b 17 with allison motors

[GregP]

Air-cooled radials made great max power, but cruise at very low power settings. Fuel flow and high drag are primary reasons. A standard B-17G could reach 287 mph at top speed but cruised at 182 mph, and slightly less for large formations. The speed was always set by the slowest planes. With Allisons it could have cruised in the 230 mph range.

My real question is much more of ... would that difference have mattered at the time?

Neither 180 mph or 230 mph would run away from any single or twin engine fighter or any potential flak. If the XB-38 was running fast, like maybe at, say, 285 mph ... it ALSO would not run away from any fighter or flak, so the speed difference is moot. If it could make a 330 mph cruise, like a fast-cruising B-29 ina shallow descent from the target, that's a different story. It materially reduces the fighter closing speed and makes the tail gunner's job a LOT easier since the fighter trying to get you is moving much slower, relatively speaking.

I love the Allison ... but really see no reason to change the engine from the R-1820 to the V-1710 unless Wright had a real quality problem at some time, and they didn't. The R-1820 is a GOOD, reliable radial.

It really was a grand experiment, but was very likely more or less an "emergency in B-17 production fallback," rather than a serious attempt to change engine suppliers.

Just my opinion ... yours may differ, sort of like EPA gas mileage estimates. In any event, it didn't get selected for production. It surely LOOKED better, but that's a personal preference, too.

Love the shot of the uncowled engines. Makes it look like they are turbocharged, but the air pipe is really just exhaust. The Allison has a downdraft carb, not an updraft carb. The aiscoop is low, and they had to get the air up to the carb somehow, and I can't see how from the pic. But I KNOW it's there. I can see the scoop ductwork make a sharp upward turn. More or less a P-40-style side-facing engine mount. Nothing else used the side mounts as far as I know, and didn't know the B-38 did until the pic. Everything else used vertical bolts.

I wonder if using both left and right-turning Allisons would have made any difference, but I doubt it would have done more than slightly improving engine--out handling, which was ... by all accounts ... not all that bad to begin with.

 

[Denniss]

They high cruise speed would considerably reduce time in enemy airspace and flight time in general but so-equipped B-17 groups would always have to be kept separate from groups with standard B-17s. This could have been achieved either with later take-off and catching-up with standard B-17 formations near the target area or by using Allison-only fast bomber streams.
A lot of additional planning and logistics effort required for this.

 

[Shortround6]

Speed difference was probably much, much closer. Max cruise with lean mixture (auto-lean) was 750hp (62.5)gal/hr for the engines in a B-17F. Max cruise for an Allison in a P-38L was 795hp in auto lean (63gal/hr). Max cruise in Auto rich was 1000hp for the Cyclone (at 103 gal/hr per engine) and 1100hp for the Allison at 113 gal/hr per engine.
Figures are from specific engine flight charts for both planes as in their manuals. your mileage may vary (especially at altitude)

comparing the B-17G to the XP-38 is not quite comparing apples to apples as the XP-38 didn't have the Bendix chin turret and in fact was the 9th B-17E off the production line.

 

[GregP]

The data from the time all state the B-38 was quite a but faster, but the top speed was only slightly faster and nobody ever said it was a B-17G replacement. In order to get an apples to apples comparison, use a B-17E to compare, simple. If they had modified a G model, the speed difference would have been almost exactly the same, though slightly slower than a B-17E. My point is the difference in speed, even if it worked out to be 25 - 35 mph, would not have mattered to the safety and survivability of the new B-17 model since it could not mitigate any single threat the B-17 faced in any way. So the cost to implement the modification was not worth the effort and the very slight change in performance.

Naturally the time over target could be slightly less if slightly faster, but the difference would amount to nothing. A flak gun that can hit a 185 mph airplane can also hit a 205 mph airplne with equal accuracy or inaccuracy; it makes no difference to the gunners and the time the targets are in range is very substantially the same. I doubt if there would be a difference in the number of rounds fired and, if so, it would amount to a one round delta at best. Again, nothing.

This MUST be similar to the thinking at the time because the B-38, pretty though it was, wasn't selected for production ... all of which leads me to conclude that the exercise was the development of a production fallback model in the event Wright had difficulties in delivering engines or if they suddenly developed a quality issue.

Sort of like the B-32 being an alternate plan in case the B-29 failed. The B-29 DID have issues and they DID build a small run of B-32s (118 in total), but they quickly decided they would stick with the B-29, so it rapidly went out of production. As a rather meaningless aside, the B-32 had the distinction of being the last Allied aircraft to be engaged in combat during World War II.