Aces High Bulletin Board
Help and Support Forums => Help and Training => Topic started by: xtinct on August 06, 2003, 08:20:35 AM
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Hello,
what's the difference between rpm and manifold pressure when it comes to use throttle? When (if evre) do I need to adjust rpm in flight since my throttle only adjusts the man pressure.
Thanks for your help
xstinct
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You can adjust the rpms with the +/- keys on the numberpad as a separate thing from the throttle (which u may have on your stick or may be using the =/- keys on the main part of the keyboard). Generally, I don't think many people adjust rpms except under very certain conditions, gliding being one of them. Most of the aircraft will glide far better with reduced rpms so if your engine is off and you want to glide, press and hold the - key on the numberpad until you see the rpms drop. When you start your engine again you can press + on the numberpad, or, you can press WEP (P) for a second and that will increase your rpms back up to full.
reducing rpms should also impact fuel use, but that is a combination of rpms and throttle position. I have some numbers I tried on the Typhoon though and the loss in speed from reducing rpms or throttle did not help extend range. It helped extend duration of engine run-time, but not range, because you were slower.
Hope that helps,
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Another area where reducing throttle and RPMs may come in handy is when escorting bombers to and from a long distance target...
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Generally, put your throttle at 75~80%, with the RPM around just 2000.. it may be placebo, but with that settings, a whole lot of fuel is conserved, it seems to me.
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Certain (few) planes on the HTC help pages have different man and RPM settings for different things. The 190A8 is the one I can think of off the top of my head.
I think it's 33" man 2100rpm for max range in the A8.
http://www.hitechcreations.com/ahhelp/models/models.html
And I think it's only the original planes that have these engine settings listed.
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I have to agree with Kweassa on this one, I was limping home with less than 25% fuel in both my RW and my Main tank in an F4U today, I was crusing about 325 so I dropped RPM's to 2k and manifold pressure to about 40lb's and I made it almost 2 sector's, tried to taxi to the rearm pad on landing. Then Iran outta fuel about 20 feet away.
I don't think I would have made it had I cruised at a higher RPM and Manifold pressure.
BTW I only lost about 25 TAS when I dropped the throttle and RPM's.
edit: fixed kweassa's name
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if you change both manifold pressure and rpm, you can't tell whether the increase in range is from reducing manifold pressure or rpm.
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Just for reference, I ran a number of tests like this on the Typhoon for the current release of AH and it showed that range was maximized at full throttle (or even WEP) and max rpms. Fuel duration was shortest but the loss in speed from lesser settings lead to an overall loss of range. Strange, it surprised me and maybe I didn't consider all the potential settings, but nothing gave me a longer range than WEP and full rpms in the Typhoon.
Data:
Typhoon
RA/LA: 13:30 (6:45 with only 1 tank burned)
tested at sea-level
rpms Man Speed Run Duration Range
3.6 (full) 9 (WEP) 370 6:45 41.64miles
3.6 (full) 8 (full) 355 6:45 39.993 miles
3.5 8 (full) 350 6:45 39.26 miles
3.6 (full) 6 327 7:15 39.558mile
3.5 7 331 7:05 39.079miles
3.0 6 303 7:50 39.512miles
You can see what happens, reduced throttle appears to extend fuel duration but the reduced speed hurts total range. I tested the durations at several throttle settings and it seems 100% was 6:45, 75% 7:18 and 50% 11:13.... at 50% though it was difficult to keep the plane airborne.
I think some planes do better (F6F? Maybe Levthan had some numbers on that) but the Typhoon was better to run at full throttle. This was a pretty quick and dirty test though but I was careful to adjust the fuel burn modifier in order to get accurate numbers. Normally I would have agreed with Kwessa et al on this, but my tests of the Typhoon didn't show what I thought they should.
(sorry about the crummy chart above... it's kinda hard to read)
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Only time it really pays IMO to drop RPM is if you have a dead engine and a ways to glide to reach home.
Feather the engine, set autoclimb (will keep the plane at its best glide speed) and let it glide.
I've done a sector & a half on 10k in a la7.
B17's will also glide a sector on 10k.
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With reference to WEP and full throttle....sure...if you have enough altitude and very little fuel, and are far from home, I agree, burn it on WEP...get as close as you can, as quickly as you can...and when out of fuel...minimize RPM's and glide as far as you can from your present altitude.
