Climb Rate Discrepancies
- Thread starter Zipper730
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- #22
Haven't we been through this? I explained how I calculated it. The time to climb figures came from the Hurricane Mk.I page on https://www.wwiiaircraftperformance.org. The time to climb figures I simply calculated the time to climb based on the altitude change (i.e. 1000-2000, 2000-3000, 3000-5000, 5000-6500;, 6500'-7600').
What I meant was the rate of climb figures that were cited, were from the article; the calculations were based on altitude versus time.
ThomasP
Senior Master Sergeant
My apologies Zipper730, like tyrodtom I also thought that you were quoting values calculated by Hawkers or some other agency back in the 1930s. I did not realize that you were asking why your method was not working out.
As PFVA63 pointed out in his post#7 you are calculating the average ROC for the altitude interval, not the instantaneous ROC at a given height. The resulting average ROCs for your method should then fall between one instantaneous ROC and the next, if the original chart values are correct. By definition the values for your method of calculating cannot equal the instantaneous ROC values.
In addition, imprecision in measurement (ie misreading of instruments), and/or imprecise or miscalculation, and/or typos, by the agency responsible will mess things up. If some of the numbers are rounded off or are imprecisely measured or typoed (even by only a few hundredths of a minute for example) then any later calculations using smaller increments than originally used will result in exaggerated variance from the original result.
A good example in the chart you are using is the imprecision in the TTC (Time To Climb) and/or instantaneous ROC values for 6500' and 7600'.
Lets assume that the instantaneous ROCs are correct for 0', 6500', and 7600'. If we back-figure the average ROC and TTC to the two altitudes, we get:
For 0' to 6500':
ROC average = (2550 + 2880) / 2 = 2715 ft/min
TTC = 6500 / 2715 = 2.3941 min
For 0' to 7600':
ROC average = (2550 + 2950) / 2 = 2750 ft/min
TTC = 7600 / 2750 = 2.7636 min
And therefore:
2.7636 - 2.3941 = .3695 min time interval from 6500' to 7600'
As you can see, the resulting interval between TTCs in the original chart (ie .33 min) is significantly off (~12%) vs the calculated interval (ie .3695 min) if the instantaneous ROC values are correct.
If you use the new time value for 6500' to 7600' calculated above in your method of calculation you will see that the results for average ROC are closer to the original instantaneous ROC chart values.
In effect we get:
7600' - 6500' = 1100' height interval
and
1100 / .3695 = 2976 ft/min average ROC from 6500' to 7600'
or ~12% less than your previously calculated value of 3333 ft/min
This answer is still not exact (or I suspect, correct) but it is significantly closer than 3333 ft/min (which is not possible at the aircraft weight/engine HP/propeller combination used in the tests).
The reason I suspect the 2976 ft/min average ROC value is still incorrect is because with the British method of using constant boost to FTH (rather than the US method using constant HP) the instantaneous ROC at FTH has to be higher than the average ROC anywhere below FTH.
Hopefully this helps (and hopefully I did not make any typos).
As PFVA63 pointed out in his post#7 you are calculating the average ROC for the altitude interval, not the instantaneous ROC at a given height. The resulting average ROCs for your method should then fall between one instantaneous ROC and the next, if the original chart values are correct. By definition the values for your method of calculating cannot equal the instantaneous ROC values.
In addition, imprecision in measurement (ie misreading of instruments), and/or imprecise or miscalculation, and/or typos, by the agency responsible will mess things up. If some of the numbers are rounded off or are imprecisely measured or typoed (even by only a few hundredths of a minute for example) then any later calculations using smaller increments than originally used will result in exaggerated variance from the original result.
A good example in the chart you are using is the imprecision in the TTC (Time To Climb) and/or instantaneous ROC values for 6500' and 7600'.
