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

[Dawncaster]

I encircled the area in a brighter shade of red than the area you encircled, and connected them together. The front area is the front of the tear-drop.

Ah, I didn't expect it to look that way.

It's flap linkage.

As for your graph on the F4U-4, I was under the impression that, up to this point, that the propeller was 13'4" instead of 13'2". You learn something new everyday. As for the gear ratio being reduced from 0.5 to 0.45, is definitely smart.

While the tip-speed is still quite high, it only goes supersonic at critical altitude in WEP and Military power settings, despite being considerably faster...

It's a cool calculation again.

Probably a similar case, for F4U-1 BuNo.50030, the recommended 2700 RPM appears to be rather disadvantageous in terms of Vmax.

Well, calculation seems to have used the detailed specification of F4U-4. Unless otherwise noted(as F4U-5's detail specification), the detail specification typically includes the estimated performance.

I recommend using the last edition of the official performance for Navy standard.

F4U-4's maximum performances

 

[Zipper730]

Ah, I didn't expect it to look that way. . . It's flap linkage.

I guess I was wrong then...

I'd guess there are two important questions to ask

1. Did they ever put the "tear-drop" arrangement on an operational aircraft?
2. Do you have any pictures of such an aircraft from the side or aft quarter?

It's a cool calculation again.

It's not a complicated math calculation (SQRT*((FPS)^2+(RPM/60)^2)))/Speed of Sound at Altitude, but it's nice to illustrate the tip-speed. It seems that the goal in such a large propeller was to drive up climb-rates (which is a low-speed domain on propeller aircraft) at low-speeds. Performance gets sacrificed at high speeds (particularly altitudes where the speed of sound is lower).

Looking at this, I would figure the first three figures were based on wartime-emergency power, with the remaining three on military-power.

I'm reminded of a pilot who found that at certain speeds, lowering the RPM on the governor actually gave him better performance. It's probably for the same reason as on this graph. I'm curious how often pilots in practice would run at 2600 in favor of 2700 at WEP settings at 18360 feet, and at 23480-23500', at 2500 in favor of 2700? It seems like it would have certainly provided more speed...

Well, calculation seems to have used the detailed specification of F4U-4.

Oh well, I could run some numbers for based on actual flight test data.

BTW: What's the WEP figures for the R-2800-18W in WWII times? I'm seeing 2380 hp & 2450 hp, and I'm not sure if either are accurate.

 

[Zipper730]

BTW: I'm not sure who said this, but somebody said that there were early problems with the F4U-1 that, in addition to issues with the supercharger, there were flights were complete engine failure occurred about 29000 feet (well below the aircraft's service ceiling) due to malfunctions where sparks would jump the gap and burn out the distributor points.

Why did this happen?

 

[Dawncaster]

I guess I was wrong then...

I'd guess there are two important questions to ask

1. Did they ever put the "tear-drop" arrangement on an operational aircraft?
2. Do you have any pictures of such an aircraft from the side or aft quarter?

Yes, it was installed on the late F4U-1s. The picture I uploaded was also service aircraft. and it appears that half of the tail wheel doors have not been opened because it's land-based configuration. therefore, so it was easy to see the streamlined block on at ground.

But in practice it was divided and attached to the tail wheel door, so if the tail wheel door is entirely opened, a slight more care should be taken to find it.

In conclusion, because I couldn't find it in the photo attached to the report of F4U-1 BuNo.17930, so I had assumed that it was a structure like a dark block wrapped around a strut, and bias that it would have been a 'special modification' would have also had slight impact. but it was a 'standard modification' and the F4U-1 BuNo.17930's photo attached to the report appeared to have been shot earlier than the test.

It's not a complicated math calculation (SQRT*((FPS)^2+(RPM/60)^2)))/Speed of Sound at Altitude, but it's nice to illustrate the tip-speed. It seems that the goal in such a large propeller was to drive up climb-rates (which is a low-speed domain on propeller aircraft) at low-speeds. Performance gets sacrificed at high speeds (particularly altitudes where the speed of sound is lower).

Looking at this, I would figure the first three figures were based on wartime-emergency power, with the remaining three on military-power.

I'm reminded of a pilot who found that at certain speeds, lowering the RPM on the governor actually gave him better performance. It's probably for the same reason as on this graph. I'm curious how often pilots in practice would run at 2600 in favor of 2700 at WEP settings at 18360 feet, and at 23480-23500', at 2500 in favor of 2700? It seems like it would have certainly provided more speed...

Oh well, I could run some numbers for based on actual flight test data.

Great, perhaps the below content of the manual is also about it.

