Top Heavy Bomber

[FLYBOYJ]

Actually a lot of the oil comes out of the crankcase oil breather. Other leaks may develop at the bottom of the cylinder heads (Jugs) where they attach to the crank case (Radial Engines). There is also leakage from the rocker arm covers. You could have several "small" leaks on a very big engine and it produces what is perceived as a big mess.

And what Pb described is known as "hydraulic lock." A build up of engine oil at the bottom of the engine. The spark plugs on the lower cylinders have to be removed and the prop pulled through - when this happens oil shoots out of the spark plug holes. If this is not done when the engine is started the crankshaft and connecting rods could bend.

That one of the reasons why you see ground crews pulling through props prior to the start of a mission.

 

[Graeme]

Actually a lot of the oil comes out of the crankcase oil breather. Other leaks may develop at the bottom of the cylinder heads (Jugs) where they attach to the crank case (Radial Engines). There is also leakage from the rocker arm covers

Acceptable loss? or an indication that its time for maintenance? Gaskets/seals/O-rings/bearings?

 

[FLYBOYJ]

Acceptable loss? or an indication that its time for maintenance? Gaskets/seals/O-rings/bearings?

It depends - some manufacturers actually state in their manuals that "minor" leaks are acceptable. Others may give provisions on how much oil loss per hour is acceptable. The most common cause of oil around radial engines is oil getting blown out of the crankcase breather and there's not much one could do for that. Many manufacturers will not allow any leakage from the prop btw...

Most of the leaks could be fixed with new gaskets and seals. Altitude changes, heat, cold and vibration play hell on seals and gaskets....

 

[seesul]

It depends - some manufacturers actually state in their manuals that "minor" leaks are acceptable. Others may give provisions on how much oil loss per hour is acceptable. The most common cause of oil around radial engines is oil getting blown out of the crankcase breather and there's not much one could do for that. Many manufacturers will not allow any leakage from the prop btw...

Most of the leaks could be fixed with new gaskets and seals. Altitude changes, heat, cold and vibration play hell on seals and gaskets....

Joe, some cars producers use the system where the oil gases going out from the crankcase go back to the engine inlet and in fact are burnt out. Do some a/c engine producers use the same or similar system or not? Or is the oil leak volume so high that it is impossible to send it back and burnt it out?

 

[FLYBOYJ]

Joe, some cars producers use the system where the oil gases going out from the crankcase go back to the engine inlet and in fact are burnt out. Do some a/c engine producers use the same or similar system or not? Or is the oil leak volume so high that it is impossible to send it back and burnt it out?

For some reason most of the engines I've seen just vent the breather oil overboard. I'm guessing that there is no return due to pressures that might build up in the crank case.

Something a lot smaller - I had a 65' Cessna 150 with a Continental O-200. It had a normal oil capacity of 6 quarts. If you filled it to capacity it would "throw out" about 3/4 of a quart from the breather. After that there was no oil from the breather and it burnt a quart maybe every 40 hours.

I've seen other GA aircraft do the same thing and yet I've seen others that are fine with a full capacity - it's almost as if they have a mind of their own!

 

[pbfoot]

The same for the T6/Harvard if the oil tank is full she throws it out like crazy if a little over half full she doesn't toss half as much . Much less wiping down

 

[kool kitty89]

Not to complain, but a few comments on the list: as said earlier, the Ju 88 could carry more than 5,500 lbs of bombs (as could the B-25 and He 111) but it is obviously not a true Heavy Bomber and likewise the Il-4 and DB-3 shouldn't be considdered as such either. Also, the Fw-200 probably should have been on the list, as should the Do 217, though the 217 was more in the weight class of a medium bomber the 8,000lb+ max bombload shows otherwise. However, none of these bombers even come close to the Top Heavy, so it's a moot point.

