P-51 top speed not at 100% rpm

[CrazyGman]

So after trying to get the maximum top speed out of the Mustang I was told that I need to reduce the RPM to 84% (around 2600) rather then have it at 100% (3000)

 

and it indeeds makes the mustang faster. I know this must be due to drag, but can anyone explain how this works in relation to thrust and drag created by the change in pitch that the constant speed propeller creates at various RPM?

[ShamrockOneFive]

Not the only aircraft like that. The Yak-1B has a similar characteristic.

 

This is probably not the best explanation but my understanding is that at the highest speeds possible propeller efficiency begins to decline and so reducing RPM actually makes the propeller more efficient in terms of the thrust that it can produce. Increasing to maximum RPM at those speeds results in more drag and less top speed. My best stab at the understanding of that phenomenon.

[RedKestrel]

P47 is like this as well. At really high speeds the propeller governors can’t maintain the  blade angle at the optimum for best speed as propeller drag increases. By reducing RPM you coarsen the prop a tad and that produces less propeller drag which gives you better speed. However your acceleration and climb will be reduced.

[Cpt_Siddy]

Its like that with dives, too. At 100% rpm, the prop acts as an air brake when you reach critical speed. This is true with all props. 

 

Ideally, in level flight this is negligible effect,, maybe 5... 10 kph at most. But in shallow dives, the rate at which you accelerate becomes quite noticeable. 

Ofc, you will hit compressability easier, too. 

[Blackhawk_FR]

I'm surprised the best RPM setting for max speed is 2600 but why not. 

 

Interesting to know: at 61inch  (max combat power), reducing rpm increase your engine timer (for example, about 4min more with 2900 instead of 3000). At max MP, it won't work. 

Edited October 27, 2019 by F/JG300_Faucon

[56RAF_Roblex]

You are not just setting RPM,  you are setting prop pitch at the same time.   As you go to higher RPMs the prop is automatically adjusting to suit your chosen engine power and there can come a point where the prop pitch is now less than ideal for maximum speed.  It is not true for all planes and when it does happen it can only be true for certain altitudes eg the Yak is usually best flown at 2700 but below 1000m 2550 gives slightly more speed.

[[DBS]TH0R]

2700 RPM is the sweet spot for the Merlin, it is like that in every sim that I've flown it. 46" and 2700 RPM equates to cruise settings.

 

Have not tested it as far as highest speed goes, normally when I am chasing someone or need to climb after him - I will run 100% or 3000 RPM.

[EAF19_Marsh]

At higher RPM the blade pitch is probably too 'fine' (more vertical / high angle of attack) as the constant speed mechanism attempts to keep up the RPM; this - as noted above - is a draggier angle for aircraft at higher speed which normally requires a 'courser' blade angle (more horizontal, lower angle of attack) rotating more slowly. As with the hoary old car / gear example you can cruise faster in 5th gear at medium RPM than in 2nd at high RPM. Or to give a different (rough comparison), you often climb faster at a shallower angle and higher speed than a steeper angle at lower speed.

[CountZero]

8 hours ago, CrazyGman said:

So after trying to get the maximum top speed out of the Mustang I was told that I need to reduce the RPM to 84% (around 2600) rather then have it at 100% (3000)

 

and it indeeds makes the mustang faster. I know this must be due to drag, but can anyone explain how this works in relation to thrust and drag created by the change in pitch that the constant speed propeller creates at various RPM?

From what i could see its not one set rpm or % value (like 84% or 2600rpm), best setings depends heavy on altitude.

And how i see it work is you need to set prop % to few % higher then when emergancy techchat shows up, and best gain is at around 5000m, it works only with emergancy techchat on, you have no gains in speed on combat or continuous by reducing rpm.

Why it works like that i dont know.

[6./ZG26_Klaus_Mann]

The Propeller only has a Secondary Effect.

 

The main Reason is the Supercharger turning Slower at lower RPMs (with it being mechanically coupled to the Crankshaft) which allows for greater Throttle Opening. Centrifugal Superchargers eat a disproportional amount of Power when throttled (throttling being required to maintain the asked for MAP).

