Peter Gunn
Master Sergeant
I'll remember that, it may come in handy when I give my scathing review of said tome, which will not arrive until the 24th of August, so you're safe until then.
P-51B Mustang - The Bastard Stepchild that Saved the 8th AF has been sent by Osprey Publications for Print.
- Thread starter drgondog
- Start date
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I read the Kindle version. It's great to get the finer details about how the Mustang came to be. One question: why do you refer several times to the V-1710-45 as having a two speed drive? All available information says it had a fluid coupling on the auxiliary stage.
Ive not read the book yet but wasn't the V-1710 the Allison engine? The Mustang P-51B/C was powered by the Packard V-1650 licence built RR Merlin?
The Allison, yes. The book mentions the proposal to use the two-stage Allison and NAA's study of it, which concluded they would need to move the wing so far forward it was unacceptable, plus NAA found Allison's customer service to be lacking (probably mostly because of early production chaos).
Dash119
Senior Airman
Interestingly, the lightweight XP-51J did fly with a two stage Allison V-1710 and I don't think it required movement of the wing. They did eliminate the carb air intake from the engine cowling, and instead drew the intake air from the belly scoop. The Allison model was the V-1710-119...
Reading between the lines of NAA's decision, it wasn't that fitting the Allison was impossible, but they had a better option in the Merlin, and maybe they suspected the two-stage Allison would perform poorly (which it did; later the NACA had to fix up basic design errors in the V-1710-93 auxiliary stage), so they pushed the unacceptability of the design changes as much as possible. FWIW I don't doubt the wing movement, as they had to move it slightly for the Merlin too, and the two-stage Allison was much more awkward.
Another question for drgondog: your book states, "Atwood approved RD-1062 to study airframe and cooling system changes that would arise from the installation of the Continental V-3420 engine into the NA-73 airframe," and it mentions a "Continental V-3420 X engine" in another couple of places. Surely you mean the Continental XI-1430 V12? Allison made a V-3420 but there's no way it was a canditate for the Mustang.
- Thread starter
- #48
drgondog
Major
You are correct - my editor made that correction to the final draft and I missed it. If there is an 'errata sheet' I will correct that plus a couple of other edits that I missed.
As to the V-1710-45. It was so described in NAA correspondence from Rice to Atwood as the 'two stage supercharged Allison with the auxiliary second stage'. No it didn't have the pressure-altitude governed two speed gearbox or intercooling of the Merlin 60 series.
Last edited:
Thanks for explaining.
The reason I ask is because it's described in your book as, "Allison's proposed two-speed/two-stage supercharged V-1710-45."
Another, very minor typo is "V-1710-39/FSR" instead of "F3R".
- Thread starter
- #50
drgondog
Major
Actually, it would have required more significant changes than the Merlin conversion and delays to re-tool totally unacceptable to BAM/RAF.
kool kitty89
Senior Master Sergeant
On the issue of the V-1710-45 (and many or possibly all auxiliary supercharger installations on the V-1710), it appears that the aux stage used a fluid coupling, like an automatic transmission's toque converter, similar to the DB-601/605/603 and some R-2800 aux stages (and the single stage one used on the F8F). This is why you get the power curves you do with the similar -47, -93, -117, and -109 used in the XP-63 and P-63 and likely why you get the somewhat confusing wording of "Allison V-1710-47 engine was fitted with a second hydraulic turbosupercharger" in Joe Baugher's XP-63 article. (a fluid coupling uses a matched pair of impeller and turbines in oil/hydraulic fluid, so the supercharger is literally driven by a hydraulic turbine)
Bell XP-63 Kingcobra
This would also explain the aux stage 1710s only having 2 supercharger earing speeds listed for 2-stage variants. (one number for the integral supercharger and one for the full/locked condition of the aux stage) Hence also why there's only a take-off and high altitude rating for those engines, just like single-stage single-speed ones, and it would refer to the full/locked position of the fluid coupling.
So those V-1710s are 2-stage, but not 2-speed: the integral stage is fixed speed and the aux stage is variable. (I also see no references to indicate neutral+high+low mechanical drive gearing on aux stages like some P&W engines used)
All those engines should be pressure-altitude governed using a throttle setting to select the manifold pressure rather than manually controlling the throttle plate. This feature was already present in single-stage V1710s with automatic boost control (throttle sets the manifold pressure and the throttle plate is automatically adjusted to provide that manifold pressure). The difference here being the throttle plate would be wide open at all but low altitude and low pressure settings (as seen in P63 testing) and beyond that the pressure is controlled by allowing more oil into the fluid coupling housing.
In USAF charts the -45 is described as "Similar to -39 except: auxiliary second stage automatic boost control, and aneroid unit, PT-13E carburetor." And the reference to the aneroid unit may be the mechanism used for boost control (the aneroid unit should be the mechanical air pressure gauge used in conjuction with manifold pressure position set by the throttle lever).
