Alex – I really appreciate all the work you’ve done developing this mod. The Beech 18 is one of my favorite airplanes in the sim (and in real life as well), and the Denarq mod really enhances the experience of flying it.
I have some observations and questions regarding fuel consumption rate, however. I had noticed that the fuel consumption rate was higher than the default Carenado plane for MSFS 2020 and seemed higher than some of the documentation that I could find. I ran across this discussion of fuel consumption rate and learned that we can adjust the “fuel_flow_scalar” parameter in the sim. But, what should it be set to? I read your post and tried to duplicate your numbers as well as try other conditions.
The attached screenshot of my Excel spreadsheet (at the bottom of this post) shows the fuel consumption rates I obtained compared to the consumption rates found in the Aircraft Operating Instructions – Expeditor and the Flight Handbook – USAF Series C-45H Aircraft. Along with the conditions (altitude, manifold pressure, rpm, mixture setting), I also show the indicated airspeed (KIAS) as a relative measure of the thrust for a given altitude, and the ratio of the observed fuel flow rate to the reference fuel flow rate. All test were carried out with the MSFS 2020 simulator with weather set to “Clear Skies” which should assure ISA (international standard atmosphere) conditions. I should also note that these observations were carried out with Denarq Mod v1.3. This may be different than what you used as your post predated the release of v1.3, but I’m not sure.
Below I show two of my observations that match two of the conditions that you posted. For these, I was using a value of 1.03 for fuel_flow_scalar parameter. I did not modify this, so presumably this was what came default with v1.3 of the mod. This is lower than what you used, so should have resulted in a lower fuel flow rate. Nevertheless, my observations were higher than yours – 58.0 gph versus your 48.4 gph for the high rpm case (30.5 in Hg MP, 1950 rpm), and 34.5 gph versus your 28.7 gph for the lower rpm case. My observed fuel flow rates are also higher than found in the C-45H flight handbook.
| Test # | altitude (ft) | MP (in Hg) | rpm | mixture1 | KIAS | fuel flow (gph) | ref fuel flow (gph)* | fuel usage ratio |
|---|---|---|---|---|---|---|---|---|
| 18 | 5000 | 30.5 | 1950 | 50°F LOP | 161 | 58.0 | 50 | 1.16 |
| 19 | 5000 | 25.5 | 1600 | 50°F LOP | 132 | 34.5 | 28 | 1.23 |
- From Flight Handbook – USAF Series C-45H Aircraft, p. 89, Figure A-8.
I’m not sure what could cause this difference – did something change in v1.3 of the mod that would cause my numbers to be different from yours? One possible cause of difference from the reference numbers is the leanng procedures. In both references I consulted, they use a manual leaning procedure that doesn’t rely on exhaust gas temperature (EGT) as they did not have an EGT gauge to use. For the Expeditor, the manual calls for a mixture just rich of peak and using their leaning procedure I found that this did result in a slightly rich of peak mixture. For the C-45H leaning procedure, I got a mixture ~100 °F or more lean of peak. These leaning procedures without reference to EGT are hard to replicate, particularly the one from the C-45H flight handbook. Nevertheless, even at 100 °F lean of peak I could not replicate the reference fuel usage rates shown in the table above (see tests #21 and #26 in the screenshot of the spreadsheet).
Looking at the observations in the spreadsheet, most of the fuel usage rates I observed (using a value of 1.03 for the fuel_flow_scalar parameter) were on the order of 15-30% higher than the reference values. The closest ones (within 5% of the reference values) were comparing my observations at 50 °F or 100 °F lean of peak to values from the Expeditor manual which, presumably, were for mixture settings rich of peak. I concluded from this that a fuel_flow_scalar value of 0.9 would give fuel flow rates closer to the reference flow rates, although they still seem a bit high.
Another observation that I am curious about is comparing fuel flow rates at mixture settings of full rich versus rich of peak (see tests #8, 9, 16, and 17 in the spreadsheet). I had forgotten to lean the mixture before taking the observation, so I thought I would include it as well. It shows that when leaning the mixture to 100 °F rich of peak, the fuel flow rate actually went up compared to the full rich case. I certainly did not expect this, but not being an expert, I did a brief google search and found a discussion thread in Reddit that discussed this. One person explained that this could happen if leaning optimizes the fuel/air mixture and results in more power from the engine – so more air and more fuel at a given fuel/air mixture (my interpretation of the explanation). However, everyone else on the discussion disagreed and were pretty adamant that leaning the mixture would always result in reduced fuel flow rate. Any thoughts on this? Could this happen IRL or would this be an artifact of the sim?
So, to summarize my questions:
- Did something change in going from v1.2.1 to v1.3 of the mod that would explain my results compared to yours (5000 ft, 30.5 in Hg MP, 1950 rpm or 25.5 in Hg MP, 1600 rpm)?
- If not the change to the new version of the mod, would you have any thoughts as to what other factors could cause the difference?
- Are you still recommending a fuel_flow_scalar value of 1.1 or has that changed?
- Is it possible that leaning a mixture from full rich to somewhere rich of peak could cause fuel usage to decrease?
Anyway, thanks again for this wonderful mod and sorry this post was so long.
