<HTML><BODY style="word-wrap: break-word; -khtml-nbsp-mode: space; -khtml-line-break: after-white-space; ">Nat,<DIV><BR class="khtml-block-placeholder"></DIV><DIV>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?</DIV><DIV>Seems pretty obvious to me.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>Jim</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV><BR><DIV><DIV>On Oct 10, 2007, at 2:21 PM, Nat Penton wrote:</DIV><BR class="Apple-interchange-newline"><BLOCKQUOTE type="cite"> <DIV><FONT face="Arial" size="2">Ron</FONT></DIV> <DIV><FONT face="Arial" size="2">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</FONT></DIV> <BLOCKQUOTE style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px"> <DIV style="FONT: 10pt arial">----- Original Message ----- </DIV> <DIV style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: black"><B>From:</B> <A title="ronlock@comcast.net" href="mailto:ronlock@comcast.net">ronlock@comcast.net</A> </DIV> <DIV style="FONT: 10pt arial"><B>To:</B> <A title="nsrca-discussion@lists.nsrca.org" href="mailto:nsrca-discussion@lists.nsrca.org">NSRCA Mailing List</A> ; <A title="nsrca-discussion@lists.nsrca.org" href="mailto:nsrca-discussion@lists.nsrca.org">NSRCA Mailing List</A> </DIV> <DIV style="FONT: 10pt arial"><B>Sent:</B> Wednesday, October 10, 2007 11:07 AM</DIV> <DIV style="FONT: 10pt arial"><B>Subject:</B> Re: [NSRCA-discussion] Weather Vane</DIV> <DIV><BR></DIV> <DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; ">I thought I'd throw in my 2 cents worth, but maybe it turned into too much. Ya been warned <G>.</DIV><P class="MsoNormal" style="MARGIN: 0in 0in 0pt"> </P><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; ">There is a strong, almost overwhelming, visual perception that airplanes weathervane into the wind as seen by an observer on the ground.<SPAN style="mso-spacerun: yes"> </SPAN><SPAN style="mso-spacerun: yes"> </SPAN></DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; "><O:P> </O:P></DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; ">If the airplane is partly connected to the ground (as in takeoff or landing roll) it will likely weathervane into the wind. <SPAN style="mso-spacerun: yes"> </SPAN>(Like a real weathervane on the barn).<SPAN style="mso-spacerun: yes"> </SPAN>Putting more side area behind the CG probably increases the weathervane effect.</DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; "><O:P> </O:P></DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; ">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.<SPAN style="mso-spacerun: yes"> </SPAN>Nor does the hot air balloon mentioned in an earlier post.</DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; "><O:P> </O:P></DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; ">Just after take off, we likely see the nose is pointed some amount into the wind.<SPAN style="mso-spacerun: yes"> </SPAN>This visual perception is true. But is it proof of weathervaning?<SPAN style="mso-spacerun: yes"> </SPAN>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.</DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; "><O:P> </O:P></DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; ">We all know stall turns are easier to do "into the wind". <SPAN style="mso-spacerun: yes"> </SPAN>Is it because they weathervane?<SPAN style="mso-spacerun: yes"> </SPAN>On the upline to a stall turn in a cross wind, does the pilot wind correct the airplanes track?<SPAN style="mso-spacerun: yes"> </SPAN><SPAN style="mso-spacerun: yes"> </SPAN>Most of us do, consciously or not.<SPAN style="mso-spacerun: yes"> </SPAN><SPAN style="mso-spacerun: yes"> </SPAN>If we are having any success at all with the wind correction, the fuselage is "leaning" into the wind a bit <SPAN style="mso-spacerun: yes"> </SPAN>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 style="mso-spacerun: yes"> </SPAN></DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; "><O:P> </O:P></DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; ">Pilots try to make heading adjustments to hold track, or hold distance in lines and maneuvers. <SPAN style="mso-spacerun: yes"> </SPAN>Those adjustments (crab angles) can appear to be a result of weathervaning, but are often pilot inputs, consciously or not.</DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; "><O:P> </O:P></DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; ">Related things to consider-<SPAN style="mso-spacerun: yes"> </SPAN></DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; "><O:P> </O:P></DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; ">Airplanes do get "hit" in the side, top and bottom when in turbulent wind, and wind shear situations.