On the note of the Lancaster, the Merlins were certainly liabillity to survivabillity as a daylight bomber as was the realitively weak defensive armament, and the single pilot was a prooven risk. However, the pilot did have the flight engeneer's extra set of eyes to help keep a look-out. The 4x Merlins could be replaced by 4x Bristol Hercules radials with similar power and altitude performance. (as around 300 Lancs were produced with) A second set of controlls could easily be added, and were in many cases, allowing the use of a copilot and a substancial risk reduction resultantly.

That said, the B-29 is still the "Top" bomber imo, with the Lancaster in second. Except maby as a dalight precision bomber, as it still didn't have the high-altitude capabillities, long-range bombloads, and defensive armament capabillities that the B-17 and B-24 had, in fact it seems no WWII bombers were as heavily armmed as those of America, though they were still no match w/out an escort, but the excellent defences helped considderably combined with fighter support.


One thing I've wondered about in the B-29's case is why not use R-2800 engines? The smaller diameter, size, and better airflow characteristics would have eliminated the overheating problems, weight was somewhat less, with the R-3350s of the B-29 rated for just 2,200 hp, even earlier R-2800s weresn't much less powerful (2000-2100 hp rated) and by the time B-29s were entering production the R-2800-59 with 2,300 hp rating (and 2,500+ hp WEP) was being used on the latest production P-47Ds and the P-47M/N's 2800 hp R-2800-57C was availiable as well.

And just look at the B-29's nacelles, its just begging for R-2800 Double Wasps. (ie it apears to have stollen the noses of 4 P-47s )

The only reason I can think of is that the R-2800 was in too high of a demand, but seeing the priorety of the B-29 this shouldn't have been a major issue. (licence production could be set up if necessary)

 

[B-17engineer]

Kool Kitty I was thinking the same thing why the IL-4 is up there as well as the DB-3. TO me the IL-4 was a medium bomber. It wasn't big at all either. 4 man crew at first it was 3.

Personally i would've added the Do-217.

 

[FLYBOYJ]

One thing I've wondered about in the B-29's case is why not use R-2800 engines? The smaller diameter, size, and better airflow characteristics would have eliminated the overheating problems, weight was somewhat less, with the R-3350s of the B-29 rated for just 2,200 hp, even earlier R-2800s weresn't much less powerful (2000-2100 hp rated) and by the time B-29s were entering production the R-2800-59 with 2,300 hp rating (and 2,500+ hp WEP) was being used on the latest production P-47Ds and the P-47M/N's 2800 hp R-2800-57C was availiable as well.

And just look at the B-29's nacelles, its just begging for R-2800 Double Wasps. (ie it apears to have stollen the noses of 4 P-47s )

The only reason I can think of is that the R-2800 was in too high of a demand, but seeing the priorety of the B-29 this shouldn't have been a major issue. (licence production could be set up if necessary)

"Further work resulted in the Model 333A of late 1938. It was to have been powered by four 1150 hp Allison V-1710 twelve-cylinder liquid-cooled engines, installed in tandem pairs. However, full pressurization of the cabin was considered impractical because of the need to open the bomb bays during high-altitude flight, and it was decided that only the crew areas in the nose and in the mid-fuselage sections were to be pressurized. The nose and mid-fuselage pressurized cabins were to be connected by a pressurized tunnel passing over the bomb bay that allowed the crew members to change positions during pressurized flight. This feature remained on all subsequent Boeing long range bomber design proposals.

Because of the poor high-altitude performance of the Allison liquid-cooled engine, variations of the project were proposed with the new flat-mounted Wright and Pratt Whitney radial engines. This led to the Model 333B project of February 1939. It was powered by four Wright engines buried in the thick wing. With a gross weight of 52,180 pounds, the maximum speed was to have been 364 mph at 20,000 feet. The range was to have been 2500 miles with a 2000-pound bombload.

In March of 1939, the Model 334 was proposed. The wingspan was extended to 120 feet in order to provide enough fuel to reach a range of 4500 miles. The Pratt Whitney radials were still buried in the wing. A twin fin-and-rudder was to have been used to facilitate the installation of tail armament. Gross weight was up to 66,000 pounds and maximum bomb load was 7830 pounds.

