Merlin 40 series performance curves??

[Engineman]

Another thing the detailed graph you posted illustrates is the way that even quite authoritative information can be in error. In this case, the RR illustration was generalised to the point of being in error. Whereas, the specific engine performance data showed the correct detail. This sort of weakness in original data is something that crops up regularly.

Eng

 

[ThomasP]

In all seriousness, that is the primary reason I decided to make the spreadsheet. There were just too many odd numbers or patterns that did not make sense, sometimes from what seemed to be authoritative sources. It has only been in the last 10 or so years that many of the question marks have been answered by the uploading of original source documents to the internet.

The secondary reason I made the spreadsheet is that I am an AR type.

 

[tomo pauk]

.... I am an AR type

For the un-initiated?

 

[ThomasP]

Anal Retentive

adjective
(of a person) excessively orderly and fussy (supposedly owing to conflict over toilet-training in infancy).

noun
a person who is excessively orderly and fussy.

Psychoanalysis
Anal retentiveness is a personality trait that is characterised by excessive concern with trivial details

 

[kool kitty89]

It's interesting to note that the Merlin 46 power curve is very close to that of the Merlin XX series in FS gear, per the Merlin 22 power chart.

Given the 9.49:1 gear drive ratio and 10.25" impeller diameter of the Merlin XX supercharger, tip speeds at 3000 RPM are 1273.294 ft/s.
And on Merlin 46, 9.089:1 drive, 10.85" impeller, so at 3000 RPM tip speeds are 1290.876 ft/s. So only slightly higher, and apparently achieving very similar mass flow and pressure ratio (and efficiency) given the critical altitude for given boost pressure and the given power at altitude.




Also, it seems like running higher boost levels at 2850 RPM would've been a reasonable workaround for the problems the RAAF had (and presumably other tropical operations with the Merlin 46) given charge heating should've been similar to the Merlin 45 at 3000 RPM (or just slightly higher), with a critical altitude of around 7500 ft for +16 PSI and around 1470 to 1515 hp. (1515 is about right with a straight line continued from the +9 PSI 2850 RPM curve, but assuming a similar lower altitude curve to the 45 at 3000 RPM, then a 45 hp loss at 7500 ft would be close to correct) The Merlin 46 at 2850 RPM should cross the Merlin 45 3000 RPM line at just above 8500 ft, and have more power below that. (Merlin 46 should be ~1455 hp 2850 rpm wide-open throttle at 8500 ft, vs 1450 hp 3000 RPM +16 PSI boost part-throttle for Merlin 45)

For climbing conditions, they'd still be limited to 2850 RPM +9 PSI if they were already overheating/detonating at those levels, unless they dropped to lower RPM to further reduce charge heating. If the Merlin 45 was tolerating 3000 RPM +9 PSI climb conditions, then +12 PSI at 2650 RPM should've worked for the 46. Likewise, if the 45 tolerated 2850 RPM at +12 PSI, the 46 should have been OK at 2650 RPM at +14 PSI.


You could mess around with other RPM and boost levels, but sticking to the existing max continuous and 30 minute climb limit RPMs would mean only modifying one variable (boost pressure) compared to established standard operating conditions.

 

[Engineman]

It's interesting to note that the Merlin 46 power curve is very close to that of the Merlin XX series in FS gear, per the Merlin 22 power chart.

Given the 9.49:1 gear drive ratio and 10.25" impeller diameter of the Merlin XX supercharger, tip speeds at 3000 RPM are 1273.294 ft/s.
And on Merlin 46, 9.089:1 drive, 10.85" impeller, so at 3000 RPM tip speeds are 1290.876 ft/s. So only slightly higher, and apparently achieving very similar mass flow and pressure ratio (and efficiency) given the critical altitude for given boost pressure and the given power at altitude.




Also, it seems like running higher boost levels at 2850 RPM would've been a reasonable workaround for the problems the RAAF had (and presumably other tropical operations with the Merlin 46) given charge heating should've been similar to the Merlin 45 at 3000 RPM (or just slightly higher), with a critical altitude of around 7500 ft for +16 PSI and around 1470 to 1515 hp. (1515 is about right with a straight line continued from the +9 PSI 2850 RPM curve, but assuming a similar lower altitude curve to the 45 at 3000 RPM, then a 45 hp loss at 7500 ft would be close to correct) The Merlin 46 at 2850 RPM should cross the Merlin 45 3000 RPM line at just above 8500 ft, and have more power below that. (Merlin 46 should be ~1455 hp 2850 rpm wide-open throttle at 8500 ft, vs 1450 hp 3000 RPM +16 PSI boost part-throttle for Merlin 45)

For climbing conditions, they'd still be limited to 2850 RPM +9 PSI if they were already overheating/detonating at those levels, unless they dropped to lower RPM to further reduce charge heating. If the Merlin 45 was tolerating 3000 RPM +9 PSI climb conditions, then +12 PSI at 2650 RPM should've worked for the 46. Likewise, if the 45 tolerated 2850 RPM at +12 PSI, the 46 should have been OK at 2650 RPM at +14 PSI.


You could mess around with other RPM and boost levels, but sticking to the existing max continuous and 30 minute climb limit RPMs would mean only modifying one variable (boost pressure) compared to established standard operating conditions.

Pie in the sky. R-R were making the absolute best of the Merlin. The power ratings reflect the compromises required by the Air Ministry for engine reliability and overhaul life while achieving the best power that fitted all the requirements.
In addition, supercharger output and efficiency is complicated, just saying it would work better at a different rating does not mean it would.

Eng