Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Stoney on March 24, 2008, 11:09:01 PM
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I appologize in advance for not researching it myself, but I had a thought mid-post in another thread and was wondering if any tests were performed on the P-51 to turbo-charge the engine instead of super-charging. Given that the P-47's impressive performance at altitude had as much to do with the extreme critical altitude of the engine as a result of turbo vice super-charging, I was curious as to whether or not the P-51 could have carried that same potential. Perhaps a difference in induction methods for in-line engines as illustrated by the P-38's problems?
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They weren't turbocharged INSTEAD of supercharged. They were turbocharged as well as supercharged. The reason the P-38 had turbochargers is that it did not have two stage two speed superchargers. It had two stage single speed centrifugal crankshaft driven superchargers, that were fed by GE B series turbochargers. The purpose of the turbocharger was to provide the supercharger with sea level air.
The P-39 originally WAS turbocharged, NACA and the USAAC felt streamlining was more important than power, so they got rid of the turbocharger, and as a result the P-39 had less than adequate performance at higher altitudes. The P-40 could have been turbocharged, in fact, I've read that plans were drawn up. Since the P-51 was originally equipped with an Allison, it too could probably have been turbocharged, and had it been, it never would have needed the Merlin. The Allison had more power at 29K when turbocharged than the Merlin could hope to.
It was not that there was a problem with turbocharging inline engines (the Allison is a Vee type, not a true inline). There were some problems with the way the system on the P-38 worked. It got better with later models, and after the H model, if the crew chief knew what he was doing, and the pilot was trained properly, the only thing a P-38 had to fear engine wise was bad fuel.
No Merlins were turbocharged that I am aware of, and I'm not sure the design lends itself to turbocharging.
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I sincerely doubt the turbocharged Allison developed more power at 29k than the Merlin 76 (used in high-alt Mossie bombers and night fighters).
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They weren't turbocharged INSTEAD of supercharged. They were turbocharged as well as supercharged. The reason the P-38 had turbochargers is that it did not have two stage two speed superchargers. It had two stage single speed centrifugal crankshaft driven superchargers, that were fed by GE B series turbochargers. The purpose of the turbocharger was to provide the supercharger with sea level air.
The P-39 originally WAS turbocharged, NACA and the USAAC felt streamlining was more important than power, so they got rid of the turbocharger, and as a result the P-39 had less than adequate performance at higher altitudes. The P-40 could have been turbocharged, in fact, I've read that plans were drawn up. Since the P-51 was originally equipped with an Allison, it too could probably have been turbocharged, and had it been, it never would have needed the Merlin. The Allison had more power at 29K when turbocharged than the Merlin could hope to.
It was not that there was a problem with turbocharging inline engines (the Allison is a Vee type, not a true inline). There were some problems with the way the system on the P-38 worked. It got better with later models, and after the H model, if the crew chief knew what he was doing, and the pilot was trained properly, the only thing a P-38 had to fear engine wise was bad fuel.
No Merlins were turbocharged that I am aware of, and I'm not sure the design lends itself to turbocharging.
Lots of poor terminology on my part then. I suppose turbo-supercharged would be the proper terminology, and the Merlin was a V-12, so inline was an improper description as well. I suppose scavenging the exhaust of the Merlin would mean the Pony's cowl would have to be totally redesigned.
Anyway, just curious...thanks.
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I sincerely doubt the turbocharged Allison developed more power at 29k than the Merlin 76 (used in high-alt Mossie bombers and night fighters).
For that, you simply compare the engines' of the P-38 (individually, of course) to the Merlin, at the desired alt. Depending on what you go by; Some will say that it was possible for factory reps' and USAAF ground crews to tune an extra 400 H.P. out of the Allisons' in the P-38.
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I'm afraid I don't have those stats. Do you have the power curves of the turbo Allison and the Merlin 76 (or another 70-series Merlin)?
