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Wild_Bill_Kelso
Senior Master Sergeant
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- Mar 18, 2022
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Are you sure about that? I have the manual for the P-40F/L from Dec 1942 and it states take off power at 54.3" Hg / + 12 lb, and "War Emergency" at 61" Hg
Screen shot:
View attachment 664808
One thing obviously wrong in this chart is that it shows 1300 horsepower for 54.3" Hg (+12 lbs), and the same 1300 hp at 61" Hg (+15 lbs)
I think both of those numbers are wrong, 54.3" Hg should be closer to 1400 hp I think, and 61" should be close to 1600 hp. Anyone know how to calculate that exactly for a Merlin XX? What is the RAF equivalent HP for Merlin XX at +12 and +15 boost respectively?
About as far as this 71 year old man can push it one handed while wearing roller skates with the engine sitting on the ground.So how far can you push an R-1830?
It was noted that the Allison didn't always break on the flight that they were over boosted on. A fair number broke on the next flight or the one after without being over boosted on those flights.I have read personal accounts of some pilots doing exactly this. Some got away with it, some didn't. I have also read many accounts of blown engines from those early days.
This took several years, Allison was reworking 277 or so of the early engines in 1940 to get them up to specs. There was a miscommunication (?) between the army and the Allison about the 1040hp vs 1090 hp rating. The next major up grade came around Dec of 1941 with hundreds of -39 engines already built. They knew the change in the reduction gears was a positive thing but the change in the crankshaft and crankcase was the 2nd one since the summer of 1940.Allison responded by strengthening components of the engines and agreeing to compromise boost levels.
Yes and no. British fuel in the summer of 1940 was 100/115-120 while American fuel was 100/100. You could over boost an Allison engine using British fuel with better chance of success than using American fuel. I have no idea what the US was using where in Dec 1941. Like what the Canal zone had or what the Philippines had or whatever. By the Spring/summer of 1942 the British and Americans were both using 100/130 fuel, the British had gone to the 100/130 before the Americans but exact dates in 1941 are hard to come by. Now since the Mustang Is in British service didn't go operational until May of 1942 it doesn't make much difference.Better fuel also became available which made higher boost more tenable.
Not really. see belowAt 56" or 57" Hg a P-39D or P-40E basically becomes a 1,500 hp fighter below 10,000 ft.
The -39 engine in the E and the -73 engine in the K operated exactly the same except the the -73 was allowed to use a bit higher boost down low.The K model P-40 was making almost 1600 hp down low. This to me really helps explain why these aircraft kind of had a split Jeckyll / Hide reputation, IMO.
If the engine was being operated at wide open throttle you might be closer to being correct.I think both of those numbers are wrong, 54.3" Hg should be closer to 1400 hp I think
Or simple human error of the technical writer and editor missing one or more parts of the manual where the fuel quantity was written - especially if the technical writer and editor was the same person.sometimes the manuals don't even agree between pages of the same manual (early version of cut and paste for manuals?)
Yes agreed. I think it became standard practice when they came down leaking oil and a fresh scorch mark on the side of the fuselage, and maybe a broken wire on the boost control, to change all the spark plugs and check the rings etc. Mechanics weren't happy to see a plane landing like that, even if they were glad to see it landing.About as far as this 71 year old man can push it one handed while wearing roller skates with the engine sitting on the ground.
I only seen any official clearance once for operating the R-1830 at higher than normal (book) settings and that may have been revoked. It was being permitted to over rev the engine for emergency carrier take-off.
Air cooled engines have several things working against them.
One, the air cooled engines were known as severe duty engines in the fuel industry. I may the adjective wrong but I hope you get the idea. Liquid cooled engines were considered mild or medium duty. The Air cooled engines were operating closer to their thermal limits to begin with and there may have been hot spots in the cylinders with some spots hotter than the liquid cooled engines had. Air cooled engines seldom operated at the pressures that liquid cooled engines did.
the air cooled engines didn't have the cooling fluid and radiator to act as a heat sink so when they were asked to make more power they overheated very rapidly.
