Well yes, since the USA formally entered the war days before 1942, you couldn't have 1942 or 1943 as late could you, it is nearer the beginning than the end. For many aspects of aviation the end was coming in mid 1944 with all sorts of people looking to the future.
XP-39: pros cons (1 Viewer)
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And horsepower was 1150? Do you have a chart that lists horsepower along with TAS along with Altitude. I'm trying to figure out how much speed would be added with the different altitudes.
And without the screens, I would speculate it'd probably be less?
P-39 Expert
Non-Expert
Yes horsepower was 1150 for the -35. I do have charts, but the wwiiaircraftperformance.org site has performance charts for the P-39D that show critical altitude of 13800' with about 370mph of ram. That's your difference in ram and no ram. Also the critical altitude for climb is 12400' and that would be with only about 170mph of ram.
The backfire screens meant less power as they reduced manifold pressure, deleting the screens gained manifold pressure.
The reason the charts can be useful is that the ones on WWII aircraft performance page don't list horsepower for speed/altitude. I was trying to compute what different critical altitude.
Do you have any idea how much less?
Why would you ask me and why do you want to know. What use is a guess anyway?
Shortround6
Major General
It is getting self defeating.
there are a chart and an explanation on Page 344 of "Vees for Victory" showing the power needed by the supercharger to flow various amounts of air using different gear ratios.
The example goes something like this, Assume 10,000lbs of air flowing through the engine per hour, that is enough to make 1672hp in the cylinders (IHP) but the engine uses 147hp in internal friction, power internal drives and pumps. This cuts the power to about 1525hp and the 6.44 gears need about 110hp to supply that much air leaving you with 1415hp to the prop.
an engine with 9.60 gears needs about 240hp to drive the supercharger leaving you with 1285hp.
You also have a 9.5in impeller spinning at 28,800rpm with the 9.6 gears. which means the tip speed of the impeller blades is 1193fps if I have done the math right.
the speed of sound at sea level on a standard day is 1125fps. the higher pressure and temperature inside the supercharger allow for higher speeds before the tips break the sound barrier but obviously things are getting a bit tight.
Then you have the heat problem, the faster you spin the impeller the more you heat the intake charge and the closer you get to the detonation limits which can cause the peak pressure to be reduced. 8.80 gear engines were allowed to use 60 in of MAP for WER while the 9.60 gear engines were limited to 57in.
This also shows up with either (or both) reduced power for take-off or at low altitudes. The 8.80 gear engines in the P-40 made 1490 hp at 4300ft using 56in of boost (and 1580hp at 2500ft using 60in with the -73 engine). the 7.48 geared engine in the A-36 made 1500hp at 5400ft using 52in of boost. the 9.60 engines were rated at 1360hp at sea level using 57in. snf by opening the throttle made 1410hp at 9500ft at 57in.
They paid for that 3-3,500ft of altitude by losing about 80-125hp at sea level.
trying to go for 16,500ft is going to get you more of the same, less power down low, pushing closer to the detonation limits and pushing the engine harder for that last 1000ft of altitude.
Curiosity, plus you can always learn something. Shortround's response was pretty solid.
wuzak
Captain
It was discussed at a meeting held in Indianapolis on Tuesday 17th October 1944, involving Allison, GM Executives, representatives of the USAAF and a couple of airframe manufacturers.
The meeting was set for 9am, but started at 8:56am because all attendees had arrived early, according to the minutes of the meeting.
The resolutions of the meeting included Allison agreeing to develop a 2 speed supercharger to maybe, possibly, suck a little bit less. The USAAF agreed to deliver a V-1650-1 to Allison so it could be copied!
The timetable was tight, the first one was to be on the dyno no more than one week of the delivery of the V-1650-1.
The USAAF delivered the V-1650-1 on Thursday 28th December 1944. Allison asked for an extension on their timeline, since their experimental shop gear cutter was out sick with the flu that week, the USAAF agreed because they didn't really give a shit about a 2 speed V-1710 at that stage.
The 2 speed V-1710 finally made it on the dyno on Sunday 4th March 1945. By that time no-one really cared, including Allison, because they were all about jet engines.
Or not.
P-39 Expert
Non-Expert
The actual horsepower was listed on the performance memorandums in 5000' increments, but not on the charts. Hope that helps.
Sometimes I revisit old threads to ask questions I should have asked earlier. I also sometimes actually find answers to some degree that at least help out with the various discussions in the thread.
Often, from what I remember: There was a tendency to see the turbochargers designed in ways that reduced efficiency quite a bit. When Hooker came around, he actually identified numerous problems in the formulas that gave rise to the supercharger design. One thing that I remember being told was that (at least with twin-staged designs), the two stages were the same diameter (later on, it would be found that having the first stage larger and the second stage smaller would give better performance).
Interestingly, I'm not sure if there was enough money to have developed turbos for mass-production and twin-stage supercharging.
From what it appears, the problems with the 9.6 designs was that they seem to have miscalculated the amount of structural strength needed for the engine to hold together while driving a larger supercharger.
I should have asked this awhile back, but out of curiosity, what kind of aerodynamic improvements would have been do-able at the time?wuzak said:
I did some checking in Warbird Tech Series: Vought F4U Corsair and it says the F4U-3 was to be fitted with a Birman type supercharger as well. From what I read, the goal was to increase the critical altitude a whole lot. They even managed to get up to altitudes of around 50,000 feet (I'm not sure if the cockpit was pressurized, but that seems to be the altitude where it'd be a very good idea!). The problem with the Birman was that it's reliability sucked, MTBF was something like one flight.
