Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Regurge on June 26, 2004, 05:57:50 PM
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While flying a 190A8 I noticed something seemed odd about the fuel consumption. It increases with altitude from sea level up to 1st gear full throttle height, where it starts to drop off. The way I understand it, a geared supercharger should be able to maintain maximum manifold pressure all the way up to full throttle height, so fuel consumption ought to be constant (as long as rpm stays constant as well).
I tried a spit9, 190A5 and a yak9U offline and they all behaved similarly. Only the Typhoon behaved like I thought it should: constant consumption up to 1st gear full throttle height, decreasing until 2nd gear engaged, then constant up to 2nd gear full throttle height.
Also, none of them appear to be geting any ram-air effect which AFAIK should show up as increased consumption/manifold pressure above full throttle height.
So can anyone explain this?
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I have no idea how consumption is modeled in the AH but in theory the engine with fixed speed mechanical supercharger works most efficiently when the throttle is open ie at FTH and above. So for a given MAP the efficiency should peak at FTH and above because below FTH some power is wasted in the supercharger
gripen
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I don't think they are finished with the fuel model yet. Pyro will get around to it.
Crumpp
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You'll see this characteristic in most fuel consumption charts that you look at. If you also look at engine power charts, you'll typically see power increasing with altitude below the FTH. An offhand example of this would be the P-51D. At 61" and 3000 RPM, it has 1490 HP at SL. At the same MP and RPM that increases by 100 HP at 8 or 9K.
As Gripen points out, the supercharger is doing wasted work below FTH but there's more going on than that. In fact, it's actually taking more power to drive the supercharger at FTH than it is at SL even though it's more efficient at FTH than it is at SL.
We tend to think of constant manifold pressure and RPM as constant power or a constant amount of fuel/air charge to the cyclinders, but manifold pressure and RPM are only two parts of the equation. There are a number of other factors involved but the other two major ones are the exhaust backpressure and the charge temperature, both of which are affected by altitude changes.
The density of the fuel-air charge is proportional to intake pressure / charge temperature. So as altitude increases and temperature decreases, your charge weight will go up for a constant intake pressure.
The other major factor is that as altitude increases and ambient pressure decreases, the lower the pressure of the residual exhaust gas in the cyclinder which means more charge can enter the cylinder during the intake stroke.
A further factor could be whether the automatic mixture control on any of these planes adjusted to this but I don't know if that was the case or not.
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Damn! I'm glad to see you and Hitech able to post more. :D
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So your saying the 109G10 is supposed to have the same range as the 190A's or Dora?
Crumpp
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In fact, it's actually taking more power to drive the supercharger at FTH than it is at SL even though it's more efficient at FTH than it is at SL.
True in the most cases; most engines had throttle valve located before supercharger so when the engine is throttled, there is lower pressure after throttle valve and therefore supercharger takes less power than throttle fully open. There is couple exceptions, some German engines with side mounted supercharger like the Jumo 211 and DB 605 had throttle valve(s) after supercharger (the Jumo 213 had vane type throttle located before supercharger).
Also the Klimov M-105 (and other HS 12Y derivates with carburators) had throttle valves (there were 6 carburators) located after supercharger.
gripen
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Yeah I wasnt thinking of the decreased backpressure at altitude. I figured throttling the intake below FTH would cool the charge somewhat, but I guess not enough to cancel the effect of warmer ambient air.
Anyway, thanks Pyro. Its good to know AH2 is taking all this into account.
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Originally posted by gripen
True in the most cases; most engines had throttle valve located before supercharger so when the engine is throttled, there is lower pressure after throttle valve and therefore supercharger takes less power than throttle fully open.
I'm not sure I follow you. I was referring to an identical MP and RPM setting at SL and FTH. It's true that the throttle will be partially open at SL and fully open at FTH, but the pressure at the supercharger intake will still be the same.
The extra power required at FTH is because of the increased charge weight. Power required to drive the supercharger is proportional to charge weight. More mass through the supercharger = more work.
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As an example, this is what fuel consumption looks like at various power settings and altitude for the P&W Double Wasp.
(http://mysite.verizon.net/vze479py/sitebuildercontent/sitebuilderpictures/r2800_8_sfc.gif)
The data is from a specific engine flight chart for the F4u posted by F4udoa.
