How does RPM affect fuel consumption?(JoeBlogs, Dtango)

[F4UDOA]

Heya's,

Not sure how you guy's are determining miles. But his is how I conducted my original test that promted me to start the thread.

Offline fuel multipier set to 10X

F4U-1A 25% fuel tested for max speed before running out of fuel as well as highest alt reached before bingo fuel

Normal power with full RPM

44"MAP 2700 RPM =2.19min duration top speed acheived 316MPH auto climb reached 4K

Normal power with appropriate RPM

44"MAP 2700RPM= 227Min duration top speed acheived 311MPH Auto climb reached 4K

Mil power full RPM

54"MAP 2700RPM= 2.10 duration top speed achieved 341MPH auto climb reached 5K

Mil power reduced RPM

54"MAP 2550RPM= 2.10 duration top speed 332MPH and auto climb= 4.5K

Max cruise

34"MAP 2150RPM= 5.24 duration top speed reahed 287MPH auto climb 5.5K.

Seems to me

1. That lower RPM has little or no effect on duration or range.

2. Lowering MAP has the most effect.

3. Lowering from Mil to Normal had very little effect.

4. Lowering from Mil to max cruise more than doubled range and endurance.

As a note I took all power settings from the flight manual. However these power setting require an Auto lean mixture to achieve the desired results in Max cruise. There is no fuel consumption listed if you were to run auto lean in Normal or Mil power hence the huge difference in duration.

[Swoop]

Do one more test at 34" MAN and full RPM.


Oh and I was measuring distance only (via sector lines) and not time or speed, the object of my tests was simply to find best settings for maximum range.

[joeblogs]

Sorry guys, did not see this post today as I am snowed under at work.

It will be interesting to go over all these test you guys have done to see what it is that AH models in terms of engine performance and fuel consumption.  I can tell you from some experiments I did on the FW190A-8 that both Manifold pressure and RPM matter in terms of endurance.  I can't tell you in what proportions they matter.

The easiest way I can think about this stuff is to think about a piston engine like it was an air pump with a little gas added into the mixture.

Holding mixture constant for a moment, MAP tells you something about the amount of air being sucked into the engine on every stroke.  If it's below 1 atmosphere the throtle is starving the engine of some air, if it is above the supercharger is pushing a lot of air into the engine on each stroke.  With a constant fuel mixture, more air means more fuel.

RPM tells us the amount of cycles per minute, so holding MAP and fuel mixture constant,  more RPM means more air pumped through the engine per unit of time implies more fuel consumed.  

Sounds like MAP and RPM should work in the same way but the relationship is not actually that linear, primarily because more heat is generated, and must be dissipated at higher MAP and higher RPM.  This necessitates a rich fuel mixture which in itself dominates any other factor in terms of fuel economy.  On real planes with piston engines, the pilot is manageing 3 variables - RPM, manifold pressure, and fuel mixture and often does this using a fourth variable - cylinder head temperature.

So the best way to evaluate RPM and MAP is to explore how the engine performs at 65% power or below at a given altitude.  I see some of you have done just that.

On a lean fuel mixture, changes in RPM will have a greater effect on fuel economy if MAP is high and less if MAP is low.  Conversley, changes in MAP will have the largest effect on fuel economy when RPM is high than when RPM is low.

Here's an experiment.  Set your plane up in a cruise at modest manifold and RPM, but above a stall.  Increase RPM by several hundred and watch the manifold pressure.  It should fall.  If it doesn't something is not modeled correctly in AH.  The reason is that at higher RPM, the engine wants to suck more air per second, but a partially closed throttle limits the amount of air that can get through in any interval of time.

On planes, there is an additional variable which is that propeller's have an ideal RPM and angle of attack - i.e. are most efficient at a certain speed and angle.  So when you are thinking of holding the plane in the air (a minimum horsepower problem) or maximizing range (best speed/fuel economy) prop RPM has to be set with this in mind.  I have no idea if that is modeled in AH.

- More later

-blogs

[LLv34_Snefens]

"Here's an experiment. Set your plane up in a cruise at modest manifold and RPM, but above a stall. Increase RPM by several hundred and watch the manifold pressure. It should fall. If it doesn't something is not modeled correctly in AH. The reason is that at higher RPM, the engine wants to suck more air per second, but a partially closed throttle limits the amount of air that can get through in any interval of time."

This is NOT modelled in AH. In normal fligth you can adjust the one without it's affecting the other in any way. You DO see a bit of this at "extreme" (not really right word) situations:
When your speed is 0 you can't increase RPM without opening the throttle some.
RPM might also go beyond "max" in a very highspeed dive, but maybe that's just something I've imagined after too much FB.

