Critical altitudes
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Thorlifter
Captain
I can understand critical altitude for an engine as it would at that level be deprived of oxygen and die. What is the definition for the plane itself? Is it when the air is no longer thick enough to provide lift?
Critical altitude, or full throttle height, is the highest altitude where the engine's supercharger can produce the maximum boost, while the engine is still able to use that boost, on a specific power setting (in US parlance, that might be military power, or, maybe, war emergency power, if it is allowed for the specific engine). Eg. for the early F series V-1710, single stage, the critical altitude for military rating/setting was some 12000 ft. That is for the engine at the test stand, ie. the air is not forced into the intake prior entering the supercharger.
The plane flying straight level will usually force, or ram the air into the supercharger (= ram effect), thereby 'elevating' the engine's critical altitude. In that case, the engine's critical altitude for, say, P-40E (having the V-1710 engine mentioned, military rating) will be maybe 14000 ft - a 2000 ft gain. The faster the plane goes, the higher the crit.alt. P-51Bs were able to add 4000-5000 ft to the engine's no-ram critical altitudes, while the P-39 and P-63 were lousy in that 'discipline', despite the decent speeds.
The plane flying straight level will usually force, or ram the air into the supercharger (= ram effect), thereby 'elevating' the engine's critical altitude. In that case, the engine's critical altitude for, say, P-40E (having the V-1710 engine mentioned, military rating) will be maybe 14000 ft - a 2000 ft gain. The faster the plane goes, the higher the crit.alt. P-51Bs were able to add 4000-5000 ft to the engine's no-ram critical altitudes, while the P-39 and P-63 were lousy in that 'discipline', despite the decent speeds.
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Shortround6
Major General
The altitude at which an engine would "die" (cease running) is thousands of feet (10s of thousands of feet?) above the altitude the plane will fly. It is generally considered to be somewhere around 55-56,000ft. That is the point at which the engine can no longer make enough power to overcome internal friction. Please note that this assumes the ignition system and fuel system are still operating properly, which they often didn't.
the max height the plane can fly is much lower because the engine has to provide enough power (thrust) for the plane to maintain enough forward speed to maintain lift. Using the ever popular Spitfire as an example MAX altitude went from about 36,000ft to over 44,000 ft on the same wing because the later engines increase in power at altitude. This is despite the plane gaining around 1/3 in weight and so requiring about 1/3 more lift.
Ceilings for planes were never really described as "critical", Specification sheets will give absolute or max ceiling (the altitude at which the plane can reach but can only fly straight and level at one speed. It is at full throttle and can go no faster and any reduction speed will cause a loss of altitude or stall), The will list a "service" ceiling which is usually the altitude at which the plane has enough power to still climb at 100ft per minute. Sometimes you can find the "operational" ceiling which is the altitude wher the plane has enough power to climb at 500 feet per minute. It is also the altitude at which a small formation of planes can operate at. Imagine a group of 4-8 planes in formation executing a 90 degree turn. The outside plane/s have to speed up in order to keep formation. They need enough extra power to do that.
Please note that many planes cannot effectively fight at such altitudes. They can dive down from such an altitude but they cannot maintain height in any sort of sharp turn (combat type) and regaining height is a very slow process. Also rates of climb tend to follow all the way up. A plane that has 200-300fpm advantage at sea level (where both planes are climbing at over 3000 fpm and such a difference is only 6-10%) may still have a 200fpm advantage where one plane is climbing at 400fpm and the other one at 600fpm (a 50% advantage). Some country's (or pilots) considered the ability to climb at 1000fpm as the max height a plane could fight at but this was not anywhere near as universally acknowledged as absolute and service ceilings.
Bombers didn't follow the combat rule as they were generally flying straight and level although it does help explain why even night bombers flew several thousand feet below their service ceiling. They wanted the engine power and maneuverability to perform evasive maneuvers like corkscrews.
the max height the plane can fly is much lower because the engine has to provide enough power (thrust) for the plane to maintain enough forward speed to maintain lift. Using the ever popular Spitfire as an example MAX altitude went from about 36,000ft to over 44,000 ft on the same wing because the later engines increase in power at altitude. This is despite the plane gaining around 1/3 in weight and so requiring about 1/3 more lift.
Ceilings for planes were never really described as "critical", Specification sheets will give absolute or max ceiling (the altitude at which the plane can reach but can only fly straight and level at one speed. It is at full throttle and can go no faster and any reduction speed will cause a loss of altitude or stall), The will list a "service" ceiling which is usually the altitude at which the plane has enough power to still climb at 100ft per minute. Sometimes you can find the "operational" ceiling which is the altitude wher the plane has enough power to climb at 500 feet per minute. It is also the altitude at which a small formation of planes can operate at. Imagine a group of 4-8 planes in formation executing a 90 degree turn. The outside plane/s have to speed up in order to keep formation. They need enough extra power to do that.
Please note that many planes cannot effectively fight at such altitudes. They can dive down from such an altitude but they cannot maintain height in any sort of sharp turn (combat type) and regaining height is a very slow process. Also rates of climb tend to follow all the way up. A plane that has 200-300fpm advantage at sea level (where both planes are climbing at over 3000 fpm and such a difference is only 6-10%) may still have a 200fpm advantage where one plane is climbing at 400fpm and the other one at 600fpm (a 50% advantage). Some country's (or pilots) considered the ability to climb at 1000fpm as the max height a plane could fight at but this was not anywhere near as universally acknowledged as absolute and service ceilings.
