An Enemy Analysis

The following excerpts are taken from "A German View on Allied Engines," an enemy report on captured types of aircraft engines, which was published in the September, 1941, issue of Aircraft Engineering. This report was written for an official German publication by P Kotzschke, German engineer, and is of unusual interest since an insight is gained in the methods used by German engineers in their examination of captured engines. The report, translated by the British Air Ministry from the publication Luftwissen, is complete with tables giving the chemical structure of the parts examined, charts, photographs and cross-sectional diagrams. Following are pertinent quotations pertaining to American engines:

"Since the outbreak of war, a number of enemy aircraft have fallen into German hands, either shot down, forced to land, or captured in occupied territory. Thus, in addition to the aircraft, their armament and equipment, various types of engines of the enemy powers — English, French and American — have become available for examination. Research on the materials has been carried out on behalf of the Reich Air Ministry by various German engine builders and makers of components, and by the DVL — the German Aeronautical Research Institute. The present report summarizes the principal results of this research ….

"The front portion of the American Wright Cyclone crankshaft is a distinctly low-alloy steel, a chrome steel with 0.4 per cent carbon and 0.7 per cent chrome, heat treated and hardened to 105 kg/sq mm. The rear portion is of a chrome-nickel steel corresponding to the German VCN 35 standard steel. The heat-treated strength is similarly about 100 kg/sq mm. The crankshaft of the American [Pratt & Whitney] Twin Wasp radial engine is a case-hardening nickel steel with about 4.5 per cent Ni, heat treated to 115 kg/sq mm core strength. The crank pins are case hardened to about 0.8-0.9 mm depth ….

"The crankshafts of the American Twin Wasp … and of the smaller [British] Armstrong twin-radial Tiger are in one piece. Contrary to the British engine, the Twin Wasp has, however, an intermediate bearing, sufficiently large to enable the inner roller race to be pushed on over pin and throw. This results in a circular intermediate web of great torsional strength, with prismatic web projections on each side. The crankpin is bored cylindrically. The oil passages are not case-hardened, but individually bushed.

"In the Wright Cyclone engine, the rear portion of the crankshaft carries a 'Sarazine-Taylor' dynamic vibration damper and is clamped on the crankpin the internal thread for the locking bolt being cut directly into the separate web. The pin is not bored right through, material being left at the rear to serve as a stiffener to the pin at the clamping point.

"The separate rear crank web is rather narrow by comparison with the pin diameter. The carbon-steel counterweights are forged in one piece, and fixed by four riveted studs ….

" … Again, it is the American Wright Cyclone which uses the lowest alloy steel. The Twin Wasp, however, agrees with the French practice in using for connecting rods a chrome-nickel steel remarkably high in molybdenum ….

"In contradistinction with the usual practice of modern radial engine design, the master rod of the Twin Wasp is in two parts. This is necessitated by the one-piece construction of the crankshaft. In the Wright Cyclone the double T section of the connecting rod is turned through 90° compared with the usual practice. The flanges of the rod are thus in line with the big end flange housing the knuckle pins of the auxiliary rods, whilst the web of the rod is in the plane of the engine axis. This has the advantage from the point of view of production that big end and rod flange can be milled in one operation on a special miller. It is, however, now not possible to use a forging approximating closely to the final shape and the amount of machining involved must be considerable. A further disadvantage to the design is the fact that the flow of the grain is now interrupted and the shaft is relatively weaker than the standard design. It must also be noted that the big-end bore of this engine is plated with hard chromium to prevent surface damage when inserting or removing the bearing bush ….

" … The cylinder liners of the foreign engines examined were of the most varied composition. It appears that each firm has had special experience with a definite grade of steel. Thus … heat-treated Cr-Mo steel for the Twin Wasp, Cr-Mo nitrided steels (containing Al) for the Hispano-Suiza and Cyclone ….

"This confirms an earlier observation that the majority of foreign engines utilize nitrided cylinder liners ….

"The materials of the inlet and exhaust valves in the captured engines are generally very similar to German practice, although it can again be noticed that the alloy content of the steels is higher than is indicated by our experience for the stress involved.

"The materials of the inlet valves … again show the tendency to standardization … irrespective of the engine …. The inlet valves of the Wright Cyclone also use a high-alloy, chrome-nickel steel, but without the addition of tungsten. Pratt & Whitney, on the other hand, in the Twin Wasp, use a chrome-tungsten steel containing about 13 per cent tungsten, the same that is already familiar from their Hornet engine ….

"The British and American engines use a steel in exhaust valves equivalent to the German Flw 1440 …. "All the engines have hollow exhaust-valve stems; the Hispano-Suiza, Pratt & Whitney and Wright engines, also hollow heads ….

