Modelling engine wear and WEP limitations

[Seadog]

Originally Posted by ICDP

I don't entirely agree with his stance, nor do I agree with yours that an engineer could just shrug his shoulders and say don't worry about it. ALL engines regardless of what boost was used were inspected at the very least at the end of the days flying and preferably at the end of each flight timing permitted. Using +12lbs boost was by your own admission enough to warrant an inspection at the earliest possible convenience. The earliest possible convenience would be that very evening in the worst case scenario. When doing the routine maintenance at the end of each evening the engineer consults the logbooks of all aircraft flown that day. He sees that this particular aircraft has an entry that the pilot used +12lbs boost. He knows from his orders (Straight from Dowding) that it is SOP to do a more thorough check for wear and tear. Do you now dispute this?

First off, Dowding's memo states that oil filter checks were mandatory, "when convenient" for aircraft that exceeded 5mins at 12lb/3000rpm:

Let's be very clear on this point.:

Originally Posted by ICDP

Ah the bias starts to show. What you want is some "click here = win" button for the RAF? +12lbs boost gave a boost in performance, it was not a massive advantage that guaranteed victory. Even with +12lbs boost the difference in performance between a Spitfire, 109E and even a Hurricane were close enough that surprise, tactics and pilot skill was the determining factor in the outcome of any engagement.

No, want I want is historical accuracy. The Luftwaffe and RAF were locked in a constant technology battle that eventually saw the Luftwaffe field the Fw-190 when the RAF still had the Spit V and then the Me-262...but in 1940 the use of 100 octane fuel gave RAFFC greater power output at medium and low altitudes, when needed. This was the historical situation and I want the sim to reflect it.

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.

[Crumpp]

First off, Dowding's memo states

Who cares? The published Operating Instructions and Technical Orders for the Spitfire are clear. Dowding was not the technical expert on Rolls Royce engines. He had people to fill that role for him and he followed their advice.

It is a fact, ANY use of +12lbs in the Merlin engines requires a log book entry and a mechanics inspection before the engine is returned to service.

You seem to think that having the engine inspected after the extreme stress of over boosting is uncommon.

Almost every fighter aircraft engine in WWII had to have it done.

BMW, Diamler, Rolls Royce, Allison, and just about every else required it. It only makes practical sense.

Even OUR P51D Merlin had the same instructions:



I think you just want a magic win button for your game.

[Seadog]

Originally Posted by Crumpp

Who cares? The published Operating Instructions and Technical Orders for the Spitfire are clear.

They certainly are clear and Dowding's memo leaves no room for doubt.

Originally Posted by Crumpp

Dowding was not the technical expert on Rolls Royce engines. He had people to fill that role for him and he followed their advice.

He certainly did, which is why the memo is worded the way it is.

Originally Posted by Crumpp

It is a fact, ANY use of +12lbs in the Merlin engines requires a log book entry and a mechanics inspection before the engine is returned to service.

You seem to think that having the engine inspected after the extreme stress of over boosting is uncommon.

Almost every fighter aircraft engine in WWII had to have it done.

BMW, Diamler, Rolls Royce, Allison, and just about every else required it. It only makes practical sense.

Even OUR P51D Merlin had the same instructions:



I think you just want a magic win button for your game.

War emergency power on a Mustang!!!? The Mustang/Merlin 60 series was not the same as 12lb boost on a Merlin III:


67 inHg = +18 lbf/in² boost
61 inHg = +15 lbf/in² boost
46 inHg = +8 lbf/in² boost
44.5 inHg = +6 lbf/in² boost

so they are talking about pulling 18lb boost and 1700hp from a Merlin Engine with a two stage SC, or about 30% more power than a Merlin III at 12lb boost.

The Normal full throttle setting on the Mustang is 61" boost or 15lb boost and at that setting no extra inspection is required.

The Merlin XII was cleared for 12lb boost on T/O, so this was simply normal operations for a Spit II

[Blackdog_kt]

All i know is that takeoff power or emergency power is called that way for a reason: it is to be reduced to lower levels under all other regimes of flight.
Otherwise they would just call it full power and be done with it.

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.

If they didn't slam the throttle to the stops on takeoff that's telling me that full power at low airspeeds was a combination for insufficient cooling, overheat and eventual engine seizure if the oil dissolved. True, this is for later mark Spits with a higher power output, but these also had an extra radiator to help with cooling which our in-game early Spits lack, so it's more or less a trade-off.

