"The case for the P-47 Thunderbolt being the greatest fighter of the Second World War "

[GregP]

I was hoping that you know the answer to a specific question:
How much of engine power was used by the turbocharger on the P-47 (for example)?

Alas.

There is a chart somewhere, that I KNOW you've seen, which shows the power coming out of a boosted aviation engine in terms of percent of power in gasoline that gets used for what purpose. Can't seem to find it just now, but I have it somewhere. We've ALL likely seen it at some point. Seems like it had typical percent of the power in the gasoline allocated to various things, including engine power, power loss due to friction, power loss due to supercharging (or turbocharging? ...maybe not), power loss to exhaust (I seem to recall it is a decently high percentage, as we'd expect). I'll look around for it and try to re-discover the graphic. I THINK it came from Rolls-Royce, but I may misremember that.

Found it at: Waste Heat Utilization

Good article.

 

[GregP]

Sir Stanley Hooker was placed in charge of supercharger design at Roll Royce and wrote many articles and memos on supercharger design and testing. One such article is here: AC Eng Perf Analysis at R-R . This is from Enginehistory.org, as you can see in the link.

This is a 1940 article on defining a new term, called shaft horsepower from the engine tests. He defined shaft horsepower as Brake Horsepower + Compressor Horsepower. Brake horsepower is what drives the propeller and compressor horsepower is the power absorbed by the supercharger. They HAVE to account for it because the crankshaft must produce shaft horsepower when the engine runs ... at LEAST in mechanical stress. Since the supercharger TAKES power from the Shaft Horsepower, Brake horsepower is what is left to drive the propeller.

To make a long story shorter, the engine they tested, was a Merlin XX. They used 90% efficiency, used the same friction and pumping characteristics as the Allison V-1710 at the time, as indicated just above table 4. The engine made 1,234 shaft hp at sea level and 1,298 shaft horsepower at 20,000 feet. At seal level, the supercharger used 216 horsepower and at 20,000 feet it used 225 hp. That leaves 1,018 Brake horsepower at sea level and 1,073 horsepower at 20,000 feet for the Merlin XX used in test.

So, at seal level the supercharger consumed 17.5% of the shaft horsepower while, at 20,000 feet, it consumed 17.33 % of the shaft horsepower. This was AFTER Hooker improved the basic single-stage engine to produce the Merlin 45, and the Merin XX used his better-matched impeller and diffuser parts.

Just for clarity, the Merlin XX used a single-stage, 2-speed supercharger.

I assume that when the 2-stage engine was developed, the second stage used a bit less horsepower since the first compressor usually had the larger impeller diameter. But, how MUCH less power it might use is up for "guestimate." I'll assume it uses something like 90% of the power used by the first compressor stage as a second-order estimate. To be conservative, as a first-order estimate, I'd assume the same power consumption for the second stage as the first stage, and further assume it to use about the same percent as this test. So far, I haven't found a supercharger analysis for a 2-stage Merlin to confirm anything, but I'd assume both stages use about 17.5% of the Shaft Horsepower and go from there.

The Merin 61 made 1,565 hp Brake hp @ 3,000 rpm, +15 psi boost, at 11,250 feet. That means shaft horsepower was about 2,407 with a 35% loss to superchargers as a 1st-order approximtion.

 

[SaparotRob]

This may be Kindergarten stuff for most of you guys but it's quite educational for me.

Please, no pop quizzes.

 

[GrauGeist]

This may be Kindergarten stuff for most of you guys but it's quite educational for me.

Please, no pop quizzes.

You best be taking notes, mister.

 

[GregP]

Das flyen machinen is nicht for gerwerken by das dumbkopfen.
Is easy blowenfusen, poppencorken, mit spitzensparken.
Das rubbernecken sightseeren, keepen der hands in das pockets.
Relaxen und watch der blinkenlightsen.

 

[SaparotRob]

Finally, a technical post I can understand.

 

[Just Schmidt]

Das flyen machinen is nicht for gerwerken by das dumbkopfen.
Is easy blowenfusen, poppencorken, mit spitzensparken.
Das rubbernecken sightseeren, keepen der hands in das pockets.
Relaxen und watch der blinkenlightsen.

Is that Dutch or homemade?

 

[drgondog]

The Spit is definitely in the conversation as it was so suitable for upgrades that it could stay relevant, but by '44-'45, even with the -K model I think the -109 is an also-ran. Certainly had a great run the first four years of the war, though.

You're right that both planes spurred progress, but by the end of the war, the Spit still had room for mods to expand its capabilities, while the basic design of the -109 limited its ability to take much further variation. Does that make sense?

