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When the real aircraft behaves differently .. what to change

Good sims require good FDMs (the "thing" that makes an aircraft behave like an aircraft).

Re: When the real aircraft behaves differently .. what to ch

Postby Darktrax » Sun Feb 07, 2016 12:31 pm

Bomber wrote in Sat Feb 06, 2016 1:18 pm:I've another question that might shed some light on my poor thinking...
When a plane is at best glide and descending at 28:1.... is it flying at 0 degrees AoA or somewhat closer to 2 degree ?

open the the floor this question.
Simon
Er.. OK, allowing that I am now getting well out of my depth, here goes..
To get opinions from from guys soaked with aeronautical knowledge, we probably have to state the question in more general terms.
For this, we are allowed to get away from the exact detail of ASK13 numbers for a bit.

As I understand it (still learning at this time)..
1. We start with knowing the polar curve. We have plots and tables of sink rate vs airspeed, and calculated glide ratios.
2. Sink rate is then easily also expressed as a glide angle. Not the same as aircraft local attitude angle at the time, but is useful concept.
3. The chord line between leading and trailing edge of the airfoil deciding angle of attack for forward progress, no matter how the rest of the airplane is built around it. It is the one thing that connects to the angle other surfaces like h-stab, or tailplane might have, relative to the wing chord.
4. We can see from the side view on the plans that the airfoil is set at some designed angle relative to the rest of the structure as shown.
Our only real grip on how much the airplane is nose-up or nose down at the time, and hence how much lift there is and where it acts, is via the chord line of wing, and it's angle relative to what we think of as forward progress.

For me, the speed vector is where in space the centre of mass is heading, no matter how the airplane is positioned around it!
The wing chord position at the time then forces where everything else is, and where the forces act, and their directions. Some may be from aerofoil action, some from good old gravity, whatever. Very likely JSBSim programmers have long ago defined how these are done.

So..
Q(1) What is the best way to take the numbers we have, and turn them into suitable inputs for a JSBSim program?
Q(2) What is the best route to adjust what the JSBSim program does to make the final ride look something like the reported behaviour of the real thing?
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Re: When the real aircraft behaves differently .. what to ch

Postby josware » Sun Jul 05, 2020 10:52 pm

I wish to take this up 5 years later. A simulator will be ideal for spin training before experiencing the real thing. I learn in a real K 7 which I believe is quite similar to the ASK 13 which I use in Flightgear. My question: Can a pilot point me to how to set up the simulator to get a close to real spin behaviour of the aircraft (ASK 13 or even another glider).

I am on Debian GNU/Linux 9.12 (stretch), Flightgear 2019.2.0

Some more videos:
K13 glider spinning and stalling at the London Gliding Club
https://www.youtube.com/watch?v=vRlpemLBh6I

Harry spinning a K13 glider at the London Gliding Club
https://www.youtube.com/watch?v=pk5rzIP2Cf8

Glider Spin / Spinning Glider L-23 !! Awesome aerial takes !!
https://www.youtube.com/watch?v=SwoBhc0i5gI
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Re: When the real aircraft behaves differently .. what to ch

Postby Thorsten » Mon Jul 06, 2020 6:31 am

Can a pilot point me to how to set up the simulator to get a close to real spin behaviour of the aircraft (ASK 13 or even another glider).


First I guess you need to use JSBSim (YaSim doesn't spin to my knowledge). Then you basically need to set up the simulation to account for what happens in reality - a spin is an asymmetric stall on one wing at high AoA.

Which means you need to do the wing aerodynamics calculations separately for each wing and do the lift/drag curves high enough in AoA to put in the stall - and that should give you at least the onset of a spin. From there you could look into moment generation what is needed to realistically sustain it.

There's a few aircraft that spin, I believe the F-15 does, probably also the C-172p and likely the P-51D - you could take a look at these examples.
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Re: When the real aircraft behaves differently .. what to ch

Postby wkitty42 » Tue Jul 07, 2020 3:41 am

the c172p definitely has spin modeled... i remember reading numerous messages about it during the development of the latest craft some 3 or 4 years ago ;)
"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: When the real aircraft behaves differently .. what to ch

Postby erik » Tue Jul 07, 2020 7:53 am

My believe is that it all comes down to getting the low Reynolds numbers for the particular aircraft and using them in JSBSim. I've sort of implemented in AeromatiC++ but it relies on a trigger to get the spinning started. When using the wind-tunnel data at low Reynolds numbers it should fall all into place I believe.

Erik
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Re: When the real aircraft behaves differently .. what to ch

Postby Alant » Tue Jul 07, 2020 10:28 am

To do a spin with JSBSim, as a very minimum, the wings need to be modeled separatly, Each one has its own incidence and airspeed. Better still split the wing into chordwise elements. See Prof Selig´s papers e.g. https://m-selig.ae.illinois.edu/pubs/RaghebSelig-2015-AIAA-2015-0548.pdf
It gets more complicated as the flow over one element is affected by the flow from its neighbours.
The accuracy of the simulation depends upon how much detail the FDM writer wants to go to, and how good is his understading of this type of non-linear airflow. I would put this at postgraduate university level.
Of course an artisitic approach could be taken instead, tweaking forces and moments until it "feels right".
Alan
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Re: When the real aircraft behaves differently .. what to ch

Postby erik » Thu Jul 16, 2020 7:37 am

I just rediscovered a link to an analysis tool for airfoils, wings and planes operating at low Reynolds Numbers which I had saved once. I did not test it myself yet but it might come in handy:
http://www.xflr5.tech/xflr5.htm

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