Why reduce RPM?

[sandpatch]

So I am trying to not break the engine of the P51. 

I'm doing quite ok now but reading the forum, everyone mentions too much pressure with too little RPM. 

Does it damage the engine to just leave it at 3000 rpm?

[davidrbarnette]

35 minutes ago, sandpatch said:

So I am trying to not break the engine of the P51. 

I'm doing quite ok now but reading the forum, everyone mentions too much pressure with too little RPM. 

Does it damage the engine to just leave it at 3000 rpm?

I may not be directly answering your question, but maybe the answer helps anyway:

Engine management in the P-51 is easy if you adhere to the following:

- Full RPM (3000) and manifold pressure on takeoff

- Other than takeoff, use 2700 rpm & 46 inches manifold pressure as a maximum for continuous use, including climb power

- When in a level cruise, put the rpm and the manifold pressure at the top of the "green arc" in each gauge

- When engaging / engaged, use full rpm & manifold pressure and keep an eye on the temps. The manual lists no more than 15 minutes at this setting.

[Nealius]

1 hour ago, sandpatch said:

Does it damage the engine to just leave it at 3000 rpm?

No but you'll suck more gas. Imagine rowing a boat. When you slap that water with the oar you take a bigger "bite" out of the water but you need more strength to push the oar through that water. If you angle the oar so it slices into the water a bit, you don't need as much strength to push the oar through the water. Kinda similar with props. At 3,000rpm the blade's pitch is taking a bigger bite out of the air but also inducing more drag, requiring more power to drive it, thus burning more gas, when you could achieve the same speed at a lower manifold pressure if your RPM were lower, thus saving gas.

At least that's how I think of it.  

Edited February 12 by Nealius

[Ercoupe]

if you run any engine at too high an rpm at a long length of time, of course you are going to do damage. Most of the maximum top speeds you read of on these airplanes were only for a short period. A quick burst of speed to catch up to an enemy aircraft, or to run from one. If you did have to go past the wire the engine was usually replaced when you got back to the field. Try sitting in your car for a half hour with the engine running, and your gas pedal floored. See how that works out.

[DD_Fenrir]

RPM and fuel efficiency are directly proportional.

More RPM = more fuel usage.

More RPM also = greater stress on the engine.

Anytime you wish to save gas, or the motor reduce RPM.

Buut remember that there are limits to how far open the throttle can be at lower RPMs; go too low with the RPM and don't reduce your throttle with it and you'll suffer from detonation or 'knock' which will also kill the motor.

[Slippa]

Good analogy Nealius .

[sandpatch]

30 minutes ago, Ercoupe said:

if you run any engine at too high an rpm at a long length of time, of course you are going to do damage. Most of the maximum top speeds you read of on these airplanes were only for a short period. A quick burst of speed to catch up to an enemy aircraft, or to run from one. If you did have to go past the wire the engine was usually replaced when you got back to the field. Try sitting in your car for a half hour with the engine running, and your gas pedal floored. See how that works out.

Well I am not keeping ot floored. 3000 RPM is just above 2700. You car will be perfectly fine at when going 10% above cruising RPM. Going past the wire of course will not be good, but I never do that. 

58 minutes ago, davidrbarnette said:

I may not be directly answering your question, but maybe the answer helps anyway:

Engine management in the P-51 is easy if you adhere to the following:

- Full RPM (3000) and manifold pressure on takeoff

- Other than takeoff, use 2700 rpm & 46 inches manifold pressure as a maximum for continuous use, including climb power

- When in a level cruise, put the rpm and the manifold pressure at the top of the "green arc" in each gauge

- When engaging / engaged, use full rpm & manifold pressure and keep an eye on the temps. The manual lists no more than 15 minutes at this setting.

That is the data I am following. It is displayed in the cockpit on the right side. 

48 minutes ago, Nealius said:

No but you'll suck more gas. Imagine rowing a boat. When you slap that water with the oar you take a bigger "bite" out of the water but you need more strength to push the oar through that water. If you angle the oar so it slices into the water a bit, you don't need as much strength to push the oar through the water. Kinda similar with props. At 3,000rpm the blade's pitch is taking a bigger bite out of the air but also inducing more drag, requiring more power to drive it, thus burning more gas, when you could achieve the same speed at a lower manifold pressure if your RPM were lower, thus saving gas.

At least that's how I think of it.  

 

Well, I understand what you mean. Except this is a constant speed prop, so higher RPM will actually reduce the bite from the engine. 

But that's a technicality. If fuel saving is the reason, then not really a problem in DCS.