What I was aluding to in my previous post was that fighters can extend their escort time and range by watching altitude and fuel consumptions rates by timing their climbs to altitude with the bombers, and when up high...20k or so, reduce throttle and RPMs to match the bombers...so that the fighters are not always running full throttle and full RPMs which they don't need once above the bombers and in a good bounce position....not sure this makes any sense....I think I know what I want to say, however, I'm not sure I'm saying it properly.
Perhaps I am over-generalizing...dunno
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Patches,
The tests I ran, mind you they were only for the Typhoon, showed that duration of engine run time was improved by throttling back. What they didn't show though was any improvement in range. Basically, if you had to fly the maximum range then you wanted to run at full throttle (and WEP)... so if you were low on gas, WEP and run for home, don't lean it out and try to cruise back slower. This was basically opposite to what I would have expected but that's what it seemed like at least in the Typhoon.
So, for bomber escort, throttling back would not help other than to make it easier to keep formation with the bombers. It might extend your loiter time, which might be a concern, but if range was the issue then you better just run at full throttle. Typically on escort you are better to keep your speed up nice and high anyway so you can react to things better. Making a zig-zag path at full speed gives you more options to counter the enemy than just throttling up before combat. The only time I cruise in escort at less than full throttle (not WEP mind you) is when I want my engine to cool.. then I cut the engine, reduce rpms, and glide a bit to let the engine cool off quickly.
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The aircraft modeled in the game have constant speed propellers. The prop turns at a set RPM based on the pitch of the propeller blades. Reducing the RPM increased the AOA (angle of attack) of the blades. This is why reducing the RPM for gliding improves the glide ratio. The Manifold pressure number is an indicator of how much power is being produced by the engine and will vary with altitude and turbo/supercharger speeds. An example of that is the P51D/B. These two aircraft have similar engines but the supercharger shift points are different. The P51D switches to hi-boost at 13K and the P51B at 16K. Watch the manifold pressure as you climb through these altitudes. (I may have the reversed.)
There are optimal settings for the aircraft depending on the flight mode.
For combat you should fly with you RPM at MAX. If you are cruising and/or need to conserve fuel a reduced RPM and throttle setting will help.
I don't have specific data; however, reducing RPM 50% (generally 2000-2400 RPM depends on airplane and range of adjustment available) and reducing manifold 40-50% results in significant fuel savings.
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Originally posted by empty
The aircraft modeled in the game have constant speed propellers. The prop turns at a set RPM based on the pitch of the propeller blades. Reducing the RPM increased the AOA (angle of attack) of the blades. This is why reducing the RPM for gliding improves the glide ratio. The Manifold pressure number is an indicator of how much power is being produced by the engine and will vary with altitude and turbo/supercharger speeds. An example of that is the P51D/B. These two aircraft have similar engines but the supercharger shift points are different. The P51D switches to hi-boost at 13K and the P51B at 16K. Watch the manifold pressure as you climb through these altitudes. (I may have the reversed.)
There are optimal settings for the aircraft depending on the flight mode.
For combat you should fly with you RPM at MAX. If you are cruising and/or need to conserve fuel a reduced RPM and throttle setting will help.
I don't have specific data; however, reducing RPM 50% (generally 2000-2400 RPM depends on airplane and range of adjustment available) and reducing manifold 40-50% results in significant fuel savings.
I was waiting to see the words" constant speed props" Just went over all of this stuff tuesday in class. Im currently taking my PPL.
FBsmokey
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If you are running low on fuel the best way to maximise range is to climb on full throttle at best climb speed until you run out of fuel (I always keep a little bit in the tank for the approach and landing). Shut down the engine, reduce engine RPM and leave auto climb autopilot on. This will give you the best glide angle = furthest gliding distance.
This gives you better range than simply going level and WEPing as the higher alt gives you better fuel economy and also a higher groundspeed (note wind layer at 16k ft!) due to the fact that the TAS you acheive is higher than lower down.
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I can't speak for AH, being new here... But, as a real world pilot I can tell you that basically, the RPM controls are much like the transmission in your car.
High RPM will deliver superior climb and accelleration performance, and lower numbers (within limits) will provide superior fuel economy.
Manifold pressure can be compared with your foot on the gas pedal in a car. More power, and greater fuel consumption.
So, High RPM and High Manifold pressure is much like driving your car in first gear with the gas pedal floored... You'll get mean accelleration or hill climbing, but you are working the engine to death.
Medium settings are like highway cruising in your car.. These settings vary from one aircraft to the next.
Generally speaking, reducing RPM and manifold pressure while in straight and level cruise flight by 10% or so should manifest fuel savings.
During a dog fight, you will definitly want full RPM and power as needed.
Hope that helps!
