Why or why not turbo chargers

[davparlr]

For what it is worth, I have generated a couple of graphs showing the thrust difference between the P-47M and the Ta-152H. I used an internet formula of T=HP*325/TAS. One graph is strictly thrust vs. alt. and the other is the ratio of thrust for the Ta-152 and P-47 vs alt. HP was assumed to be Thrust HP for simplicities sake. I don't really know if this is useful, but it does show some comparison of supercharger vs. turbo-supercharger performance. The Ta-152 maintains about 70% of the P-47 thrust up to about 25k ft but drifts down to about 50% at 35k ft. This seems to indicate that the turbo-supercharger has little if any advantage at lower altitude, but does behave better at higher altitudes. It also indicates that the power/thrust curve for the turbo-supercharger is better behaved over the envelop than the supercharger alone. This data was taken below the altitude where the GM-1 was engaged on the Ta-152, which would have negated the data.

Sorry I couldn't paste the picture.

 

[HoHun]

Hi Davparlr,

>This seems to indicate that the turbo-supercharger has little if any advantage at lower altitude, but does behave better at higher altitudes. It also indicates that the power/thrust curve for the turbo-supercharger is better behaved over the envelop than the supercharger alone.

Interesting comparison!

Two effects that affect the practical impact of the supercharger type selection:

a) Exhaust thrust of the aircraft with jet exhausts has to be added to the propeller thrust. Exhaust thrust depends on indicated power, not on shaft power, and increases with decreasing atmospheric pressure, so that it reaches its maximum at the highest supercharger gear's full throttle height. For the Jumo 213E at WEP, this amounts to 188 kg of thrust at 9.1 km altitude in addition to its 1630 shaft HP.

b) The exhaust thrust acts directly, while the shaft power has to be converted into thrust by a propeller that suffers losses. At high speeds, these losses increase with Mach number which in turn increases with altitude.

So while the turbo-supercharger gives greater shaft power for the same core engine and seems to be less affected by altitude effects, the mechanically-supercharged jet-exhaust engine has a hidden advantage that is most telling at high altitude.

Accordingly, a diagram comparing the absolute thrust of the two engines in a similar way as you compared the propeller thrust would show a relationship more favourable for the jet-exhaust engine than the one you posted above.

Regards,

Henning (HoHun)

 

[renrich]

Dav, am enjoying your analysis. Another factor to consider is the nice smooth power curve of the turbo charged engine versus the peaky power curve of the super charged two stage, two speed P51s and the Hellcat and Corsair. Another factor was that although the Mustang supercharger operation was automatic, the USN fighters had a manually operated clutch and it seems like sometimes the pilots needed three arms. I had an Audi A6 2.7 Twin Turbo that had a V6 with a turbo for each bank. It did a good job at high altitudes. My house was at 9300 feet.

 

[SoD Stitch]

Dav, am enjoying your analysis. Another factor to consider is the nice smooth power curve of the turbo charged engine versus the peaky power curve of the super charged two stage, two speed P51s and the Hellcat and Corsair. Another factor was that although the Mustang supercharger operation was automatic, the USN fighters had a manually operated clutch and it seems like sometimes the pilots needed three arms. I had an Audi A6 2.7 Twin Turbo that had a V6 with a turbo for each bank. It did a good job at high altitudes. My house was at 9300 feet.

That's the way to do it, one (smaller) turbo per bank; when Porsche first installed a turobcharger on their Turbo RSR race car, they had originally used one big KKK turbocharger, but they soon found that two smaller turbos (one per bank of cylinders) worked much better, because they "spooled up" faster than the one big turbo (less inertia) and, therefore, there was less turbo "lag".

 

[davparlr]

Hi Davparlr,

Interesting comparison!

Accordingly, a diagram comparing the absolute thrust of the two engines in a similar way as you compared the propeller thrust would show a relationship more favourable for the jet-exhaust engine than the one you posted above.