Lets assume that the instantaneous ROCs are correct for 0', 6500', and 7600'. If we back-figure the average ROC and TTC to the two altitudes, we get:
For 0' to 6500':
ROC average = (2550 + 2880) / 2 = 2715 ft/min
TTC = 6500 / 2715 = 2.3941 min
For 0' to 7600':
ROC average = (2550 + 2950) / 2 = 2750 ft/min
TTC = 7600 / 2750 = 2.7636 min
And therefore:
2.7636 - 2.3941 = .3695 min time interval from 6500' to 7600'
As you can see, the resulting interval between TTCs in the original chart (ie .33 min) is significantly off (~12%) vs the calculated interval (ie .3695 min) if the instantaneous ROC values are correct.
If you use the new time value for 6500' to 7600' calculated above in your method of calculation you will see that the results for average ROC are closer to the original instantaneous ROC chart values.
In effect we get:
7600' - 6500' = 1100' height interval
and
1100 / .3695 = 2976 ft/min average ROC from 6500' to 7600'
or ~12% less than your previously calculated value of 3333 ft/min
This answer is still not exact (or I suspect, correct) but it is significantly closer than 3333 ft/min (which is not possible at the aircraft weight/engine HP/propeller combination used in the tests).
The reason I suspect the 2976 ft/min average ROC value is still incorrect is because with the British method of using constant boost to FTH (rather than the US method using constant HP) the instantaneous ROC at FTH has to be higher than the average ROC anywhere below FTH.
Hopefully this helps (and hopefully I did not make any typos).
DarrenW
Staff Sergeant
Here's both an entertaining and educational wartime video concerning USAAF flight testing at Wright Field...enjoy!
ThomasP
Senior Master Sergeant
Pretty cool video, thanks.
- Thread starter
- #26
No, I was using their figures. The fact was that the climb-rates listed didn't match up with the time it took to get to the given altitudes.
That's correct, but I was using the time between altitude intervals. It was the best precision I could manage, and the figures didn't add up.
Looking at your proposal, the idea seems to measuring the time intervals based on averaging everything below that, then subtracting the differences, and using that to establish a percentage, which can be used to calculate for a more accurate RoC?
If I do these numbers right, I end up with the following to the FTH...
- 0' to 1000'
- RoC Average: 2575 fpm
- Computed TTC: 0.3883 minutes
- Listed TTC: 0.38 minutes
- 0' to 2000'
- RoC Average: 2600 fpm
- Computed TTC: 0.7692 minutes
- Listed TTC: 0.76 minutes
- 1000' to 2000'
- Computed TTC: 0.3809 minutes
- Listed TTC: 0.38 minutes
- Computed RoC: 2625.5 fpm
- 0' to 3000'
- RoC Average: 2630 fpm
- Computed TTC: 1.1407 minutes
- Listed TTC: 1.15 minutes
- 2000' to 3000'
- Computed TTC: 0.3715 minutes
- Listed TTC: 0.39 minutes
- Computed RoC: 2692.1 fpm
- 0' to 5000'
- RoC Average: 2680 fpm
- Computed TTC: 1.8657 minutes
- Listed TTC: 1.89 minutes
- 3000' to 5000'
- Computed TTC: 0.725 minutes
- Listed TTC: 0.72 minutes
- Computed RoC: 2758.7 fpm
- 0' to 6500'
- RoC Average: 2715 fpm
- Computed TTC: 2.3941 minutes
- Listed TTC: 2.43 minutes
- 5000' to 6500'
- Computed TTC: 0.5284 minutes
- Listed TTC: 0.54 minutes
- Computed RoC: 2838.6 fpm
- 0' to 7600'
- RoC Average: 2750 fpm
- Computed TTC: 2.7636
- Listed TTC: 2.76 minutes
- 6500' to 7600'
- Computed TTC: 0.3695 minutes
- Listed TTC: 0.33 minutes
- Computed RoC: 2976.8 fpm
Wait, I thought the limitation was set by boost rather than HP?
