What of the Republic P-47 Turbo-Supercharger

[jmcalli2]

Still doesn't make sense to me; this engine is available to many aircraft; yet the P-47 utilizes this system of duct work and turbines to capture engine exhaust, cool and convert into supercharged air for the engine intake.

There are no engine exhausts headers on the the P-47 sticking out the side of the airframe, the exhaust is ducted to the turbo-supercharger, then there are controllable 'waste gates' of the turbo-supercharger.

The exhaust goes from the engine to the turbo, then exhausts from under the turbo well behind the wings. The waste gates are used to regulate the exhaust pressure to the turbo. The two flaps behind the waste gates are oil cooler exits.

 

[soaringtractor]

Due to its size, the Turbo-Supercharger(Turbocharger in modern parlance) was located in the aft fuselage. The exhaust was ducted underneath the pilot to the Turbo-Supercharger and then exited underneath the lower fuselage. The compressed(boosted) intake air was returned to the Supercharger of the R-2800 via additional ducting underneath the pilot. All of the plumbing and ducting, as well as the Turbo-Supercharger itself, in part dictated the large size of the plane itself. There is a pronounced hump underneath the rear fuselage where the exhaust exited from the Turbo-Supercharger. The Waste Gates regulated the amount of exhaust gases sent back to the Turbo-Supercharger, and I believe all the exhaust escaped through the Waste Gates during engine start.

Instead of a two stage Supercharger , which was common on other high altitude fighters, the R-2800 in the P-47 had a single stage Supercharger which was fed by the Turbo-Supercharger. This is the same set-up that was used in the P-38 Lightning, but the Lightning used the Allison V-1710 rather than the P&W R-2800.

It was unique because I believe the P-47 was the only single engined Turbo-Supercharged fighter in the Allied inventory. It was valuable because the Turbo-Supercharger generated more boost above 25,000 feet, and didn't require some of the engine horsepower to turn it. There was some loss because there was no thrust generated by the exhaust gas, but it was more than made up by the higher boost.

Actually the turbocharger was the second stage in that system in that the turbo fed the mechanical supercharger !!! Same as in the P38 !!!

 

[tommayer]

As a kid reading Robert S. Johnsons' "Thunderbolt", I was captured by his exploits and the aircraft he flew. I really became fond of the P-47; I found out it was huge for a fighter, didn't climb all well (not counting zoom climbs), wasn't all that good maneuvering (depending upon profiles of flight). But, was extremely durable, had powerful armament, was very one of the fastest WW2 aircraft at 25K alt and above, could dive like a home sick demon (and not break apart)...and, the top European Theater aces (Gabraski, Johnson, Muhurin) all flew P-47's and survived.

I used to wonder why there where no engine exhaust stacks from the massive PW R-2800 engine on the P-47? Those two portals on either side just in front and a little lower of the wings was the exhaust? Huh? I gather those two ports where called "Waste Gates"? Doesn't seem right compared to other supercharged aircraft where their exhaust stacks are all but clear to the eye.

So, what's up with the turbo-supercharger system of the P-47? Why so unique? Why valuable?

 

[tommayer]

Go to Greg's airplanes & automobiles. He has a four or five part series on the 47. The turbocharger is covered extensively and accurately. BTW, 47 definitely had engine exhaust stacks. You can see some of them in pics in the Johnson book, if you know where to look.

 

[GregP]

In the nice diagram above, the waste gate LOOKS like it is behind the turbocharger exhaust outlet, but it can also be in front and the pipe leading from the waste gate can be curved and go around the top of the turbocharger, as it does in the P-38. if you have the waste gate behind the turbo physically, then some small exhaust will always go through the turbo and make it spin at a low speed even when the waste gate is open. That can be a good thing because it avoids shock heating and cooling of the exhaust fan blades ... but even if the waste gate is open, the turbo won't spin fast enough to generate boost.

 

[Dash119]

Actually the turbocharger was the second stage in that system in that the turbo fed the mechanical supercharger !!! Same as in the P38 !!!

I would think that since the Turbo-Supercharger feeds the Supercharger we would refer to the Turbo-Supercharger as the first stage of a two stage system.

But I have been wrong before...

Kim

 

[wuzak]

I would think that since the Turbo-Supercharger feeds the Supercharger we would refer to the Turbo-Supercharger as the first stage of a two stage system.

But I have been wrong before...

Kim

I would agree with that.

 

[Denniss]

And on sealevel the Turbo feeds what? Nothing as that's purely done by the mechanical supercharger of the engine. The additional feeding of the Turbo is the second stage.

 

[P-39 Expert]

In a nutshell, turbos were better in multi-engined planes (P-38, B-17 and B-24) because there was much more internal room for the turbo, intercooler and ducting.

