Fuel consumption vs. altitude

[Regurge]

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?

[joeblogs]

Again using the R2800 for comparison, I've just plotted fuel consumption against altitude at various power settings:



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?

[gripen]

Here  is a page from the NACA report 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

[Regurge]

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?

[fats]

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

[gripen]

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

[joeblogs]

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  is a page from the NACA report 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

[joeblogs]

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?