What's interesting is that can find this in the F4U-1 manual, but not in the F4U-4 manual.

BTW: What's the WEP figures for the R-2800-18W in WWII times? I'm seeing 2380 hp & 2450 hp, and I'm not sure if either are accurate.

It's not clear. documents during World War II specify 2450 BHP of 60"hg and 2650 BHP of 70"hg. 2380 BHP was the figure specified by 1946 ACP.

BTW: I'm not sure who said this, but somebody said that there were early problems with the F4U-1 that, in addition to issues with the supercharger, there were flights were complete engine failure occurred about 29000 feet (well below the aircraft's service ceiling) due to malfunctions where sparks would jump the gap and burn out the distributor points.

Why did this happen?

It looks like the contents of a post I wrote a few pages ago.

According to Barrett Tillman, It was traced to faulty pressurization from the Pratt and Whitney supercharger, which sometimes aloowed the spark to "jump the gap" and burn out the distributor points.

 

[Zipper730]

Yes, it was installed on the late F4U-1s.

That's pretty cool.

Great, perhaps the below content of the manual is also about it.

That seems to cover it, though normal rated power usually involves a lower manifold pressure, which is easier to achieve, even at higher altitudes.

The fact that it's not included in the F4U-4 might just be that it performs well enough to not really require the need for fine-tuning the RPM.

It's not clear. documents during World War II specify 2450 BHP of 60"hg and 2650 BHP of 70"hg. 2380 BHP was the figure specified by 1946 ACP.

So 2450 @ 60" and 2650 @ 70".

It looks like the contents of a post I wrote a few pages ago.

According to Barrett Tillman, It was traced to faulty pressurization from the Pratt and Whitney supercharger, which sometimes aloowed the spark to "jump the gap" and burn out the distributor points.

Does this have to do with the air having an insulating effect against electricity? I remember another member talking about this on high altitude designs because with less air, it makes it easier for electrical currents to jump.

 

[DarrenW]

It had to do with the basics of cowl design (and there might be errors here, so bear with me -- I'm used to being the village idiot when it comes to propeller driven aircraft).

1. Propeller: Since almost all aircraft were tractor props, the propeller would be in front of the cowling. The propeller accelerates the airflow and increases the pressure of the flow.
2. Cowl-Lip: The cowl is divergent in shape, which slows down the airflow going through the cowling: This effectively provides more pressure and more air in a given area to absorb the heat from the engine. Provided the velocity is slowed down and builds up pressure, yet remains fast enough to carry away the heat, you have a design that seems workable. While increasing pressure does produce heat, it's not particularly massive (this becomes more significant when supersonic), and the air outside is very cold, and the air temperature is high.
3. Air-Cooled Engine: Air flows from the cowl-lip to the engine, and is heated up substantially by the engine. This causes the air to absorb the heat, and bringing down the engine temperature. It also causes the air to heat up, and expand. Provided the expansion goes more rearwards than forwards, you have a degree of "thrust" that is produced (unfortunately, it almost never equals the amount of drag, but it will negate some of the drag produced by the assembly).
4. Aft Cowl / Cowl-Flaps: I'm not sure the exact term but it's basically the shape of the cowl/fuselage behind the engine, which form a convergent shape. Theoretically you want to make the shape quite convergent as it accelerates the airflow the most, but pressure plays a role in imposing limits on this. The pressure gets too low, and you'll see the air dam up, so you'll have cowl-flaps to allow extra area for the air to escape. This arrangement reminds me a bit of the nozzles on jet-fighters, which are fully open at idle to allow the airflow plenty of area to escape; as the engine spools up, the exhaust pressure goes up, and the nozzle can be narrowed down, and produce a faster exhaust velocity. When 100% power is produced, the nozzles are narrowed in as much as possible. This set-up seems the same on cowling flaps.

Some aircraft have tighter cowlings than other aircraft, being barely big enough to encompass the engine they're built around; a more prominent bell-mouthed shape, a fatter spinner, and a narrower aft-cowling, and so on. These are generally aimed at reducing cooling drag, though they have a tendency to reduce cooling. I figure, if the top-cowling flap was faired over by a metal-plate, then the airflow would have to escape out the other flaps, or the cowling would simply be "tighter" in the back.

Thank you Sir for the info......

 

[DarrenW]

OK, now that we've been buried in the endless minutiae of countless test reports for most of this thread, I have a small question which has alluded me for quite some time. From the performance figures listed in the F4U-1D ACP which was discussed earlier, there is a further decrease in Vmax when pylons were uncapped (409 vs. 401 mph):

By comparison the speed figures for the F6F-5 were taken with pylon sway bracing installed, but no mention of a capped or uncapped condition:

Does anyone have photographs or drawings of a capped pylon? They must have been covers which could be easily placed over the pylon to improve performance (possibly so the pylon didn't require complete removal when not in use):

 

[Dana Bell]

Hi Darren,

Hope these images help!

