Aces High Bulletin Board
Help and Support Forums => Aces High Bug Reports => Topic started by: Dawger on September 02, 2010, 03:07:27 PM
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In the 190D9 (I havent tried in any other aircraft) the fuel flow does not increase with RPM in a dive when diving with propeller RPM set below maximum.
High speed dive causes RPM to increase above selected RPM with no corresponding increase in fuel flow.
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why should it?
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why should it?
For a given throttle setting the fuel flow (measured in gallons per hour) increases with RPM (revolutions per minute)
Every revolution of the engine sucks in X amount of fuel. If you produce more revolutions per minute you increase the number of gallons you burn in an hour.
Easy math Example: At full throttle and 1 revolution per minute you burn 60 gallons per hour (one gallon per revolution)
If you double the revolutions to 2 per minute and the throttle remains the same you will now burn 120 gallons per hour. The turning engine sucks in the fuel. It doesn't really matter what is making it turn.
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For a given throttle setting the fuel flow (measured in gallons per hour) increases with RPM (revolutions per minute)
Every revolution of the engine sucks in X amount of fuel. If you produce more revolutions per minute you increase the number of gallons you burn in an hour.
Easy math Example: At full throttle and 1 revolution per minute you burn 60 gallons per hour (one gallon per revolution)
If you double the revolutions to 2 per minute and the throttle remains the same you will now burn 120 gallons per hour. The turning engine sucks in the fuel. It doesn't really matter what is making it turn.
I'm not a mechanic, but this doesn't sound right at all. Wouldn't engine RPM and prop RPM be separated by a gear system?
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I'm not entirely sure that's always true, Dawger.
If you are rolling down hill in an automobile with throttle closed, but engage the clutch which brings up the engine rpm while slowing the car, I don't believe that you draw in any more fuel than if you had simply let the engine idle and braked instead.
On the other hand, if I had a nickel for every time I've been wrong they'd have to round every price to the nearest dime.
<S>
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as far as I know, fuel is metered to how much air is going in the engine..
with throttle closed, you have low manifold pressure = less air = less fuel.. has nothing to do with RPM.
if it were sucking more fuel, it would be making more power.. if it were making more power, it would turn faster (your example dawger), and suck more fuel, and make more power, etc. etc... you are forgetting the effect of the throttle, I think.
but now I've got myself confused now that I reread the original post. you may be right.. :) but then I wonder would increased rpm against a closed throttle decrease MP even more. :headscratch:
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For a given throttle setting the fuel flow (measured in gallons per hour) increases with RPM (revolutions per minute)
Every revolution of the engine sucks in X amount of fuel. If you produce more revolutions per minute you increase the number of gallons you burn in an hour.
Easy math Example: At full throttle and 1 revolution per minute you burn 60 gallons per hour (one gallon per revolution)
If you double the revolutions to 2 per minute and the throttle remains the same you will now burn 120 gallons per hour. The turning engine sucks in the fuel. It doesn't really matter what is making it turn.
NO!
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IIRC most planes adjust prop pitch and the RPM gauge is a representation of what adjustment you have. I agree that acceleration in a dive should increase the RPMs of the prop, but is the gearing between the engine and prop direct or indirect. I would think it has an inertial clutch and would not force the engine to increase. But if it is not, than the engine would counter the increase of airflow somewhat and the difference would be minuscule<<<just a guess on my part.
Any Aircraft Engineers in here?
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IIRC most planes adjust prop pitch and the RPM gauge is a representation of what adjustment you have. I agree that acceleration in a dive should increase the RPMs of the prop, but is the gearing between the engine and prop direct or indirect. I would think it has an inertial clutch and would not force the engine to increase. But if it is not, than the engine would counter the increase of airflow somewhat and the difference would be minuscule<<<just a guess on my part.
