# Climb Rate Discrepancies



## Zipper730 (Mar 4, 2020)

I'm looking at the Hawker Hurricane Mk.I, just the prototype and I've noticed something unusual

Altitude.....Time to Climb.....Time to Climb Interval.....Listed Rate of Climb.....Calculated Rate of Climb
0'..............0 .....min..............0......min..........................2550 fpm.......................N/A
1000'........0.38 min..............0.38 min..........................2600 fpm.......................2632 fpm
2000'........0.76 min..............0.38 min..........................2650 fpm.......................2632 fpm
3000'........1.15 min..............0.39 min..........................2710 fpm.......................2564 fpm
5000'........1.89 min..............0.74 min..........................2810 fpm.......................2703 fpm
6500'........2.43 min..............0.54 min..........................2880 fpm.......................2778 fpm
7600'........2.76 min..............0.33 min..........................2950 fpm.......................3333 fpm
10000'......3.63 min..............0.87 min..........................2680 fpm.......................2759 fpm
13000'......4.8...min..............1.17 min..........................2370 fpm.......................2564 fpm
15000'......5.7...min..............0.9...min..........................2150 fpm.......................2222 fpm
16500'......6.4...min..............0.7...min..........................2000 fpm.......................2143 fpm
18000'......7.25 min..............0.85 min..........................1840 fpm.......................1765 fpm
20000'......8.4...min..............1.15 min..........................1620 fpm.......................1739 fpm
23000'......10.4 min..............2......min..........................1310 fpm.......................1500 fpm
26000'......13.0 min..............2.6...min............................990 fpm.......................1154 fpm
28000'......15.2 min..............2.2...min............................790 fpm.......................909...fpm
30000'......18.1 min..............2.9...min............................570 fpm.......................690...fpm

Why are there these discrepancies?


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## tyrodtom (Mar 4, 2020)

What discrepancies ?
That a particular aircraft, on a particular day, doesn't exactly perform as it was calculated it would do ?

Welcome to the real world, no mechanical device performs exactly the same, day in day out. And no two complex mechanical devices can be exactly duplicated .
They're usually made by humans, and operated by humans.
Even today with all our supercomputers, they still test, and still the aircraft may fall short of, or exceed calculations.
Calculations are estimates only, what you hope it'll do.
Sometimes the designers are close, sometimes they look like idiots.

I remember when NASCAR used to have a race series called the International race of champions, IROC.
They'd take , I think, 12 cars, all of them made by the same maker, equip , them with identical parts.
Every effort was made to get the same performance out of each car.
The drivers from all race series , Nascar, Indy, F1, etc, were put in cars, chosen by lot, and ran several different races. No driver got the same car twice.
Every year, there would always be some cars that was faster than the others, no matter who drove it, and some that were dogs.

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## taly01 (Mar 5, 2020)

> I'm looking at the Hawker Hurricane Mk.I, just the prototype and I've noticed something unusual



Fixed pitch propeller?!


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## pbehn (Mar 5, 2020)

How is the data collected? Does the pilot scribble it down on a note pad or call numbers out on the radio.


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## jetcal1 (Mar 5, 2020)

Perhaps failure to hold consistent and exact AOA and rudder inputs would have some influence on climb performance, add in tolerances from pitot static system.......


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## K5083 (Mar 5, 2020)

It doesn't mention what the pilot was instructed to hold as best climb speed. That's an indicated value and would in physics be only best at one speed/height combination. The pilot's notes will usually say something like 160kts to 8000' then 150 kts. It is simplified for the pilot. Maybe the test pilot is trying to hit best climb numbers regardless of some set 'best speed' but by looking at the rate of climb indicator (which has a built-in lag). The fixed pitch prop will make a difference and be ideal at only one speed at any given height. The pilot is more likely to have a pen recorder for time vs altitude, not having to write it down. 

I find the 3333 ft/min rate at full throttle height to be pretty good for a Hurricane, but K5083 was lighter than the production Mk1. It's pretty good at 30000'too.


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## PFVA63 (Mar 5, 2020)

Hi,
In looking at your data it appears that the last column is equal to the difference in altitudes from one step to the next divided by the time interval in covering that step (as listed in column three). As such I'd think that the data in column four is more of an average rate of climb over that step than at actual rate of climb at the altitude listed in column 1. 

I tried a quick plot where the average rate of climb for a given step is plotted at the mean height of that step, and the points appeared to fall closer to the other data for lower altitudes but not higher ones. So there might be other factors in play here, but I suspect that one may be that we are trying to compare average rates of climb over a range of height steps, to a plot that potentially trying to relate the estimated rate of climb at given altitudes.