ThomasP
Senior Master Sergeant
Hey Zipper730,
re: "Wait, I thought the limitation was set by boost rather than HP?
The UK used constant boost pre-war and early-war, at least for the aircraft fitted with constant speed propellers. They used it for most of the rest of the war also.
A good example of the relationship I am talking about can be found on the website "WWII Aircraft Performance" under Seafire L Mk II.
Table II here: " Seafire L Mk. IIC Trials" for the tabulated Normal climb rate,
and the Normal and Combat climb chart here: " http://www.spitfireperformance.com/mb138climb.jpg"
On the climb chart HP is controlled by boost at +12 lbs to FTH for the Normal climb rate and +16 lbs to FTH for Combat climb. In both cases the climb rate increases slight from SL to FTH, giving a line that slants to the right as altitude increases to FTH.
The US used torque meters on their engines, from pre-war, to prevent generating more HP than one part or another of the engine could sustain safely. In effect the US used a fixed BHP as the limiting factor. If you look at some of the US engine charts, the boost will decrease slightly as altitude increases, upto the altitude at which the max safe HP can be generated.
If you use a given HP as the control, then the max sustained climb rate will decrease slightly and the climb rate line will slant slightly to the left as altitude increases. Look at the SAC climb charts on this site for the F8F Bearcat. The climb charts lean to the left from SL to max sustained HP altitude.
The exception (at least sometimes) to this for the US seems to be the aircraft using turbo-supercharged engines.
At some time, the UK started to use torque meters for at least some of the tests, giving similar left hand slanted lines, either that or they corrected the charts for constant HP instead of constant boost. In some cases I think they used both and simply did not note this.
re: "Wait, I thought the limitation was set by boost rather than HP?
The UK used constant boost pre-war and early-war, at least for the aircraft fitted with constant speed propellers. They used it for most of the rest of the war also.
A good example of the relationship I am talking about can be found on the website "WWII Aircraft Performance" under Seafire L Mk II.
Table II here: " Seafire L Mk. IIC Trials" for the tabulated Normal climb rate,
and the Normal and Combat climb chart here: " http://www.spitfireperformance.com/mb138climb.jpg"
On the climb chart HP is controlled by boost at +12 lbs to FTH for the Normal climb rate and +16 lbs to FTH for Combat climb. In both cases the climb rate increases slight from SL to FTH, giving a line that slants to the right as altitude increases to FTH.
The US used torque meters on their engines, from pre-war, to prevent generating more HP than one part or another of the engine could sustain safely. In effect the US used a fixed BHP as the limiting factor. If you look at some of the US engine charts, the boost will decrease slightly as altitude increases, upto the altitude at which the max safe HP can be generated.
If you use a given HP as the control, then the max sustained climb rate will decrease slightly and the climb rate line will slant slightly to the left as altitude increases. Look at the SAC climb charts on this site for the F8F Bearcat. The climb charts lean to the left from SL to max sustained HP altitude.
The exception (at least sometimes) to this for the US seems to be the aircraft using turbo-supercharged engines.
At some time, the UK started to use torque meters for at least some of the tests, giving similar left hand slanted lines, either that or they corrected the charts for constant HP instead of constant boost. In some cases I think they used both and simply did not note this.
- Thread starter
- #28
Yeah, I figured you'd see a constant boost setting up to the critical altitude, with an increase in horsepower up to that point due to a throttling loss.
The Hurricane prototype (where I got this data from) seemed to include a progressively increasing rate of climb until critical altitude is reached.
That's pretty odd. Throttling loss would occur most at low altitude, then go away at FTH. It seems with a setting like that, you'd be underperforming quite a bit.
