As always, thank you all. What I read at wikipedia was, that single-stage supercharger could hurt an aircrafts performance at low altitudes. IIRC, it was because the compressed air would become hotter, not a problem high-up where it is cold, lower down though, it apparently caused issues (pre-ignition?). Guess the superchargers could not simply be turned off, at least not initially.
Groundhog Thread Part Deux - P-39 Fantasy and Fetish - The Never Ending Story (Mods take no responsibility for head against wall injuries sustained) (3 Viewers)
- Thread starter greybeard
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Greg Boeser
1st Lieutenant
54th Fighter Squadron received its first P-38Es in February 1942. It deployed to Alaska in late May. Flew the first operational mission on 3 June, a local patrol over Cook Inlet near Anchorage.
MIflyer
Major
A supercharger could "hurt" an aircraft's low altitude performance if it was set to a high gearing ratio so that pressure produced by the supercharger got to the maximum manifold pressure allowed before the throttle was advanced very far. It would be rather like trying to drive a manual transmission car fast in 1st gear; you would hit the redline RPM before you reached a high speed. Similarly, you would hit the redline on the manifold pressure before you had advanced the throttle very far.
Single stage superchargers in WWII did not have a means of cooling the air coming out of the supercharger, such as with an air-to-air heat exchanger. Light aircraft today usually use turbos for supercharging and sometime employ what they call an "intercooler" but which actually is an "aftercooler."
Single stage superchargers in WWII did not have a means of cooling the air coming out of the supercharger, such as with an air-to-air heat exchanger. Light aircraft today usually use turbos for supercharging and sometime employ what they call an "intercooler" but which actually is an "aftercooler."
Shortround6
Lieutenant General
A difference was the two stage supercharger in the F4F was good for 1000hp at 19,000ft. The same P & W R-1830 when given a turbocharger (and using the same engine driven supercharger) was good for 1200hp at 25,000ft in the last versions of the P-43. It did require a bit more weight and bulk and the turbo (and controller) wasn't really ready for front line use.
The problem with single stage single speed superchargers is that they can only be optimised for one altitude. The Best engine I know of for illustrating this is the Merlin. The Merlin III used in the Hurricane I Spitfire I and a several other British aircraft would hold 6lbs of boost (around 42in ) to 16,250 ft with the engine turning 3000rpm which spun the impeller at 25,764rpm.
Please note that anytime the engine was doing 3000 rpm the impeller was spinning at 25,764rpm regardless of altitude. The air at 16,000ft is at 16.21 inches of Hg so the Merlin supercharger was compressing almost 2.6 times. If the throttle had been opened all the way at sea level the supercharger might have been able to produce 77in of manifold pressure which would have certainly either caused massive detonation with the fuel available or destroyed the engine even without detonation. By restricting the amount (volume of air) coming into the supercharger at sea level the engine made 880hp for take off at the same 3000 rpm and same 6lbs of boost (some books give 6 1/4lbs).
There are two sources of heat rise in a supercharger. One is the simple heat rise of the work of compressing the air/intake charge even assuming the compressor is 100% efficient.
The other is the fact that most pre-WW II and WW II superchargers were not very efficient. If 70% of the power applied to the input shaft of the supercharger actually went to compressing the air it was a very good supercharger indeed in early WW II. The other 30% of the power didn't just disappear. All but a tiny percentage (under 1% for friction) went into heating the air over and above the heating done in actually compressing the air. You wound up with some very hot air going into the engine.
A supercharger that was 65% efficient saw 35% of the power applied to the input shaft turned into excess heat of the intake air/charge and so on for lower efficiencies.
the power needed to turn the supercharger impeller was proportional to the speed of the impeller (or more properly to the speed of the tips but if we are talking about the same supercharger the impeller speed will do). Obviously reducing the impeller speed will require much less power and also much less power going into heating the intake charge.
The supercharger on the Merlin VIII engine was the same as the Merlin III except it used a set of 6.313 gears instead of 8.588 gears the overall change was an increase in take-off power 50 1080hp at 3000rpm using 5 3/4lb boost. Less power used to turn the supercharger, throttle plate open further reducing pumping loss and a much cooler/denser intake charge. trade of was engine topped out at 1275hp at 3000rpm at sea level suing 9lbs of boost.
