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Question about RPM management


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Hello,
since i'm trying to get better at dogfighting i'm also now trying to get a better and more intuitiv understanding of engine management.

i know how to set up rpm and throttle (and boost) in all flight regimes for all warbirds that i own (p-51, p-47, bf-109), however i have some questions.

1.(a) i always thought of the variable pitch props as analogues to a car's gearbox, where you would exchange torque for speed, but now i think that this is not entirely correct, is it?

1. (b) intuitively i pull back rpm a little bit when i'm near max speed, but i never scientificially tested i'f that's really faster. is there an aerodynamic performance advantage of a prop spinning slower comapred to one spinning at max rpm (you have that at ships, where propellers loose effectivness above a certain rpm, but those are fixed pitch of course...)

1. (c) if there is not, would that mean that max rpm is always the fastest AND best for acceleration, if you do not consider engine wear and fuel consumption?

2. if you run prop RPM too high, this will damage the engine, the prop or both? i always think of "overspeeding the prop", but it's actually more like over-reving the engine, correct?

3. coming back to the car's gearbox analogy. is it correct to say that rpm mangement is more about striking a balance between over-pressuring the engine and over-reving the engine?

4. are there relevant fuel-saving aspects to rpm mangement?

 

Thanks 🙂

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1.a You're right, it's not exactly like that since governor's job it to keep a constant rpm in the gauge, so no gear change at all, on the contrary constant rpm even though at a constant rpm you can get different power outputs due to manifold pressure changes. That's the weirdness of constant speed props when you fly them at first, you get different power outputs but rpm are constant and hence sound pitch is constant either.

1.b In a constant speed propeller there's no difference theoretically, governor keeps the optimal pitch for you and you don't need to care about that since you have no direct control of pitch itself but an rpm selector in your hand.

1.c In a constant speed propeller, yes, it's supposed to be the best, even though theoretically you could squeeze some extra mile per hour out of a manual pitch, but since you're flying not optimally all the time with a variable pitch propeller I'm not so sure how much in a simulator a difference would be. That anyway is something to test if we get some day a manual variable pitch propeller aircraft available and modelled to DCS standards, otherwise pointless at all. Yes, max rpm are the fastest you can get in P-51, the most optimal setting certainly.

2. Prop overrev is only a thing when your governor gets hit (quite easy on P-51), but generally speaking overreving in the P-51 is not a thing, it won't happen in most cases, governor does a pretty good job. Anyhow what gets damaged is the engine itself out of shear temperature, the problem with piston engines is always temperature and cooling, if you manage to keep it cool you'll be good to go almost all the time you want to, if you don't you'll kill the engine fast as hell.

3. Not exactly. Max rpm are tested by engine manufacturer, if they tell you it's safe to run 27-46 forever it's because they've tested and they know it's safe. Usually what kills an engine with a constant speed prop is overboosting since governor will manage to keep the rpm selected but pressure inside the engine isn't constant, that kills an engine relatively easily, but it's not like you can't overboost even an inch over your rpm "limit", you can and it's also a long term thing, indeed the problem with overboosting should be more for the ground crews after you land rather than while flying most of the time, so mostly not a thing in a simulation since you don't see the engine leaking oil and having to be overhauled with new cylinder heads and seals. Overreving isn't a thing as said, governor will take care of that, overboosting on the contrary is easily performed by the careless pilot. The balance you look for are the engine settings at the manual, a rev selection is like a stop you should never go beyond in order to keep things cool (no pun intended, but double meaning here indeed), so you already know, 27-46, you never should go far beyond 46 inches manifold pressure with a 2700rpm setting, 24-36, same, etc. 3000 rpm is your no limit for manifold, but then you have to keep an eye on anemometer even before temps, as long as you keep speed up it'll be fine so that's why you can kill the engine in combat, speed isn't constant and refrigeration either while you're getting the most out of the engine, that's where the engine dies.

4. Yes, there are. The thing with P-51 is it boasts a so good range you could fly around almost all of the maps we have available and not having a problem with fuel (unlike Spitfire, or 109/190 with way smaller fuel tanks). You can indeed get an even more conservative setting that would allow you to fly all day long if you want, those are usually low rpm settings and "high" (relatively high) manifold and you can squeeze lots of miles, useless miles since you can't go Berlin and back in the maps we have, but it's there and it actually works. It does work either for Spitfire indeed. I wouldn't remember actual numbers you have to run for those settings in P-51 but they should be somewhere, and no those aren't specified in the manuals, those appeared out of field tests by pilots. There's a story indeed about a guy "stealing" a P-51B, with full fuel and all, and he made it to Norway and back only to probe their bosses it works. Japanese pilots with Zeros knew those settings indeed and that was the "secret" for a6m amazing ranges, not any other.

5. All I said here could be mostly the very same in any engine management, I mention P-51 since that's the one you ask for, but the kind of management explained here (numbers aside) could be for any piston engine aircraft sporting a constant speed prop.


Edited by Ala13_ManOWar
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"I went into the British Army believing that if you want peace you must prepare for war. I believe now that if you prepare for war, you get war."