In July of 1939, Boeing revised the design still further to produce the Model 334A. It eliminated the buried engine installation and the twin fin-and-rudder assembly of the Model 334 and replaced them with four conventionally-mounted Wright R-3350 radials and a single vertical tail. A high aspect-ratio wing of 135 feet span was to be used. The Boeing proposal had finally begun to take the recognizable shape of what was eventually to emerge as the Superfortress.

In January of 1940, the Army issued the formal requirements for the VLR "superbomber". The requirements called for a speed of 400 mph, a range of 5333 miles, and a bomb load of 2000 pounds delivered at the halfway-point at that range. This became the basis for Request for Data R-40B and Specification XC-218. On January 29, 1940, the War Department formally issued Data R-40B and circulated it to Boeing, Consolidated, Douglas, and Lockheed. The official specification was revised in April to incorporate the lessons learned in early European wartime experience, and now included more defensive armament, armor, and self-sealing tanks.

The Boeing company had a leg up on its competition, since it had been thinking along the very same lines. In August of 1939, Boeing had started work on the Model 341 project, which featured a new high-lift airfoil for a high aspect-ratio wing of 124 feet 7 inches in span. The Model 341 offered a maximum speed of 405 mph at 25,000 feet. It was to have been powered by four 2000 hp Pratt Whitney R-2800 radials. Weighing 85,672 pounds, the range was to have been 7000 miles with one ton of bombs. A maximum load of 10,000 pounds could be carried over shorter distances.

In order to meet the requirements of Data R-40B, the Boeing Model 341 design was reworked into the Model 345. The Model 345 envisaged a pressurized aircraft, four Wright R-3350 engines replacing the R-2800s of the Model 341, a twelve-man crew, a double-wheeled tricycle undercarriage which retracted into the engine nacelles instead of sideways into the wing as on previous projects, four retractable turrets each carrying a pair of 0.5-inch machine guns, and a tail turret with two machine guns and a 20-mm cannon. The retractable Sperry power turrets were operated under remote control by gunners sighting through periscopes. The Model 345 was to be capable of carrying a ton of bombs over the stipulated 5333 miles at a cruising speed of 290 mph. The maximum bombload was to be 16,100 pounds. The maximum speed was estimated to be 382 mph at 25,000 feet. The weight was to be 97,700 pounds. "

 

[kool kitty89]

Still I don't see why they didn't considder the R-2800s as an alternative in the 345 model, especially considdering that the power outputs of the R-2800 were better than those of early R-3350s of the same time, pluss the 2800 had reliabillity and prooven design on its side.

Seeing as 2100 hp Allison V-3420s as well as the more powerful R-4360s were both chosen and tested as alternate powerplants for the basic 345 design, I don't see why the R-2800 wasn't considdered: except the 2800 was nearing the end of its development for power output while the V-3420, R-3350, and R-4360 all had far more development for increase in power output and performance. Still, the 2800 should have been considdered as an intrim powerplant, and still outperformed (at least in the 2800-57C,and 59) most R-3350 models used on the B-29, and certainly those used durring the war.

 

[HoHun]

Hi Koolkitty,

>Still, the 2800 should have been considdered as an intrim powerplant, and still outperformed (at least in the 2800-57C,and 59) most R-3350 models used on the B-29, and certainly those used durring the war.

Very good point. Maybe it is a question of fuel consumption? I haven't looked at the R-2800 figures yet, but if the engine was more thirsty for the same power settings, the ambitious goals for the B-29 regarding range might have been out of reach ...

Regards,

Henning (HoHun)

 

[syscom3]

The R3350 weighed less than the R2800, as well as having a lower drag. Not to mention it carried a larger prop.

 

[FLYBOYJ]

Still I don't see why they didn't considder the R-2800s as an alternative in the 345 model, especially considdering that the power outputs of the R-2800 were better than those of early R-3350s of the same time, pluss the 2800 had reliabillity and prooven design on its side.