The USAF charts also only have a 9.5" diameter listed for several aux stage V-1710 models (including the -45 and -47) but 9.5" and 12.18" for some later models (and 10.25 and 12.18 for later yet). However, this may simply imply that both stages are of 9.5" diameter on the examples that only list 9.5" (note: while the diameter may be the same, the impeller dimensions could still differ, particularly in height/chord of the impeller/vanes, making overall volume larger for the same diamter; that would make sense for providing higher volume at lower pressure from the aux stage to the integral stage).
There's no intercooler, but you have reduced charge heating due to the variable supercharger speed and avoid throttle plate losses at all wide-open conditions (Junkers also avoided that with use of a swirl throttle), but you do need additional oil cooling and an oil/air separator to account for the oil heating from the fluid coupling and the air churned up inside it at medium altitudes.
In any case, the significant increase in length of the auxiliary supercharger installation is what made it more difficult to fit to existing aircraft and would have delayed production compared to the P-51B's merlin installation. (presumably using a bevel geared drive at 90 degrees from the accessory drive shaft on the V-1710 could've allowed a side-mounted aux stage similar to the DB and Jumo engines, and avoided the increase in length, but for whatever reason Allison didn't do that, maybe due to a potential increase in width/frontal area and/or to minimize weight, but that's pure conjecture: albeit if the USAF chart is correct, the -45 was only 1515 lbs, lighter than the 1520 lbs for the single-stage V-1650-1 merlin, let alone the 1690 to 1745 lbs for the 2-stage merlins)
Bell XP-63 Kingcobra
This would also explain the aux stage 1710s only having 2 supercharger earing speeds listed for 2-stage variants. (one number for the integral supercharger and one for the full/locked condition of the aux stage) Hence also why there's only a take-off and high altitude rating for those engines, just like single-stage single-speed ones, and it would refer to the full/locked position of the fluid coupling.
So those V-1710s are 2-stage, but not 2-speed: the integral stage is fixed speed and the aux stage is variable. (I also see no references to indicate neutral+high+low mechanical drive gearing on aux stages like some P&W engines used)
All those engines should be pressure-altitude governed using a throttle setting to select the manifold pressure rather than manually controlling the throttle plate. This feature was already present in single-stage V1710s with automatic boost control (throttle sets the manifold pressure and the throttle plate is automatically adjusted to provide that manifold pressure). The difference here being the throttle plate would be wide open at all but low altitude and low pressure settings (as seen in P63 testing) and beyond that the pressure is controlled by allowing more oil into the fluid coupling housing.
In USAF charts the -45 is described as "Similar to -39 except: auxiliary second stage automatic boost control, and aneroid unit, PT-13E carburetor." And the reference to the aneroid unit may be the mechanism used for boost control (the aneroid unit should be the mechanical air pressure gauge used in conjuction with manifold pressure position set by the throttle lever).
The USAF charts also only have a 9.5" diameter listed for several aux stage V-1710 models (including the -45 and -47) but 9.5" and 12.18" for some later models (and 10.25 and 12.18 for later yet). However, this may simply imply that both stages are of 9.5" diameter on the examples that only list 9.5" (note: while the diameter may be the same, the impeller dimensions could still differ, particularly in height/chord of the impeller/vanes, making overall volume larger for the same diamter; that would make sense for providing higher volume at lower pressure from the aux stage to the integral stage).
There's no intercooler, but you have reduced charge heating due to the variable supercharger speed and avoid throttle plate losses at all wide-open conditions (Junkers also avoided that with use of a swirl throttle), but you do need additional oil cooling and an oil/air separator to account for the oil heating from the fluid coupling and the air churned up inside it at medium altitudes.
In any case, the significant increase in length of the auxiliary supercharger installation is what made it more difficult to fit to existing aircraft and would have delayed production compared to the P-51B's merlin installation. (presumably using a bevel geared drive at 90 degrees from the accessory drive shaft on the V-1710 could've allowed a side-mounted aux stage similar to the DB and Jumo engines, and avoided the increase in length, but for whatever reason Allison didn't do that, maybe due to a potential increase in width/frontal area and/or to minimize weight, but that's pure conjecture: albeit if the USAF chart is correct, the -45 was only 1515 lbs, lighter than the 1520 lbs for the single-stage V-1650-1 merlin, let alone the 1690 to 1745 lbs for the 2-stage merlins)
- Thread starter
- #53
drgondog
Major
The problem of detonation at boosts higher than 63" was never solved for the F/82 V-1710-143/-145. According to NAA Schmued, NAA fitted a backfire screen and sent the data to Allison - but Allison refused to modify the engine and effectively got away with telling both AAF/USAF and NAA to pound sand. IIRC the MP was set at 61" thereafter, dramatically reducing the performance of the F-82 vs the XP-82 and P-82B with Merlins.
The length was a 'killer'. Allison may have been encouraged to make the changes if they believed that NAA and the orders for Packard Merlins would be replaced by the Allison. By that time, AAF-MC/RAF/NAA were in firm agreement with P-51B/1650-3 and no hope for Allison to recapture Mustang business.
Wild_Bill_Kelso
Senior Master Sergeant
- 3,231
- Mar 18, 2022
Congrats, and great title!
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