F4U vs F6F & Top-Speed: Let's Settle This

[DarrenW]

So you could basically say the following

1. All other things equal: An F6F & F4U of the same period will see the F4U with a speed advantage over the F6F?
2. Variables that affected the performance of the F4U would have included
   • Quality of paint-job
   • Removal of hook & covering over it for land based units
   • Smoothing & puttying over the fold-lines for land-based units, and the wing-fold mechanism
   • Redesign of the later F4U-1's canopy
   • Replacing the 13'4" propeller with the 13'1" propeller when possible.
   • Redesign of the tailwheel from short to high
   • Addition of a streamlined fairing behind the taller tailwheel on some desigins
   • Replacement the 13'4" propeller with the F6F's smaller 13'1" propeller
   • Improvement of pylon design

Others would be improvements to the supercharger, a pressurized ignition harness, higher MAP ratings and allowances for higher engine temp (lean mix), water injection, and higher carburetor impact pressure.

Yes, that's a great summation of what we have discussed so far....

 

[Reluctant Poster]

During a test climb to Hellcat service ceiling, Corky Meyer had the engine quit at 32,640 feet "as though someone turned off the ignition switch." Fortunately he got a windmill start after losing about half his altitude. By this time the engine was cold and running poorly so he immediately landed. His next two attempts to reach service ceiling had the same result. Remembering that Grumman's neighbors at Republic were regularly flying the R-2800 at 40,000, Meyer got on the phone. Republic's chief test pilot told him straightaway that they'd never get over 32,000 without a pressurized ignition harness.

"As P-38, P-47, Hellcat and Corsair fighter operating altitudes increased to well above 35,000 feet, the atmospheric pressure 'insulation' of the wires decreased so much from sea level to these altitudes that it allowed the spark electrical energy to easily jump through the normal rubber insulation protection and short out prior to reaching the spark plugs."

"The pressurized harness was developed by sealing each of the wires into flexible tubes from the magnetos to the spark plugs and then pressurizing the wires, magnetos and spark plugs by an air pressure pump driven by the engine."

"We then found out that the Army Air Corps had a very high priority and requisitioned all of the pressurized harnesses that Pratt and Whitney had built for the 40,000 foot ability of the Republic P-47 because it was soon going into high altitude combat in Europe... Needless to say we got the Navy brass in Washington to get us one of the pressurized harnesses for our Hellcat and I made a most anti-climactic climb to 39,455 feet altitude, which was just what we had predicted for the Hellcat's service ceiling."

Incidentally, for flights above 30,000 feet Grumman had their pilots pre-breathe oxygen for 30 minutes while riding a stationary bicycle in order to work the nitrogen out of their blood.

Meyer and Ginter, "Grumman F6F Hellcat," 2012.

The British didn't have the same problems. They had better spark plugs which were less susceptible. From the Fedden Mission Report that I posted recently:

According to the book "The Vital Spark" the British used ceramic in lieu of mica for the insulator and platinum for the electrode. The British manufacturers designed spark plugs to fit the American engines and produced them to allow fitting to American aircraft as they arrived. Roosevelt even mentioned this in a speech to Congress:

" After the United States Eighth Air force began operation from Britain the summer of 1942, the British undertook to double their production so that they could provide all our Eighth Air ice Fortresses with these plugs. Since early in 1943 virtually every United States Flying Fortress has taken off from British bases with these plugs in each of its four engines."

 

[DarrenW]

The British didn't have the same problems. They had better spark plugs which were less susceptible. From the Fedden Mission Report that I posted recently:


View attachment 582100

According to the book "The Vital Spark" the British used ceramic in lieu of mica for the insulator and platinum for the electrode. The British manufacturers designed spark plugs to fit the American legginess and produced them to allow fitting to American aircraft as they arrived. Roosevelt even mentioned this in a speech to Congress:

" After the United States Eighth Air force began operation from Britain the summer of 1942, the British undertook to double their production so that they could provide all our Eighth Air ice Fortresses with these plugs. Since early in 1943 virtually every United States Flying Fortress has taken o ff from British base with these plugs in each of its four engines

This is very interesting, thanks for sharing it.

