Board index FlightGear Development Aircraft Flight dynamics model

JSBsim Gear is not responsive

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

JSBsim Gear is not responsive

Postby Octal450 » Sat Jun 24, 2017 4:36 pm

Hi all
I'm having a really annoying problem I try to fix for about 3 monthes now -- the gear does not have enough authority. When you steer, it is sluggish, and when you try to straighten, it keeps going in that direction for a a while, as if the gear doesn't have any friction.

- Aircraft is balences nicely in flight
- Aircraft behaves nice in every other way except ground steering.

How do I fix something like this? (Still somewhat new to JSBsim)

(Single engine taxi is IMPOSSIBLE)

Kind Regards,
Josh
Last edited by Octal450 on Thu Feb 15, 2024 8:32 am, edited 1 time in total.
Skillset: JSBsim Flight Dynamics, Systems, Canvas, Autoflight/Control, Instrumentation, Animations
Aircraft: A320-family, MD-11, MD-80, Contribs in a few others

Octal450's GitHub|Launcher Catalog
|Airbus Dev Discord|Octal450 Hangar Dev Discord
User avatar
Octal450
 
Posts: 5583
Joined: Tue Oct 06, 2015 1:51 pm
Location: Huntsville, AL
Callsign: WTF411
Version: next
OS: Windows 11

Re: JSBsim Gear is not responsive

Postby Bomber » Sat Jun 24, 2017 11:28 pm

Yeh I think you answered your own question..
"If anyone ever tells you anything about an aeroplane which is so bloody complicated you can't understand it, take it from me - it's all balls" - R J Mitchel
Bomber
 
Posts: 1933
Joined: Fri Dec 14, 2007 8:06 pm
OS: Windows XP and 10

Re: JSBsim Gear is not responsive

Postby Octal450 » Sun Jun 25, 2017 1:10 am

No -- I have no idea how to fix this.
Skillset: JSBsim Flight Dynamics, Systems, Canvas, Autoflight/Control, Instrumentation, Animations
Aircraft: A320-family, MD-11, MD-80, Contribs in a few others

Octal450's GitHub|Launcher Catalog
|Airbus Dev Discord|Octal450 Hangar Dev Discord
User avatar
Octal450
 
Posts: 5583
Joined: Tue Oct 06, 2015 1:51 pm
Location: Huntsville, AL
Callsign: WTF411
Version: next
OS: Windows 11

Re: JSBsim Gear is not responsive

Postby Thorsten » Sun Jun 25, 2017 7:33 am

I've had a short look, but I can't see anything insane - unless the coordinate system is very weird, the gear is the lowest contact point (we may assume you've tested that the gear compression values are sane and that the plane actually rests on the gear, not on the structure contact points?) Eye-balling the values compared with the weight, it doesn't look wildly off.

Friction coefficients are different from what works for me, but they're in fact higher than mine so that doesn't look like the culprit.

Seems subtle...

Have you tested whether the FCS properties take the values you expect them to when you use tiller/rudder on the ground?
Thorsten
 
Posts: 12490
Joined: Mon Nov 02, 2009 9:33 am

Re: JSBsim Gear is not responsive

Postby Richard » Sun Jun 25, 2017 11:08 am

After some more investigations[1] I think there are a number of different issues all coming together.

The main manifestation of the problem is that it can't perform engine out taxiing; there are other manifestations but these are less definitively wrong (as I'm fairly sure that an aircraft that can't perform single engine taxiing would never make it off the drawing board, or pass flight testing, or gain approval).

The way I'm testing is to load the aircraft, use the config dialog for "ready for taxi", then shutoff an engine, gradually increase power and use the keyboard rudder controls to stay in a straight line. The result is a very excessive yaw moment about a point that seems to be aft of the CoG.

So to identify the problem I've:
1. Checked the geometry using http://www.aircraft.airbus.com/fileadmi ... 20_WTF.zip [2] - maybe the nose gear should be a bit further back, but basically the geometry seems ok.
2. Checked the nose and main gear configurations. Apart from the nose wheel having no brake group it looks ok.
3. Checked the moments of inertia, against p44 of https://www.diva-portal.org/smash/get/d ... TEXT01.pdf - revised based on these figures.
4. Added a flight control system for nose wheel steering with rudder
5. Removed all contact points to ensure that it's not an errant ground strike (it isn't)
6. Remove all of the pointmasses (by setting to minimum values).

The main problem is (4) and (6). If I set the masses to the minimum values (especially those aft of the CG), and the revised NWS I can control in a reasonably straight line up until 120kts (when I run out of tarmac).

