Greg's Airplanes: Nakajima Ki-84 Hayate "Frank" History (1 Viewer)

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Mar 1, 2022
Greg just published a video on the Nakajima Ki-84 "Frank" Hayate fighter. It's a solid summary of the history, development, and controversy on its top speed.

EDIT:
Much of the information in the video is covered in this thread on the Homare engine.

Some highlights of the video:

  • The Homare was the only major engine of WW2 that used high RPM and compression ratio to achieve its high horse power rating. According to Greg, the only other engine to use high RPMs, rather than manifold pressure, was the Napier Sabre.
  • Japanese aircraft used very few electrically powered components , although had a lot of wiring in them, possibly because they intended to use wooden construction at some point.
  • Greg seems to agree that the 427 MPH number for the top speed of the Frank is accurate.
  • Japanese water-methanol systems are not well understood, but it seems likely that the Frank's system used 50% water, mixed with 25% ethanol and 25% methanol in order to achieve optimal cylinder head temperatures.
  • The system was designed to automatically kick in at around 37-inches of manifold pressure; that's well below the maximum manifold pressure of the Homare.
  • Greg thinks that the Frank's oil cooler seems like it was stapled on as an afterthought, although IMO, it seems to be a Meredith effect type radiator

There's a part II of this video coming out soon as well as a part III. However, Greg is ill at the moment, so this might be delayed. Best wishes to Greg and hope he gets well soon!

Now there's a part III! Greg goes into the Ki-84's top speed controversy.

Finally, there's the conclusion

EDIT 2: Added part II. It's a wild ride. Greg's video mostly covers the armor plate behind the pilot, which was something like 12.4mm thick and used Ni-Cr-Mo hardening, making it more effective than the plate used in the Oscar. Overall, it's a great update although it doesn't cover some of the performance questions that we've had.

EDIT3: Added part III.
EDIT4: Added the conclusion to Greg's work on the Hayate.
 
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Mar 1, 2022
My opinion, which is shared by others, is that the 427 mph figure shouldn't be trusted but I'll withhold passing judgement and give Greg the benefit of the doubt by waiting to see what he has to say in video 2 on the subject.
Greg, unfortunately, doesn't cover all of the details that we've discussed on the Frank's performance other than mentioning that:

  • Nakajima constantly introduced refinements to the Frank on the production line as quality decreased due to the use of unskilled "conscripted" (he doesn't mention the child labor) labor rather than using already trained machinists. Therefore as improvements were introduced, quality decreased. So these factors complicate the maximum-speed estimates.
  • Greg also goes over the 388/390 MPH number, which was on a "expert" assembled prototype aircraft that had an 1,800 HP engine without thrust stacks or the additional aerodynamic refinements that later aircraft had. He said to not underestimate the impact of ram air and thrust, in reference to the Frank's ram-air intake. He also discussed some of the air intakes on the Frank as well as its smaller frontal profile.
  • German aircraft suffered from the same fuel quality, and engine quality problems and implied that if we use the prototype speeds for German aircraft we should also use the prototype speeds for Japanese aircraft, although that picture is complicated by the lack of records from Japan
  • Greg didn't mention that the TAIC report used calculated performance, although he did seem to read the engine analysis on the Homare 21, which implied he understood why TAIC arrived at the speed that they calculated. If he's aware of the engine's limitations and performance data, his understanding of the TAIC report is probably pretty accurate.

Overall, the videos are really interesting and I hope he gets into the top speed in more detail. He also did not get into the anecdotal report that the main failing of the Frank was its propeller design. According to one report from Australia's equivalent of TAIC, when they visited Japan, Allied groundcrews claimed Japanese propeller design was "five years" behind the Allied standard. This might be true although it's only referenced in a single article from Australian TAIC and it's a second-hand account.

But if the Frank did have a lower top speed than 427 MPH it would have been because of some serious aerodynamic design limitations that the Allies weren't aware of at the time of the TAIC report. However, the Japanese were using modern VDM Hamilton-Standard props, so I don't know how true that is. Does anyone know what prop the Frank used?
 
