Merlin vs. DB601

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cimmex said:
Here I disagree. Only the lower Prop axis of an inverted V-engine allowed the use of an annual radiator. I don't know any plane with an upright V-engine which had an annual radiator
cimmex
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That is not the case at all.

In the case of both the upright and inverted vees, at least of WW2 vintage, the prop drive tended to be offset to the centre of the frontal area of the engine. There is nothing precluding the use of an annular radiator with an upright V-12.

The V-12s with annular radiators in the 1920s didn't, in reality have annular radiators - they had car type radiators mounted ahead of the engine with the prop drive through the radiator.
 

Bingo exactly what I am trying to get to only you put it more succinctly. I couldnt care less if a Merlin had more parts than a Space Shuttle and the Allison and DB had the same number of parts as a clockwork toy.
 
wuzak said:
The V-12s with annular radiators in the 1920s didn't, in reality have annular radiators - they had car type radiators mounted ahead of the engine with the prop drive through the radiator.
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What is the definition of an annular radiator does it have to be a circular radiator wrapped round the propshaft.
 
If you don't see the relationship between parts count and ease of maintenance, then you've never taken one apart and put it back together. 4,000 more parts take a LONG time to install or remove, and there is a LOT more opportunity to drop one inside an engine. That requirs even MORE time to disassemble it and find the part if it goes inside the engine.

There is a strong relationship between number of MOVING parts and relaibility. More moving parts means more parts to fail, simple math of reliability engineering. Parts count matters in reliability and MTBF.

I do not have an estimate of the number of MOVING parts in an Allison versus a Merlin, but I KNOW from being inside both the Allison has fewer moving parts.

When properly assembled, tuned and operated, both engines give excellent service, with the Merlin requiring more maintenance. For instance, you have to torque the Merlin cylinder liners every 25 hours or so while the Allison cylinder liners must be torqued once when being assembled and are never touched until disassembly after that. The Merlin method of torqueing in the valve seats is just plain stupid and involves turning it in until the new seat shank snaps off. There is some minimum torque, but no defined torque. That they used a method that produces no defined torque value for the seat does NOT make the Merlin a bad engine, it means there no defined torque for the valve seats, that's all. In operation, it doesn't seem to make much difference, but most good engineers don't design that way.
 
More moving parts?

Which would those be Greg?

Clutches for the multiple speed supercharger drive? Not needed on the Allison since it didn't have a multi-speed drive.
Crankshaft torsional vibration damper? Well, the Merlin didn't have one, but the Allison did.
Roller cam followers? Again, featured on the Allison, and not the Merlin.

Do you count retaining bolts as moving parts?
 
fastmongrel said:
What is the definition of an annular radiator does it have to be a circular radiator wrapped round the propshaft.
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Well, yes.

It has to be in the form of an annulus.

"In mathematics, an annulus (the Latin word for "little ring", with plural annuli) is a ring-shaped object, especially a region bounded by two concentric circles. The adjectival form is annular (as in annular eclipse)."

Annulus (mathematics) - Wikipedia, the free encyclopedia
 
cimmex said:
never heard, what type?
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Well, there was this: Verville VCP - Wikipedia, the free encyclopedia and this: Curtiss XP-22 - fighter

This one was British: Bristol Scout E / F

It was also pretty common for aircraft with Liberty V-12s to have radiators in front of the engine, with the propshaft passing through the radiator. This is, of course, topologically identical to an annular one

Outside of Germany, annular radiators were not particularly popular. I suspect this was because they were viewed as equally as draggy as a radial installation, because they didn't permit the nice pointy nose that a V-12 with a radiator under the wing permits. Even in Germany, it seems to have had relatively few proponents -- Focke Wulf (or Kurt Tank at FW), Junkers, and Kurt Tank. I don't think the annular radiator is a bad idea, as it permits nice, short coolant lines, but I don't see it has any other particular benefits.
 
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Hi Wuzak,

Bolts, nuts, and washers that are attached to moving parts (think of bolts, washer, and nuts on a crankshaft) ARE moving parts. They are subject to more vibration and stress than non-moving parts which, by defintion, may be moving through the sky, but are NOT moving relative to the crankcase (think of engine mount bolts).
 
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Merlin used 6 bolts per pair of connected rods. One pair on the center rod end cap and one pair on each of the forked rod's end caps. Pretty much the same as the Allison?