The heat of compressing the air, especially when factoring in the waste heat, rose faster than the outside air temperature dropped with rise in altitude.
The heat rise of a Merlin XX engine with Hooker supercharger (more efficient than earlier models) was figured to be 148 degrees centigrade at 3000rpm in high gear almost regardless of altitude. at 35,000ft the intake charge temperature in the intake manifold wass about 95 degrees centigrade. In part due to the fact that much less work was being done. Only 84.5lbs of air and fuel per minute going through the supercharger ( and outlet pressure of 26.56 in) compared to the 144lbs at 20,000ft and the output pressure of 48.24in.
I hope this isn't too confusing. But superchargers, despite only having one moving part are not always simple to figure out
varsity07840
Airman 1st Class
- 182
- Jun 25, 2013
If it was such a game changer, why didn't the AAF go for it?
I am not a technical minded person sadly, but what I take from this is that " single stage single speed superchargers is that they can only be optimised for one altitude". Though two questions:A difference was the two stage supercharger in the F4F was good for 1000hp at 19,000ft. The same P & W R-1830 when given a turbocharger (and using the same engine driven supercharger) was good for 1200hp at 25,000ft in the last versions of the P-43. It did require a bit more weight and bulk and the turbo (and controller) wasn't really ready for front line use.
The problem with single stage single speed superchargers is that they can only be optimised for one altitude. The Best engine I know of for illustrating this is the Merlin. The Merlin III used in the Hurricane I Spitfire I and a several other British aircraft would hold 6lbs of boost (around 42in ) to 16,250 ft with the engine turning 3000rpm which spun the impeller at 25,764rpm.
Please note that anytime the engine was doing 3000 rpm the impeller was spinning at 25,764rpm regardless of altitude. The air at 16,000ft is at 16.21 inches of Hg so the Merlin supercharger was compressing almost 2.6 times. If the throttle had been opened all the way at sea level the supercharger might have been able to produce 77in of manifold pressure which would have certainly either caused massive detonation with the fuel available or destroyed the engine even without detonation. By restricting the amount (volume of air) coming into the supercharger at sea level the engine made 880hp for take off at the same 3000 rpm and same 6lbs of boost (some books give 6 1/4lbs).
There are two sources of heat rise in a supercharger. One is the simple heat rise of the work of compressing the air/intake charge even assuming the compressor is 100% efficient.
The other is the fact that most pre-WW II and WW II superchargers were not very efficient. If 70% of the power applied to the input shaft of the supercharger actually went to compressing the air it was a very good supercharger indeed in early WW II. The other 30% of the power didn't just disappear. All but a tiny percentage (under 1% for friction) went into heating the air over and above the heating done in actually compressing the air. You wound up with some very hot air going into the engine.
A supercharger that was 65% efficient saw 35% of the power applied to the input shaft turned into excess heat of the intake air/charge and so on for lower efficiencies.
the power needed to turn the supercharger impeller was proportional to the speed of the impeller (or more properly to the speed of the tips but if we are talking about the same supercharger the impeller speed will do). Obviously reducing the impeller speed will require much less power and also much less power going into heating the intake charge.
The supercharger on the Merlin VIII engine was the same as the Merlin III except it used a set of 6.313 gears instead of 8.588 gears the overall change was an increase in take-off power 50 1080hp at 3000rpm using 5 3/4lb boost. Less power used to turn the supercharger, throttle plate open further reducing pumping loss and a much cooler/denser intake charge. trade of was engine topped out at 1275hp at 3000rpm at sea level suing 9lbs of boost.
The heat of compressing the air, especially when factoring in the waste heat, rose faster than the outside air temperature dropped with rise in altitude.
The heat rise of a Merlin XX engine with Hooker supercharger (more efficient than earlier models) was figured to be 148 degrees centigrade at 3000rpm in high gear almost regardless of altitude. at 35,000ft the intake charge temperature in the intake manifold wass about 95 degrees centigrade. In part due to the fact that much less work was being done. Only 84.5lbs of air and fuel per minute going through the supercharger ( and outlet pressure of 26.56 in) compared to the 144lbs at 20,000ft and the output pressure of 48.24in.
I hope this isn't too confusing. But superchargers, despite only having one moving part are not always simple to figure out
1) "single-speed", I would assume that if your supercharger could work at different speeds, it meant that it could compress the air to different levels, making it more flexible at different altitudes, no?