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I don't. I'll look for it, or maybe someone such as Widewing might have the data? Give me some time, I'm sure it's all lurking online somewhere...Here's a little something on the Allison:http://www.unlimitedexcitement.com/Miss%20US/Allison%20V1710%20Engine.htm (http://www.unlimitedexcitement.com/Miss%20US/Allison%20V1710%20Engine.htm)
And a little info on the Merlin: http://www.unlimitedexcitement.com/Pride%20of%20Pay%20n%20Pak/Rolls-Royce%20Merlin%20V-1650%20Engine.htm (http://www.unlimitedexcitement.com/Pride%20of%20Pay%20n%20Pak/Rolls-Royce%20Merlin%20V-1650%20Engine.htm)
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Wrong Merlin and I don't think any war time Allisons had a WEP rating of 2,250 hp, but don't bother searching unless this is important to you. My remark was directed at Captain Virgil Hilts comment hoping he could offer some insight into his claim. It's no big deal.
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An Allison in a P-38J could maintain sea level power to at least 26K feet. And about 90% to 95% all the way to about 30K feet. No Merlin in any P-51, at least in factory tune, could maintain sea level power that high. The Merlin in the P-51D was good below 18K, and then again around 22K, right after the supercharger gear drive shifts to the high speed gearing. Some tables say the Merlin in the P-51D was down to around 1250 or so at 26K, and down to 900 or so by 29K.
We were not discussing Merlins in Spitfires or Mosquitos, we were discussing P-51s. Yes, there were Merlins tuned specifically for high altitudes. They had different supercharger gearing. Those engines could not operate at peak output any where other than those high altitudes, or they'd overboost and blow up.
The limitation on ANY crank driven supercharger, especially in aircraft, is that the supercharger must be both sized AND geared for a specific output at a specific altitude. To a limited extent, this can be overcome with a two speed gear box, as the Merlin had. However, you still have only two narrow peak operating ranges, and performance suffers outside those operating ranges. With the Merlin, you have two ranges of about 5K or so where it has good efficiency. Where those two ranges are depends on the supercharger gearing. Further, you can only spin a supercharger to a certain speed, beyond that point, efficiency drops dramatically, and it begins to absorb a much higher percentage of the power it adds.
A turbocharger will also over speed, there were warning lights and buzzers on both the P-38 and the P-47. Usualy, around 30K or so, the GE B series turbocharger on the P-38 and the P-47 could no longer maintain full pressure without exceeding the RPM limit. However, the turbocharger has a far wider operating range than a supercharger, and it also has far less parasitic drag, since it uses exhaust pressure and heat to power it, as opposed to directly robbing HP at the crankshaft as a crank driven supercharger does.
Yes, if you added a turbocharger to the Merlin in the P-51, the exhaust would change drastically. The only place to mount a turbocharger on the P-51D would be behind the fuel tank in the fuselage. That would mean the exhaust would go back there, under the cockpit, over the radiator, to the turbocharger, and then exhaust there. The pressurized air would then have to be ducted forward, through an intercooler, then into the supercharger intake. You'd also have to run oil from the engine to the turbocharger and back. I do not think the Merlin could deal with it, the burden on the oil system would likely be fatal, as the oil system is marginal at best. I doubt the cooling system would handle it either.
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P-51
http://www.wwiiaircraftperformance.org/mustang/mustangtest.html
P-51B fitted with the V-1650-3 version of the Merlin had better high altitude performance than later B and D model Mustangs fitted with the V-1650-7. V-1650-3 put out 1275hp at 29800ft at 60.5" giving 441mph. (67" gave a higher power and top speed, but at a lower altitude)
P-51H had a different version of the Merlin which had a somewhat higher power output at altitude, but was also using water injection.