The Merlin may very well have a few dozen (or even a few hundred) more moving parts than the Allison, that is a far cry from the 3-4,000 "total" number of parts.

I won't argue that the Allison was easier to take apart or put back together, shear number of parts argues otherwise. But to argue that the "number" of parts affected reliability or durability to any great extent leaves out too many other variables to be taken seriously. Maybe the number did parts did and maybe it didn't but since a number of the parts were NOT made of exactly the same materials, or heat treated/processed the exact same, or in some cases perhaps not "finished" the same (surface finish or polish prevented the start of fatigue cracks, how much polish was enough?) the variables are too great to dismiss.
 
No they aren't SR. 4,000 more parts take a long time to both remove and reinstall. The time to complete an overhaul HAS to be greater for a Merlin, even if everything else was equal, just due to that many parts.

Military TBO may have been similar, but use in peacetime shows the Allison to last quite a bit longer than a Merlin. Of course, in military service the engines were overhauled at very similar intervals, so it didn't make that much difference at that time. It doesn't help if the engine life was 1,000 hours but you overhauled it at 400 hours anyway.

Rolls Royce generally uses more parts to do the same thing that Allison does or, perhaps from another point of view, Allison generally uses less parts to do what Rolls-Royce does. Go watch a teardown and rebuild somewhere near you and you may well come away with the same opinion I have after having done that, including helping with the allison function.

This is not a "one is better than the other" argument here. It is just noting that one has many more parts than the other, and that is a fact, not a question. Wartime service generally required rebuild before wearout was reached anyway, so the difference in lifetimes was never very apparent until after military service.

Both are good engines.
 
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Obviously you don't see what I mean. Of course all V engines had a reduction gear which shifts the prop axis offset to the crank axis but at the upright V to the wrong direction. Imagine a radiator around the actual spinner position at a Mustang or a Spitfire and then try to imagine how the view over the nose would be.
cimmex
 

Gotta agree with cimmex; the reduction gears on an upright vee wouldn't allow a conventional annular radiator installation. There is a myth which says that annular radiators were used on the Avro Lincoln's Merlin 80 series "universal" power-plant installation, but these used semi-circular "chin" radiators. Looking at this installation, its clear that an annular radiator would restrict vision over the nose:

universal power | power plant | flight universal | 1947 | 0233 | Flight Archive

AFAIK the only annular radiator installations tested by the British were on Napier Sabres which, being of H-24 configuration, lent themselves to the configuration.

 

There are obviously many here who have some experience maintaining or servicing engines of one type or another. Even I have stripped and rebuilt a single cylinder Ducati engine (complete with 'desmodronic' valve system). Actually I gave up and took it to a nice man in Wisbech who did it for me but that's another story
Most engines can be 're-built' without being reduced to component parts. The total number of parts in an engine, down to the last fibre washer and circlip has nothing to do with its ease of maintenance. How many Merlins or Allisons were stripped to their thousands of individual components between leaving the factory and being loaded on a truck bound for the scap heap?
The way an engine is put together and also how it is installed are FAR more relevant than the total number of parts. This is something Davebender touched on a while ago.
The lengths someone might go to today to recover or restore an old aero engine would be very different to those that a service depot in WW2 might go to in order to fix something that was essentially a disposable item. Stripping for spares isn't reducing to components either.
Cheers
Steve
 
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In an inverted vee the prop shaft is shifted down relative to the crankshaft, which is at the top. In an upright vee the prop shaft is shifted up from the crankshaft, which is mounted at the bottom.

The annular radiator wouldn't know if it were an upright or an inverted V-12 behind it.

As regards to the Spitfire and Mustang, if they were to have an annular radiator the engine and thrust line would have to be moved down.

The view over the nose would be worse than the standard planes, but that is true of any V-12 behind an annular radiator - being round they are wider than the engine is by some margin.

An upright vee behind an annular radiator:





I believe that is an (upright) Allison V-1710 powering the Flugwerks Fw 190D replica.

You can also see that the annular radiator is much bigger than the engine. That is because it was fitted to an aircraft originally designed for a radial, and the point of using an annular radiator was to be able to bolt up either the radial or in-line engine module.
 
And when you think about it, if you take the inverted V-12/annular radiator module and rotate it 180° about the prop axis you have an upright V-12 with an annular radiator.
 
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