2) What is a "stage" in a supercharger? I would assume that two-stage superchargers supercharge the air twice, essentially running it through one compressor first and then through a second one. Or skip the second if for example the aircraft is at a lower altitude where this would not be helpful.
Sorry for such basic questions, but apart from not being technically minded, you undoubtedly noticed that English is not my first language. I can do reasonably well in conversations (I hope), but technical subjects get confusing.
Shortround6
Lieutenant General
To answer,
1, you are correct. there were a number of 2 speed superchargers. a gear change mechanism allowed for two optimum altitudes. Merlin X engine used the same supercharger as the III and VIII but used different gear ratios and the pilot had a choice. Some aircraft used a variable drive but only between certain limits. No 3 speed supercharger every made it to service in any numbers.
2. again you are correct, except for the last part which you have a bit backwards. the first compresser is the one that could be skipped on some aircraft. On a turbocharged aircraft the air still went through the compressor before going to the engine driven supercharger, the compressor in the turbo just wasn't spinning very fast. All turbocharged aircraft used two compressors in series.
problem with two stage superchargers is the intake charge gets really hot, several hundred degrees more than a single stage so some method of cooling the intake air/charge is needed or the extra compression of the 2nd stage is largely wasted (air is less dense and less manifold pressure can be used before detonation sets in)
detonation (like knocking in a car only much worse) has quite a number of factors but for our understanding it is the fuels ability to not spontaneously combust as the piston is rising in the cylinder before the spark plugs ignite the fuel/air mixture. low octane fuel will ignite before high octane fuel in the same conditions of which the pressure and temperature of the mixture are the most important. This is why all the talk and interest in intercoolers and aftercoolers and water injection, anything that would help cool the intake charge allowed higher boost as it helped keep the temperature down.
1, you are correct. there were a number of 2 speed superchargers. a gear change mechanism allowed for two optimum altitudes. Merlin X engine used the same supercharger as the III and VIII but used different gear ratios and the pilot had a choice. Some aircraft used a variable drive but only between certain limits. No 3 speed supercharger every made it to service in any numbers.
2. again you are correct, except for the last part which you have a bit backwards. the first compresser is the one that could be skipped on some aircraft. On a turbocharged aircraft the air still went through the compressor before going to the engine driven supercharger, the compressor in the turbo just wasn't spinning very fast. All turbocharged aircraft used two compressors in series.
problem with two stage superchargers is the intake charge gets really hot, several hundred degrees more than a single stage so some method of cooling the intake air/charge is needed or the extra compression of the 2nd stage is largely wasted (air is less dense and less manifold pressure can be used before detonation sets in)
detonation (like knocking in a car only much worse) has quite a number of factors but for our understanding it is the fuels ability to not spontaneously combust as the piston is rising in the cylinder before the spark plugs ignite the fuel/air mixture. low octane fuel will ignite before high octane fuel in the same conditions of which the pressure and temperature of the mixture are the most important. This is why all the talk and interest in intercoolers and aftercoolers and water injection, anything that would help cool the intake charge allowed higher boost as it helped keep the temperature down.
MIflyer
Major
I believe the first P-38 to see actual combat was an F-4 recon bird. Despite having an engine shot out over the Pacific, the Lightning outran the Japanese fighter anyway.
By the way, I wrote an article about recon P-38's and had the editor of Aviation History tell me that airplane was not the first P-38 in combat because it did not have guns!
By the way, I wrote an article about recon P-38's and had the editor of Aviation History tell me that airplane was not the first P-38 in combat because it did not have guns!
MIflyer
Major
The XP-41 was an attempt to produce a "better" P-35, getting rid of that horrible MLG and generally cleaning up the airplane. But it used a 2 stage mechanically supercharged R-1830, like the F4F. Seversky had a better idea and built at his own cost the AP-4, which was basically an XP-41 with a turbosupercharger in the large baggage compartment. The XP-41 was slower than the P-40 and lost that competition but the AP-4 impressed the Air Corps and led to the P-43 as the production version. Then scale up the P-43 by about 50% and you've got a Thunderbolt.
