Just so that you know: I just finished building and running FG 2.8 on an old (5 years) computer, and the default settings left FG basically unusable (even when using just the ufo), the largest impact had disabling the AI traffic/model system (which was causing severe stutter, which also showed up in the system monitor) and ALL the shader support. Starting FlightGear in a no-scenery location gives me about 350 fps in 800x600, starting at KSFO is well above 60 fps (frame throttling disabled), usually between 50-90 fps and frame spacing between 35-45 ms.
So, I'd suggest to customize your FG version accordingly and keep these settings disabled for starters.
There are probably a number of other opportunities to customize FG without touching the C++ code (even though optimizing the C++ code would definitely seem like a good idea in my opinion). Custom texture packs and custom-built scenery tiles were already mentioned. Making use of frame throttling and adjusting the FDM update interval (model-hz) would also seem to make sense.
I didn't check the specs of the Android phones, but I'd also make sure to set up the threading mode properly.
In other words, even if you don't have access to an Android phone, there are certain things that can be done, and which would help (using an Android emulator via VirtualBox would be another option). Just customizing FG for limited targets like netbooks would also help, and if you have an old computer with an nvidia 6x/7x generation card, you could just as well try running FG.
EDIT:
http://wiki.flightgear.org/index.php?ti ... le_devices