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DarrenW

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Dec 24, 2017
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Greg, unfortunately, doesn't cover all of the details that we've discussed on the Frank's performance other than mentioning that:

  • Nakajima constantly introduced refinements to the Frank on the production line as production simultaneously as quality decreased due to the use of unskilled "conscripted" (he doesn't mention the child labor) labor rather than using already trained machinists. Therefore as improvements were introduced, quality decreased. So these factors complicate the maximum-speed estimates.
  • Greg also goes over the 388/390 MPH number, which was on a "expert" assembled prototype aircraft that had an 1,800 HP engine without thrust stacks or the additional aerodynamic refinements that later aircraft had. He said to not underestimate the impact of ram air and thrust, in reference to the Frank's ram-air intake. He also discussed some of the air intakes on the Frank as well as its smaller frontal profile.
  • German aircraft suffered from the same fuel quality, and engine quality problems and implied that if we use the prototype speeds for German aircraft we should also use the prototype speeds for Japanese aircraft, although that picture is complicated by the lack of records from Japan
  • Greg didn't mention that the TAIC report used calculated performance, although he did seem to read the engine analysis on the Homare 21, which implied he understood why TAIC arrived at the speed that they calculated. If he's aware of the engine's limitations and performance data, his understanding of the TAIC report is probably pretty accurate.

Overall, the videos are really interesting and I hope he gets into the top speed in more detail. He also did not get into the anecdotal report that the main failing of the Frank was its propeller design. According to one report from Australia's equivalent of TAIC, when they visited Japan, Allied groundcrews claimed Japanese propeller design was "five years" behind the Allied standard. This might be true although it's only referenced in a single article from Australian TAIC and it's a second-hand account.

But if the Frank did have a lower top speed than 427 MPH it would have been because of some serious aerodynamic design limitations that the Allies weren't aware of at the time of the TAIC report. However, the Japanese were using modern VDM Hamilton-Standard props, so I don't know how true that is. Does anyone know what prop the Frank used?

I'm waiting for a breakdown of his reasons for accepting the TAIC data, as I think it warrants a explanation. I'm also a bit curious about the depth of his supposed "hottest" data on the US types will amount to. I know of performance specs for the F6F Hellcat that are not generally known to most aviation enthusiasts (it is my favorite aircraft after all) and ignoring it wouldn't allow for a fair comparison to be made IMHO.

I will have to take a wait and see mentality and hope for the best...
 

DarrenW

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Dec 24, 2017
Warren, MI USA
According to one report from Australia's equivalent of TAIC, when they visited Japan, Allied groundcrews claimed Japanese propeller design was "five years" behind the Allied standard. This might be true although it's only referenced in a single article from Australian TAIC and it's a second-hand account....However, the Japanese were using modern VDM Hamilton-Standard props, so I don't know how true that is. Does anyone know what prop the Frank used?
I've heard this about Japanese propellers as well, mainly they were generally not as efficient as US, British, or German types. It had a diameter of 10.2 feet, with what seems to be somewhat narrow blades, their shape reminding me of the early "toothpick" P-47 propellers. The Middletown report on the Ki-84 mentions this:

1670456181093.png
 
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Mar 1, 2022
I've heard this about Japanese propellers as well, mainly they were generally not as efficient as US, British, or German types. It had a diameter of 10.2 feet, with what seems to be somewhat narrow blades, their shape reminding me of the early "toothpick" P-47 propellers. The Middletown report on the Ki-84 mentions this:

View attachment 697699
Great find! I completely missed this.

I've heard the prop referred to as the Pe-32, or similar to the Pe-32, from Hiromachi on the War Thunder forums, before he departed for Il-2. The Pe-32 was a poorly documented, short and lightweight prop used only on the Hayate. We know almost nothing about it, other than that it was used on the Hayate. So it clearly wasn't one of the Sumitomo VDMs or even one of their Hamilton-Standards. Apparently, it was similar to or based on a Curtiss Electric prop.

According to Hiromachi, it was significantly shorter than a Shiden's prop, so it probably couldn't efficiently absorb the output from a Homare 21, 23, or 25. Seems like a poorer option compared to the other props that were available to the Japanese. But he speculates that there must be some reason for why a high-performance aircraft like the Frank would use a obscure prop from an unknown manufacturer (which did not make props normally).
 
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Mar 1, 2022
Greg just published Part III this morning. I hope everyone watches it.

