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Help me understand P-51D propulsion


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alright so, i think i understand the most basic principle at work here- the engine has a fuel reservoir - the engine draws it, provides mechanical power which it conveys somehow to the propeller - the propeller which is basically a sideways helicopter blade system yanks the plane through the air by varying the angles of it's airfoils.

 

 

 

so, the question: the P-51 seems to have a "chained" engine power/propeller RPM system that loosely increases/decreases together depending on your prop pitch / throttle setting. at 500 ASL~ if you push the throttle past 45" pressure your RPM will climb until it reaches it's max operating speed of 3000 propeller RPM. so, you can continue raising the throttle until 61" (or 67" WEP) the question is then: is there any actual benefit to raising the throttle higher than the throttle setting that allows 3000 RPM and if so, where is the thrust coming from if the propeller is staying at 3000 RPM? is there a prop pitch (and pursuant thrust increase) past 45" / 3000 RPM mark?

 

 

 

I've always assumed that running the engine higher produces more thrust (otherwise why takeoff at 61" MAP instead of 40" MAP?) but recently i've begun to wonder if there is any actual benefit.

 

 

 

interested in any answers, thanks.

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P-51 has a constant speed propeller. The system tries to maintain a set RPM, which is commanded by the pilot via a lever in the cockpit, by increasing or decreasing the propeller blade pitch angle.

 

The propeller RPM is limited to 3000, because past that the speed of the blade tip is approaching the speed of sound and the efficiency of the propeller starts to drop rapidly. Because the system is adjusting the angle of the blade, more power allows bigger angle to be set and that pushes more air past the propeller disk, which means more thrust. Even though the RPM of the propeller doesn't change.

 

All that means that the pilot doesn't need to know what pitch angle he needs to set the propeller to. All he cares are the RPM of the propeller and the engine manifold pressure, which he needs for managing the fuel economy and safe engine operation.

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Although the RPM remains constant (RPM which was set by the pilot), the pitch does not. As the RPM approaches the RPM set by the pilot, the governor changes the pitch (increasing the area, which also increases thrust and drag) in order to keep a constant RPM.

 

As to why pressure increases power, this has to do with the heat released by the combustion (oxygen and fuel), as you increase the pressure, the work output is increased, generally speaking. Hence the higher horsepower (work rate) created by the engine, so more pressure equals more horsepower available to the propeller.

 

As mentioned above, one of the problems with an unducted propeller is that as you increase the RPM, depending on diameter, the tip tends to approach the speed of sound, which is a bad thing, but that's not the only way to increase thrust, fortunately, instead of increasing the speed, you can increase how much air the propeller "grabs" by changing the pitch angle.

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If you fly with engine setting 45inHg/3000RPM, you will get nothing compared to 45/2700.

Only your engine will be warmer, but you will not fly faster...

 

Table with the most efficient engine settings (manifold pressure with proper RPM) is in manual, and the best is follow it.

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so what i'm hearing is that pitch continues to increase with throttle setting (towards max) IE thrust should continue to increase as throttle rises towards 61" MAP, makes sense to me.

 

 

it does say in the manual that WEP at sea level is useless (i guess that prop pitch is maxed at 61" MAP at ~sea level) is there a rule of thumb as to when 67" MAP becomes useful? how often do you guys use WEP in general?

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it does say in the manual that WEP at sea level is useless (i guess that prop pitch is maxed at 61" MAP at ~sea level)

That is not true, it is wrong understanding and rewriting from manual from 1952.

Where is written that military power (61inHG) gives to a pilot enough power to damage engine and this is why there is not reason to increase power (and mainly possibility to kill engine) using power above rated (means 67inHG WEP).

If you need extra power/speed, it is very usefull from 0m AGL up to critical altitude.

 

how often do you guys use WEP in general?

I personaly try to avoid it, because it kills* manifold pressure regulator.

 

*actualy not kills, only shifts up range where it works for 5inHG..

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I personaly try to avoid it, because it kills* manifold pressure regulator.

 

*actualy not kills, only shifts up range where it works for 5inHG..