- Thread starter
- #29
ThomasP
Senior Master Sergeant
Hey Zipper730,
This is an example of an early-war P-40E V-1710-39 SEFC that shows what I am describing:
Note the boost at Military at 11,800 ft FTH and at TO at SL. Both ratings are 1150 HP at 3000 rpm. The MP is only 1.3"Hg lower (45.5 - 44.2 = 1.3) at FTH but it allows the HP to remain constant. I have a whole bunch of pre- and early-war charts made by the various testing agencies that show the same rating methods, but they are on the laptop that crapped out on me a few months ago and I still have not recovered the data. The chart above is the best I can do at the moment.
I have read some USAAF/USN test and service pilot accounts from pre- and early-war in which they describe their actions in climb and combat conditions, and they specifically mention backing off on the MP by small amounts as altitude increased, to stay below/at a given HP. It seems to me that as the war progressed the charts were less likely to show these differences. I have read that reduced pilot workload was the main reason the UK adopted constant boost as the limiting factor. I do not know for sure that this is true for the US, but I would guess that it became so as the war progressed. Whether this was also due to sturdier models of engines, or the recognition of already existing strength in the engines, I cannot say.
This is an example of an early-war P-40E V-1710-39 SEFC that shows what I am describing:
Note the boost at Military at 11,800 ft FTH and at TO at SL. Both ratings are 1150 HP at 3000 rpm. The MP is only 1.3"Hg lower (45.5 - 44.2 = 1.3) at FTH but it allows the HP to remain constant. I have a whole bunch of pre- and early-war charts made by the various testing agencies that show the same rating methods, but they are on the laptop that crapped out on me a few months ago and I still have not recovered the data. The chart above is the best I can do at the moment.
I have read some USAAF/USN test and service pilot accounts from pre- and early-war in which they describe their actions in climb and combat conditions, and they specifically mention backing off on the MP by small amounts as altitude increased, to stay below/at a given HP. It seems to me that as the war progressed the charts were less likely to show these differences. I have read that reduced pilot workload was the main reason the UK adopted constant boost as the limiting factor. I do not know for sure that this is true for the US, but I would guess that it became so as the war progressed. Whether this was also due to sturdier models of engines, or the recognition of already existing strength in the engines, I cannot say.
Last edited:
- Thread starter
- #31
I figured the T/O setting was the same as the maximum setting. Military power is usually a little lower (usually it's emergency power, military power, and normal rated), but I see what you mean.
That's a good question, eventually the V-1710 would be beefed up quite a lot. W wuzak , do you have any figures as to engine strengthening throughout WWII?
ThomasP
Senior Master Sergeant
Hey Zipper730,
One way to estimate the TTC value for climb to Service Ceiling (SC), and any altitude in between, is to figure the average ROC from SL to FTH, and then do the same for FTH to SC. The resulting ROC curve should be a nearly straight line from SL to FTH (regardless to using boost or BHP as the limiting factor - if boost is the limit then it will lean to the right, if BHP is the limit the it will lean to the left). The precision calculated line from FTH to SC should also be a nearly straight line (but not as straight as the line from SL to FTH) with s slightly convex curve (ie sag in the middle).
PFVA63 plotted a graph which used the actual values from the test and is typical for a ROC curve. It is probably as correct as we can get with out actually doing the tests ourselves using more accurate measurement systems. In this case, whatever curve we generate should be close to this.
If we use your calculated values from the Hurricane tests above, we can use a technique called "smoothing" (also sometimes referred to as "normalizing") to reduce the errors in a methodical way. In statistical applications using actual measurements, it is customary and acceptable in most cases, to plot the test values on a graph, then draw a line from point to point for the values, and discard the values that are too far off the straight line. The idea behind this process is that for any significant number of measured points, there will a number of errors that will result in an erroneous values. But, "enough" of the measurements have to be considered to be accurate enough to use, or the whole test has to be considered meaningless. These accurate/more accurate test points point toward the correct shape of the curve.
