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Unusually poor climb performance at altitude

Postby Paul_Buller » Fri Oct 12, 2018 11:45 pm

Hello all, newbie here. First post. The obligatory pre-apologies are in order. I poked around and couldn’t see any other post where this topic was addressed, and I’ve done my best to find an appropriate place to list this concern.

I just started into Flightgear and was confused at the very poor climb performance at altitude. I have found that multiple light prop-driven aircraft are incapable of climbing past about 8,000 or 9,000 ft ASL; including aircraft that reportedly have service ceilings closer to 20,000 ft ASL. I suspected it may have something to do with my lack of familiarity with constant-speed props, so I did this experiment with the 172R with the same results. To date I have tested the 172R, the 182, the 310 and the Seneca II. All the same; they climb well at lower altitudes but for the life of me I cannot get them to break 9,000 ft ASL.

First question; has anyone been able to climb above 9,000 ft ASL in these aircraft? Perhaps it’s something I’m doing wrong. That would be simple and I'm ok with feeling stupid.

I took a deeper dive into the C310 model, and investigated the internal properties. With the information FG provided I took a snapshot of the properties at around 5,000 ft ASL and 8,000 ft ASL and did some calculations. Using the speed, RPM and diameter of the prop I calculated the advance ratio. The propC10v.xml file provides C_THRUST tables for different advance ratios and pitches. The pitch was 17 in the model, so I interpolated between 15 and 19 in the table. With that information (using propeller thrust equations provided in the wiki ( and more internal properties from FG) I calculated that the theoretical thrust under those conditions – and at 5,000 ft ASL – should have been around 544 lbf. The internal properties indicated that the thrust was only around 473 lbf.

I did the same experiment, and calculations, at 8,000 ft ASL. The thrust I calculated was 403 lbf and the thrust reported by FG was 274 lbf. Similarly, the HP provided by the prop was substantially lower as reported by FG than as I calculated from the values in the tables. Having the thrust and the power substantially lower than theoretical might explain the lack lustre climb performance, but why are they so far off?

Then I suspected maybe FG wasn’t accessing the values in the XML file I was calculating from, so I quickly created a second XML file and doubled the C_THRUST numbers in it. It climbed like a bat out of hell, so clearly FG is accessing data from the file I think it is accessing data from.

Any thoughts? Ideas? Or is there something stupidly easy that I’ve overlooked?

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Re: Unusually poor climb performance at altitude  

Postby Thorsten » Sat Oct 13, 2018 6:11 am

Just to check one obvious suspect you did not mention - did you adjust mixture with altitude?
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Re: Unusually poor climb performance at altitude

Postby Paul_Buller » Sat Oct 13, 2018 5:38 pm

Yep, we have a winner. Apparently it has been too many years since I've flown. I just got the Seneca II up to 12,000 ft.

Thanks. Feeling dumb, but enjoying the sim now.
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