[NSRCA-discussion] Weather Vane

[Jim Alberico]

> -----Original Message-----
> From: James Oddino [mailto:joddino at socal.rr.com] 
> Sent: Friday, October 12, 2007 7:14 PM
> To: alberji at charter.net; NSRCA Mailing List
> Subject: Re: [NSRCA-discussion] Weather Vane
> 
> Hi Jim A,
> 
> Thanks for your input.  Now you are going to have to answer 
> all our questions.  Welcome to the club.

Uh-ohhh. <g>

Club indeed.  
As of today: NSRCA #4205

> 
> 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.

There just seems to be a little confusion on the topic, and lots of fun
tangents.  I believe Mr. Shulman actually nailed the answer back on Tuesday
afternoon!

> 
> 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?

Absolutely. 

> 
> 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.
> 

It will turn into the wind only if it "sees" a relative wind on the windward
side (i.e., a sideslip).  If the fuse is truly pointed straight up, and is
allowed to track downwind at the speed of the crosswind, then there will be
no sideslip and thus no turning.  Again, this is the idealized case of a
steady wind.  The story is different for a gust ... Injecting your fixed
missile into the crosswind is equivalent to a gust.

> 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.

Ouch.  Well "recovering" is the keyword here.  Get well.  Meanwhile, the
rest of us can seek out that perfect steady state day for our experiment.
No gradients, no fluctuations.  :-D

It's great to see such an active community forum!!

Jim