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Aircraft engines: Turbochargers vs. superchargers


Doc72

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8 hours ago, Doc72 said:

My old post has experienced quite a revival.

Still I wonder why turbochargers were (almost) exclusively limited to US designs. Can this be explained by raw-material supply/metallurgy and simple path dependency (nations stuck with whatever technology they had chosen in the 1930s)?


It wasn't just US designs, but USAAC/USAAF designs, as the USN also preferred superchargers in their aircraft.

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Yes, good point. I suppose, the reason was that the Navy was more focussed on low and medium altitude operations. Dive bombers and torpedo bombers flew and attacked much lower than the USAAF's B-17, -24 and -29s. Turbochargers were in general used to give good high altitude performance. The security/reliability concerns mentioned above might have added to this.

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17 hours ago, VMA131Marine said:

 

The F4U Corsair with the supercharged Pratt & Whitney R-2800 and the AD Skyraider with a supercharged Wright R-3350 both served long after WWII ended as did the supercharged P-51D/K/H and the P-82. The PW R-4360 Wasp Major was also supercharged and found significant post-war use in a slew of aircraft; B-36, B-50, C-97, C-74, C-124, Martin JRM Mars, and C-119.

 

The combat types mentioned may have had long careers but they were designed during WW2 and used WW2 technology. As such they can't be really considered postwar types, even if they did serve postwar. When I say that the jet engine quickly replaced the piston types I mean on new builds, older aircrafts may have been used for a long time after that but even if the last Mustangs were retired in the '60s we can't really consider an RR Merlin as a typical engine of that era

 

2 hours ago, Doc72 said:

Yes, good point. I suppose, the reason was that the Navy was more focussed on low and medium altitude operations. Dive bombers and torpedo bombers flew and attacked much lower than the USAAF's B-17, -24 and -29s. Turbochargers were in general used to give good high altitude performance. The security/reliability concerns mentioned above might have added to this.

 

I wouldn't want to have slow throttle response when having to land an aircraft on a carrier... superchargers are (or were) worse from this point of view. Of course early jets (and even some not so early ones) may have been even worse but the performance advantage was such that in the end it made sense to accept their more dangerous behaviour. I'd be curious to see some accident rate numbers for early carrierborne jet powered types, I'd not be surprised to see some horrific data

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All early jet fighter ops were horrifically dangerous by modern standards: naval jet ops even more so (all the land-based risks, plus the inherent dodginess that comes from the higher speeds and wing loadings around deck take-offs and landings, and low fuel endurance being a poor accompanyment to carrier ops)

 

The RAF's deeply alarming post-war Meteor losses speak for themselves, and US naval aviation in particular took a real pasting.

This is an excellent background paper on the subject

https://www.usnwc.edu/getattachment/76679e75-3a49-4bf5-854a-b0696e575e0a/The-U-S--Navy-s-Transition-to-Jets.aspx

 

A stark extract makes the point:

"...... 1954, when naval aviation (that is, Navy and Marine combined) lost 776 aircraft and 535
crew, for an accident rate well above fifty per hundred thousand flight hours—
and the rate for carrier-based tactical aviation was much higher than that..."

 

Edited by Work In Progress
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On 6.6.2017 at 14:28, Jure Miljevic said:

Hello, vppelt68

Why would collecting exhaust gasses from inline or V-engines be so difficult? Cheers

Jure

Jure, not in any way difficult. I was just wondering what the exhaust system would look, and how it would affect the aerodynamics in a slim and tightly confined shape of a Spit or a 109 cowling. V-P

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23 minutes ago, vppelt68 said:

Jure, not in any way difficult. I was just wondering what the exhaust system would look, and how it would affect the aerodynamics in a slim and tightly confined shape of a Spit or a 109 cowling. V-P

I guess it would look pretty good:

curtiss-xp37.jpg

Of course it didn't turn out any faster than the supercharged P-40..

Cheers

Will

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Hello

Vppelt68, I understand your point, but I do not think this had been particularly pressing issue. After all, P-38's Allison engines had been packed even tighter into their cowlings than Merlins or DB.601/605 into Spitfires' and Bf 109s' noses. Weight and question where to build in turbochargers is another matter, though.