An R-1830 in order to make 1200hp was already operating at 9lbs of boost. and there they stayed until 1943-44 ?
A few 1350hp engines running at 2800rpm were produced but they had a modified auxiliary section, a modified supercharger and used trickle down knowledge from the cylinder fins and cooling muffs from the later R-2800 engines. Some of these were used on the Convair P4Y2 which shows the time scale.
It was noted that the Allison didn't always break on the flight that they were over boosted on. A fair number broke on the next flight or the one after without being over boosted on those flights.
This took several years, Allison was reworking 277 or so of the early engines in 1940 to get them up to specs. There was a miscommunication (?) between the army and the Allison about the 1040hp vs 1090 hp rating. The next major up grade came around Dec of 1941 with hundreds of -39 engines already built. They knew the change in the reduction gears was a positive thing but the change in the crankshaft and crankcase was the 2nd one since the summer of 1940.
Until they got out in the field for a number of months it would have been hard to justify upgrading them.
In the US literature they mention 98 or 96 fuel and 100 fuel. The latter was authorized for higher boost settings. Gasoline doesn't last too long in the field so presumably they were on the better stuff fairly soon after it started being produced.Yes and no. British fuel in the summer of 1940 was 100/115-120 while American fuel was 100/100. You could over boost an Allison engine using British fuel with better chance of success than using American fuel. I have no idea what the US was using where in Dec 1941. Like what the Canal zone had or what the Philippines had or whatever. By the Spring/summer of 1942 the British and Americans were both using 100/130 fuel, the British had gone to the 100/130 before the Americans but exact dates in 1941 are hard to come by. Now since the Mustang Is in British service didn't go operational until May of 1942 it doesn't make much difference.
I'm sure you are right, but I've never seen hard data on overboosting the -33s on the Tomahawks. The Tomahawks, especially the P-40B types, were closer to their optimal weight, especially if they flew for about half an hour before combat, so the need to increase the standard boost settings may not have been as pressing.A lot of people had over boosted the Tomahawks, in part because they had no boost control/limiter on the throttle. but since the -33 engine had the older crankshaft/s and crankcases you weren't getting good data on the -39 engine and later -39 engines
1310 hp is still quite a bit better than the 1050 or 1150 hp they are usually quoted as having in hundreds of books, which is also incidentally what you get with a V-1710-39 at 42" Hg - 1150 hp. Which is the military power rating.Not really. see below
The -39 engine in the E and the -73 engine in the K operated exactly the same except the the -73 was allowed to use a bit higher boost down low.
Yes it could use 60in at 2500ft (no ram so add a few thousand feet) HOWEVER it was back down to 1490hp at 4300ft just like the -39 engine and both engines were tapering off (in a straight line) to the 1150hp level at 11,700-12,000ft. Like without RAM the engine/s will be making about 1310hp at 8,000ft.
RAM decreases as the aircraft speed decreases. In other words if you are doing a hard climb or exiting a hard turn and your air speed is down around 200mph your power available is several thousand feet lower than it would be when flying straight and level at max speed or near max speed.
This was part of gain with the 9.60 gears, they may have lost some of the impressive horsepower figures down low (but since the air at those low altitudes sucks up power anyway it may not be so bad) but the -81 engine could give the P-40 pilot an extra 100hp 1,500ft higher than the -73 engine. and could keep giving that extra power as the altitude climbed.
We seem to be confusing the different Allisons.1310 hp is still quite a bit better than the 1050 or 1150 hp they are usually quoted as having in hundreds of books, which is also incidentally what you get with a V-1710-39 at 42" Hg - 1150 hp. Which is the military power rating.
I used to think that the higher boost settings were only available at near sea-level but that is clearly not the case.
The 60" may not have been available above 4 or 5,000 feet but from the flight tests it's clear that they were boosting to 57" Hg up to about 9,000 feet and still at 50" well above 12,000 ft. Obviously depending on atmospheric conditions, temperature, fuel, and other factors.
Somebody has been using the rose tinted glasses.yes they could maintain pretty high power up to about 16 -17,000 ft compared to around 12-13,000 for the earlier types with the 8.8-1 superchargers.
