B-17 and B-24: plausible upgrades?

wuzak · Mar 22, 2013

GregP said:
In any case, they didn't proceed with the V-12 B-17, so it's pretty much a moot point. I like the XB-38's looks but, in the end, only the radial variants saw service. I prefer the two-row Pratt 1830 to the single-row Wright 1820, but that's personal preference only. The specific fuel consumption is better for the Pratt by about 20%.

The XB-38 fell, like so many projects of the time, to the alter of production. The V-1710 was suggested as an alternative to the R-1820 in case there was a supply shortage of the latter. There wasn't. The XB-38 was intended to explore possibilities for performance improvements, but the prototype was delayed by higher priority works for Vega (ie production, and setting up their B-17 line).

Basically the XB-38 would have needed to show a much larger performance increase over the B-17 for it considered for production.

Shortround6 · Mar 22, 2013

wuzak said:
Greg, here is the data drom Joe Baugher:..................

So, the XB-38 was 9mph faster at 25,000ft than the B-17E, 2mph faster than the B-17F (when it was using WER).

On the face of it the XB-38 was 26mph faster than the B-17F's cruise speed, but the cruise speed of the E is between 195mph and 223mph - that is between 3mph and 31mph slower than the XB-38. The range would, probably, be the difference between cruise settings - maximum cruise, best economy cruise.

I do think that the cruise speeds employed by B-17s in combat were the result of formation flying and the need to maximize range.

Thank you for the work of typing out all that information.

The report you posted the link to is also very interesting as it shows that an "F" could do 309mph running light (42,452lbs) and 299mph at 49,463lbs at 25,000ft at Military power. it also shows a possible cruise of 219mph at 650hp per engine with a specific fuel consumption of .485 lb/hp/hr. unless there is a misprint ( and there is at least one on that chart).

Operational cruising speeds were often different than book or tested cruise speeds.

GregP · Mar 22, 2013

The 0.485 lb/hp-hr would be for the Pratt engined version of the B-17. I think the Wright 1820 was right at 0.6 lb/hp-hr.

I can't find a single reference that says any G model B-17 could get to 325 mph. It is usually listed as 287 - 295 mph with maybe 302 at WER, which was never used in real life unless they were trying to get the bird to a landing spot on 1 or 2 engines after the others were shot out. We happen to have a B-17 that we are returning to flight status. I wish I had a proposed flight date, but that depends on money. All aircraft fly on money, despite the occasional references to oil and gasoline / jet fuel.

I don't believe the chart above. If it were true, we'd surely have reports of 10,000 pound bomb loads being used in the war from B-17's on a regular basis. I daresay they might have flown a few, but not many. Once we had airfields in France, if the chart were true, we'd be regularly hauling 10,000 pound bomb loads to Germany ... and we didn't, even at the end of the war, when we surely were within range.

But, I'll keep an open mind and look at the flight manual for our B-17 tomorrow ... if Steve Hinton is there and lets me see it ... we'll see.

wuzak · Mar 22, 2013

GregP said:
The 0.485 lb/hp-hr would be for the Pratt engined version of the B-17. I think the Wright 1820 was right at 0.6 lb/hp-hr.

219mph TAS @ 25,000ft, 48,583lb weight, engine rating - 650hp @ 1900rpm, 27.5inHg MAP, 0.485lb/hp.hr
299mph TAS @ 25,000ft, 49,643lb weight, engine rating - 1190hp @ 2500rpm, 45.5inHg MAP, 0.670lb/hp/hr

One is lean and one is rich thus giving the discrepancy between the specific fuel consumptions.

Which P&W engined B-17?

SR, was the discrepancy you found the difference between MAP settings for two conditions when the engines were at 1200hp @ 15,000ft?

wuzak · Mar 22, 2013

GregP said:
I don't believe the chart above. If it were true, we'd surely have reports of 10,000 pound bomb loads being used in the war from B-17's on a regular basis. I daresay they might have flown a few, but not many. Once we had airfields in France, if the chart were true, we'd be regularly hauling 10,000 pound bomb loads to Germany ... and we didn't, even at the end of the war, when we surely were within range.

I wonder if the 10,000lb "bomb load" is actually 10,000lb military load, including nearly 6000 rounds of ammo?

GregP · Mar 23, 2013

You got me there Wayne. I was thinking of the DC-3 / C-47 since I worked on it last weekend and typed it into a B-17 thread. Duhhhhh ....

Production B-17's were Wright 1820's while the DC-3 / C-47 was produced with both engines before settling on the Pratts. I have seen a post about a turboprop B-17 powered by four Darts, but it was a fire bomber conversion. Engage brain before posting ...

By the way, our DC-3 is nearing flight status again. It is unlikely to be mistaken for a B-17.

wuzak · Mar 23, 2013

I proposed a twin engined B-17 previously, using V-3420s. I don't think the R-2800 has enough grunt to be used in the B-17 as a twin, and is probably too heavy and uses too much fuel in a 4 engined version, so what about a triple engined B-17 with R-2800s?

The bombadier would have a window below and behind the fuselage engine for sighting his bombs.

Forward defence would be taken car of by the front upper turret, perhaps a secons one to the rear helping with rear defence too. Perhaps a couple of synchrnised 0.50s firing through the prop to make the pilot feel better!

cimmex · Mar 23, 2013

wuzak said:
, so what about a triple engined B-17 with R-2800s?

