JU 252 352

Shortround6 said:

Kind of depends on how much time you want to spend redesigning the whole thing. Trading three 1350hp engines that weigh around 13-1400lbs for two 22-2400hp engines that weigh around 2400lbs, shouldn't be too hard but then you done't get much, if any performance increase. You also have a bit of a problem if one quits.

Keeping 3 engines gives more performance but also increases the empty weight by well over 3000lbs. You aren't going to use Jumo 211 radiators to cool Ju 222 engines and you sure aren't going to use Jumo 211 propellers. If you use the extra power you are going to burn more fuel per mile/km. Payload is cut significantly. Now with the extra power you can certainly lift a bigger load but are the tires, landing gear, brakes up to it? Is the wing up to handling not only the extra weight but whatever extra speed you are using? and so on. Most passenger/cargo planes were built with much lower reserves of strength than Fighters or even bombers.

ANd do you really need a faster transport? If you go for the 66-7200hp installed power option is the plane now volume limited? Can lift more weight than the normal cargo items use up in volume?

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Wait a second. How are you going to lose range with the 222's, didn't they have good fuel efficiency, wouldn't the removal of drag in the nose increase performance?

Removing drag increases speed ... assuming you don't pass the flutter limit. But you DO have to know the drag decrease to quantify it. Since nobody DID one and documented it, the drag decrease would be a guess. There's not much point in calculating a guess unless it's just for fun since there's no way to verify it.

Koopernic said:

The DC3 flew and was in service by 1936, even the DC2 was an excellent aircraft. The best hope the Germans might have had was a He 111 with the fuselage physically widened out but using the same wings and empennage. similar to how the DC2 became the DC3. The great economies of production then make the transport viable and as the He 111 bomber was superseced production could shoft to the transport version. One then ends up with a conflict between He 111 bomber production and He 111 Transport production. The bomber would tend to win in the early part of the war. One might have to put radials on that He 111 Transport to spare Jumo 211 production.

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Given the development timing of the Fw 200, and its wide (3 seat row) fuselage design, having a smaller twin engined counterpart using a similar, but shorter fuselage and smaller wing and tail (like the Fw 206 later did) would have made sense to develop in parallel with the larger trans-atlantic airliner. (and far more likely to actually be profitable ... or close to it -let alone strategically useful as a mass produced military transport)

I suppose the same argument could be made for the Ju 90's development though. (perhaps more so given its greater passenger capacity than the Fw 200, though also possibly better in a shortened tri-motor arrangement rather than a twin)

But yes, the He 111 seems one of the better options as well, and in development earlier than either of the above. Its design is closer to the Lockheed Model 10 or Model 14 than the DC-3, but it also had a much larger wing area and possibly more potential fuel capacity and perhaps even more potential than the Model 14 to be stretched/expanded as the case with the Lodestar. (granted, the Passenger He 111 only had the capacity of the Model 10 as it was, so it could use all the expansion it could get)

As far as engines go, the Jumo 211 has an advantage in drag and specific fuel consumption compared to the BMW and (especially) Bramo radials along with significantly more take-off power than the former. But given the number of designs (particularly transports) that resorted to using Bramo 323s, that seems the likely alternative. (especially once the 2-speed supercharged models are available in number -let alone C-2/C-3 rated ones or MW/50 equipped models)

Yes but who was thinking of resupplying whole divisions and even armies enough so that they would write a spec. There were some superbly thought out airliners planned (they often appear in Luft46 style aircraft) but they would have started production in the war years. Aint going to happen. Even the Ar 232 was effectively cancelled.

Any transport would need to be in production by 1937 else it comes into conflict with military priorities I would think.

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Larger numbers of Ju 90s and Fw 200s built used as transports wouldn't have hurt either, but more cost effective 2 or 3 engine transports should have been the priority.

If you don't know the real payload, you can't do a very good comparison. If you do, it comes up laughably wrong.

SpicyJuan11 said:

Wait a second. How are you going to lose range with the 222's, didn't they have good fuel efficiency, wouldn't the removal of drag in the nose increase performance?