Bombers didn't follow the combat rule as they were generally flying straight and level although it does help explain why even night bombers flew several thousand feet below their service ceiling. They wanted the engine power and maneuverability to perform evasive maneuvers like corkscrews.
Per power setting you have two critical alts, one static and one with RAM effect (air pressure increase from forward movement), the latter was often ~500m higher than static alt.
With a methanol/water boosting agent the FTH is reduced a lot (~1500m or more) because engine requires a lot more air to deliver the increased power but you are still able to use the unboosted FTH. AFAIR rated alt of german engines was for climb&combat power (30 min rating). Take-off power rating (up to 5 min) reduced this by ~100m
Bf 109E: early with 4km, later with 4.5km
Bf 109E/F with DB 601N: 4.9km
Bf 109F-4 with DB 601E: 4.8 km
Bf 109G: 5.8km
Bf 109G with DB 605AS or DB: ~6.8km
Bf 109K: ~6.8km
With a methanol/water boosting agent the FTH is reduced a lot (~1500m or more) because engine requires a lot more air to deliver the increased power but you are still able to use the unboosted FTH. AFAIR rated alt of german engines was for climb&combat power (30 min rating). Take-off power rating (up to 5 min) reduced this by ~100m
Bf 109E: early with 4km, later with 4.5km
Bf 109E/F with DB 601N: 4.9km
Bf 109F-4 with DB 601E: 4.8 km
Bf 109G: 5.8km
Bf 109G with DB 605AS or DB: ~6.8km
Bf 109K: ~6.8km
Shortround6
Major General
For the Spitfire MK I the critical altitude was 16,250ft no ram at 6lbs of boost with 87 octane fuel. 1030hp.
6lb boost could be had at max forward speed (350mph?) at 18,500ft.
with the coming of 100 octane fuel and 12lb of boost the critical was much lower. The supercharger could not supply 12 boost at 16,000ft. The engine made as much power as it ever did at 16,000ft and above but the throttle could be opened further at the lower altitudes for more power without wrecking the engine. Max power and "Critical" altitude became 1310hp at 9,000ft and later Sea Hurricanes were allowed to use 16lb of boost at 5,500ft for 1440hp. As the Hurricane climbed the power would fall to 1310 at 9,000ft (+ ram) and as it climbed higher it would fall to 1030hp at 16,500-17,000ft. As the Hurricane was a bit slower than the Spitfire it could not keep up the 6lb s of boost all the way to 18,500ft. 17,750 ft recorded in one test. (British actually going for 6.25in boost.)
Many British climb tests do not show the actual combat capability of the aircraft as they do not use the full throttle (max power) for the climb test. It is more of an operational climb test. Early Spitfires and Hurricanes were tested at a climb of 2600rpm and 6.25lbs boost instead of the 3000rpm and 6.25lbs boost use for Max level flight. It does tell you how fast a plane (or group of planes) could reach a given operational altitude.
Many American tests use full military power for the first 5 minutes and then max continuous power for the remainder of the time needed to reach the desired altitude if it cannot be reached in 5 minutes. This gives a more impressive number but not one often used in service unless the situation was dire. It does give a better indication of combat capability at altitudes were Military power was used. but tells no more than the British numbers at altitudes were max continuous was used.
These complications make answering the original question difficult. Many of the answers are available but they take digging.
6lb boost could be had at max forward speed (350mph?) at 18,500ft.
with the coming of 100 octane fuel and 12lb of boost the critical was much lower. The supercharger could not supply 12 boost at 16,000ft. The engine made as much power as it ever did at 16,000ft and above but the throttle could be opened further at the lower altitudes for more power without wrecking the engine. Max power and "Critical" altitude became 1310hp at 9,000ft and later Sea Hurricanes were allowed to use 16lb of boost at 5,500ft for 1440hp. As the Hurricane climbed the power would fall to 1310 at 9,000ft (+ ram) and as it climbed higher it would fall to 1030hp at 16,500-17,000ft. As the Hurricane was a bit slower than the Spitfire it could not keep up the 6lb s of boost all the way to 18,500ft. 17,750 ft recorded in one test. (British actually going for 6.25in boost.)
Many British climb tests do not show the actual combat capability of the aircraft as they do not use the full throttle (max power) for the climb test. It is more of an operational climb test. Early Spitfires and Hurricanes were tested at a climb of 2600rpm and 6.25lbs boost instead of the 3000rpm and 6.25lbs boost use for Max level flight. It does tell you how fast a plane (or group of planes) could reach a given operational altitude.
Many American tests use full military power for the first 5 minutes and then max continuous power for the remainder of the time needed to reach the desired altitude if it cannot be reached in 5 minutes. This gives a more impressive number but not one often used in service unless the situation was dire. It does give a better indication of combat capability at altitudes were Military power was used. but tells no more than the British numbers at altitudes were max continuous was used.
These complications make answering the original question difficult. Many of the answers are available but they take digging.
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