Pat

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## Shortround6 (Mar 5, 2020)

Part of the discrepancy is that column 4 (Listed Rate of Climb.) is the rate of climb at that altitude listed in column 1. 

Column 5 appears to be the _average _rate of climb between two altitudes. 

as in the 28,000ft to 30,000ft climb. The discrepancy can be explained by the plane climbing at 790fpm at 28,000ft but the climb rate slowly declines to 570fpm at 30,000ft giving you the average of 690fpm for the 2.9 minute climb. 

690fpm is neither climb rate at 28,000ft or the climb rate at 30,000ft. It _might_ be the climb rate somewhere close to 29,000ft. 

There may be some other discrepancies that could either be typos or errors in recording data. 
Picture of Hurricane altimeter. 





Picture of rate of climb and decent indicator 





Unless fitted with a special recording instrument you are also depending on the pilots ability to interpret the gauges as they are not marked do any great degree of precision.


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## Zipper730 (Mar 6, 2020)

tyrodtom said:


> What discrepancies?


The climb-rate figures don't match. Based on the time it took to reach each altitude mark the plane appears to have reached that rate faster than the VSI reads.



K5083 said:


> It doesn't mention what the pilot was instructed to hold as best climb speed.


Uh, I just didn't list that because I was curious about discrepancies in climb-rate.

Regardless, the figures are as follows

Altitude....TAS...............IAS...............CAS
0'.............151.5 mph....150....mph....151.5 mph
1000'.......154....mph....150....mph....151.5 mph
2000'.......156....mph....150....mph....151.5 mph
3000'.......158.5 mph....150....mph....151.5 mph
5000'.......163.5 mph....150....mph....151.5 mph
6500'.......167....mph....150....mph....151.5 mph
7600'.......170....mph....150....mph....151.5 mph
10000'.....173.5 mph....148....mph....149.1 mph
13000'.....177.5 mph....144....mph....145.4 mph
15000'.....181....mph....142....mph....143.5 mph
16500'.....183....mph....140....mph....141.6 mph
18000'.....186.5 mph....139....mph....140.7 mph
20000'.....189.5 mph....136.5 mph....138.3 mph
23000'.....195....mph....133....mph....135....mph
26000'.....199.5 mph....129.5 mph....131.2 mph
28000'.....204....mph....127.5 mph....129.4 mph
30000'.....208....mph....125....mph....127....mph


> Maybe the test pilot is trying to hit best climb numbers regardless of some set 'best speed' but by looking at the rate of climb indicator (which has a built-in lag).


By built in lag, you mean the altimeter will move slower than the altitude of the plane goes up? If that's the case, wouldn't that actually mean the climb-rates would be higher than I even calculated for? After all, the altitudes would be higher than what the pilot recorded...


> The pilot is more likely to have a pen recorder for time vs altitude, not having to write it down.


So, that's how he gets accurate data...


> I find the 3333 ft/min rate at full throttle height to be pretty good for a Hurricane, but K5083 was lighter than the production Mk1.


Actually it sort of was, I just used it because it was a chart that was handy and had a discrepancy I noticed.


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## Zipper730 (Mar 6, 2020)

PFVA63 said:


> In looking at your data it appears that the last column is equal to the difference in altitudes from one step to the next divided by the time interval in covering that step (as listed in column three).


Altitude in column 1 divided by time in column 3.


> As such I'd think that the data in column four is more of an average rate of climb over that step than at actual rate of climb at the altitude listed in column 1.


That's the figures that were listed for the aircraft. I'm not sure how you'd calculate the exact rate of time at any one period. So I just calculated altitude change by interval in time. That said, the climb rate was highest at the aircraft's FTH, so I figured that I was probably right, since R/C was even listed in the chart.


> I tried a quick plot where the average rate of climb for a given step is plotted at the mean height of that step


Can you show me what you came up with?



Shortround6 said:


> Part of the discrepancy is that column 4 (Listed Rate of Climb.) is the rate of climb at that altitude listed in column 1 . . . Column 5 appears to be the _average _rate of climb between two altitudes


Well, the discrepancy is that the numbers don't agree with each other (and in some cases it's by over 100 fpm): The calculation I made was simply based on altitude vs time-interval. While calculating over huge altitudes (i.e. 30000 feet / 18.1 minutes = 1657.5 fpm would give figures that are ridiculous -- they'd be far higher than the actual rate of climb at high altitude, and far too low for sea level climb), but since I calculated over several intervals (1000-2000, 2000-3000, 3000-5000, 5000-6500), which range from 1000 feet to 2000 feet between the interval. From 7600-10000 there's a 2400 foot interval, and the highest interval is 3000 feet.


