The Sky is Not the Limit

By N F Silsbee

Major, United States Army Air Forces

The interceptor-bomber struggle for altitude supremacy is carrying aerial combat ever higher in the troposphere.

Who rules the upper air will rule the world. Shortly after the Battle of Britain, when Luftwaffe medium bombers and fast bomber-fighters began coming over England at 30,000-foot levels, Sir Frederick Banting, co-discoverer of insulin, made this significant remark: "Whichever power gets up to 40,000 feet first and can stay there longest with the heaviest guns will win the war."

A few months later war in the sub-stratosphere became a reality. Three Boeing Flying Fortresses (B-17Cs) in July, 1941, dropped bombs from about 35,000 feet on the German battleships Scharnhorst and Gneisenau in French ports.

(Popularly termed sub-stratosphere, the 30,000- to 50,000-foot range is more accurately know as the troposphere. At about 35,300 feet, for some reason as yet not fully accounted for, the temperature becomes fixed at 67 degrees below zero. This is called the tropopause. The stratosphere proper begins at 50,000 feet.)

Although midsummer, the Fortresses were so high they were covered with frost. The British Air Ministry announced that the attack took place at such fantastic height that the scream of the bombs was probably the first warning. Yes, war is moving up into the higher regions. The see-saw between high-flying bombing planes, then higher-flying fighter planes to knock them out of the sky, then, in turn, new techniques to get the bombers up even higher, goes on at a furious pace.

In other words, every new offensive weapon brings an almost immediate reaction in the field of offense. This is what Major General Ira Eaker, Chief of Bomber Command in the European theater, meant when he said that every week — even every day — of delay in pressing home our smashing air offensive on the industrial and communications system of the Reich allows that much more opportunity for them to develop effective defensive measures against our fast, high-flying heavy bombers. At present the speed, ceiling, ruggedness, and especially the effectiveness of long-range high velocity .50-caliber guns of our Fortresses and Liberators give us an edge over the best fighter defense the Luftwaffe can muster. Delay will prove costly in terms of precious American lives and valuable equipment.

Bombing Varieties

Of course, it is not to be assumed that all bombing from now on is to be at these high levels. Weather conditions over continental Europe are admittedly a limiting factor and there still will be plenty of bombing in the 10,000- to 25,000-foot altitude range. Heavy night raids of the "area-bombing" variety will continue to play a part in the overall air strategy. Fast low-flying attack bomber sweeps with Douglas Boston attack bombers, deHavilland Mosquitoes and such fighter planes as the Hawker Hurricane and North American Mustang are raising hob with hundreds of small but highly strategic targets. Similar bombing, in cooperation with Allied ground forces, has been proving tremendously effective on all war fronts. In the sea-air war, high-level, low-level, dive and torpedo bombing have their place and are being used effectively.

By and large, however, the trend is up. When in early 1939 the Boeing YB-17 met its first successful tests in high altitude with the American-developed turbosupercharger, the leaders of the Luftwaffe knew they had a date with destiny somewhere around 40,000 feet. Already Germany had been working night and day on this problem in their great research establishments, but now they went all-out. Their star single-seater fighter, the Messerschmitt 109, thoroughly proved in the Spanish Civil War, greatly improved in the Me-109E, was in high-quantity production — a fast, all-around fighter, maneuverable, good rate of climb, well-armed, but not a high-altitude job. By late summer, 1941, the Me-109F began to make its appearance with rounded wing tips and generally cleaned up and streamlined. Most of all, however, its Daimler-Benz 1,150-hp liquid-cooled engine was provided with an improved supercharger, bringing the announced ceiling of the Mef to 37,000 feet. Reports indicate, however, that it lost its effectiveness very rapidly beyond 30,000 feet. This is confirmed by the next turn of the wheel.