<SPAN style="mso-spacerun: yes"> </SPAN>We fly low, and are often in turbulence.<SPAN style="mso-spacerun: yes"> </SPAN>There may be short, nearly random, turbulent air effects that result in a weather vane type effect.</DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; "><O:P> </O:P></DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; ">A free flight glider does not weather vane into the wind.<SPAN style="mso-spacerun: yes"> </SPAN>The glider may fly in circles, but it will drift downwind at the overall average of the wind speed.<SPAN style="mso-spacerun: yes"> </SPAN><SPAN style="mso-spacerun: yes"> </SPAN>No matter how much side area is put behind the CG, it will not find and maintain a heading into the wind.</DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; "><O:P> </O:P></DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; ">Given flight in no wind situation, a string tied on nose of an airplane blows exactly backwards and parallel to the fuselage.<SPAN style="mso-spacerun: yes"> </SPAN>(unless the airplane is out of rig, or has control surface inputs). <SPAN style="mso-spacerun: yes"> </SPAN><SPAN style="mso-spacerun: yes"> </SPAN>If flown in a cross wind direction, the airplanes track across the ground changes, but the string stays straight. <SPAN style="mso-spacerun: yes"> </SPAN>(not counting any effect of propeller induced spiral airflow)</DIV><P class="MsoNormal" style="MARGIN: 0in 0in 0pt"><SPAN style="mso-spacerun: yes"></SPAN> </P><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; ">The old full scale stories about loosing airspeed and stalling while doing a "downwind turn" fit into this discussion to a degree. <SPAN style="mso-spacerun: yes"> </SPAN>Does wind hit the tail of an airplane as it turns downwind thereby reducing airspeed?<SPAN style="mso-spacerun: yes"> </SPAN><SPAN style="mso-spacerun: yes"> </SPAN>In the "old" days, pilots often flew low and partly judged airplane speed by visual observation of ground speed. <SPAN style="mso-spacerun: yes"> </SPAN>(similiar to our situation of observing from the ground)<SPAN style="mso-spacerun: yes"> </SPAN><SPAN style="mso-spacerun: yes"> </SPAN>This could lead to the perception of plenty of ground speed being plenty</DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; "><O:P> </O:P></DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; ">Sorting out the visual perception of the "obvious" weather vane effect is tough.<SPAN style="mso-spacerun: yes"> </SPAN>Lots of things complicate the observation - pilot inputs, turbulence, paralax, and more.<SPAN style="mso-spacerun: yes"> </SPAN></DIV><P class="MsoNormal" style="MARGIN: 0in 0in 0pt"><SPAN style="mso-spacerun: yes"></SPAN> </P><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; "><SPAN style="mso-spacerun: yes">IMHO, bottom line, it's not wind hitting side of airplane, no matter how much side area is behind the CG.</SPAN></DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; "><O:P> </O:P></DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; "><O:P>Later, Ron</O:P></DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; "><O:P> </O:P></DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; "><O:P> </O:P></DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; "><O:P> </O:P></DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; "><O:P> </O:P></DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; "><O:P> </O:P></DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; "><O:P> </O:P></DIV><DIV style="margin-top: 0in; margin-right: 0in; margin-bottom: 0pt; margin-left: 0in; ">In summary, I agree that as observers on the ground, we "see" effects that appear to be weathervaning.<SPAN style="mso-spacerun: yes"> </SPAN>But the "real" cause</DIV></DIV><DIV> <BR class="khtml-block-placeholder"></DIV><HR><DIV><BR class="khtml-block-placeholder"></DIV>_______________________________________________<BR>NSRCA-discussion mailing list<BR><A href="mailto:NSRCA-discussion@lists.nsrca.org">NSRCA-discussion@lists.nsrca.org</A><BR>http://lists.nsrca.org/mailman/listinfo/nsrca-discussion</BLOCKQUOTE><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">_______________________________________________</DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">NSRCA-discussion mailing list</DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; 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