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Old 07-01-2019, 09:30 AM   #1
grafspee
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Default Engine preformance

I got couple concerns about p-51 engine preformance when i look at speed proformance charts for p-51 v-1650-7 hitc crit alt at around 26k ft
in dcs there is no way that you hit top speed at 26-27k ft you need to climb much higher to something around 33k ft which from the charts is completly unaccurate.
Any one have any thoughts about it ??
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Old 07-01-2019, 02:07 PM   #2
Ala13_ManOWar
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That depends on atmospheric conditions. 26-27K ft in the charts are most probably meaning ISA conditions or whatever standard they used back then. But that might mean you get that same performance either at 36K indicated in a higher pressure or 16k in a lower pressure due to density altitude.

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Old 07-01-2019, 03:32 PM   #3
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Quote:
Originally Posted by Ala13_ManOWar View Post
That depends on atmospheric conditions. 26-27K ft in the charts are most probably meaning ISA conditions or whatever standard they used back then. But that might mean you get that same performance either at 36K indicated in a higher pressure or 16k in a lower pressure due to density altitude.

S!
hmm i m not sure about it but still engine should develop max power at crit alt for its supercharger(unless the air temp starts droping that fast that it can compensate for lost of boost maby then but i dont know is it posisble ) and reach top speed here. i can assume that top speed can change depending on air temp at crit alt but i m not sure can it change tops speed alt. unless there is soem extream hot air at 27k and very very cold air at 36k than maby, but when temp drops consistenly through alt this should not change top speed alt.
it can change top speed for sure

Last edited by grafspee; 07-01-2019 at 03:38 PM.
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Old 07-01-2019, 06:16 PM   #4
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Quote:
Originally Posted by grafspee View Post
hmm i m not sure about it but still engine should develop max power at crit alt for its supercharger
And that's exactly what it does, just that critical altitude is relative due to density altitude so you read whatever in your altimeter (set by QNH or QFE obviously) but density altitude might be whatever else. Did you read DA article I posted before?



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Originally Posted by grafspee View Post
i can assume that top speed can change depending on air temp at crit alt but i m not sure can it change tops speed alt.
It's the other way around, top speed at optimal altitude doesn't change, what changes is that optimal altitude compared to your altimeter reading which is set to read AGL or ASL, but density altitude can be anything else.


I think you missed what DA means. For instance, I used to fly in an aerodrome not far away and here it's a 45ºC during summer place. That means, I set my altimeter to QNH and read the same as always, platform altitude ASL, but in that hot conditions the atmosphere even at ground level was equivalent to say 6000ft in an ISA conditions atmosphere. So DA was 6000ft, it is like you're flying at 6000ft at ground level. In Winters a C152 took off in 400m, but in summers per manual the C152 in that DA conditions needs 1200-1300m long to take off. The runway is 1000m so it were unable to take off in hotter hours.


Altimeter says what it says, altitude ASL or AGL depending on what you set, but real equivalent atmosphere due to DA is a very different story.


Putting that example in P-51, supercharger changes stage 2 at 19K climbing, and 14K descending IIRC manual. That means in (probably) ISA conditions, but in a different DA those can be 25K and 20K, or 17K and 12K. Top speed optimal altitude is the same, achieved at DA equivalent, not altimeter reading (which is mere distance to ground).



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Old 07-01-2019, 07:17 PM   #5
grafspee
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Quote:
Originally Posted by Ala13_ManOWar View Post
And that's exactly what it does, just that critical altitude is relative due to density altitude so you read whatever in your altimeter (set by QNH or QFE obviously) but density altitude might be whatever else. Did you read DA article I posted before?



It's the other way around, top speed at optimal altitude doesn't change, what changes is that optimal altitude compared to your altimeter reading which is set to read AGL or ASL, but density altitude can be anything else.


I think you missed what DA means. For instance, I used to fly in an aerodrome not far away and here it's a 45ºC during summer place. That means, I set my altimeter to QNH and read the same as always, platform altitude ASL, but in that hot conditions the atmosphere even at ground level was equivalent to say 6000ft in an ISA conditions atmosphere. So DA was 6000ft, it is like you're flying at 6000ft at ground level. In Winters a C152 took off in 400m, but in summers per manual the C152 in that DA conditions needs 1200-1300m long to take off. The runway is 1000m so it were unable to take off in hotter hours.


Altimeter says what it says, altitude ASL or AGL depending on what you set, but real equivalent atmosphere due to DA is a very different story.


Putting that example in P-51, supercharger changes stage 2 at 19K climbing, and 14K descending IIRC manual. That means in (probably) ISA conditions, but in a different DA those can be 25K and 20K, or 17K and 12K. Top speed optimal altitude is the same, achieved at DA equivalent, not altimeter reading (which is mere distance to ground).



S!
supercherger in p-51 will change speed at difrent alt there is automatic switch which will make it happen depends on couple variables so you can get switch to high blower maby at 12k up to 17k-18k ft. Ram air is factor too, whne you climb with low airspeed you will get switch earlier then climbing with high speed
i understand this idea but tests are performed at standard atm and standard air temp wich ir around 1013.25hPa and 15 celcius
and durign my tests in dcs supercharger hit crit alt at 26k ft so pressure and temp are quite close so why results are so difrent. BTW air temp plays no role in supercharger critical alt it will hit its crit alt no matter of air temp around is it be -20 or -40 doesnt matter but power of the engine will be quite difrent and i expect difrent top speeds

Last edited by grafspee; 07-01-2019 at 07:27 PM.
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Old 07-01-2019, 07:22 PM   #6
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Quote:
Originally Posted by grafspee View Post
i understand this idea but tests are performed at standard atm and standard air temp wich ir around 1013.25hPa and 15 celcius
and durign my tests in dcs supercharger hit crit alt at 26k ft so pressure and temp are quite close so why results are so difrent
Depending on the charts you're looking, those might be done in other conditions. ISA atmosphere wasn't standard since ever and they could had a different standard.



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