> The discrepancy can be explained by the plane climbing at 790fpm at 28,000ft but the climb rate slowly declines to 570fpm at 30,000ft giving you the average of 690fpm for the 2.9 minute climb.


I was under the presumption that at 1000' the R/C listed would be exactly the rate of climb showing on the VSI at that specific altitude, not an average rate of climb between the two of them.


> There may be some other discrepancies that could either be typos or errors in recording data.


It seems that the altimeter gauge is reliable by marking to around 50-100'; the VSI seems good to 100-200 fpm increments.


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## PFVA63 (Mar 6, 2020)

Hi,
Since a climb rate graph based on the first couple columns would like something like this;




But you appear to be calculating average rates of climb over certain altitude bands, if you then plot the average rate of climb that you calculated for each time step at the average altitude covered by that time step, you end up getting something similar to what is shown below.




In general the points on the blue line tend to fall fairly close to the grey curve (from above), except near the break in the curve at 7600 ft, where this "knee in the curve" represents a change in available power due to the way the supercharger works.

It is not clear to me yet why the data at/near 7600ft altitude seems a bit high but, I believe that most the other points fall fairly close to the original curve.

Possible reasons for the discrepancies seen at the points (other than near 7600' altitude) may stem from a number of issues, including the accuracy of how the data was measured and any the possibility that corrections/margins that may have been applied during the testing, recording and presentation of the data.

Pat


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## Zipper730 (Mar 7, 2020)

P
 PFVA63


What I find puzzling is that climb rate would fall off at 3000 feet, nor why you'd see higher rates from 5000-6500 feet. Especially with 
S
 Shortround6
speculating as to it being the average figure between the altitude range. One would expect that the climb figures would only go up until the FTH and then drop off, even if they were averaging the climb-figures at specified altitudes.

As for the effects of the supercharger performance, I don't know. On one hand throttling losses do occur, but I figure they'd be less extreme on a propeller that doesn't have variable pitch.

RPM of the engine would vary across a wider range from low to high altitude on a single-pitch propeller because of efficiency issues
Constant-speed propellers usually see around the same RPM across the board: Boost levels vary, but RPM varies little, with the power absorbed by the propeller. With arrangements like that, you end up with a supercharger spinning at the same RPM with huge amounts of restriction to the airflow.
That said: I do remember something to the effect of the supercharger being geometrically (exponentially) related to RPM. This does seem to conform to the power output of jet-engines (almost nothing at low RPM, with almost all the power coming up at high RPM). Regardless, RPM only varies from 2100-2305 throughout the entire altitude range listed (during speed trials, you see around 2505-2960), which isn't a massive change in RPM: When the Constant-Speed propeller was fitted to the Hurricane, general ranges cited for climb and speed trials were 2600-3000


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## DarrenW (Mar 7, 2020)

Shortround6 said:


> ..Unless fitted with a special recording instrument you are also depending on the pilots ability to interpret the gauges as they are not marked do any great degree of precision....



There is also situations where a pilot's unfamiliarity with certain instruments and the use of different correction data may skew test results:

_....In an independent trial the U.S.A.A.F. obtained slightly higher figures, possibly owing to the *difficulty of reading the Standard American airspeed indicator* *and to the different methods of reduction*. At this Unit the British Performance Reduction Methods for Modern Aircraft (A. &A.E.E./Res/170) were used.... _

_P-47 Tactical Trials _


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## tyrodtom (Mar 7, 2020)

When I took rotary flight training in the earl 70's, and fixed wing in the 90's, I was told both times by instructors that all pressure reading flight instruments ( altimeter, rate of climb, air speed ) have a lag between what they display and exactly what's going on in that instant of time. 
Especially if you're in a rapidly climbing aircraft and see 5000 feet on the altimeter, you were actually there a few seconds before that.

I doubt the flight gauges of the late 30's were any better.

I doubt those notes were taken by a test pilot on a clipboard in the cockpit, because if it was he'd be awfully busy in the first part of the climb, making estimates of rate of climb off a indicator incremented in 500 fpm when it's above 2000 fpm, and making these notes every 20-25 seconds. While flying at full power, so he'd also have to be monitoring the engine instruments pretty close too.

Just how stable would the automatic recording instruments have been in the late 30's, wouldn't they just as lag prone as the regular flight instruments ? 