The easiest way to "smooth" the curve is to draw a line from one graphed end point to each successive point in the series, then do the same starting from the opposite end point. When you are done, find the two longest lines, one generated from each direction, that reach the farthest from each end, and are closest to being parallel to the other. If you cannot find any lines that are parallel to each other, then we discard one or both of the end points, and do it all over again leaving those points off of the graph. If you then draw a line down the middle of the resulting two parallel lines, you should be as close to accurate as reasonably possible within the limits of the test data.
If we believe that a particular point (or group of points) is more accurate, then we can use these points to generate the lines mentioned above. An example in this case would be using the 2550 ft/min climb rate value at SL and whatever the maximum achieved average ROC below FTH. Then do the same for the curve above FTH using the maximum average ROC above FTH and some value above that. If we do the same "smoothing" as described in the paragraphs above, and the resulting curves are not close to the curve we generated using the points we believe to be more accurate, then the points we believe to be more accurate are probably not more accurate. In some cases we might be able to discard certain points by looking at where thee curves should intersect. For example, if the lines for above and below FTH do not cross at ~7600 ft, then the data points that prevent this are probably inaccurate.
If we do the above correctly, then a chart generated using instantaneous ROC and one using average ROC, should be very close to each other.
One way to estimate the TTC value for climb to Service Ceiling (SC), and any altitude in between, is to figure the average ROC from SL to FTH, and then do the same for FTH to SC. The resulting ROC curve should be a nearly straight line from SL to FTH (regardless to using boost or BHP as the limiting factor - if boost is the limit then it will lean to the right, if BHP is the limit the it will lean to the left). The precision calculated line from FTH to SC should also be a nearly straight line (but not as straight as the line from SL to FTH) with s slightly convex curve (ie sag in the middle).
PFVA63 plotted a graph which used the actual values from the test and is typical for a ROC curve. It is probably as correct as we can get with out actually doing the tests ourselves using more accurate measurement systems. In this case, whatever curve we generate should be close to this.
If we use your calculated values from the Hurricane tests above, we can use a technique called "smoothing" (also sometimes referred to as "normalizing") to reduce the errors in a methodical way. In statistical applications using actual measurements, it is customary and acceptable in most cases, to plot the test values on a graph, then draw a line from point to point for the values, and discard the values that are too far off the straight line. The idea behind this process is that for any significant number of measured points, there will a number of errors that will result in an erroneous values. But, "enough" of the measurements have to be considered to be accurate enough to use, or the whole test has to be considered meaningless. These accurate/more accurate test points point toward the correct shape of the curve.
The easiest way to "smooth" the curve is to draw a line from one graphed end point to each successive point in the series, then do the same starting from the opposite end point. When you are done, find the two longest lines, one generated from each direction, that reach the farthest from each end, and are closest to being parallel to the other. If you cannot find any lines that are parallel to each other, then we discard one or both of the end points, and do it all over again leaving those points off of the graph. If you then draw a line down the middle of the resulting two parallel lines, you should be as close to accurate as reasonably possible within the limits of the test data.
If we believe that a particular point (or group of points) is more accurate, then we can use these points to generate the lines mentioned above. An example in this case would be using the 2550 ft/min climb rate value at SL and whatever the maximum achieved average ROC below FTH. Then do the same for the curve above FTH using the maximum average ROC above FTH and some value above that. If we do the same "smoothing" as described in the paragraphs above, and the resulting curves are not close to the curve we generated using the points we believe to be more accurate, then the points we believe to be more accurate are probably not more accurate. In some cases we might be able to discard certain points by looking at where thee curves should intersect. For example, if the lines for above and below FTH do not cross at ~7600 ft, then the data points that prevent this are probably inaccurate.
If we do the above correctly, then a chart generated using instantaneous ROC and one using average ROC, should be very close to each other.
wuzak
Captain
Boost wasn't really constant, but rather a boost limit. The boost was maintained as constant to the full throttle height, after which it would fall off.
Automatic boost controllers were adopted to ease pilot work load, these were used whatever the boost pressure was set.
The boost limit was set by two factors: detonation and engine strength.
