[NSRCA-discussion] Weather Vane

[Jim Alberico]

> -----Original Message-----
> From: James Oddino [mailto:joddino at socal.rr.com] 
> Sent: Saturday, October 13, 2007 5:29 PM
> 
> I think we've found our expert.  Two questions:

LOL.  I wish my thumbs had more expertise!!

> 
> 1) Is it not considered a sideslip if the relative wind is on 
> the leeward side?

<VBG>
Maybe file "a relative wind on the windward side" with the Department of
Redundancy Department. -- maybe --

I meant "windward side" in the larger sense relative to described maneuver.

One can cause a sideslip on either side of the vehicle, of course, at any
time.  

My stall turns sometimes have quite a bit of sideslip on the opposite side
when I turn way too early and the fuse has already rotated 90 deg with the
CG still going upward.  On a calm day, that is 90 deg of sideslip, meaning
the relative wind is coming straight downward onto the side.  


> 
> 2) How does a gust cause the plane to yaw when a steady wind will not?

By the same reasoning exquisitely detailed by Ron Lockhart in the "Weather
Vane or Crab" thread.  The rotation (or "correction" or "weathervane") due
to directional stability can only occur whenever the airflow is not aligned
with the fuse (i.e., a sideslip).  The assumption in our example is the cg
is moving downwind at the same rate as the wind.  Therefore, no sideslip,
and no turn. 

Consider a steady wind of 10 mph, and our "vertical" is tracking downwind at
that rate.  Then consider a sudden increase to 15 mph.   There is a delay
time for the CG to accelerate from 10 to 15 mph. During that acceleration,
the aircraft "sees" a relative wind component at its side.  Initially that
component is 5 mph.  That is when the rotation into the wind would begin in
our example.

Hope this helps.


> 
> Thanks in advance, Jim
> 

You are quite welcome.