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

[F4UDOA]

The subject pretty much says it all.

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

I did some testing and before I start spouting off I was just wondering what affect this should have.

[HoHun]

Hi F4UDOA,

>Is reduced RPM supposed to increase or decrease fuel consumption? Or not have any affect at all?

All else being equal, consumption is proportional to RPM :-) But that's probably not what you wanted to know!

From a British WW2 memorandum on engine handling:

"Use full throttle and minimum revs above full throttle height for any desired A.S.I. This gives the best combination of fast cruising and minimum consumption."

A positive side effect is mentioned, too:

"When you're travelling at full throttle, and full power is suddenly wanted, it is only necessary to push the constant speed lever fully forward to get full revs and boost. To return to high speed cruising at best economical conditions, reduce your revs and not your boost."

(From Price' "Spitfire")

By contrast, the US practice was to teach the pilots "matched" boost/revs settings to use. You may have heard of Lindbergh's visits to the PTO in order to instruct the USAAF pilots in efficient long-range flying - he basically taught the same techniques as the RAF.

This and some USN comments suggest that the "matched" settings were only meant as a learning aid for novice pilots, but of course the US institutions were turning out large numbers of these during WW2.

Regards,

Henning (HoHun)

[F4UDOA]

HoHun,

I was aware of the Lindberg method but I had not seen the Brit reccomendations.

Actually it is what I thought, however...

I just ran a series of test on the F4U-1 trying to figure out what the best consumption rate is using real Mil, Normal, and fast cruise sttings from the flight manual.

What I found is that with the exception of loosing performance reducing RPM has virtually no positive effect on fuel consumption in AH. I say virtually because there is no more than a fractional difference. However by reducing MAP you can achieve much greater flight duration. In fact at 34" MAP you can fly indefinitely on 25% fuel in the F4U-1 it seems.

I will post my results tommorrow. But you may want to check it out for yourself.

Thanks

[ChuBBs]

i seen a thing on history channel  and it said about p38s using like 24 RPM setting and 35 Man. Something like that. And they could go over twice the distance.  I've tried it an it worked.

[HoHun]

Hi F4UDOA,

>However by reducing MAP you can achieve much greater flight duration.

The concern in real life was range, and the power for maximum range would be higher than the power for maximum endurance. The same considerations with regard to boost and revs should still apply though :-)

Regards,

Henning (HoHun)

[F4UDOA]

I agree HoHun,

My point is however that lowering RPM in AH makes little or no difference in range or duration. Duration is the more important factor anyway because decreasing RPM reduces performance. Less speed and less time/duration at that speed means less range.

Basically I am saying this

Lower MAP= greater duration/range less performance

Lower RPM= lower performance with no gain in Range/duration

[FTJR]

If you check the P38L under the Help section it has some power settings i.e

 Normal crz  44"@ 2600 RPM = 113 gph consumption
 Max Range crz 35" @ 2300 RPM = 63 gph consumption.

Haven't flown it for a while, but the max range has you around 40 mph slower than the normal crz.

[F4UDOA]

FTJR,

I have the F4U and P-38 pilots manuals. That's where I'm getting my MAP and RPM settings. However the reduction of RPM in AH seems to have no real measurable effect of fuel consumption.

Hence my post.

[214thCavalier]

Hmm i seem to remember similar conversations in the past.
Anyway i am sure some are fed up of seeing this link but here it is again as tested in AH with an F6F fully loaded.

http://www.cavalier.cwc.net/dlfiles/f6f5fuel.htm

Compare charts 2, 3 and 4.

Same configuration used in climbout to 12k each test, then different cruise settings, best cruise speed for F6F factually was 168.

Whatever else you do keep the speed the same for all tests and juggle map and rpm.

It clearly shows rpm DOES affect fuel consumption in AH, but you MUST keep to the same constant airspeed in all tests for the test to have any validity.

However it also shows if you reduce rpm too much and push the map too high then consumption increases dramatically.

Personally i would call a worst of 160 to a best of 360 a considerable difference when flying at the same airspeed.

Worst was  Map 48 and rpm 950, second best was Map 32.5 and rpm 2000, however best was Map 35.5 and 1500 rpm.

Note an increase of Map from 32.5 to 35.5 BUT reducing rpm was more efficient to the tune of 68 miles extra range.

