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Engine performance 10,000 - 13,000 ft


ekg
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What happens between 10,000 and 13,000 ft? I do a sustained climb ~2700 RPM boost at 12 as described in the take off tutorial. The boost starts dropping starting at 10,000 ft. I suppose this makes sense that the air is thinner so performance is worse. I try adding more power via the throttle but it has no effect which is strange. However around 13,000 ft the boost jumps way past 12 (most likely due to added throttle) almost like some sort of switch was turned on. The engine warning light turn on after I pass 13,000 ft.

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That jump is the second speed of the supercharger kicking in order to counter that very effect of power loss you describe and is perfectly normal. And the light isn't a warning per se; merely an indicator that you're FS (fast speed) on the supercharger.

 

In any supercharged piston aircraft you'll find there are altitude bands between speeds that you get a deterioration in engine performance. Could you kick the second FS speed in earlier to compensate? Sure, but the your performance at much higher alt would be compromised, plus you could overtax the engine trying to drive that amount of compression at the lower alt.

 

As with any engineering solution there is no one fix all so compromises have to be made and with the Merlin 66 it was determined that 13,000ft (or so depends on air pressure rather than fixed alt) is the optimum to kick in the second speed of the supercharger.

 

Just try to avoid the 10,000-13,000ft band when getting in a fight...!

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In any supercharged piston aircraft you'll find there are altitude bands between speeds that you get a deterioration in engine performance.

 

Not any, though it is true for most multi-speed supercharged engine's.

 

On the DB 600 series engines (i.e. K-4) the supercharger's 2nd speed is variable and has infinite number of speed gradually increased with altitude by a hydraulic clutch, rather than fixed ratio gears, hence the supercharger always delivers roughly the same amount of air corresponding to the engine's needs. There's a slight overpressure which is throttled but otherwise you do not get anywhere near to huge engine outpit hit between supercharger speeds.

 

Another smart solution for improving supercharger control effiency was the SZYDLOWSKI - PLANIOL supercharger, which used tilting vanes on fixed supercharger speed, however it had the limitation that it was not very suitable for very high tip speeds. It was a French design - but I believe the Jumo 213 in our D-9 follows a similar idea. Basically as I understand as opposed to the DB design (which controlled air pressure with the driving the supercharger itself at optimum speeds), the S-P supercharger effectively had an air throttle before the supercharger itself, thus preventing too much air being entering into the supercharger in the first place.

 

The two stage Merlin (and most engines in WW2) in contrast just lets in as much air as possible into the supercharger (which runs at either of the two fixed supercharging speeds, so in effect its only operates optimally at two fixed altitude points), cools the supercharged (and very much heated) air with an intercooler and then throttles the excess air . There is much unnecessary temperature rise with air and a lot of engine power or air 'wasted' in this kind system, hence the phenomenon you encountered.


Edited by Kurfürst

http://www.kurfurst.org - The Messerschmitt Bf 109 Performance Resource Site

 

Vezérünk a bátorság, Kísérőnk a szerencse!

-Motto of the RHAF 101st 'Puma' Home Air Defense Fighter Regiment

The Answer to the Ultimate Question of the K-4, the Universe, and Everything: Powerloading 550 HP / ton, 1593 having been made up to 31th March 1945, 314 K-4s were being operated in frontline service on 31 January 1945.

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Happily.

 

Oh and I forgot the most obvious solution. Turbochargers.

 

Its one straight power curve all the way to the top. No need to loose power to having to mechanically drive the supercharger. Good for fuel economy, too.

 

Its technically the best solution. Well, mostly, unless we consider other than the engine department's point of view, as you also need half-a-ton of turbo ducting that fits only into a 6 ton plane of the size of a locomotive, and requires generous application of rare alloys so critical in wartime and the loss of engine exhaust thrust.

 

Great for bombers though, where most of this is not an issue.

http://www.kurfurst.org - The Messerschmitt Bf 109 Performance Resource Site

 

Vezérünk a bátorság, Kísérőnk a szerencse!

-Motto of the RHAF 101st 'Puma' Home Air Defense Fighter Regiment

The Answer to the Ultimate Question of the K-4, the Universe, and Everything: Powerloading 550 HP / ton, 1593 having been made up to 31th March 1945, 314 K-4s were being operated in frontline service on 31 January 1945.

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Not any, though it is true for most multi-speed supercharged engine's.

 

On the DB 600 series engines (i.e. K-4) the supercharger's 2nd speed is variable and has infinite number of speed gradually increased with altitude by a hydraulic clutch, rather than fixed ratio gears, hence the supercharger always delivers roughly the same amount of air corresponding to the engine's needs. There's a slight overpressure which is throttled but otherwise you do not get anywhere near to huge engine outpit hit between supercharger speeds.

 

Another smart solution for improving supercharger control effiency was the SZYDLOWSKI - PLANIOL supercharger, which used tilting vanes on fixed supercharger speed, however it had the limitation that it was not very suitable for very high tip speeds. It was a French design - but I believe the Jumo 213 in our D-9 follows a similar idea. Basically as I understand as opposed to the DB design (which controlled air pressure with the driving the supercharger itself at optimum speeds), the S-P supercharger effectively had an air throttle before the supercharger itself, thus preventing too much air being entering into the supercharger in the first place.

