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

[ICDP]

Originally Posted by Seadog

Fighters, by definition cannot run for more than about 40 - 50 minutes at 12lb boost/3000rpm because of fuel limitations, and yet if a fighter could only be flown for a few moments at 12lb/3000rpm then there would be no need to point out the increased fuel consumption at 12lb/3000rpm (see point 4 in AP1590G above at 12:27PM)

Cooling on the ground and cooling in the air are two separate issues. This is a test of a Spit V at the normal and combat ratings, and the plane was flown repeatedly to its service ceiling at 16lb boost:
http://www.spitfireperformance.com/aa878.html
and the aircraft was specifically tested for cooling suitability at 16lb boost and was found acceptable for English summer conditions and this required considerably more cooling than at 12lb.

I am not saying the engines should suffer from major overheating issues if pushed for a few minutes but they should and did overheat if pushed to high for too long. The Spitfire Mk Vc had a larger radiator and had better cooling than a Mk I and Mk II, so while it is very relevant it is not a perfect example. I know these engines could take more punishment than just a few minutes at full power, but they weren't immune to breaking if pushed. Definately more punishement than a mere admin warning. I also know that cooling on the ground and cooling in flight behave differently, I am just pointing out the fact that CoD does not model overheating on the ground very well. I can sit indefinately with the radiator open on the ground at 1200 rpm, something that couldn't be done in real life in a Mk I Spitfire.

Originally Posted by Seadog

I don't have any "agenda" other than historical accuracy. Every Hurricane and Spitfire in RAFFC could use 12lb boost, and there was no 'magic' limit after which the engine caught fire or blew up or seized up. Using 12lb boost simply increased the wear and tear on the engines and probably kept the ground crews up late at night doing engine checks but this probably caused little concern to pilots whose lives and/or aircraft were saved or who ensured kills by "pulling the plug" and going all out. It is quite telling that the Merlin III could be modded to accept 16lb boost on the Sea Hurricane I with essentially identical engines and cooling as per BofB Hurricanes.

I know these planes ran at +12lb boost, I have no problem with that. My only issue is with the fact that you seem to be advocating they should run without problems at +12lbs boost until the fuel ran out. ALL of the scenarios you refer to would not require +12lbs boost for more than a few minutes. In reality it would be unlikey that someone would follow your full power dive for more than a few minutes. I know if I was in a Spitfire Mk Ia in 1940 I would not dive after a 109E in a full power dive for very long, especially considering that in a few short minutes I would most likely be flying over France at low altitude with an engine starting to run hot.

In the Mk Vc full power test you linked to the radiator temperature reached 124 C at 12.000ft after 3 minutes, OAT was +5 C. This is already 4 C higher than the recommended maximum, at 130 C the Merlin had to start venting to reduce coolant pressure (small hole on right side of engine cowling just behind the prop). If this test had been done at lower altitude in the hotter air then it would almost certainly have overheated without pilot intervention. It was easier to keep these planes cool at 12,000 or 24,000 ft than it was at SL.

Originally Posted by Seadog

If COD is going to be an accurate simulation then it has to allow 12lb boost at the pilot's discretion, and then factor in some kind of admin penalty for using it without justification.

See my response above. I do not want unrealistically hard overheating modelling for any of the aircraft. What I do want is the fact that sometimes if you push too hard for too long that there MAY be repurcussions that go beyond an admin reprimand. Apart form the ground cooling issues CoD seems to do quite well in this regard.

[Seadog]

An aircraft might fail if run too long at 12lb/3000rpm, but then again it might not, and failure is not an automatic consequence, and in fact unless the engine has suffered battle damage it probably won't fail if run until fuel exhaustion, but of course running out of fuel is likely to ruin your day, in any event.

Regarding the Spit Vc test:

.A.A.878 is fitted with a temperate type cooling system. The engine oil inlet and coolant outlet temperatures were measured on two climbs to 24,000 ft. This takes considerably longer than the 3 minutes for which the combat power may be used, a concession for test purposes. The observed temperatures are therefore likely to be higher than would normally be obtained in a 3 minute period at the same outside temperature. Even so, the oil inlet temperature is within requirements (100 deg C) for temperate summer conditions and does not exceed the emergency maximum temperature of 105 deg.C when corrected to tropical summer conditions.

