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New physics system of flight

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Re: New physics system of flight

Postby Thorsten » Tue Aug 27, 2019 2:51 pm

Let me get this straight:

* you pull full elevator
* the plane starts pitching up rapidly, but thenat some point stops doing that
* we'd expect that to happen in reality
* but you conclude based on the observation that there is no elevator stall in YaSim that if the plane behaves as expected in reality, there's really something wrong in the FDM?

I'm baffled. :shock:
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Re: New physics system of flight

Postby Icecode GL » Tue Aug 27, 2019 2:58 pm

If you pull the elevator, the aircraft increases its angle of attack, which increases drag, which decreases horizontal speed, producing a stall if uncorrected. Am I correct? I got a fluid mechanics exam next week, gotta get the facts straight. :mrgreen:
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Re: New physics system of flight

Postby V12 » Tue Aug 27, 2019 3:02 pm

Thorsten :
Another example with much smaller elevator deflection :

Image

Result similar...

In any video of real Cit X (and other aircrafts) I never observed this behaviour. Somewhere is some error. It should be in Citation X FDM, or YASIM model. And this is not problem only on Cit X. Similar behaviour I observed on many YASIM aircrafts, but none with JSBSIM.

Icecode GL :
Did You check data and my previous posts ? Sample rate is 10 Hz, not 1 Hz...
Speed loss from Vmo to stall speed in split of the second ??? Possible only at Coffin Corner. And there is reversed situation, I push control forward, speed drop is impossible. All this tests was in speed ranges 200-250 kts.
Last edited by V12 on Tue Aug 27, 2019 4:02 pm, edited 1 time in total.
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Re: New physics system of flight

Postby V12 » Tue Aug 27, 2019 3:40 pm

C550 - very similar results, with uncommanded barel roll. But overall fly much better than Cit X...

Image

Conditions as previous, 10 Hz sample rate, Elevator, Ailerons from -1 to 1, RollRate degs per second
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Re: New physics system of flight

Postby V12 » Tue Aug 27, 2019 3:58 pm

And one of the best flying YASIM aircraft DHC6 wheeled version :

Image

Here is for me strange fact - normalized elevator position is shifted -0.2 - probably some autotrim ?
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Re: New physics system of flight

Postby D-ECHO » Tue Aug 27, 2019 4:15 pm

So when you make large elevator inputs probably leading to a high-speed stall the aircraft makes sudden roll movements? I agree that the stall and post-envelope simulation of YASim is not very good, but an aircraft having a wing drop at stall seems fairly normal.
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Re: New physics system of flight

Postby Thorsten » Tue Aug 27, 2019 4:17 pm

If you pull the elevator, the aircraft increases its angle of attack, which increases drag, which decreases horizontal speed, producing a stall if uncorrected. Am I correct?


At least one person thinks it through... :D

Edit: Make that two.

C550 - very similar results, with uncommanded barel roll.


You know what? I've been flying some YaSim aircraft for more than 10 years in FG. Do you know how often I saw an uncommanded barrel roll?

Never ever.

I'm quite sure you have some idea what you think is going wrong in these plots - but let me assure you - attitude change rates are never ever a function of only airfoil position - so the fact that they do not always follow airfoil positions is nothing to worry about.

Only in Mickey-Mouse FDMs is the elevator the only thing that can cause a pitch rate. In the real world, all sorts of things can.
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Re: New physics system of flight

Postby V12 » Tue Aug 27, 2019 4:45 pm

This is JSBSIM 140 tons liner :

Image

This is JSBSIM 75 tons 737-800YV

Image

As You can see, elevator movement is nicely copied to pitch rate. For me, JSBSIM aircrafts flying significant better than YASIM. That is all. This my statement is not applicable on helo. I have not any experiences with helo in any sims.
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Re: New physics system of flight

Postby Thorsten » Tue Aug 27, 2019 5:12 pm

As You can see, elevator movement is nicely copied to pitch rate.


Actually, I only need to look into the last part of the first JSBSim plot to see nothing of that sort - blue curve is constant, red curve does interesting things.

So whatever you believe you see is not actually there in these plots, sorry.

For me, JSBSIM aircrafts flying significant better than YASIM.


Given how frequently you seem to be pushing them into a stall, I'm kind of surprised...
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Re: New physics system of flight

Postby V12 » Tue Aug 27, 2019 5:21 pm

Many times, some times crashed. My last crash was half year ago with Concorde on night land at rwy 05, Funchal, Madeira. Reason : Stall after speed loss on base turn (VOR-DME approach rwy 05) with more than 30 tons FOB. IDG A32x has Alpha Floor protection, it is near impossible to stall her. I do not fly other aircrafts in FG.

And about charts - Check frame ranges and pitch rates for that "interest things"...
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Re: New physics system of flight

Postby Thorsten » Tue Aug 27, 2019 6:00 pm

I do not fly other aircrafts in FG.


So, to summarize - you have a lot of practice on the Concorde but still stall the bird now and then, and when you fly other aircraft you basically have zero practice handling them, drive them into a stall as a result and wonder why they react in unexpected ways, ascribe that to FDM deficiencies, settle finally for only flying craft which are so automated you can't really stall them, declare their FDM okay and all others bad.

I wouldn't really mind, except for your general tendency to bad-mouth FG and for the fact that less experienced users might actually take you seriously.

I'd suggest to learn to fly the Citation X or the Twin Otter properly - then they don't do 'barrel rolls'.
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Re: New physics system of flight

Postby WoodSTokk » Tue Aug 27, 2019 6:07 pm

V12 wrote in Tue Aug 27, 2019 3:40 pm:C550 - very similar results, with uncommanded barel roll. But overall fly much better than Cit X...

Image

Conditions as previous, 10 Hz sample rate, Elevator, Ailerons from -1 to 1, RollRate degs per second

Was that test with the xml file that i link you?
What speed and altitude are present at the time of test?
I have already turned the C550 up side down only with pull the yoke and the aircraft goes stable up side down.
I'm flow up side down for more then a minute and it was stable.
Sure, thats not realistic because the C550 isn't a acrobatic aircraft, but it shows that full elevator doesn't end in an uncomented barrel roll.
The C550 has a maximum celling of FL430, but this is hard to reach.
Everybody can feel that above FL300 the behaviour is changing because of the thin air and less surface authority.
Make sure that your (acrobatic) tests are made below FL200 and more then 220 kts speed.

I have also flowen the Citation X (only from FGaddon) and i'm also not happy with the FDM.
But i'm concentrate on the C550.
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Re: New physics system of flight

Postby V12 » Tue Aug 27, 2019 6:32 pm

Not, that test was with default fgaddon xml. Altitude 3000 feets, speed from 200 to 250 KIAS. Default 550 fly better than Cit X. I hope, with new FDM it will be even better.
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Re: New physics system of flight

Postby WoodSTokk » Tue Aug 27, 2019 6:47 pm

V12 wrote in Tue Aug 27, 2019 6:32 pm:Not, that test was with default fgaddon xml. Altitude 3000 feets, speed from 200 to 250 KIAS. Default 550 fly better than Cit X. I hope, with new FDM it will be even better.

Yes, it will. If i'm back at home i upload my latest xml file. I'm allways happy to receive test results from other pilots.
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Re: New physics system of flight

Postby legoboyvdlp » Tue Aug 27, 2019 6:56 pm

IDG A32x has Alpha Floor protection, it is near impossible to stall her


There isn't any protection except automatic setting of TOGA... if you pull back you'll still stall. Seems like some Airbus pilots still need to learn that it will stall when AOA protection is disabled - or missing ;)
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