Detonation was the main factor, at least pre-war and early in the war where the octane ratings were lower (87 or 100) and ADI was not available.
The engine could sometimes be allowed increased boost without strengthening after higher octane fuels became available.
The engine manufacturer would test their engine with the new fuel and determine whether higher boost could be used or the engine needed strengthening.
As to "figures for engine strengthening", what do you mean? Actual details of the changes?
Strengthening usually involved rods, pistons and crankshaft, sometimes it would mean strengthening aspects of the block(s). Cooling design may need revision, especially with air-cooled engines.
The Merlin finished the war with a maximum continuous cruise rating boost higher than the all out maximum boost it had at the start. There were a lot of changes during that period.
- Thread starter
- #34
Sag in the middle? It just seems to curve up and to the left instead of form a diagonal line to the right.
I did my math wrong by the way: The averaged time to climb figures worked out pretty close, if I time from 0 to 7600'. That said. The climb rate from sea level to 1000' feet wouldn't average out to 2575 feet per minute and still be 0.38 minutes. It'd be closer to 0.39 minutes (0.3883 min), and generally one would round 0.3883 to 0.39, particularly considering that the chart only goes to two decimal places. I suppose one could divide by 0.9785 (0.38/0.3883), or 0.9891 (average of 0.38 & 0.3883), and start with something, as it would better cover the discrepancy.
Is it possibly to invert an average? Let's say you have 5200 as an average, can you pull out of that 2550 & 2650?
That's what I was under the impression of as well. And yet they were pulling back a skosh as they went up. I'm not sure if this was to correct for ram or something.
wuzak
Captain
It was a sinister pre war tactic to get one over post war internet discussion forums, the lizard shape shifters were behind it mainly inspired by Prince Philip, all well documented and proven by science.
Shortround6
Major General
closely related to but not identical to a "smidgen"
ThomasP
Senior Master Sergeant
Hey Zipper730,
re: "Sag in the middle? It just seems to curve up and to the left instead of form a diagonal line to the right."
The sag I was talking about only occurs in a precisely measured or calculated line from FTH to SC, not from SL to FTH. It is not shown in most charts/graphs, as it is very small, and the lines were often drawn as straight lines from FTH to SC for simplicity. Measurements in WWII were also usually not precise enough to show this slight convexity either (hence at least part of the reason for the current discussion) and the lines would be 'smoothed' to a straight line
re: "Is it possibly to invert an average? Let's say you have 5200 as an average, can you pull out of that 2550 & 2650?"
Yes, although you have a typo above, if 5200 should be 2600?
If we have the average ROC value, multiply it by 2, and then subtract the a known instantaneous ROC from one end or the other of the climb interval. In your example above, let us say we know the average ROC from SL to 2000' is 2600 ft/min, and that we do not know the instantaneous ROC at SL, but do know the instantaneous ROC at 2000' is 2650 ft/min. Then the instantaneous ROC as SL would be:
(2600 x 2) - 2650 = 2550 ft/min
re: "That's what I was under the impression of as well. And yet they were pulling back a skosh as they went up. I'm not sure if this was to correct for ram or something."
The reason for the required decrease in boost with increased altitude upto FTH, so as not to exceed max allowable HP, is due to the following:
A supercharger will lose efficiency compressing air if the intake air pressure is less, ie if intake air pressure is 22.5727"Hg vs 29.9213"Hg for example.
A supercharger will gain efficiency compressing air if the intake air temperature is lower, ie if intake air temp is 491.577°R vs 518.670°R for example. (R is an absolute temperature using the Rankin scale, aka temperature Fahrenheit above absolute 0 using Fahrenheit degree increments.)
As we all know, increasing altitude results in decreased temperatures and pressures. But, the resulting small increase in weight of charge entering the cylinder due to the lower temperature will allow an increase in engine HP output, making up for a small amount of the increased HP needed to drive the supercharger due to the decrease in pressure. This is why using constant boost will result in increased HP as you climb. (Not sure if I have phrased this clearly?)
