Austin's Blog...

[Paraffin 2,197]

1 hour ago, Murmur said:

This is the default L5 modified with a jet engine to eliminate propeller effects. The behaviour on takeoff and landing seems better, pointing to a flawed modeling of propeller slipstream. Would be interesting to hear opinions. (Since the aerodynamics have not been changed, the weathervaning tendency on crosswind is still the same).

https://drive.google.com/open?id=0B5fyJwRhjar5OGt1bDNaRl91c1k

Did a quick test and it does seem to handle better. As you said though, the weathervaning is still there with just a very light crosswind.

I set a direct crosswind of just 5 kts from the left, and applied a gentle amount of throttle to keep moving at taxi speed but not a full takeoff roll. With feet off the rudder pedals, the plane will slowly turn left into the wind and eventually run off the runway. When I change the 5 kt crosswind so it comes from the right side instead, moving forward at slow taxi speed with feet off the rudder has the plane steering off the runway to the right. The effect is stronger as the wind velocity is raised. 

As a check for "weight on wheels" that might affect this, I tried the same tests with the Cirrus jet to see if the behavior changed with a heavier (if still small) aircraft. And yep, it will eventually self-steer off the runway with a slow taxi and a 5 kt crosswind from either direction, turning into the wind.

The Cirrus jet behaved about the same as the Stinson L-5 at something like three times the aircraft weight, and there is no way a measly 5 kt crosswind should push around a plane like this. So maybe we're looking at not enough tire friction (in the sideways direction of applied wind force). It doesn't seem to be related to amount of weight on the wheels.

All this of course, is in addition to whatever is going on with prop slipstream and anything else, but that weathervaning will really mess up a small plane when it gets up around 10-15 knots of crosswind. I don't know why Austin can't seem to acknowledge this. It has never showed up on the official known bug list.

[Murmur 3,941]

1 hour ago, Paraffin said:

Did a quick test and it does seem to handle better. As you said though, the weathervaning is still there with just a very light crosswind.

If I remember correctly, from experiments I did some time ago on the default C172, X-Plane tends to overestimate the directional stability (weathervaning tendency) a lot. And I mean a lot. The cause is unclear.

I should modify the default C172 or L5 to give it proper directional stability, and then see how it behaves in crosswinds.

 

[strider1 575]

15 hours ago, jcomm said:

P-factor is indeed active at any AoA, provided you're not aligned with the relative wind. 

Yo-Yo ( ED / DCS ) tried to achieve the best possible modelling of it's effects starting with the P51d, where inflight, in a sideslip situation you would clearly feel the pitching up or down moments ( in that case they manifest themselves in pitch instead of in yaw... ) depending on if the sideslip was left or right.

And, on a different matter,  also think Murmur's "thesis" about the origin of the poor ground behaviour of aircraft under x-wind is probably due to an overdone / overcalculated weathervane effect. Maybe it's also being calculated twice :-)

And yes, if the propwash update was in since beta4 of 11.0, than I also hope Austin continues to work on that... The "Visual Flight Model" mode should also allow us to glimpse some of the effects.

 

 

P-Factor should increase with a higher angle of attack ? Combine that with slip stream and you should be stepping on the rudder hard to correct for yaw. Thats my experience in a real 172 !

I emailed Austin last night about P-Factor and his response was "we do have p factor it will be more accurate in 11.10"

I did not ask about poor ground behavior, I will leave that up to you guys !

[jcomm 5,890]

So, apparently, many things will get fine tuned by 11.10.... All we can do is wait and see :-)

[GusRodrigues 1]

16 hours ago, strider1 said:

P-Factor should increase with a higher angle of attack ? Combine that with slip stream and you should be stepping on the rudder hard to correct for yaw. Thats my experience in a real 172 !

I emailed Austin last night about P-Factor and his response was "we do have p factor it will be more accurate in 11.10"

I did not ask about poor ground behavior, I will leave that up to you guys !

Austin is aware of a lot of issues were left unchecked from the transition from XP10 to XP11.

the 11.1 will be a great upgrade for the FM for JetEngines too. But I want a better documentation with planemaker.

[Humpty 751]

9 hours ago, jcomm said:

So, apparently, many things will get fine tuned by 11.10.... All we can do is wait and see :-)

hmmmm

[Humpty 751]

19 hours ago, Murmur said:

This is the default L5 modified with a jet engine to eliminate propeller effects. The behaviour on takeoff and landing seems better, pointing to a flawed modeling of propeller slipstream. Would be interesting to hear opinions. (Since the aerodynamics have not been changed, the weathervaning tendency on crosswind is still the same).

https://drive.google.com/open?id=0B5fyJwRhjar5OGt1bDNaRl91c1k

I will try it out Murmur