GregP
Major
Hi Miflyer,
1) If the P-38 in question was getting shot at, I submit it WAS in combat, whether or not it could return fire. If that isn't the case, then I suppose a C-47 that got shot down wasn't in combat either. I bet any survivors would dispute that it wasn't in combat! But, an editor usually has the power to say whatever they want to say and make it stick.
2) What is MLG? Main Landing Gear? I kind of like the P-35 landing gear because there was very little damage in the event of a gear-up landing and, since retracting landing gear was somewhat "new" when the P-35 came out, there were a LOT of gear-up landings. I think it was only when the advanced trainers got retractable gear (I'm thinking AT-6) that they began to be less of an issue because new fighter pilots were already used to raising and lowering landing gear. But ... I agree the XP-41 DOES look better than the P-35.
Cheers.
1) If the P-38 in question was getting shot at, I submit it WAS in combat, whether or not it could return fire. If that isn't the case, then I suppose a C-47 that got shot down wasn't in combat either. I bet any survivors would dispute that it wasn't in combat! But, an editor usually has the power to say whatever they want to say and make it stick.
2) What is MLG? Main Landing Gear? I kind of like the P-35 landing gear because there was very little damage in the event of a gear-up landing and, since retracting landing gear was somewhat "new" when the P-35 came out, there were a LOT of gear-up landings. I think it was only when the advanced trainers got retractable gear (I'm thinking AT-6) that they began to be less of an issue because new fighter pilots were already used to raising and lowering landing gear. But ... I agree the XP-41 DOES look better than the P-35.
Cheers.
CORSNING
Tech Sergeant
Slightly off topic but, the following comes from page 147, AHT by Francis
H. Dean:
Dec. 7 '41 USAAF inventory of P-38s at the time of Pearl Harbor is
69 in active service including some D models; the only unit fully
equipped is the First Pursuit Group, Selfridge Field.
Dec. 8 '41 Initial elements of the First Pursuit Group from Selfridge
arrive at Sand Diego to defend the west coast from Japanese attack.
Group movement is complete by December 22, 1941.
Apr. 7 '42 F-4 Photo Lightnings of Flight A, 8th Photo Squadron
with 75 gallon drop tanks arrive in Australia under Maj. K. Prolifka
and by April 16 start reconnaissance missions over eastern New
Guinea and New Britain. These are the first Lightnings in the
Pacific. Later F-4s operate very successfully with the 435th
Bombardment Squadron.
2 more cents worth.
H. Dean:
Dec. 7 '41 USAAF inventory of P-38s at the time of Pearl Harbor is
69 in active service including some D models; the only unit fully
equipped is the First Pursuit Group, Selfridge Field.
Dec. 8 '41 Initial elements of the First Pursuit Group from Selfridge
arrive at Sand Diego to defend the west coast from Japanese attack.
Group movement is complete by December 22, 1941.
Apr. 7 '42 F-4 Photo Lightnings of Flight A, 8th Photo Squadron
with 75 gallon drop tanks arrive in Australia under Maj. K. Prolifka
and by April 16 start reconnaissance missions over eastern New
Guinea and New Britain. These are the first Lightnings in the
Pacific. Later F-4s operate very successfully with the 435th
Bombardment Squadron.
2 more cents worth.
MIflyer
Major
GregP: Yep, I thought that was a ridiculous comment to make, especially from a publication that prides itself on accuracy. Aviation History has experts in the appropriate fields review articles for accuracy. The first article I sent them I got a call from an author who asked me why I thought George fighters had attacked two USN PB4Y-2's instead of the Tojos that had been accepted. I explained that the noted Japanese author Henry Sakida had confirmed that and even confirmed the two kills the USN airplanes claimed.
The P-35 Main Landing Gear had those huge "airscoops" when the gear doors came down. I am amazed they could even stay in the air long enough to get on the ground.
I think that if you have been shot at you have been in combat and if you've been hit and lost an engine you sure have. I guess all those Liberty ships that were sunk were never in combat.
I sent that article to Air Classics and they published it withiout even tell me they were doing so in advance - and they never paid me for it. Since it was a tribute to a friend of mine, I really did not care too much if I got paid.
You can read it at: 9th Photo Reconnaissance Squadron
CORSNING: Yes, that matches my info as well. The F-4's sure got into it early. We did not have a better recon aircraft for some time.
The P-35 Main Landing Gear had those huge "airscoops" when the gear doors came down. I am amazed they could even stay in the air long enough to get on the ground.