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The 222's didn't have miracle efficiency. And if you are turning the Ju 252 into a twin lets look at it. The three Jumo 211s had 4050hp. two Jumo 222's were supposed to have 5000hp. That looks pretty good but unless the 222s are a miracle of efficiency (like 20-25% more power per unit of fuel burned) if you use more than 4000hp you will burn more fuel per minute or hour. The twin engine installation isn't any lighter. Two bare Jumo 222s weigh about 2200kg. Three 211Fs weigh ??
Now lets say we have the plane off the ground and have throttled back a bit (long climb?) to 900 hp each for the 211s (2700hp), yes the 222s can match that easily but they want to climb faster they will use more fuel. If they climbe using 1350hp apiece they need enough airflow through the radiators and oil coolers to matchthe power output, basically they need the same amount of air ( and roughly the same cooling drag) as the 3 smaller engines.

BTW Jumo 211s could get around 0.45 lbs per HP hour for a fuel burn in cruising conditions and NO WW II piston engine in service got below 0.40lbs per hp hour and most did not get into the low .40s. Make sure you are comparing like to like when comparing fuel consumption. (Wright got the R-3350 down into the high 0.30s post war but needed three blow down exhaust turbines geared to the crankshaft to do it). A quick look at the 222 will also revel that it would take a miracle for it to have better fuel consumption than the 211 in cruising conditions. General rule of thumb is that about 80% of the friction in an engine comes from the pistons and piston rings scrubbing the cylinder walls. A Jumo 222 (135 X 135 cylinders) has about 46% more cylinder wall scrub area per revolution than a Jumo 211 so that pretty much washes out the 2 engine vs 3 engine argument right there. Throw in the facts that 1, the Jumo 222 is running at higher rpm and 2, friction goes up with the square of the speed (10% increase in speed means 21% more friction) and it is very hard to figure where the increase efficiency is supposed to come from. They both use fuel injection, they both use about the same compression ratio.

The three engines have 105 liters displacement, the early 222s have 100 liters combined but it is 36 cylinders to 48 cylinders. I will grant you that the original Ju 252 had a rather less than elegant engine installation on the nose but you need radiators about 50% bigger in each wing nacelle ( if not 75% bigger if you want to use full power) if you are sticking 222 engines in them.

One reason the Jumo 222 was favoured over the DB604 and the BMW802 was its expected superbly high high speed cruise and cruise efficiency. This is of course of interest to a bomber which needs a sustained high speed cruise to minimise interception.

Koopernic said:

One reason the Jumo 222 was favoured over the DB604 and the BMW802 was its expected superbly high high speed cruise and cruise efficiency. This is of course of interest to a bomber which needs a sustained high speed cruise to minimise interception.

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Yes, I knew that which is why what Shortround stated, baffled me.

Ok, I am baffled too, WHY was the Jumo 222 expected to have "superbly high high speed cruise and cruise efficiency"?

The normal Jumo engines used fuel injection, no advantage. They both used about the same compression ratio (unlike the later high compression DB engines) so no advantage there.

We have gone over the internal friction problem. Now it was a very compact engine and they may have been anticipating a significant reduction in drag but that has little to do with the mechanics of the engine. The whole He 177 fiasco was an attempt to save about 3-4% in drag over 4 separate engines. Now perhaps to some people that is "superbly high high speed cruise and cruise efficiency" and justifies the mechanical shenanigans of the coupled engines or 6 bank 24 cylinder engines. TO be fair Wright went even further down a similar rat hole with the 42 cylinder Tornado engine.

I thought it was just the high maximum continuous RPM expected, allowing relatively high power settings under lean mixture conditions. (while maintaining specific fuel consumption close to that of the Jumo 211 in its economical cruise range)

This might not matter so much for a transport, unless it was operating over heavily contested airspace and likely to be intercepted. (and was a low drag airframe that allowed reasonably efficient high-speed cruise)

I'm not sure that puts it at an advantage over coupled (or H block, or double V block, or vertically opposed double V block -similar to H but with V angled banks) V-12 engines. Jumo had both the existing 210 and 211 to work off of there depending on the size and power they were interested in targeting.

I remember reading some where that the JU-252 was replaced by the 352 because the high command wanted to use lower rated French engines and less strategic material. The 352 was not as good as the 252 and since it was so late in the war, there was no reason to produce very many of them. But you have to admit that JU-252 was a very attractive plane.