New High-Altitude Fighters

Early in the autumn of 1942, reports began coming in from the Russian and other fighting fronts of a new high-altitude fighter, the Messerschmitt Me-109G. The Germans claimed a 40,000-foot ceiling for this job, naively remarking that it was 10,000 feet better than the previous model. British sources report that power plant of the new fighter is the Daimler (Mercedes)-Benz 603 inverted V-engine of about 1,500 hp; speed of 325 mph at 13,000 feet; ceiling, 40,000 feet; top speed 380 mph; much more heavily armed than previous models, carrying three cannon and two machine guns and possibly provided with a pressure cabin. We shall find out more about this fighter if we are lucky enough to catch one alive, like the highly touted Focke-Wulf 190 captured last July in the south of Kent by RAF Coastal Command fighters before the Nazi pilot could smash it up. Its claim of 375 mph at 37,000 feet was proved to be an exaggeration, as the Fw-190 falls off sharply above 25,000 feet.

The improved Focke-Wulf, the Fw-290, recently reported in action, may provide the answer. It is said to be powered by a BMW 802 radial air-cooled 18-cylinder engine, turning up close to 2,000 hp. Its 3280 cubic-inch displacement and rated horsepower bring it into the same class as our Pratt & Whitney R-2800 Double Wasp (2,000 hp) and Wright R-3350 Double Cyclone (2,200 hp). There are also reports of a three-speed, two-stage supercharger which gives this new engine good performance at extremely high altitude.

Nor are we through with the indefatigable Messerschmitt line with the 1942 Me-109G, either. For some time there has been under development an Me-209 with considerably enhanced performance over the 109 line, though still substantially the same airplane in its basic design. A far cry from the 650-hp, 300-mph original which first saw the light of day in late 1935, and — regarded objectively — our aeronautical hats are off to such an achievement, as indeed they are to the equally long-lived and highly developed British Spitfire and American Flying Fortress heavy bomber. Reliable British sources report the speed of the Me-209 as 398 mph at 21,000 feet; ceiling of 42,000; and a pressurized cabin; Daimler-Benz 603 engine, 1,600 hp. This is not in the reports but, as a scientific guess, it may be conjectured that the difference in performance between the DB-603 in the Me-109G, and the DB-603 in the Me-209 is that the former has a two-speed, two-stage supercharger, and that the latter has a turbosupercharger. (It is hardly possible that America will be able to have a permanent corner on this vital contribution to high-altitude flying.)

Be that as it may, the fact that late model Spitfires are tangling with German fighters at better than 42,000 feet indicates that the newest fighters on both sides are certainly getting "up there" with a vengeance. Possibly these are Me-109Gs and FW-290s as the Messerschmitt 209 is not yet reported in full production and combat action. That would make these two high-altitude fighters a formidable pair indeed. Their immediate predecessors, still much in evidence in European skies, the Mef and Fw-190, has thus far battered their heads against a stone wall in their efforts to knock out our Fortress and Liberator heavy bombers. Their 20-mm cannon, with 400- to 500-yard effective range, were able to take a fair toll of British heavies — the Lancasters, Stirlings and Halifaxes — with their lighter .303-caliber guns, but so far they have been hopelessly outclassed by our .50s. We do not know what armament improvements Germany may be able to effect in the new fighters, even though they appear to have licked the high-altitude problem. Nor do we know how they will stand up against such promising high-altitude United Nations performers as the new Spitfire (Mark IX), Hawker Typhoon, Lockheed Lightning (P-38) and Republic Thunderbolt (P-47). All of these are in production and some of them have already seen highly successful action.

United Nations Meet Challenge

No details are releasable on the new Spitfire, but it may be stated that its enhanced performance is largely due to a radical improvement in the supercharging of its Rolls Royce Merlin engine. Speed of better than 400 mph is reported and ceiling of around 45,000 feet. The Hawker Typhoon has been under development for some time. It existence as a type was announced last spring and its engine, the H-type Napier Saber of 2,200 hp, was made public in April, 1941. The Typhoon has been reported as a very fast, high-flying, heavily armed fighter.

