Help me understand P-51D propulsion - ED Forums
 


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Old 12-06-2018, 09:56 AM   #1
Cik
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Default Help me understand P-51D propulsion

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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Old 12-06-2018, 10:15 AM   #2
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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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Old 12-06-2018, 10:42 AM   #3
Vitormouraa
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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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Old 12-06-2018, 10:48 AM   #4
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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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Old 12-06-2018, 11:04 AM   #5
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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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Old 12-06-2018, 11:41 AM   #6
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Quote:
Originally Posted by Cik View Post
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.

Quote:
Originally Posted by Cik View Post
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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Old 12-06-2018, 08:07 PM   #7
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Quote:
Originally Posted by saburo_cz View Post
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

Last edited by voodooman; 12-06-2018 at 08:13 PM.
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Old 12-06-2018, 09:38 PM   #8
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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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Old 12-07-2018, 10:13 AM   #9
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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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Old 12-11-2018, 02:24 PM   #10
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Quote:
Originally Posted by saburo_cz View Post
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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