 

Since they deliver a linear increase in Boost with RPM, decreasing RPM requires larger Throttle Opening for the same Manifold Pressure thus despite the Manfold Pressure you may actually get more Power out of a lower revving Engine.

 

 

 

Edited October 27, 2019 by 6./ZG26_Klaus_Mann

[Cpt_Siddy]

17 minutes ago, 6./ZG26_Klaus_Mann said:

The Propeller only has a Secondary Effect.

 

The main Reason is the Supercharger turning Slower at lower RPMs (with it being mechanically coupled to the Crankshaft) which allows for greater Throttle Opening. Centrifugal Superchargers eat a disproportional amount of Power when throttled (throttling being required to maintain the asked for MAP).

 

Since they deliver a linear increase in Boost with RPM, decreasing RPM requires larger Throttle Opening for the same Manifold Pressure thus despite the Manfold Pressure you may actually get more Power out of a lower revving Engine.

 

 

 

 

Yeah, this is prolly closer to the truth in a level flight. Dives are different animal, because you assume speeds that are above what you can maintain by engine power alone. 

[JtD]

18 minutes ago, 6./ZG26_Klaus_Mann said:

The Propeller only has a Secondary Effect.

 

Not true, on a Merlin engine the major benefit comes from the increased propeller efficiency. The power difference of a Merlin at 2850 and 3000 rpm is marginal.

[Darkmouse]

As had been stated above, at 100% rpm the propellor is at 'fine' pitch and has a relatively low angle of attack relative the airflow. As your aircraft speed increases that angle of attack is progressively reduced to zero, where the prop is no longer able to take a 'bite' of air. In the diagram at the bottom of this post as the forward airspeed increases angle of attack decreases. 

 

In order to overcome this, you need to coarsen the pitch of the propellor blades in order to allow them to take a relatively bigger 'bite' of air. 

 

Before variable pitch propellors were invented, high speed racers (think Schneider trophy) had a fixed coarse pitch, giving them a good top speed because the prop could take a good bite of air. However, try pulling away in your car in 5th gear - you are asking your engine to work extremely hard and it bogs down. The same is true of a fixed coarse pitch blade - great at high speed, but it's asking an aweful lot of the engine to take that huge bite of air at low airspeeds. This is why the racers were seaplanes - it took a very long time for the speed to built to a point where the propellor wasn't putting a huge aerodynamic load on the engine - basically the prop was almost stalled until the aircraft got moving. That meant that they had very long takeoff runs, which meant that large sea tracts were a more practical proposition than an airfield. 

 

So, we know that coarse pitch is great for high speed, but terrible for quick power changes at low speed, due to the aerodynamic load it places on the engine - the answer is a variable pitch prop. In its simplest terms, this could be a two position prop, giving fine and coarse pitch - fine pitch, taking a smaller bite of air allowing greater acceleration and more effeciency at lower speed, and a coarse pitch allowing higher speed flight.  

 

In addition, any aerofoil, of which a propellor is one, will have a most efficient angle of attack, giving maximum lift for drag - refering back to the diagram, you will see that as speed changes, angle of attack on a fixed propellor changes - if you could somehow create a mechanism that constantly alters prop pitch to keep it at its most efficient angle of attack for any given speed, you have a much more efficient propellor. Hence traditional variable pitch props. 

Edited October 29, 2019 by Darkmouse
Spelling

[Lusekofte]

I always used prop pitch for how efficient the propellers is to grasp the air. 
climbing up to thinner air one have to make the propeller be more effective. In level flight , my understanding higher rpm is most ineffective. 
I look at max settings as a way to accelerate and climb. Level flight is more a balance between power and pitch. 
I never really thought of it in any other way. We learn something every day

[Voyager]

Here's a write-up on propeller efficiency.