A turbo in a single engined plane has to end up looking like a P-47, not really any other way to do it.

The P-47 sacrificed almost every fighter performance figure (takeoff run, climb rate, maneuverability) for top speed over 25000'. The only time performance was needed at that altitude was bomber escort, and the P-47 didn't have the range for that. All that extra weight and structure made the P-47 a very tough airplane, but the object was to be doing the shooting, not receiving it.

Mechanical two stage (Merlin 60 series, Allison -93) were far more compact resulting in a smaller, cleaner airframe and much better fuel efficiency. The loss of horsepower to drive the mechanical supercharger was partially recovered by exhaust thrust which was not available with a turbo.

Hindsight is a bitch. Just my opinion.

 

[Juha3]

In fact P-47D had good roll-rate, very good at high speeds, so it had a ace against 109Gs in maneuverability, it was also fast at all altitudes, in 1943 -47D-10 was faster than 109G-1 (1.3 ata) from SL upwards and faster than Fw 190A-5 over 8,000 ft, difference was small up to 22,000ft but over that marked.

 

[Shortround6]

In a nutshell, turbos were better in multi-engined planes (P-38, B-17 and B-24) because there was much more internal room for the turbo, intercooler and ducting.

A turbo in a single engined plane has to end up looking like a P-47, not really any other way to do it.

The P-47 wound up as fat as it was for several reasons. One is, as everyone says, the turbo installation. but lets remember that the P-47 also carried 305 US gallons of fuel in the fuselage. about 3 times what a Spitfire or 109 carried. and about twice what a P-40 carried.

The P-47 sacrificed almost every fighter performance figure (takeoff run, climb rate, maneuverability) for top speed over 25000'. The only time performance was needed at that altitude was bomber escort, and the P-47 didn't have the range for that. All that extra weight and structure made the P-47 a very tough airplane, but the object was to be doing the shooting, not receiving it.

Again using hindsight. the P-47 was designed and ordered in 1940, the actual need for a long range escort at the time was near non-existent. The first B-17E was ordered in Aug of 1940 and didn't fly until Sept of 1941, this is the first B-17 with a power turret. The P-47, despite the big engine, had an operational radius about 50% to twice that of the smaller fighters with none of them carrying drop tanks. True it turned out to be a lot less than what was needed but P-47 was not a short ranged fighter when first designed/adopted.

A major part of the P-47s "problems" was the armament, as designed it carried about 570lbs worth of guns and and over 1000lbs worth of ammunition. You simply can't carry that load in a small airplane. The volume of the guns and ammo forced the positioning of the fuel tanks into the fuselage despite the 300 sq ft wing. A few other factors here, one being Republic's lousy experiences with integral wing tanks in the P-43.

Mechanical two stage (Merlin 60 series, Allison -93) were far more compact resulting in a smaller, cleaner airframe and much better fuel efficiency. The loss of horsepower to drive the mechanical supercharger was partially recovered by exhaust thrust which was not available with a turbo.

The fuel efficiency is debatable. The smaller planes were more fuel efficient due to size. Wither the engine set ups were more fuel efficient is another story.
Exhaust thrust works well at full or nearly full throttle. It is a lot less useful at cruising speeds. the trust is the mass of the exhaust times the velocity. At cruise settings the mass could be under 1/2 of the thrust at full throttle, sometimes well under.
Next is power vs thrust. you get the most power from the same amount of thrust when you have the closest match to the speed of the exhaust jet/s to the speed of the aircraft.

to try to keep comparisons as equal as possible try comparing the P-47 to the F4U and F6F which used the same basic engine but with the two stage mechanical supercharger and which use six .50 cal guns with about 400rpg or about 3/4s the weapons load of the P-47 as designed.

 

[GregP]

The P-47 turbocharger doesn't actually feed the supercharger. It is connected to the intercooler which, in turn, feeds the carburetor. The carburetor feeds the supercharger impeller.

In the case of the P-38, early P-38s with the streamlined cowling had hollow leading edges that were used as intercoolers for the turbocharger. Later P-38s had the deeper cowling and the intercooler was in the center of the drooped part of the cowling with the oil coolers on either side. So, the turbocharger fed the intercooler, which fed the carburetor which fed the supercharger impeller. The early P-38s with the leading edge intercooler could only cool about 1,050 hp. If you were making more horsepower than that, then the temperature would slowly rise until you either throttled back or the engine failed. Later P-38s with the deeper cowling could cool the entire engine power output. That helped the P-38 a lot at higher altitudes (P-38J and onward).

