Quote:
Originally Posted by Blackdog_kt
The mustang's 67" of MP is equivalent to the Spits +12lbs, the 109's 1.45 Ata and so on and so forth, take your pick, in the sense that they are not meant to be ran for eternity because things will start to break. Maybe not on this sortie or the next, but definitely something will give after a few missions, especially if i push it that way on every single sortie and the mechanics follow your reasoning of not inspecting it afterwords 
As another example, for later mark Spitifires like the Mk.IX it was advised to take off with a mere +9lbs no matter if it could do +12, +16 or +25 and that's a pretty critical phase of flight in terms of power reserves in case something goes wrong.
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The Spit 9 was approved for 12, or 18lb boost, on TO depending on the variant:
https://lh6.googleusercontent.com/-U...ngine_data.jpg
The Merlin 61 and 63 were approved for 12lb on T0 and 5MIN at 15.25lb for Combat.
The Merlin 66 was approved for 18lb for TO and 5min at 18lb for combat.
The V-1650-3 and -7 used on the P51-B, C and D was approved for 15.25lb (61") on TO and was basically equivalent to the Merlin 63 but had a special WEP rating of 18lb for 5min, not present on the Merlin 63, so the V-1650 -3,-7 were running beyond the equivalent of 12lb on a Merlin III.
If you try and use 18lb boost on TO, for example, you will end up with major engine torque issues that will make the whole process extremely dangerous, while 9lb will get the Spit off the ground in a very short space and was a lot safer.
Regarding 12lb/3000rpm, I made a solid proposal which is based upon operational data:
Quote:
I have read through every source on the Merlin engine that I have, and all the combat reports at:
http://www.wwiiaircraftperformance.org/
From what I can gather, Merlin engine failures, were primarily random events and the main culprit was manufacturing defects/design faults that eventually break the engine. The 50 hour 12lb/3000rpm test is an example of this, where the engine was cycled 100 times at 5min/20min at 12/4.5lb boost and eventually developed a coolant leak from a defect that plagued service engines that were not being run past 6.25lb.
The Merlin in Perspective states that fighters had a higher propensity for coolant leaks than bombers because fighters were cycling engine power from very low to very high much more frequently, but this was still not a common occurrence.
The next greatest problem was bearing failure from oil starvation, and again 12lb boost had little to do with this except for prolonged steep climbs, as per Dowding's memo, but probably the greatest cause was inverted flying and prolonged dives that caused excessive (~3600) RPM.
1939 Merlin TBO:
Fighters: 240 hrs
Bombers: 300 hrs
repair depots:
1942 onward: 35% of engines were there due to time expiry.
1942 onward: average engine under repair had 60% of nominal life, or 144 hrs for a fighter engine and 180 hrs for a bomber engine.
I would propose the following:
Any engine has a 65% probability of random major engine failure, during 240 hrs of operation, or about 160 sorties. Another way to express that would be a 6.5% probability of one aircraft out of 16 having major engine failure on a typical mission. I don't know how to model the use of 12lb/3000 rpm for more than 5mins, but a simple way would be be multiply the failure probability by, say 1.15, to simulate the increased RPM and stress on the engine.
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