 

http://www.epi-eng.com/propeller_technology/selecting_a_propeller.htm

 

Basically as people have pointed out above, as you get to the speed of sound, the propeller gets there first, and the tips get there even faster, so the higher the RPM, the faster the onset do wave drag at the propeller tips. Once that happens you're not getting any thrust out of it anymore, so backing off on the RPM gets you more thrust after wave drag losses than you lose by the power reduction. 

[Cpt_Siddy]

13 hours ago, JtD said:

 

Not true, on a Merlin engine the major benefit comes from the increased propeller efficiency. The power difference of a Merlin at 2850 and 3000 rpm is marginal.

 

On the engine output, maybe, but on the supercharger RPM it is substantial. This is, ofc altitude dependent, if you are at alt where you need to throttle your charger in order to maintain manifold, this can yield considerable savings in horsies. 

 

Prop inefficiencies kick in when the  prop pitch is too fine. That usually happens at speeds that are faster then what the plane can generate in a level flight. This is all highly situational and changes with altitude, ofc. 

 

At altitudes where supercharger is not throttled at all, all gains are from prop, on altitudes where lowering engine RPM opens throttle, you gain power from there too. 

[JtD]

1 hour ago, Cpt_Siddy said:

this can yield considerable savings in horsies

 

It doesn't on a Merlin which is reduced from 3000 to 2850. It's like 1% of brake horse power. Far more power is generated from more efficient combustion and cooler induction air, in region of 6%. All of this of course has to be weighed against the 5% reduced rpm, which in turn means you're taking about a ~2% net power increase. Which is not going to change much with changing altitude up to full throttle altitude.

 

2% extra power translate to roughly 0.7% speed increase, so like 2mph at 300mph IAS. In game, at around 16k, I'm gaining 9mph or 3% in a P-51. That's prop efficiency, just as it is in real life.

[Cpt_Siddy]

42 minutes ago, JtD said:

 

2% extra power translate to roughly 0.7% speed increase, so like 2mph at 300mph IAS. In game, at around 16k, 

 

Yeah, at 16k that's 99% prop efficiency, due to being at open throttle alt and speed of sound decreasing (hence different prop efficiency). 

 

At altitudes below 5000 feet its whole different story. This is multi variable problem with different solution depending on changing circumstances... like i pointed out in my post that you quoted.

 

The things that you need to consider are:

 

IAS - at certain speeds the incoming air just makes your prop in to air brake at finer pitch, this is factor in dives.  

Speed of sound - the higher you climb, the less efficient the prop becomes at higher RPM's, even if your IAS decreases, the blade tips can exceed the speed of sound. 

Throttling - more you have to throttle your supercharger, the more hp you waste to due to throttling losses. 

 

This is by no means easy problem to solve for at any given altitude and speed, so proclaiming it is X, when it is totally dependent on your local condition, is misleading. The OP did not state his initial conditions, so both you and Klaus can be right when you chose the right conditions. 

[Ehret]

Another effect in the plane like the P-51D (compressor is driven by a fix ratio of engine RPM) is that if you lower the RPM then you also lowering supercharger boost thus the manifold pressure governor may open throttle more to maintain the same MP (assuming you are at low enough altitude). Thanks to that there will be less excess heat from compression and more useful HP available. Radiators may also close by some amount reducing drag further.

 

One way to extract extra performance from engine if you have some extra MP control ability like the P-47D is to open the throttle fully and use combination of turbo-boost/RPM to get the desired MP. In the other game where the P-51D has the ram-air boost lever implemented you can do a similar thing.

 

There is funny trick with the in game's P-38J too - set RPM to about 50-60% range (the green range on the RPM dial) and open throttle 100% (and if temps are allowing set mixture to lean as well - the trick is awesome in winter maps). You will be amazed with the speed it gives and how much it extends the timer both at once...

 

IRL low RPM, high boost + lean mixture is inviting combination for a detonation, thought. However, that's ok in the game so there is no reason to not use it.

Edited October 28, 2019 by Ehret

[CountZero]

5 hours ago, Cpt_Siddy said:

 

Yeah, at 16k that's 99% prop efficiency, due to being at open throttle alt and speed of sound decreasing (hence different prop efficiency). 