The observed oil inlet temperature has been corrected to temperate summer and tropical summer conditions of A.D.M.491 by adding 70% of the difference between the appropriate standard and the observed air temperature.



.......Under tropical summer conditions, the oil inlet temperature exceeded the maximum permissible (100 oC) above 15,000 ft. and rose to 105 oC at 22,000 ft., and remained at this figure to 24,000 ft. with no tendency for the temperature to fall. The temperature exceeded 100oC for about 4 minutes.

You will note that the test made allowances for increased temps during English and Tropical (IE Egypt) summers.

Dowding did take his pilots to task for probably abusing the use of 12lb boost:
http://www.spitfireperformance.com/dowding.pdf
but this was only one facet of abuse that could lead to engine failure, as Dowding's memo points out, but RAFFC had lots of spare aircraft and engines, and I doubt that pilots felt that they were using 12lb boost unnecessarily.

[ICDP]

Originally Posted by Seadog

An aircraft might fail if run too long at 12lb/3000rpm, but then again it might not, and failure is not an automatic consequence, and in fact unless the engine has suffered battle damage it probably won't fail if run until fuel exhaustion, but of course running out of fuel is likely to ruin your day, in any event.

Exactly, pushing too far for too long COULD lead to consequences a pilot would not like to face. That is what I want modelled, not some arbitrary limit of say 5 minutes = bang, conversely I do not want it modelled that I can run at +12lbs boost constantly without some potential consequneces. Your whole point seems to be that a Merlin could run at +12lbs boost until the fuel ran out. No pilot in real life WOULD EVER do such a thing from take-off until landing.

Originally Posted by Seadog

Regarding the Spit Vc test:

You will note that the test made allowances for increased temps during English and Tropical (IE Egypt) summers.

Read what I said again. My point was that at 12,000ft or 24,000ft it is easier to keep an engine cool than at SL. Had the test been done at SL the temperatures would have been much higher. Or to put it another way, flying full power +12lbs boost without taking circumstances and consequences into account is not possible. It is silly to assume there would be no potential issues running +12lbs boost until the fuel runs out.

Originally Posted by Seadog

Dowding did take his pilots to task for probably abusing the use of 12lb boost:
http://www.spitfireperformance.com/dowding.pdf
but this was only one facet of abuse that could lead to engine failure, as Dowding's memo points out, but RAFFC had lots of spare aircraft and engines, and I doubt that pilots felt that they were using 12lb boost unnecessarily.

OK, where did I say we shouldn't be allowed to use +12lbs boost? Also what use is a spare engine sitting in a workshop when you are 12,000ft over enemy territory? Like it or not the pilot notes have engine limitations for a reason, yes they may be conservative numbers but it doesn't mean they could be ignored.

These engines required carefull attention, not constant but they certainly weren't balls to the wall and forget about the consequences.

[TomcatViP]

Give them back their Starwar cruise ship and let's end this discussion. They won't give up until they got back their 25lb spit. Years of stupid mods have to get a justification.

[Viper2000]

Originally Posted by ICDP

Read what I said again. My point was that at 12,000ft or 24,000ft it is easier to keep an engine cool than at SL. Had the test been done at SL the temperatures would have been much higher.

This doesn't follow. The air temperature is lower, but so too is the air density.

The worst case for cooling is generally likely to be minimum speed at FTH.

Above FTH the power decreases in proportion to air density, but the problem is reduced because:

• FTH is likely to be <11 km and so the air temperature continues to decrease during the climb.
• minimum TAS will be higher, and mean TAS will also be higher because airframe drag is less.

BTW, the last time I tested the Spitfire II it was just wrong, and if anything was faster than it should be with a Merlin XII at +12, with the wrong FTH.

Speed is a pretty meaningless metric IMO. It's much better to plot a TAS vs altitude graph and then take the error to be the area of the gap between the lines of real test data and sim data. Of course, this should be an absolute figure, because +ve and -ve errors obviously don't cancel each other out.

In the end, this thread wasn't intended to be an argument about +12 boost.

It was intended to point out that given that we have 87 octane boost indications, we should have 87 octane boost cutout behaviour, which was very different from 100 octane boost cutout behaviour. IMO it's worth simulating this simply because it's interesting.

Obviously we should have 100 octane fuel and +12 boost, but that's for another thread (indeed it's pretty sad that such a thread should be needed at all, but obviously it is).

*edited to remove engine wear stuff to Kurfürst's thread*