 

 

Could you elaborate what you mean by "shifts up range where it works for 5inHG"?

Do you mean by just cutting the wire and going to wep something changes in the operation of the manifold pressure regulator?

 

 

I have had this strange feeling of, that 61" doesnt feel the same 61" after cutting the wire...but as you know, these "feelings" could be anything, like nonsense of imagination ;)


Edited by voodooman
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I mean for example,

 

if i fly at 100m above sea and set max. continuous setting 46inHg/2700RPM the manifold pressure regulator keeps 46inHg manifold pressure up to certain altitude without ANY touch of the throttle lever (in this case up to 1900m with 400km/h TAS).

 

Then, during this one flight, i use WEP, then descent, slow down and repeat procedure.

Again i set max. continuous engine setting and start climb. And what happen.

Manifold pressure starts nearly immediately falling down (in this case at 1900m above sea i had only 38inHg instead 46 before WEP, the same speed of course).

 

In past i had to set at least 47inHg to make the manifold pressure regulator working (41 is minimal before WEP engagement according to manual).

 

I hope it is understandable.

 

But now, it seems to not work anymore after WEP. So, probably no shift for 5inHg but total malfunction..

No have time for more tests now.

 

I tried report it in past, but with no effect...

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Thank you for your explanation. i allways felt that after cutting the wire it's not the same (engine behaviour) anymore.

 

 

Especially when cutting the wire in flight i have noticed this. I allways tought that it's just me imagining things, so partly due that feeling i have cut the wire on ground before starting the engine so it stays the same trough out the whole flight. Partly to remove the sudden "jerk" if you cut the wire in flight.

 

 

It would be interesting to see if there are difference to your findings when cutting the wire on ground before engine start. Sadly I dont have that much time in my hands right now, but will test that for sure when the opportunity presents it self (christmas holidays most likely).

 

 

Thanks again Saburo.

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If you fly with engine setting 45inHg/3000RPM, you will get nothing compared to 45/2700.

Only your engine will be warmer, but you will not fly faster...

 

Have you tried it? Same MAP with higher RPM should give you more power and thus more speed.

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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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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.

When you hit the wrong button on take-off

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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


Edited by grafspee

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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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yes i did, and flew slower with 3000rpm...

it is understandable

 

Okay, that's a statement :thumbup:

When you hit the wrong button on take-off

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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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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 :huh:

When you hit the wrong button on take-off

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Can't wait for that new AI damage model for this kind of thing to matter i m single player. Probably my #1 most wanted DCS feature.
Are you just going down the list of topics in the P-51 thread? haha

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In past i had to set at least 47inHg to make the manifold pressure regulator working (41 is minimal before WEP engagement according to manual).

 

That doesn't even make sense. The impeller is a mechanically driven two-speed supercharger. MAP above a selected power setting's critical altitude should only be dependent on ram air effect and engine RPM for the same altitude since the power required for actually turning that impeller gets deducted from the engine's gross hp output.

 

 

E: I did some testing just now. The reason you're encountering what at first seems to be a discrepancy is because once you engage WEP, the game redefines the throttle axis - what was the throttle position for 61" up to critical altitude just shy of the wire break will give you around 66.5" once you switch emergency power on. The throttle position for 61" at SL is now at around the 2/3 open position on the throttle.

 

As you climb, you can regulate the throttle to give you 61" up to and through the wire break, which will top out at around 11k ft, the same as with full throttle without switching over to emergency power. THe question I feel should be asked is whether the throttle behavior is an accurate representation, or if the last 6" of MAP were actually condensed down into what little additional travel the throttle could do past the wire break.

 

Funnily enough, I'm actually getting a higher critical altitude during climb at 67" than what this test says it should be - around 7900 ft compared to just shy of 5000 ft. That said, as the emergency power climb test was a standalone, it should be taken with a massive grain of salt. The ones where I was running the engine at 61" - once with and once without the wire cut - were done under the same meteorological conditions and can be compared with each other without issue.


Edited by antagonist
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