Error Propagation

[Admin]

A Wright R-3350 engine on display. Many believe the engines easily caught fire because the crankcase was magnesium—not so. Incorrect info has propagated throughout the internet. (United States Air Force/)

In the formal mathematical or engineering sense, error propagation tells us how a range of errors in the variables determining the solution to a function will affect the uncertainty in that solution. The errors “propagate” through the function to its solution. But in our case, I am using the expression to describe how an error made by one author may be dutifully repeated by later commentators using that author as a reference. You may think this is a new phenomenon, spawned by social media. It is not.

When I first became interested in the Wright R-3350 aero engine that powered the B-29 bomber, I read that the radial easily caught fire in operation because its crankcase was made of magnesium. A friend recently decided to investigate further, and found nine pages of Google citations agreeing that the R-3350 had a magnesium crankcase.

Not true. Anyone who has actually been around these engines or has read original primary sources of information on them will know that the three-section wartime crankcase and the four-piece case of the postwar engine (sharing the same name but no parts) are made of forged steel. One of the reasons I can tell you this is because I have one sitting in front of me.

The New England Air Museum has a B-29 on display, and I spent many hours in its restoration shop during the assembly of that aircraft’s four engines. I can assure any interested party that its crankcases are steel, as is the case in my shop. Another air-cooled radial with a forged-steel crankcase was the BMW 801, which powered the Focke-Wulf 190 fighter.

In radial engines such as these, as a cylinder fires and combustion pressure rises to its peak, a force of more than 30,000 pounds attempts to uproot each cylinder in turn. Steel was chosen for its resistance to fatigue, as waves of high stress circulate around a radial’s crankcase during operation.

Pratt & Whitney radials, which have forged-aluminum crankcases, retain their cylinders by studs and nuts. On the other hand, the cylinders of Wright’s big engines are retained by bolts which screw into the steel case—21 individual 7/16 NF bolts per cylinder. The late John Minnich, a Wright engineer during the war, told me that at the level of the intake port, just over a foot away from the cylinder’s upset-forged steel base flange, rod angularity at each firing caused the cylinder to kick sideways by twenty-thousandths of an inch. Because of that flexure, each base bolt sat on a part-spherical washer, fitting into a matching cup in the base flange. Without this feature base-bolt breakage was constant. Everything was constantly flexing. Try to imagine magnesium threads surviving such motions.

The Origin of the Crack

So how did the idea arise that the 3350 crankcase was magnesium? In World War II and the Korean War, 3350 engines were susceptible to induction fires caused by backfiring (usually on takeoff). Such a fire would burn back upstream through the affected cylinder’s intake pipe to the supercharger diffuser. The diffuser was a magnesium casting on both Wright and P&W engines. There the fire would continue to burn the gasoline-air mixture being supplied by the supercharger until it ignited the magnesium diffuser.  Why didn’t they just install anti-backfire screens to prevent this? While they were tested, these screens reduced output by several hundred horsepower.

At this point the “scanner” on that side of the aircraft, under orders to stare out through his Plexiglas gun-aiming bubble like a human Check Engine light, would report to the aircraft commander upon seeing a nacelle-access door darken, then burn away as flame emerged, streaming back as far as the tail. The on-board extinguisher system was helpless because the fire originated inside the engine. With the bail-out alarm sounding, the crew knew they had 30 to 60 seconds in which to leave the aircraft before the fire weakened the wing’s main spar and it folded. Once a large aircraft spins, its occupants are immobilized by the rotation and cannot reach the exits.

A B-29 engine fire, at this point the crew had less than 60 seconds to bail out before the wing would fail. (United States Air Force/)

R-3350s did not catch fire because their construction employed magnesium parts—all the major radials employed lightweight magnesium castings for items like the nose case (containing the propeller reduction gear), the aforementioned supercharger diffuser (which converted the velocity of mixture emerging from the supercharger impeller into pressure), the super housing and the accessory drive behind it. No, R-3350s had a special vulnerability to induction fires because fuel was not distributed equally to all of its 18 cylinders. Which cylinders were lean, which were rich, and which received a close-to-correct mixture was determined by each change of engine rpm or throttle angle. The 3350s burned because an excessively lean cylinder’s backfire could ignite an induction fire. Engines with more uniform fuel distribution did not backfire, and had no induction fires.