All that is enough explanation for me to convince me that operating limits are there for a reason. I want a difficulty setting that imposes penalties if i exceed them, that's all. If you don't like it, feel free not to use it.

However, the majority of people in the community won't stand for implementing changes to the FM/DM that have all other aircraft adhering to some kind of limits while the Spitfires suffer none and it's not even for balancing reasons. It's because it's common sense to assume that emergency and takeoff power are named that way for a reason.

Can we please get back on the topic of how such a game mechanic/feature could be implemented? If you want to continue debating if +12lbs classified as emergency power, feel free to start a different thread about it, you're just being off topic in this one:

The current thread is not about "what can the Merlin run with impunity?". The topic is "how do we punish the player that exceeds what the engine can reasonably run, if he chooses to enable the relevant difficulty/realism settings". It's about ALL engines, not just the Merlin.

[Crumpp]

All i know is that takeoff power or emergency power is called that way for a reason: it is to be reduced to lower levels under all other regimes of flight.
Otherwise they would just call it full power and be done with it.

Exactly.

how such a game mechanic/feature could be implemented?

The most realistic thing your game could do to simulate it is just have the engine start losing power gradually.

The most common sign of engine wear in RL is inability to develop full rpm and loss of compressions.

This is not a dramatic event and is why compression checks are done at annual at a minimum. A compression check would be in order after running an engine at Emergency power.

The longer a player left the "emergency rating" engaged beyond Operating limits, the more rpm and compression he would lose.

My 2 cents...

Here is what Lycoming's Key Reprints have to say about following the Operating Procedures in their engines:

Pilot technique is another factor affecting engine life. Following manufacturer recommendations could be categorized as good operation. On the other hand, complete disregard for these recommendations could conceivably cause engine destruction in as little as 100 hours. Although this would be extreme and unusual, pilots who regularly climb at steep angles, make abrupt throttle changes, improperly lean the engine in climb, exceed maximum specified manifold pressure and/or RPM, chop throttles abruptly and let down rapidly causing rapid contraction of metals that have been up to operating temperatures are using techniques that may shorten TBO.

http://www.lycoming.com/support/tips...%20General.pdf

[Crumpp]

They certainly are clear and Dowding's memo leaves no room for doubt.

Then you should find those same instructions in the Operating Instructions.

However that is not the case.


The Operating Instructions are clear. The pilot must log the event, the mechanic must inspect the engine and return it service.

[Seadog]

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.



[/b]

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:

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.

[TomcatViP]

One thing that I wld like to see is a parameter including the way player move the throttle fwd and back to simulate the damaging effect of walling the throttle (just like it was on initial release ?)

[Crumpp]

One thing that I wld like to see is a parameter including the way player move the throttle fwd and back to simulate the damaging effect of walling the throttle (just like it was on initial release ?)

That is good idea to reward game players who operate their virtual machine correctly.

Real pilots follow published procedures because they understand the engineering margins are narrow and the procedure is in place to get the best performance from your engine. The procedures are there for a reason, to keep your engine developing the maximum power it can deliver.

In the real world, the penalty for abusing your engine is reduced power and an increased risk of catastrophic failure. It should be the same in your game.

You start out with a given potential for maximum power and as you abuse the engine, that power potential is reduced. By the time you finish slamming the throttle, improper propeller use, mixture control use, supercharging gearing misuse, not adhering to temperature rates, and running the motor over-boosted, you could end up with a 900 hp engine at the end of the sortie. No funny noises, just a gradual loss of power. That means your performance will noticeably degrade as you abuse the engine. That loss would effect the airplanes combat performance.

It is not realistic to consider hours flown or previous abuse. It is a game and cannot simulate such things. You get one airplane and at the beginning of the sortie it is new. Depending on how you treat the engine you could land with the same power potential or you could land a worn out aircraft that is no longer combat effective.

This would have to be clearly published in the game manual along with "proper operating procedures" or the whining would be legendary.

Such a system would reward the players who understand their aircraft and can integrate proper operations with proper tactics. You would have a much better simulation of WWII air combat.

[TomcatViP]

+1

Great! Now who wld make a FM model resulting from a kick in the a$$ from an angry Chief mechanics ?!