Then the aircraft was flown in a real world .
How was the weather? wind gusting ?


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## Zipper730 (Mar 7, 2020)

DarrenW said:


> There is also situations where a pilot's unfamiliarity with certain instruments and the use of different correction data may skew test results:
> 
> _....In an independent trial the U.S.A.A.F. obtained slightly higher figures, possibly owing to the *difficulty of reading the Standard American airspeed indicator* *and to the different methods of reduction*. At this Unit the British Performance Reduction Methods for Modern Aircraft (A. &A.E.E./Res/170) were used...._


How did they reduce the data?



tyrodtom said:


> When I took rotary flight training in the earl 70's, and fixed wing in the 90's, I was told both times by instructors that all pressure reading flight instruments ( altimeter, rate of climb, air speed ) have a lag between what they display and exactly what's going on in that instant of time.
> Especially if you're in a rapidly climbing aircraft and see 5000 feet on the altimeter, you were actually there a few seconds before that.


So, there was clearly a lag in the time to climb figures. 

Was there any evidence to suggest that there were differences in the lag-rates used by different gauges in different nations?


> I doubt those notes were taken by a test pilot on a clipboard in the cockpit, because if it was he'd be awfully busy in the first part of the climb


That makes sense, do you have any idea how they recorded the data?


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## PFVA63 (Mar 7, 2020)

Hi,
I didn't mean to imply that the supercharger was a potential cause for the variation in the points. What I was trying to say was that there is a "knee" in the curve because of the supercharger. In other words, below the altitude that the "knee" occurs, I believe, that you are able to apply full power from the engine (because of the supercharger), but above that altitude the amount of power that the engine is able to put out begins to drop with altitude.

Regards
Pat

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## Zipper730 (Mar 9, 2020)

PFVA63 said:


> I didn't mean to imply that the supercharger was a potential cause for the variation in the points.


Well you said it. That said, I figured it was an attempt at a guess.


> In other words, below the altitude that the "knee" occurs, I believe, that you are able to apply full power from the engine (because of the supercharger), but above that altitude the amount of power that the engine is able to put out begins to drop with altitude.


Actually, the power output below that altitude would be less, owing to throttling loss (they occur because the airflow to the supercharger is sub-optimal with the airflow-restriction).

Manifold pressure would be maxed out until critical altitude is reached.


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## PFVA63 (Mar 9, 2020)

Hi,
I disagree. Here is a direct quote of my earlier post (with underlining and bolding for emphasis);

"In general the points on the blue line tend to fall fairly close to the grey curve (from above), except near the break in the curve at 7600 ft, *where this "knee in the curve" represents a change in available power due to the way the supercharger works.*

It is not clear to me yet why the data at/near 7600ft altitude seems a bit high but, I believe that most the other points fall fairly close to the original curve.

Possible reasons for the discrepancies seen at the points (other than near 7600' altitude) may stem from a number of issues, including the accuracy of how the data was measured and any the possibility that corrections/margins that may have been applied during the testing, recording and presentation of the data."

Nowhere in the above text (that I can see) do I appear to state anything about the supercharger being "a potential cause for the variation in the points".

Regards
Pat

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## ThomasP (Mar 9, 2020)

One of the things I have noticed over the years is the presence of typos in the original documents. In this case the 3333 ft/min in the calculated climb rate column should be 2833 ft/min, and the 2564 ft/min should be 2654 ft/min. It is remotely possible that this is due to miscalculations, but I would bet it is simply typos - either in the original calculation results chart, or when the results were transcribed to the performance test chart.


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## Zipper730 (Mar 10, 2020)

ThomasP said:


> One of the things I have noticed over the years is the presence of typos in the original documents.


The numbers I posted came from calculating altitude versus time. The listed climb-rate figures are on column four. There wasn't any errors except the fact that I rounded...


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## tyrodtom (Mar 10, 2020)

Your calculations ???
I thought the first post was from original sources.
How much of that first post is from original documents and how much is your calculations?


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## Zipper730 (Mar 10, 2020)

tyrodtom said:


> Your calculations ???


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.


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## ThomasP (Mar 11, 2020)

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).


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## DarrenW (Mar 11, 2020)

Here's both an entertaining and educational wartime video concerning USAAF flight testing at Wright Field...enjoy!


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## ThomasP (Mar 11, 2020)

Pretty cool video, thanks.


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## Zipper730 (Mar 12, 2020)

ThomasP said:


> 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.


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.


> 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.


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

The time intervals still seem to be off, almost 37 minutes versus 33 minutes.


> 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.


