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trying to spin

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Re: trying to spin

Postby dany93 » Sun Jan 26, 2014 4:01 pm

C172P: my current FDM C172P_FDM_20140126 [EDIT] Rather look for the last one in this thread [/EDIT]

Flat spin (75 - 80 deg) is now possible, possibly hard to recover (although always possible, with patience and altitude loss).
To get a flat spin (engine at idle):
- pull the yoke backward, up to stall,
- full rudder in the aimed direction,
- when the spin roll has started, cross the yoke like to counter the spin roll,
- and wait with full rudder and yoke backward, till the spin becomes flat (several turns).
Full throttle facilitates (worsens IRL) the leftwards flat spin.
Recovery: cut throttle, counter rotation with rudder opposite to the spin, yoke at neutral or pushed forward and be patient (several turns). The rudder is not very efficient at all. If the spin does not seem to slow down, yoke opposite to rudder (that is, yoke toward the spin!) helps. It takes at least 1000 - 2000 ft height.

As already written in that post, this FDM includes:
- forward-slip bank angle (with drop of the higher wing at stall),
- cross-wind landing (for 20 kt) with decrabbing (by crossing the commands),
- slip-skid ball response (more sensitive).

Spin roll at stall in turns is rather gentle, maybe too much I don't know, although the C172 has this reputation. For now I've not found how to make it more rough without degrading something else.
The 'stall-hysteresis' makes stalls less easy to recover.

Of course, as pointed out by hvengel, this rather simple code is for docile aircraft: no Reynolds number effect, fixed critical stall angle, no inverted flight.
Positively, for those who would like, the principle is easy enough to understand (more than P51D's), which can be nice for a first approach. The code is mainly by (six) patches, inlaid in but not entangled with the standard FDM, and the tables are much more simple. The file is commented in order to help understanding.
However, as can be expected, tuning of the parameters for a particular aircraft needs many attempts and patience (like every FDM).

Technical notes:
- I've increased a bit the adverse yaw (Yaw_moment_due_to_aileron) because IRL it may provoke a spin departure. With this, when stalled, an attempt to correct the spin departure (the bank angle) with the yoke instead of the rudder can provoke the opposite effect.
- @ Hvengel: like you did and wrote, I've been led to apply a feedback loop on yaw to get flat spin. However, I've used the yaw rate itself as the 'engine' for the feedback. According to my current understanding :?: , this can be justified by the idea that an important source of yaw maintenance is the differences in drag and lift between the two wings during the almost vertical fall (these differences are connected to the yaw rate). Not sure, clear explanations are scarce. Tricky to tune, and very sensitive to a change in other parameters of the FDM.
Last edited by dany93 on Sat Mar 01, 2014 2:40 pm, edited 2 times in total.
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Re: trying to spin

Postby hvengel » Mon Jan 27, 2014 4:24 pm

dany93 wrote in Sun Jan 26, 2014 4:01 pm:...
Technical notes:
- I've increased a bit the adverse yaw (Yaw_moment_due_to_aileron) because IRL it may provoke a spin departure. With this, when stalled, an attempt to correct the spin departure (the bank angle) with the yoke instead of the rudder can provoke the opposite effect.
- @ Hvengel: like you did and wrote, I've been led to apply a feedback loop on yaw to get flat spin. However, I've used the yaw rate itself as the 'engine' for the feedback. According to my current understanding :?: , this can be justified by the idea that an important source of yaw maintenance is the differences in drag and lift between the two wings during the almost vertical fall (these differences are connected to the yaw rate). Not sure, clear explanations are scarce. Tricky to tune, and very sensitive to a change in other parameters of the FDM.


I think you are correct that the yaw rate is a key component for making the spin self sustaining and in modeling spins in general. Yaw rate is what drives the lift and drag differences between the two wings once in a spin which in turn creates forces that drive the yaw rate higher. That is the main part of the feedback loop. So the higher the yaw rate the higher the roll rate which tends to increase the yaw rate ...at least up to a point otherwise it diverges into an unrecoverable spin.

Increasing aileron adverse yaw also does the same thing when an attempt is made to use the ailerons to counter the spin. The pilot will tend to push the stick away from the spin to counter the spin induced roll which will deflect the aileron down on the retreating wing which will increase the drag on that side more than the other wing which in turn will increase the yaw rate which leads to increased spin/roll rate.