 

The two stage Merlin (and most engines in WW2) in contrast just lets in as much air as possible into the supercharger (which runs at either of the two fixed supercharging speeds, so in effect its only operates optimally at two fixed altitude points), cools the supercharged (and very much heated) air with an intercooler and then throttles the excess air . There is much unnecessary temperature rise with air and a lot of engine power or air 'wasted' in this kind system, hence the phenomenon you encountered.

On the Merlin the carburettor and throttle body are positioned before the supercharger, so below critical altitude the impeller is effectively creating a vacuum to suck air past the throttle butterfly. This is the cause of power loss at altitudes below the full throttle height- in fact as I understand it placing the throttle body after the supercharger is more efficient, but causes other problems.

 

The D-9's supercharger (and presumably the S-P supercharger it is similar to) uses variable inlet guide vanes, a concept later seen on compressors for jet engines. The vanes aren't a throttle (though the Jumo had one of those too) but instead aerodynamic devices that modify the airflow into the impeller, effectively changing the angle of attack of the impeller blades and thus the overall pressure ratio of the supercharger.

 

A little trivia: the variable speed drive of the DB600 series is also seen on some versions of the Allison V-1710. When it became obvious that the V-1710 wasn't going to be used only in turbocharged planes (as had originally been planned, hence the weak supercharger) Allison designed a bolt-on auxiliary supercharger driven by hydraulic clutch to bring altitude performance more in line with the Merlin. These engines powered the P-63.

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  • ED Team
On the Merlin the carburettor and throttle body are positioned before the supercharger, so below critical altitude the impeller is effectively creating a vacuum to suck air past the throttle butterfly. This is the cause of power loss at altitudes below the full throttle height- in fact as I understand it placing the throttle body after the supercharger is more efficient, but causes other problems.

 

The D-9's supercharger (and presumably the S-P supercharger it is similar to) uses variable inlet guide vanes, a concept later seen on compressors for jet engines. The vanes aren't a throttle (though the Jumo had one of those too) but instead aerodynamic devices that modify the airflow into the impeller, effectively changing the angle of attack of the impeller blades and thus the overall pressure ratio of the supercharger.

 

A little trivia: the variable speed drive of the DB600 series is also seen on some versions of the Allison V-1710. When it became obvious that the V-1710 wasn't going to be used only in turbocharged planes (as had originally been planned, hence the weak supercharger) Allison designed a bolt-on auxiliary supercharger driven by hydraulic clutch to bring altitude performance more in line with the Merlin. These engines powered the P-63.

 

It's not correct for Merlin. There is no significant difference between pre- or post-blower throttle, and, by the way, we are talking about full open throttle. Any blower and constant MP automatics has it's critical altitude where the throttle that was partly closed at lower altitudes opens fully. So, the MP begins to lower.

The R-R type regulator has a feature: it can not maintain accurately the MP if it is lower than maximal, so, for example, climbing at plus 8 boost you need to advance the throttle gradually to have it fully open at true critical altitude. Then, as the second speed is on, the throttle is to be retarded abit to return 8 lb.

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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It's not correct for Merlin. There is no significant difference between pre- or post-blower throttle, and, by the way, we are talking about full open throttle. Any blower and constant MP automatics has it's critical altitude where the throttle that was partly closed at lower altitudes opens fully. So, the MP begins to lower.

The R-R type regulator has a feature: it can not maintain accurately the MP if it is lower than maximal, so, for example, climbing at plus 8 boost you need to advance the throttle gradually to have it fully open at true critical altitude. Then, as the second speed is on, the throttle is to be retarded abit to return 8 lb.

What Kurfurst said suggested that the Merlin's throttle body was after the supercharger impeller- I corrected him, as any examination of the outside of a Merlin will reveal that the carburettor is positioned between the air intake on the aircraft's underside and the supercharger impeller. I agree that the differences between different throttle body locations are probably minimal- What I read was from a old Hot-rodder's engine tuning guide, and those guys aren't always scientific about what they do.

 

The feature of the R-R regulator you described is interesting. How much error is there? Would one notice a difference in engine power, or is the variance small enough for the pilot to ignore under standard use?

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What Kurfurst said suggested that the Merlin's throttle body was after the supercharger impeller- I corrected him, as any examination of the outside of a Merlin will reveal that the carburettor is positioned between the air intake on the aircraft's underside and the supercharger impeller. I agree that the differences between different throttle body locations are probably minimal- What I read was from a old Hot-rodder's engine tuning guide, and those guys aren't always scientific about what they do.

 

The feature of the R-R regulator you described is interesting. How much error is there? Would one notice a difference in engine power, or is the variance small enough for the pilot to ignore under standard use?

 

No, it was directly mentioned for, at least, P-51 in a number of manuals. It's not an error... to be accurate, it's a distinctive feature. As the throttle valve position was a sum of the lever displacement and the servo piston acting only for closing, any regulator setting for MP lower than max leads to not fully open valve position. It does not matter at low altitudes but can reduce critical altitude without advanc7ng throttle.


Edited by Yo-Yo

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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No, it was directly mentioned for, at least, P-51 in a number of manuals. It's not an error... to be accurate, it's a distinctive feature. As the throttle valve position was a sum of the lever displacement and the servo piston acting only for closing, any regulator setting for MP lower than max leads to not fully open valve position. It does not matter at low altitudes but can reduce critical altitude without advanc7ng throttle.

We appear to be having some issues understanding each other: by error I meant 'deviation from desired value' rather than 'software malfunction'. For example, a control system might have an error of 2%, in which case the output of the controller would deviate from the correct value by up to 2%.

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