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

[ICDP]

Originally Posted by Seadog

Dowding's memo reports a number of combat situations that led to increased engine wear and thus to bearing failure and basically these centre on high G/low oil pressure manoeuvres and inverted flight, both of which led to oil starvation due to low oil pressure and obviously have nothing to do with 12lb/3000rpm flight. Prolonged steep climbs at high boost/rpm was a potential problem area (but why would Dowding mention this if pilots weren't doing it historically?). I previously posted a test report showing that 8.5 hours (in repeated 5min intervals) at 12lb boost/3000rpm did not cause increased engine wear:
http://www.wwiiaircraftperformance.o...bs-14nov39.jpg
Basically, it is a question of modelling the cooling and lubrication capability of the aircraft and in straight and level flight 12lb/3000rpm will not cause problems but pilots need to keep a watch on their oil pressure, temp and engine coolant gauges during prolonged steep climbs, and their engine RPM during dives.

So if I am chasing/being chased by a 109 and I'm in level flight and I pull the boost override, I have a very high probability of being able to run at 12lb/3000rpm until I run out of fuel. If I make repeated steep climbs and let my temps and pressures stay in the red, then my probability of engine failure increases. It is a question of accurate flight modelling.

Then it seems I owe you an apology, I have assumed all along that you meant +12 lbs boost was always available without consequence in all flight regimes, combat or otherwise.

[Seadog]

Originally Posted by ICDP

Then it seems...

I should have been clearer from the start in stating that straight and level and/or turning flight was unlikely to be problematic at 12lb/3000rpm because the cooling and lubrication system could cope with it (gauges stay within normal parameters). I suspect that there might be differences between the various aircraft in steep climbs and 12lb/3000rpm in terms of cooling capability but accurate flight modelling will provide danger warnings to the pilot as his gauges go into the red.

[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.

[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.

[Crumpp]

Well there you go.

I am sure that did not magically happen either. I believe one of the other snippets talks about measures to prevent the cylinder head cracks that appeared during the test in the form of replacing existing parts with new ones that were not prone to cracking.

That probably paved the way for an increase to 5 minutes.

Either way, the notion of +12lbs being using continuously is pure fantasy.

However, I previously provided a quote:

Who cares....

I am sure a few pilots on all sides rolled the dice and took their chances by violating limits placed on their engines.

We hear about the ones who gambled and won. Those who did not win are not around to tell their side of the story.

Facts are, according to the FAA's statistics in aircraft accidents, not adhering to published limits is a factor in ~85% of the engine failures in aviation accidents.

Believing that pilots routinely violate published limits is gamer thinking and not grounded in reality.

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

Ahh, no the trials were not successful. They pointed the way to modifications which allowed +12 to be used for 5 minutes just like the previous Emergency rating of +9lbs. In comparison most endurance trial engines have tolerance issues or things like worn supercharger blades. They don't have cracked cylinder heads like in the case of the Merlin trials.

...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.

Yes...Normal being the life of the engine in comparison to using +9lbs of boost for 5 minutes. It is not the gamer notion that 12lbs could be used continuously. It is just as the FC directs, when used, it must be dead-lined and cleared by a mechanic for return to service.

[Seadog]

Originally Posted by Crumpp

Well there you go.

I am sure that did not magically happen either. I believe one of the other snippets talks about measures to prevent the cylinder head cracks that appeared during the test in the form of replacing existing parts with new ones that were not prone to cracking.

That probably paved the way for an increase to 5 minutes.

Either way, the notion of +12lbs being using continuously is pure fantasy.

Believing that pilots routinely violate published limits is gamer thinking and not grounded in reality.

Ahh, no the trials were not successful. They pointed the way to modifications which allowed +12 to be used for 5 minutes just like the previous Emergency rating of +9lbs. In comparison most endurance trial engines have tolerance issues or things like worn supercharger blades. They don't have cracked cylinder heads like in the case of the Merlin trials.



Yes...Normal being the life of the engine in comparison to using +9lbs of boost for 5 minutes. It is not the gamer notion that 12lbs could be used continuously. It is just as the FC directs, when used, it must be dead-lined and cleared by a mechanic for return to service.

First off, I don't think it necessary to have the same discussion on two threads.

I pointed out to you that the gycol gasket leak reported at 49.5 hours of the 50 hr test was unrelated to 12lb/3000rpm operation as the memo states.

11. The use, in an emergency, of this high boost pressure is a definite overload condition on Merlin engine and therefore all occasions on which it is essential to make use of the + 12 lb must be reported by the pilot and recorded in the engine log book so that the engineer officer may be able to assess the reduction in life between overhauls and the need for special inspections.
http://www.wwiiaircraftperformance.org/ap1590b.jpg

Again this does not state a requirement for automatic inspection or grounding of aircraft until the engine is inspected.

Dowding's memo states otherwise, and he states "...that some pilots were pulling the plug with little excuse on every occasion..."
http://www.spitfireperformance.com/dowding.pdf

An engine was flight tested for 49.5 hours of which 8.5 hours were at 12lb boost/3000rpm and the engine life was found to be little effected. I think that you might be having trouble translating some of the documents,
http://www.wwiiaircraftperformance.o...bs-14nov39.jpg
as there is nothing in the above to indicate excessive wear resulting from 12lb/3000rpm operation. The mods required for 12lb operation:
http://www.wwiiaircraftperformance.org/ap1590b.jpg
does mention the need for modified cylinder top joints, but then by definition these changes were all made prior to having the boost cutout modded for 12lb operation and the 14 nov 39 memo states that cylinder head problems were unrelated to 12lb operation, but I guess it was felt that more reliable cylinder head gaskets were a desirable feature in combat aircraft and the Merlin in Perspective discusses the problem with coolant leakage in early Merlins and the design changes implemented to correct it. Rapidly changing cylinder head temps were likely to cause leakage problems but by definition prolonged running at 12lb will result in a stable cylinder head temp.

[TomcatViP]

Originally Posted by Seadog

First off, I don't think it necessary to have the same discussion on two threads.

I pointed out to you that the gycol gasket leak reported at 49.5 hours of the 50 hr test was unrelated to 12lb/3000rpm operation as the memo states.




Again this does not state a requirement for automatic inspection or grounding of aircraft until the engine is inspected.

Dowding's memo states otherwise, and he states "...that some pilots were pulling the plug with little excuse on every occasion..."
http://www.spitfireperformance.com/dowding.pdf

An engine was flight tested for 49.5 hours of which 8.5 hours were at 12lb boost/3000rpm and the engine life was found to be little effected. I think that you might be having trouble translating some of the documents,
http://www.wwiiaircraftperformance.o...bs-14nov39.jpg
as there is nothing in the above to indicate excessive wear resulting from 12lb/3000rpm operation. The mods required for 12lb operation:
http://www.wwiiaircraftperformance.org/ap1590b.jpg
does mention the need for modified cylinder top joints, but then by definition these changes were all made prior to having the boost cutout modded for 12lb operation and the 14 nov 39 memo states that cylinder head problems were unrelated to 12lb operation, but I guess it was felt that more reliable cylinder head gaskets were a desirable feature in combat aircraft and the Merlin in Perspective discusses the problem with coolant leakage in early Merlins and the design changes implemented to correct it. Rapidly changing cylinder head temps were likely to cause leakage problems but by definition prolonged running at 12lb will result in a stable cylinder head temp.

And what did the Pope eat that day ?