So if I understand this completely, these "inferior" spark plugs actually forced the American engineers to improve the associated electrical components (in order to help make up for this deficiency), and in the end created equipment which was superior to what was being developed in Britain at the time. I suspect that the combination of British spark plugs and American magnetos/harnesses made for a winning combination indeed. Pretty neat stuff.

 

[Zipper730]

It seems that the reports from 1942-1943 seem to indicate top speeds of 388-395 mph at critical altitudes of 23800' to 24350': It would appear at this point, the airplane was pretty much in the baseline configuration. I'm not sure why the critical altitude varied by 550', though if I made a guess, it might be a lower manifold pressure (388 mph @ 24350' vs 395 mph @ 23800') seen in the earliest designs.

There was a report dated February 8, 1943 which included a variety of modifications that seem to be based on the NACA report that suggested various ways of cleaning up the F4U (smoothing over wing panels, access doors, skin irregularities, removal of the wing-walkways, arrester and catapult-hooks, as well as fairing over the arrester hook cut-outs). It would appear that the aircraft (BuNo #02334) was an F4U-1 (corrected). The engine was re-geared from the normal 0.5:1 to 0.4:1, and fitted with a 4-bladed 13'0" propeller in lieu of the normal three-bladed 13'4" propeller, and fitted with water-injection aimed at bumping up manifold pressure to 58" (early aircraft seemed to top out around 52-54").

According to estimates they could get the design up to 432 mph if horsepower was increased from 1650 to 1875, and 442 mph, if 2000 horsepower could be achieved with the modifications listed: Testing carried out on on January 31, 1943 showed a top speed of 431 mph @ 23825' and 428 mph @ 22840'.

What's of interesting note (in addition to keeping the propeller tip speed subsonic at maximum speed), is the claim of the F4U-1 reaching 404 mph @ 25300': While I read that this was an estimate -- I'm curious how they came up with such numbers in still air.

While I know there's methods to increase speed at altitude by lowering the RPM, and ways to increase critical altitude by leaning out the fuel/air mixture (something which might be dangerous), the RPM was reading 2700, so short of some inexactitude of terminology (i.e. bullshit), they appeared to have eked out performance that bested the normally listed figures for the F4U-1.

 

[Dana Bell]

Hi Zipper,

BuNo 02334 was the 182nd Birdcage F4U-1, though that doesn't change the importance of the report.

Cheers,



Dana

 

[DarrenW]

What's of interesting note, in addition to that, is a statement of the F4U-1A reaching 404 mph @ 25300....

Part of the issue is that 404 mph was an estimate and not accomplished during actual testing. The earliest F4Us were hard pressed to reach 400 mph under operational conditions, but this was eventually overcome by various modifications and thus the -1A variant was born.

It must also be recognized that when the report was written (February 1943) a "standard F4U-1" was the birdcage variant and not the modified F4U-1A.

 

[DarrenW]

During wind tunnel testing NACA determined that the speed of the F6F-3 could be improved by as much as 20 mph with various air frame modifications (to reduce aerodynamic drag), but of those mentioned in the report Grumman only deleted the lower cowl flap. I believe that given the performance edge already held by the Hellcat over the enemy it was more critical to keep production as simple and cost effective as possible in order to provide as many fighters for the US Navy as they could.

 

[DarrenW]

I'm not sure why the critical altitude varied by 550', though if I made a guess, it might be a lower manifold pressure (388 mph @ 24350' vs 395 mph @ 23800') seen in the earliest designs.

A variance such as that could be attributed to slight differences found in the natural operation of the auxiliary stage regulators involved and/or piloting technique.

 

[Zipper730]

BuNo 02334 was the 182nd Birdcage F4U-1, though that doesn't change the importance of the report.

The reason I came up with that was because there was a picture of the F4U-1A on the top of the page, and the link and summary were the first on the list.

Regardless, the aircraft depicted couldn't have been the intended aircraft because it had a three-bladed propeller, and the aircraft for this test had a four-bladed propeller.

