XP11 winds, aircraft and weathervaning ...

[Murmur 3,941]

Been tracking this thread and I am confused. Did Austin really focus on tire force modelling when NOT moving as opposed to tire force modelling when moving and in weather? That seems like an absurd prioritization. Or am I comparing apples to oranges here?

 

Here is the thread I am referring to: http://forums.x-plane.org/index.php?/forums/topic/111423-coming-in-xp11-beta3-tire-force-modelling-when-the-plane-is-not-moving/

 

Someone tell me I am missing something because this seems like a much more pressing concern than the one he tackled?

 

The tire model when the aircraft is moving is pretty much correct. Maybe it could be tuned a little for things like slip angles, etc., but the forces the tires can apply when the aircraft is moving are accurate.

 

Austin just changed the tire model X-Plane uses when the aircraft is not moving. Of course this is relatively unimportant, and mainly effects the behaviour of the aircraft during engine run-ups, etc.

 

Now, the issue of excessive weathervaning discussed in this thread, is not, as previously thought, due to an inadequate tire model, but it's apparently caused by an overestimation of tailfin sideforce by X-Plane flight model.

 

So that is the issue that should be corrected, not the tire model.

Can someone give feedback on the modified C172 .acf and its behaviour in crosswind? Especially comparing it to the original one.

[strider1 575]

Austin works on what he's interested in at the time. Apparently it has bugged him for a while that running up the engine while standing on the brakes doesn't lower the nose, so he decided to improve that aspect of the sim. 

 

I wonder if maybe Austin hasn't noticed or cared about this weathervaning issue because he never flies tailwheel planes in the sim or real life, where the problem is more noticeable.

 

Anyway, if enough people mention unrealistic weathervaning -- and this is the PERFECT time for it, during a beta before major release -- then maybe it will get addressed. It can take a while when the parameters of the problem aren't known, but it sounds like Murmur has made good progress here.

 

Filed a bug !

The tire model when the aircraft is moving is pretty much correct. Maybe it could be tuned a little for things like slip angles, etc., but the forces the tires can apply when the aircraft is moving are accurate.

 

Austin just changed the tire model X-Plane uses when the aircraft is not moving. Of course this is relatively unimportant, and mainly effects the behaviour of the aircraft during engine run-ups, etc.

 

Now, the issue of excessive weathervaning discussed in this thread, is not, as previously thought, due to an inadequate tire model, but it's apparently caused by an overestimation of tailfin sideforce by X-Plane flight model.

 

So that is the issue that should be corrected, not the tire model.

Can someone give feedback on the modified C172 .acf and its behaviour in crosswind? Especially comparing it to the original one.

 

I setup a 30 knot crosswind. No weathervaning with the brakes on, and very little weathvaning with breaks off ! I did not notice any excessive weathervaning on takeoff either ! Seems like a fix to me !

[jcomm 5,939]

Murumur, I will only be able to test tomorrow :-/

[Humpty 751]

I tried Murmur .acf file , @KPSP using NOAA , on the runway with the breaks ON the aircraft is still , when i release the breaks she starts going to the left slowly, now NOAA wx for KPSP has wind from 040 @4kt and the current rwy I am on is 13R so that 4 kt wind is hitting the tail so hard ?

 

So is that tail over reacting ? 

 

BTW the Palm Springs area does look good with the lighting. I just need 10 fps from somewhere to reach 20 fps

 

Now something else , not sure what that is, i busted the G limit so it put me on Rwy 31R same wind conditions , I release the brakes and she slowly moves forward and a bit to the right, throttle to idle , now here the wind is hitting the right side of the tail fin,

 

One another thing i noticed, I changed the airport to KOAK rwy 11 , again NOAA wind is 30009KT , when i am on the take off roll and even at 10kts i pull the pitch up she prances like a horse.

 

About the prancing horse , does it mean the COG of the 172 is in the tail ? i ask because of the tail dragger post from a CFI regarding tail draggers in DCS. tricycles have the COG in the front right. 

 

But Austin did give her new shock absorbers very nice.