Size matters. The Spit and the Bf 109 were arguably the best (conventional) point interceptors but overall size and capacity for internal fuel reduced their military flexibility. IMO, in complete agreement your summary of the utility of the 109K.

The Spit is always in the conversation.

That said, I woud be hard pressed to dismiss either Fw 190, P-47, P-51, F4U/F6U or last 18mo of P-38J/L. Range combined with high performance provided miitary planners with far greater tactical footprint, flexibility of platform from CAS, tactical and Strategic recon, interception, LR escort. True that P-47, F6F/F4U/Fw 190 ot used as recon in reasonable % - but easily capable.

Whenever the P-47 is raised as a serious challenger to the P-51, the facts ar that whenever possible or feasible - the Mustang replaced the Jug in the inventory for all the strategic Air Forces (save a few P-47Ns), was used side by side in tactical role, exclusively retained for recon - and survived post VJ-Day drawdown through Korean War.

As you mentioned earlier, top speed combined with range and maneuverability, as well as maintainability, low unit cost and low cost of ownership will usually turn the head of the severest critics.

 

[drgondog]

The fact that the Spitfire had almost every part altered shows that it had growth potential. The Mk.21 had a new wing but that only entered operational service in January 1945. Prior to that variant, all Spitfires had adaptations to the same basic wing form. Again, that's growth potential intrinsic to the design. Singling out the Mk.21 or P-51H completely ignores the growth inherent in both original airframes.

The fact that every part was altered is not 'growth potential'. Many highly skilled labor hours are consumed by new design, new tooling, new procurment, new production processes. The fact that most parts were changed and all the labor intensive costs were accepted is because the original and succeeding 'very similar to previous models' were deemed not adequate for the new mission deemed very important.

The growth inherent in the Mustang from Mustang I through P-51D were incremental changes with few relative lines changes and commonality of many parts and assemblies. The XP-51F/G and H were a complete departure in both lines and parts and systems (save Merlin and Meredith based cooling system) from previous Merlin based models.

In order to have the capacity for growth, you must first have a design that doesn't just barely do its primary job. It must have the structural integrity to absorb more powerful engines, more armament, or take on different roles...and remain competitive while doing it. Some aircraft have those attributes and some don't. The Mosquito is another example of a type that did have growth potential...while the Blenheim is a type that didn't.

If the design can migrate from say a Merlin C to a Griffon and increase max internal Gross Weight a couple of thousand pounds - without significant internal changes to spar, empennage attach, and empennage dimensions - then the claim of implied growth potential from original design is sound and fair. That said, I have no idea what design changes to spar/longerons/aft bulhead/empennage were made that required significant changes to either tooling/parts or reduced allowable Angle of Attack loads.

Contrast the wing and aft frame and empennage of the NA-73 Mustang I through the P-51B - basically the same. For the latter, the internal coolig system arrangement, engine mounts, lower cowl and WL of wing location changed but retained many NA-73/83 parts and assemblies. What did change was allowable Limit Load and Ultimate Load which reduced from 8G and 12g respectively to 7g and 10.4g. That compromise was deemed more accepable than making the necessary internal changes to structure and panel thicknesses.

Except for mods to forward fuselage and lower position of the wing, the Lines and most parts/subassemblies experienced small incremental changes.

 

[drgondog]

Hypothetical, how would a P47 go head to head in a one on one dogfight after the merge with a 190A or MkIX LF at low altitude?, the Jug pilot would haul the big girl around only to find the other two already in a firing position, same for high altitude, at 30,000ft the Jug pilot would turn losing height only to once again look over his shoulder to see a TA152/Spit XIV bearing down on him from above, the P47 was a B&Z fighter which is fine if you are in a position to bounce your enemy but in a straight dogfight it's weight puts it at a serious disadvantage.

I think if the P-47D pilot was aware that a turn would nullify the inherent advantages of high speed/energy, the Jug would extend. For the same two a/c the P-47M would be ata decided advantage >28K. The Ta 152 is somewhat meaningless in a discussion of contribution, but it would certainly be able to get altitude on a P-47 but would the Spit XIV with Griffon not be running out of top speed (and Hp avail to Hp required beginning at 22K?

 

[drgondog]

Merlin engined Mustangs couldn't have flown those missions without the Spitfires Typhoons Tempests of 2nd TAF and P47's of the 8th AF giving them a clear run. The Mustang was an awesome aircraft but the best?, no chance.

Of course they could have flown the same missions but there would been more bomber losses as LW contollers vectored fighters to unescorted boxes. What do you mean 'clear run'?

 

[BobB]

"The engine made 1,234 shaft hp at seal level and 1,298 shaft horsepower at 20,000 feet. At seal level..."