BTW: I'm not sure what modifications were intended for the canopy on this design, though it was listed in the report.

Part of the issue is that 404 mph was an estimate and not accomplished during actual testing.

That's a good point, and for all I know, the might have chosen to risk a degree of terminological inexactitude, as Winston Churchill would say.

 

[Dana Bell]

When did the F4U-1A first start to appear, and when did it become standard?


My production delivery lists end on 24 July 1943 with the 873rd Vought Birdcage. The 950th Vought Corsair was BuNo 17647 - the first raised cockpit aircraft (or "1A"). Since that's only 75 airframes later, I'd guess that the first 1As began rolling off the assembly line in August or early September 1943.

Cheers,


Dana

 

[DarrenW]

Do you have any graphics that depict proposed mods?

They can all be found here in this report:

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930092668.pdf

I know that you're at least somewhat familiar with this document, as we discussed it earlier on in this very thread.

If you look at figures 7c, 23, 29a, 30b, 31, and 39b for airplane 10 (F6F-3) you will see drawings of the proposed changes and their effect on speed. Granted one modification suggested by NACA would not be suitable for carrier aircraft (removal of wing fold joints) but it does give a pretty clear indication that there was a lot of room to improve the aerodynamics of the F6F and thus increase it's maximum speed. All told the F6F-3's Cd was reduced by more than 28 percent. By comparison the P-51B, which was aerodynamically a much cleaner airplane, had it's Cd reduced by slightly less than 23 percent with suggested modifications.

Be aware that a few of the suggestions made by NACA couldn't be accomplished in the "real world" (such as completely enclosed exhaust stacks), and were only there to show where aerodynamic improvements could possibly be made that had the greatest effect on a particular airplane's maximum speed.

 

[Zipper730]

If you look at figures 7c, 23, 29a, 30b, 31, and 39b for airplane 10 (F6F-3) you will see drawings of the proposed changes and their effect on speed.

Looking at what I've see, one proposal was to minimize the gaps in the cowl-flaps (+3 mph); another was to seal over the gaps at the wing-fold joint and gun-access & ammunition doors (+4 mph); add a full-length fairing for the main-landing gear doors (+5 mph); internally seal the bulkhead and/or add external fairings from the openings at the tail-wheel and arrester hook (+3 mph), and; remove radio antenna.

The removal of the radio antenna is ridiculous: Radio is needed; fairing over the wing-fold joints would be unsuitable for carrier-ops, and fairing over the gun access/ammo doors would render the plane unusable (unless the doors could be covered when not in use).

 

[DarrenW]

The removal of the radio antenna is ridiculous: Radio is needed; fairing over the wing-fold joints would be unsuitable for carrier-ops, and fairing over the gun access/ammo doors would render the plane unusable (unless the doors could be covered when not in use).

I believe they were concerned about the mast and it's drag invoking qualities. If you look at figure 39b there are examples of radio antenna installations that basically have zero effect on speed. Applying these to the Hellcat and other aircraft would improve speed by 1.5 - 2 mph.

NACA found that reducing panel gaps in the wings (possibly with special seals) would allow for smoother air flow without actually removing the panels all together.

 

[Zipper730]

I believe they were concerned about the mast and it's drag invoking qualities. If you look at figure 39b there are examples of radio antenna installations that basically have zero effect on speed. Applying these to the Hellcat and other aircraft would improve speed by 1.5 - 2 mph.

That sounds reasonable, as far as I can see. Would that have affected production seriously?

NACA found that reducing panel gaps in the wings (possibly with special seals) would allow for smoother air flow without actually removing the panels all together.

Would that have been hard to implement, and how would the speed reduction compare to removing the panels altogether?

Also, I'm curious which exacted a greater penalty: The drag produced by the wing-fold, or drag produced by the ammo-bay doors? The ammo bay doors look like they'd be a bigger offender because they were just open gaps.

How many land-based F6F's were used?

