[NSRCA-discussion] Weather Vane

[James Oddino]

Hi Jim A,

Thanks for your input.  Now you are going to have to answer all our  
questions.  Welcome to the club.

Your description of the vertical portion of a stall turn sounds good  
to me .  I never did find the beginning of the thread and only jumped  
in because it sounded like some folks didn't believe that air  
vehicles weathervane in flight.

My experience is with antitank missiles that are launched at  
relatively low velocity from a tube pointed roughly at the target.   
The low velocity is a result of a short launch motor burn time,  
completed within the tube so the gunner doesn't get a blast in the  
face.  A flight motor ignites when the missile is a safe distance in  
front of the gunner.

If we launch this missile with locked controls, we'd expect it to yaw  
slightly into any crosswind (and drift downwind off the line of  
sight) as it exits the tube.  Sound right?

Back to the vertical portion of the stall turn with a pattern plane,  
only this time with locked rudder.  We all agree it will drift off  
the track as it slows to a stop.  The question is, will it yaw into  
the wind on its own as it slows to a stop?  If it does, this is my  
definition of weathervaning.

I'm recovering from an emergency appendectomy so I'm depending on all  
you other guys to go try a cross wind stall turn with no rudder  
command tomorrow and report on your results.

Best Regards, Jim O


On Oct 11, 2007, at 8:19 PM, Jim Alberico wrote:

> 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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