Why or why not turbo chargers

[renrich]

A question has been bugging me and I hope some of you more technically minded can answer it. The P47 had a turbo supercharged engine which allowed the engine to hold up it's power from sea level to quite high altitude and it seemed to work well. The P38 and some bombers like the B17 also used turbo chargers. AFAIK, the Spitfire, BF109, FW190, Corsair, Hellcat, etc. all used superchargers which had peaks in the performance curves and did not keep the power up entirely much above 25000 feet. Why did not those premier fighters use turbo superchargers operationally?

 

[Marshall_Stack]

One reason that I read is that the US had trouble getting enough turbos from GE in the first place. I read that the P-70 night fighter (an A20 with radar) needed turbos but priority was for the fighters you mention and all of the heavy bombers.

Turbos add weight and complexity too. They tried putting turbos on the P-40, Corsair, and P-39 at various points but had troubles with them. That surprises me because Allison and P&W engines have been installed with turbos on the P-38 and P-47 respectively.

 

[SoD Stitch]

A question has been bugging me and I hope some of you more technically minded can answer it. The P47 had a turbo supercharged engine which allowed the engine to hold up it's power from sea level to quite high altitude and it seemed to work well. The P38 and some bombers like the B17 also used turbo chargers. AFAIK, the Spitfire, BF109, FW190, Corsair, Hellcat, etc. all used superchargers which had peaks in the performance curves and did not keep the power up entirely much above 25000 feet. Why did not those premier fighters use turbo superchargers operationally?

I'm no expert on forced induction, but so-called "turbo-superchargers" took up a lot more space than a regular single-speed or two-speed supercharger. You may have noticed that the P-47 had the largest fuselage for a single-engine fighter of the War, there was a reason for that; the fuselage had to be big enough to contain all of the ducting involved in using a turbo-supercharger, especially since the turbo-supercharger was located in the rear of the fuselage, behind the pilot. In the case of the P-38, the twin booms provided a convenient place to put the turbo-superchargers without a lot of extra ducting. There may have been room to put a turbo-supercharger in the Corsair or the Hellcat, since they had fairly roomy fuselages already, but there was simply nowhere to put them in a Spit or an F-190.

Here's a good drawing of all the plumbing involved in the induction system for the P-47:

 

[davebender]

Not to mention that WWII era turbocharged aircraft cost through the nose. Nobody but the USA could afford to pay three times as much to purchase a turbocharged fighter aircraft that was arguably inferior to the competition.

1941 aircraft cost (assume 2.5 marks per dollar)
http://www.maxwell.af.mil/au/afhra/aafsd/aafsd_pdf/t082.pdf
$ 134,284. P-38.
$ 113,246. P-47.
$ 34,388. Me-109. (85,970 marks)

 

[tomo pauk]

???
What US planes w/ turbocharger were inferior to the competition?

 

[delcyros]

It´s also a matter of the desired performance envelope.
A turbo-supercharger requires the exhoust gase of the engine to operate. A mechanically driven supercharger eats up engine power but does not use up the energy stored in the exhoust gases.
These could be utilized for some significant amount of extra power by jet stacks.
I rephrase what I wrote with regard to the Jumo-213 / RW-2800 discussion in the B-29 vs Luftwaffe thread, as it examplifies the problem:

Turbocharging is not necessarely an efficiant instrument to augment performance at high altitude and high speed, altough it appearently worked very well in the RW-2800.
The Jumo-213E at critical altitude developed 200 Kp of exhoust jet thrust.
That´s quite a lot when You consider that the Jumo-004D was producing a mere 380 Kp thrust at this altitude when running at 100% load.
This equates to 200kg or 2000N which at a speed of 200m/s(440mph) from Power = force x velocity = 400kW at the shaft.
Factoring in the propellor inefficiencies at high altitudes this equates to the aequivalent of ~500kW or ca. 670 hp more power developed by the engine in the first place.
Second order maybe but still to substantial to be ignored.
As you know, to get a 10% increase in speed requires approximtely a cubed increase in power whereas it requires only a squared increase in thrust! It does not come in with increased cooling requirements, too (which would add drag in return)! So it´s maybe not worth to waste that source of power to drive a turbocharger, that´s at least the reason why Rolls Royce invested so much in turbocharging. They tried to find a solution to keep the jet exhoust thrust.