[sandpatch]

22 minutes ago, DD_Fenrir said:

RPM and fuel efficiency are directly proportional.

More RPM = more fuel usage.

More RPM also = greater stress on the engine.

Anytime you wish to save gas, or the motor reduce RPM.

Buut remember that there are limits to how far open the throttle can be at lower RPMs; go too low with the RPM and don't reduce your throttle with it and you'll suffer from detonation or 'knock' which will also kill the motor.

Fuel usage isn't a big problem when fighting, unless I am doing CAP or something similar. So I see no reason to lower the RPM, unless I know it a chill phase of flight. 

How does greater RPM stress the engine? Is 10% higher than max cont. really such an extreme difference?

[Insonia]

In a constant-speed prop system. Increase throttle with low RPM setting, which increases MP and BMEP(Brake mean effective pressure). At a low attitude, the air and oxygen are abundant. Just like your piston engine car. If you put too much air and fuel into the chamber per stroke, detonation , boom. your engine trashed.

Increased RPM via prop handle reduces MP and BMEP pressure. It allows you to deliver needed power spread over more cycles. reducing the risk of detonation. 

The risk of *excess RPM is heat and wear. High RPM increases friction coefficient, fuel burn, and both increase temperature. has a risk of pre-ignition. as long as your cooling is okay and you do not excessively abuse it for too long. You can get away with it. 

When it gets too hot, pre-ignition and even detonation happen as well.

Edited February 12 by Insonia

[grafspee]

2 hours ago, Ercoupe said:

if you run any engine at too high an rpm at a long length of time, of course you are going to do damage. Most of the maximum top speeds you read of on these airplanes were only for a short period. A quick burst of speed to catch up to an enemy aircraft, or to run from one. If you did have to go past the wire the engine was usually replaced when you got back to the field. Try sitting in your car for a half hour with the engine running, and your gas pedal floored. See how that works out.

Running engine at 3000rpm won't damage it, it will increase engine weare and shorten engine life time but it will not damage it.

And P-51 manual states that engine goes for full tear down inspection after 5 hours of accumulated WEP usage so NO, engines weren't replaced after single use of WEP. You could probably could go for more then hour of continuous wep before engine would give up. That would shorten engine life time to couple hours but life time of p51 engine was aimed for couple hundreds hours so equivalent time limitations were used

Edited February 12 by grafspee

[Gunfreak]

18 minutes ago, grafspee said:

Running engine at 3000rpm won't damage it, it will increase engine weare and shorten engine life time but it will not damage it.

And P-51 manual states that engine goes for full tear down inspection after 5 hours of accumulated WEP usage so NO, engines weren't replaced after single use of WEP. You could probably could go for more then hour of continuous wep before engine would give up. That would shorten engine life time to couple hours but life time of p51 engine was aimed for couple hundreds hours so equivalent time limitations were used

 

Yet in every warbird in every simulator, WEP or even just slightly to high engine settings for slightly too long. Will brick the aircraft and lead to sitting out the war in a POW camp.

If real ww2 engines really were that sensitive. I'd expect to read far more instances of bricked engines. Yet I don't think I've seen one reference to an engine breaking form to much high power use.

I've read many instances of engines not working right, usually early after take off and leads to turning around and landing. But it seems real pilots in combat basically flew WEP from start of fight until end. However long that fight was.

[grafspee]

@Gunfreak This is why i countless times bring this issue on this forum since i joined it. I just can't imagine situation when after let say 15 minute of intense combat pilot had to drop power leave battle and his friends.

Famous WEP issue in DCS P-51 starting since i joined here with hard coded script which kills engine once you use WEP, i remember ppl were recording this famous loud bang after they landed and shut off engine. I don't know if this issue is fixed or not but DCS successfully discouraged me from ever breaking WEP wire in P-51, i just don't do that so i don't get irritated by engine seizing  

I can bring another over sensitivity The famous P-47's main engine bearing, which in first introduction was insane sensitive. Till now it isn't fixed completely because engine gives up way before 3060rpm red line which according to manuals this 3060 rpm red line is cleared for up to 30s. In DCS you pass 2850 or so and engine is toasted.

In arcade games those hard limits are introduced to prevent players from firewalling engines because in gaming you don't care if your engine will last 400 hours, 1 to 2 hours is the max and every time you take fresh plane, ofc there are players who are trying to go back to base refuel and take off again as many times as they can but this arcadish modeling prevent them from succeeding in this endeavor. I'm not sure if those engines in DCS can survive 10 hours of life time.