Any Aircraft Engineers in here?
the props in the planes here are constant speed props.. the prop adjusts pitch automatically to maintain the selected RPM.. there is no clutch. at the extremes of prop pitch, there is no more pitch available, so the prop/engine will turn at a RPM not selected. i.e. in a crazy dive, it cant go coarse enough to stay below the selected rpm (not sure this is possible in game), or at idle on the ground, cant go fine enough/engine doesn't make enough power to turn it to its selected rpm
the issue is whether at closed throttle, changing RPM should change the fuel flow.. this I am not sure of.
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ok i been thinkin about this all afternoon now :) .. I think dawger was correct all along. :) (sorry dawger)
even with the throttle at idle (dont know why i was stuck on this).. the faster the rpm, the more air going thru the engine = more fuel..
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ok i been thinkin about this all afternoon now :) .. I think dawger was correct all along. :) (sorry dawger)
even with the throttle at idle (dont know why i was stuck on this).. the faster the rpm, the more air going thru the engine = more fuel..
Mixture control?
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I'm not a mechanic, but this doesn't sound right at all. Wouldn't engine RPM and prop RPM be separated by a gear system?
Only in a geared engine. Most aircraft engines that drive propellers are direct drive. The prop RPM and engine RPM are the same. But even in a geared engine the engine has turn faster if you make the prop go faster by diving faster than the propeller governor can handle.
An engine turning draws in fuel. The amount of fuel for each revolution is metered by carburetor or a fuel injection system. The intake stroke of each piston draws in the fuel air mixture. That amount of fuel times the number of cylinders is remarkably unchanging. The ratio of fuel to air is fairly constant. The ideal stoichiometric ratio is 14.7.
What that means is that there is 14.7 times as much air as fuel by MASS. It changes a little for various circumstances but not an incredible amount. A little richer for high power situations to lower the combustion temperature and the risk of detonation and leaner during periods of low demand for fuel economy.
But for most purposes the amount of fuel sucked into a given engine for ONE revolution does not change very much.
What we change is the number of times per minute the engine turns by giving the engine more air. More air means the engine can accelerate to a new RPM level with its attendant increase in fuel consumption over time.
The car example rolling down a hill is analogous to the aircraft. In a car you can separate the engine from the drive train using the clutch. The engine runs at idle RPM, burning X amount of gas per hour. When you engage the clutch then the engine must turn at the RPM of the drive wheels. The throttle is still set to deliver the fuel for idle. Fuel use for each RPM doesn't change but the number of RPM increases so fuel use has to increase when viewed over time.
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Mixture control?
The original bug report is that the fuel flow does not change when engine RPM increases for a given throttle setting. Increasing RPM is done by diving the aircraft.
In game, we do not control mixture so we will just say that it does not change.
So the conditions for the bug report are as follows:
Throttle - FULL POWER
Mixture - FULL RICH
Propeller-2000 RPM
In level flight with these conditions set the fuel flow is 106 gallons per hour. Diving the aircraft without changing any of the above settings, the propeller RPM increases to 3000 plus RPM with no increase in fuel flow measure in gallons per hour.
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nevermind
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We had just discussed the rpm thing in another thread. The rpm gauge is reading engine rpm so if you are seeing a spike in rpm in a dive (and Im not sure why you are) then it is the engine that is rising in rpm. I dont think any of the planes in AH have direct prop/engine drives so the prop will always be at some ratio to engine rpm. kvou had it right about the constant speed props adjusting load to maintain a given engine rpm through a prop governor system (as explained by Hitech).
So I suppose your bug-report/question is a valid one although I think the question should also be why the props dont overrun or wreck when driven so hard in a dive.
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Maybe this would be more relevant in the WWI arena if the bug exists there where you can overrun your engine and they are probably direct drive
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We had just discussed the rpm thing in another thread. The rpm gauge is reading engine rpm so if you are seeing a spike in rpm in a dive (and Im not sure why you are) then it is the engine that is rising in rpm. I dont think any of the planes in AH have direct prop/engine drives so the prop will always be at some ratio to engine rpm. kvou had it right about the constant speed props adjusting load to maintain a given engine rpm through a prop governor system (as explained by Hitech).
