Strain on an engine and RPMs and PROP PITCH

[Uriah]

I think I must have something confused.  I was thinking prop pitch would affect RPM.  But then I tought how could changing the angle of attact of the prop make the engine have more or less revolutions per minute.   And of course keeping the RPM high will burn out any engine.

[dburne]

I think I must have something confused.  I was thinking prop pitch would affect RPM.  But then I tought how could changing the angle of attact of the prop make the engine have more or less revolutions per minute.   And of course keeping the RPM high will burn out any engine.

 

What confuses me more is the low angle is called coarse, and the high fine - seems in my mind they would be just the opposite.

[andyw248]

Same as with any airfoil - increasing AoA will increase drag which will reduce speed, and vice versa. Also, lift of a prop blade translates to thrust for the plane..

[Requiem]

What confuses me more is the low angle is called coarse, and the high fine - seems in my mind they would be just the opposite.

No need to be confused, you're thinking correctly as they are opposite to how you wrote.

 

Coarse = higher blade angle, lower RPM, max airspeed

Fine = lower blade angle, higher RPM, max thrust

[Uriah]

lets say I had a prop with just two blades.  If those blades from leading edge to trailing edge were in the same direction as from noes of plane to tail - what would that be?  Not sure of the name but as SYN_Requiem describes it - it must be course.  I can image this is good for a dive as that would alow the air to pass through the prop from nose to tail with the least amount of resistance.  And if the blades were turned on edge so on one blade the edge would be down towrad the ground and trailing edge toward the sky  that would cause a much bigger bite out of the air but when diving faster that what the prop could speed you through the air then the nearly flat side of the blades are facing the wind head on and so acting as air breaks.

 

I like to think of it like my old jeep.  If I want to go up a steep hill I put the thing in low gear which from what SYN_Requiem said sounds like fine pitch.  And if I want to travel down the interstate through flat lands I would want to shift to High gear (I guess COURSE pitch).

 

But I am still confused how turning the pitch of the plade mades the engine pistons turn faster of slower.  Which by the way, maybe that is because I am asuming the shaft off the propeller is directly connected to the engine shaft.

[Flanker15]

If you have a course pitch, that is, a high angle of attack for the propeller air flow there's more drag on the propeller blade (it's called torque in this case but that's not important!) as the blade is effecting a greater volume of air.  This drag acts against the torque generated by the engine on the shaft, holding it back thus lowering the RPM. 

If you imagine an indestructible engine in a vacuum which can produce a maximum amount of torque on a prop since there's no drag it would have the highest possible RPM that engine could output on that throttle setting, fuel, pistons etc etc as you add drag to the prop the RPM is slowed down proportionally until the drag exceeds the torque of the engine which would stop it.

If we use your car analogy, you have a car running flat at a set throttle and gear it will produce a particular RPM, if you lift the drive wheels off the ground removing the force acting against the engine the RPM increases (fine pitch), drive up a hill and the force against the engine increases lowering the RPM (course pitch).

[=AH=_Sid_]

[dburne]

No need to be confused, you're thinking correctly as they are opposite to how you wrote.

 

Coarse = higher blade angle, lower RPM, max airspeed

Fine = lower blade angle, higher RPM, max thrust

 

Yikes, see I told ya I was confused !

 

Ok so when I am getting ready to takeoff, and I move the prop pitch lever all the way forward, I am setting it to Fine correct?

[DD_bongodriver]

Yes correct.

[=AH=_Sid_]

Only indirectly, your setting the RPM you want, the constant speed unit is setting fine to start with to achieve that, but as you takeoff and increase speed the constant speed unit will automatically adjust the pitch to maintain the desired RPM.

[dburne]

Yes correct.

 

Sweet thanks, I am getting there...

[Finkeren]

It has been said before, but it really is essential to keep in mind: In planes with a constant speed propeller, you do not control propeller pitch.

 

The automatic system constantly regulates the pitch to keep a constant RPM, which is what you're controling with the prop governor. The system doesn't make adjustments instantly tough, which is why rapid throttle movements could potentially cause the engine to stall or overrev, especially with early systems.

 

In the Bf 109 it was posible to override the automated system and control prop pitch directly, allowing a skillful pilot to achieve marginally better performance, but requiring constant adjustments during maneuvers. This system will be modelled in BoS, but is not implemented yet.

[DD_bongodriver]

In planes with a constant speed propeller, you do not control propeller pitch.

