[NSRCA-discussion] Weather Vane

[Jim Alberico]

Jim Oddino said:

>>>> ====
Wish we had a real aero guy jump in here. ...

Best Regards, Jim
==== >>>>

OK, Mr. Oddino, I'll take the bait.

I qualify as a real aero guy much more than as a pattern pilot.  ;-)

This whole thread started with the dynamics of a stall turn in a crosswind
(I think).

Point 1:
Initial Conditions 
-- steady wind from the right (constant velocity and direction).
-- constant aircraft velocity (or vertical rate of the cg)

To track a vertical line, the fuse must be pointed to the right.  Under
these conditions, the thrust line counters gravity as well as the components
of drag in the vertical and horizontal directions.

This is similar to the familiar "crabbing" in horizontal flight to maintain
direction.  However in the vertical geometric plane, gravity complicates
things a bit. ...enough about that.

Note that in crosswind, rudder trim is required to maintain the horizontal
line, as well as the vertical line.  This is important.   Any "weathervane"
effect under these conditions is through control inputs.  It doesn't come
for free.

Point 2:
Dynamics Near Stall  (dynamics are always more complicated)
-- wind still steady from right
-- aircraft is decelerating to zero, then accelerating again.

As the vertical speed (and overall velocity) decreases, the fuse must be
pointed more and more to the right to maintain a vertical track. Here is
where it is very hard to generalize.  So much depends on the details of the
aircraft shape, mass properties, control inputs (including throttle), the
wind speed, and etc.  At zero speed, for example, the ONLY force countering
the wind will be thrust.  Sideslip forces certainly enter the picture on the
final path up and the initial path down. 

Note also that the fuse must point leftward during the downward phase to
maintain the line (hence turning into the wind requires less than 180 deg
rotation).  Again, pilot provides the rudder trim to maintain the line.
Here fuselage side force plays greater role, as thrust is significantly
reduced.  

...

Regarding weathervane, the phenomenon is certainly present, but only in
dynamic situations.  

... But again, depends on what you mean by weathervane ... In 3D, is a
steady angled hover in a steady wind considered weathervaning?

I think not, but I also do not even think a weathervane  "weathervanes"
after steady state is established...

I understand the physics fairly well...I just haven't learned to harness it
nearly as well as just about everyone here. ;-)

The graceful way you guys fly is amazing.

Thanks for reading.  Be sure to catch the movie version someday.

Best regards,

Jim A
(ultra-newbie pattern flyer)