Considering that You get basically comparable high speed performance by a lot simplier and subtsantially lighter powerplant design without turbocharger, the supercharger option is attractive for lightweight, high speed high altitude A/C such as single engined, single seat fighters.
The turbo-supercharger offers significantly better poweroutput at high altitudes and lower speeds, so it´s the more attractive solution for slower flying, high altitude planes, such as bombers and load carriers.
Jet exhoust stacks were introduced in the early 40´s, when the P-47/P-38 design was already advanced. For a considerable time, there was an uncertainity whether or not turbocharged engines offer the best high altitude performances compared to jet augmented superchargers or slave engine driven* versions wrt to weights and power.

* Compare the design of the Do-217P to the Do-217M. The-P high altitude subversion had a slave engine (a turbocharged db-605T) mounted in the bombbay to drive compressors for both Db-603 engines (which were jet augmented). The main engines didn´t dropped in performance until about 33.000 ft. altitude were reached and developed full exhoust jet power at this altitude. Claims are raised that the Do-217P would fly very fast in the coffin corner between Mach tuck and stall, exceeding 425 mp/h (claims go high as 488 mp/h at 9600m with the Do-217P V04, which I consider less believable) at very high altitude with a service ceiling in excess of 50.000ft.

 

[renrich]

I knew that one reason the P47 had as big a fuselage as it had was partly because of the space used for the ductwork and I suppose that additional drag caused by the big fuselage was the reason for it's indifferent performance at low altitude at least until the P47M. The engines on the B17 had turbo superchargers, I believe. Why no space penalty there? I believe the Mustang achieved a little more thrust with exhaust jetting but what other US fighters had that feature. Are youall saying that the Spitfire, for example was just to small to accomodate turbo supercharging?

 

[delcyros]

The B-17´s turbos are partly ducted externally and of a less sophisticated design than those of the P-47 with all it´s after- and intercoolers.
The Spitfire, Bf-109 and Fw-190 appears to have not enough space for a complete internal installation of the ducting required for an effective turbocharger.

B-17 turbocharger detail on Flickr - Photo Sharing!

 

[renrich]

Thanks Del, that explains things nicely. I have seen photos of the XF4U3, I think, which was the turbo charged Corsair and it seems there were some bulbous exterior features which would have added drag.

 

[davparlr]

It´s also a matter of the desired performance envelope.
A turbo-supercharger requires the exhoust gase of the engine to operate. A mechanically driven supercharger eats up engine power but does not use up the energy stored in the exhoust gases.
These could be utilized for some significant amount of extra power by jet stacks.
I rephrase what I wrote with regard to the Jumo-213 / RW-2800 discussion in the B-29 vs Luftwaffe thread, as it examplifies the problem:



Considering that You get basically comparable high speed performance by a lot simplier and subtsantially lighter powerplant design without turbocharger, the supercharger option is attractive for lightweight, high speed high altitude A/C such as single engined, single seat fighters.
The turbo-supercharger offers significantly better poweroutput at high altitudes and lower speeds, so it´s the more attractive solution for slower flying, high altitude planes, such as bombers and load carriers.
Jet exhoust stacks were introduced in the early 40´s, when the P-47/P-38 design was already advanced. For a considerable time, there was an uncertainity whether or not turbocharged engines offer the best high altitude performances compared to jet augmented superchargers or slave engine driven* versions wrt to weights and power.

According to Spitfire Performance, test on the Merlin shows about 20% thrust horsepower is gained by exhaust gas thrust.

If we compare the P-47M to the Ta-152H, one with turbo supercharger and the other with supercharger and both designed for high altitude combat, we can see some of the tradeoffs of the turbo supercharger in performance.

At SL, hp, airspeed
Ta-152H 2050hp, 370 mph
P-47 2600hp, 365 mph
It is apparent that the Ta-152H is a much cleaner airframe

10k
Ta-152H, 1890 hp, 397 mph
P-47M 2800 hp, 405 mph

20k
Ta-152H, 1690 hp, 436 mph
P-47M, 2800 hp, 437

30k
Ta-152H, 1340 hp, 463 mph
P-47 2800 hp, 467

33k
Ta-152H, 1300 hp, 458 mph
P-47 2800 hp, 475 mph

35k
Ta-152H, 1200 hp, 455 mph
P-47, 2600 hp, 475 mph


Now assuming the Jumo 213E engine has an advanced and efficient supercharger, it is apparent that the turbo-superchargers are significantly better capable of maintaining horsepower than superchargers by themselves. Now if we do some manipulations with the P-47 engine we should be able to somewhat compare the turbo with the non-turbo. Assuming an imaginary P-47 with a similar supercharger set up as the Ta-152H and a max SL hp of 2600, and assuming it has the same power profile as the Ta-152H, we find that the imaginary P-47 would generate approximately 1690 hp at 30k ft., or 1110 hp less than the turbo-supercharged version. I don't believe that exhaust thrust can make that up. I would have preferred a comparison to the F4U-5, a PW2800 powered supercharged high altitude fighter, but I did not have the necessary data.

