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<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Hi Ron:</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>I wonder if the weathervane effect is
there because of the greater distribution of side area aft of the CG? I’m
not sure of that, just an idea.</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'> </span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Ed</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'> </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=2 face=Tahoma><span
style='font-size:10.0pt;font-family:Tahoma'>-----Original Message-----<br>
<b><span style='font-weight:bold'>From:</span></b> nsrca-discussion-bounces@lists.nsrca.org
[mailto:nsrca-discussion-bounces@lists.nsrca.org] <b><span style='font-weight:
bold'>On Behalf Of </span></b>ronlock@comcast.net<br>
<b><span style='font-weight:bold'>Sent:</span></b> Wednesday, October 10, 2007
11:08 AM<br>
<b><span style='font-weight:bold'>To:</span></b> NSRCA Mailing List; NSRCA
Mailing List<br>
<b><span style='font-weight:bold'>Subject:</span></b> Re: [NSRCA-discussion]
Weather Vane</span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<div>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>I thought I'd throw in my 2 cents worth, but maybe it
turned into too much. Ya been warned <G>.</span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>There is a strong, almost overwhelming, visual
perception that airplanes weathervane into the wind as seen by an observer on
the ground. </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>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.</span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>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.</span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>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.</span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>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. </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>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.</span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>Related things to consider- </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>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.</span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>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.</span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>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)</span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>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</span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>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.
</span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>IMHO, bottom line, it's not wind hitting side of
airplane, no matter how much side area is behind the CG.</span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>Later, Ron</span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>In summary, I agree that as observers on the ground,
we "see" effects that appear to be weathervaning. But the
"real" cause</span></font></p>
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