[NSRCA-discussion] Weather Vane

[Jim Alberico]

> -----Original Message-----
> From: James Oddino [mailto:joddino at socal.rr.com] 
> Sent: Friday, October 26, 2007 12:13 AM
> To: alberji at charter.net; NSRCA Mailing List
> Subject: Re: [NSRCA-discussion] Weather Vane
> 
> Hi Jim,
> 
> You have the picture.  The airplane is essentially hovering 
> horizontally with zero ground speed in any direction just 
> before touch down.  I don't call that crabbing.  And yes I 
> did twiddle the ailerons and the throttle all the way down.  
> As I see it, as soon as I steer with ailerons the airplane 
> starts side slipping, and as I recall from your explanation, 
> an airplane will weathervane if it is side slipping.  I 
> believe this is what we all experience when landing in a 
> cross wind and why we believe the airplane starts to crab all 
> by itself.  Make sense?
> 
> Jim O
> 

Yup.  Makes good sense, Jim.  I would only add for clarity the fine points
that sideslip is not a visual thing (except if you are inside the plane,
looking at a piece of yarn taped to the windshield), and that sideslip can
be caused by dynamics other than rudder or aileron inputs.  I hope the fine
points don't cause more confusion.  :-)

A side note:
For grins, I experimented with this in G3.5 with the Venus II model.  Great
sim, but the tailwheel sideforce (due to friction) is not realistic.  The
plane weathervaned when just sitting on the runway like it was on ice, or
more like the tailwheel was freely castering.  Illustrates the weathervane
effect very nicely, though.

I do know from experience that accurately simulating all the physics of the
gear/tires is a challenge.  It would be interesting to know how well the
other sims deal with it, although that should certainly not be much of a
criterion for choosing one sim over another.    

A second side note:
Long ago, I used to fly full scale gliders, and always did have a piece of
yarn to look at.  That was the turn coordination instrument, still commonly
used.  A "coordinated turn" has no sideslip.

Jim A

> 
> On Oct 25, 2007, at 6:38 PM, Jim Alberico wrote:
> 
> >
> >
> >>
> >>> If you didn't crab into the wind and the airplane is
> >> aligned with the
> >>> runway but drifting sideways, you'd have to time the
> >> touchdown to land
> >>> on the runway.
> >>
> >> At touch down the plane isn't crabbed. It is perpendicular to the 
> >> runway headed into the wind, and it got there without me 
> touching the 
> >> rudder.
> >>
> >
> > :-D
> > LOL.  It's crabbed 90 degrees, then.  So-calling crabbing has no 
> > physical meaning except for reference to some straight 
> ground track.  
> > Here we'd have to assume the reference is the line running down the 
> > runway centerline.
> >
> >>> Either way, without cross-controlling, you're going to put
> >> side loads
> >>> on the landing gear when you touch down.
> >>
> >> There is no side load on the landing gear as there is no velocity 
> >> vector along the runway.
> >>
> >
> > At those high wind speeds, that makes sense.  ...but, I bet you 
> > twiddled with the ailerons and throttle all the way down.  :-)
> >
> >
> > Jim A
> >
> >
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