[NSRCA-discussion] Weather Vane

Thu Oct 11 11:03:24 AKDT 2007

Hey Ron;

I haven't read all of this fine discussion but I totally agree with your
explanation.  I also think we turn into the wind at the top of a stall
turn because it makes the turn appear to be tighter form the ground.  If
we turn away from the wind the drift of the aircraft with the wind makes
the turn appear to have a greater radius.

I think we need to put the nonbelievers in the middle of the Gulf Stream
in a row boat and see if they weather vane.

Ron Needham

-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of george w.
kennie
Sent: Thursday, October 11, 2007 12:38 PM
To: NSRCA Mailing List
Subject: Re: [NSRCA-discussion] Weather Vane

Hey Jim,

I think you might need to say ,  "32 ft/sec/sec" so that the cross stays
constant as the descent speed increases.

----- Original Message ----- 

From: James Oddino <mailto:joddino at socal.rr.com>  

To: NSRCA Mailing List <mailto:nsrca-discussion at lists.nsrca.org>  

Sent: Wednesday, October 10, 2007 7:20 PM

Subject: Re: [NSRCA-discussion] Weather Vane

Nat, 

If you hold an arrow (or a bomb) horizontal and drop it, what happens to
it's attitude on the way down and why?  Then drop it with a crosswind of
32 ft/sec.  What happens to its attitude?

Seems pretty obvious to me.

Jim

On Oct 10, 2007, at 2:21 PM, Nat Penton wrote:

Ron

You are correct - an airplane does not weathervane or weathercock - it
fliies straight into the freestream unless given rudder. BUT, as Kennie
says, he hopes to be here next year to reargue the point !!
Nat

----- Original Message ----- 

From: ronlock at comcast.net 

To: NSRCA Mailing List <mailto:nsrca-discussion at lists.nsrca.org>  ;
NSRCA Mailing List <mailto:nsrca-discussion at lists.nsrca.org>  

Sent: Wednesday, October 10, 2007 11:07 AM

Subject: Re: [NSRCA-discussion] Weather Vane

I thought I'd throw in my 2 cents worth, but maybe it turned into too
much.  Ya been warned <G>.

There is a strong, almost overwhelming, visual perception that airplanes
weathervane into the wind as seen by an observer on the ground.   

If the airplane is partly connected to the ground (as in takeoff or
landing roll) it will likely weathervane into the wind.   (Like a real
weathervane on the barn).   Putting more side area behind the CG
probably increases the weathervane effect.

But once airborne, an airplane no longer feels wind on side of the
airplane and does not weathervane in the sense that the weathervane on
the barn does.  Nor does the hot air balloon mentioned in an earlier
post.

Just after take off, we likely see the nose is pointed some amount into
the wind.  This visual perception is true. But is it proof of
weathervaning?  It probably weathervaned into the wind during take off
roll. Once airborne the weathervane is now what we call a crab angle
into the wind.

We all know stall turns are easier to do "into the wind".  Is it because
they weathervane?    On the upline to a stall turn in a cross wind, does
the pilot wind correct the airplanes track?   Most of us do, consciously
or not.    If we are having any success at all with the wind correction,
the fuselage is "leaning" into the wind a bit   It's certainly easier to
get a stall turn by continuing into the established lean, than it is to
go against the lean to the downwind direction.  

Pilots try to make heading adjustments to hold track, or hold distance
in lines and maneuvers.  Those adjustments (crab angles) can appear to
be a result of weathervaning, but are often pilot inputs, consciously or
not.

Related things to consider-   

Airplanes do get "hit" in the side, top and bottom when in turbulent
wind, and wind shear situations.  We fly low, and are often in
turbulence.  There may be short, nearly random, turbulent air effects
that result in a weather vane type effect.

A free flight glider does not weather vane into the wind.  The glider
may fly in circles, but it will drift downwind at the overall average of
the wind speed.   No matter how much side area is put behind the CG, it
will not find and maintain a heading into the wind.

Given flight in no wind situation, a string tied on nose of an airplane
blows exactly backwards and parallel to the fuselage.  (unless the
airplane is out of rig, or has control surface inputs).   If flown in a
cross wind direction, the airplanes track across the ground changes, but
the string stays straight.   (not counting any effect of propeller
induced spiral airflow)

The old full scale stories about loosing airspeed and stalling while
doing a "downwind turn" fit into this discussion to a degree.  Does wind
hit the tail of an airplane as it turns downwind thereby reducing
airspeed?   In the "old" days, pilots often flew low and partly judged
airplane speed by visual observation of ground speed.  (similiar to our
situation of observing from the ground)   This could lead to the
perception of plenty of ground speed being plenty

Sorting out the visual perception of the "obvious" weather vane effect
is tough.  Lots of things complicate the observation - pilot inputs,
turbulence, paralax, and more.  

IMHO, bottom line, it's not wind hitting side of airplane, no matter how
much side area is behind the CG.

Later, Ron

In summary, I agree that as observers on the ground, we "see" effects
that appear to be weathervaning.  But the "real" cause

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