 

At altitudes below 5000 feet its whole different story. This is multi variable problem with different solution depending on changing circumstances... like i pointed out in my post that you quoted.

 

The things that you need to consider are:

 

IAS - at certain speeds the incoming air just makes your prop in to air brake at finer pitch, this is factor in dives.  

Speed of sound - the higher you climb, the less efficient the prop becomes at higher RPM's, even if your IAS decreases, the blade tips can exceed the speed of sound. 

Throttling - more you have to throttle your supercharger, the more hp you waste to due to throttling losses. 

 

This is by no means easy problem to solve for at any given altitude and speed, so proclaiming it is X, when it is totally dependent on your local condition, is misleading. The OP did not state his initial conditions, so both you and Klaus can be right when you chose the right conditions. 

 

OP is talking about deck with P-51 and 150 octan fuel mod, and if you use 84% prop you will have best setings for max speed at that alt and that mod, you can do 624kmh IAS

but that setting only work on deck with 150 P-51, when you fly normal P-51 then best is 75%, and higher you go % is changing, so at 1km best is 81%, 2km 88%, at 3km 93%, 4km 100%, at 5km 83% and so on... but if your using 150 oct mod, then at 1km best is 89% at 2km 96% at 3km 100% and so on... rpm value is differant, its not like on P-47 where you should be at 2550rpm up to 7km to get best speed

Edited October 28, 2019 by 77.CountZero

[Stoopy]

Hah! I just thought of one more way to beat this subject into a senseless bloody unrecognizeable pulp with its internal organs oozing all over the sidewalk, check it out:

 

We wouldn't have to rehash this so much or use car transmission analogies etc. if the original name for the propeller hadn't gotten all mucked up and changed around from it's original, and far more accurate, name of "airscrew".  All of the above would all seem far more intuitive for most folks who are at least semi-handy around the house.

 

It's a screw.  So finer pitch = finer threads, easier to turn, doesn't go as far per turn. But easy to get started quickly (takeoff).

 

At cruise we want coarse threads which provide max travel distance out of each turn, once the aeroplane is moving along.   But not too coarse as to be unworkable or inefficient. 

 

To slow down on descent to landing, we want to throw her back into fine pitch, which presents more frontal area to the onrushing airstream than the aircrew can slice through in one revolution at zero angle of attack, meaning angle of attack is actually negative in that air is pressing against the front of the airscrew blade area, such that it acts as a brake to help slow the ship down.

 

A feathered airscrew is essentially a screw of max coarseness such that the threads are aligned as parallel as possible with the axial centerline of the screw.  Basically it's turned into a funky lookin', and fairly expensive, nail. Or should I say, it's now an airnail and no longer an airscrew. 

 

Taken literally, a propellor is just something that propels the aeroplane, so in that sense those newfangled jet engines I'm hearing about are also propellers.  With airscrews on the inside! 

 

If that doesn't makes sense, that the turny-thing at the front of the aeroplane is an air screw, and the aeroplane is also an air ship, but with screw(s) at the front rather than the rear where they belong, then I will gladly buy a round of refreshing sarsaparillas for all you hep cats down at the local juice joint.   

 

Edited October 28, 2019 by =[TIA]=Stoopy

[[APAF]VR_Spartan85]

^^^^^^I like this guy  

[JtD]

21 hours ago, Cpt_Siddy said:

Yeah, at 16k that's 99% prop efficiency, due to being at open throttle alt

 

No, it is far below full throttle altitude of F.S. gear, which I was in.

 

21 hours ago, Cpt_Siddy said:

At altitudes below 5000 feet its whole different story.

 

True, at sea level you basically don't gain any speed reducing from 3000 to 2850. That's because prop pitch isn't as fine to start with and less Mach effects, so hardly any gains here, and the basically nonexistent extra engine power has pretty much zero effect, as it does on all other altitudes.

