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New physics system of flight

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New physics system of flight

Postby Bowblend » Fri Aug 23, 2019 11:10 pm

Where is the best planes physics system? Xplane11 have the best. Here's my idea. If developers want to make the most realistic flight experience as they can they need to figure out how physics works in xplane11 and recreate in flight Gear.

I know that is not possible to charm data out of the manufacturer. Maybe there is some way to copy aerodynamic tunnel data(or some plane physics datas) from Xplane11 and paste it to Flight Gear planes.


Here is some video to help with it:
https://youtu.be/O_NDeSPCMks
https://www.x-plane.com/desktop/how-x-plane-works/
Last edited by Bowblend on Tue Aug 27, 2019 8:10 am, edited 3 times in total.
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Re: New physics system of flight

Postby wkitty42 » Sat Aug 24, 2019 1:44 am

i guess you're not aware of the three or four different FDMs in flightgear, are you? at least one is also used by NASA on a daily basis... you should probably study a bit more before making such silly statements ;)
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Re: New physics system of flight

Postby Parnikkapore » Sat Aug 24, 2019 3:57 am

TLDR: We can use actual numbers from wind tunnels, but X-plane can only approximate them from the specifications. That said, we also support an approximation engine (YASim) which is used by lower-quality planes. That one is worse than X-plane.
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Re: New physics system of flight

Postby Thorsten » Sat Aug 24, 2019 4:24 am

Where is the best planes physics system? Xplane11 have the best.


I recommend a basic course in aerodynamics - you clearly confuse physics with marketing announcements. When you sit down a few minutes and think about it, there would not be any reason engineers make wind-tunnel tests and conduct hour-long fluid dynamics simulations to derive a few coefficients when there's another scheme which can get it all in 1/60 of a second at better quality.
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Re: New physics system of flight

Postby V12 » Sat Aug 24, 2019 7:26 am

Bowblend :
JSBSIM FDM in FG is very good. There is other problem - in FG is too few planes with properly implemented FDM. Many airplanes in FG flying like RC models with mass 10 kilograms, not hundreads tons. But this is aircraft developer's problem, not FG FDM.
What plane did You tested to make this conclusion ?
Check Boeing 707, or IDG Airbus 32x.
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Re: New physics system of flight

Postby legoboyvdlp » Sat Aug 24, 2019 7:56 am

This is ridiculous - XPlane's method is at best a very good guess (with a lot of very odd things). If you want to plug in documents with thousands of coefficients - too bad, you can't!
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Re: New physics system of flight

Postby LesterBoffo » Mon Aug 26, 2019 11:51 am

V12 wrote in Sat Aug 24, 2019 7:26 am:Bowblend :
JSBSIM FDM in FG is very good. There is other problem - in FG is too few planes with properly implemented FDM. Many airplanes in FG flying like RC models with mass 10 kilograms, not hundreads tons. But this is aircraft developer's problem, not FG FDM.
What plane did You tested to make this conclusion ?
Check Boeing 707, or IDG Airbus 32x.


Where exactly are these FG planes flying with 10 kilogram weights? If you are going to post hyperbole you should back this up with links.. :roll:

I always use the unloaded weight of the aircraft to calculate my FDMs when I make an aircraft. When you can build an airplane from scratch like I do, maybe we'll have something to discuss.
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Re: New physics system of flight

Postby V12 » Mon Aug 26, 2019 5:01 pm

For example, most of YASIM aircrafts have very strange FDM.
Or short engine spool up time - for example all 737s, or Bus 340, another example - rudder reaction with large accelerations - again all 737s,
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Re: New physics system of flight

Postby fmg » Mon Aug 26, 2019 5:59 pm

V12 wrote in Mon Aug 26, 2019 5:01 pm:Most of YASIM aircrafts have very strange FDM.

Examples please, for that we could improve them.
There are also some rather strange JSB planes. DC-6B for example. Impossible to land with flaps out. I made a Yasim-FDM for it that works quite well. It fit's the (few) data that I have for that plane. Tried one with JSB first with Aeromatics, what was also bad.
Guess it's not always the FDM-system, but garbage in, garbage out.
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Re: New physics system of flight

Postby V12 » Mon Aug 26, 2019 6:20 pm

CitationX, Cessna 550, Citation Bravo, DHC6 - strange behaviour on all control inputs
747-400, 747-8i - near Vmo very unstable
Cessna 172P (in 2018.3.1, it is JSBSIM) - too sharp reactions on all control inputs, too strong P - factor

I can continue all night, there are many aircrafts in default hangar...

For me, perfect developed airliners :
- IDG 3xx families (still missing MCDU)
- B707 (i miss real INS)
- Lockheed Super Constellation
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Re: New physics system of flight

Postby Thorsten » Mon Aug 26, 2019 6:30 pm

You can't gauge control input reliability, because essentially the HW you use is very different from what the plane uses, and you control the sensitivity factor of your own HW yourself.