Once the feedback loop is in place tuning the spin is very delicate and it is difficult to create self sustaining spins without impacting the flight characteristics leading up to the stall/spin. This is where the 'stall-hysteresis' or something like it comes into play. There actually needs to be two sets of curves for generating the spin forces. One for spin departure (or initiation) and one for spin decay. The spin departure force curves need to start doing things right at the stall AoA (for a very docile aircraft perhaps somewhat after the stall AoA - for a less docile aircraft perhaps slightly before the stall AoA). But the decay curves need to create spin forces into the AoA region below the stall point to make the spin sustained. How strong these spin decay forces are and how far they extend below the stall AoA is what determines how hard spin recovery is.
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Re: trying to spin

Postby dany93 » Sat Mar 01, 2014 2:32 pm

C172P_20140301 full aircraft with current FDM

Improvements:
- (Stall and) Spin departure in a turn: more frank
- Tables for lift and drag supplemented between 20 and 90 deg AoA (were only up to 20 deg)
- Spiraling propwash effect added (thus, pulls leftwards instead of wrongly rightwards at acceleration to takeoff)

[EDIT] I'm afraid that MediaFire is now spoiled by Advertisement and Kapchas. Please tell me if you can't download.
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Re: trying to spin

Postby dany93 » Fri Mar 07, 2014 4:19 pm

I have also implemented this stall and spin (same as C172P, up to flat spin) in DR400 JSBSim 120 - 180 HP and DR400 Dauphin (aircraft with Patrouille de France).
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AoA

Postby Robertfm » Thu May 23, 2019 10:32 am

I know this is an old topic but I can't get past the set up not wanting me to post a new topic.

I know what AoA is and how it affects the plane etc and stall angle etc at 17%. What I don't know is the function of the AoA in AP. By that I mean what number do you put it. Is it your preferred 'below' stall angle. As I said I understand what it is but not what goes in the box & when it is necessary.
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Re: trying to spin

Postby legoboyvdlp » Thu May 23, 2019 10:50 am

As far as I know it holds that angle of attack. If you enter 5 degrees it will hold 5 degrees AOA. That's just a guess though, I've never used it.
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Re: trying to spin

Postby Robertfm » Thu May 23, 2019 11:21 am

Thanks for your prompt reply. I know in Real World the computer controls all this so was just wondering. Like us I haven't used it.
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Re: trying to spin

Postby Octal450 » Thu May 23, 2019 1:14 pm

AoA hold is a bizarre thing that doesn't exists in any A/P
IRL I've ever heard of because it makes no sense. AoA with throttle makes sense, but with pitch no. It also doesn't work very well, but it's the generic A/P and that thing is a mess anyways.

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Re: trying to spin

Postby Robertfm » Thu May 23, 2019 1:18 pm

Thank you. To honest as a newbie I find AP & Route Manager very useful.
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Re: trying to spin

Postby wkitty42 » Thu May 23, 2019 3:56 pm

Octal450 wrote in Thu May 23, 2019 1:14 pm:AoA hold is a bizarre thing that doesn't exists in any A/P
IRL I've ever heard of because it makes no sense. AoA with throttle makes sense, but with pitch no.

FWIW: in the sim i use AoA to drop down the glide scope or when taking off to maintain the same angle until i reach my desired altitude... in the UFO it works fine but the UFO is not a real craft, either... i regularly come into airports at high speed and stop on a dime over the runway at which point i then set a low speed (15kts) and move on to parking or maybe even just blast on down the runway and take off again to fly elsewhere... doing this in a simulated or real aircraft is not recommended :mrgreen:
"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."
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Re: trying to spin

Postby wkitty42 » Thu May 23, 2019 4:02 pm

Robertfm wrote in Thu May 23, 2019 1:18 pm:Thank you. To honest as a newbie I find AP & Route Manager very useful.

they are extremely useful... especially when you don't know the craft and you are simply flying around for something to do to stave off boredom... i find myself not liking craft that disable the AP and force you to learn the myriad of buttons, their placements, and their procedures with little readily available and easily accessible documentation... some may have documentation in the craft's install directory but that doesn't do one any good when they do not know where the craft are installed in the first place... if there were links in the menus, it would be a bit easier but then there's the problem of a pdf reader or document writer popping up over the sim and the user knowing how to switch between them... no one thinks about these early steps/stages once they have figured FG out and how to work the craft they fly...

my 2 cents with still vivid memories of my initial newbie time with FG back in 2015...
"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."
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Re: trying to spin

Postby Robertfm » Thu May 23, 2019 5:59 pm

I think you summed it up nicely. I am sure I can't be the only newbie that finds all the technical jargon pretty intense. I look through reams and reams of flightgear stuff and it blows my mind.

It reminds me of when I was a Police officer when they first introduced the Police National Computer. Some of us were sent on a course to learn how it worked so we could instruct others. Problem was this guy was talking Bit & Bites and 0 & 1's.
After a while I said, all we need to know is what buttons do what. Not how the inner workings work. I received a round of applause, it seems everyone was thinking the same but didn't want to say.
Clearly there are many knowledgeable people putting FG together and I admire their knowledge, like my son who writes complex systems in his role in the Royal Navy.
I like FG and simply want to use it and seek simple answers.
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