I think that if you have been shot at you have been in combat and if you've been hit and lost an engine you sure have. I guess all those Liberty ships that were sunk were never in combat.
I sent that article to Air Classics and they published it withiout even tell me they were doing so in advance - and they never paid me for it. Since it was a tribute to a friend of mine, I really did not care too much if I got paid.
You can read it at: 9th Photo Reconnaissance Squadron
CORSNING: Yes, that matches my info as well. The F-4's sure got into it early. We did not have a better recon aircraft for some time.
P-39 Expert
Non-Expert
Regarding the P-38 there were a few P-38s on station around the world in the last half of 1942, but most historians say that Operation Torch in November '42 was their European debut. They began operating in strength in the Pacific in December '42. Vast majority of 1942 was fought with the P-39, P-40 and F4F.
Weight could have been easily reduced in the P-40 and F4F by deleting two of the six .50cal MGs. This would have saved around 350# per plane. Climb would have increased on the order of 400 feet/minute which both the P-40 and F4F sorely needed. The P-40E could barely reach 20000' and the F4F pilots bitterly complained when the two .50cal MGs were added saying that the previous F3F with four MGs climbed and handled much better.
The P-39D/F/K/L would have lost around 300# by deleting the four useless .30cal MGs in the wings (200#) and the nose armor (100#) which didn't protect anything. This increased climb by about 360fpm which would have allowed those early P-39s to outclimb the contemporary Zeros. P-39s already enjoyed a 40mph speed advantage up to 25000' over the Zero. The remaining 37mm cannon and two .50cal MGs left plenty of firepower as the Russians proved against the Luftwaffe.
Four .50cal MGs were plenty anyway. The AAF was already going to four MGs in their newer P-51A/B/C Mustangs. The USN was using 4 .50cal MGs in their newer FM2 Wildcats and F8F Bearcats.
MIflyer
Major
The P-40D had four .50 cal guns and RAF Ace Caldwell, I believe it was, preferred it that way. There was a case where he was covering a convoy and two 109's came down out of the Sun and hit him good. He turned around, shot down one of the 109's and sent the other one packing.
Deleting the four .30 cal would have decreased the value of the P-39 for strafing, and on the 'Canal it w as sorely needed for that. The Marines credited the P-400's of the 67th with saving Henderson Field with their close support. And I think deleting the 37MM and putting in a third .50 cal would have been a good idea for optimized air to air P-39's. With the "roman candle" firing rate of the cannon, the chance of hitting anything with the 37MM other than a bomber going straight and level was pretty slim and its trajectory was so different from that of the .50 cal and .30 cal that hitting something other than the ground with all three was unlikely.
Deleting the four .30 cal would have decreased the value of the P-39 for strafing, and on the 'Canal it w as sorely needed for that. The Marines credited the P-400's of the 67th with saving Henderson Field with their close support. And I think deleting the 37MM and putting in a third .50 cal would have been a good idea for optimized air to air P-39's. With the "roman candle" firing rate of the cannon, the chance of hitting anything with the 37MM other than a bomber going straight and level was pretty slim and its trajectory was so different from that of the .50 cal and .30 cal that hitting something other than the ground with all three was unlikely.
Greg Boeser
1st Lieutenant
Interestingly, local practice was to mount extra guns in the factory designed four gun models of the P-40. (P-40L, M, early N) The wings had the mounts already, and the pilots preferred the extra firepower.
It is true that the USN complained about the 6 gun F4F-4. It used the same engine as the F4F-3, but the added guns, armor, and folding wing mechanism increased the weight by over 500 lbs. The Navy pilots were most upset about the decrease in firing time from 34 second to 20 seconds. The FM-1 and FM-2 returned to the four gun armament.
It is true that the USN complained about the 6 gun F4F-4. It used the same engine as the F4F-3, but the added guns, armor, and folding wing mechanism increased the weight by over 500 lbs. The Navy pilots were most upset about the decrease in firing time from 34 second to 20 seconds. The FM-1 and FM-2 returned to the four gun armament.
Shortround6
Lieutenant General
The benefit of the four gun armament in the F4F and perhaps the P-40 rather depends on how the planes were loaded.
The F4F-3 was listed as having 286lbs worth of guns with 360 lbs of ammo (300rpg) 'standard' and 516lbs of ammo (430rpg) in overload condition.
