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

[ICDP]

Originally Posted by Seadog

Historically, if you were in a Merlin III powered fighter and you needed to use 12lb boost, you did so, and you damned the consequences because staying alive and/or destroying the enemy was more important than explaining away a broken engine. 12lb boost/3000rpm will not cause overheating in level flight. It might cause overheating in prolonged max angle of attack climbs as per Dowding's memo of Aug 1, 1940 but other combat manoeuvres were probably the prime culprit in causing increased engine wear leading to bearing failure, also as per Dowding's memo.

I must admit to being kind of lost on what your stance/opinion is. Now you are saying you want to be able to fly at +12 lbs boost becuase there was a war to win and that was more important than a broken engine. So you accept that flying around in combat at +12 lbs boost could wreck your engine, but that the pilots didn't care?

[Kurfürst]

Originally Posted by Seadog

12lb boost/3000rpm will not cause overheating in level flight.

I am curious on what do you base the above, especially as the engine's coolant system was designed for some 30% lower engine outputs..

[Viper2000]

Because +12 in level flight = fast, and the cooling system was designed for the max climb case. Flying twice as fast doubles the mass flow rate through the radiator, and thus the heat rejection capability at constant radiator matrix temperature.

If you've got enough radiator for the climb case, you've almost always got too much for high speed level flight - hence the need for variable geometry.

[Kurfürst]

Pity it doesn't work as such in CoD... I mean temp goes up like mad in a moment if you set the radiator flaps too narrow, almost regardless of aircraft speed. :/

[Viper2000]

The list of bugs/questionable model behaviours is quite a long one... As has been mentioned earlier, at the moment the cooling behaviour is also far too forgiving on the ground; you can idle a Spitfire for ages without boiling the coolant...

[Seadog]

Originally Posted by ICDP

I must admit to being kind of lost on what your stance/opinion is. Now you are saying you want to be able to fly at +12 lbs boost becuase there was a war to win and that was more important than a broken engine. So you accept that flying around in combat at +12 lbs boost could wreck your engine, but that the pilots didn't care?

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.

[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.

[ICDP]

Originally Posted by Viper2000

The list of bugs/questionable model behaviours is quite a long one... As has been mentioned earlier, at the moment the cooling behaviour is also far too forgiving on the ground; you can idle a Spitfire for ages without boiling the coolant...

Indeed it is far too forgiving on the ground. I just sat on the ground in a Spitfire Mk I idling at around 1200 RPM. If I left the radiator closed the temperature would slowly rise too 120 C, if I opened the radiator it would drop to 80 within seconds. It seems that the radiators have the exact same level of effectiveness regardless of airspeed or airflow.

[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.