The bombadier would have a window below and behind the fuselage engine for sighting his bombs.

Forward defence would be taken car of by the front upper turret, perhaps a secons one to the rear helping with rear defence too. Perhaps a couple of synchrnised 0.50s firing through the prop to make the pilot feel better!

I'm aware that the "what if"-guys have strange ideas but this statement is ridiculous. It is simply not possible to install a motor cannon in a R-2800 or another two row radial.
cimmex

Neil Stirling · Mar 23, 2013

Greg,

I don't believe the chart above. If it were true, we'd surely have reports of 10,000 pound bomb loads being used in the war from B-17's on a regular basis. I daresay they might have flown a few, but not many. Once we had airfields in France, if the chart were true, we'd be regularly hauling 10,000 pound bomb loads to Germany ... and we didn't, even at the end of the war, when we surely were within range.

Altitude, and weight. The flight manuals for the B-17 and B-24 show a marked increase in consumption above about 20,000ft. Consumption testing here B-24 Performance Air miles per gallon fell from IIRC about 1.1 ampg to about .65. Also tactics I believe the practise was to fly about over the UK and get into formation before setting out.

Neil.

wuzak · Mar 23, 2013

cimmex said:
I'm aware that the "what if"-guys have strange ideas but this statement is ridiculous. It is simply not possible to install a motor cannon in a R-2800 or another two row radial.
cimmex

Who said anything about a motor cannon? I said synchronised machine guns. Mounted on the cowl.

cimmex · Mar 23, 2013

sorry, I missed that.
cimmex

Shortround6 · Mar 23, 2013

wuzak said:
SR, was the discrepancy you found the difference between MAP settings for two conditions when the engines were at 1200hp @ 15,000ft?

No, it was just above the 1190hp setting at 25,000ft. The two 425hp settings, one at 1900rpm and 18.78in and the next one at 2800rpm and 20.2in.

2800rpm would definitely be over revving . 1800rpm might make sense and is a simple typo.

tomo pauk · Mar 23, 2013

Looking at the wing area of B-24 ( 1,048 ft², span 110 ft 0 in ), the Avro Manchester actually have had more (1,131 ft², span 90 ft 1 in ). Developing it into Lancaster also involved increase of wing area span, now being 1,297 sq ft and 102 ft 0 in. The wing was also greater in later B-26 vs early models. Maybe a wing root plug would not be too much for the B-24? Say, 3-5 feet at each side, so the wing grows in span and area, thus lowering the wing loading?

drgondog · Mar 23, 2013

With 10 extra feet of wing at root, there is probably at least 5% extra profile drag - not good for cruise speed and definitely not for range - but better take off performance and heavier load capacity

tomo pauk · Mar 23, 2013

Bill,
What was the usual altitude of bombed-up B-17s and B-24s, cruising toward targets in ETO?

GregP · Mar 24, 2013

Didn't get anywhere near the office today to look at the B-17 pilot's manual. Spent the whole day riveting and making stiffeners for the fillets on the tail of our Bell YP-59A. Sorry ... space-cased the load chart. Will try to correct that next weekend.

drgondog · Mar 24, 2013

tomo pauk said:
Bill,
What was the usual altitude of bombed-up B-17s and B-24s, cruising toward targets in ETO?

24-26,000 for B-17, 20-22,000 for B-24s.

bob44 · Mar 24, 2013

What was the usual altitude of bombed-up B-17s and B-24s, cruising toward targets in ETO?

Here is some interesting reading on the B17 and B24.
Typical altitude for B-17 missions? | ArmyAirForces
8th AF - B-17 vs B-24 | ArmyAirForces

tomo pauk · Mar 25, 2013

Thanks for the feedback, Bill and Bob.

The altitude of 20-22000 ft makes the B-24 just within of the envelope for the most numerous Flak, the 56 caliber 8,8cm. Even the ones with well worn barrels could reach it? Parsifal, maybe you could chime in, about the realistic envelopes of the Flak?
The R-1830 was able to make same power from SL up to 25000 ft (no ram), so the wing area seems likely to be main culprit for the Lib not cruising higher? Data from Wikipedia, for B-24J and B-17G:

Wing area: 1,048 ft²
Max. takeoff weight: 65,000 lb

Wing area: 1,420 sq ft
Max. takeoff weight: 65,500 lb

The B-17 did have about 35% lighter wing loading, according to that. So the B-24 with a wing of 10% greater wing area might cruise maybe 2000 ft higher; the engine have enough power for that. The air at, say, 23000 ft is thinner than at 21000 ft, ie. less drag, so the speed and range would remain the same? The wing 'plugs' can allow for increased fuel tankage, if the range needs to be further increased (to seal the Atlantic gap?).
The increased cruising altitude can make the job harder for interceptors, too.

From here, web 'version' of Ray Wagner's book "American Combat Planes of the 20th Century":

The Eighth Air Force in Britain favored the B-17 because it was easier to fly, had a higher ceiling, and seemed more resistant to enemy gunfire (see Table 5, page 243 for comparisons). Both heavy bombers had added much weight, but the smaller area of the B-24's narrow wing handicapped it at high altitudes.

tomo pauk · Mar 30, 2013

Mike Williams has updated his site with new data about B-17, B-24 and B-26

B-17 and B-24: plausible upgrades?

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