The American Lightning and Thunderbolt, while very different in many respects, have several interesting things in common — apart from the singularly appropriate combination of nicknames which were arrived at independently. Lightning was originally the British name for the P-38. Both airplanes weigh more than 13,000 pounds and are heavyweight sluggers of the upper air. Both are in the 400-mph class and are wickedly armed, with plenty of rounds of ammunition to fight it out to a finish with any opposition may encounter. The Lightning has one 37-mm cannon and four .50-caliber machine guns, and the Thunderbolt has eight .50s. Both have a fuel capacity to stay long enough in the air to convoy bombers to a distance of several hundred miles and return, instead of having to leave them part way as other present model fighters. Both are equipped with turbosuperchargers for top performance at high altitude. Both take off so fast and have such a breathtaking rate of climb that they are almost out of sight by the time you have lighted a cigarette and half smoked it. Finally, despite their weight, both are surprisingly maneuverable and can take sharp turns, and land reasonably slow despite their great speed. Incidentally, it may be noted that point for point (except for weight) the above would pass for a description of the Navy's new smashing fighter, the Vought-Sikorsky F4U Corsair, with its 2,000-hp turbosupercharged P & W Double Wasp engine and bristling array of .50-caliber guns.

At this point, however, the Lightning and Thunderbolt part company. The Lightning is a thoroughly unorthodox design, with no fuselage, but twin booms, looking for all the world like a darting dragonfly. With two V-type liquid-cooled engines and tricycle landing gear, it has the fastest rate of climb of any airplane in the world and the longest range of any single seater fighter now in the air. The Thunderbolt, despite many modern and highly ingenious features, is of a thoroughly tried and proved fighter design, tracing its lineage back through the P-43 and the P-35, a charging bull let loose in the upper skies, with one large radial air-cooled engine. The Lightning was designed in 1936-37; prototype test-flown in February, 1939; bugs removed; thousands of changes and improvements in plane, engine and supercharger made; in production late 1941, and in successful action by mid-1942.

The Thunderbolt, on the other hand, was designed in September, 1940, as a result of an informal conference at Wright Field as to what leading experts and test pilots would like to see in a dream fighter plane. Within 8 months (May, 1941) the prototype P-47B was test-flown. A year later and the improved P-47D was in production at the main factory and now is being turned out in a midwest branch, with another important company tooling up to increase production on these big, tough packages of firepower. The OWI report speaks of the Thunderbolt as a design of the highest promise and probably destined to become one of the outstanding planes of the war. A high-ranking authority, recently returned from London, stated: "The Republic Thunderbolt, built for high-altitude performance, will open the eyes of the world. I have watched the RAF and the Luftwaffe at work and am convinced that the Thunderbolt will astonish them both."

How Superchargers Work

The theory of the supercharger is fairly simple. The gasoline in the airplane engine burns oxygen from the air. The higher the altitude, the less oxygen there is in the air; therefore the higher the engine goes, the less efficient it is. Engines at higher altitudes become short-winded, like the traveler climbing Pike's Peak. A Pratt & Whitney Twin Wasp air-cooled engine, developing 1,200 hp for take-off, would turn up less than 600 hp at 20,000 feet without supercharging, would only turn up about 400 hp at 30,000 feet. However, with the supercharger, extra air is pumped into the carburetor, and that does the trick.

How does it work? The most common type of supercharger is driven by the airplane engine itself and is called a "gear-driven" supercharger. It consists of a small air compressor of blower which forces greater quantities of air into the cylinders than the normal piston suction could obtain. In this way the "critical altitude," or maximum height at which the engine will develop its rated horsepower, is increased. However, as the engine begins to revolve more slowly, so does the supercharger; hence the "two speed" supercharger, which permits it to be shifted from low into high as the engine starts to slow down. The next refinement consists of two superchargers, taking the already compressed air and forcing it into a second auxiliary supercharger. Engines so equipped are know as two-speed two-stage engines. Good examples are such radial engines as the Wright Cyclone R-1820 in the Navy Grumman Wildcat which has been polishing off Jap Zeros to a merry tune, and the BMW 801 engine in the Focke-Wulf 190. In the V-type liquid-cooled field, the Daimler-Benz 603 in the Me-109G, and the Rolls Royce Merlin LXI in the newest Spitfire, are current examples. According to the OWI report, the Allison is also now in production on this type which, with the Merlin LXI, undoubtedly will be used to step up the high-altitude performances of some of our present Army fighters, such as the P-39 Airacobra and the P-51 Mustang.