P-38
http://www.wwiiaircraftperformance.org/p-38/p-38.html
P-38L:
"True speed at critical altitude, 26,200 feet, at military power, 1385 BHP and 3000 RPM, was 408 MPH"
"True speed at critical altitude, 32,200 feet, at normal rated power, 1138 BHP, was 400 MPH"
P-38J:
http://www.wwiiaircraftperformance.org/p-38/p-38-67869.html
when level: 1312hp at 30000ft giving 413.5mph.
in a climb: 1220hp at 3000ft giving 1830fpm climbrate.
Mosquito
http://www.wwiiaircraftperformance.org/mosquito/mosquito.html
http://www.wwiiaircraftperformance.org/mosquito/merlin72-curve.jpg
Can extrapolate Merlin 72,73 power curve to get ~1200hp at 29000ft.
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Thank you for replying Captain Virgil Hilts, but there are a couple of things I'd like to address:
Yes we're talking about the P-51, but it would be much easier to just fit a different Merlin into the Pony than to add turbo charging to and Allison engined Pony. How much weight does the turbo system add? And you're wrong about the Merlin's super charger having "ranges of about 5K or so where it has good efficiency". In later models it didn't just have a two speed charger, it was also two stage with two separate impellers (Merlin 61 and later (including the Pony's Packard Merlins)).
http://www.wwiiaircraftperformance.org/mosquito/mm748.pdf
As you can see from this document the Merlin 72 is able to achieve sea level boost (+17.5 lbs) in two ranges covering no less than 21,000 feet. The first stage/small-impeller provides full pressure from SL up to 12,200 feet, while the second stage/large-impeller provides full pressure from 16,000 feet to 24,800 feet. Full pressure heights are at 12.2k and 24.8k for each stage respectively. Couldn't find info on the Merlin 76 though. It was supposed to have a second-stage full pressure height of 29k or so.
Your use of the term "parasitic drag" in that context is incorrect and unfortunate.
Also I find your notion that you have to run oil from the engine to the turbocharger and back impractical. Why couldn't the turbo have its own little oil system powered by the turbine itself? In most turbo applications the turbo has a separate lubrication system.
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If TUXC's numbers are correct, and I have no reason to suspect they are not, then the Merlins and turbo charged Allison seem to be remarkably similar in power at around 30k. Also I assume those hp figures for the Merlins do not include exhaust thrust?
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Thank you for replying Captain Virgil Hilts, but there are a couple of things I'd like to address:
Yes we're talking about the P-51, but it would be much easier to just fit a different Merlin into the Pony than to add turbo charging to and Allison engined Pony. How much weight does the turbo system add? And you're wrong about the Merlin's super charger having "ranges of about 5K or so where it has good efficiency". In later models it didn't just have a two speed charger, it was also two stage with two separate impellers (Merlin 61 and later (including the Pony's Packard Merlins)).
http://www.wwiiaircraftperformance.org/mosquito/mm748.pdf
As you can see from this document the Merlin 72 is able to achieve sea level boost (+17.5 lbs) in two ranges covering no less than 21,000 feet. The first stage/small-impeller provides full pressure from SL up to 12,200 feet, while the second stage/large-impeller provides full pressure from 16,000 feet to 24,800 feet. Full pressure heights are at 12.2k and 24.8k for each stage respectively. Couldn't find info on the Merlin 76 though. It was supposed to have a second-stage full pressure height of 29k or so.
Your use of the term "parasitic drag" in that context is incorrect and unfortunate.
Also I find your notion that you have to run oil from the engine to the turbocharger and back impractical. Why couldn't the turbo have its own little oil system powered by the turbine itself? In most turbo applications the turbo has a separate lubrication system.
You can call it what you want, but a crank driven supercharger absorbs HP from the crank that could be used to drive whatever the crank is driving. Parasitic drag is as good a term as any. As an example, in a current Top Fuel dragster engine, that generates around 8000HP (that's 1000HP per cylinder), the supercharger absorbs almost 1000HP.
Most centrifugal superchargers for aircraft engines are two stage, the Allison uses a two stage. The Merlin supercharger is also two speed, as well as two stage. I didn't bother to mention that because it is obvious to most people.
