[NSRCA-discussion] Weather Vane

[Jim Alberico]

Thanks, Ron ...

.... but ...  Oops! An error.

In the steady state case, rudder trim is required to establish the line, not
to hold it.  I'm sure most of the full-scale folks would jump on that one
right away.

And I should have put more emphasis on the fact that it is reasonable that
"weathervaning" contributes to the turning as the apex is reached, *if* the
maneuver has been flown correctly.  This is simply a stable airplane
correcting itself for sideslip. However, any crosswind effect beyond the 90
deg rotation will contribute in the opposite direction. 

...and we all know weathervaning causes the fuse to rotate towards nose
down, regardless of cross wind.  As the CG begins to fall, the relative wind
becomes incident on the right side.  Weathervaning is all about directional
stability.

Jim

-----Original Message-----
From: Ron Lockhart [mailto:ronlock at comcast.net] 
Sent: Friday, October 12, 2007 8:03 AM
To: alberji at charter.net; NSRCA Mailing List
Subject: Re: [NSRCA-discussion] Weather Vane

Yep, I think Jim Alberico's discussion is accurate.

Later, Ron Lockhart

----- Original Message -----
From: "Jim Alberico" <alberji at charter.net>
To: "'NSRCA Mailing List'" <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)
>
>
>
>
>
>
>
>
>
>
>
>
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