In hardcore simulator like DCS i won't accept those arcade engine limiters. 

[Gunfreak]

44 minutes ago, grafspee said:

@Gunfreak This is why i countless times bring this issue on this forum since i joined it. I just can't imagine situation when after let say 15 minute of intense combat pilot had to drop power leave battle and his friends.

Famous WEP issue in DCS P-51 starting since i joined here with hard coded script which kills engine once you use WEP, i remember ppl were recording this famous loud bang after they landed and shut off engine. I don't know if this issue is fixed or not but DCS successfully discouraged me from ever breaking WEP wire in P-51, i just don't do that so i don't get irritated by engine seizing  

I can bring another over sensitivity The famous P-47's main engine bearing, which in first introduction was insane sensitive. Till now it isn't fixed completely because engine gives up way before 3060rpm red line which according to manuals this 3060 rpm red line is cleared for up to 30s. In DCS you pass 2850 or so and engine is toasted.

In arcade games those hard limits are introduced to prevent players from firewalling engines because in gaming you don't care if your engine will last 400 hours, 1 to 2 hours is the max and every time you take fresh plane, ofc there are players who are trying to go back to base refuel and take off again as many times as they can but this arcadish modeling prevent them from succeeding in this endeavor. I'm not sure if those engines in DCS can survive 10 hours of life time.

In hardcore simulator like DCS i won't accept those arcade engine limiters. 

I've had my P47 engine die with water injection, because I dived a little too far down and the MP shot up for minute or so and didn't pay attention.

I imagine if you're a 21 year old man and you are chasing a 190 or have a 109 om your ass. Your not really thinking about "oh I should lover the MP or my engine is toast in seconds "

Another example not from DCS, but have similar feel. You can break the engine of your Camel by pointing your nose down for literally seconds as you try and get the nose on an enemy 100 feet below you. A few seconds past max RPM that engine is dead.

Now compare that to the description Arthur Lee gives in his book No Parachute, he is jumped by several huns. He puts the nose down and full throttle for several minutes as he outrun the enemy.

Full throttle several minutes in a long dive vs full throttle a few seconds as you dip your nose down...

I just can't believe anyone could fight with so delicate equipment be it ww1 or ww2. I think very few plots would get home if their engines were that delicate.

Mabye if the F4 wear and tear can be transferred to warbirds. Mabye they'd give us more realistic engine damage.

[=475FG= Dawger]

6 hours ago, Nealius said:

No but you'll suck more gas. Imagine rowing a boat. When you slap that water with the oar you take a bigger "bite" out of the water but you need more strength to push the oar through that water. If you angle the oar so it slices into the water a bit, you don't need as much strength to push the oar through the water. Kinda similar with props. At 3,000rpm the blade's pitch is taking a bigger bite out of the air but also inducing more drag, requiring more power to drive it, thus burning more gas, when you could achieve the same speed at a lower manifold pressure if your RPM were lower, thus saving gas.

At least that's how I think of it.  

 

This is incorrect. 
 

The propeller used to also be called an airscrew. 
 

This is a useful analogy. 
 

Higher RPM means lower blade angle or, in screw terms, a finer thread pitch ( threads closer together) Fine Pitch, a term you will see in some aircraft manuals  

The screw is taking a smaller bite with each rotation with fine pitch  

Lower RPM means higher blade angle or a coarser thread pitch ( threads further apart)

The screw is taking a bigger bite with every rotation in coarse pitch  

Manifold pressure is the torque or force you are applying to the screw. MP is your screwdriver. 
 

A fine thread screw with threads closer together is easier to drive than a coarse thread screw. 
 

So, higher RPM ( ) is always going to be easier on your engine. 
 

The situation to avoid is coarse pitch and high torque so you want to avoid high MP and low RPM.

3000 RPM should not be a factor in engine wear, at least not enough to ever be noticeable in less than a thousand or more hours of operation  

There could be some loss of efficiency at 3000 RPM as maximum RPM is determined by the RPM that cause the propeller tips to exceed the speed of sound  

This is the primary reason there is an RPM reduction after takeoff  

Hopefully, this makes the relationship between propeller RPM and Manifold Pressure slightly less murky  

https://skybrary.aero/articles/blade-pitch#:~:text="Fine" pitch refers to a,and fuel economy (cruise).
 

 

[grafspee]

Engine RPM affects peak working pressure and how long it act on piston.

Fuel has effective octane rating this is why WW2 fuel are rated 100/130 or 100/145 first number is effective octane rating for lean mixture and second is for rich mixture.