~1980 Standard Day temperature and pressure
491.577°R and 22.5727"Hg at 7600'
518.670°R and 29.9213"Hg at SL
Hey wuzak (and others),
re: "Automatic boost controllers were adopted to ease pilot work load, these were used whatever the boost pressure was set."
My understanding is that automatic boost controls were mostly used to prevent overboost as an aircraft descended, ie for example a pilot might increase boost to max at 7600' when entering combat and in the ensuing maneuvers descended to SL. At least the in the early-war. Were they used/operated differently later? I am not particularly familiar with the various mechanisms used by the different nations, so any info would be appreciated.
re: "Sag in the middle? It just seems to curve up and to the left instead of form a diagonal line to the right."
The sag I was talking about only occurs in a precisely measured or calculated line from FTH to SC, not from SL to FTH. It is not shown in most charts/graphs, as it is very small, and the lines were often drawn as straight lines from FTH to SC for simplicity. Measurements in WWII were also usually not precise enough to show this slight convexity either (hence at least part of the reason for the current discussion) and the lines would be 'smoothed' to a straight line
re: "Is it possibly to invert an average? Let's say you have 5200 as an average, can you pull out of that 2550 & 2650?"
Yes, although you have a typo above, if 5200 should be 2600?
If we have the average ROC value, multiply it by 2, and then subtract the a known instantaneous ROC from one end or the other of the climb interval. In your example above, let us say we know the average ROC from SL to 2000' is 2600 ft/min, and that we do not know the instantaneous ROC at SL, but do know the instantaneous ROC at 2000' is 2650 ft/min. Then the instantaneous ROC as SL would be:
(2600 x 2) - 2650 = 2550 ft/min
re: "That's what I was under the impression of as well. And yet they were pulling back a skosh as they went up. I'm not sure if this was to correct for ram or something."
The reason for the required decrease in boost with increased altitude upto FTH, so as not to exceed max allowable HP, is due to the following:
A supercharger will lose efficiency compressing air if the intake air pressure is less, ie if intake air pressure is 22.5727"Hg vs 29.9213"Hg for example.
A supercharger will gain efficiency compressing air if the intake air temperature is lower, ie if intake air temp is 491.577°R vs 518.670°R for example. (R is an absolute temperature using the Rankin scale, aka temperature Fahrenheit above absolute 0 using Fahrenheit degree increments.)
As we all know, increasing altitude results in decreased temperatures and pressures. But, the resulting small increase in weight of charge entering the cylinder due to the lower temperature will allow an increase in engine HP output, making up for a small amount of the increased HP needed to drive the supercharger due to the decrease in pressure. This is why using constant boost will result in increased HP as you climb. (Not sure if I have phrased this clearly?)
~1980 Standard Day temperature and pressure
491.577°R and 22.5727"Hg at 7600'
518.670°R and 29.9213"Hg at SL
Hey wuzak (and others),
re: "Automatic boost controllers were adopted to ease pilot work load, these were used whatever the boost pressure was set."
My understanding is that automatic boost controls were mostly used to prevent overboost as an aircraft descended, ie for example a pilot might increase boost to max at 7600' when entering combat and in the ensuing maneuvers descended to SL. At least the in the early-war. Were they used/operated differently later? I am not particularly familiar with the various mechanisms used by the different nations, so any info would be appreciated.
wuzak
Captain
Automatic boost control was used to control the boost, um, automatically.
It didn't matter if the pilot was going up, down or sideways, in combat, climbing or cruising, the pilot set the boost and the automatic boost control maintained that boost.
Otherwise the pilot would be watching the boost gauge and constantly changing the throttle position as the altitude changed.
Say you wanted a combat climb at +18psi. You set the +18psi and start the climb. If the throttle plate in the carburetor is not adjusted the boost would fall away and the power would too, so the climb performance would suffer. So either the pilot has to adjust that or it has to be adjusted by other means - namely the automatic boost control.
