Blumlein's Plane Crash

[Kalee20]

Exactly. And from the OP's diagram, this is an Over Head Camshaft engine, no pushrods... I'd have said that a loose locknut would make the valve fail safe, after all, the force on the valve adjuster is such that it would tend to screw outwards away from pressure, rather than inwards into pressure.

The locknut could work itself free and drop down an oilway into the sump, that bit makes sense.

Engine output would drop, but it's only one cylinder down, and only on the one engine... Hazardous if relying on full power for climbing steeply after take-off, but not so otherwise.

[chriswood]

There is some information which repeats the assertion re tappet problems damaging the valve. I do wonder if as the tappet screw comes undone sufficiently, it catches the top of the valve stem putting a side force on it causing the valve to bend or snap. In the article below it says that aircraft Halifax V 9977 had just been through a precautionary procedure to adjust the tappets probably increasing the possibility of an error.
https://www.newscientist.com/article/mg1...tors-life/
Also I found a scan of the original RR manuls posted here
https://ww2aircraft.net/forum/threads/ro...938.38267/

Chris

[peter scott]

Chris,

Finding the Merlin manual is superb! Many thanks for that. I've been searching for any history of Merlin inlet valve failures but have so far found nothing. I still have no understanding of the increased stresses due to excessive tappet clearance. It might be more understandable if it were an exhaust valve failure given that they are hollow but as far as I know the inlets were not sodium filled.

   

Peter

[chriswood]

Peter I must say I find it difficult to understand how the valves get damaged, but it does seem a strange co-incidence that the tolerances on the tappets were changed apparently to avoid a problem. It would be great if a technical note or addendum to the manual could be found.

For those that want to follow this further if you are coming down the M5 past Jct 7 Croome Hall the National Trust Property use to be part of RAF Defford and on site there is museum devoted to the work at Defford including Halifax V 9977. It seems the museum is now available with limited hours 11.00-14.00 after a long closure.

https://www.nationaltrust.org.uk/croome

[peter scott]

When you look at the length of the tappet adjustment screw and the amount of freedom that the cam follower arm would have in the event of a fully unscrewed adjustment it's difficult to understand how any excess stress is placed on the valve stem. I think the head of the adjuster can't pass through the threaded hole in the follower arm. One little note is interesting: "On later type valves the split taper collets fit under the shoulder at the top of each valve, and not around it as on early type valves." Does this suggest that the valve stem could slip through the collets on early type valves?

   

Peter

[Kalee20]

Well, the split collets would be held in place by spring pressure on the round item that the upper end of the spring presses against (can't remember its name! )

The New Scientist article by Barry Fox that Chriswood gave the link to, also says that the clearances were readjusted tight, as they tended to increase in service - that's exactly what would be expected as the valve adjuster winds itself away from pressure. And not that they were readjusted loose, as they tended to close-up (which would be the unfavourable direction - fortunately not credible).

Any loss of life is tragic, but what a strategic disaster losing all that expertise.

Barry also mentions the pilot had only 13 hours experience in the Halifax - I'll bet he was selected to clock-up more flying hours in the Halifax on a low-risk mission, "Just take up the geeks, fly around the Severn estuary, it's daytime, not over enemy country, weather is good, you won't even need to do any weird manoeuvres." It would all make perfect sense.

[BusyBee]

My thoughts would go to a mechanic having set the tappets too tight. When I have seen people doing this I always think it an hit or miss process. Even a good mechanic I know has mentioned (some years back) that tappets are often only set on a car by garages if they are noisy. How does that allow for one or more that are tight? I think I have heard of dropped valves on engines too. Obviously that would make a nasty mess.

Tracy

[peter scott]

There shouldn't be a dropped valve in the Merlin because there are circlips to restrict valve movement in the event of broken valve springs.

Peter.

[ppppenguin]

Whatever the cause of the failure, this was wartime. Lot of people were dying for all sorts of reasons. Lots of planes crashed, not all due to enemy action.

The tragedy was having several of the top engineers in one aircraft. But again in wartime the niceties get overlooked.

[peter scott]

The tragedy was having several of the top engineers in one aircraft.

This is of course true but there is something very strange in the analysis of this failure. There are always lessons to learnt from past mistakes and I don't mean in the nitty gritty technical detail.

Peter