"The following methods, agreeing with German practice, are used to prevent wear at the tip of the valve stem:

  1. Insertion of a compression member of case-hardened steel.
  2. Insertion of a compression member of tool steel.
  3. Hard metal deposit by welding ….

"All exhaust valve seat inserts except those of the Hispano-Suiza are of a high-nickel, chromium, or chromium-nickel steel ….

"With regard to valve spring materials, the foreign engine builders again show considerable diversity. Whereas up to now oil-hardened carbon-steel springs are German practice, the captured engines show every type of spring steel, with the exception of the oil-hardened material. Rolls Royce, Merlin, Cyclone and Twin Wasp have chrome-vanadium steel springs, similar to those used originally on the Pratt & Whitney Hornet built under license in Germany ….

"In no case does the surface-finish of the springs approach German standards. In the French engines, the springs are not even burnished. Shot-plating was found to be used only on the springs of the Cyclone. It must, therefore, be assumed that the spring loads have been kept lower than in German practice, either by engine design, or by modifying the design of the springs themselves. Of course, this leaves out of account the possible extent to which difficulties have been encountered with valve springs in foreign engines ….

"A number of gear wheels from the main and auxiliary drives of several engines were examined, with these results:

"Case-hardened gears predominate and, consequently, chrome-nickel or nickel case-hardening steels, from molybdenum, are mainly employed. Nitrided steels containing aluminum are only used where grinding of the flanks of the teeth would be difficult, eg, on internal teeth. If nickel case-hardening steels are used, the usual alloy is 5 per cent nickel for highly stressed gears, and 1.7 per cent for moderate stresses. Such gears are usually heat treated to about 120 kg/sq mm. The chrome-nickel steel employed is similar to the former German ECN 45 steel, but with the percentage of nickel at the lower and the percentage of carbon at the higher limit allowed by the German specification. For highly-stressed gears, a chrome-nickel-molybdenum is also used corresponding to the German Flw 1409 specification. These two alloys are heat treated to 120-140 kg/sq mm. The case-hardening depth (0.8-1.3 mm) is greater than in German practice. The gears have a surface hardness between 590 and 620 kg/sq mm corresponding to 60-62 units of Rockwell scale C.

"The tooth flanks are generally ground, and this applies particularly to case-hardened power transmitting gears …. However, the finish leaves much to be desired. The very small radius at the transition from flank to bottom of the tooth in the Twin Wasp gears may be remarked, as well as the absence of grinding at the bottom of the tooth resulting in a burr being left in that region. Similar rough finish at the bottom will be noticed in the ground gears of the Merlin X auxiliary drive ….

"The rough workmanship to be found is...shown by...the base of the thread of heavily-stressed crankcase assembly and cylinder holding-down bolts of the Twin Wasp engine. It is, of course, impossible to say whether these examples are exceptions which escaped control or whether better workmanship was not called for ….

"The following light alloys were used in the engines examined: Cast alloys — the subeutectic, silumin-gamma type; also Al-Cu-Ni alloys. Wrought alloys — exclusively 'Y' and 'RR' (Al-Cu-Ni).

"Contrary to German practice, the pistons of all captured engines are of Y or RR alloy, cast or wrought ….

" … The American Twin Wasp uses a cast Al-Mg-Si alloy with about 1 per cent silicon and 0.6 per cent magnesium. This is contrary to German practice. An addition of about 0.25 per cent cobalt may be noted. The object is probably to reduce creep.

"The Wright Cyclone has a crankcase made of low-percentage chrome-nickel-molybdenum steel heat treated to about 80 kg/sq mm.

"For the cylinder heads of radial engines, the Y alloy predominated. While French and American engines have cast heads, the Bristol Mercury uses forgings … .

"Three in-line, five twin-radial, and three single-radial engines of British, French and American origin were examined for the type of materials used, and the design features of highly-stressed parts. The foreign steels used for crankshafts, connecting rods, valves and gear wheels contained higher percentages of chromium, nickel, tungsten, etc, than those considered necessary in German practice for the same stressing. It is significant that the lowest percentages were in general found in American steels. The quality corresponds to equivalent German materials. The surface finish was frequently not as good as in German engines and confined, particularly in the American designs to the most highly-stressed parts. Nitriding is employed to a greater extent than in German practice. In addition to the bearing metals used in Germany, the foreign designers made use of Cd-Ni alloys and lead bronzes with high tin or silver content.

"Light alloys are utilized in very much the same way as in German practice."

This short report was originally published in the June, 1942, issue of Flying and Popular Aviation magazine, vol 30, no 6, pp 56, 90, 92.