Regards,

Henning (HoHun)

Alas, there was something nagging at my brain as wrong! Thanks, you are right, of course. I thought about how to compare two different aircraft with thrust, drag, whatever, but I went screaming out the door. However, I did decide to compare top speed against altitude, which should compare thrust/drag performance. As can be seen, the graph somewhat reflects the original graphs although I would guess not as severe. It seems apparent that the Ta-152 does indeed maintain airspeed performance up to about 25k ft and then performance starts to drop off. It appears that the loss of propeller efficiency with altitude of the P-47M does not offset the reduction in power available to the Ta-152H, also, I suspect that exhaust thrust also reduces as hp is reduced. Of course, this is all affected by the configuration; another stage of supercharger on the Ta-152H would change the data. The Ta-152 overcomes this disadvantage above 36k ft. by nitrous injection, GM-1. I would suspect that the P-47M turbo is pretty big.

Data base is as previously shown except for altitude of 40k, Ta-152H 431 mph (does not include GM-1 usage), P-47M 472 mph (estimation based on P-47N flight test data corrected for M performance, this data point only).

 

[davparlr]

Oops, forgot the graph. Sorry

 

[delcyros]

Well done, Dave.
I might add in defense of the turbo-supercharger that the power aviable by this installation is not affected by speed. So high slow (thinking of a turn at high altitude) it should have been an advantage versus the supercharger + exhoust thrust system. Comparable situation in accelearation. The exhoust thrust increases with the speed (or vice versa, it drops with the speed, too!). Gives good zoom-climb but perhaps not that convincing acceleration. Some perople argue that acceleration is perhaps more important than all out level speed, which I tend to agree with.
Another interesting point raised by Gersdorfs graphs (Thanks Hennig!) touches the specific fuel consumption, which seems to be lower in cruise speed for the turbo-spercharger installation. This should translate to increased range.

I am more and more convinced that the USAAF and Luftwaffe respectively applied the technology best suited to their specific needs.

 

[davparlr]

I am more and more convinced that the USAAF and Luftwaffe respectively applied the technology best suited to their specific needs.

I wholeheartedly agree. The military knew what they wanted in aircraft and the people who designed these planes knew what they were doing to meet the requirments specified. The proof is in the results, many outstanding aircraft.

 

[billswagger]

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?

P-47 had a Turbo supercharger which included an inter cooler that helped with cooling and compressing the air so it would always be at max density no mater what altitude.
So you have a combination of the turbine pushing more air, as well as the intercooler which made it denser.

Other planes had mechanically operated superchargers that required adjustment at different stages, based on air density. The higher you got the more air needed to be pushed because it was thinner. So you needed a turbine that adjusted its speed to account for changes in air density with altitude. There
There was no inter cooler. if mechanically operated.

The only real trade off is size. The inter cooler is a bulky component to have, and requires additional ducting. For that you get much more consistent performance.
I expect that as planes and fire power grew more advanced, it was recognized that having a smaller sleeker design was more beneficial than having an inter cooler. Especially when range and fuel consumption became the issue.

 

[Shortround6]

Actually if range and fuel consuption are the issue then turbos are the way to go.

The jet effect from the exhaust works better the higher and faster the airplane flies.
Higher because there is a greater pressure differential between the exhaust gases and the atmosphere.
faster becasue of the better matching of velocities of the gas stream.
In cruise flight the velocity match is off leading to lower effiecincy.
The thrust is from the exhaust gases which is mass times velocity. While the velocity of the gases stays pretty much the same the mass goes down considerably in cruising flight.

Using the turbo to provide some of the low manifold pressure required for cruising flight does increase the effiecincy of the engine over a mechanical drive two stage supercharger.

ALL two stage super chargers NEED an intercooler to work effectively. Or they need massive amounts of anti-detonation injection.
The R-2800 used BOTH to get it's higher ratings. ADI without the intercoolers would not have allowed the 2500-2800hp ratings.
Corsairs, Hellcats and P-61s all used intercoolers for their 2 stage engines.

 

[Clay_Allison]

Turbochargers: Too big, too heavy, too expensive, technology only reached its potential late in the war. I think it was a gamble that the army took that they barely broke even with.