As the plane climbs the throttle plate is opened until it reaches the full throttle height, where the throttle plate is wide open.
It is true that if the plane is descending without changing the throttle plate the boost would increase and could, depending on the boost at the start of descent, cause overboosting.
(In a dive the propeller will drive the engine, which may be over the normal operating maximum rpm. Engines usually had some sort of overspeed limit, which was ~3,300rpm for Merlins IIRC. If the engine overspeeds the supercharger will also go faster than normal and produce even more boost. This would exacerbate overboost if not controlled by the throttle plate.)
ThomasP
Senior Master Sergeant
Hey wuzak,
Sorry, I was not clear in my question. Moving the boost lever to +16 lbs (for example) at take-off, even if the system is not fitted with automatic boost, would (of course) maintain +16 lbs as long as the engine/supercharger rpms did not change and as long as the aircraft did not climb above FTH. Turning and such would not affect this.
The situation I was wondering about was for an engine/supercharger combination such as the Merlin III in the pre- and early-war period, I may be misremembering/misunderstanding what I have have read, but what I think I read in many of the pilot accounts was that they had to monitor their boost as they descended to prevent overboost. I got the impression that the early Merlins (for example) did not have any built in any bypass/vent ability for the compressed air coming from the supercharger. What I am thinking is that with the engine at Combat rating (say 3000 rpm and +6.75 lbs), if the aircraft descended to SL the effective boost could?/would? increase to somewhere around +19 lbs at SL if 3000 rpm was maintained and if the boost lever was not pulled back. So I am not referring to an overspeed condition, although the same problem could occur if I am thinking right. Yes/No? Clarification would be appreciated (from anyone) please.
Also, my understanding (which again could be wrong) is that pre- and early-war the engine/supercharger did not always have the ability to safely descend without the pilot pulling back on the boost in order to keep the IHP in the safe range. An example of this is the 2-speed Merlin XX in 1942, the pilot could set the boost to +16 lbs at altitudes above ~12,000 ft, but had to manually reduce the boost to a max of +14 lbs at lower altitudes in order to prevent detonation and/or overstressing the engine. I think the early 60 series engines had the same problem? Admittedly, this was due to the change in supercharger rpm, but still.
Sorry, I was not clear in my question. Moving the boost lever to +16 lbs (for example) at take-off, even if the system is not fitted with automatic boost, would (of course) maintain +16 lbs as long as the engine/supercharger rpms did not change and as long as the aircraft did not climb above FTH. Turning and such would not affect this.
The situation I was wondering about was for an engine/supercharger combination such as the Merlin III in the pre- and early-war period, I may be misremembering/misunderstanding what I have have read, but what I think I read in many of the pilot accounts was that they had to monitor their boost as they descended to prevent overboost. I got the impression that the early Merlins (for example) did not have any built in any bypass/vent ability for the compressed air coming from the supercharger. What I am thinking is that with the engine at Combat rating (say 3000 rpm and +6.75 lbs), if the aircraft descended to SL the effective boost could?/would? increase to somewhere around +19 lbs at SL if 3000 rpm was maintained and if the boost lever was not pulled back. So I am not referring to an overspeed condition, although the same problem could occur if I am thinking right. Yes/No? Clarification would be appreciated (from anyone) please.
Also, my understanding (which again could be wrong) is that pre- and early-war the engine/supercharger did not always have the ability to safely descend without the pilot pulling back on the boost in order to keep the IHP in the safe range. An example of this is the 2-speed Merlin XX in 1942, the pilot could set the boost to +16 lbs at altitudes above ~12,000 ft, but had to manually reduce the boost to a max of +14 lbs at lower altitudes in order to prevent detonation and/or overstressing the engine. I think the early 60 series engines had the same problem? Admittedly, this was due to the change in supercharger rpm, but still.
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