Help me understand P-51D propulsion - Page 2 - ED Forums
 


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Old 12-11-2018, 03:00 PM   #11
Vitormouraa
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Quote:
Originally Posted by sydost View Post
Have you tried it? Same MAP with higher RPM should give you more power and thus more speed.
More horsepower (work rate) doesn't necessarily mean a higher top speed. With a higher RPM, you have more induced drag. I have tested this myself, all I could see is a difference of 2, 3 km/h. So his argument is valid. A few km/h isn't going to save your life most of the time, but running a cooler engine sometimes will.
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Old 12-11-2018, 03:15 PM   #12
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If you are running the same MAP, won't the engine temperature be the same? You will be running more RPM's, but burning less per cycle. So actually burning the same amount of fuel over the same period of time.

Isn't the real question, at which RPM the propeller is most efficient/effective?

Also, the question of overall resistance of turning over the engine --the faster it is running, the more resistance to overcome.
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Old 12-11-2018, 03:23 PM   #13
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I guess it's around 2850, 2700 RPM. Just guessing though... That's a great question.
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Old 12-11-2018, 06:15 PM   #14
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46/2700 is max/climb power for p-51 while flying 46inch at 3000rpm you make your engine less efficient becouse v-1650-7 has gear driven supercharger(-7 has 2 stage supercharger but it doesnt matter principle is the same) which means that at higher rpm supercharger impeler spins faster that mean that supercharger want to make higher boost so boost pressure regulator have to throttle spurcharger more then at lower rpm that meant that supercharger eats more power form engine to drive it self. At 2700 /46 inch boost you actualy have more power on propeler then at 3000 rpm
and pitch of blade in constant speed prop depends not only from power settigns like rpm or boost but depends on velocity of plane faster you fly higher pitch of blade become at the same power settings
i dont know much about aerodynamics but i would gues that at high speeds and high power setting 3000rpm is the most efficient
so if you going for max speed 3000rpm is what you want.
Most fuel efficient setting is probably around 2400rpm
I know that reno racers are running about 3500rpm in modded p-51 and something around 150inHg of boost reaching speed above 500mph in level flight

Last edited by grafspee; 12-11-2018 at 06:27 PM.
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Old 12-11-2018, 06:24 PM   #15
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Quote:
Originally Posted by sydost View Post
Have you tried it? Same MAP with higher RPM should give you more power and thus more speed.
yes i did, and flew slower with 3000rpm...
it is understandable
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Old 12-12-2018, 07:06 AM   #16
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Quote:
Originally Posted by saburo_cz View Post
yes i did, and flew slower with 3000rpm...
it is understandable
Okay, that's a statement
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Old 12-12-2018, 09:46 PM   #17
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i've started running my engine up for a little while before taking off, and it's ceased seizing. even running WEP for reasonable periods of time (5-10 minutes on/off) it hasn't seized all week in probably ~10 hours of flying -51D.



additionally, it feels to me like altitude advantage means way more than throttle. i've just been dogfighting at takeoff (~61" MAP and 3k RPM) which seems a lot more manageable temp-wise when making high yoyo climbs. been having a lot of success recently, sometimes bagging several 109s per sortie.


best practice seems like: attack from high angle against unaware target. often can gain victory in single .25-.5 second gunburst (at certain ranges .50 seems like an instant death chainsaw) in if you are co-energy just dive out as it is unwinnable.



engine seems quite tameable, i think before it was high oil pressure causing the engine kills (from not warming oil up to 40C) once you do that it behaves like a dream, if you can accept that you are just fated to be a double inferior plane and recognize your limitations.
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Old 12-13-2018, 11:46 AM   #18
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The 109 only has armor for about 10-15° cone to the rear and front. Any attack from outside those cones are not protected by any armor, so any .50's hitting anything of any importance will do major damage, including to the pilot
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