We seem to be confusing the different Allisons.
The "military Power rating" of 1150hp at 11,700ft or 12,000ft depending on backfire screens and whatnot, was at wide open throttle. There was no more boost to be had.
There was no 50" well above 12,000ft in an engine with 8.80 gears. Not unless you are winding the engine well above 3000rpm. And you better have played with the prop governor or blade pitch adjustments to do that. RAM is not going to give you over 5.6 in of manifold pressure at 12,000ft or above.
I have said it before, you can draw a line on a graph and plot all the points on one straight line on the graph. 1150hp at 11,700ft becomes 1090hp at 13,200ft continuing on the line you get 1040hp at 14,300ft and the chart goes up to 25,000ft. going the other way the engine will make about 1360hp at 6500ft at 50 inches (no ram ) and continue down 61 in at sea level.
I repeat, this is at ZERO ram. The supercharger can flow less by being throttled back. It cannot flow more air unless RAM provides more air going into the intake.
RAM is not fixed. It has two variables, one is the airspeed of the airplane. The faster the plane the higher the pressure in the duct before the carb.
Which is plenty of powerSomebody has been using the rose tinted glasses.
The "High power" was 1125-1150hp.
The P-40N-1 was good for 1480hp at 10550ft with the 57in of MAP.
Nominal altitude was 9,500ft...................however when climbing the 1480hp level was only maintained to 8,000ft before the 57in pressure could not be maintained.
The Nominal power rating of 1125hp at 15,500ft was maintained to 17,300ft
The rose colored glasses come in with the fact that out of well over 2000 P-40Ns built only about 400 (at best) were the N-1 version and many of them were modified in the field which slowed them down and they weren't getting quite the RAM effect.
A P-40N-5 recorded a max speed of only 350mph instead of 371mph and could only hold the 1125hp setting at 16,400ft (44.5in) in high speed flight and when climbing it could only hold the 44.5in setting (1110hp) at 14,000ft.
Depending on Ram for hi power is a bit dicey as I have said before as many people want to take the power rating for high speed and forget about what happens when trying to climb or accelerate out of a speed robbing maneuver.
I'm going by this chart which I posted previously, from an Australian test with IIRC a Kittyhawk Ia so a -39 or maybe up-engined to a -73. This shows WEP of 57" right up to 9200', (and it certainly looks like more is available right up to that point) then slowly tapering off and it appears to still be 50" Hg at 13,000 ft. Military power (probably 42") is available right up to 17,000 ft.
That chart uses an engine with 9.60 supercharger gears.That chart is with full ram effect, ie. the aircraft is at max speed (5 min worth for war emergency power applied). If the aircraft is climbing or cruising, the boost level (and with it the power) will be lower, much closer to the no-ram-effect power levels as posted at post #114 here.
Un-sporty thing with ram effect is that the enemy also benefits from it.
The chart you provided looks like what a V-1710-81 would provide.
the date of the test is March of 1944 which makes it a little bit suspect as to why they would be testing a -39 engine well over 1 1/2 years since the stopped making them.
What is even more suspect is that the serial number of the aircraft belongs to a P-40M (KIttyhawk III) which as built had a -81 engine.
The test weight of the plane is about 400lbs below that of a normal P-40M which means under 1/2 the internal fuel or they had lightened the aircraft by pulling a couple of guns and some ammo.
Test weight of aircraft has little to do with engine performance.
That chart is with full ram effect, ie. the aircraft is at max speed (5 min worth for war emergency power applied). If the aircraft is climbing or cruising, the boost level (and with it the power) will be lower, much closer to the no-ram-effect power levels as posted at post #114 here.
Un-sporty thing with ram effect is that the enemy also benefits from it.
But in combat you won't be in a slow climb or cruise, or anyway not for long, right? Fighter pilots would build up energy by diving and zooming, extending at full power and so on.
So long as we compare like with like, it's fine to me. The point I'm making is that higher (not just WEP) power settings were not only available at Sea Level, and in fact for this aircraft were available almost to 10,000 feet, or considerably higher depending on the type.