The default steering of +/- 70 degrees means 3.5 degrees per keypress; this isn't enough to control with any precision, and more than 15 degrees tends to lead to skidding at anything more than 10kts.

Inspecting the nosewheel compression (with the default mass configuration) the compression is lower than I'd expect[3] - I now think this is the main problem - but the NWS is aggravating the situation, and possibly there is a pendulum affect of the rear masses, however this isn't as easy to determine.

I can either set PAX C to the minimum, or configure as in the picture and (with the NWS change) accelerate contrallably.

Image

The NWS is pretty much out of the F-14, but it's near enough for test purposes.
Code: Select all
<flight_control name="FCS">


    <channel name="Nose Wheel Steering">
        <switch name="position/aircraft-on-ground">
            <description>Aircraft on ground</description>
            <default value="0"/>
            <test value="1" logic="OR">
                gear/unit[0]/WOW ne 0
                gear/unit[1]/WOW ne 0
                gear/unit[2]/WOW ne 0
            </test>
        </switch>
        <switch name="systems/NWS/engaged">
            <default value="0"/>
            <test value="1">
                gear/unit[0]/wheel-speed-fps le 135
                <!--systems/hydraulics/combined-system-pressure ne 0-->
                position/aircraft-on-ground ne 0
                <!--systems/holdback/launchbar-engaged eq 0-->
            </test>
            <output>systems/NWS/engaged</output>
        </switch>

        <scheduled_gain name="fcs/scheduled-steer-pos-deg">
            <input>fcs/steer-cmd-norm</input>
            <table>
                <independentVar lookup="row">gear/unit[0]/wheel-speed-fps</independentVar>
                <independentVar lookup="column">systems/NWS/engaged</independentVar>
                <tableData>
                    0   1
                    0   0   87
                    80  0   15
                    135 0   0
                </tableData>
            </table>
            <output>fcs/steer-pos-deg</output>
        </scheduled_gain>
    </channel>
</flight_control>


-------------
[1] already been trying to help Josh with this.
[2] Airbus Drawings: http://www.aircraft.airbus.com/support- ... -drawings/
[3] Josh mentioned earlier that it might be nose light.
Richard
 
Posts: 810
Joined: Sun Nov 02, 2014 11:17 pm
Version: Git
OS: Win10

Re: JSBsim Gear is not responsive

Postby Octal450 » Sun Jun 25, 2017 2:27 pm

@Thorsten
Yes, I've checked that, all is correct.

@Richard
I really appreciate what you've done here. I'll test it out shortly.

Kind Regards,
Josh
Skillset: JSBsim Flight Dynamics, Systems, Canvas, Autoflight/Control, Instrumentation, Animations
Aircraft: A320-family, MD-11, MD-80, Contribs in a few others

Octal450's GitHub|Launcher Catalog
|Airbus Dev Discord|Octal450 Hangar Dev Discord
User avatar
Octal450
 
Posts: 5583
Joined: Tue Oct 06, 2015 1:51 pm
Location: Huntsville, AL
Callsign: WTF411
Version: next
OS: Windows 11

Re: JSBsim Gear is not responsive

Postby Bomber » Sun Jun 25, 2017 6:51 pm

Weight on the nose wheel varies as you pick up speed and rotate slightly, so it could be that as you pick up speed you lose authority... Observe compression property as pick up speed.
"If anyone ever tells you anything about an aeroplane which is so bloody complicated you can't understand it, take it from me - it's all balls" - R J Mitchel
Bomber
 
Posts: 1933
Joined: Fri Dec 14, 2007 8:06 pm
OS: Windows XP and 10

Re: JSBsim Gear is not responsive

Postby Octal450 » Sun Jun 25, 2017 8:06 pm

But the issue happens also at 5kts... so I doubt it.

Kind Regards,
Josh
Skillset: JSBsim Flight Dynamics, Systems, Canvas, Autoflight/Control, Instrumentation, Animations
Aircraft: A320-family, MD-11, MD-80, Contribs in a few others

Octal450's GitHub|Launcher Catalog
|Airbus Dev Discord|Octal450 Hangar Dev Discord
User avatar
Octal450
 
Posts: 5583
Joined: Tue Oct 06, 2015 1:51 pm
Location: Huntsville, AL
Callsign: WTF411
Version: next
OS: Windows 11

Re: JSBsim Gear is not responsive

Postby wkitty42 » Sun Jun 25, 2017 10:02 pm

if the craft doesn't sit with enough weight on the nose wheel, i would expect it... it is like trying to drive a truck towing a trailer... if the trailer is overloaded, the trucks front wheels are lifted and it looses steering traction... adjust the load of the trailer so that it is better balanced and the truck's front wheel sit on the ground properly or maybe are even forced tighter to the ground (weight at back of trailer lifting truck rear end)... this last would result is easier spinning of the drive tires...
"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."
User avatar
wkitty42
 
Posts: 9148
Joined: Fri Feb 20, 2015 4:46 pm
Location: central NC, USA
Callsign: wk42
Version: git next
OS: Kubuntu 20.04

Re: JSBsim Gear is not responsive

Postby Alant » Sun Jun 25, 2017 11:49 pm

It took me ages to make nose-wheel steering and castering work. You can blame my aged brain, the documentation,or both.