These tests were not done at MA fuel burn rates but for a special event using a different fuel modifier of 1.5.

However they are perfectly valid for this purpose.

Just checked F6F video at  Zenos they recommend 1500 rpm and 30 Map below 5000ft for minimum fuel consumption of approx 40 gals per hour.

They do not state which airspeed this will give and note thats for MINIMUM fuel consumption not cruise settings of 168 which i tested at.

But it does seem to confirm that AH is very close at least for the F6F.

[LLv34_Snefens]

Cav, you are changing RPM and MAP at the same time in those tests. How can you say that it's the RPM that's causing better range and not MAP?
How far does the F6F go in that configuration, with max RPM  and MAP of 35.5? I'll see if I can do the test when I get home.

[FTJR]

F4UDOA,


Oh.. well sorry about that. Was only pointing out the help section. I haven't really checked the consumption rates under different power settings. I just presumed if it is written it must be so (refering to the AH help section)

[214thCavalier]

Snef i actually tried out many more configurations including same Map of 35.5 and full rpm.
It only got worse, Those are the only ones i bothered to input the data for because they show the pertinent facts that at 950 rpm consumption is awful at 2000 rpm its much better but it has already gone past the sweet spot of 1500 rpm.

If you wish to duplicate the tests remember i ran them on a 1.5 fuel modifier.
I also discovered it was better to get to alt as fast as possible this included climbout on wep as the longer time you could then spend in cruise configuration more than made up the extra fuel burnt climbing on wep.

Map reductions do give you a much bigger saving than rpm alone, up to a point but notice a small Map increase of 3 from 32.5 to 35.5 but reducing the rpm by 500 actually gained 68 miles extra range.

So basically rules for best possible range are climb to cruise alt on wep, use the aircrafts historic cruise speed if possible and go for Map reductions first.

Also as these were tests of a fully loaded F6F to judge max operational range for a scenario in AH the mileage recorded is not the point at which fuel ran out but the range at which you would ditch, ie includes glide range.

These tests were done a long time ago so i dont think i still have the rest of the data but will have a look.

If you wanted the best range while still under power then it would have to be the test done at 6000ft which gave the same overall distance.

[LLv34_Snefens]

I am truly puzzled here.

I did the test you have in "chart 3", only with max rpm. I reached 250miles before engine cut after approx 60 min.

I then did testings with x30 mulitplier at SL.

#1: Full man, full RPM
#2: Full Man, reduced RPM
#3: Reduced man, full RPM
#4: Reduced Man, Reduced RPM

With the F6F I found that the first two burned fuel approx the same, while you got some increase in duration with reduced man, and even more if you also reduced RPM.
Now the reason this puzzled me is that I seemed to remember I had tested using reduced RPM and finding it had no effect.

So I took out the 109G6 and did the same 4 tests as above. Now the funny thing is that I found #1 and 2 to give same endurance and #3 and 4 also similar. So for the G6 RPM have no effect on endurance.
The FM-2 got results like F6F, Reduced RPM at Reduced MAN was clearly better than the rest, even #3.
190A8 was like G6, no effect of RPM.

For all the planes the reduced setting was 50% of max of my controllers setting (my X45 throttle set vertical for instance). It might be that you need other settings for the G6 or A8 for it to show effect.

[214thCavalier]

So you think it may affect some aircraft and not others ?

Snef try and find a cruise speed for the 109's and test at that setting.

To be clear this is how i did the tests,

Fully loaded fuel and weapons.

Use wep to get airborne as soon as climb speed achieved go auto climb and shut down wep.

When target altitude reached go level.

Set your chosen Rpm and leave it alone do not move it for duration of test.

Adjust Map until airspeed stabilises, for the F6F speed of 168 i found it easiest to set the RED TAS marker on 200.

You will need to make small adjustments to the Map setting as the fuel load decreases otherwise you will overspeed and get false results.

If i get bored i may try a 109 test but i do them in real time cos i am stoopid, so it takes a while.

[Swoop]

it does effect only some aircraft.

I tested in P51D once.  25% fuel, take off, auto on, fuel multiplier up to max and wait.


On full power I got 12 miles.
On 35" MAN and max RPM I got 18 miles.
On 35" MAN and 2200 RPM I got 25 miles.


Doesnt work in a spit though......