Higher peak pressure combine with longer piston action (in terms of time = Lower RPM ) results that engine makes more out of air-fuel mixture charge.

Higher RPM = more cycles per time = more power / Higher RPM lowers peak working pressure so engine can be operated safely at higher manifold pressure 

Lower RPM = higher working pressure and expansion time = higher efficiency / Lower RPM increase working pressure so engine can not be operated safely  at  high manifold pressure / High MP at low rpm will result in working pressure exceeding effective octane rating parameters of the fuel resulting in detonations and quick engine death.

RPM is only related to engine, optimal Prop RPM is set by engine to prop reduction gear, if engine could operate at 5000 rpm prop rpm would be the same because optimal prop rpm depends on prop diameter.

Edited February 12 by grafspee

[Nealius]

What causes the induced drag when going to high pitch settings? I frequently go into full pitch to act as a speedbrake when rejoining on AI formations or dumping excess speed in the downwind when I've fudged my overhead break.

[grafspee]

8 hours ago, Nealius said:

What causes the induced drag when going to high pitch settings? I frequently go into full pitch to act as a speedbrake when rejoining on AI formations or dumping excess speed in the downwind when I've fudged my overhead break.

Do you really think that reducing rpm increase drag so plane slows down faster?

It maybe shock you but going high pitch or feather position (which is the highest pitch you can set) reduce drag and make that plane can glide much further.

Going in to low pitch (rpm lever full forward) prop generates the highest drag possible thus slowing plane down much faster. 

Higher windmill rpm = higher power required = higher kinetic energy drain simple as that.

Edited February 13 by grafspee

[some1]

Just to get everyone on the same page, as I have some doubts if you all talk about the same thing.

http://essentialpilot.co.za/wp-content/uploads/2019/02/f0268-01.gif

low pitch = fine pitch. That's high rpm setting. Also higher drag if the engine is windmilling.

high pitch = coarse pitch. That's low rpm setting. Also lower drag if the engine is windmilling.

 

9 hours ago, =475FG= Dawger said:

There could be some loss of efficiency at 3000 RPM as maximum RPM is determined by the RPM that cause the propeller tips to exceed the speed of sound  

This is the primary reason there is an RPM reduction after takeoff  

Maximum RPM is determined only by engine redline. The propeller is driven by reduction gear, which in case of Mustang/Spitfire keeps prop RPM below 1500 when the engine does 3000. If the engine spun faster, or the prop was bigger, they would simply install larger reduction gear. The engine redline is usually determined by the engine ability to handle inertia forces of its pistons and valves. But as you've said, the engine is not supposed to immediately disintegrate from running at rated max rpm.

High RPM low throttle setting is the worst for economy not because of the propeller, but, among other things, because the fixed engine driven supercharger robs the engine of a few hundred HP at such speed, while at the same time the supercharger output is immediately wasted because of closed throttle. It's burning extra fuel for nothing.

[=475FG= Dawger]

2 hours ago, some1 said:

 

Maximum RPM is determined only by engine redline. The propeller is driven by reduction gear, which in case of Mustang/Spitfire keeps prop RPM below 1500 when the engine does 3000. If the engine spun faster, or the prop was bigger, they would simply install larger reduction gear. The engine redline is usually determined by the engine ability to handle inertia forces of its pistons and valves. But as you've said, the engine is not supposed to immediately disintegrate from running at rated max rpm.

 

One of the problems with discussions like this is the inability to simplify for the purposes of understanding.
 

Yes, the engine is rated for a maximum RPM for its intended application and then a reduction gearbox is engineered to meet the requirements of the propeller in order to keep it from exceeding the speed of sound at the tips. 
 

From the pilot’s point of view, the important information is the number (3000 in this case) and anything faster will greatly reduce propeller efficiency. 
 

That is a simplification that serves to aid in understanding the concept and it is the one widely used in the real world. 
 

Is it good to understand the engineering behind this? Probably. 
 

However, repeating over and over that RPM is driven by the engine limits and propeller gearboxes are used to slow the prop down only serves to muddy basic understanding. 
 

We see the direct result of this in how piston props get modeled in games, with many putting the engine on a self destruct timer any time the propeller is above the maximum continuous rating, which is ridiculous. 

[some1]

1 hour ago, =475FG= Dawger said:

 

However, repeating over and over that RPM is driven by the engine limits and propeller gearboxes are used to slow the prop down only serves to muddy basic understanding. 