Cheers,



Dana

 

[DarrenW]

Hi Darren,

Hope these images help!

Cheers,



Dana
View attachment 577615View attachment 577616View attachment 577617

Thanks Dana for the great pictures, they definitely help. Does the last photo show the pylon in the capped condition? I'm also assuming that there was some sort of attachment that included sway bracing, the type dependent on whether bombs or drop tanks were to be carried.

 

[XBe02Drvr]

According to Barrett Tillman, It was traced to faulty pressurization from the Pratt and Whitney supercharger, which sometimes aloowed the spark to "jump the gap" and burn out the distributor points.

Does this have to do with the air having an insulating effect against electricity? I remember another member talking about this on high altitude designs because with less air, it makes it easier for electrical currents to jump.

Yes. Air is a pretty decent insulator at sea level, but the thinner it gets, the thinner its insulation value gets. Engines operating above 18-20K generally need to have the Magneto's, and sometimes the entire ignition system, pressurized. All it takes is one leaky seal amongst the many in the system, and arcing and sparking set in. Not only hurts performance, but generates radio interference as well.
Cheers,
Wes

 

[Zipper730]

Yes. Air is a pretty decent insulator at sea level, but the thinner it gets, the thinner its insulation value gets. Engines operating above 18-20K generally need to have the Magneto's, and sometimes the entire ignition system, pressurized.

When was this starting to become a common feature on aircraft engines?

 

[XBe02Drvr]

When was this starting to become a common feature on aircraft engines?

When did engines with high tension ignition start operating for extended periods above 20K? That's your history assignment.

 

[Zipper730]

When did engines with high tension ignition start operating for extended periods above 20K? That's your history assignment.

Searching...

Update 4/24, I haven't been able to find anything. That said, if I had to make a guess, it's probably when altitude records started exceeding 30000' which would be 1919-1920. That said, I'm not sure when mass-produced systems came around...

 

[Joe Broady]

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.

 

[Laurelix]

F6F-5:
Empty Weight: 4780kg
Loaded Weight: 5630kg
Wing Area: 31.0m2
Engine: Pratt Whitney R2800-10W
Take Off: 2135hp (WEP)
2000hp at SL / 1755hp at 1650m / 1800hp at 4800m / 1650hp at 6400m
-
Max Speed: (Military Power)
547km/h at Sea Level
629km/h at 7050m
-
Max Speed: (WEP)
559km/h at Sea Level
644km/h at 5700m
-
Rate of Climb: (Military Power)
Time to 6000m: 8:24
-
Sustained Turn Time: (WEP, Sea Level)
21 seconds
-
Stall Speed: (At Sea Level, No Flaps)
163km/h IAS
-
Armament:
6x 12.7mm M2 (400 rounds per gun)

References:
• http://alternatewars.com/SAC/F6F-5N_Hellcat_ACP_-_1_July_1944.pdf

• http://www.wwiiaircraftperformance.org/f6f/f6f-5-72731.pdf

• http://www.wwiiaircraftperformance.org/f6f/f6f-5-58310.pdf

 

[DarrenW]

F6F-5:
Empty Weight: 4780kg
Loaded Weight: 5630kg
Wing Area: 31.0m2
Engine: Pratt Whitney R2800-10W
Take Off: 2135hp (WEP)
2000hp at SL / 1755hp at 1650m / 1800hp at 4800m / 1650hp at 6400m
-
Max Speed: (Military Power)
547km/h at Sea Level
629km/h at 7050m
-
Max Speed: (WEP)
559km/h at Sea Level
644km/h at 5700m
-
Rate of Climb: (Military Power)
Time to 6000m: 8:24
-
Sustained Turn Time: (WEP, Sea Level)
21 seconds
-
Stall Speed: (At Sea Level, No Flaps)
163km/h IAS
-
Armament:
6x 12.7mm M2 (400 rounds per gun)

References:
• http://alternatewars.com/SAC/F6F-5N_Hellcat_ACP_-_1_July_1944.pdf

• http://www.wwiiaircraftperformance.org/f6f/f6f-5-72731.pdf

• http://www.wwiiaircraftperformance.org/f6f/f6f-5-58310.pdf

Hi Laurelix,

Your maximum speed is correct for an F6F-5 in a 'clean' condition but in reality most combat missions were flown with an assortment of bombs, rockets, and drop tanks. The wing and fuselage racks required for this normally lowered top speed by 11-16 mph (16-26 km/h), depending on altitude (without munitions and drop tanks of course). Climb rate seems to be with pylons installed.