Kurt
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We're left with these ballpark estimations and guesstimations. HTC probably doesn't try to model engine performance too detailed it seems. I would love to see range increase with relaxed RPM and MAP settings, as it definately does in real life, but Soda's testing shows that (on the Tiffie) it doesn't. I haven't done any testing, but it seems to me that high alt cruising increases range (as it should). If you increase range by climbing at full performance and then glide, then it shows that we cannot expect high fidelity in AH's engine model. In real life this tactic wouldn't work - as a general rule.
It seems to me that the AH planes burn a lot of fuel on idle throttle. This leads to silly engine off gliding to conserve fuel. Anyone have any thoughts on this?
Real life prop AC "brake" quite a bit on throttle idle/max RPM. I don't know much of WWII AC in this regard, but I don't get a feel that it is very pronounced. Any thoughts?
Has anybody tested high versus low alt fuel consumption?
Can you wreck an engine by setting high MAP at low RPM?
Hmm, looks like i need to to some testing ;)
Frankly, I haven't looked into any of this before reading this post. I simply found AH too simplistic (engine wise) to even consider it.
BTW: I've found massive help in your AC descriptions, Soda. Thanks for putting in so much "work" ;) in them. (No pun intended, I just expect you enjoyed it too)
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You're welcome.
My only tests on this were on the Typhoon so I don't know that those apply across all aircraft. I haven't had a chance to run them on any other aircraft but I'm assuming that the results may be different (at least slightly).
I don't think this detracts from the game all that much though as fuel is easiler to judge in this fashion and people have created "fuel duration" images you can put in your clipboard to give you exactly the length of time your engine will run. It's not like you need to lean out or reduce rpms in AH to travel very long distances 99.9% of the time. If you are simply looking to extend your loiter time though you can simply cut throttle and it will improve your loiter time a lot.
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Agreed!
You guys just set me off;)
I think it is deliberate that the system, as it is now, is simple. Then there no learning curve on engine management on top of everything else. (I know I have my hands full on even basic ACM)
Where do I find the images you mentioned? Are they posted somewhere?
If you have stuff that is not included in your guides feel free to send it to me: dagfinn@start.no
I've made some simple and easy to read speed/alt XLS charts. Help yourself at netaces.
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REDUCING MP & RPM WILL GREATLY INCREASE RANGE -- HERE'S HOW:
You first need to be at cruising speed at an altitude at which your engine gets good performance. Then reduce MP (manifold pressure, or boost in UK) and RPMs. You'll go much further than someone who uses full power.
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Here are some test results:
How far will I go on the notoriously small aux tank on a P-47D-11?
Fuel burn rate was 2, the same as in MA. (At least that's what I've heard.)
Flight #1:
20k, entered course level at 245 mph IAS (indicated airspeed), dropped DT and hit WEP. After WEP was used up I left if off. WEP accerated the plane to just over 400 mph TAS (true airspeed) and then plane gradually slowed to 385 TAS. This is in accordance with the flight charts from HTC.
Distance Flown: 82 miles
Flight #2
Same as above, but as I entered the course, I reduced the MP to 32.5 (65%) and RPMs to 2100. The plane slowed gradually to 230 IAS, which at 20k meant my TAS was 310 mph. I maintained this speed for the entire flight.
Distance Flown: 130 miles
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Range was increased by 59%.
MRPLUTO VMF-323 ~Death Rattlers~ MAG-33
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Hi
With regards to rpms and what settings, in most ww2 aircraft the rpms beleow 2000 metres was set about 2000 -25000, higher above that the less rpms the plane needs to use, it is like going up a gear in the car, Max rpms is 1st and the less you use the higher the speed, Yes in this context you will use less fuel and the engine will manage better. However one of the posts says fighters used to match the rpms with the bombers, to save fue. I do not think this is correct because a fighter is alot faster than a bomber at high altidue.
Say a P51d was flying at 25k the rpms would be alot lower than at 6k and why this what gives the planes the speed and fuel effiecency. If the plane matched the bombers speed and rpms the engine would not perform at this attitude burn more fuel and proabably over heat eventually. The job of a fighter is to scan for enemy planes and some would follow the bombers in close formation but this was done sparingly and not for any length of time most fighers where abover the bombers, at hieght behind or front and they would be travelling on average about 350 + doing fighter sweeps on the look out for the hun from the sun.
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If manifold (throttle) is at maximum and RPM's less than maximum, will the plane fly faster or slower than max RPM and max throttle?
HONK!
Gooss
(Yeah, yeah, I will test it next time online. Just curious.)
