Fuel Burn bug

[curry1]

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

[321BAR]

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.

[Dawger]

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.

[The Fugitive]

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.

[Jayhawk]

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?

[WWhiskey]

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!

[Chalenge]

I was going to say... dawger must have a different definition of 'direct drive' than any I have heard.

[Edgar]

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.

[Tigger29]

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.

[Infidelz]

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.

[kvuo75]

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?

[RTHolmes]

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.

[WWhiskey]

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!
 

[kvuo75]

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?

[RTHolmes]

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.)