Conversely, if we take the Ta-152H and add a turbo-supercharger that performed as the P-47M turbo-supercharger did, it would be generating 2050 hp at 30k ft, or 710 more hp than the supercharged version, about 50% more power. I don't think exhaust thrust can come near making up for this difference.

So, the question is whether this performance makes up for the added weight of the turbo-supercharger, about 940 lbs minus the added complexity of the supercharger. Apparently the engineers at Focke-Wulf felt that the performance did not warrant the increase in weight and complexity, and the engineers at Republic, with the knowledge of the exhaust thrust by the time the P-47M/N came along, felt that the turbo-supercharger still provided the best answer for their performance envelop.

This is a light overview of a complex topic of which I have little knowledge, but it might show some of the issues facing a designer.

The turbo-supercharged PW R-2800-57 was one of the most impressive piston engines of WWII with 2800 hp flat rated to 33k ft and with probably 2000 hp available at 40k. There were few engines that flew operationally, if any, able to match it.

 

[drgondog]

According to Spitfire Performance, test on the Merlin shows about 20% thrust horsepower is gained by exhaust gas thrust.

If we compare the P-47M to the Ta-152H, one with turbo supercharger and the other with supercharger and both designed for high altitude combat, we can see some of the tradeoffs of the turbo supercharger in performance.

At SL, hp, airspeed
Ta-152H 2050hp, 370 mph
P-47 2600hp, 365 mph
It is apparent that the Ta-152H is a much cleaner airframe

10k
Ta-152H, 1890 hp, 397 mph
P-47M 2800 hp, 405 mph

20k
Ta-152H, 1690 hp, 436 mph
P-47M, 2800 hp, 437

30k
Ta-152H, 1340 hp, 463 mph
P-47 2800 hp, 467

33k
Ta-152H, 1300 hp, 458 mph
P-47 2800 hp, 475 mph

35k
Ta-152H, 1200 hp, 455 mph
P-47, 2600 hp, 475 mph


Now assuming the Jumo 213E engine has an advanced and efficient supercharger, it is apparent that the turbo-superchargers are significantly better capable of maintaining horsepower than superchargers by themselves. Now if we do some manipulations with the P-47 engine we should be able to somewhat compare the turbo with the non-turbo. Assuming an imaginary P-47 with a similar supercharger set up as the Ta-152H and a max SL hp of 2600, and assuming it has the same power profile as the Ta-152H, we find that the imaginary P-47 would generate approximately 1690 hp at 30k ft., or 1110 hp less than the turbo-supercharged version. I don't believe that exhaust thrust can make that up. I would have preferred a comparison to the F4U-5, a PW2800 powered supercharged high altitude fighter, but I did not have the necessary data.

Conversely, if we take the Ta-152H and add a turbo-supercharger that performed as the P-47M turbo-supercharger did, it would be generating 2050 hp at 30k ft, or 710 more hp than the supercharged version, about 50% more power. I don't think exhaust thrust can come near making up for this difference.

So, the question is whether this performance makes up for the added weight of the turbo-supercharger, about 940 lbs minus the added complexity of the supercharger. Apparently the engineers at Focke-Wulf felt that the performance did not warrant the increase in weight and complexity, and the engineers at Republic, with the knowledge of the exhaust thrust by the time the P-47M/N came along, felt that the turbo-supercharger still provided the best answer for their performance envelop.

This is a light overview of a complex topic of which I have little knowledge, but it might show some of the issues facing a designer.

The turbo-supercharged PW R-2800-57 was one of the most impressive piston engines of WWII with 2800 hp flat rated to 33k ft and with probably 2000 hp available at 40k. There were few engines that flew operationally, if any, able to match it.

This is all correct.

It is important to note, that for Republic to put out a high altitude/high performance interceptor in 1941 they chose the only route available at the time for engines available.

Fw (Tank) never had 30,000 performance as high priority when designing the Fw 190 or probably would have accepted the weight/cost penalty.

 

[delcyros]

Thanks Davparl.
Your numbers appear to be correct. I just want to add that I really don´t see that much difference between these two specific cases. At 30K, Your estimation of a turbo-supercharged Ta-152 yields 710 hp more power. The jet thrust generated at this altitude did came close to that number. It was the aequivalent of ~670 hp generated by the powerplant in the first place.
Thrust is more "efficient" than power in translation to speed for any aircraft.
It does not reach the levels of the turbocharged P-47M, though.
However, a supercharged and thrust augmented P-47M may have been as fast, too. It might not achieve the exceptional flat powercurve typical for turbocharged powerplants but it still would augment speed significantly.
The question is how much weights are involved here. Does anyone have a good weight breakdown for the supercharger / turbocharger installations?

