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There was a prototype F6F-4 with a single stage R-2800-27:Unfortunately some of these time tables require engines that either were never built or were at least contemplated and either didn't work or never made it through testing.
P&W had been working on their two stage supercharger since 1938, They skipped right over the 2 speed supercharger until sometime in 1940.
The R-2000 supercharger was sort of short of breath. How much was common to the R-1830 I don't know. But the R-1830s were good for 1200hp from sea level to 4900ft in low gear.
The R-2000s were good for 1350hp up to 2,000ft. The R-2000 weighed just about as much as an R-1830 with the 2 speed supercharger and at over 18-19,000ft the two stage engine would give better performance. Putting a similar supercharger on an R-2000 is going to add at least 100lb. Problem is that is just for the dry engine, Trying to put more power through tooth pick props doesn't work well.
Some of the Avenger problems were because of the Navy, why it took 6 months to order the two prototypes after the 7 month design selection process I don't know.
Flight trials began on 7th Aug 1941, Getting the navy to actually order the Prototypes even 3 months earlier might have meant more than 6 Avengers at Midway (still would not have fixed the torpedoes)
Navy also wanted two stage superchargers. What they were expecting to torpedo at 30,000ft I have no idea. Now keep this in mind, Wright announced, but only delivered about 2 two stage R-2600 engines. Which did not work very well and soon disappeared never to be heard from again.
Wright R-2600 engines,
1600hp for take-off. Available in large numbers in 1940-41.
1700hp for take-off, none in 1940, 443 built in 1941, 206 of them in Dec, Production in 1942 is rather sketchy in early part of the year, it takes until June to beat the Dec total, In the worst month of 1942 (Feb) they only built 60 engines between two plants.
1900hp for take-off. Does not show up in double digit numbers until July of 1943.
Now for the reality check.
The 1700hp T-O rating R-2600 was good for 1450hp at 14,100ft no ram.
The engine in the F6F-3 was good for 1800hp at 15,500ft but with ram could hold to 18,700ft
In a British test they got 371mph in low gear at 18,700ft.
I doubt that at 16,000-17,000ft the R-2600 powered Hellcat was going to make 360mph.
What the two stage supercharged R-2600 might have done I don't know, they yanked that engine out of the F6F-1 prototype in about one month.
Grumman was, as shown by the list, working on a lot of other things than the F5F/XP-50.The 'now' part is covered by the historical F4F. The perfected part is what the people in Grumman will be working instead of on the F5F and P-50.
Wow so the solution may just be get FM-2 into production earlier. That could have been very helpful for the Marines and the FAA as well.
Shortround's point about the 'toothpick propellers' is also interesting. Can you put a broader bladed propeller on an F4F-3 or FM-2?
Thank you,There was a prototype F6F-4 with a single stage R-2800-27:
Few comments on superchargers.
You may very well be right about 37% but that seems awfully low. Efficiency of the supercharger is measured by the power it take to compress the air the desired amount by the power it takes to drive the compressor.
As in it takes 65hp to get the desired airflow and pressure but it takes 100hp to drive the compressor you have compressor operating at 65% efficiency. The problem is where does the other 35hp go? Only 2% or less goes into friction/drag in the supercharger gears. The rest is converted to heat in the intake mixture.
A 75% efficient supercharger would take about 87hp to get the same airflow/pressure (65hp required to do the work) as the less efficient supercharger leaving the engine with 13 more HP going to the crankshaft and a lot power turning into heat, just over 1/3 less. Now this is the extra heat over and above what is going to happen from a strict mathematical formula for the heat of compression.
This does not tell us anything about the airflow or pressure ratio of the supercharger, except that trying to get high boost from a poor supercharger is going to take a lot of power and very hot hot air.
