[NSRCA-discussion] Weather Vane

[Terry Beachler]

The boat analogy previously used would only be valid for a submarine. 
Wind force and water force are two separate forces. Wind shear in the 
air may some application, but that is not what we are talking about. 
We'll have to ask a submariner if a submarine weathervanes or er ah 
watervanes. It doesn't.

Terry Beachler

At 07:17 10/12/2007, you wrote:

>Hi Jim.
>
>     Thanks for taking the time to post your reply.  Wonder how many 
> will agree with your clarification.  Some people are amazed with 
> magic also..  <tic>
>
>     Del
>
>----- Original Message -----
>From: "Jim Alberico" <<mailto:alberji at charter.net>alberji at charter.net>
>To: "'NSRCA Mailing List'" 
><<mailto:nsrca-discussion at lists.nsrca.org>nsrca-discussion at lists.nsrca.org>
>Sent: Thursday, October 11, 2007 11:19 PM
>Subject: Re: [NSRCA-discussion] Weather Vane
>
> > 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)
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > _______________________________________________
> > NSRCA-discussion mailing list
> > <mailto:NSRCA-discussion at lists.nsrca.org>NSRCA-discussion at lists.nsrca.org
> > http://lists.nsrca.org/mailman/listinfo/nsrca-discussion
> >
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