The other way to look at it is to compare the fuel consumption curves across the power range at the three different supercharger settings (neutral, low, and high):
(http://mysite.verizon.net/vze479py/sitebuildercontent/sitebuilderpictures/r2800.gif)
The dip at the highest output setting is due to WEP. -blogs
Originally posted by Regurge
While flying a 190A8 I noticed something seemed odd about the fuel consumption. It increases with altitude from sea level up to 1st gear full throttle height, where it starts to drop off. The way I understand it, a geared supercharger should be able to maintain maximum manifold pressure all the way up to full throttle height, so fuel consumption ought to be constant (as long as rpm stays constant as well).
I tried a spit9, 190A5 and a yak9U offline and they all behaved similarly. Only the Typhoon behaved like I thought it should: constant consumption up to 1st gear full throttle height, decreasing until 2nd gear engaged, then constant up to 2nd gear full throttle height.
Also, none of them appear to be geting any ram-air effect which AFAIK should show up as increased consumption/manifold pressure above full throttle height.
So can anyone explain this?
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What happens if you you advance the throttle beyond what is required for reccomended MAP?
Will you over boost? Or overspeed the supercharger?
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Originally posted by Crumpp
So your saying the 109G10 is supposed to have the same range as the 190A's or Dora?
Crumpp
I think it should have even better range. Definetely the light DB605/Bf109 combination was more effiecient than the heavier BMW801/FW190 combo, both that it had less drag and the engine was also smaller capacity.
Recalling from memory, the Bf 109G w. 605A had equal or better range than the FW 190A at economic powers even when the the fuel capacity was some 25% lower; in case of the G-10 with its higher rated altitude 605D powerplant which allows significantly higher ruising speeds at the same power, I believe the G-10/K-4 probably had 10-30% more range than the equivalent 190A/D. I am not sure about the amount of difference, but I am pretty sure it had more range based on the known techspecs. The K-4s max. possible range should be even higher by about 25%, as it could use its rear MW tank as an auxilary fuel tank as well.
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What are the numbers for the 109 Barbi? Percentages don't cut it.
On internal fuel only, the A-8 had a best range of 1035km/644mi @7km. There was still a 12.5% reserve and range includes climb and desent.
The best I have seen for a 605 engine is 300l/h or 1.33 hr flight time for a 109(internal fuel). The above range for the A-8 was flown in 2.18hr.
http://mitglied.lycos.de/luftwaffe1/aircraft/lw/DB605_varianten.pdf
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Originally posted by F4UDOA
What happens if you you advance the throttle beyond what is required for reccomended MAP?
Will you over boost? Or overspeed the supercharger?
Automatic boost control prevents overboosting. Supercharger speed is a function of engine rpm and gearing.
To others in this thread - Why do topics in here have to get continously threadjacked? Is hitting "new topic" that much more difficult than hitting "reply"?
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Originally posted by Pyro
I'm not sure I follow you. I was referring to an identical MP and RPM setting at SL and FTH. It's true that the throttle will be partially open at SL and fully open at FTH, but the pressure at the supercharger intake will still be the same.
Yep, I should have written that when the throttle valve is located before supercharger, it keeps pressure in the supercharger constant. Opening of the throttle at given altitude (below FTH) would naturally result higher MAP.
If the throttle valve is after supercharger, the pressure in the supercharger varies depending on altitude and RAM effect, in this case output loss is higher below FTH because supercharger produces higher pressure than required.
I just tried to point out differences between throttle systems.
Originally posted by Pyro
The extra power required at FTH is because of the increased charge weight. Power required to drive the supercharger is proportional to charge weight. More mass through the supercharger = more work.
Yes, that's fully correct.
This (http://naca.larc.nasa.gov/reports/1947/naca-tm-1169/naca-tm-1169.pdf) NACA report is a good reference how throttle type affects efficiency. This system was copied to the Jumo 213 and late DB developements used this throttle system too. The airflow over throttle valve heats air some what when the engine is throttled below FTH and this kind of vane type throttle reduced heating of the air before supercharger if compared to the conditional throttle valve. Naturally supercharging itself heats air too.
gripen
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One more thing I noticed was that in the 190A's the consumption keeps on decreasing with altitude above 1st gear FTH, even after 2nd gear is engaged. And as I mentioned before the Typhoon also behaves slightly differently from other 2-speed supercharged planes.
Is there some difference in the supercharging systems that causes this?
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Again using the R2800 for comparison, I've just plotted fuel consumption against altitude at various power settings:
(http://mysite.verizon.net/vze479py/sitebuildercontent/sitebuilderpictures/r2800_8_cons.gif)
As Pyro suggested, fuel consumption rises (slightly) with altitude for this engine. I don't know why this wouldn't be true of the BMW801D. But Grippen may be right that it has to do with carburetor design.
Note this chart does not control perfectly for horsepower at different altitudes as the SFC chart (above) does. But controlling for power settings seems to do a pretty good job.