So I suppose your bug-report/question is a valid one although I think the question should also be why the props dont overrun or wreck when driven so hard in a dive.
some planes in the game have rpm change at different alts. my a6m at 15k set at 2000rpm cannot sustain that rpm below 10k approx. and on the deck, the rpm cannot be pulled back from near maximum like manifold pressure can.
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Maybe this would be more relevant in the WWI arena if the bug exists there where you can overrun your engine and they are probably direct drive
WWII airplanes are direct drive. There aren't very many reciprocating engine propeller driven airplanes the are not direct drive. Direct drive means engine RPM and propeller RPM are the same.
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I'm not an engineer or even a plane mechanic but in a basic fuel system fuel/air mixture is only "sucked" in from the manifold. If the throttles are cut and are only supplying a minimal amount of of fuel/air mixture the engine can only suck what is available. So if your not supplying the engine with more fuel your engine can't suck any more.
Also full throttle means full throttle, you can not get more fuel through as it is wide open allowing max fuel. Once you hit max, how can you draw more? So once you hit max gallons per minute even should the engine be able to "suck" fuel through the fuel management system it can still only pull what the max is due to the physical size of the lines and pressures.
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There isn't any kind of free spin mechanism that would allow the prop to spin faster but not reversely effect the engine is there?
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WWII airplanes are direct drive. There aren't very many reciprocating engine propeller driven airplanes the are not direct drive. Direct drive means engine RPM and propeller RPM are the same.
Just wondering where you got this information from, I can't think of any WW2 direct driven propeller fighters!
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I was going to say... dawger must have a different definition of 'direct drive' than any I have heard.
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Dawger is correct in the fact that if the engine remains at full throttle and you increase the RPM's in a dive, the engine will suck in more air and more fuel during that dive. This is up to the point that you exceed the airflow capabilities of the induction system (carb or injection). After you reach the maximum flow rate of the induction system, no more air can be drawn into the engine. There is also another limitation and that would be the fuel delivery system to the induction mechanism. The fuel pump can only deliver a finite number of gallons per hour to the carb or injection system. When this limit is reached, again, fuel burn rate cannot increase past this point.
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Theoretically, yes Dawger is correct.
However, I'm almost POSITIVE that there is a mechanism that can allow the prop RPM top exceed the engine RPM so that it doesn't over-rev itself (unlike the WW1 planes that are very easy to blow the engine on in a dive for this exact reason). Because of this, I'm not convinced that an increased RPM due to being in a dive really has anything to do with the engine's RPM.
Also there are design differences between carbed and injected systems. I know we're not talking about electronic injection here, but maybe the mechanical injection systems of the day had ways of preventing fuel flow during such maneuvers?
I do know that in the old days, one way to increase fuel economy in a car was to hold the clutch down while going downhill, this way the engine would idle instead of rev during these times... but on electronic fuel injected cars the opposite it true. With your foot off the throttle and engine RPM staying high, it can actually cut all fuel flow at the injectors (or at least below the amount it would use at idle) which means that even at 3000 rpm, it is possible for today's cars to get better fuel economy going down a hill than at idle. Whether or not this has any relation at all to WWII aircraft is beyond me.
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In the 190D9 (I havent tried in any other aircraft) the fuel flow does not increase with RPM in a dive when diving with propeller RPM set below maximum.
High speed dive causes RPM to increase above selected RPM with no corresponding increase in fuel flow.
hmm. Think. Your driving your car downhill in 4th gear with the engine at 3500 RPM your gas milage is
NOT the same as
your driving your car up hill in 4th gear with your engine at 3500 RPM.