 

Yes but whichever way you look at it you are still controlling the system that governs the prop pitch and in high RPM range the governor is keeping the pitch in a fine range and vice versa, so it's a simplified way to explain the same process.

[Rama]

Yes but whichever way you look at it you are still controlling the system that governs the prop pitch and in high RPM range the governor is keeping the pitch in a fine range and vice versa, so it's a simplified way to explain the same process.

But then, with the same simplified way, since all the systems (or almost) will have an effect on the prop pitch (every modification of airspeed will modify prop pitch), then all levers, wheels and sticks in the cockpit should be named "pitch control"...

I agree with Finkeren, it's much more understandable to everybodyt (and especially to those not knowing about CSP, and there are a lot on all simulation forums) to talk about RPM control than about pitch control.

[DD_bongodriver]

controls are only named after their 'primary' function

[Rama]

And the "primary" fonction of the "prop pitch" lever, on a CSP, is to regulate the propeller rotation speed, not the prop pitch.

 

I'll stop there, but I think it's important to avoid confusing the simmers, when a real manual prop pitch control will be available on the Bf109, some will not understand that it will behave totally differently from the RPM control, if you name it the same way.

In the past years, It was quite painfull to explain that on forums... and the same questions allways come again.

[DD_bongodriver]

But then, with the same simplified way, since all the systems (or almost) will have an effect on the prop pitch (every modification of airspeed will modify prop pitch), then all levers, wheels and sticks in the cockpit should be named "pitch control"...

 

 

I was really responding to this little gem.

[Uriah]

And the "primary" fonction of the "prop pitch" lever, on a CSP, is to regulate the propeller rotation speed, not the prop pitch.

 

I'll stop there, but I think it's important to avoid confusing the simmers, when a real manual prop pitch control will be available on the Bf109, some will not understand that it will behave totally differently from the RPM control, if you name it the same way.

In the past years, It was quite painfull to explain that on forums... and the same questions allways come again.

Oh yeah man.  Now I am daised.  Guess I will just fly the planes.  But as far as actual pitch of the prop blade - from looking at the A2A vid post it seems COARSE is a higher angle of attach and good for fast speed until you are going so fast as in a dive you end up with an air break.  Fine pitch of the blade is a much smaller angle of attack and good for faster take off and at low speeds but at faster speeds it more quickly becomes an air break.  Thank God for true automatic change of the prop angle.  That is what the bf 109f has right?

[Finkeren]

It'll really be fascinating to learn proper prop pitch control in the 109. I never fully mastered it in any of the previous IL2 sims, but it sure felt like you were doing something dangerous. This is one of the few controls where mishandling it IRL can actually kill your engine more or less instantly.

[Wolger]

 

Variable Pitch Constant Speed Propeller - changing the pitching of propeller, higher angle of attack (coarse, minimum lever like in LaGG-3, lowering RPM, cruise, to maintain airspeed, dive, engine temp decrease), lower angle of attack (fine, max lever, increasing RPM, for combat, climb, takeoff/landing, engine temp increase).

 

Fixed 2-Pitch Propeller - only can set the propeller to either coarse or fine. Early Spitfires and Hurricanes only has non-variable, 2-pitch propeller before they were converted to De Havilland or Rotol propeller unit. Just like the fuel mixture in those planes and P-51, there's only dual input Auto Rich and Auto Lean while LaGG-3 has variable fuel mixture.

[BigMotor]

One important operational consideration from RL- 1. When reducing power (as in level-off after climb) always reduce Manifold Pressure first and then reduce RPM.  2. When increasing power (as in initiatiing a climb) always increase RPM first and then increase Manifold Pressure.

 

If done incorrectly, the engine will be severly damaged. Don't know if this is modeled in the sim.

[Requiem]

One important operational consideration from RL- 1. When reducing power (as in level-off after climb) always reduce Manifold Pressure first and then reduce RPM.  2. When increasing power (as in initiatiing a climb) always increase RPM first and then increase Manifold Pressure.

 

If done incorrectly, the engine will be severly damaged. Don't know if this is modeled in the sim.

I think I attempted to damage the engine by doing this and don't remember anything happening. It may not be modeled yet but I'll look for it when access re-opens.

Edited December 18, 2013 by SYN_Requiem

[Finkeren]

The wild swings in rpm is there, when you slam the throttle with a low rpm setting, but I don't think it can actually cause damage as of yet.