Sunday, April 17, 2011

Running Away to the Air, II: Magic Planes

The magic plane thesis is simply this: American builders achieved what the Air Staff couldn't even imagine: building a Very Long Range bomber (the B-24), and a Long Range fighter (the P-51), and thus won the war. The Air Staff is stupid! Depending on taste, so is the entire British aviation industry. John Terraine laid this argument out for the P-51 in a separate appendix to Right of the Line. This being a naked example of an appeal to the authority of the history of technology, you would expect to see some history of technology following --at the very least, of the tendentious kind that Correlli Barnett usually supplies. But nada.

Don't worry, though, John. I loved The Smoke and the Fire when I was 19, so I'll save you!

Or not.

Let's start with the "no British VLR." We can agree that the Short Stirling was a bit of a disappointment and talk about the reasons why at length, although weight control would be the basic issue here. The Air Staff specification  (B. 12/36) specified a maximum range (overload at 15,000 ft) of 3000 miles carrying 8000 lbs at a cruising speed of 235 mph. This certainly isn't ignoring range. The problem is that by January, 1939, the  estimated maximum takeoff weight, determined by engine power and wing area, was 69,100 lb, with empty weight at 44,000lbs. This ate seriously into this range, operationally specified at 1500 miles with 1000 lbs. bombs with maximum range proved out at 2200 miles in a test flight on 8 June, 1940. (Numbers from Chaz Bowyer's book on the Stirling, so as usual with the Bowyers, this hardly counts as a citation at all.) 

Clearly, the earlier we go, the more likely we are to find a design group struggling. Move up to a contemporary with the B-24 and we get the Handley Page Halifax, another VLR misfire.The difference is that Dr. Handley Page was a controversial figure in the industry, and people were willing to speak much more frankly about the problems with the Halifax. But first, here's a summary of the Aircraft and Armaments Experimental Establishment's assessment of the Halifax.

AAEE saw a great deal of the  Halifax, as it had constant operational troubles in its first 2 years.. These problems culminate in a test series begun in February 1943. The final report, six months later, primarily focussed on the plane's morale-damaging tendency to go into irrecoverable dives, and established that there were serious aerodynamic problems with the rudders. This was fairly common in the larger wartime double-rudder designs and was solved by the move to single rudders, only technically practicable because of (just a minute while I saddle up my hobby horse here) developments in servo technology. Incidentally, however, it added that the aircraft suffered from persistent overweight problems, and many heroic measures were undertaken over time to control and reduce the weight. At least one aircraft turned over to Boscombe Down was assessed as unsafe to fly at normal load in summer (3–10). A late and thorough redesign associated with a new variant operating Bristol Hercules engines finally fixed this.

So what went wrong? Here's the frankness boiling over from the attendees at D. C. Robinson's 1949 talk to the Royal Aeronautical Society, “Some Developments in Aircraft Production,” (J. Roy. Aero. Soc. 43 (1949): 39–66.)
Robinson, the works manager at Handley Page, talks about H.P.’s split constructional methods as they evolved from the Harrow through the Halifax. The details of this method of assembling the internals of a fuselage by making it in two split halves have been much discussed,
Sydney Camm of Hawker goes first, critiquing jointing construction methods and other production friendly but weight adding methods such as stamping, while the Air Ministry's W.O. Manning provides numbers, noting that the joints used to assemble complete a/c add so much weight that the Halifax’s 8 mainframe and “say” 3 in tailplane might easily add 400 lbs. (62). So there's trouble with the Halifax, but it has nothing to do with British industry being backwards. On the contrary, it is Handley Page's adoption of "modern mass-production methods" that screwed up the plane. Correlli Barnett, call your office!  Incidentally later models of the B-24 and, so far as I know, the Ju-88 (woah --this looks interesting) had the exact same problem for the exact same reason.
Weight control being such a persistent problem, are we perhaps just raging in vain? No, the AAEE was much more positive about the Lancaster. Looking at these reports, even in secondary sources, shows the limits of our current technical history. Francis Mason's history of the Lancaster notes that the original prototype was a rebuilt Manchester (the Lancaster's precursor), and that it had an auw of 41,000lbs, and this number used to show up in the secondary literature. Thank Heavens for Wikipedia! When Terraine was writing, he would have had to seek out my source (pp. 30--35) to discover the tare weight for the production Lancaster I that it test (38,000 lbs), well under the commonly cited 41,000. This would demystify AAEE's extraordinary findings for that plane, including a Vmax of 310 mph (30), a maximum takeoff weight of 63,000 lb in the fall of 1943, later raised to 78,000 lbs, and a certified operational range, as early as June–July, 1942, of 2370 mile maximum still air range at 15,000 feet, 2340 at 1000 feet. It looks like (I'm going to grab some crunch and bring in another hobby horse, just a second!) the remarkable cruciform assembly of press-extruded aluminum high duty alloy spars at the heart of the Lancaster structure is doing its work!