You cannot ignite a large magnesium casting with a match—it easily conducts such minor heat away, never reaching ignition temperature. Only an intense and continuing fire, such as an induction fire fueled by the entire air-fuel output of the supercharger, can rapidly heat a major casting to its ignition temperature. Our late friend Bob Nichols (he of the specially modified 9,000-rpm prewar Indian Scouts) had related hearing a B-29 engine start to backfire on takeoff and knowing what was likely to happen next. He survived, but many did not.

See for Yourself

Go to an air museum near you which has a B-29 or an R-3350 wartime engine on display. Look around to be sure no one’s watching, then pull out your handy mechanic’s magnet, the one mounted on a telescoping stalk. Reach up and try the magnet on the crankcase. The magnet will stick…because the case is forged steel.

This propagation of errors occurs often. One researcher (or internet commentator) writes down nonsense and later researchers repeat it. Stop! Go back to original documents and get it right. Don’t propagate errors. Don’t introduce a crack into your own credibility. Educate yourself and get it right.

So endeth today’s sermon.

Atomic Consequences

And now, a little lagniappe for the congregants who have stuck around for the post-service coffee and doughnuts.

What happened when a B-29 backfired on takeoff and suffered an induction fire? If the airplane was moving slowly enough, a fortunate pilot could brake to a halt and the crew could run for it, leaving behind up to 20,000 pounds of bombs and 6,700 gallons of fuel. If not, they were committed to flight.

Airborne, the affected engine’s torque would fall to near zero, as it’s hard for an engine to make power when its intake charge is already burning. Robbed of that engine’s 8,000 pounds of propulsive thrust, the airplane would yaw into the failure. The wing, now producing reduced lift because it no longer benefited from that engine’s high-speed slipstream over it, would drop.

Pilot and copilot would heave on the controls in an attempt to stop the combined yaw and roll, but their control deflections just increased drag, causing the “slow” wing to drop even more. What onlookers saw was an unstoppable increase in roll to near 90 degrees, at which point the aircraft cartwheeled into the ground.

The man in charge of the Hiroshima atomic bomb before its delivery over Japan, Navy officer William Parsons, had seen such crashes. He knew that an atomic-armed B-29 exploding on takeoff could destroy all the facilities on Tinian Island and kill its population of roughly 100,000. He decided that the takeoff would be made with the bomb inactivated by partial disassembly. Only once the takeoff was successful did he make his way into the bomb bay, reinstall the missing parts, and verify they were installed correctly. Then he returned to the forward pressure compartment and closed its hatch.

View the full article

[Bender]

Well that made a change from kev..... 

 

It's a rather tenuous link to motorbikes but it's there  

[bonio]

Well knock my socks off - it was by Kevin. 

[Bender]

Anyone not wanting to read it, don't use magnesium intake, with a supercharger and a lean mixture or your engine may catch fire if you have a backfire. 

 

Ohhh and don't put aeroplane engine on your bike. 

[MikeHorton]

Kevin keeps godin in these other articles now we must be hurting his feelings! We love you really Kevin 

[Bianco2564]

With more than a passing interest in ww2 aviation and piston engines, I looked into what our Kev was saying.

I agree about a lot of research just plagarising others work and how a small untruth can grow.Indeed what I have written is all based on others work from numerous sources, but none of us were there so what can you do?

From what I can gather, the early Wright 3350 did have an aluminuim alloy crankcase with a high magnesium content this was later changed ( can't determine when) to a forged steel unit. The engines was put into service before fully developed as was the case with much machinery in the war, it later went on to successfully power several airliners post war,the Lockheed Constellation being one.

This rush to put into use resulted in many failures, its written that more B29 crews were lost to mechanical failures and accidents than by by the Japanese forces.

Sources.

https://www.historynet.com/superbombers-achilles-heel.htm

 

WWW.enginehistory.org

KD McCutcheon

 

[MikeHorton]

The Constellation that was a machine. I believe the breitling one still flies