[flakbait]

Does RPM have any effect on range? Yes, it does. What that effect gives you isn't much though. By reducing RPM in any prop plane, you also lower the fuel flow because the fuel pump is geared off the engine. Lower engine revs = less gas roasted. The big way to increase range is to run LOP (lean-of-peak). You end up burning something like one-third less fuel, run with cooler CHT (cylinder head temps), and make 8-hour hops possible.

If you've got FS2k2, or some other sim with a flow gauge, you can see the reduction in fuel flow by running any plane up to 6k and pulling off RPMs. It isn't much of a difference, but the difference is there.



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Flakbait [Delta6]
Delta Six's Flight School
Put the P-61B in Aces High

[joeblogs]

Fuel pumps are not geared to an engine.  Most are electric.  Fuel pumps are designed to deliver a certain amount of pressure on the fuel line.  

What determines fuel consumption is one or more mechanical valves in the carburator.  

By WWII, you set a fuel mixture and the amount of fuel consumed then depended on the rate at which air moved through the engine.  That was determined by manifold pressure and engine RPM.

You are right about the addage of running lean of peak.  What that means is that you set fuel mixture so that cylinder head temperature is a certain margin away from the peak that can be attained by altering fuel mixture alone.

-Blogs

Originally posted by flakbait
Does RPM have any effect on range? Yes, it does. What that effect gives you isn't much though. By reducing RPM in any prop plane, you also lower the fuel flow because the fuel pump is geared off the engine. Lower engine revs = less gas roasted. The big way to increase range is to run LOP (lean-of-peak). You end up burning something like one-third less fuel, run with cooler CHT (cylinder head temps), and make 8-hour hops possible.

If you've got FS2k2, or some other sim with a flow gauge, you can see the reduction in fuel flow by running any plane up to 6k and pulling off RPMs. It isn't much of a difference, but the difference is there.



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Flakbait [Delta6]
Delta Six's Flight School
Put the P-61B in Aces High

[flakbait]

Actually, fuel pumps are geared off the engine, though not directly. And in hindsight it was bad word choice on my part. What I meant was the fuel pump is geared off the engine in that by lowering engine revs you lower the fuel burned. There isn't much of a real diff by doing it that way though.

In the FS2k2 tests I ran in the B58 Baron and in a Cessna 182, dropping RMP from 2700 to 2200 only reduced flow by, on average, 4 gallons an hour. Considering a Cessna running ROP (rick-of-peak, the wrong way) torches off 17 galls an hour, it's not much of a drop. Running lean of the peak EGT (exhaust gas temp) saved, besides the cylinder heads, around 8 gal/hr. My personal best in the Baron test was leaning back to 13 gallons an hour for both engines combined. Roughly half the normal flow rate of 24+ an hour.

In WW2 AC, at least of US make, putting the mixture lever in Auto-Lean automatically dropped the engine into LOP. Reducing engine revs lowered engine wear and reduced fuel burn by a bit more. Foriegn AC usually had either a manual mixture knob (like a Cessna) or something similar to the auto-lean/rich lever of US planes.

If you want to read about how running LOP really works in real AC, along with turbo operation, check out this guy's babbling.....

http://www.avweb.com/news/columns/182146-1.html



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Flakbait [Delta6]
Delta Six's Flight School
Put the P-61B in Aces High

[frank3]

Originally posted by F4UDOA
Is reduced RPM supposed to increase or decrease fuel consumption? Or not have any affect at all?

I think when the RPM gets higher the fuel consumption will be higher too.
but the fuel consumption had much to do with the mixture, the mixture is pretty much the same as a gearbox on a car.
The mixture controls the angle of the props so they can work more effecient at high speeds or lower ones.

for example: If you have a low airspeed, you want the props to have the biggest angle possible (mixture is rich) so they can chobb trough a maximum ammount of air.

I hope you're satisfied with this.

Frank

[LLv34_Snefens]

Ehh, mixture has not anything to do with what angle the propellers are at.
I'm no expert, but I believe mixture describe the ratio between fuel and air in the combustion.

[dtango]

Hi Guys:

Just saw this post.  Things are really complicated around all this.  

F4UDOA - I think the answer is that it depends as you can see by what blogs and flakbait have pointed out.  As Joe Blogs says we've got 3 variables to consider - only 2 of which we have any control over in AH- 1) MP(throttle setting), 2) RPM (prop pitch), 3)Fuel Mixture.  From what I can gather throttle setting and fuel mixture has the majority of the impact on fuel economy while RPM has a lesser impact.

The aerodynamic effects of RPM/prop pitch are modelled in AH.

Remember also that endurance and range are two different things as well.  Flying at max endurance will not give you max range.