When Boscombe Down got its hands on the B-24 (and B-17), results proved more-or-less comparable: the first 7 Lancasters delivered to the RAF in March, 1941 had a manufacturer-declared operational range of 2400 miles, with actual Vmax at 263 mph, “normal” Vmax of 253' cruise at 216 mph at 45,500 lb auw; (99-100) the claimed range of the B-17 (3300 miles) was even higher. The American planes certainly score well on low all up weight, but had several advantages over British, including high pressure tyres not suitable to light pavements or grass; no turrets; and no self-sealing tanks. Fortunately, as hard as it was to pave Keflavik (and Akureryi, Gander and Goose Bay) airfields, it was just those four fields that needed to be done. There was no-one shooting at the planes, and this not only allowed them to go without new fuel tanks and defensive armament, but allowed the "VLR Liberators" to fly with fuselage ferry tanks, contraindicated in combat planes for stability control reasons.  

As for the P-51, on testing, in the summer of 1942, early range trials at an auw of 8300 lb, 180 mph IAS, gave a fuel consumption 8.75 ampg, fuel capacity of 130 gallons (note that later a/c of the same type have listed fuel capacity of 140) showing an est. maximum range of 990 st. miles, end. 4.1 hours. This is not an aerodynamic miracle; it is big gas tanks. If anything, the plane looks a little bloated, with perhaps some redundant structure weight (a rather controversial issue: here's Wikipedia arguing that it all comes down to different service load factors.) Of course later the P-51 got another 85 US Gallon fuel tank. Magic!

Actually, not really.

The way that we have made aviation gasoline has changed a great deal over the years, and the results are sen in the "octane" rating of the fuel, which is a measure of the pressure conditions in a test cylinder at which a given gasoline stock begins to experience pre-ignition, or knock. A reference fuel of pure octane has a "100 octane" rating, while the real fuels available for testing when this method was developed in the 1920s might give as little as 60%, hence 60 octane. As technology advances we get to fuels giving as much as 145%. (History of technology and sportswriting: the unsuspected convergence.)  

The traditional method was simple distillation, but even before WWI, producers began to "crack" petroleum feed stocks  to extract additional gasoline from the “gas oil” left over after distillation. This raised the percentage of crude oil marketed as gas from 25% in 1918 to 45% in 1940, and while straight-run gas averaged 60-63 octane (Research Method, unleaded), cracked averaged 72-5 (ditto).  This was the standard military avgas of the interwar period, and the fuel behind the records of the 1920s.
  By the late thirties, catalytic cracking was introduced to obtain 92-95 octane gas (with the addition of tetraethyl lead).  Allied catalytic cracking capacity was 928,000  barrels a day in 1946. Gasoline with octane ratings of 94.4-98.2 could be achieved with the addition of 3 ml.  TEL and cracked East Texas gas oil gave 3.2% dry gas, gasoline 31.1%, “cycle stock,” 65.2% and coke 2.04%, and a Research Method unleaded octane rating of 78.7. Unless something is being left out, the stocks used in cracking do matter].  Higher doping will get you to 100 octane at considerable risk of lead attack. At this point we get to the French versus American priority dispute. The Franco-American Houdry firm pioneered catalytic cracking, incidentally making the first commercial use of a (Brown Boveri) gas turbine to do it. Exxon vociferously disagreed, not least because the Houdry plants were fundamentally impractical.

As 100 octanes go, cracked fuels are pretty weak. Reforming, a process of turning low octane natural gasoline distillates into high-octane by reforming their molecules, is the way to go.  (343-4) Here's a table cribbed from my sources showing how much avgas you can get out of the reforming process, and the octane rating.  6( Mid-continent Naptha).
vol% butane
Debutanized gasoline, %
Octane Number, Research, unleaded

As you can see, there's a pretty stark inverse relationship between the amount of gas you can get out a given amount of feedstock and the octane rating. This is a technology for coalitions with petroleum to spare.

Catalytic reforming involves “processing with catalysts in the presence of hydrogen (to minimize coke deposits.” This produces gasolines high in aromatics and isoparaffins, low in olefins, with ON (Research method, unleaded) of 85-102 or higher.  These were developed “prior to WWII”  (quoting my source here, E. V. Murphree, B.S., M.S., D. Sc.h.c. F.Inst.  Pet.  And G. Cirprios, “chapter 9: Cracking and Reforming,” in Modern Petroleum Technology, 3rd.  Ed., Hon.  Ed.  E. B. Evans (London: By the Institue of Petroleum, 1962): 313-64, 348.) 

