[NSRCA-discussion] Weather Vane

[ronlock at comcast.net]

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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