-blogs
Originally posted by Regurge
While flying a 190A8 I noticed something seemed odd about the fuel consumption. It increases with altitude from sea level up to 1st gear full throttle height, where it starts to drop off. The way I understand it, a geared supercharger should be able to maintain maximum manifold pressure all the way up to full throttle height, so fuel consumption ought to be constant (as long as rpm stays constant as well).
I tried a spit9, 190A5 and a yak9U offline and they all behaved similarly. Only the Typhoon behaved like I thought it should: constant consumption up to 1st gear full throttle height, decreasing until 2nd gear engaged, then constant up to 2nd gear full throttle height.
Also, none of them appear to be geting any ram-air effect which AFAIK should show up as increased consumption/manifold pressure above full throttle height.
So can anyone explain this?
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Here (http://naca.larc.nasa.gov/reports/1945/naca-wr-e-192/index.cgi?page0051.gif) is a page from the NACA report (http://naca.larc.nasa.gov/reports/1945/naca-wr-e-192/naca-wr-e-192.pdf) on the BMW 801. It can be seen that at high RPM fuel flow is about constant up to the FTH of the first gear. At lower RPM however fuel flow increases up to the FTH which is the same phenomena as Regurge noted on AH Fw 190.
What I was talking earlier in this thread on efficiency affects mostly on consumption when measured as kg/km ie output at propeller shaft. The throttle system can affect to charge temperature and depending on fuel metering system, it can also affect fuel flow.
gripen
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Thatnks for posting that chart gripen. Thats exactly what I was interested in. The AH 190A5 matches that chart reasonably well for low gear, its not even close for high gear. I did a quick test of fuel consumption at military power from SL to 20k.
Alt GPH
0k 150
2 151
4 153
6 155
8 148
10 141
12 137
14 136
16 135
18 134
20 133
I cant make a graph at home but as you can see the fuel flow doesnt jump back up when hi gear is engaged, like in the chart. A bug I guess?
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Regurge,
In case you're lacking MS Office suite to make charts with, then you might be interested in checking out OpenOffice.org suite. It's free. Something like 70MB download.
http://www.openoffice.org/
It has quite the same functionality as MS Office as far as I've used them, which is to say I haven't used either that much. Here's a screenshot of their spreadsheet:
http://www.openoffice.org/product/pix/calc-big.png
// ville
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While that NACA report is quite convincing, it should noted that after all they were testing captured equiment without proper manuals. So results should be taken with grain and salt.
gripen
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Now that I think about it for a moment, why wouldn't fuel consumption fall above full throttle height? Holding constant the fuel-air mixture, as the density of air falls above critical alt, shouldn't the amount of fuel?
Is it possible what we observe for the 190 simply an engine that can't be run flat out at low altitudes, but with a single stage supercharger will peak at an altitude around 20k?
-blogs
Originally posted by gripen
Here (http://naca.larc.nasa.gov/reports/1945/naca-wr-e-192/index.cgi?page0051.gif) is a page from the NACA report (http://naca.larc.nasa.gov/reports/1945/naca-wr-e-192/naca-wr-e-192.pdf) on the BMW 801. It can be seen that at high RPM fuel flow is about constant up to the FTH of the first gear. At lower RPM however fuel flow increases up to the FTH which is the same phenomena as Regurge noted on AH Fw 190.
What I was talking earlier in this thread on efficiency affects mostly on consumption when measured as kg/km ie output at propeller shaft. The throttle system can affect to charge temperature and depending on fuel metering system, it can also affect fuel flow.
gripen
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Having read the NACA report Grippen mentions, the phenomenon described is exactly what is observed in the NACA charts. I suspect it is so noticeable for this engine because full throttle height of the 1st gear is rather low and the supercharger will not shift into its second gear for about another 5,000 feet.
-blogs
Originally posted by Regurge
While flying a 190A8 I noticed something seemed odd about the fuel consumption. It increases with altitude from sea level up to 1st gear full throttle height, where it starts to drop off. The way I understand it, a geared supercharger should be able to maintain maximum manifold pressure all the way up to full throttle height, so fuel consumption ought to be constant (as long as rpm stays constant as well).
I tried a spit9, 190A5 and a yak9U offline and they all behaved similarly. Only the Typhoon behaved like I thought it should: constant consumption up to 1st gear full throttle height, decreasing until 2nd gear engaged, then constant up to 2nd gear full throttle height.
Also, none of them appear to be geting any ram-air effect which AFAIK should show up as increased consumption/manifold pressure above full throttle height.
So can anyone explain this?