The upshot? Here's N. O. Rawlinson, “Chapter 13: Aviation Fuels,” 490-547 in Ibid.  (492) with the UK Avgas Grades Specification D. Eng.  R. D. 2485, Issue 3.
Max TEL content/gallon

(492, 509) (The 115/145 specification was subsequently amended on account of between the aromatic content and the TEL, it was eating its way through tanks and engines.) The reason for the double rating is that fuels respond differentially depending on whether you are adding just enough fuel to burn all the air that you can cram into the cylinder ("lean burning") or all the fuel that you can induce to burn ("rich" rating.) The “100 octane” of 1939 was a lean mixture rating, even though we usually encounter it in the secondary literature in the implicit context of rich mixture rating --it's making Spitfires go like a rocket. Nevertheless, performance at both ratings is important for long range performance. The higher the rich mixture, the higher the supercharger boost rating on the ground, giving more power and a higher takeoff weight. Once up and cruising over the Atlantic, it is the weak mixture rating that counts, giving you maximum fuel economy. However, your engine has to be able to shed the heat produced (engines burning lean mixtures run hot because of the lack of vapourisation cooling). 

The upshot? Per the corporate history, 100/130 came into (American) production in the winter of 1943. Got yer "VLR" miracle right there, if this information isn't updated by someone with access to Air Ministry records as opposed to corporate. (2: 503.) As for the P-51 "miracle," A. C. Lovesay, “Development of the Rolls-Royce Merlin,” Aircraft Engineering, July, 1946, 218–25 is quite interesting. What Lovesay vaguely refers to as 100/150 octane was first tried out on a testbed by the simple expedient of fill it full of lead. This was not on, so other additives were looked for. This would be 2.5% mono methyl aniline produced at ICI in the UK was set in motion. Lovesay tells us that the production facility came on line in time for its first operational use “against the flying bombs in the middle of 1944" .....” He then goes on to say that it was "later" used by the "Second Tactical Air Force during and after the invasion of the Continent. The Americans promptly followed suit, and used this British produced fuel in their escort fighters of all kinds.” Lovesay's interest is mainly in the way that  higher octane fuels required strengthened crankcases, main bearings, end oil feeds crankshaft, and the addition of a deep top land to the pistons, cutting the number of scraper rings by 1, omission of some core plugs from the cylinder heads, a strengthened cylinder skirt, improving the bearing capacity of the reduction gear pinion bearings (I believe --I get to ride another hobby horse-- by nitriding, but I may be overintepreting my evidence) and a new supercharger ball bearing. 

Going to an enginebuilder for operational details is fraught with risks. Lovesay's chronology is confused, but for now I'm going to go with the argument that "Mustangs over Belin" were possible because of the introduction of 115/145 octane into service --and, of course, the Merlin engines that went with them. When, exactly, the rating revision was carried out, and with what consequences, is left as a question for further research. As is, quite frankly, the chronology that would establish my claim for sure.

Magic? Nothing of the kind: As Andrew Nahum established, it was a policy change. (in “Two-Stroke or Turbine: The Aviation Research Committee and British Aero Engine Development in World War II,” Technology and Culture 38 (April, 1997): 336-48.) The Air Ministry did its best to resist the move to higher octane gas, fearing supply and price problems, but by 1935 accepted their inevitability during the transition to jet engines. Air Commodore F. R. Banks laid out the Air Ministry policy change by implication in “Fuels and Engines,” Flight, 28 September, 1939, 269–70,, and restated his views, with numbers, in his James Clayton Lecture to the Institution of Mechanical Engineers postwar (Banks, “The Aviation Engine, J. Inst. Mech Eng. 162 (1950): 433–45. It is observed that in 1936 the Merlin I gave 1030hp at 16,250ft on 87 octane fuel at a dry weight of 1,335lb. The “modern” Merlin engine [no mark number given] gives, on 100–130 octane fuel, at a dry weight of 1740lb, 1760hp (at 16,250ft?). “Using an engine with a two-stage supercharger in high gear on the test bed, a maximum corrected power of 2,620bhp was obtained at 3,150rpm with a boost pressure of 101.46 lbs Hg (35.73lb/sq. in at the gauge) on a fuel of 100-150 grade, plus water injection. A ten-hour flight approval test was made with a similar engine in a “Mustang” and, at a limiting boost of 91 inches of mercury (30.5
lb per sq. in. gauge) on 100-150 octane fuel only, the maximum power developed was 2129 bhp at 3150rpm. The former power is equivalent to a b.m.e.p. of 400 lb per sq. in. and the latter to one of 326 lb. per sq. in., calculated on net power delivered at the propeller shaft.” (434). He further notes that the Merlin successor engine, the Griffon 57, achieved 0.825lb/bhp on 100/130 octane. Better fuels and better metallurgy delivered range increases.

I guess I've taken the magic out of history now. Sorry.

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