[Murmur 3,941]

I tried Murmur .acf file , @KPSP using NOAA , on the runway with the breaks ON the aircraft is still , when i release the breaks she starts going to the left slowly, now NOAA wx for KPSP has wind from 040 @4kt and the current rwy I am on is 13R so that 4 kt wind is hitting the tail so hard ?

 

So is that tail over reacting ?

 

If the engine is at idle, it will pull the aircraft forward and then it will turn into the wind. Have you tried with engine off? In that case, mine stands still up to 30kts of crosswind.

 

Also, have you tried a takeoff with crosswind? I can takeoff and land with a 25 kts crosswind using this modified C172.

[Humpty 751]

If the engine is at idle, it will pull the aircraft forward and then it will turn into the wind. Have you tried with engine off? In that case, mine stands still up to 30kts of crosswind.

 

Also, have you tried a takeoff with crosswind? I can takeoff and land with a 25 kts crosswind using this modified C172.

 

Not with the engine off, i will try that later in the morning and post the feedback. 

 

Yes i did try the x-wind take off and it did seem ok but that was around 4kts it did pull a bit to the left though.  I can add more speed in the wind when i run the test again.

 

Hmm why would an idle engine cause that effect ? is it due to the disc spinning at a low rpm ? Sorry to ask but i don't understand.

 

BTW i have mentioned the 172 pitches up like crazy when i apply a good amount of back pressure during the take off roll even at 10kts. My stick settings are all to the left. Is that a glitch ? it's like doing a wheely on a bike. 

[Murmur 3,941]

Uhm... I don't know, I haven't touched the center of gravity, so it should not pitch up like that. Make sure the CoG is set to 0.0 in the weight and balance screen.

[Humpty 751]

Uhm... I don't know, I haven't touched the center of gravity, so it should not pitch up like that. Make sure the CoG is set to 0.0 in the weight and balance screen.

 

Sure will check it out and run the tests. thanks.

[jcomm 5,939]

Well,

 

for me the weathervanning, even at only 8 knot x-wind components it's still present in it's overdone way Murmur, and I do use stick / yoke fwd pressure during the takeoff / taxi...

 

I remember that with my 744 experiments I defined a wing, actually a surface ( rectangular ) all along the fuselage, and latter reshaped it to be a bit higher near the first thir of the fuselage to mimic the upper stack of the 744. It was a "wall", with a symmetric airfloil, and produced the closest to what I had in PSX behaviour under x-winds...

[Humpty 751]

i ran the test again in the morning with a x-wind of 30 kts, with the engine ON and brakes applied 172 is standing still, once brakes are released it starts to move forward and then into the wind. With engine OFF same moves forward and then into the wind with the brakes OFF.  x-wind take off was good.

 

I need to try the tail draggers too

 

BTW forgot to check the weight and balance will do that once i get back home from work in a few hours.

[jcomm 5,939]

But I think that under a 30knot x-wind a C172 unbraked would certainly weathervane IRL too....

[Humpty 751]

But I think that under a 30knot x-wind a C172 unbraked would certainly weathervane IRL too....

 

yeah could be but it was also happening @4kts which i tested last night , gotta try taildraggers as these are pretty horrible in XP during these conditions

[Murmur 3,941]

In my case, if unbraked and with engine off, it stands still up until to about 30 kts crosswind (if you gradually increase the wind).

[Humpty 751]

In my case, if unbraked and with engine off, it stands still up until to about 30 kts crosswind (if you gradually increase the wind).

 

Well i can try with a 20kts x-wind with brakes off and engine off  and then increase to 30

[jcomm 5,939]

In my case, if unbraked and with engine off, it stands still up until to about 30 kts crosswind (if you gradually increase the wind).

 

Ok Murmur, that made the difference - started with 5knot fro portside, and increased 5 knot for each test until 30.

 

- The struts show the force, but the nose wheel stays put;

- At 33 knot the nose wheel moved a bit;

- at 37 knot and above the aircraft weathervanes with the nosewheel sidesliping always aligned with the aircraft longitudinal axis,