A little preamble...

 

I have spent many evenings adjusting the base C172 that comes with XPIX in order to get it to behave properly.

 

It took a while to get all the wings, objects, weight and balance and control geometries all set up to match the real aircraft. I must say that I am very impressed at how well the initial results looked. However, the aircraft was still extraordinarily twitchy and unrealistic until I added radii of gyration.

 

At this point the aircraft handles very much like the C172 I fly within the normal flight envelope when in calm weather.

 

I am struggling to get the aircraft to not be blown around like a paper bag when taking off in a 9G12kt nearly direct crosswind. No matter how much rudder or aileron correction I use the airplane will weathervane violently into the wind and drift off the runway. Having just flown in the same conditions that same day, I am 100% certain that this is not correct behavior.

 

Is there something that I have missed in Planemaker that will make the aircraft behave realistically in these conditions when on the ground? Once in the air, it does "feel" about right, but the ground handling is a place where I am stumped!

Hi Dave,

 

I tested the default C172 with wind up to 13kt G17kt on ground and the aircraft is pretty stable. What changes did you make to the model? Maybe you have gone too far...

Is there something that I have missed in Planemaker that will make the aircraft behave realistically in these conditions when on the ground? Once in the air, it does "feel" about right, but the ground handling is a place where I am stumped!

 

Try going (inside Plane-Maker) to Standard->Landing Gear->Gear Data and increase the "maximum co-friction" value e.g. to 0.9. See if that works.

 

Marco

I made a few changes.

 

The wind conditions I am testing with are runway 01, 090 @ 9KT, Shear strength 3 shear direction 1. This is not quite what I stated earlier, as it is gusting, but the gust component is a headwind and not a crosswind.

 

Added Radii of Gyration. This made the aircraft respond with an appropriate amount of inertia.

 

Lowered the nosegear spring tension to 10 and removed the phase out to 5 degrees. This increased the authority of the nosewheel.

 

Moved the Hstab back a few inches bit and slightly increased sweep and control surface relative chord to match the real AC.

 

Modified Rudders to properly model size and shape and reduce control surface travel.

 

Decreased the propeller's mass. The prop was taking too long to change RPMs and the RPMs were too high during final approach at idle power with full flaps at 65 knots.

 

Increased the basic empty weight and shifted the default CG about 3 inches rearward and 6 inches lower. I wanted this to match the AC I use.

 

Adjusted the upward limit of elevator trim travel. Setting the trim to the TO position now results in a Vy climb.

 

I find that even in a 5kt wind w/o gusts, the plane will readily weathervane into the wind and requires a decent amount of rudder to compensate. In fact I need to put in quite a bit of left rudder to compensate for the right side crosswind weathervane tendency.

 

I tried increasing rolling friction as suggested, but all that does is prolong the TO roll and not do much about weathervaning.

 

This modified aircraft is far more stable than the default and very much like what I expect (great job on the implementation of the Planemaker utility!). I just wish the wheels would be more effective when they are touching the ground.

I spent the whole evening testing various maneuvers and configurations in the stick and my custom 172.

 

The biggest impact to handling was from adding the radii of gyration. Every other tweak was merely fine tuning.

 

Unfortunately, I couldn't figure out any way to prevent weathervaning even at taxi speed in a 5 knot wind. I did some more reading on old threads and discovered that this is an old problem. The explanation of the dilemma on computation cycles and frames makes sense to me.

 

I also found p-factor to be too weak during slow flight or soft field take offs. I could not observe asymmetrical thrust with the flight model shown when in slow flight at under 40 knots with flaps fully deployed. Oddly I could maintain a 700fpm climb in this config and attitude at full power. Again this is with either plane.

 

So add long as I remain well within the envelope and don't get too hung up on performance takeoffs, landings or generally getting close to the edge of the envelope, I do like the way the sim handles things.

 

I also tried uploading the plane into the XP10 demo and the behaviors were just about the same.

 

I tried increasing rolling friction as suggested, but all that does is prolong the TO roll and not do much about weathervaning.

 

You do not have to increase the "rolling friction", you have to increase the "co-friction"!!!

 

Marco

Use default gear deflection (spring constants) and damping. The default numbers

makes the gear to soft and the acft banks on the ground in xwind and the problems start.

 

You do not have to increase the "rolling friction", you have to increase the "co-friction"!!!

 

Marco

 

Apologies, I mistook the field name in your post. The max co-friction is already 1, but I can try increasing that a tiny bit more and/or adjusting gear deflection constants to see if I can get a little more grip.

 

I am wondering if we are seeing very different performance out the same default model in XP 9. Is the sim known to behave differently on specific machines?

 

I am surprised that alfletcher said that the model can be stable in 13G17 conditions? I would actually expect it to be very difficult to control under those velocities.

I recorded a second test using these wind conditions, first 10G18 then 15G25 (wind come from the bottom right corner). Look at the full windsock just before the final turn. Here is the result (HD720, 90 Mb file).

http://www.talisman-illustration-design.com/turbulence/files/windyC172.m4v

 

As you can see, the aircraft is pretty stable and weathervane as soon as it leave the ground on takeoff. BTW, the aircraft is AI controlled.

I have never looked into letting the AI fly the plane, but the conditions you are using are dangerous for a RL 172 if they are crosswinds.

 

Does the AI in x plane truly fly an airplane? Is it a fair test?

You are right, those conditions are very hazardous for the C172. The fact that the AI control the aircraft have no impact of the handling performance so I think you should expect same results piloting it manually. It’s just for the purpose of this demonstration.

 

I tested again with more wind and manual control. Same result.

Keep safe and do not fly with more than 15 Kt direct crossing in the C172.

I made one change that had an immediate impact on the handling. I turned down all the rendering options to default settings. The biggest changes were too Texture Res, Number of Objects and Airport Detail.

 

The aircraft is far more controllable now thought the FPS impact is not really all that much IMHO. It was definitely smooth before, so I guess the sim was prioritizing rendering the world over simulating flight to some degree.

 

However, I am still annoyed that the nosewheel of either the stock C172 or my modified C172 are completely ineffective at steering straight ahead when taxiing along at a snail's pace in a 5 knot direct crosswind. I know that this should not be the case at all.

 

All I do is release the brakes and give just enough throttle to start the plane moving. It will immediately rotate into the wind and stop rotating once it is facing into the wind. The only way to stop this is to apply quite a bit of opposite rudder input. The nosewheel should totally counteract this tendency when the wings are generating next to no lift.

 

Is it safe to say that this is just the way it is? It is very strange to taxi and even takeoff with a lot of left rudder in a 5 knot crosswind from the right. Is this the experience others are having? I'm glad that the stability has improved by reducing rendering options, but the ground handling is still strange.

I was doing some more reading on the subject and discovered that out is possible to use plug-ins to override the ground handling behavior. I haven't had any luck on finding an "off the shelf" plug-in to do this.

 

I did find a scripting package called Gizmo that can be used build my own overrides. I also noticed that some 3rd party developers use this approach.

 

Is anyone familiar with off the shelf plugins for this purpose? I just can't find any using Google. :(

Maybe ask Ben Russel himself. He is the mastermind behind the Guizmo project. He is active sometime on AVSIM forum’s.

https://sites.google.com/a/x-plugins.com/www/home