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Variable constant speed propeller has couple jobs main jobs.

1. First noticeable improvement is at take off, it allow to start with very low pitch at initial roll and with higher pitch at lift off with constant optimal engine rpm. This improve take off performance a lot, you should experiment with P-47 which has fixed pitch mode, set minimum pitch and try to take off you will be amazed how quickly you have to retard throttle to avoid engine over rev.

Early spitfires has fixed pitch prop with 2 positions low pitch and high pitch. Pitch was set optimal for take off but whole take off not initial run, so first pilot pushed the throttle but engine could not get to max rpm which was 2800 rpm, so boost for initial roll was limited due to low rpm, when plane got some speed pilot could advance throttle more but after take off when plane got some speed he had to throttle back or switch prop mode. Pilots later found out that they could set intermediate position between low pitch and high pitch setting.

2. It allow plane to be operated at wide range of speeds

3. Is related to variable speed, this allow fuel consumption reduction and increase engine life time.

Running engine at high rpm low MP expose engine to high rotational forces which are not required for this MP.

Running engine at low rpm / high MP induce very high peak pressure during combustion which can cause detonations to happen, this kills engine from inside so no indication from oil or coolant temp until it is over. Ofc due to a lot of noises in cockpit knock/detonations are impossible to hear by the pilot.


Edited by grafspee
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To add what guys said above, popular comparison to manual car gearbox is a bit dangerous and misleading, 'cause it only applies 1:1 to our 109, 190 and P-47 when they're switched to manual prop mode (the only mode in which you control blade pitch directly). In all other cases governors do the physical blade adjustment, not the pilot, who is rather rpm- or power-manager.

If we wanted to compare constant speed props to cars, I guess the only cars that fit would be old DAF variomatics, which could accelerate/decelerate/drive at different speeds while maintaining constant engine rpm.

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i've got yet another question. maybe you guys can help me with this one also, or tipp me in teh right direction... @Ala13_ManOWar@grafspee@Art-J

i believe that i've got a decent understanding of how an engine in a car behaves concerning throttle, torque and power, but i'm not 100% there with our beloved warbirds.

 

what does actually happen in the engine if i open up the throttle in a constant speed prop plane? and what happens if i then increase the rpm?

so openign the throttle would allow the engine more air and through the carburator it would also get more fuel, so i increase the amount of fuel/air mixture in the cylinder. so on ignition there is now a much bigger bang while the piston moves at the same speed as before. therefore there is a higher pressure in the cylinder (bad if too high) and also more torque on the crankshaft.
does that mean that manifold pressure is a measure of torque?

when i keep the throttle but now increase RPM, shouldn't the pressure go down equally? the throttle is the same, so the engine cannot get more air/fuel, but it has more cycles per second, so each ignition should result in a smaller bang. so torque would be lower, but because of the higher rpm power output should - in theory - be similar.
this is however not what one sees in the sim, so the relation between throttle and rpm must be more complex than i imagine it to bee. can anyone help?

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It is more complex, for example lets take P51.

P51 has automatic boost which start working at MP higher then 41 or 42 inch of MP. Out side of this range when you increase rpm less  air fuel goes to cylinders so you get less torque but the power remain about the same. And this is simulated in DCS. Set 30inch and start playin with rpm you will notice that MP goes down when you increase rpm and vice versa. Now things are quite difrent when MP is above 41 inch. From this point pilot do not have direct control over throttle valve. If you flying at 46inch oh MP and 2700rpm and you increase RPM boost unit will open throttle as well to maintain ammount of air fuel mix introduced to cylinders so torque is about the same but power goes up.

What i said is truth if power curve of the engine tops out at around 3000rpm, but things may have be different. Full Power chart for v-1650-7 is needed to determine it, could be possible that engine power tops out at 2000rpm and this additional 1000rpm above max power point is only to allow engine to take high MP like 67inch, then things will be quite different. Unfortunately we have only rudimentary power charts, so we know what power engine makes at 2700rpm and 46inch or 3000rpm

and 61inch but we don't know what power engine makes at 3000rpm and 46inch.

 


Edited by grafspee
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System specs: I7 14700KF, Gigabyte Z690 Aorus Elite, 64GB DDR4 3600MHz, Gigabyte RTX 4090,Win 11, 48" OLED LG TV + 42" LG LED monitor

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25 minutes ago, grafspee said:

It is more complex, for example lets take P51.

P51 has automatic boost which start working at MP higher then 41 or 42 inch of MP. Out side of this range when you increase rpm less  air fuel goes to cylinders so you get less torque but the power remain about the same. And this is simulated in DCS. Set 30inch and start playin with rpm you will notice that MP goes down when you increase rpm and vice versa. Now things are quite difrent when MP is above 41 inch. From this point pilot do not have direct control over throttle valve. If you flying at 46inch oh MP and 2700rpm and you increase RPM boost unit will open throttle as well to maintain ammount of air fuel mix introduced to cylinders so torque is about the same but power goes up.

ok. that makes totally sense. thanks.
but if you take away boost and other effects, it is - at least in theory - the way i described it, where more throttle means higher torque and higher rpm means less torque but roughly same power, right?

also does a set amount of manifold pressure always mean a similar amount of torque produced? so does taking-off with X units of manifold pressure means the engine is creating the same amount of torque as it does with the same manifold pressure at 20.000 feet?
does this then also mean that a set manifold pressure at a set RPM would always produce the same amount of power independent of altitude? so every warbird's engine can run at peak power independent of altitude as long it's intake/turbo setup can provide enough air?