By the way, what about fuel? With high octane fuel engine is less susceptible to knock at altitude and its performance is thus improved. By 1939 USA refineries had been producing 100-octane fuel and with addition of lead its performance number (by definition the highest octane number is 100) reached as high as 130. This allowed USAAC to develop daylight high-altitude mass bomber formations, a doctrine into which turbochargers fitted nicely. Of course, early on it was UK who benefited the most from US inter-war fuel development, as imported 100-octane fuel had been supplied to RAF in early 1940, just in time for Battle for Britain. I think until that time 87-octane fuel had been in use. Years earlier, competing for Schneider Trophy called for low-altitude performance and it is hardly surprising that Rolls-Royce directed its efforts toward superchargers. During WWI Germany, short on man-power, materiel and desperate to avoid war of attrition, took several shortcuts towards better aircraft performance at high altitude (predecessors of aromatic fuels, over-compressed and oversized engines), as proper turbochargers had still not been developed. In 1919 Versailles Treaty put an end to all military aviation in Germany and also severely curtailed civilian aviation. During WWII Luftwaffe almost completely standardized on 86-octane fuel out of necessity, as this was the highest octane number synthetic fuel plants could produce. Nevertheless, such fuel was fine for low and medium altitude flying. Just my thoughts. Cheers

Jure

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The fuel quality question is one reason why the German V12 engines tended to be of larger swept capacity than their UK counterparts, for similar power outputs. (A tendency rather than a hard and fast rule, other factors were also in play.) Lower fuel quality means lower maximum boost pressures, all else being equal, so you then have a choice between revving the engine harder or making it bigger capacity. Bigger capacity is easier, and probably gives you a lighter and more reliable engine than building a smaller one which is required to rev a lot harder.

e.g.

A Merlin is 27 litres, and went roughly from 900 to 2000 hp during the course of the war. The DB601 in the Bf109 and Bf110 was a hair under 34 litres for about 1100 bhp and they didn;t get it up to anything like late Merlin power levels.

A Griffon is 37 litres, and went roughly from 1730 to 2370 hp during the course of the war. The 2100 hp in a Ta-152C came from 44.5 litres capacity.

Edited by Work In Progress
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On 6/7/2017 at 5:25 AM, Giorgio N said:

 

I wouldn't want to have slow throttle response when having to land an aircraft on a carrier... superchargers are (or were) worse from this point of view. Of course early jets (and even some not so early ones) may have been even worse but the performance advantage was such that in the end it made sense to accept their more dangerous behaviour. I'd be curious to see some accident rate numbers for early carrierborne jet powered types, I'd not be surprised to see some horrific data

I think you might have misspoken. I can tell you from practical experience that it is turbochargers, not superchargers, that have a lag in power output in the event of a go-around. Superchargers are operating in low blower mode for takeoff and landing on a radial engine and have zero material effect on throttle response time.

 

On a takeoff, planes like the P-38 (or my Piper Chieftain) need to have the throttles "stood up" and then you wait for the turbos to spool up before increasing manifold pressure to the stops for takeoff. Supercharged engines like the R-2800 do not require this step, but the NATOPS/ POH recommend that for takeoff on land the throttle be opened up incrementally so the pilot has enough time and rudder authority to counteract torque.

 

-d-

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39 minutes ago, David H said:

I think you might have misspoken. I can tell you from practical experience that it is turbochargers, not superchargers, that have a lag in power output in the event of a go-around. Superchargers are operating in low blower mode for takeoff and landing on a radial engine and have zero material effect on throttle response time.

 

On a takeoff, planes like the P-38 (or my Piper Chieftain) need to have the throttles "stood up" and then you wait for the turbos to spool up before increasing manifold pressure to the stops for takeoff. Supercharged engines like the R-2800 do not require this step, but the NATOPS/ POH recommend that for takeoff on land the throttle be opened up incrementally so the pilot has enough time and rudder authority to counteract torque.

 

-d-

 

Of course, I wrongly used super instead of turbo in the sentence above , Thanks for pointing this out

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