I never said it'd be a good idea to turbocharge a P-51. In fact, if you read my post, I listed numerous reasons NOT to do it. Regardless of whether it is Merlin or Allison powered.
Just because a supercharger can maintain boost does not make it efficient. You left out how much power it absorbs while doing so, and how much it heats the air while doing so. The faster you spin it, whether by gearing or not, the more power it absorbs, and the more it heats the air. And the further you get out of its range, the worse the problem gets.
By your own document, the Merlin cannot hold sea level power between 12,200 feet and 16,000 feet, and not above 24,800 feet. Where as the turbocharged Allison maintains sea level power from sea level to well over 26,000 feet. The higher altitude you require sea level power at, the bigger the hole in the power curve for the Merlin.
My "notion" is not impractical, it is the way it is done. My turbocharged Buick Regal is pretty much the standard for turbocharger systems, and the engine supplies the oil for the turbocharger. In most applications, all of them that I have ever seen or worked on, in about 30 years or so, the engine supplies the oil to the turbocharger (and the turbocharger returns heated oil, after use for cooling and lubrication, back to the crank case of the engine). That includes aircraft, passenger cars, race cars, and trucks, light, medium, and heavy duty. As well as heavy equipment and tractors. A turbocharger requires a great deal of oil, for both lubrication AND cooling. NO, it is NOT practical to have a separate lubrication system for the turbocharger. Got any idea how much HP it takes to drive an oil pump to create 70 pounds of pressure, and move several gallons of oil per minute? I do, and if you forced a turbocharger to try and drive such a pump you'd shear the shaft off of it immediately.
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Lumpy, Stoney, Virgil, and everyone else;
I believe that the limitations' in TC'ing the P-51 were not so much in the Merlin or Allison engines' themselves, but rather the airframe. It was Turbocharging that dictated much of the design of the P-47, because a Turbo large enough to work with the P&W R-2800 was huge. The deep fuselage on the Thunderbolt was due to Intake and Exhaust ducting running under the cockpit, to the Turbo itself mounted in the rear fuselage.
Now, the P-38's Twin Allisons' were both Turbocharged, but I don't believe that the Turbo's for the smaller displacement Allison's were quite as large. However, the P-38 was a large fighter, with extra space in the boom's for the Turbos'. Which it had, from it's first design. The P-51 would be somewhat space-constricted to fit the turbo anywhere, IMHO.
BTW Lumpy...If the turbo uses' the same oil as the engine, the extra oil lines' would be nowhere near as much a problem as a seperate oil system just to run the turbo, which would add much more weight than just the lines, and would also rob power from the Turbo itself, just to spin the oil pump. This could cause quite a bit of Turbocharger "lag", or a reduction in turbine speeds' that would impair turbo function.
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Captain Virgil Hilts, parasitic drag is a term used in aircraft nomenclature for something completely different. As such your misuse of the terms is unfortunate. The Allison engine also uses a supercharger that leaches power from the crank, and in physics you don't get anything for free; the turbo creates back pressure that the engine must overcome using HP and with a turbo the exhaust thrust effect is almost negligible.
Unless you can provide documentation to the contrary, the performance number presented in this thread has shown your claim that "the Allison had more power at 29K when turbocharged than the Merlin could hope to" is a gross exaggeration. The numbers show that the turbo Allison and the Packard Merlin V-1650-3 already in use in P-51B's are almost identical in power at 29k, and I still assume the Merlin's hp rating is without exhaust thrust.
As for the efficiency of the Merlin's supercharger, with the power chart posted by TUXC it is easy to see that the difference in power between SL and 1st stage full pressure height is ca. 100 hp, or less than 10% of the engine's power. The power difference between the start of second stage to full pressure height is about 50-60 hp (edit: more like 70 hp), less than 5% of the engine's power.