First check that the nose-wheel does steer as expected using an external view and examining the property tree.

I can post my xml you wish, but it is complicated as I also have a nose-wheel castering mode, which is needed if you want differential brakes and differential engines to be effective. Nose-wheel steering can be more powerful than either of these.
Alant
 
Posts: 1219
Joined: Wed Jun 23, 2010 6:58 am
Location: Portugal
Callsign: Tarnish99
Version: latest Git
OS: Windows 10/11

Re: JSBsim Gear is not responsive

Postby Octal450 » Sun Jun 25, 2017 11:57 pm

Yes it's correct.

The issue is strictly with the steering itself.

The problem seems to be mass is distrubted through the airframe, but the engines are missing from the mass!!! So it is tail heavy!!! Will fix an report.

Kind Regards,
Josh
Skillset: JSBsim Flight Dynamics, Systems, Canvas, Autoflight/Control, Instrumentation, Animations
Aircraft: A320-family, MD-11, MD-80, Contribs in a few others

Octal450's GitHub|Launcher Catalog
|Airbus Dev Discord|Octal450 Hangar Dev Discord
User avatar
Octal450
 
Posts: 5583
Joined: Tue Oct 06, 2015 1:51 pm
Location: Huntsville, AL
Callsign: WTF411
Version: next
OS: Windows 11

Re: JSBsim Gear is not responsive

Postby wkitty42 » Mon Jun 26, 2017 8:08 pm

are you adding the engines to the mass when you feed it to the FDM? are you also including the mass of the current fuel quantity?

i remember the c172p having to do certain things depending on which engine was used in the craft so that the weight of the craft work properly for wheeled as well as float configurations... the one engine weighs more than the other but has more power... this affects steering as well as liftoff and general flight...
"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."
User avatar
wkitty42
 
Posts: 9148
Joined: Fri Feb 20, 2015 4:46 pm
Location: central NC, USA
Callsign: wk42
Version: git next
OS: Kubuntu 20.04

Re: JSBsim Gear is not responsive

Postby Octal450 » Mon Jun 26, 2017 8:17 pm

Yes.

Kind Regards,
Josh
Skillset: JSBsim Flight Dynamics, Systems, Canvas, Autoflight/Control, Instrumentation, Animations
Aircraft: A320-family, MD-11, MD-80, Contribs in a few others

Octal450's GitHub|Launcher Catalog
|Airbus Dev Discord|Octal450 Hangar Dev Discord
User avatar
Octal450
 
Posts: 5583
Joined: Tue Oct 06, 2015 1:51 pm
Location: Huntsville, AL
Callsign: WTF411
Version: next
OS: Windows 11

Re: JSBsim Gear is not responsive

Postby AndersG » Mon Jun 26, 2017 10:20 pm

Have you double checked that you have put the right length unit on all positions in the JSBSim configuration?
E.g. a long long time ago some positions I (thought I) had specified in meters where read as in inches by JSBSim - giving rather unexpected results.
Callsign: SE-AG
Aircraft (uhm...): Submarine Scout, Zeppelin NT, ZF Navy free balloon, Nordstern, Hindenburg, Short Empire flying-boat, ZNP-K, North Sea class, MTB T21 class, U.S.S. Monitor, MFI-9B, Type UB I submarine, Gokstad ship, Renault FT.
AndersG
 
Posts: 2525
Joined: Wed Nov 29, 2006 10:20 am
Location: Göteborg, Sweden
Callsign: SE-AG
OS: Debian GNU Linux

Re: JSBsim Gear is not responsive

Postby Bomber » Mon Jun 26, 2017 11:31 pm

The trick here is to have a mass ballance spreadsheet which you use to check against the cog of the jsbsim flight model...... pretty sure I've included such in my previous work.
"If anyone ever tells you anything about an aeroplane which is so bloody complicated you can't understand it, take it from me - it's all balls" - R J Mitchel
Bomber
 
Posts: 1933
Joined: Fri Dec 14, 2007 8:06 pm
OS: Windows XP and 10

Next

Return to Flight dynamics model

Who is online

Users browsing this forum: No registered users and 2 guests