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Spin and recover the c172

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Spin and recover the c172

Postby danielHL » Wed Apr 08, 2020 4:52 pm

Hi,

so today I was trying to spin and recover the c172 on FG (fgdata, next). My spin introduction worked, throttle back, pitch up, let left wing drop and apply left rudder. First thing that seemed odd to me was that instead of spinning with the nose pointing to the ground, I got into a "flat spin" (wings horizontal, spinning around vertical axis) first, which only later on developed in a downwards spin. From what I know it should immediately go nose down and spin around the longitudinal axis?

Second thing is that the recovery procedure (power down, ailerons neutral, opposite rudder, stick forward) had almost no effect for another 5-10 rotations and then kicked in all at once. Also the rotation rate seemed too high, I estimated it to be about one revolution per second...

Is this too much to ask from the FDM to handle this or am I doing it incorrectly?
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Re: Spin and recover the c172

Postby wlbragg » Wed Apr 08, 2020 5:37 pm

Good question, are you an actual pilot that has tested this spin/recovery in real life? If so then we should discuss this in the c172p development repository system on GitHub. Please open an issue and you will reach the developers that have the highest chance of implementing any needed changes.
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Re: Spin and recover the c172

Postby V12 » Wed Apr 08, 2020 5:47 pm

Can c172 withstand g forces in spin and recovery phase ?
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Re: Spin and recover the c172

Postby D-ECHO » Wed Apr 08, 2020 5:53 pm

During spin, there is quite little airspeed and also quite little "g-force", so this should be no problem. For the recovery phase, it actually is a bit tricky to react fast enough not to overspeed while being gentle enough to not break the aircraft. Normally, when flying below Vmo (the manoeuvering speed, sometimes also called Va), you can use full deflections while not over-stressing the airframe.
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Re: Spin and recover the c172

Postby D-ECHO » Wed Apr 08, 2020 5:54 pm

For those interested: here is a video of spin recovery training in a C172, should give some basic idea of how it should behave: https://www.youtube.com/watch?v=4dSrjVR0MvE

Adding to this, from my own experience doing FDMs, getting the behaviour in extreme situations like spins right is one of the most difficult things about designing an FDM
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Re: Spin and recover the c172

Postby wlbragg » Wed Apr 08, 2020 5:57 pm

In sim, yes as well.
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Re: Spin and recover the c172

Postby D-ECHO » Wed Apr 08, 2020 6:01 pm

Sorry wlbragg, I don't quite understand what you mean, to which comment/question did you reply? :D
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Re: Spin and recover the c172

Postby wlbragg » Wed Apr 08, 2020 6:03 pm

I was commenting that the FG c172p in the sim can handle the stresses of a stall spin and recovery as well.
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Re: Spin and recover the c172

Postby D-ECHO » Wed Apr 08, 2020 6:46 pm

Ah :) yup
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Re: Spin and recover the c172

Postby danielHL » Wed Apr 08, 2020 9:35 pm

No, I'm not a pilot. I've just watched some youtube videos and wanted to try this on FG. The c172 seemed the obvious choice because it has by far the most complete FDM. I know that it's an edge case and very difficult to model. I was just wondering if that is considered at all...

From the video D-ECHO posted it's also clear to see what I expected to happen. Nose down attitude and ca. 3-4 seconds per rotation (again, estimated) and an instant stop of rotation once rudder is applied.
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Re: Spin and recover the c172

Postby wlbragg » Thu Apr 09, 2020 2:21 am

After testing the current version using the technique to start the stall/spin that is highlighted in the video, my version (current master version) is behaving really close to what I see in the video. I am able to get out of the spin right away with power off. Also the stall drops the nose immediately into the spin.
I'm impressed that it performed as well as it does and that it appears to match what I am seeing in the video.
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Re: Spin and recover the c172

Postby wlbragg » Thu Apr 09, 2020 2:53 am

For demonstration I recorded my attempt to duplicate what is in the video. It's close but not perfect. I need to practice. I did one test earlier I didn't record that was almost identical to the video. This video's first attempt was recovered almost immediately. The second attempt was closer to the video but i forgot to use full opposite rudder to pull out and so it took an extra turn or two.
Both attempts were sloppy and I didn't follow the instruction in the video exactly nor time the full back pressure and full rudder correctly as I did the test before studying the video for the correct procedure. Actually the second attempt almost put me into a "flat" spin, but I think that was mostly my fault.
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Re: Spin and recover the c172

Postby danielHL » Thu Apr 09, 2020 11:21 am

Okay, thanks :) I too managed to get it done correctly one or two times. Strangely, most tries result in a flat spin for me. So this seems to be something that I do wrong.

I'm curious about the flat spin, too. I heard it's possible to get into that attitude/situation and that it's almost impossible to recover from. How does one get into that deliberately, avoid it and - if possible - recover?
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Re: Spin and recover the c172

Postby Johan G » Fri Apr 10, 2020 9:09 am

IIRC one of the reasons why one should reduce power if entering a spin is to not get into a flat spin. If one get into a flat spin the best way is to apply power, meaning that if you lose the engine in a flat spin you are practically done.

Regarding entering and recovering from a flat spin, though in a completely different aircraft, a Tipsy Nipper, these two videos comes to mind:

goncstube, "26 Turn Flat Spin from The Aviators TV show (from LOOP TV)" (5 min, uploaded February 11, 2012)


goncstube, "26 Turn Flat Spin in a Tipsy Nipper" (2 min, uploaded September 30, 2008)


Edit: Regarding stalls, spins and recovery, a good resource is probably the FAA Airplane Flying Handbook, FAA-H-8083-3B, Chapter 4: Maintaining Aircraft Control: Upset Prevention and Recovery Training (PDF)
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Re: Spin and recover the c172

Postby wlbragg » Fri Apr 10, 2020 4:46 pm

Nice video Johan, good find! That explains a lot and explains it well. Not being a pilot myself I have wondered why sometimes I can easily control the spin and sometimes not. Apparently the difference between a flat spin and a nose down spin is not always that noticeable other than pressure on the surfaces, ie; feedback on the stick and rudder and of course the difficulty in getting out of it. I have to say the c172p FDM models both types of spins fairly accurately. I have inadvertently experienced them both on many occasions, always pushing the limits.
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