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Sunday, March 12, 2017


Arctic Cat Snowmobile
A couple of weeks ago there was a story in the WSJ about snowmobiles that mentioned some of technical advances that have been made, and one of them was turbochargers. Turbochargers have been around for a while, you can buy a production car right off the showroom floor that is equipped with a turbocharger or even two. Hot rodders have been putting them on all kinds of machines, but as far as I know no one has put a turbocharger on a small, mass production engine, like those found in motorcycles or snowmobiles, until now.

P-38 Turbocharger Installation
Anyway this started me musing on the subject and the turbochargers from the WW2 P-38 fighter aircraft popped into my head, so I went digging, and some strange stuff surfaced.

First of all, almost all WW2 aircraft engines were supercharged, which gave them enough boost that they were able to reach altitudes of 25,000 feet (five miles high). When engines were equipped with turbochargers, the turbochargers were added to the existing supercharger arrangement so the engine now had two stage supercharging. The turbo provided the first stage. This moderately compressed air was fed to the supercharger, which compressed it to the point that is was as dense as the air you find at sea level, and fed it to the engine. This two stage compression gave the aircraft enough power to reach 40,000 feet (eight miles high), more or less.

Shane Christopherson and his General Electric turbocharger
The General Electric turbocharger (it is always referred to as the General Electric turbocharger) was a bit of a beast. It was also secret, kind of like the Norden bombsight or RADAR. Early on in the war, we sent some P-38's to Britain, but for some reason (perhaps because we didn't trust the Brits to be able to keep a secret), they weren't equipped with General Electric turbochargers, which had a sizable impact on their performance. I imagine the Brits weren't too happy about this, but beggars can't be choosers, so they took them anyway.

P-47 engine, turbocharger and associated duct work
The P-47 was also equipped with a General Electric turbocharger. The P-47 always looked a little plump, but I figured the designers knew what they were doing, and who knows what kind of complicated stuff was buried in the fuselage. Well, now I know. It was puffed up to accommodate all the duct work needed to connect the engine in the nose with the turbocharger and intercooler in the tail.

The De Havilland Mosquito

Now I'm watching a YouTube video about the De Haviland Mosquito which seems to have been quite an aircraft. It is a British aircraft, so it was not equipped with a General Electric turbocharger, but it still managed to get to high altitudes. How did it do that without a turbocharger? It had a two-stage supercharger, that's how.

Merlin Two-Stage Supercharger
The two-stage superchargers on the Merlin engine were driven by gears. General Electric turbochargers are driven by exhaust gas from the engine. Gears are dense and compact. The duct work needed to connect the engine to the turbo is bulky, but not dense. The weight penalty is probably similar. Duct work is easy to fabricate out of sheet metal. Gears require machine tools and castings to hold them alignment, which means the drive mechanism for a supercharger is going to be more expensive than the duct work for the turbo. On the other hand, the turbo needs to be made from unobtanium in order to withstand the heat from the exhaust gas. Superchargers can be made from regular old metal like aluminum or even iron, if you don't mind the extra mass. I think it is safe to say that the General Electric turbocharger was the forerunner of the General Electric jet engine program.

Bombing of the Gestapo headquarters in the Shellhus, Copenhagen, Denmark in March 1945. A Mosquito pulling away from its bombing run is visible on the extreme left, centre.
de Haviland Mosquito was an all wood airplane, and it you believe everything in the video, you might conclude that it was vastly superior to all of the big, heavy, four-engined bombers (like the Flying Fortress, the Liberator and the Lancaster) that the Allies used for the majority of their bombing raids on Europe. That superior performance was entirely the result of its high speed (fastest WW2 military aircraft), which it was able to attain because of its light weight. It was light because it didn't carry any armor, or gunners or gun turrets. Originally it did not have any guns at all, but once they found out what it could do, they mounted a bunch of guns in the nose.

Twelve O'Clock High

I suppose it was 12 O'Clock High, and my dad's role as a gunner in a B-24, that locked the heavy bomber into the leading role (in my mind) in our war against the Axis. I remember quite vividly how the Japan's Mitsubishi Zero was a light weight, high-performance aircraft whose Achilles' heel was its lack of armor. What all this illustrates is the 'design by committee' syndrome. When the enemy builds a high performance aircraft without any armor, it's because they are not concerned about their pilot's well being, but when we do it and the aircraft is hugely successful, we shove it under the rug because it doesn't fit the narrative that the military industrial complex is trying to sell us.

There are advantages to this approach. The story is vital in order to garner popular support, which you really need in order to win. And building big, heavy airplanes means you need a big, heavy, industrial machine to produce them. And if push comes to shove, you can drop a lot of weight by getting rid of the armor, which is going to make your aircraft perform better, which is what you really need in combat. It's kind of like sandbagging. You carry a bunch of extra weight around with you which makes you look slow and cumbersome, but it makes you stronger, and when it comes to crunch time, you can drop those sandbags and really drop the hammer on your enemies.


Rich P said...

It's my understanding that in the beginning, most or all of the WWII allied fighters had no armor, the designers having decided that it was not necessary. Shortly thereafter, numbers of intense exchanges of views with pilots altered their outlook. See Robert Johnson's "Thunderbolt" for a supporting account.

Chuck Pergiel said...

Toward the end of the war, they did start putting armor into Mosquitos, so maybe the Brits also changed their outlook. Or maybe they got a boost from the US that enabled them to do so.