Tango, XO
412th FS Braunco Mustangs

[bj229r]

I was about to say earlier...but saw this addressed toward the end...in AH...ya can only mess with RPM or manifold pressure...in reading Bob Johnson's P47 bio....he endlessly fiddled with rich/lean mixture for mileage (can that be considered same as manifold prssure?) also...when changin RPM's ya would want to change prop pitch...I assume pitch in AH is merged into RPM adjust? EVERY plane in WW2 except hurricane had pitch adjustment..makes heap much difference at alt

[joeblogs]

The better analogy to cars is that the throttle is the gas pedal and the RPM setting is the gear box.  Mixture, if it varies at all depends on springs in the carburator or an electronic ignition system.

There is no relationship between fuel mixture and propeller pitch.  

Most planes in AH have some modeling for fuel mixture as fuel consumption will about double once you cross 60% to 70% of rated power.  Of course we don't have a power meter, we infer it from manifold pressure...

-blogs

Originally posted by frank3
I think when the RPM gets higher the fuel consumption will be higher too.
but the fuel consumption had much to do with the mixture, the mixture is pretty much the same as a gearbox on a car.
The mixture controls the angle of the props so they can work more effecient at high speeds or lower ones.

for example: If you have a low airspeed, you want the props to have the biggest angle possible (mixture is rich) so they can chobb trough a maximum ammount of air.

I hope you're satisfied with this.

Frank

[frank3]

joeblogs,

hmm, could be right, but I thought it had something to do with the mixture, since you can get a better acceleration with it I thought

well, I just opened up a new subject

[flakbait]

Uhh, no frank. Mixture just sets what the fuel/air mix is. If you want to go faster or accelerate quick, you go to full rich, max RPM, full manifold pressure.  A good read on mixture...

http://www.avweb.com/news/columns/182084-1.html

A quick note on mixture:
As your altitude increases your mixture must be leaned out. Why? Fuel/air mix. If you leave the mixture control in full rich and climb to 10k (sans turbo anyway) there isn't enough air to burn off all that gas. Too lean to burn. Add in a turbo and you get full sea-level air density clear up to 20k. So you really don't have to fiddle with the mixture a whole lot. The automatic systems (in nearly every WW2 US AC) worked by sensing what the altitude pressure was, and setting the mixture automatically based on that. While modern four-bangers (like HiTech's RV-8) have to fiddle with mixture, anyone in a P-38/47/51 had to do was drop it into Auto-Lean and leave it.

As for prop control, yes it is modeled. Lower engine RPMs mean a much sharper pitch on the prop blades. At high altitude, an RPM setting of 2,700 would let you go, but not fast. The air is too thin for that "fast" of a blade pitch. The higher you go the lower you need the RPMs to be, so the prop can take a nice-sized bite of the thinner air.

I'm running a test in FS2k2 at the moment regarding props and mixture on a Mooney Bravo with a turbo system. I'll post the results shortly.

-=Edit=-

Test results:

35" MAP used (max 40") for test. Pressure 29.92, 54º F sea-level temp, no wind.

5k: 2200 RPM, rich, 15.84 GPH, 151.7 knots
5k: 2200 RPM, lean, 12.1 GPH, 154.4 knots
12k: 2200 RPM, lean, 9.93 GPH, 132.5 knots
12k: 2500 RPM, lean, 10.49 GPH, 138 knots
15k: 2500 RPM, lean, 14.83 GPH, 156.5 knots, TAS 189 knots
15k: 2200 RPM, lean, 13.27 GPH, 148.4 knots
15k: 2500 RPM, very lean, 8.68 GPH, 122.3 knots, TAS 151 knots


Rich fuel burn for the Bravo is 18-22 GPH producing 270 HP. The mixture had to be set manually, but otherwise the AC did fine when I left it alone. 2200 RPM checks were for a comparison between cutting RPM and leaving it at max. Max (2500) produced higher speeds, but also burned a touch more gas. Leaning it to 14 GPH or so, then pulling the RPM back ended up giving the best range/speed combo. Fuel burn could've been lowered further if I pulled MAP back to around 30". The only diff between operating the Bravo's engine, and running a Packard-Merlin, is the auto-mixture function on the Merlin.

The turbocharger doesn't really cut in until around 9k or so. Below that there's a gap where pulling RPM is the only way to run it lean enough to fly a long ways. Above 9k the turbo really kicked in and I started testing at full RPM. In a real cruise situation you'd probably lean it to 14 GPH, pull the prop back to 2300 RPM, and reduce MAP to around 30". Doing this at 15k would let you go a long ways! The Bravo has an 89 gal fuel capacity, with 84 being useable. 84 gallons at 8.68 GPH, 151 knots TAS, would let you fly 1464.7 nauticle miles in 9 hours and 40 some-odd minutes.



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Flakbait [Delta6]
Delta Six's Flight School
Put the P-61B in Aces High

[frank3]

uh...I don't intend to read the whole topic Flakbait, but since there's so much I assume you're right