Seeing as 2100 hp Allison V-3420s as well as the more powerful R-4360s were both chosen and tested as alternate powerplants for the basic 345 design, I don't see why the R-2800 wasn't considdered: except the 2800 was nearing the end of its development for power output while the V-3420, R-3350, and R-4360 all had far more development for increase in power output and performance. Still, the 2800 should have been considdered as an intrim powerplant, and still outperformed (at least in the 2800-57C,and 59) most R-3350 models used on the B-29, and certainly those used durring the war.

The engineers responsible for propulsion could of had a number of reasons why they changed their minds. Weight, size, reduction gearing, and accessory accommodation could of all been a factor, as well as availability, producibility and last but not least - costs!

 

[kool kitty89]

The R-2800 was more expensive than the R-3350?! Either in maintenence costs or production costs I'd think the 2800 would be cheaper due to the large number produced, and the realitive reliabillity and serviceabillity.

I don't see how it would be heavier and streamline more poorly, all the info I've seen on the 2800 shows it to be ~300 lbs lighter and only ~52 in in diameter compared to the 3350's ~55 in. I don't see why the R-3350 could use a larger prop though...

Availabillity would be a factor certainly, but I don't think the early carborated versions of the 3350 had SFC any better than the 2800, though the direct injection later used likely would have. Judging by the performance of the P-47's layout and the accesories it carried it would seem to have adaquate performance for the intended long-range high-altitude role. The smaller R-2800 shouldn't have had as much of a problem with oil leaks either.


Still though, the R-4360 is the ultimate engine for the design, with excelent reliabillity in the air (compared to the R-3350, and no real fire issues), though a maintenence mess on the ground.


Overall: 1.) Weight and size is better if anything in the 2800, 2.) reduction gearing may have been an issue, 3.) accessory accommodation maby though it had alot of versitility in this area(seen in the variety of areas it was used, with turbos, 2-stage superchargers, water-injection, use in low-alt and high-alt, and long-range applications; used in fiighters, night-fighters, bombers and transports for a few), 4.) availability definitly would have been a factor due to the high demand for a number of uses, 5.) producibility I'm not sure of but shouldn't have been a big problem (licenced production if necessary), 6.) and costs shouldn't have been an issue.

 

[FLYBOYJ]

The R-2800 was more expensive than the R-3350?! Either in maintenence costs or production costs I'd think the 2800 would be cheaper due to the large number produced, and the realitive reliabillity and serviceabillity.

I don't see how it would be heavier and streamline more poorly, all the info I've seen on the 2800 shows it to be ~300 lbs lighter and only ~52 in in diameter compared to the 3350's ~55 in. I don't see why the R-3350 could use a larger prop though...

Availabillity would be a factor certainly, but I don't think the early carborated versions of the 3350 had SFC any better than the 2800, though the direct injection later used likely would have. Judging by the performance of the P-47's layout and the accesories it carried it would seem to have adaquate performance for the intended long-range high-altitude role. The smaller R-2800 shouldn't have had as much of a problem with oil leaks either.


Still though, the R-4360 is the ultimate engine for the design, with excelent reliabillity in the air (compared to the R-3350, and no real fire issues), though a maintenence mess on the ground.


Overall: 1.) Weight and size is better if anything in the 2800, 2.) reduction gearing may have been an issue, 3.) accessory accommodation maby though it had alot of versitility in this area(seen in the variety of areas it was used, with turbos, 2-stage superchargers, water-injection, use in low-alt and high-alt, and long-range applications; used in fiighters, night-fighters, bombers and transports for a few), 4.) availability definitly would have been a factor due to the high demand for a number of uses, 5.) producibility I'm not sure of but shouldn't have been a big problem (licenced production if necessary), 6.) and costs shouldn't have been an issue.