Wait, I thought the limitation was set by boost rather than HP?


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## ThomasP (Mar 13, 2020)

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.


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## Zipper730 (Mar 13, 2020)

ThomasP said:


> 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.


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.


> 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 Hurricane prototype (where I got this data from) seemed to include a progressively increasing rate of climb until critical altitude is reached.


> 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.


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.


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## Zipper730 (Mar 13, 2020)

T
 ThomasP


Time to climb comes out about right: 2.764 versus 2.76 minutes. I misread the graph and got a bogus number.


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## ThomasP (Mar 13, 2020)

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.


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## Zipper730 (Mar 13, 2020)

ThomasP said:


> Note the boost at Military at 11,800 ft FTH and at TO at SL. Both ratings are 1150 HP at 3000 rpm.


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.


> 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 was. Whether this was also due to sturdier models of engines, or the recognition of already existing strength in the engines, I cannot say.


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?


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## ThomasP (Mar 14, 2020)

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.


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## wuzak (Mar 14, 2020)

ThomasP said:


> 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.





Zipper730 said:


> 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?



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.


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## Zipper730 (Mar 14, 2020)

ThomasP said:


> 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 slightly convex curve (ie sag in the middle).


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?



wuzak said:


> 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.


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.


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## wuzak (Mar 14, 2020)

Zipper730 said:


> 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.



Who was pulling what back? A

And what the fuck is a "skosh"?

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## pbehn (Mar 14, 2020)

wuzak said:


> Who was pulling what back? A
> 
> And what the fuck is a "skosh"?


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.

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## Shortround6 (Mar 14, 2020)

wuzak said:


> And what the fuck is a "skosh"?


closely related to but not identical to a "smidgen"

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## ThomasP (Mar 15, 2020)

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.


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## wuzak (Mar 15, 2020)

ThomasP said:


> 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.



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.)


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## ThomasP (Mar 16, 2020)

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.


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## wuzak (Mar 16, 2020)

ThomasP said:


> 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.



If the engine does not have an automatic boost control system, the boost will fall as the aircraft climbs unless the pilot adjusts the throttle to maintain the boost.



ThomasP said:


> 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.



Not sure if the early Merlins had automatic boost control.




ThomasP said:


> 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.



+16 was for FS (high gear) and +14 for MS (low gear). Not sure if there was a system which would restrict boost in low gear with the automatic boost controller.

Soon enough the 2 speed engines were rated the same in both gears.

I think maybe teh Merlin 61 and 63 had different boost allowance for MS and FS gears, but thereafter it was the same.

This may have been more to do with the increased power required to drive the supercharger at the same boost in FS, possibly too much for the drive shaft or gear sets.


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## Zipper730 (Mar 19, 2020)

wuzak said:


> And what the fuck is a "skosh"?


It means a tiny bit.



ThomasP said:


> The sag I was talking about only occurs in a precisely measured or calculated line from FTH to SC


That's what I was talking about...


> 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.


Oh, so it's not a perfect diagonal line, but actually a curved line from left/right going up?

The thing I'm getting confused by, and I'm not that good at math, so it shouldn't be a big surprise: Why when I measured by interval (1000/0.38) I got around 2630 fpm, and when I averaged the numbers I ended up with around 2575 fpm, but an average time to climb of 0.3883 minutes? Ultimately I'd get around 2.76 minutes but I'd get a slower time to 1000 feet, and a slower time from 6500-7600 feet. Was this the result of a sag in the curve, or was this an error in the way they were timing things?


> 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


But what If I know neither? I just have the average...


> The reason for the required decrease in boost with increased altitude upto FTH, so as not to exceed max allowable HP


I thought boost was a far larger limitation on engines than the horsepower?


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## wuzak (Mar 19, 2020)

Zipper730 said:


> It means a tiny bit.



You made that up.

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## pbehn (Mar 19, 2020)

wuzak said:


> You made that up.


It is a gnat's whisker more than a little bit.

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## Zipper730 (Mar 19, 2020)

wuzak said:


> You made that up.


Definition: skosh


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## pbehn (Mar 19, 2020)

Zipper730 said:


> Definition: skosh


From the Japanese for a little "skoshi"

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## Zipper730 (Mar 19, 2020)

pbehn said:


> From the Japanese for a little "skoshi"


That's correct.


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## pbehn (Mar 19, 2020)

Zipper730 said:


> That's correct.


Appears in the 1950s probably brought back by US military who had been in Japan. There is a huge lexicon of words in English brought back from WW1 France especially in aviation because British aviation grew up there.


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