The supercharger on the Kestrel V gave 1.5lbs boost at 14,000 but could give 6lbs at sea level or higher, 6lbs seems to be detonation limit the British were not going to exceed using their 87 octane fuel. The Kestrel V at altitude would run at 2900rpm and used an 8.8 gear ratio on the supercharger, however I have no information (others may have a lot of information) on the impeller size, number of blades, size/shape on the inlet and diffuser vanes/housing.
At any rate Kestrel V at 14,000ft was compressing the air about 1.88 times the standard atmospheric pressure (17.57 in hg/8.63lb/sq/in)
The Kestrel XV at 14,500ft running at 3000rpm and using a 9.4 supercharger gear could give 6lbs and was compressing the air about 2.45 times.
The Merlin III supercharger at 16,250ft had a pressure ratio of about 2.62 (no RAM as are the others). In the late 30s this was as good as it got.
The engine in the F4F-4 was limited to 7.66lbs of boost at 19,000ft.
The F4F may have been limited by it's cooling fins but basically the two stage R-1830 with intercooler roughly equaled the Merlin X/XII single stage supercharger.
Intercooler design is another ballgame. The needed airflow is highly dependent on the amount of cooling desired.
One simple chart says that an equal amount of cooling air to intake air to get a 40 degree drop in charge temp, you need 1.5 to 1 ratio to get just over 50 degree drop and a 2 to 1 ratio gets you just over a 60 degree drop.
The R-2800 used higher supercharger pressures but these were intercooled so we don't know what the pressures and temps were between the stages, We do know that the R-2800s were moving a lot more air through the intake system.
Grumman was, as shown by the list, working on a lot of other things than the F5F/XP-50.
They were already working on the F6F in fall of 1940.
That made me laugh, that's so US Navy of the day lol.Navy also wanted two stage superchargers. What they were expecting to torpedo at 30,000ft I have no idea
Thanks for the details. Was looking into the XF4F-8, any info as to when work started on it? Weight was down about 500 lb, what/how did they lightened it? Obviously the engine is not ready in 1942, but for this TL the light airframe must be ready sooner, and powered by the regular 1830-86 by necessity. The folding F4F-4 flew in spring 1941 while the light airframe in late 1942, so to get the light airframe ready earlier something has to give, OTL they were working on the rather useless F4F-7, so perhaps instead of that they focus on weight reduction since the F4F-4 is so overweight. Perhaps it can be done in stages so as not to disrupt production, let's say they manage to pare down say 200-250 lb by late 1941, so this will be an F4F-4A, then by summer another 200-250 lb, so this will be an F4F-4B. coupled with small improvements such as jet exhausts, this would at least restore the speed /climb to the F4F-3 level, a welcome boost.Unfortunately some of these time tables require engines that either were never built or were at least contemplated and either didn't work or never made it through testing.
P&W had been working on their two stage supercharger since 1938, They skipped right over the 2 speed supercharger until sometime in 1940.
The R-2000 supercharger was sort of short of breath. How much was common to the R-1830 I don't know. But the R-1830s were good for 1200hp from sea level to 4900ft in low gear.
The R-2000s were good for 1350hp up to 2,000ft. The R-2000 weighed just about as much as an R-1830 with the 2 speed supercharger and at over 18-19,000ft the two stage engine would give better performance. Putting a similar supercharger on an R-2000 is going to add at least 100lb. Problem is that is just for the dry engine, Trying to put more power through tooth pick props doesn't work well.
Some of the Avenger problems were because of the Navy, why it took 6 months to order the two prototypes after the 7 month design selection process I don't know.
Flight trials began on 7th Aug 1941, Getting the navy to actually order the Prototypes even 3 months earlier might have meant more than 6 Avengers at Midway (still would not have fixed the torpedoes)
Navy also wanted two stage superchargers. What they were expecting to torpedo at 30,000ft I have no idea. Now keep this in mind, Wright announced, but only delivered about 2 two stage R-2600 engines. Which did not work very well and soon disappeared never to be heard from again.
Wright R-2600 engines,
1600hp for take-off. Available in large numbers in 1940-41.