Basically, Greg explains the wide range of speed estimates for the Ki-84 Frank. There are three numbers that he elaborates on:

  • The 388 MPH number comes from an older translation of a Japanese technical document for an earlier version of the Ki-84 which had a 7:1 compression ratio. The later versions of the Hayate had an engine with an higher compression ratio of 8:1. Furthermore, the older versions of the Homare engine had been run at 250mm of mercury, which was far below its 400mm of mercury.
  • The 394 MPH numbers were derived from unsourced and unreliable documentation.
  • The 427 MPH is the best sourced and understood documentation because it's derived from a 1946 report which is based on interviews with aircrews, captured aircraft, and testing, although the engine was diassembled and reassembled using high tolerances equal to or superior to what the Japanese had available. In other words, these numbers represent the absolute maximum that a Frank with a -21 Homare was capable of.
Interestingly, Greg noticed that the US military tested the Ki-84 with 96 octane, instead of 92 octane. He's not sure, but he thinks this did not impact testing. According to a test pilot who flew the Ki-84 on 100 octane fuel, apparently the problem with reliability vanished which means it's likely that the lower octane fuel caused issues with knock. 96 octane might have allowed the engine to run more smoothly.

The implication here is that Ki-84s flying out of Formosa and mainland China were likely faster and more reliable than those encountered in the Philippines.

Furthermore, Greg uses a calculation that can estimate top speed to contextualize and validate other speed ratings. His conclusion is that the 427 MPH number is valid and that the top speed of a well maintained Frank was probably more like 419 MPH.

He also does a speed comparison chart between the Corsair 1D, -4, The F6F-3 (I wish he had done the -5), the P-51D, and a P47N. As you would expect, the P-47N beats the pants off all the planes and the -4 Corsair (being a total beast) similarly outspeeds the Ki-84.
 

tomo pauk

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The 388 MPH number comes from an older translation of a Japanese technical document for an earlier version of the Ki-84 which had a 7:1 compression ratio. The later versions of the Hayate had an engine with an higher compression ratio of 8:1. Furthermore, the older versions of the Homare engine had been run at 250mm of mercury, which was far below its 400mm of mercury.
I've also read about the CR of 8:1 in some American docs. However, Japanese note just the 7:1 CR, while this excerpt notes that assumed 8:1 CR is not the case, they measured 7.17:1. We also have a consequantial 'evidence' that makes the 8:1 CR less likely to happen, namely that Japanse will know that such a high CR engine will not work well with the fuel they had, even if it was actually the 'full spec' 91 or 92 octane.
Unfortunately, Greg does not mentions this detail, despite the detail being from the same doc (TAIC report #31) from where is the power chart.
Anyway:

hom 2111.jpg

The influence of extra 100 rpm (3000 vs. 2900) is barely touched upon in his video, too. Difference is about 100 HP (although the lines at around 6000m show even more, ~150 HP).

hom chart2.jpg

On the other hand, it is hard not to agree with his conclusions - an early Ki-84 and imperfect fit & finish, powered by the lower powered engine will be lucky to beat 390 mph mark, while the later engine combined with an impeccable aircraft might result in the high speeds of above 420 mph.
 
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Mar 1, 2022
I've also read about the CR of 8:1 in some American docs. However, Japanese note just the 7:1 CR, while this excerpt notes that assumed 8:1 CR is not the case, they measured 7:17. We also have a consequantial 'evidence' that makes the 8:1 CR less likely to happen, namely that Japanse will know that such a high CR engine will not work well with the fuel they had, even if it was actually the 'full spec' 91 or 92 octane.
Unfortunately, Greg does not mentions this detail, despite the detail being from the same doc (TAIC report #31) from where is the power chart.
Anyway:

View attachment 699854

The influence of extra 100 rpm (3000 vs. 2900) is barely touched upon in his video, too. Difference is about 100 HP (although the lines at around 6000m show even more, ~150 HP).

View attachment 699855

On the other hand, it is hard not to agree with his conclusions - an early Ki-84 and imperfect fit & finish, powered by the lower powered engine will be lucky to beat 390 mph mark, while the later engine combined with an impeccable aircraft might result in the high speeds of above 420 mph.
It's interesting that the -21 Homare was regarded as having anywhere from a 1:8 to a 1:7 compression ratio based on the model and the time period in which it was manufactured.

Greg mentioned in an earlier video that the reason they use MW50 (it's more like M25E25W50, not MW50) is because of the compression ratio. In other words, the poor fuel quality caused issues with knock at higher manifold pressures which is why water was injected below WEP performance was reached (as you shared with us earlier, thanks again for providing that information).