As far as muddying basic understanding, it's you who brought propeller tips exceeding the speed of sound into this discussion. Despite what you wrote, this is not the reason for RPM reduction after takeoff, or for the continuous RPM limit lower than MAX.

Propeller going supersonic may be a factor in high speed flight at altitude, but not during slow climb in a dense air near the ground.

Edited February 13 by some1

[grafspee]

I doubt that tip of the prop exceeds speed of sound ever. Tip velocity may go close to speed of sound. 

P-51 manual also states that in case of prop governor failure pilot can operate engine up to 3250rpm even at those rpm tip isn't supersonic.

[some1]

It can be at high altitude near published top speeds. The prop speed through the air is the sum of its rotational and forward movement.

[grafspee]

Maybe during high alt high speed dives, but for level flight can't be, going supersonic instantly kills thrust generated by this part of the blade and if prop reduction gear got small adjustment top speed would increase significantly.

[kablamoman]

A while back I had been curious about high-speed performance and how it related to engine RPM settings, and I actually ended up running the numbers for the Mustang prop tip speed just to see if it could play a role in reduced prop efficiency at high speed. That is to say, I was wondering if I could get better performance by reducing engine RPM slightly in max level flight or a high-speed dive.  This was based on the reduction gear outputting 1437 prop RPM at 3000 engine RPM (0.479:1), and a 134" propeller diameter:

At sea level and roughly max level speed (355 MPH or 308.5 KTAS), it came out to around 0.88 mach for the prop tip speed. For that same TAS at 35,000', the tips would be at 1.02 mach.

0.88 is starting to get to the point at which a conventional propeller design starts to bump up against inefficiencies due to wave drag/supersonic flow over the thicker portions of the prop blades -- it is my understanding that these start to creep in and become increasingly worse anywhere from mach .84 to .88 depending on prop design (more modern designs with thin, swept blade tips, try to delay this onset as much as possible). It doesn't seem like a coincidence that the Mustang has been tuned to operate very close to this threshold at the top end of its speed range.

All of this to suggest that wave drag and tip speed problems with efficiency should only really factor in at the higher end of the speed range, and more so at higher altitudes. Whether DCS models this and to what extent, I am not sure -- but it shouldn't be a factor low to the ground at lower speed (eg. takeoff, climb).

@Nealius, with regards to windmilling, and how the mechanism by which this creates drag in the absence of power works, this video sums it up nicely (see time 12:10): 

 

It could be thought of as a kind of auto-rotation that creates lift, but in the direction opposing the aircraft's normal thrust line because the blade ends up flying with a negative angle of attack -- almost as if it were a reverse autogyro. That entire video is pretty well done and worth a watch.

The prop and all of its widgets and doodads can be thought of mechanism for transferring energy between the engine and the relative airflow. It's important to remember that it can go both ways: In the absence of engine power, it'll start to extract energy from the relative airflow in the form of drag.

With a constant speed prop, to think of the prop lever as something you use to select a blade pitch directly is wrong, as the actual blade angle/pitch will depend on airspeed and engine power output (both MP & RPM) and will vary dynamically in response to those things changing -- instead, think of the lever as something you use to select the rate of energy transfer between the engine and the airflow. You choose the direction of that transfer with the throttle. For example, if you want max power to accelerate (transferring from engine to airflow), push both levers forward. If you want to extract the most energy from the airflow in the form of drag, push the prop lever forward, but pull the throttle to idle. If you wanted to minimize that transfer at idle throttle to reduce drag from the airflow, pull the prop back all the way as well. With this analogy you can also see how high engine MP settings with a low transfer rate (reduced RPM) might be problematic -- the energy the engine is generating has nowhere to go, as its transfer to the airflow is inhibited, so you end up with an excess of potentially damaging heat and pressure.

Edited March 2 by kablamoman

[Nealius]

On 2/13/2024 at 1:31 PM, grafspee said:

Do you really think that reducing rpm increase drag so plane slows down faster?

It happens in DCS. As stated, I do it frequently and watch my airspeed needle plummet faster than it would if I had not increased the prop pitch. If that's not supposed to happen, then what's the explanation for higher prop pitch causing a dump of airspeed in DCS? It's clearly observable in all the Allied warbirds. I haven't messed with manual pitch on the Axis warbirds so can't comment on those.

  

15 hours ago, kablamoman said:

with regards to windmilling, and how the mechanism by which this creates drag in the absence of power works, this video sums it up nicely (see time 12:10): 

So by going fine pitch at a low power setting, I'm inducing windmilling and that's what causes the speed dump?

Edited February 14 by Nealius