 

[seesul]

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

There´s no fuselage in the car so no prob with aerodynamic drag. The car has its own design and mostly it has a water cooler in front. You put the turbo intercooler (or aftercooler) in front of the watter cooler (or behind) without any need of changing the the car design. In the aircraft you never have enough space and mostly not enough surface for the intercooler (aftercooler). The more surface of the intercooler=the more intercooler efficiency=the lower air temperature=the more oxygen in the air.

 

[fibus]

Very interesting comparisons.
The whatiffs might include all aircraft designers were aware of the Merrick radiator thrust effect as used on the P 51?
And the P 47 with paddle props and water injection could beat any German fighter at almost any altitude.

 

[trackend]

The Ideal would be combination of both with the super charger disengaging once the turbo kicks in this would give a very wide power band but the techknowledge and amount of space needed would be prohibitive but it would be interest to see a 1935 Auto Union race car with a turbo added to its double superchargers Tiff Needel took one for a test drive a few years ago on the TV and with just the chargers was wheel spinning at 130mph

 

[Marshall_Stack]

There´s no fuselage in the car so no prob with aerodynamic drag. The car has its own design and mostly it has a water cooler in front. You put the turbo intercooler (or aftercooler) in front of the watter cooler (or behind) without any need of changing the the car design. In the aircraft you never have enough space and mostly not enough surface for the intercooler (aftercooler). The more surface of the intercooler=the more intercooler efficiency=the lower air temperature=the more oxygen in the air.

Pure speculation on my part, but I wonder if today's metallurgy has anything to do with creating a more compact and lighter design. The turbo is exposed to high temps and rpm.

 

[billswagger]

Turbochargers: Too big, too heavy, too expensive, technology only reached its potential late in the war. I think it was a gamble that the army took that they barely broke even with.

with out turbo systems in planes you wouldn't be able to contend. They are necessary to get the performance needed out of a war plane at higher altitudes. Higher than 10,000ft, higher than 20,000ft, higher than 30,000ft.
It was also how the war progressed. The higher planes often were the ones to return home.
The battle for altitude ensued, and technology that allowed planes to fly higher was developed.

There was a lot of teething in the process, but it would've been a bigger gamble to not use turbochargers.

 

[Butters]

Advances in metallurgy and material technology do play a vital role in the development of more efficient TC's. Smaller, lighter turbos spool up more rapidly than larger ones due to lower inertia. However, since the goal is to pump the maximum amount of air thru the engine in the shortest possible time (all air-breathing engines, whether reciprocating or turbine, are essentially air pumps) the lower volumes of small TC's means that they must spin much faster to push thru the same mass. High speeds lead to more friction, and hence, heat. When your talking about 100,000 + rpms, there are huge stresses on both the blades and the bearings. That requires extremely strong and durable materials with low thermal expansion rates.

JL

 

[Clay_Allison]

with out turbo systems in planes you wouldn't be able to contend. They are necessary to get the performance needed out of a war plane at higher altitudes. Higher than 10,000ft, higher than 20,000ft, higher than 30,000ft.
It was also how the war progressed. The higher planes often were the ones to return home.
The battle for altitude ensued, and technology that allowed planes to fly higher was developed.

There was a lot of teething in the process, but it would've been a bigger gamble to not use turbochargers.

I'm not suggesting that they should have been naturally aspirated. I think Multi-Stage supercharging is more of a way to go.

 

[Shortround6]

Every single American aircraft that used a Turbo-charger in WW II was using multi-stage supercharging.


No American aircraft engine of 450hp or greater was without a supercharger providing some level of boost.

 

[Clay_Allison]

Every single American aircraft that used a Turbo-charger in WW II was using multi-stage supercharging.


No American aircraft engine of 450hp or greater was without a supercharger providing some level of boost.

I don't think you know the difference between supercharging and turbo-supercharging.

Mechanical, two-stage superchargine was less developed in America than it should have been.

Exhaust-powered TURBO-charging was emphasized above all else and never ended up hitting stride until 1944 when the P-47 and the P-38 really started becoming game-changing aircraft.