1700hp for take-off, none in 1940, 443 built in 1941, 206 of them in Dec, Production in 1942 is rather sketchy in early part of the year, it takes until June to beat the Dec total, In the worst month of 1942 (Feb) they only built 60 engines between two plants.
1900hp for take-off. Does not show up in double digit numbers until July of 1943.
Now for the reality check.
The 1700hp T-O rating R-2600 was good for 1450hp at 14,100ft no ram.
The engine in the F6F-3 was good for 1800hp at 15,500ft but with ram could hold to 18,700ft
In a British test they got 371mph in low gear at 18,700ft.
I doubt that at 16,000-17,000ft the R-2600 powered Hellcat was going to make 360mph.
What the two stage supercharged R-2600 might have done I don't know, they yanked that engine out of the F6F-1 prototype in about one month.
We can see here the NACA report from late 1940 (tests were made in Spring of 1940) where the XP-41 - firmly in class of the F4F speed-wise - gained far more than 2-3 mph with better exhaust stacks fited. Granted, this is a propulsion 'tweak', not an aerodynamic one, but still.I am just not seeing 30mph worth of aerodynamic "tweaks" to the F4F.
It is more the area of 2-3mph per tweak, not 5 mph.
Note: The 500 lbs. was gross weight, not empty.Was looking into the XF4F-8, any info as to when work started on it? Weight was down about 500 lb, what/how did they lightened it?
The prop on the FM-2 went 394lb.
The prop on the F4F-3/F4F-4 went about 315lbs, individual props could vary a few pounds.
You need enough prop to handle the power, too much prop just adds weight and drag.
I feel like I am repeating things a lot.
Grumman or Eastern Aircraft CANNOT get the FM-2 into production until Wright is making the 1300hp version of the R-1820 engine in quantity. like 200-300 a month.
Having a couple of tool room samples for testing does not mean you are ready to go into production.
Wright needed to figure out what they needed to get 1300hp out of the R-1820 and THEN they needed to figure out how to make the engine AND get the new tooling and factory space for it.
Once again, the 1300hp version (the -56) used
Different cylinder heads than the older versions,
Different cylinders with a totally new way to make the fins
A different crankcase
Different crankshaft.
assorted other changes. Next to NO parts on the 1200hp version would work on the 1300hp version.
You need a lot of new jigs and fixtures and you need some new types of tools.
Thank you,
The F6F-4 had about as much power running "normal"/max continuous as the 1700hp R-2600 did running at military power (5 min).
So the 3rd column might be closer to the performance of an R-2600 powered plane.
The F6F-4 was rebuilt out of the XF6F-1 after it belly landed in Aug 1942. It was later used as the prototype for the four 20mm gun installation.
I would also like to call attention to the much lower ceiling and times to climb to 20,000ft of this machine.
IN fact this R-2600 powered F6F would be within a few mph and 30 seconds of time to 20,000ft of a P-40E.
Does spending time, money and effort on improving the F4F, beyond what they did historically, impact the development of the F6F?
This is what i have on the XF4F-1, from here:Care to post a picture of that aircraft?
Thanks for the details, didn't realized there were no structural mods like lightened fuselage or lightened folding wings.Note: The 500 lbs. was gross weight, not empty.
230 lbs. is the weight reduction of the experimental Wright R-1820 compared to the std P&W R-1830-86.
20 lbs. is the moving/consolidation the wing to cowl oil cooler, and the redesign of cowl.
130 lbs. is the removal of outer 2 guns (back to 4 gun wing)
The rest is the removal of ammunition for said guns. (inner guns retain the 240 rpg of the F4F-4, not the 450 rpg of F4F-3))
Other than rebalancing the airframe to account for the lighter engine, Grumman didn't do much.* (The slotted flap wings weren't a success and were replaced with regular split ones).
* The XR-1830-56 in addition to being lighter made 150hp more. Grumman increased fin and rudder size to account for this.