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In real life too much MAP over to little RPM would overstrain the engine and eventually damage/break it. The exact settings for this would or will vary with different engines - this is not so in AH, the engines are indestructible... Unless you throw in shells ;)
Generally, in real life, max speed is almost always attained with max RPM and MAP. In AH I think this is also so.
Compare it to rowing: Do I take many, quick and "light" strokes (?) or less but heavy strokes. Actually, if you're on a certain throttle setting and then decrease RPM, MAP will rise (on most engines). AH doesn't model these finer issues, and the only reason to fiddle with RPM would be to save fuel (I haven't tested this to be true) But it appears that some have found they can save fuel this way.
I do not think you will find a speed increase down the RPM road.
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fyi,
I ran additional tests on aircraft range/duration after my Typhoon tests. I tested two other aircraft, the P-47D11 and P-38 though I didn't run a full set of settings numbers on either (I used a couple of generic rpm/throttle settings as compared to 100% values). My results indicated that the P-47D11 had dramatic improvements ( in line with MRPLUTOs observations) though my P-38 numbers showed No improvements, in line with my Typhoon tests. I can't claim these tests were foolproof but they seemed very strange as they were done using the exact same method in each case yet the P-47 showed the expected improvements while the P-38/Typhoon showed no improvement.
I know someone posted some "Actual" P-38 cruise settings at one point so it would be interesting to test those numbers out and see if it was an issue with the settings I used or whether some aircraft have a range/duration advantage at lower power settings.
-Soda
The Assassins.
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While all planes are different I found a good rule of thumb for gliding is expect 1k loss of altitude for each 1 mile covered with rpm at idle, engine off.
When I get carried away dogfighting (just have'n way too much fun) and notice I'm about outa fuel start a climb for home and immediately do a distance check to the nearest friendly field. Once I determine how far I gotta go try and grab to the alt needed for the glide. If I gotta glide 10 miles I go for 10k minimum and I alwasy try to leave a lil gas in the tank for taxi to the hotpad. In most planes you will find you you arrive overhead your field with alt to spare.
However, you may want to check out your gliding distance in your favorite steed.
Ren
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I tested the A-5. To my surprise it flew the exact same distance on WEP as on Cruise!
The test:
Climb to 10,000 ft on DT. Level out, let speed settle. At grid line drop the DT, go to WEP. Distance covered 3.2 sectors.
Repeat, but instead of WEP use the Max Cruise setting from the HTC Help pages for the A-8; 33"man @ 2100 rpm. Distance covered 3.2 sectors, but took 50% longer.
Lesson: In A-5 use WEP for home run. You'll cover the same distance, but when you run out of fuel you'll be going almost 100mph faster, therefore additional glide distance.
This is clearly not right. On cruise all planes should be able to fly greater distances.
This also gives the P-47 another advantage in that it can "cruise" where other planes can't.
Historical Note on Charles Lindbergh regarding cruise settings from http://www.ww2pacific.com/lindbergh.html:
Republic had him test fly the P-47 Thunderbolt. United had him test the F4U Corsair. Working below the visibility of those in the upper Washington circles, he arranged to test the Corsair under combat conditions. Arriving at Guadalcanal, he showed how to take off with double the rated bomb-load and then showed that dive bombing with that load was out of the question, so he wiped out a gun emplacement with horizontal bombing.
MacArthur immediately heard of his unannounced arrival in the theater and ordered him to Australia where he was assigned to extend the range of the P-38 Lightning in New Guinea operations. Lindberg was able to return from combat missions with his tanks half full when others returned empty. He was able to teach how to add 500 miles to the P-38's range. It had been considered to be 400 miles, Lindbergh's techniques let the Lightning appear hundreds of miles from where the Japanese expected to find them. He flew missions to Balikpapan, Mindanao, even led a 4 plane raid on Palau, considered out of range of land based fighters, a base defended by 200 enemy fighters.
And from http://p-38online.com/lindy.html:
He flew along combat missions as an observer, and quickly calculated that the combat radius could be extended by 30%. A standard technique at the time was cruising at 2200 - 2400 rpm's in auto-rich at low manifold pressure. Lindbergh called for only 1600 rpm in an auto-lean mixture with a high manifold pressure. This reduced fuel consumption to 70 gallons per hour, and resulted in a cruising speed of 185 mph. By comparison, in July 1944, P-38s would fly a five-hour mission and come back on fumes, but after taking Lindbergh's advice, Colonel Jack Jenkins landed with over 160 gallons of fuel.