As for the turbo oil system, I'm sure you're right with your experience.
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Both a supercharger and a turbocharger have parasitic drag.
However......a turbocharger requires a lot less energy to do the same amount of work.
A supercharger has multiple shaft seals, high thrust loading, bearings,gears, oil seals, and rotor tip seals (depending on the design). A supercharger also requires a heavy gearbox and drive system increasing the power loss. A turbo charger usually only has a few seals and bearings. It doesnt require a seperate drive system and everything is usually engine independent except oil supply. For a given horsepower used a turbocharger will out perform a supercharger in almost all circumstances. (This is assuming you have properly designed components through out.)
Strip(er)
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The P-51B with the V-1650-3 outperformed P-47Ds and P-38s at high altitude when it entered service late 1943. Only the late P-47 versions (M and N) offered somewhat better performance at high altitude and that time there were even better high altitude versions of the Merlin available if better performance had been really needed.
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Both a supercharger and a turbocharger have parasitic drag.
However......a turbocharger requires a lot less energy to do the same amount of work.
A supercharger has multiple shaft seals, high thrust loading, bearings,gears, oil seals, and rotor tip seals (depending on the design). A supercharger also requires a heavy gearbox and drive system increasing the power loss. A turbo charger usually only has a few seals and bearings. It doesnt require a seperate drive system and everything is usually engine independent except oil supply. For a given horsepower used a turbocharger will out perform a supercharger in almost all circumstances. (This is assuming you have properly designed components through out.)
Strip(er)
That's because the turbo uses exhaust pressure and heat to drive the compressor. The exhaust pressure is normally wasted on non-turbo car engines, and is usually not powerful enough to have an effect on the car's performance anyway. In big WWII aircraft engines however the exhaust thrust was very important, often being a significant part of the total engine power.
(http://www.wwiiaircraftperformance.org/p-38/p-38-wayne-fig3c.jpg)
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My "notion" is not impractical, it is the way it is done. My turbocharged Buick Regal is pretty much the standard for turbocharger systems, and the engine supplies the oil for the turbocharger. In most applications, all of them that I have ever seen or worked on, in about 30 years or so, the engine supplies the oil to the turbocharger (and the turbocharger returns heated oil, after use for cooling and lubrication, back to the crank case of the engine). That includes aircraft, passenger cars, race cars, and trucks, light, medium, and heavy duty. As well as heavy equipment and tractors. A turbocharger requires a great deal of oil, for both lubrication AND cooling. NO, it is NOT practical to have a separate lubrication system for the turbocharger. Got any idea how much HP it takes to drive an oil pump to create 70 pounds of pressure, and move several gallons of oil per minute? I do, and if you forced a turbocharger to try and drive such a pump you'd shear the shaft off of it immediately.
Virgil, just an FYI for you.
The turbos on the P-38, P-47, and B-17 all get their oil from a system seperate from the engine's oil supply. The B-24 uses the same oil supply as the engine.
On the P-38, it is located and filled in the nacelles just to the aft of the turbos, and is a three gallon tank.
On the P-47, it is located and filled just aft of the right side of the cockpit, and is roughly a three gallon system.
On the B-17, it is located just aft of the engine nacelles in the wing, and is filled from the top of the main wing, also a three gallon tank.
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Captain Virgil Hilts, I retract my previous comment regarding oil systems and your experience.
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Bohdi,
just outta curiousity,what drives the pump for this sepperate oil system??
It makes sense to have the charger oil sepperate from engine oil as this would
take a large burden off the engine cooling system.
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On the type B-33 in the P-38, the tank is connected to a gear type combination scavenging and pressure pump mounted on the supercharger.
This is from the P-38L erection and maintenance manual revised 15 January 1945.
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Wasn't there a test done with Merlins in the P-38?
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Wasn't there a test done with Merlins in the P-38?
No, it was only a study exercise. Result was switching to Merlins provided no advantage to the a/c.