Your empty weight looks more like an aircraft with a basic loading, which included fuel, oil, and the pilot. According to Grumman actual empty weight was 9079 lbs (4118 kgs).

And while I have seen WEP ratings at S/L as high as 2250 hp, I think your figure of 2135 hp is more accurate. I derive this from WEP testing performed from 58 - 64 inHg, where unauthorized levels were required to achieve the 2250 hp output.

Not to be too critical, but the given stall speed should state a 'power off' condition. With 'power on' and no flaps the stall speed was closer to 97 mph (156 km/h) and 80 mph (129 km/h) with flaps.

Where did you get that turn rate from? Just curious how it was determined.....

 

[Laurelix]

Hi Laurelix,

Your maximum speed is correct for an F6F-5 in a 'clean' condition but in reality most combat missions were flown with an assortment of bombs, rockets, and drop tanks. The wing and fuselage racks required for this normally lowered top speed by 11-16 mph (16-26 km/h), depending on altitude (without munitions and drop tanks of course). Climb rate seems to be with pylons installed.

Your empty weight looks more like an aircraft with a basic loading, which included fuel, oil, and the pilot. According to Grumman actual empty weight was 9079 lbs (4118 kgs).

And while I have seen WEP ratings at S/L as high as 2250 hp, I think your figure of 2135 hp is more accurate. I derive this from WEP testing performed from 58 - 64 inHg, where unauthorized levels were required to achieve the 2250 hp output.

Not to be too critical, but the given stall speed should state a 'power off' condition. With 'power on' and no flaps the stall speed was closer to 97 mph (156 km/h) and 80 mph (129 km/h) with flaps.

Where did you get that turn rate from? Just curious how it was determined.....

It's estimated by me. I have a list of planes with documented turn time. Then I compare F6F-5's power to weight ratio, stall speed and drag coefficient compared to them to give a rough estimate value for its sustained turn time.

 

[Laurelix]

Hi Laurelix,

Your maximum speed is correct for an F6F-5 in a 'clean' condition but in reality most combat missions were flown with an assortment of bombs, rockets, and drop tanks. The wing and fuselage racks required for this normally lowered top speed by 11-16 mph (16-26 km/h), depending on altitude (without munitions and drop tanks of course). Climb rate seems to be with pylons installed.

Your empty weight looks more like an aircraft with a basic loading, which included fuel, oil, and the pilot. According to Grumman actual empty weight was 9079 lbs (4118 kgs).

And while I have seen WEP ratings at S/L as high as 2250 hp, I think your figure of 2135 hp is more accurate. I derive this from WEP testing performed from 58 - 64 inHg, where unauthorized levels were required to achieve the 2250 hp output.

Not to be too critical, but the given stall speed should state a 'power off' condition. With 'power on' and no flaps the stall speed was closer to 97 mph (156 km/h) and 80 mph (129 km/h) with flaps.

Where did you get that turn rate from? Just curious how it was determined.....

With Pylons you're looking at F6F-5 doing
523km/h at SL (WEP)
608km/h at 5700m (WEP)

 

[DarrenW]

It's estimated by me. I have a list of planes with documented turn time. Then I compare F6F-5's power to weight ratio, stall speed and drag coefficient compared to them to give a rough estimate value for its sustained turn tim

That's interesting. Do you have the sustained turn time for the F4U-1?

With Pylons you're looking at F6F-5 doing
523km/h at SL (WEP)
608km/h at 5700m (WEP)

Those look pretty good to me.

NAVAER performance figures dated both November 1945 and January 1949 for the F6F-5 under similar conditions (dual bomb racks and belly shackles) are 318 mph (512 km/h) at S/L and 378 mph (608 km/h) at 18000 ft, which matches your figure at altitude but to me the speed at S/L looks closer to performance while in Military power, which I expect to be at least 10 mph faster (there are BuAer wartime test reports that support this conclusion). But I like to trust the wartime data as much as possible.

It must also be noted that Grumman performance numbers were always greater than those given by the US Navy, but some of this obviously had to do with aircraft configuration and condition. The manufacturer normally tested aircraft without pylons and such, while the opposite was normally true for the military. For instance, Grumman report 2422C dated 15 Mar 1945 states a maximum speed of 350 mph (563 km/h) at S/L and 400 mph (645 km/h) at 20000 ft, which are very close to your figures in post #135.

 

[Zipper730]

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.