Greg also addressed the variance in compression ratio in some respect. He mentioned that engines could vary in their various tolerances, which is why mechanics stripped the Homare 21 down before reassembling it to its factory spec (or better) "Blueprint" condition. The implication here is that they were also likely custom machining components to get the engine up to its best condition. It may have been possible that Homares had a high range of tolerances for their compression ratios, given that Nakajima employed a higher number of machinists rather than using tooling, for making engines, which is why their per-plane labor ratios are so high.

Interesting note about the RPMs! I'm now curious as to how the -23 or -25 Homares performed. The -23 was probably a better performer at altitude given better control over its fuel-air mixture. But IDK.
 

tomo pauk

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Greg mentioned in an earlier video that the reason they use MW50 (it's more like M25E25W50, not MW50) is because of the compression ratio. In other words, the poor fuel quality caused issues with knock at higher manifold pressures which is why water was injected below WEP performance was reached (as you shared with us earlier, thanks again for providing that information).

ADI, like MW50 etc. is/was a 'legit' way to circumvent the low octane value of the fuel (up to a point; best thing was to have both high-octane fuel and ADI). Low octane fuel and high CRs dont mix well, even on the non-supercharged engines.

Interesting note about the RPMs! I'm now curious as to how the -23 or -25 Homares performed. The -23 was probably a better performer at altitude given better control over its fuel-air mixture. But IDK.

TAIC gives a tad better power at higher altitudes for the -22 vs. -21. There is nothing in the big TAIC manual about the -23. They do mention the -41, that was supposed to have a 2-stage S/C.
 

DarrenW

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He also does a speed comparison chart between the Corsair 1D, -4, The F6F-3 (I wish he had done the -5), the P-51D, and a P47N. As you would expect, the P-47N beats the pants off all the planes and the -4 Corsair (being a total beast) similarly outspeeds the Ki-84.
Using the same power settings the F6F-5 was roughly 10-15 mph faster at all altitudes than the F6F-3 but this was largely negated as the -5 was normally equipped with wing pylons and rocket launchers (although the -3 carried external ordinance as well on occasion). There is test data out there which shows a top speed of the F6F-5 at 409 mph but this was most likely in a clean configuration and an air frame in basically flawless condition. My conclusion is that an average F6F-5 was capable of somewhere between 380-390 mph with pylons/rocket launchers and possibly 400 mph in a clean condition.

I did like that he gave his own estimate of the Ki-84's "real" maximum speed but I'm puzzled that he still used the figure of 427 mph in the comparisons. Why go through all that trouble and end up using data from what he considers to be a hot-rodded aircraft?
 
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Mar 1, 2022
ADI, like MW50 etc. is/was a 'legit' way to circumvent the low octane value of the fuel (up to a point; best thing was to have both high-octane fuel and ADI). Low octane fuel and high CRs dont mix well, even on the non-supercharged engines.



TAIC gives a tad better power at higher altitudes for the -22 vs. -21. There is nothing in the big TAIC manual about the -23. They do mention the -41, that was supposed to have a 2-stage S/C.
I don't have access to the full journal article (it's behind a paywall), but a designer of the Homare wrote that one of the more challenging parts of the design process was adapting the engine to use 87-92 octane fuel, down from 100 octane. It appears that the ADI was added on in 1941 for this reason as earlier models did not have it. (A tangent here, but he also mentioned that the most difficult problem was finding a suitable lubricant for the Homare as well as finding a suitable alloy for the main bearing rod. I think this is because the Homare was a high RPM engine which complicates lubrication and service life.)

The Homare appears to have been ready in 1941 but the delays and set backs might have had to do with the ADI addition in the -21 in 1941 as well as other issues with mettalury and lubrication. In other words, Nakajima's engineers were continuously modifying the design to meet the changed octane rating. It would make sense that the compression ratio varied depending on what version of the -21 was used in an aircraft. Of course this is speculation on my part.

Using the same power settings the F6F-5 was roughly 10-15 mph faster at all altitudes than the F6F-3 but this was largely negated as the -5 was normally equipped with wing pylons and rocket launchers (although the -3 carried external ordinance as well on occasion). There is test data out there which shows a top speed of the F6F-5 at 409 mph but this was most likely in a clean configuration and an air frame in basically flawless condition. My conclusion is that an average F6F-5 was capable of somewhere between 380-390 mph with pylons/rocket launchers and possibly 400 mph in a clean condition.

