 hp, having a speed of 1,000 kilometers an hour and a ceiling of 30 kilometers. No doubt the French were optimistic and impractical but they were only being cognizant of current research in the jet field.
This article was originally published in the November, 1942, issue of Flying including Industrial Aviation magazine, vol 31, no 5, pp 49-50, 94, 99.
The original article includes 3 photos
Photos are not credited.