Does seal level change depending on whether the seal is diving or coming up for air?

 

[Greg Boeser]

Does seal level change depending on whether the seal is diving or coming up for air?

Don't forget tides. The seal level is affected by whether it is intide or outtide.

 

[BobB]

Don't forget tides. The seal level is affected by whether it is intide or outtide.

View: https://www.youtube.com/watch?v=U5G_t6znMtw

 

[pbehn]

Don't forget tides. The seal level is affected by whether it is intide or outtide.

Beware the tides of April, as the saying goes.

 

[buffnut453]

The fact that every part was altered is not 'growth potential'. Many highly skilled labor hours are consumed by new design, new tooling, new procurment, new production processes. The fact that most parts were changed and all the labor intensive costs were accepted is because the original and succeeding 'very similar to previous models' were deemed not adequate for the new mission deemed very important.

The growth inherent in the Mustang from Mustang I through P-51D were incremental changes with few relative lines changes and commonality of many parts and assemblies. The XP-51F/G and H were a complete departure in both lines and parts and systems (save Merlin and Meredith based cooling system) from previous Merlin based models.


If the design can migrate from say a Merlin C to a Griffon and increase max internal Gross Weight a couple of thousand pounds - without significant internal changes to spar, empennage attach, and empennage dimensions - then the claim of implied growth potential from original design is sound and fair. That said, I have no idea what design changes to spar/longerons/aft bulhead/empennage were made that required significant changes to either tooling/parts or reduced allowable Angle of Attack loads.

Contrast the wing and aft frame and empennage of the NA-73 Mustang I through the P-51B - basically the same. For the latter, the internal coolig system arrangement, engine mounts, lower cowl and WL of wing location changed but retained many NA-73/83 parts and assemblies. What did change was allowable Limit Load and Ultimate Load which reduced from 8G and 12g respectively to 7g and 10.4g. That compromise was deemed more accepable than making the necessary internal changes to structure and panel thicknesses.

Except for mods to forward fuselage and lower position of the wing, the Lines and most parts/subassemblies experienced small incremental changes.

Well, let's compare apples to apples if we're discussing the Spitfire and the Mustang. The latter never received the Griffon engine operationally, so using the fact that the Spitfire needed mods to accommodate it is tilting the analysis, IMHO.

The Mustang design moved the wing 2in forward just to accommodate the Merlin and its heavier prop. The Spitfire, meanwhile, used exactly the same main fuselage (aft of the firewall) and same wing mounting position for every variant from MkI thru MkIX. Even after the Griffon was introduced, the wing mounting position remained unchanged, as did the basic planform of the wing.

Regarding the wing itself, most changes were driven by evolving requirements for heavier weapons (see acquisition context below). Similarly, the Mustang Mk1a was often fitted with 4x20mm cannon which clearly drove internal changes compared to the initial wing armament of 4x30 cals or the later, 4x50 cals and then 6x50cals. The early Mustangs were also not plumbed for drop tanks whereas later ones were.

Also, the Spitfire took on roles, like long-range, high-altitude PR, that the Mustang never fulfilled.

The Mustang also benefitted from 4 years' worth of lessons learned and technology development that Mitchell and his team didn't have when they first designed the Spitfire.

The Mustang remains a tremendous design. It was clearly much better suited to mass production, and the NA team did an amazing job creating a truly world-beating design. However, I think you're underplaying the changes that took place during its WW2 production, and overstating changes wrought upon the Spitfire. We also can't ignore the very different acquisition conditions. The Spitfire was procured at a time when it was hoped war could be avoided, whereas the Mustang was acquired when it was clear we were going to be in a war for a number of years.

Your argument that the Spitfire didn't have "growth potential" doesn't stack up against its proven ability to remain at the forefront of air combat for a decade. Both the Spitfire and the Mustang were phenomenal designs and both evolved. I'm not sure playing a game of "my cat's blacker than your cat" achieves much.

 

[GregP]

Is that Dutch or homemade?

Late-night mumbling ... likely in the arcane Greman language.

 

[GregP]

Does seal level change depending on whether the seal is diving or coming up for air?

It depends on whether or not the seal is in offensive or defensive attack mode. If it is in MS gear, then yes. If it is in FS gear, then no.

sic, tansit, groliosky.

Correction made ... thanks!

 

[GregP]

Beware the tides of April, as the saying goes.

I thought it was "beware the tides of March!" not April. Oh yeah, that's the "ides of March." Ides, tides. What's the difference? Both happen on the first new moon!

 

[33k in the air]

Does seal level change depending on whether the seal is diving or coming up for air?

Depends on how many Newfoundlanders were in the area.