[NSRCA-discussion] Weather Vane

[James Oddino]

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 ; NSRCA Mailing List
> 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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