Config oddities on your side do not equal FDM deficiencies.

I can continue all night, there are many aircrafts in default hangar.


You sure seem to have memorized plenty of aerodynamical data and combine that with real flight experience on pretty much the whole FG hangar - there's not many people in the world who can claim that :shock:

Okay, just kidding.

Aka - it's pretty damn hard to benchmark FDMsby 'just flying' - not even real-life pilots can do that accurately by the feel of it. But it's pretty damn easy to produce a few dismissive sentences because it doesn't react like FSX (where your flight experience really seems to come from...) - but who knows - maybe FSX is wrong :evil:
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Re: New physics system of flight

Postby V12 » Mon Aug 26, 2019 7:00 pm

I'm not comparing only with FSX. Default FSX planes are too easy to fly. I have some experience from EASA FTD Level 2 sim 737-800NGX. Then I virtualy flied with PMDG737 on FSX and P3D 3.4 too, all have smoother reactions than all 737 in FG.
This is Cessna X, is that behaviour OK ?

Ofcourse, barel roll has been fixed, but acceleration after elevator inputs is still abnormal.

Again, there is not problem with FDM (JSBSim) in FG. Problem is not ideal implementation of that FDM in many FG aircrafts.
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Re: New physics system of flight

Postby Thorsten » Mon Aug 26, 2019 7:27 pm

Then I virtualy flied with PMDG737 on FSX and P3D 3.4 too, all have smoother reactions than all 737 in FG.


Yeah, so you know one simulator doesn't react like the other - what you do not know from the exercise is which one is wrong. What you don't know is what happens if you re-calibrate your control device - maybe then FG reacts smoother and the others feel wrong.

This is Cessna X, is that behaviour OK ?


How should I know - video doesn't show on my computer and I can't judge FDMs by looking at a video anyway, the whole idea is silly.

(That's not to say that there are no lower rated FDMs in FG - sure they are - but you can't judge that by the feel of it on the controls).
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Re: New physics system of flight

Postby V12 » Mon Aug 26, 2019 8:49 pm

OK, I will try to tell it with another words - PMDG 737 behaviour on P3D and FSX is closer to the EASA FTD Level 2 sim 737-800NGX than all 737 in FG.
If You think that looking at video is silly, then You can make "blackbox" recording all control surface positions and aircraft accelerations along all axis and then analyze recorded data.
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Re: New physics system of flight

Postby Alant » Mon Aug 26, 2019 10:15 pm

It is easy to to complain, but FG is a free system, with contributions from anyone who feels free.

As far as aerodynamics is concerned I can only comment on JSBSim, as this follows the same methods that I was taught at university, and after that when I worked in flight simulation. (I am now retired)

The main difficulty is finding a source of aerodynamic flight data. The aircraft manufacturers hold detailed data close to their chest, After all it cost them a small fortune to generate using wind tunnels, flight test, and more recently CFD. They can also license it to flight simulator manufacturers.

So , unless he can charm data out of the manufacturer, any JSBSim FDM developer has to fall back on alternative sources.

In order of complexity these include:
Flightgear´s own Aeromatic, which produces a generic estimate - probably equivalent to YASIM. Unlike the other methods below it requires little or no knowledge of aerodynamics.
Google will bring up lots of data, but very rarely will this cover the whole flight envelope - usually the data is valid for single case such as cruise or approach.
Datcom which only requires basic geometric data describing the aircraft and its components (wings, tail fuselage etc.) Sadly it gives no estimate of rudder effects, but this is not a great problem as a generic estimate (eg from Aeromatic) is usually sufficient . A limited knowledge of aerodynamics is required.
Panel and Vortex Lattice methods such as OpenVSP, Panukul, XFOIL, AVL which use similar geometric data to that used by Datcom, but much more care and aerodynamics knowledge is needed in setting up the problem. If we are to believe Xplane´s blurb they use a real time version of these methods, but to be real-time it must be very simplified.
CFD is the ultimate, but this requires large computer resources and a good (i.e. post graduate) understanding of fluid dynamics and mathematics. It is very easy to get, without warning, very inaccurate results as to make computer processing time acceptable there are a number of CFD methods, each of which is only relevant for limited range of geometry and speed . Pick the wrong one, or set the fluid conditions slightlynwrong and the output is garbage.

The Flightgear community does not have sufficient experienced aeronautical engineers to do this kind of work.

Many JSBSim FDMs have been fiddled with until they "feel right" - with various degrees of success. Unless these changes were done carefully, and the results checked against known aircraft performance data this technique is somewhat unreliable and usually has undesirable side effects.

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