[NSRCA-discussion] Weather Vane

[Del K. Rykert]

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" <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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