 

[davparlr]

Thanks Davparl.
Your numbers appear to be correct. I just want to add that I really don´t see that much difference between these two specific cases. At 30K, Your estimation of a turbo-supercharged Ta-152 yields 710 hp more power. The jet thrust generated at this altitude did came close to that number. It was the aequivalent of ~670 hp generated by the powerplant in the first place.
Thrust is more "efficient" than power in translation to speed for any aircraft.
It does not reach the levels of the turbocharged P-47M, though.
However, a supercharged and thrust augmented P-47M may have been as fast, too. It might not achieve the exceptional flat powercurve typical for turbocharged powerplants but it still would augment speed significantly.
The question is how much weights are involved here. Does anyone have a good weight breakdown for the supercharger / turbocharger installations?

It is definately an engineering trade off of performance and technology available and mission requirements.

The weight of the engine accessories for the P-47 is 940 lbs. This probably includes turbo system, generators, hydraulic pumps, etc. I do not know if it includes the engine mounted supercharger. Engine accessories for the F6F, with a similar engine, was 314 lbs without a turbo. So a guess is that the turbo system of the P-47 would have been about 600 lbs. Maybe somebody else has more detailed data.

 

[HoHun]

HI Davparlr,

>It is definately an engineering trade off of performance and technology available and mission requirements.

Here is a diagram von von Gersdorff et al., "Deutsche Flugmotore und Strahltriebwerke".

Caption: "A comparison calculated by the DVL demonstrates that the exhaust-gas-driven turbo-supercharger only yields a performance advantage over the mechanically-driven supercharger at great altitudes respectively at low airspeeds."

Left hand scale: "Propulsive power", right hand scale: "Fuel consumption", bottom scale, "Air speed".

Hatched graph: "Engine with jet exhausts", solid graph: "Engine with exhaust-driven turbo-supercharger".

Top graph in each diagram is propulsive power, bottom graph is fuel consumption.

Of course, the exact comparison will depend on the specific supercharger setup which is not given in the book, so we can only understand it as an illustration of the basic principles of each propulsion variant.

Regards,

Henning (HoHun)

 

[davebender]

a guess is that the turbo system of the P-47 would have been about 600 lbs.

Plus a larger and heavier aircraft to contain all the plumbing.

Lots of modern day cars have turbochargers. How do they keep the installation so compact?

 

[tomo pauk]

If I may:

Today's cars (most of them) do have abundant space under the bonnet, so the turbocharger can be neatly situated by the engine. Plus turbochargers are much smaller then engines that use them.

Plus a larger and heavier aircraft to contain all the plumbing.

Yep; the Jug is known to be designed around the engine with a sizable turbocharger.

Here is the what happens if the 'turbo' is as big as and 1.5 L gasoline engine
BMW Turbo F1 Engine

 

[Sweb]

Engines normally fitted with 2-stage superchargers were really adequate to the mission requirements of the time. To employ the use of space-greedy turbos weights had to increase, as did size, and power-to-weight ratios would decrease accordingly.

Combat engagements were short. 15 minute engagements would be considered a very long combat op. On both sides in the ETO water-methanol was used to provide a significant boost and lasted about 10 minutes. Once the boost tank was empty the plane was approx. 350 lbs lighter without adding into the gain the higher fuel consumptions during its use. Turbos could not deliver the (temporary) power increases that water injection could. The 51 was about as lean a machine as could be designed around a pilot and powerplant. It's weight and size would have significantly increased with turbo-charging and performance losses in maneuverability alone would most certainly have been a sacrifice to turbo-charging. That's why the F4U3 was dropped (aside from the glitchy turbo). So, the short and best answer was water injection, or MW50, to boost performances in combat.

 

[davparlr]

Plus a larger and heavier aircraft to contain all the plumbing.

Lots of modern day cars have turbochargers. How do they keep the installation so compact?

I would guess that generating one tenth the hp would be a clue.

 

[Clay_Allison]

Plus a larger and heavier aircraft to contain all the plumbing.

Lots of modern day cars have turbochargers. How do they keep the installation so compact?

plastic, ceramic, and alloy technology unavailable at the time. I think.

Plus it's a smaller engine.

 

[Marshall_Stack]

Here is a nice write-up on turbos....


The Turbosupercharger and the Airplane Power Plant