From what it would appear, the closest thing I can find is that Oliver Echols seemed to have a bias against twin-stage supercharging, and allocated all funding to turbochargers.P-39 Expert said:
Often, from what I remember: There was a tendency to see the turbochargers designed in ways that reduced efficiency quite a bit. When Hooker came around, he actually identified numerous problems in the formulas that gave rise to the supercharger design. One thing that I remember being told was that (at least with twin-staged designs), the two stages were the same diameter (later on, it would be found that having the first stage larger and the second stage smaller would give better performance).
Interestingly, I'm not sure if there was enough money to have developed turbos for mass-production and twin-stage supercharging.
This assumes the intake was revised with more spacing to eliminate turbulent airflow, correct?
From what it appears, the problems with the 9.6 designs was that they seem to have miscalculated the amount of structural strength needed for the engine to hold together while driving a larger supercharger.
P-39 Expert
Non-Expert
Regarding the 9.6 design, the engine held together just fine but the actual supercharger gears themselves couldn't take the strain of the step up from 8.8. The actual drive gear had one more tooth and the driven gear had one less tooth to go from 8.8 to 9.6. Same thing in reverse to go from 8.8 to 8.1 (for two stage engines). Allison was hoping to just substitute the 9.6 gears for the 8.8 gears and reap the benefit of more horsepower at higher altitude, but that was not to be without strengthening the gears by widening them which took about a year. Dang. The gear ratio would go all the way up to 11.6 without increasing the diameter of the gears, but Allison never made production engines higher than 9.6.
So, they kind of were suffering a bit of wishful thinking, combined with miscalculation?
P-39 Expert
Non-Expert
I don't know exactly, but you have to think that they weren't expecting the gears to wear that quickly. Cost them a year to redesign the accessories case to accommodate the wider gears. Could have had the new engine in early '42 instead of late '42. Of course they still had to deal with the backfire screens etc until mid '42 when they adopted the aluminum intake manifold and deleted the backfire screens. And raised the military power limit from 5 minutes to 15 minutes.
wuzak
Captain
You quoted me without a link to the original post so the question lacks context.
Shortround6
Major General
The supercharger was the same size. The engine would hold together, the supercharger gears would not.
As a rule of thumb a supercharger's required power to drive the impeller goes up the square of the speed of the impeller.
So one might expect the Allison supercharger drive gears to need to handle abou 19% more power using the 9.6 ratio than the 8.80 ratio and indeed an Allison chart shows a bit over 210hp needed to flow 10,000lbs per hour with the 8.80 gears and just over 250hp to flow 10,000lbs per hour using the 9.60 gears. Allison may have decided not to make the same mistake twice and widened the gears to handle any foreseeable increase in required power.
There is an upper limit as to how fast you can spin an impeller. If the tip speed approaches the speed of sound the efficiency goes to pot in a hurry, The speed of sound is higher in the high pressure, high temperature environment inside the supercharger but the limit is there.
One can understand why Allison designed a new supercharger for the later engines (the G series) rather than try to simply increase the gear ratio.
Ironically, had they done more work early on, they'd have saved themselves a lot of trouble later.
That would have had a significant effect on their usefulness in the war. As for the differences of ram compression by using a redesigned intake, as seen on the P-63 seems to increase critical altitude by around 900 feet over the P-39D (2000 vs 2900)
Sorry about that.
- Lighter weight and improved Aerodynamics: Page 2, Post 32
- Use of the Birman supercharger: Page 5, Post 88
I remember this discussion coming up regarding the amount of power related to the square of the supercharger RPM. I figure, this was already known prior to the decision to redesign the gear, so I figure the culprits would be...
- The new supercharger gear would do better than it did: Miscalculation
- The desire to make minimal design changes: Laziness/Over-ambition
- An improperly calibrated and polished crystal-ball.
Shortround6
Major General
I would note the British ran into a somewhat related problem with the Merlin once higher boost levels were being put into use. The original supercharger drive (exact parts I don't know but I think it was a quill shaft?) was good for up to 15lbs of boost (perhaps 16lbs depending on gear ratio/altitude?) but had to modified for 18lb of boost, this is for the singel stage engines.
Allison may have been using the RR development technique, flog it until it breaks, modify/fix it and flog it again.
Allison may have been using the RR development technique, flog it until it breaks, modify/fix it and flog it again.
wuzak
Captain
In terms of aerodynamics there are many improvements that could have been made with the XP-39 given time and effort.
However, the turbo was not reliable at the time of the XP-39 (with issues on the XP-37 and YP-37) and reworking the aerodynamics was easier without the turbo.
But it seems that it took them a lot quicker to overcome their problems than Allison did with their higher geared superchargers...
I'm pretty sure it didn't make production either. I'm curious what variables caused such a short service life. I'm curious where would one even look to find information on the Birman supercharger?
Do you have any examples?
That makes sense. There was this book I got recently which showed a bunch of aircraft designs, including proposals for the P-39. Interestingly, it actually showed one design with the turbo lying flat in the wing. It was a very unusual configuration.