Edited by twistking
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39 minutes ago, twistking said:

ok. that makes totally sense. thanks.
but if you take away boost and other effects, it is - at least in theory - the way i described it, where more throttle means higher torque and higher rpm means less torque but roughly same power, right?

also does a set amount of manifold pressure always mean a similar amount of torque produced? so does taking-off with X units of manifold pressure means the engine is creating the same amount of torque as it does with the same manifold pressure at 20.000 feet?
does this then also mean that a set manifold pressure at a set RPM would always produce the same amount of power independent of altitude? so every warbird's engine can run at peak power independent of altitude as long it's intake/turbo setup can provide enough air?

 

Ammount of MP does not mean same power. At SL P51 develops 1490hp but at critical alt for low blower power increase to 1590hp for 61inch and 3000rpm. At 20k ft it will make much less because at this alt high blower will be engaged and at 20k we can expect something like 1200hp ar 61/3000 don't remember exactly. Another thing we can't generalize here because diffent planes has diffent forced induction systems and those behave diffent.

Power/alt chart for Bf-109 looks different then for P-51 or P-47. P-51, Spitfire and Mosquito will have similar looking power charts only difference is in alt where supercharger speed changes and power outputs.


Edited by grafspee

System specs: I7 14700KF, Gigabyte Z690 Aorus Elite, 64GB DDR4 3600MHz, Gigabyte RTX 4090,Win 11, 48" OLED LG TV + 42" LG LED monitor

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24 minutes ago, grafspee said:

Ammount of MP does not mean same power. At SL P51 develops 1490hp but at critical alt for low blower power increase to 1590hp for 61inch and 3000rpm. At 20k ft it will make much less because at this alt high blower will be engaged and at 20k we can expect something like 1200hp ar 61/3000 don't remember exactly. Another thing we can't generalize here because diffent planes has diffent forced induction systems and those behave diffent.

Power/alt chart for Bf-109 looks different then for P-51 or P-47. P-51, Spitfire and Mosquito will have similar looking power charts only difference is in alt where supercharger speed changes and power outputs.

 

why though?
is it because manifold pressure does not account for temperature (turbo makes air hot, hotter air gives less oxygen molekules at same pressure) ? this is the only reason i can see why power can differ at altitude (when MP and RPM stay the same).

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17 minutes ago, twistking said:

why though?
is it because manifold pressure does not account for temperature (turbo makes air hot, hotter air gives less oxygen molekules at same pressure) ? this is the only reason i can see why power can differ at altitude (when MP and RPM stay the same).

Exactly MP is just absolute pressure in intake manifold. When you climb higher, air temp goes down so power goes up. I just realized with another thing when engine revs high supercharger revs high as well so if you uses low MP it is pointless to waste power on high revving superchargers. This eat engine power as well. So what happens with power if you move rpm from 2700 to 3000 at constant MP is even more complex. 

One thing is certain if your mp is no more then 46 you don't need more rpm then 2700. 

If you have time take p51 reach level top speed at 46/2700 and then increase to 3000 sea what happens with speed


Edited by grafspee
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System specs: I7 14700KF, Gigabyte Z690 Aorus Elite, 64GB DDR4 3600MHz, Gigabyte RTX 4090,Win 11, 48" OLED LG TV + 42" LG LED monitor

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10 minutes ago, grafspee said:

Exactly MP is just absolute pressure in intake manifold. When you climb higher, air temp goes down so power goes up. I just realized with another thing when engine revs high supercharger revs high as well so if you uses low MP it is pointless to waste power on high revving superchargers. This eat engine power as well. So what happens with power if you move rpm from 2700 to 3000 at constant rpm is even more complex. 

 

interesting. that should be the same with turbo-superchargers, right? (in the p-47 you can of course just manually hold back the turbo so you can easily avoid that wasted energy)

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4 minutes ago, twistking said:

interesting. that should be the same with turbo-superchargers, right? (in the p-47 you can of course just manually hold back the turbo so you can easily avoid that wasted energy)

In p47 you can adjust boost via turbo rpm not like in p51 via throttle valve so there is no waste at lower alt. From what i have seen p47 has flat power staring SL to very high alt. At 52/2700 sl-2000hp same at 10k and 15k and so on.


Edited by grafspee

System specs: I7 14700KF, Gigabyte Z690 Aorus Elite, 64GB DDR4 3600MHz, Gigabyte RTX 4090,Win 11, 48" OLED LG TV + 42" LG LED monitor

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