Board index FlightGear Support Flying

Cessna 172P Vertical Flight Stability

Controlling your aircraft, using the autopilot etc.

Cessna 172P Vertical Flight Stability

Postby FFGU » Wed Nov 15, 2017 4:27 pm

I've been practising on the Cessna 172P. What I have found at time is that the vertical airspeed can oscillate widely at times, when I am attempting a level flight with a sensible throttle setting (say 2,200 RPM) at a couple of thousand feet. I know from experience real light aircraft don’t behave like that (currently learning to fly)! The oscillations are say +1,000 FPM to -1,000 FPM

One thing I have wondered if my HW is partially responsible for this. FG is running on a 3.2Ghz Pentium with 8GB of RAM, a SSD and NVidia GT1030. I have noticed that the CPU is running at 100%, but that could just be FG grabbing all the cycles! I’m loathed to spend a lot of money upgrading as it’s really just a ‘scratch’ machine. I’d rather change some of the settings to reduce the CPU load or make the aircraft more stable. My aim is really to get the plane more stable so I can practise navigation etc. Not to have the ultimate graphiical experience.
Posts: 5
Joined: Mon Oct 23, 2017 7:11 pm

Re: Cessna 172P Vertical Flight Stability

Postby wlbragg » Wed Nov 15, 2017 5:21 pm

The oscillations are say +1,000 FPM to -1,000 FPM

Can you describe more as to when this happens and what your doing at the time.
I don't think I have ever encountered this in normal level flight.
Kansas(2-27-15)/Ohio/Midwest scenery development.
KEQA (2-27-15), 3AU, KRCP Airport Layout
Intel i5 3570K AMDRX480
User avatar
Posts: 5727
Joined: Sat Aug 25, 2012 11:31 pm
Location: Kansas (Tornado Alley), USA
Callsign: WC2020
Version: next
OS: Win10/Linux/AMDRX480

Re: Cessna 172P Vertical Flight Stability

Postby AndersG » Wed Nov 15, 2017 7:00 pm

It is also worth noting that FlightGear by default (if there isn't any system global vsync setting) will try to push as many frames per second as possible - so it will occupy either one of the CPU cores or the graphics card fully, depending on which is the bottleneck.

For your flight stability question: have you trimmed the aircraft first? It is, to me, somewhat unclear if joystick neutral in FG corresponds to a stick-free or stick-fixed scenario for the real aircraft, but it should be possible to trim the c172p to keep speed and altitude relatively stable.
Callsign: SE-AG
Aircraft (uhm...): Submarine Scout, Zeppelin NT, ZF Navy free balloon, Nordstern, Hindenburg, Short Empire flying-boat, ZNP-K, North Sea class, MTB T21 class, U.S.S. Monitor, MFI-9B, Type UB I submarine, Gokstad ship, Renault FT.
Posts: 2477
Joined: Wed Nov 29, 2006 9:20 am
Location: Göteborg, Sweden
Callsign: SE-AG
OS: Debian GNU Linux

Re: Cessna 172P Vertical Flight Stability

Postby wkitty42 » Wed Nov 15, 2017 9:29 pm

if one is using Active Weather (F10->Environment->Weather->Detailed_Weather) they may want to check the winds in their area if they are using NOAA METARs or possibly have a weather scenario that has windy conditions... i know that the heavy storm scenarios can easily flip a c172p from the tarmac to the top of a terminal when the winds are strong enough... especially when there's a lot of thunder and lightning ;)

additionally, if F10->AI->Traffic->Enable AI areodynamic wake is checked, then flying near (at least) AI craft can lead to experiencing wake effects... i'm not sure if this also carries over to craft flown by others in multi-player, though...
"You get more air close to the ground," said Angalo. "I read that in a book. You get lots of air low down, and not much when you go up."
"Why not?" said Gurder.
"Dunno. It's frightened of heights, I guess."
User avatar
Posts: 6603
Joined: Fri Feb 20, 2015 3:46 pm
Location: central NC, USA
Callsign: wk42
Version: git next
OS: Kubuntu 14.04.5

Re: Cessna 172P Vertical Flight Stability

Postby Clive2670 » Thu Nov 16, 2017 8:57 pm

That seems a lot of CPU / GPU usage for the plane you are flying and the amount of RAM you have! Look at my set up in my signature, I have the AMD R7-M260 with the same amount of on-board GPU RAM as you (And AMD is notoriusly bad on F/G) and 2 Gb less motherboard RAM and fly the 777, which on F/G is the most "hungry" plane out there when it comes to RAM usage and FPS. But I can still get a "Stable" 15-20 FPS! Maybe more Info' What OS are you running? Are your GPU drivers up to date? Try turning off a few "Bells & Whistles" within thhe shaders, AI, and real time weather, does this help with the FPS,CPU, and RAM usage? I am flying as I write this and F/G is using 28% of my CPU and 38% of my RAM and I am downloading tiles in terramaster and have 2 tabs open in my firefox browser. Total CPU 38% and 65% RAM, so there sre other issues causing your problem I suspect! Please put down your OS as well and accurate CPU specs I might (and others) be able to help further with your CPU/RAM problems, maybe a screenshot of your task manager as well?
Thanks Clive aka: G-BLS01
Can be seen flying the Boeing 777-200LR
Toshiba laptop,
Intel i5 4210U,
16Gb RAM,
AMD R7-M260 graphics 2Gb RAM
User avatar
Posts: 596
Joined: Mon Feb 08, 2016 7:11 pm
Location: Anywhere in the World at some point on Flightgear!
Callsign: G-BLS01
Version: 2018.3.1
OS: Windows 10 64bit

Re: Cessna 172P Vertical Flight Stability

Postby stuart » Fri Nov 17, 2017 9:23 am

On a related note, what frame-rate are you seeing, and how are you controlling the aircraft (joystick, mouse, keyboard)?

If you've got low frame-rates (< 10fps), and are using the mouse of keyboard, you might be PIO'ing the aircraft because of the lag between your inputs and apparent effect, though that seems a bit unlikely if you've got experience in real aircraft.

You can reduce the "eye candy" from the View->Rendering Options menu - just disable lots of things and move the quality bar to the left.

User avatar
Posts: 1596
Joined: Wed Nov 29, 2006 9:56 am
Location: Edinburgh
Callsign: G-MWLX

Re: Cessna 172P Vertical Flight Stability

Postby FFGU » Fri Nov 17, 2017 2:31 pm

Hi All

Thank you for all your feedback. I have a Thrustmaster T-Flight Hotas X Joystick (PC/PS3). It's a Winfdows 10 (64 bit), and yes all the drivers are up to date. The graphics card is only 2 months old. I will give View->Rendering Options menu a try too.

Yes I have adjusted the trim, which certainly does help. I did wonder partially that the instruments on the C172P respond much more quickly than the rest of the plane does, so it appears that it's oscilating widely when it's not..On a real life plane the instrument of course lag what the plane is phyiscally doing, but it might be less so the case on the FG C172P.

How do I do the following:
1) Check the frame rate, and if necessary change it?
2) Turn off the real time weather
Posts: 5
Joined: Mon Oct 23, 2017 7:11 pm

Re: Cessna 172P Vertical Flight Stability

Postby legoboyvdlp » Fri Nov 17, 2017 3:51 pm

1) View > View Settings > Show Framerate
User avatar
Posts: 7748
Joined: Sat Jul 26, 2014 1:28 am
Location: Northern Ireland
Callsign: G-LEGO
Version: next
OS: Windows 10 HP

Return to Flying

Who is online

Users browsing this forum: No registered users and 0 guests