I did like that he gave his own estimate of the Ki-84's "real" maximum speed but I'm puzzled that he still used the figure of 427 mph in the comparisons. Why go through all that trouble and end up using data from what he considers to be a hot-rodded aircraft?
That's a really great question. My guess is for the purposes of standardization. It's easier to standardize these numbers based on superlatives rather than using numbers that aren't easily compared. For example, if Greg compared a Brewster Corsair to a Blueprint Ki-84, that wouldn't generate useful data. However, a pre-production F4U-1D is a valid benchmark because it's using a similar standard of manufacture. Maybe not quite comparable but close enough.

AFAIK, most of the top speed measurements that we're familiar with for US aircraft were taken in conditions with good fuel quality as well as good construction standards, typically measured by crack test pilots, engineers, and maintenance teams. Greg mentioned that he tried to use such data, when available, for US aircraft. Although I remember the -3 Hellcat getting higher numbers from prototype aircraft so it might be that he wasn't able to get such data for the -3.

It's also a bit unfair (Greg mentioned this) to compare a Dash 3 Hellcat to a Ki-84 as they both had different roles. And the Dash 3 was available a full year earlier. I think a more valid comparison would be between the N1K1-J (or Ja) and the Dash 3 Hellcat as they were both designed and flown around the same time, although the George 11 (N1K1-Ja) had significant engine development issues as wel as an engine change.

EDIT: Merry Christmas everyone!
 
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DarrenW

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I don't have access to the full journal article (it's behind a paywall), but a designer of the Homare wrote that one of the more challenging parts of the design process was adapting the engine to use 87-92 octane fuel, down from 100 octane. It appears that the ADI was added on in 1941 for this reason as earlier models did not have it. (A tangent here, but he also mentioned that the most difficult problem was finding a suitable lubricant for the Homare as well as finding a suitable alloy for the main bearing rod. I think this is because the Homare was a high RPM engine which complicates lubrication and service life.)

The Homare appears to have been ready in 1941 but the delays and set backs might have had to do with the ADI addition in the -21 in 1941 as well as other issues with mettalury and lubrication. In other words, Nakajima's engineers were continuously modifying the design to meet the changed octane rating. It would make sense that the compression ratio varied depending on what version of the -21 was used in an aircraft. Of course this is speculation on my part.


That's a really great question. My guess is for the purposes of standardization. It's easier to standardize these numbers based on superlatives rather than using numbers that aren't easily compared. For example, if Greg compared a Brewster Corsair to a Blueprint Ki-84, that wouldn't generate useful data. However, a pre-production F4U-1D is a valid benchmark because it's using a similar standard of manufacture. Maybe not quite comparable but close enough.

AFAIK, most of the top speed measurements that we're familiar with for US aircraft were taken in conditions with good fuel quality as well as good construction standards, typically measured by crack test pilots, engineers, and maintenance teams. Greg mentioned that he tried to use such data, when available, for US aircraft. Although I remember the -3 Hellcat getting higher numbers from prototype aircraft so it might be that he wasn't able to get such data for the -3.

It's also a bit unfair (Greg mentioned this) to compare a Dash 3 Hellcat to a Ki-84 as they both had different roles. And the Dash 3 was available a full year earlier. I think a more valid comparison would be between the N1K1-J (or Ja) and the Dash 3 Hellcat as they were both designed and flown around the same time, although the George 11 (N1K1-Ja) had significant engine development issues as wel as an engine change.

EDIT: Merry Christmas everyone!
I appreciate your analysis. No matter what speed data Greg utilized concerning the F6F it simply wouldn't measure up to the numbers TAIC provided for the captured Ki-84. I guess the biggest take-away here is that the standard in-service Ki-84 obviously had a much lower performance or the allies wouldn't have scored so easily against it (pilot quality not withstanding).

As a side note the 64" Hg Greg used for the F6F wasn't an authorized manifold pressure as 60" Hg was standard for WEP operation (same for F4U). The data he presented was from testing performed to discover maximum safe levels for the airplane. As a result of these tests no real issues were found, which isn't surprising seeing that the P-47 was running at these levels in Europe for quite some time, and it of course had the R-2800 too. At 60" Hg the maximum speed would be about the same but obtained at 17,000 ft (as opposed to 15,000 ft as shown on his graph). As a matter of fact an October 1944 TAIC chart indicates a speed of 386 mph @ 17,400 ft for the F6F-5, which by all indications was configured with pylons and rocket launchers.

