Reliability of WW2 fighters.

Crumpp said:

Over a fixed and finite time period such as determining the reliability of WWII aircraft engines it is impossible to only consider mature designs.

Hence my statement that MTBUM has little bearing in the context of comparing WWII engines.

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No problem here. This is what I agreed.

An engine with a 1500hour TBO has a shorter MTBF than an engine with a 2000hour TBO.

A 2000hour TBO is a 2000hour TBO whether it is on a 4 cylinder engine or a 28 cylinder radial. FYI a Lycoming IO-360 4 Cylinder air-cooled engine, PW R-2800 series, and the BMW801 series all have a TBO of 2000hours. The MTBF of these engines is comparable and the TBO is a function of that MTBF.

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No problem here.

Certainly the more complex engines designer worked to increase the MTBF and that workload was higher than the team that worked on a simpler engine.

The complexity of the engine is irrelevant if both engines last 2000hours and we are looking to compare reliability.

If we want to compare reliability as a function of complexity then your point is valid. The R-2800 required much more development time than the Lycoming IO-360.

If the engine we are comparing are reliable and shows little or consistent wear, then the Overhaul Man-Hours will remain consistent as well.

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All of this is fine.

If the engine is not reliable, then we will see a large reduction in overhaul man-hours as that reliability is increased by the designers as well as end user's input.

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I am not sure of what this says. But this is my main point.

Say that both the R-2800 and the IO-360 (hypothethically, since I do not know what the TBO of either is), has identical TBO, say 2000 hrs. This would imply that both have similar reliability. However, I am quite sure that the manhours to rebuild is much higher for the highly complex R-2800 than the manhours for the simpler IO-360. In other words, manhours to rebuild is a good reflection on complexity, but a poor reflection of reliability.

Even similar types of engines can have the same reliability but one can be more maintainability friendly and thus have less maintenance manhours to rebuild. I am sure you have seen designs in aircraft and cars that were not maintenance friendly, and, some that were.

(hypothethically, since I do not know what the TBO of either is),

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The TBO for both engines is 2000hours, davparir.

However, I am quite sure that the manhours to rebuild is much higher for the highly complex R-2800 than the manhours for the simpler IO-360.

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True. The much simpler IO-360 requires less man-hours to overhaul.

In other words, manhours to rebuild is a poor reflection of reliability.

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I have edited your sentence. This is not a true statement. Manhours is a good reflection of reliability.

I have explained why already. The problem is in how you are viewing the data I believe and what you expect to see from the data.

You cannot look at the specifics but rather examine the trends. The data is excellent for trend development because it represents the extent of wear under normal usage of the engine as a function of it's MTBF. It's not tainted by combat or prototypical testing.

To use this data for example:

If both our engines are reliable, then the difference in man-hours will remain constant. If one engine is not reliable, then the trend will be our average man-hours decreases as our engine becomes more reliable during it's developmental lifecycle. This is a function of both the end user's and design teams working to overcome problems and improve the engine. The reliable power plant will remain generally fixed in average man-hours to overhaul. It's already reliable within the constraints of physics.

Understand?

Now when you get to a certain level, the complexity of the engine is academic and it becomes a matter of just picking your poison. There is little to choose in the complexity of a V-1650 and R-2800.

We can examine the data and see that the R-2800 experienced a ~286% reduction in overhaul man-hours during the war and the V-1650 experienced a ~170% reduction in overhaul man-hours.

That tells me that the R-2800 started out as considerably less reliable a power plant than the V-1650 series.

We can also conclude by examining the overhaul man-hours that by the end of the war, the R-2800 had developed into at least an equally reliable power plant as the V-1650. It probably reached the extent of its reliability within the constraints of physics.

All the best,

Crumpp

I may be off track here but from what I've gleaned from various sources is that after 200hours of combat flying they rotated the Spits back to OTUs or moved them to less strenuous duties

Crumpp said:

You cannot look at the specifics but rather examine the trends. The data is excellent for trend development because it represents the extent of wear under normal usage of the engine as a function of it's MTBF. It's not tainted by combat or prototypical testing.


Understand?

Crumpp

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Aaaah, I see!

Let me make an editorial change to your editorial change to my comment that I think correctly reflects your position,

"Manhours trends are a good reflection of reliability."

I agree with this! Good observation. 8)

pbfoot said:

I may be off track here but from what I've gleaned from various sources is that after 200hours of combat flying they rotated the Spits back to OTUs or moved them to less strenuous duties

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That does suprise me. The Mustangs in the 8th AF generally trended to 500 airframe hours before being declared War Weary and assigned to OTU or in Group Clobber Colleges. From memory the highest time P-51B I found for that category in the 355th was 640 hours and it flew steadily from March 1944 through October 1944 before declared WW.

The A/B/C/D/K Mustangs were designed to 8g limit and 12g ultimate which I believe was higher than the Spits. The P-51H was designed to 7 1/2 and 11.25 to reflect more to Brit standards to conserve weight.

I suspect the lower limit load envelope may have something to do with the lower airframe hours?

I agree with this! Good observation.

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Thank you.

I suspect the lower limit load envelope may have something to do with the lower airframe hours?

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It does, Bill. The lower load limits means the airframe is experiencing more wear and tear in normal flight. Gust and density effects of the atmosphere cause the normal "bumpiness" of the air to exert more relative stresses on the lower limitations of the airframe. This in turn causes increased wear.

I suspect the RAF's decision to remove the Spitfire from the line at 200hours has more to do with Logistical system of the RAF than airframe design. The CRO was a major source of repair for RAF airframes. This loose organization did an outstanding job and was a key factor in the RAF victory in the Battle of Britain. Being a loose organization of civilian repair shops the organizational answer to the slight loss of control on repair quality control would be to reduce the airframes frontline service time.

I imagine too this removal was nothing more than an airframe overhaul to serviceable standards with placement right back to service.

It is a possibility that the aero elasticity of the thin wing of the design affected the airframe limits as well but I would tend to be skeptical of such major departures from aircraft norms until I saw definitive proof.

All the best,

Crumpp

Never particularly understood these comparisons of prescribed overhaul periods, usually put forward to claim superior quality of some sort.

It`s odd, because generally there was not too much difference between the 'lifespan' (or really, the amount of time between major inspections) of contemporary engines and airframes, and statistically, airframes and engines were lost or damaged in combat well before being worn out would become a concern..

It`s odd, because generally there was not too much difference between the 'lifespan' (or really, the amount of time between major inspections) of contemporary engines and airframes,

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Exactly. It's a function of the physics of aircraft.