 

21 hours ago, Cpt_Siddy said:

This is by no means easy problem to solve for at any given altitude and speed, so proclaiming it is X, when it is totally dependent on your local condition, is misleading. The OP did not state his initial conditions, so both you and Klaus can be right when you chose the right conditions.

 

You don't gain speed from extra engine power below full throttle altitude. That is not misleading, its a fact. Because there is no extra engine power to speak of with a P-51 when you reduce rpm.

[Cpt_Siddy]

12 minutes ago, JtD said:

 

You don't gain speed from extra engine power below full throttle altitude. That is not misleading, its a fact. Because there is no extra engine power to speak of with a P-51 when you reduce rpm.

 

You are absolutely right, there. There is, however, less throttling losses to said engine power when the blower runs at slower RPM's.

 

 

[JtD]

OK, let me be more specific, there's no extra brake horsepower to speak of with a P-51 when you reduce rpm. In case I'm still not clear, it means that there is no extra power to speak of arriving at the propeller. All extra losses due to supercharger, throttles and other things (like more fricition, higher revs on pumps and so on) included.

 

Still, the main difference between 2850 and 3000rpm beneficial for the 2850rpm is the more efficient combustion, not the throttle or the supercharger. A Merlin fed by an external supply of supercharged air produces 1 - 1.5% less power at 2850rpm than at 3000rpm, depending on boost. And that's about what you lose feeding the extra supercharging at 3000rpm, for an about zero difference. Plus you gain a percent or two from cooler induction air, for a tiny net advantage. I've stated that already, but it's apparently better to ignore things than to deal with them.

 

Read up on the subject (i.e. P-51, Merlin, a Hamilton standard with a 1:0.477 reduction gear ratio) and you can stop turning a specific issue into a general debate. There have been tests on that subject. Loads. Individual components, and various packages. No need for assumptions.

[Cpt_Siddy]

Can you point me to your sources? 

 

I find it hard to believe that sea level 3000rpm throttling losses were negligible, mainly due the fact that Russians managed to save ~100hp in throttling losses with more efficient throttle design. 

[[APAF]VR_Spartan85]

I gotta contribute some how...

um,  counter weights, oil in, fine.

 

 

ok that’s all I got.

 

You guys are way too impressive arguing you’re points here.  And kinda envious of your knowledge...

 

i think it would be awesome if you had a virtual race to show your reasons

 

would be sick if we had a Reno air race style pylons to place in game

[JtD]

On 10/29/2019 at 6:38 AM, Cpt_Siddy said:

Can you point me to your sources?

 

No, not completely. I looked into this many years ago and simply don't have everything available. But, as an easily available source, you can take a look at the below HP chart of a V-1650-3. Unfortunately this particular chart only goes up to 15lb boost, but it's still valid. It contains lines of power at constant boost, of power at constant rpm and lines of what comes out after you regulate them. If you want to know the power output at a paticular altitude, rpm and boost, regulated by automatic boost control, you need to draw a line from the full throttle altitude parallel to the nearest regulated line. I've added two lines, red one 3000rpm, 62" boost, down from 11500' and yellow one, 2850rpm, 62" boost, down from 10000'. Both parallel to the nearby 60.5" boost line. At sea level, power output at 3000rpm is very slightly higher than at 2850 rpm, 1420hp vs. 1410hp.

 

If you were to do the same thing with a 44" boost line, you'd find power output at 3000rpm less than at 2850rpm, as a matter of fact not more than also available at 2500rpm (draw a line parallel to 44.2" boost regulator line).

 

There are more curves like this around, and while this at 62" boost shows a tiny bit more power available at 3000rpm than at 2850rpm, I ended up my research years ago with a minimum of extra power at 67" when changing from 3000 to 2850 at sea level. I can't show you the source, because I can't find it. But I hope we can agree that either way, 10 hp give or take don't really make a difference.

 

 

Edit: I'd really would have liked to get some feedback on this post. Does it help, are things still unclear? Thought there was at least one person interested enough to justify the time I put into it. Lesson learned. Thanks.

Edited November 4, 2019 by JtD