Don't get me wrong, I'm pretty certain that US Navy pilots exceeded 60" Hg on occasion but they were conscious of the official limit and only went higher if the situation was dire enough to warrant it.
 
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79
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Mar 1, 2022
appreciate your analysis. No matter what speed data Greg utilized concerning the F6F it simply wouldn't measure up to the numbers TAIC provided for the captured Ki-84. I guess the biggest take-away here is that the standard in-service Ki-84 obviously had a much lower performance or the allies wouldn't have scored so easily against it (pilot quality not withstanding).

As a side note the 64" Hg Greg used for the F6F wasn't an authorized manifold pressure as 60" Hg was standard for WEP operation (same for F4U). The data he presented was from testing performed to discover maximum safe levels for the airplane. As a result of these tests no real issues were found, which isn't surprising seeing that the P-47 was running at these levels in Europe for quite some time, and it of course had the R-2800 too. At 60" Hg the maximum speed would be about the same but obtained at 17,000 ft (as opposed to 15,000 ft as shown on his graph). As a matter of fact an October 1944 TAIC chart indicates a speed of 386 mph @ 17,400 ft for the F6F-5, which by all indications was configured with pylons and rocket launchers.

Don't get me wrong, I'm pretty certain that US Navy pilots exceeded 60" Hg on occasion but they were conscious of the official limit and only went higher if the situation was dire enough to warrant it.
I am skeptical of the 386 MPH speed on a dash five as well. Given the Hellcat's weight, horsepower, and wing loading, I assume its top speed was probably over 400, even for a bog-standard production version. Regardless, there's a reason why Eric Brown considered it among the best aircraft of WW2 and slightly above the Frank. Likely because it was easier to fly and more likely to get someone home. The Frank notoriously had difficult controls and less automation. Typical of Axis design philosophy.

Regarding the Frank's high loss rate, at the stage of the war in which it went into combat, the Frank was not just piloted by neophytes, the Japanese were strategically and tactically being overwhelmed by the Allies in the Philippines and in most other theaters of war. For example, the Allies would set up multiple waves of ambushes on fighters that were trying to return to base. That's because the Allies had the advantage of superior radar performance, superior intelligence gathering on the ground, and superior utilization of intelligence.

In short, the Allies knew when the enemy was leaving their bases and when they were arriving. And they deployed their overwhelming forces to the fullest advantage. The Japanese and Germans didn't stand a chance of winning, although their engineers sometimes made dazzling aircraft, it just wasn't nearly enough.

EDIT: And I appreciate your analysis as well!
 
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tomo pauk

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As a side note the 64" Hg Greg used for the F6F wasn't an authorized manifold pressure as 60" Hg was standard for WEP operation (same for F4U). The data he presented was from testing performed to discover maximum safe levels for the airplane. As a result of these tests no real issues were found, which isn't surprising seeing that the P-47 was running at these levels in Europe for quite some time, and it of course had the R-2800 too.

One wonders how much being turbo-charged made it easier for the P-47D's R-2800 to sustain a bit greater boost than what the F4U or F6F had.
The 1st water-injection kits allowed for just 56 in Hg on the P-47D, however. Later kits upped this to 64 in Hg, and the R-2800s themselves were a bit different.

I am skeptical of the 386 MPH speed on a dash five as well. Given the Hellcat's weight, horsepower, and wing loading, I assume its top speed was probably over 400, even for a bog-standard production version.

Drag is cruel, in the words of drgondog drgondog :)
Oftentimes the heavy aircraft were the faster ones.
 

DarrenW

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One wonders how much being turbo-charged made it easier for the P-47D's R-2800 to sustain a bit greater boost than what the F4U or F6F had.
The 1st water-injection kits allowed for just 56 in Hg on the P-47D, however. Later kits upped this to 64 in Hg, and the R-2800s themselves were a bit different.
Yes the different induction systems would definitely bring up new questions concerning the adaptability of each engine to higher manifold pressures but testing on both the F6F F4U concluded that higher ratings were definitely possible. My belief is that the upgrade wasn't necessary in the PTO due to the lower performance of Japanese aircraft compared to those found in Europe. Even fuel octane ratings remained 100/130 while 100/150 was standard on a host of combat aircraft operating in the ETO by the later half of 1944.
 

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