Aces High Bulletin Board
General Forums => Aces High General Discussion => Topic started by: Swoop on November 20, 2002, 07:29:55 AM
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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.
(http://image1ex.villagephotos.com/extern/626629.jpg)
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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..
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I have the most experience with it in an F6F. It definately improves range in that plane.
AKDejaVu
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In short Swoop, yes it does :)
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I did some tests on the F4U awhile back, said it did, funked disagreed, my data showed otherwise. (Shrugs)
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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
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It most certainly does in the Mustang
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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.
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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?
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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
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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.
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"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.
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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.
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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.
(http://image1ex.villagephotos.com/extern/640697.jpg)
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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.
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I want to know what the best settings are for every plane to obtain maximum distance covered.
Sounds like you need to get to testing if you plan expect to get the information done in the next year :)
Seriously it would be very time consuming to test even a few planes that all most folks will do is speculate.
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If you want economy find out the factual cruise speed for your ride and use it, but as a rule that will be too slow for most people.
If all you want is to cut back on fuel used while keeping fast thats a different story, Funked could well be correct if all your gonna do is reduce rpm BUT keep map at max.
In that case i would go for a map reduction over rpm, if you check my link below you will see at 12000 ft reducing rpm to minimum and keeping map high is the worst result possible for economy.
Have some excel stuff that i ran up for the F6F-5 but the result is that engine Map and rpm both have an effect on fuel economy when in cruise mode.
Heres a rough version,
www.cavalier.cwc.net/dlfiles/f6f5fuel.htm (http://www.cavalier.cwc.net/dlfiles/f6f5fuel.htm)
Aim was to use the F6F-5's recommended cruise speed and see if a strike was feasible for a scenario we were involved with.
NOTE the scenario ran a 1.5 burn rate but the results are still valid for the purpose of showing the huge differences involved at different settings.
Check the values for 12000 ft, rpm was altered for each run and map was adjusted to maintain the IAS of 168.
This was a fully loaded F6F-5 2 1kg bombs + Roks.
The 100% fuel readings relate to the wing tanks, aux was burned off at that point.
It also proves fuel efficiency is better at 12000ft than 6000ft if you compare at 12000ft it covers less distance in the early stages per fuel % used while climbing, but once at cruise altitude it soon recovers.
Not shown here is the fact that its much more fuel efficient to do the climbout on wep than at Mil or any other reduced map.
The longer time spent in the cruise mode by getting up there faster is more important than trying to climb on any reduced map and taking longer to achieve the cruise alt.
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hitech wrote once that it does not have an effect - i will try to find that.
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found it (got busy at work and almost forgot I promised to find it)
Originally posted by hitech
No difference in AH.
In real life there might be a very very small drag difference , would tend to be less drag with trim tab centered in the control surface and holding the stick.
http://www.hitechcreations.com/forums/showthread.php?s=&threadid=55570
Search was made much harder as the serch function will only look for words with 5 letters or more - so a search for "RPM" was not possible.
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Originally posted by Turbot
found it (got busy at work and almost forgot I promised to find it)
http://www.hitechcreations.com/forums/showthread.php?s=&threadid=55570
Search was made much harder as the serch function will only look for words with 5 letters or more - so a search for "RPM" was not possible.
I'm pretty sure Hitech was only answering the question about drag induced by the trim tabs, not fuel economy of lower RPM settings.
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AFAIK nobody has collected any data which contradict my findings (which Dinger sums up nicely). There are some who verbally contradict my findings, but their data do not support their verbal arguments.
Keep in mind that all of my testing is about a year old and I did not test every plane in the set. So there is the possibility that HTC have improved the model or that there are planes with good models that I don't know about.
So the best way to answer this question for a particular aircraft is to check it yourself, using the test methodology in Dinger's post. Fly two test conditions at the same airspeed. One with max RPM and one with reduced RPM. If you get more miles per gallon with the latter setting, my name ain't Nathan Arizona.
(Unless you're in a Mustang)
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ok, expect results in about a year......
(http://image1ex.villagephotos.com/extern/640697.jpg)
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Note:
I just looked at 214th's data and it may contradict my findings. It looks like he has three tests at a 12,000 foot, 168 mph cruise, with identical climb profiles, and the 1500 rpm cruise setting gives better range than the 950 rpm or 2000 rpm settings. He didn't test it at maximum RPM at that altitude though, so I am not certain that it contradicts my rule of thumb about RPM. But it looks like it probably would. So probably the F6F must be considered along with the P-51 as part of the group of planes for which RPM reduction has benefits.
So the answer remains - test it yourself if you want to be sure.
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I tried the real life optimal cruise settings for the f4u1d (1350rpm, 29" manifold) at 15,000ft and the plane fell out of the sky at approx. 750fpm .
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Yeah I have had very little luck matching AH performance to real world cruise charts. Even the Mustangs are well off the "book" numbers.
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Was a long time ago i did that and pretty sure i tested it in every configuration just never put that particular data into excel as it was a looong way away from what i wanted to achieve.
I will check tomorrow see if i still have it.
Nope dumped it all after getting the data i wanted.
Samm at those F4U-1 settings does the aircraft achieve the cruise speed recommended or not ?
If it does achieve the speed then that suggests not enough lift is being produced, but thats another story.
Anyway Swoop ya cloggie this really belongs in the aircraft thread ;)
Cleaned the data in the link above to show fuel used and mileage in graphs.
http://www.cavalier.cwc.net/dlfiles/f6f5fuel.htm
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Just found this in 'Spitfire' - A complete fighting history by Alfred Price.
'In Aug 1942 the Air Tactics department at the Air Ministry issued the document which follows as a guide to Spitfire pilots on the optimum engine settings to use when flying over enemy territiory'...
I'm not going to type it all it's too long. The relevent information is this:
SPITFIRE VB AND C (MERLIN 45 AND 46): APPROXIMATE PETROL CONSUPTION FIGURES AT VARIOUS BOOST AND REV SETTINGS
(http://homepage.ntlworld.com/stant.family/Aceshigh/rpmboost.gif)
* = FLY AT THESE SETTINGS
Interesting huh :)
One final quote from that document:
Low revs and high boost will bring you safely back to roost.
Gatso