 

[Reluctant Poster]

According to Dietmar Hermann's "FW190 Long Nose" as a part of the performance enhancing experiments conducted on FW 190 V53 (a D-11 prototype), all gaps on the engine cowling were sealed with rubber. This resulted in a 17 km/h increase in maximum speed. These mods never made it into production. A lot of these types of enhancements never made it into production because it would slow production down. Those that did would often not survive in the field, an example would be the retractable tailwheel of the 109 which tended to be locked in place. In a lifetime of tinkering with cars I have developed an intense dislike of gaskets which tend to fall apart when disassembled or refuse to reseal. Remember that materials back in 40s were not as durable as today

 

[Zipper730]

According to Dietmar Hermann's "FW190 Long Nose" as a part of the performance enhancing experiments conducted on FW 190 V53 (a D-11 prototype), all gaps on the engine cowling were sealed with rubber. This resulted in a 17 km/h increase in maximum speed. these mods never made it into production. A lot of these types of enhancements never made it into production because it would slow production down. Those that did would often not survive in the field. an example would be the retractable tailwheel of the 109 which tended to be locked in place. In a lifetime of tinkering with cars I have developed an intense dislike of gaskets which tend to fall apart when disassemble or refuse to realest. Remember that materials back in 40s were not as durable as today

So, you suspect that attempts to keep the cowl-flaps sealed right would either be impossible to keep in the right condition, and might be difficult to implement on a wartime footing?

 

[Reluctant Poster]

So, you suspect that attempts to keep the cowl-flaps sealed right would either be impossible to keep in the right condition, and might be difficult to implement on a wartime footing?

I think once you take it apart its is difficult to re-seat. Years ago I took out the dash board of one of my VW Corrados. Getting it back together was not a trivial task. I had the luxury of time, I can imagine that under wartime conditions a lot of the parts would left out. A Corrado in some ways is a good example as only 97,000 were produced and although it was based on a mass produced car it was largely hand assembled by Karmman. Similarly while working on my friends BMW CSi 3.0 (also assembled by Karmann) I found myself cajoling things into place. The engines in both cars are different story, the parts do fit readily if you can get at them.

 

[DarrenW]

That sounds reasonable, as far as I can see. Would that have affected production seriously?

Maybe. Once a concept is finalized and production is underway something even as minor as changes in the color of paint will effect production output enough to be noticeable.

Would that have been hard to implement, and how would the speed reduction compare to removing the panels altogether?

Not really sure, the increase in speed would probably be somewhat less because the surface wouldn't be as smooth as if they were completely fared over. As I said earlier the performance of the F6F was high enough "as is" and there really wasn't a need to make many of the changes suggested by NACA. If Japanese aircraft performance was greater it would have been more imperative to implement these modifications, but it wasn't.

How many land-based F6F's were used?

There were four US Navy squadrons which operated for a time on land, VF-1, 33, 38, and 40. VF-1 was stationed in Tawara, while the other three were in the Solomons. All of these units were rotated out by early 1944. There were also five US Marine night fighter units which were land based. VMF(N)-541 was stationed in the Philippines and VMF(N)-534 in Guam, while VMF(N)-533, 542, and 543 served in Okinawa. There's also VMD-354 which flew recce Hellcats out of Falalop in the later stages of the war.

Unlike the F4U, the F6F was always considered "suitable" for carrier use so those that were land based maintained the folding wings and tail hook for possible operations at sea. If you figure 20-30 airplanes on average per squadron we would have maybe about 200-300 Hellcats exclusively operating from land, but this is only a guess on my part. I can do further research into this if you are interested, as I have the source material available (USN records) in order to examine this more closely.

 

[Hiromachi]

Apologies for reheating this thread, but I found discussion very interesting and helpful in understanding a few things.

So based on the above discussion and estimates, I was wondering what would be prop efficiency of that 13'11" unit used on F4U / F6F ?
I've tried searching myself for the data of 23E50 constant speed unit with 6501A-0 blades, especially in form of some blade noise research by NACA or maybe prop efficiency evaluation but NASA archive did not help me here and I found nothing.

 

[Zipper730]

Apologies for reheating this thread, but I found discussion very interesting

No biggie, we do this all the time