During WWII, a turbocharger was called a turbosupercharger. That is not a "system term" at all, it is what, at the time, they called the device we call a turbocharger today. The engine still had a standard mechanical supercharger internally. The term turbosupercharger is more clear to me because the supercharger part compresses the air exactly the same is in a mechanical unit connected to the crankshaft, and the turbo part reminds me that the compressor is operated by exhaust pressure. But, I call it a turbocharger today because that is the term currently in use. No point in confusing people, even if they do forget the original terms. It's sort of like calling a capacitor a "condenser." The term is accurate, but obsolete.

 

[tomo pauk]

I would think that since the Turbo-Supercharger feeds the Supercharger we would refer to the Turbo-Supercharger as the first stage of a two stage system.

But I have been wrong before...

Kim

And on sealevel the Turbo feeds what? Nothing as that's purely done by the mechanical supercharger of the engine. The additional feeding of the Turbo is the second stage.

Turbo was the 1st stage.
Even with waste gates open, there was enough of exhaust gasses to turn the turbo, so the 2nd stage (engine-stage) receives airflow.

 

[Shortround6]

Turbo was the 1st stage.
Even with waste gates open, there was enough of exhaust gasses to turn the turbo, so the 2nd stage (engine-stage) receives airflow.

Not a lot, airflow goes up with the square of the speed impeller speed (sort of)

see, http://www.wwiiaircraftperformance.org/p-47/P-47B_41-5902_PHQ-M-19-1417-A.pdf

for the impeller/turbine speeds of P-47B in both level flight and in climb.

At low speeds there is a lot of turbulence and backflow. These are not positive displacement pumps.

I would also note that the power to drive the compressor goes up with the square of the speed so while the turbo may be supplying 300 or more HP to the compressor at 18,250rpm it may only be supplying 30 hp at 5600rpm.

Note in the report that at 5,000ft when climbing the turbine has to turn 1400rpm (20%) faster to get the same manifold pressure as high speed level flight. 20% faster to make up for the loss of RAM effect.

 

[Tony Kambic]

Power Recovery Turbine in a Wright 3350.

 

[Dash119]

Unless there is some type of bypass between the Turbo-Supercharger and the Supercharger(including the intercooler and carburetor...), then the impellers in the Turbo-Supercharger should be turning. Otherwise the intake air must be drawn past a stationary impeller, which seems like it would severely restrict the air flow.

Even if a minimum amount of exhaust gases flow past the wastegate to the Turbo-Supercharger and help spin the impellers, that seems beneficial to intake airflow. Otherwise vacuum on the intake side, and no pressure on the exhaust side of the Turbo-Supercharger are the only forces in play.

 

[tomo pauk]

Not a lot, airflow goes up with the square of the speed impeller speed (sort of)

see, http://www.wwiiaircraftperformance.org/p-47/P-47B_41-5902_PHQ-M-19-1417-A.pdf

for the impeller/turbine speeds of P-47B in both level flight and in climb.

At low speeds there is a lot of turbulence and backflow. These are not positive displacement pumps.

I would also note that the power to drive the compressor goes up with the square of the speed so while the turbo may be supplying 300 or more HP to the compressor at 18,250rpm it may only be supplying 30 hp at 5600rpm.

Note in the report that at 5,000ft when climbing the turbine has to turn 1400rpm (20%) faster to get the same manifold pressure as high speed level flight. 20% faster to make up for the loss of RAM effect.

Thank you.
Still, whatever the air the engine needs on a P-47 comes to the engine-stage S/C, it will be going through the turbo 1st

 

[Big Jake]

To provide top cover for B-17s flying at 25,000 ft and more, the designers of the P-47 needed all the power they could get at 30,000 and above. The only way to get it was by using a turbocharger system for the R2800. A supercharger gets its power directly from the engine so the higher you go, the less power the engine generates, and less is "available" for driving the supercharger resulting in lower boost generations and thus less power - you get the picture. A turbocharger is powered by the exhaust gas - as long as the engine runs, you get the driving power. The R2800 in the F4U or the F6F only had a two stage supercharger and no turbocharger and thus couldn't match a P-47 at altitude. Nothing could.

 

[P-39 Expert]

To provide top cover for B-17s flying at 25,000 ft and more, the designers of the P-47 needed all the power they could get at 30,000 and above. The only way to get it was by using a turbocharger system for the R2800. A supercharger gets its power directly from the engine so the higher you go, the less power the engine generates, and less is "available" for driving the supercharger resulting in lower boost generations and thus less power - you get the picture. A turbocharger is powered by the exhaust gas - as long as the engine runs, you get the driving power. The R2800 in the F4U or the F6F only had a two stage supercharger and no turbocharger and thus couldn't match a P-47 at altitude. Nothing could.

Too bad it used so much gas.

 

[GreenKnight121]

Was the weight of the turbocharger system in the P-47 charged to the engine or airframe?

I've been trying to find the weight of just the turbocharger system myself.