Your in a dive with your RPM retarded, your engine needs less power because the prop is more aerodynamic. The prop does not bite as much and is not pulling or straining the engine. you can actually go faster this way. In some aircraft, ,the Spit XIV, you can fly level at 17,000 feet with the RPM fully retarded and it will pick up speed.
Infidelz.
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hmm. Think. Your driving your car downhill in 4th gear with the engine at 3500 RPM your gas milage is
NOT the same as
your driving your car up hill in 4th gear with your engine at 3500 RPM.
his point is, at the same throttle setting it will use more fuel at higher rpm.
i.e. do you think it uses the same fuel at 1000rpm downhill as at 3500rpm?
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do you think it uses the same fuel at 1000rpm downhill as at 3500rpm?
at the same throttle setting, yes.
the fuel/air flow rate is controlled by the carb via its butterfly valve. for a given manifold pressure (throttle setting) the rpms are irrelevant, higher rpms just mean each cylinder charge has lower mass, the overall flow rate of fuel/air mixture is the same.
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if the fuel metering valve is closed it will burn zero fuel no matter what the RPM's are!
if the fuel metering valve is controlled by a Governor, it will close as the RPM limit is exceeded, stopping the flow of fuel in an attempt to reduce RPM's, even if the throttle setting are not changed!
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at the same throttle setting, yes.
the fuel/air flow rate is controlled by the carb via its butterfly valve. for a given manifold pressure (throttle setting) the rpms are irrelevant, higher rpms just mean each cylinder charge has lower mass, the overall flow rate of fuel/air mixture is the same.
ok i was thinkin same thing at first. hence my first few posts in this thread..
but then I figured how about full open throttle (which is a throttle setting)... you're saying the same amount of air is going thru the engine at 3500 vs 1000 rpm?
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well if you can pull FT at that rpm then yes, but you generally cant. however we're talking about diving, which means throttle at idle, so 1000rpm or 3000rpm its same flow rate (ie. very little.)
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well if you can pull FT at that rpm then yes, but you generally cant. however we're talking about diving, which means throttle at idle, so 1000rpm or 3000rpm its same flow rate (ie. very little.)
but the engine doesn't know the difference between closed(barely cracked open) throttle and full throttle. theoretically it could be a fully open little tiny throttle body that lets thru the same air as the closed throttle.
as long as it's metering the proper ratio of fuel to air, it will suck more fuel at higher rpm because it's sucking more air... i'm assuming that the carb/injection systems always are trying for a proper ratio of fuel to air, and don't go lean when someone chops the throttle :) (which I've always understood is bad for engine internals)
and the OP point was, at idle, with rpm back to 1000 or whatever minimum you can get at low airspeed, as you dive, the rpm comes back up up to 1800 or whatever, and there is no increase in fuel flow on the e6b in the game. this is a bug (the OP thinks, and I think now also)
the whole thread is a little weird, i love thinkin about this stuff tho!
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I'm not an engineer or even a plane mechanic but in a basic fuel system fuel/air mixture is only "sucked" in from the manifold. If the throttles are cut and are only supplying a minimal amount of of fuel/air mixture the engine can only suck what is available. So if your not supplying the engine with more fuel your engine can't suck any more.
if the engine is being windmilled(driven by external forces) higher than the rpm you selected, does it not suck more ?
full open throttle / idle throttle are both throttle settings. it should work the same both ways.
the way i figured it out in my mind is to think of a plane in level flight with wide open throttle.
bring back the rpm now.
the MP will decrease.
so will fuel flow.
in game, with throttle at idle, fuel flow does not vary with RPM. it should.
another way is to think of a big theoretical starter hooked up to an engine (glorified air pump) parked on the ground.. turn off the mags (no ignition), throttle butterflies full open (or missing), turn on the fuel system. spin it to 800 rpm.. it will be sucking a certain amount of fuel and spitting it out the exhaust... now spin it up to 2750 rpm at the same missing butterfly condition. you think it still sucks the same amount of fuel per hour? Ahha!