Classic cars forum & vehicle restoration.

[peter scott]

When EMI were testing the performance of klystrons versus magnetrons for use in the H2S airborne radar system there was a reluctance to use the magnetron because of the risk that a bomber over enemy territory could be shot down and the Nazis who only had the klystron at that time could learn about the magnetron. Up to this point the magnetron had proved very successful in pin-pointing U boats and destroying them with minimal risk of magnetron capture.

Unfortunately the klystron was not nearly powerful enough for H2S to display bombing targets over land and it was a magnetron testing session in a Halifax bomber that led to the crash killing all on board including the EMI development team members, Blumlein, Browne and Blythen. As it happens the magnetron was selected on account of the very poor accuracy obtained from visual bomb aiming.

Being a nice day yesterday for sitting out in the garden I decided to dig into R.C. Alexander's biography of Blumlein again and the cause of the crash had always puzzled me. There were a number of factors in the design of the Halifax that compounded the fatal outcome but according to the Rolls Royce strip report: "The tappet screw had screwed right back and the excessive tappet clearance so set up had fractured the inlet valve stem at the reduced collet section. The remaining safety circlip prevented the valve from falling right into the cylinder. Following this, the valve would be allowed to drop so far into the cylinder and the hot gases would impinge on the flame trap element causing the latter to burn, finally igniting the incoming charge in the induction manifold."



In my motoring history I've never experienced this type of problem but this could be because I have never had a supercharged car to play with. In the case of the Merlin with the fuel/air mixture being forced into the inlet manifold it's not so difficult to imagine the exhaust gases being pumped out of the open inlet valve and igniting everything in the induction system and the continuous fuel supply feeding the fire. In a normally aspirated engine you could see the backfire exploding the inlet tract but then not being fed further fuel.

In this case the fire rapidly burnt through the casings and ignited adjacent fuel tanks before causing structural failure of the wing.

I just happen to have a valve from a Griffon that may be slightly but not dramatically bigger just to show you the size of the "reduced collet section" that fractured.



Peter
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http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

[alastairq]

With a supercharger, one needs to fit a blowback relief valve on the inlet, downstream of the supercharger.
This to save the supercharger, methinks?

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Dellow Mk2, 1951 built, reg 1952.
Fiat 126 BIS
Cannon special [1996 registered. Built in 1950's]
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Ford Pop chassis, Ashley 1172 bodyshell, in pieces.