Well, then, what is a turbosupercharger? This gadget, invented by Dr Sanford B Moss of General Electric, has the great advantage of not using up the airplane engine's power at all. On the other hand, it reclaims the lost power of the engine's exhaust gases and uses them to spin the compressor at any speed the operator wishes. This is the most advanced and satisfactory type of supercharger in existence and, as already indicated, is used on the Wright Cyclone R-1820s which power the Fortress and the P & W Twin Wasp R-1830s on the Liberator, the P & W Double Wasp R-2800 in the Thunderbolt and the Allison V-1710 in the Lightning, possibly the latest Daimler-Benz 603 in the Me-209, and reported in the Jumo 207, powering the Ju-86P high-altitude bomber.

The relative efficiency of these various types of superchargers may be seen from the following comparison as shown in the recent and illuminating study by Allison Division of General Motors entitled "Some Fundamentals of Aircraft Engine Design." Service ceiling may be defined as the maximum altitude at which an airplane can climb at the rate of 100 feet per minute; critical altitude of an airplane is the altitude at which maximum speed in level flight is obtained. The figures are approximate only, and may vary under differing conditions.

An airplane equipped with a particular engine, unsupercharged, may have a critical altitude as low as 1,000 or 1,500 feet and a service ceiling of nearly 25,000 feet. With a single-stage supercharger, critical altitude may be around 15,000 feet and service ceiling 35,000. With turbosupercharger, critical altitude may be nearly 28,000 feet and service ceiling above 40,000 feet. This tells the story, as far as the engine is concerned. Now go back and read the first part of this article!

The closest approach to a solution to the human problems in high-altitude flight is the pressurized cabin, in which the air or oxygen is compressed sufficiently to prevent oxygen lack and aviator's "bends," and also heated sufficiently to minimize the effects of extreme cold.

The first use in this country of pressurized cabins for experimental flight above 30,000 feet was in the mid-thirties in an Army Air Corps Lockheed XC-35 transport. It was in this plane that "Tommy," now Lieutenant Commander Tomlinson, formerly of TWA, did much of his pioneer work "upstairs" in preparation for the Boeing Stratoliner — a commercial adaptation of the Flying Fortress — with pressurized cabins for transcontinental passenger flights at 20,000 feet, and for Pan American's new fast flights over the Andes.

Germany has been known to have been working on these pressure cabins for years, and recently a Junkers Ju-86P has been reported by the British Air Ministry as shot down by a Spitfire over Egypt at an altitude of 50,000 feet. This is a bomber version, without pressure cabin, but with turbosupercharged engines of the Junkers 86 airliner. Maybe the tape measure ran out, and it was only nine miles up (47,500 feet), but that is high enough. Anyway, it indicates that the Spitfire IX has a sealed cabin, like the Me-209. This is a noteworthy achievement, as pressure problems in a small fighter plane are much greater than in a large bomber. It is the high-altitude big bombers, and still larger ones on the way, that constitute the real aerial threat of the future.

The Spitfires, Lightnings, and Thunderbolts may have plenty of fighting to do away up there in the sub-stratosphere before this thing is finished.

Of course, there can be two-way traffic in the stratosphere airways. It doesn't make sense that the country that had a successful high-altitude bomber in the air a couple of years before all comers, and had the first high-altitude commercial airliners with pressurized cabins, would let it go at that. Remember that Tokyo broadcast early in the fall, referring to super-bombers with 10,000-mile range that might bring air war to Nippon's shores? I wonder if they had something there? And do you suppose it might be worked from North Africa with heavier bomb loads?

Victory in practically every case goes to the plane "on top" and American research and engineering skill definitely may be counted on to "keep 'em flying" — higher.

This article was originally published in the February, 1943, issue of Skyways magazine, vol 2, no 2, pp 28-33, 78.
The PDF of this article includes a cockpit photo of a B-17C, photos of an advanced trainer, A-20, P-51, B-17F, Hurricane II, Me-109 (4 photos), Lancaster, and a chart showing approximate operational altitudes of Corsair, Me-209, Spitfire, Thunderbolt, Ju-86P, Me-109G, Flying Fortress, Fw-290, Lightning, Typhoon, Fw-190, Mustang, Hurricane, Mosquito,Airacobra, Havoc.
B-17C photo credited to Warner Brothers Pictures; other photos credited to International News, Press Association, North American Aviation, Boeing Aircraft, British official photo, British Combine, Europlan.