Does reducing RPM reduce fuel consumption or not?

[Swoop]

Done a BBS search and no-one seems to agree.  

Most (well ok, mainly Funked) say reducing RPM does nothing except slow you down on all planes except the P-51D.......and others saying they can fly clean across the map in a F6F at 40" MAN, 1600rpm.

[Strange]

As being one of the jug heads.. Pulling back on the manifold and rpm do cut back on fuel use.  Thus we can stay aloft for quite some time.

Hope this answers the question..

[AKDejaVu]

I have the most experience with it in an F6F.  It definately improves range in that plane.

AKDejaVu

[Mark Luper]

In short Swoop, yes it does

[Ripsnort]

I did some tests on the F4U awhile back, said it did, funked disagreed, my data showed otherwise. (Shrugs)

[RAS]

It makes a difference in the Yak-9T......reducing manifold from 40 to 30 increases time aloft from 27 min to 37 minutes.  Reducing manifold from 40 to 25 ( half throttle) increases time aloft from 27 min to 50 minutes.  These times assume manifold pressure setting for entire flight duration, but you can see that adjusting it DOES make a difference.  Can only assume (sorry) that it would make a difference in ALL planes, but can't say with certainty.

Hope this helps.....

RASCAL

[Rude]

It most certainly does in the Mustang

[Zippatuh]

Originally posted by Rude
It most certainly does in the Mustang

I agree.

An eighth of a tank will run for some time with the RPM’s knocked back and the manifold low.

[AtmkRstr]

The question was if RPM effects fuel consumption.

We all know reducing manifold presure effects fuel consumption.

Has anyone done fuel consumption tests where the only variable is the RPM setting, and the MAN is held constant?

[phaetn]

Well, by definition, doesn't the boost have to be reduced in conjunction with the prop pitch to achieve a savings in fuel economy?

I liken it to gears in a car...  the *only* measure of fuel flow is how far the accelerator is pressed down.  By shifting up gears, however, it is possible to maintain the same velocity as in lower gears while depressing the accelerator less, thereby achieving an economy of usage.

Is that a fair comparison with RPM and manifold pressure?

Cheers,
phaetn

[Ripsnort]

Originally posted by AtmkRstr
The question was if RPM effects fuel consumption.

We all know reducing manifold presure effects fuel consumption.

Has anyone done fuel consumption tests where the only variable is the RPM setting, and the MAN is held constant?

The test I did I ran manifold at a consistent 75%--- with 100% rpm, and 50% rpm as the test...the latter producing better milege.

[popeye]

"Well, by definition, doesn't the boost have to be reduced in conjunction with the prop pitch to achieve a savings in fuel economy?"

I read somewhere that Charles Lindberg worked out a "best range" protocol for the USAAF that used a high MP and low RPM.  The only NACA tests that I could find on the subject, seemed to agree.

[ergRTC]

That would make the most sense popeye.  As RPM is important in the number of times fuel gets injected into the cylinder per minute.  You could be running at 100% manifold, but half your fuel consumption by reducing your rpms 50%.  Problem comes in how much your reducing your progress across the map.  Even in a carburated engine you can floor it, but after the acclerator pump is empty, if the engine is not producing vacuum (ie running), then no fuel will leave the carb.


Now, whether this is modeled in the game, I do not know.  I have never seen a good response from htc on this, although hitech has responded before with some incomprensible jibberish.  A good old 'yes, it is just like it is in real life' or 'no, it wasnt worth modeling like rl' would be nice.  hint hint.

[Swoop]

OK, so it seems that everyone bar Funked says it does.  :-/

Lemme give a more exact question:   Saving fuel is not just what I'm looking for, I know the best settings for minimum fuel consumption.....engine off.  I want to know what the best settings are for every plane to obtain maximum distance covered.  

[Dinger]

Now hold on,
having done some experiments in the field, and having spoken with Funked, let me state:

As far as I recall, Funked's position is not that reducing RPM has no effect on fuel economy, rather that greater economy can be had by reducing throttle and keeping RPM at max, and this is true for most, but not all AH planes.

Now, here's how you do the tests:
A. Get a stopwatch (mine broke, so I'm out of the business)
B. Load up the dueling terrain, and go to the 15k field.
C. Pick your test plane, and load 25 % fuel.
D. Run tests at each throttle and RPM setting (I use 5 Man/2 Boost increments and 500 RPMs) that the plane will fly at.


Test Run:
   1. Set FuelBurnRate to minimum (.001 or whatever it is).
   2. Take off, and make a shallow dive to 10,000 feet, accelerating to what you think is going to be the speed at alt.
   3. Level off at 10k
   4. Set Throttle and RPM settings and let the airspeed stabilize
   5. Select the smallest fuel tank on board
   6. Set FuelBurnRate to maximum (10) and start your stopwatch
   7. Record the airspeed. (IAS and True)
   8. WHen the engine quits, record the time.

E. The numbers you want are:
   1. Best Airspeed (that one's easy: Wep and full rpm)
   2. Best Economy (highest time * airspeed)
   3. Best Loiter (highest time)
   4. Best Cruise -- as the aircraft approaches top speed, you'll see the fuel consumption skyrocket.  This should be "pretty fast" without drinking the gas.

I usually "zero" in on the best settings by using the initial test as  a bracket.



From these tests, it becomes clear that some planes, like the P51s, benefit from a reduction in RPM along with Manifold; others like the P51's merlin-packing cousin, the Spitfire, should be flow at full throttle all the time.

It's also clear that these tests take time, and few people have the patience to do it.