Here's the TCDS for each engine. Although the 3350 weighed 1000 lbs more, the differences are apparent.

http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/28543f501b3e3cee8525670d0060471a/$FILE/E-231.pdf

http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/d78d8c1a3822fd4e8525670b006c8f72/$FILE/E-272.pdf

 

[kool kitty89]

The figures on that R-3350 are for the turbo-combine version, which definitely wasn't available for the war. It's also rated for a higher minimum octane fuel than the 2800 you sited. IIRC, the early 3350s used in the war were only ~300-500 lbs heavier than the 2800, the turbine equipment adds alot more weight. That model 2800 wasn't one of the ones rated for higher power either, I was thinking along the lines of the 2800-59, used in late P-47Ds with 2,300 hp take-off/millitary power and over 2,500 hp WEP with water injection. Though these ae figures for civilian engines too, so I'm not sure how they differ. (WEP obously wouldn't be listed though)

And though I don't have the figures, I suspect the 2800 would have been cheaper.

But I still think availabillity and high demand for other uses is the most likely reason, but I'm not sure.

 

[davparlr]

I don't know much about these engines, but typically, at a rated horsepower, a smaller engines works harder than a larger engine (higher RPM?). Since I am sure the B-29 engines were working harder for longer periods of time than a P-47 staying with B-17 and 24s, long endurance relaibility may have been a factor.

 

[HoHun]

Hi again,

>Maybe it is a question of fuel consumption? I haven't looked at the R-2800 figures yet, but if the engine was more thirsty for the same power settings, the ambitious goals for the B-29 regarding range might have been out of reach ...

The R-2800-21 vs. the R-3350 installed in the B-29:

R-2800: 200 g/HP/h minimum (at 950 HP, 15000 ft)
R-3350: 212 g/HP/h (at 930 HP, 14000 ft)

R-2800: 286 g/HP/h economical maximum (at 1200 HP, 25000 ft)
R-3350: 215 g/HP/h (at 1180 HP, 25000 ft)

R-2800: 352 g/HP/h maximum continuous (at 1625 HP, 29000 ft)
R-3350: 256 g/HP/h (1620 HP, 29000 ft)

R-2800: 374 g/HP/h take-off/emergency power (at 2000 HP, 27000 ft)
R-3350: 312 g/HP/h maximum continuous (at 2050 HP, 28000 ft)

(Data based on P-47 Specific Engine Flight Chart and the B-29 Airplane Commander Training Manual)

So the R-2800-21 can compete with the R-3350 only at very low power settings, which yield a speed that is below the speed for optimum range on the B-29. At higher power settings, which might be necessary when carrying a large fuel load, the fuel consumption of the R-2800-21 is rouhgly 30% greater, with a resultant impact on range.

So it looks as if the R-2800-21 at least couldn't have been used for the B-29 without sacrificing the quality for which the bomber was designed - extreme range.

It might be interesting to check the consumption data for a more modern R-2800 variant than the -21, but unfortunately I don't have any charts for these variants.

Regards,

Henning (HoHun)

 

[kool kitty89]

Any data on the R-2800-59 used on later P-47D's, or the R-2800-57C would be more useful. The later model engines (ie 50 series) had improoved ignition and fuel systems and fuel consumption would have likely been significantly different.

Plus your stats show the 2800's SFC going down from 1200 to 1625 hp to the point where it is again more efficient and cruising with ~1625 hp per engine would give low power/weight of less than .06 hp/lb and cruise speed would likely drop below 200 mph.

But the only model I was seriously considdering was the R-2800-59 of 2,300 hp take-off (2,535 hp with water injection). It would allowed take-off of about the same load and length (if not better) as the R-3350-23 and with higher max power and probably similar continuous power. But I don't have any real data for the specs of this engine other than power. It would also be more practical than the R-2800-57C which had considderable teething problems as well and wouldn't have had much of a reliabillity advantage, if any, over the R-3350.

Also the R-2800-59 engine was in the highest demand as the engine used by all late model P-47D's and availabillity would have been a problem, inless licensed production set-up specifically for the B-29 project was establashed.

 

[FLYBOYJ]

Look at the date it was decided to use the 3350....