Effect of boost control cutout prior to +12 psi boost modifications

[Seadog]

Originally Posted by Crumpp

The Merlin was cleared for +12lb on Take off and that could be used for "short duration" in an emergency. It was so limited, that it was not cleared for even ONE full minute much less FIVE.

It clearly states that the engine is highly overloaded when using +12lb boost. Furthermore, using it for short duration outside of take off, immediately deadlines the engine until it is inspected by a mechanic and cleared for re-entry into service.

How do those very clear instructions get translated into "could use +12lbs continuously"?



The endurance testing quoted in this thread is extremely limited for an endurance test. Most engines are ran continuously for far longer time periods at the emergency conditions during endurance trails with resulting tolerance wear. The Merlin was run for only 5 minutes at a time with a 20 minute rest period between. In that context, the Merlin endurance trials at +12lbs were not successful and the results are far from the "idea" that the Merlin was cleared to run +12lb continuously.

Dowding states in his memo of Aug 01 1940:



so it was certainly cleared for 5 minutes. However, I previously provided a quote:

"I had to return from Nuremburg in a Wellington II on one engine and used maximum boost and revs on a Merlin X for five hours with no sign of distress..." = +10lbs at 3000rpm.
The Merlin in Perspective,p25.

additionally
http://www.wwiiaircraftperformance.o...bs-14nov39.jpg
states:

...the life of the Merlin engines under the emergency 12lbs boost conditions should be very little reduced from the normal, it being observed that the Merlin XII engine has been type tested and approved for 12.5lb boost take off conditions.

The engine is not deadlined after using 12lb/3000rpm but such use must be noted in the log book and use beyond 5mins may prompt mechanics to check the engine's oil filters for signs of premature wear and/or bearing failure.

The above memo states that the endurance trials were successful and prompted the recommendation for the use of 12lb boost ASAP.