[NSRCA-discussion] Weather Vane

[Jim Alberico]

Thanks, Don!
 
I think I follow, mostly.
(quite a challenge presenting those details without pictures and equations,
I know)   ;-)
 
Here's how I interpret:
1.) Increasing counter rudder required throughout vertical deceleration.
2.) Amount of counter rudder ("steepness" of curve) is proportional to
amount of directional stability.  
 
As a totally green pattern flyer, I think I learned something practical here
to take to the field.

No wonder my rotations often occur too early!  :-)
 
Jim A
 

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From: tocdon at netscape.net [mailto:tocdon at netscape.net] 
Sent: Saturday, October 27, 2007 8:01 PM
To: alberji at charter.net; nsrca-discussion at lists.nsrca.org
Subject: Re: [NSRCA-discussion] Weather Vane


The physical effect of weather vaning, as best explained in a vertical
upline (most profound and easily seen example).

Measure the distance front of the aircraft to the center of gravity,
subtract the difference from that point to the center of side area lift
(fuselage side lift), times velocity squared, divided by the co-tangent of
the angle from vertical upline (this can be calulated by the second
differential equation at airspeed "a", for the horizontal component of
velocity"b", during a verical up-line of length "c"- which approaches zero
in a perfectly wind corrected crab angle- at least at any given instant)
This is used to calculate side- intertial force at each discrete point in
time "t" (otherwise the same lift-inertial phenomenon we feel during the
dreaded "downwind turn").  So the corrective input becomes a parabolic arc
(charted on a graph with the rudder input being the vertical axis and
inverse of velocity on the horizontal axis) for the rudder stick to
counteract the lift (minus x-axis (side force) gravitational pull as the
aircraft slows), such as that at the top of a stall turn, just before the
pivot.

In other words, differing rudder inputs are needed to maintain the correct
crab angle during each maneuver.  Some planes that have a center of lift far
behind the center of gravity, requiring more rudder correction to keep the
plane from weather vaning too much.  Thats why you see some weather-vaning
pattern designs moving the canopy forward, to bring the two closer together
(but ideally not at the same point) and therefore make the plane feel as
though it is self-correcting in a cross wind.

All the above is referenced with respect to where the judges (are) firmly
planted.

All the best,

Don

 
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