The F4F-4 was listed as having 288.7lbs worth of guns (four guns )with 240 lbs of ammo (200rpg) 'standard' or 422lbs worth guns (six guns) and 432lbs of ammo (240rpg) in overload condition.
Please note that an F4F-3 with 430rpg was carrying about 54 lbs less weight than an f4F-4 with six guns and 240rpg.
The P-40D was likewise designed with a ridiculous amount of ammo capacity, but since only about 30 were built ( I don't know how many British planes showed up with only 4 guns)
we have a weight of 256lbs for 4 guns * and 300lbs for 250rpg designed weight capacity but the ammo bins would hold 738lbs of ammo (620 rpg) in overload condition.
The P-40E had a design load of 384lbs worth of guns (six) and 423lbs worth of ammo (235rpg) amd an overload of 561lbs of ammo (1870 rounds total).
Please note the "design" fuel load (for design gross weight) was 120 US gallons of fuel while overload included both an extra 25.5 gallons in the fuselage tank plus the drop tank.
Please remember that both the F4F-4 and the P-40D/E were ordered in the summer of 1940 and the high rate of fire M2 gun had not yet been approved.
For the WIldcat if you pulled the two extra guns but loaded in the extra ammo to extend the firing time you are not going to see much of change in performance.
Some of the light weight P-40s resorted not only to four guns but 235rpg or even 201 rounds per gun. Which is why they gained weight in squadron service. By the time they were in service they were no longer tasked (at least not often) with the high performance interceptor role.
The F4F-3 was listed as having 286lbs worth of guns with 360 lbs of ammo (300rpg) 'standard' and 516lbs of ammo (430rpg) in overload condition.
The F4F-4 was listed as having 288.7lbs worth of guns (four guns )with 240 lbs of ammo (200rpg) 'standard' or 422lbs worth guns (six guns) and 432lbs of ammo (240rpg) in overload condition.
Please note that an F4F-3 with 430rpg was carrying about 54 lbs less weight than an f4F-4 with six guns and 240rpg.
The P-40D was likewise designed with a ridiculous amount of ammo capacity, but since only about 30 were built ( I don't know how many British planes showed up with only 4 guns)
we have a weight of 256lbs for 4 guns * and 300lbs for 250rpg designed weight capacity but the ammo bins would hold 738lbs of ammo (620 rpg) in overload condition.
The P-40E had a design load of 384lbs worth of guns (six) and 423lbs worth of ammo (235rpg) amd an overload of 561lbs of ammo (1870 rounds total).
Please note the "design" fuel load (for design gross weight) was 120 US gallons of fuel while overload included both an extra 25.5 gallons in the fuselage tank plus the drop tank.
Please remember that both the F4F-4 and the P-40D/E were ordered in the summer of 1940 and the high rate of fire M2 gun had not yet been approved.
For the WIldcat if you pulled the two extra guns but loaded in the extra ammo to extend the firing time you are not going to see much of change in performance.
Some of the light weight P-40s resorted not only to four guns but 235rpg or even 201 rounds per gun. Which is why they gained weight in squadron service. By the time they were in service they were no longer tasked (at least not often) with the high performance interceptor role.
Shortround6
Lieutenant General
According to figures in AHT the F4F-4 gained almost 300lbs in the wing alone.
I have no idea if that was due to modifications to fit the extra guns (the guns are not counted in the weight of the wing) the hinge mechanism, general beefing up to handle higher gross weights or some combination of those factors or others, however, once the structural weight is there the squadron mechanics cannot take it out. They can yank two of the guns and limit the ammo but the extra 300lbs is going to remain.
folding wing
MIflyer
Major
The P-39 could theoretically carry 2000 rounds of .30 cal ammo. On the 'Canal they probably liked that capability for ground attack.
Yes, lots of heavy parts in the F4F and FM-1 folding wing.
Yes, lots of heavy parts in the F4F and FM-1 folding wing.
Greg Boeser
1st Lieutenant
There is always the question of how a plane is loaded. Geoff Fisken reported that to improve the chances of survival in the Buffalo, they stripped them of all excess weight and reduced the ammo to about 5 seconds worth. They had learned that they were only going to get one pass at the enemy, and adapted accordingly.
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