Low Altitude Troubles

On Sunday, February 2, 2014 12:33:10 PM UTC-8, wrote:

I would be very interested in understanding:

First- what "pivotal altitude" is (definition).

The pivotal altitude is the altitude at which the line along the wingspan of a turning glider, if extended to the ground, will intersect the ground at the vertical projection of the center of the turning circle of the glider. Below this altitude the line extending out from the wing will intersect the ground at a point between the center of the circle and the vertical projection of the glider on the ground and will appear to trace a circle on the ground that is going in the same direction as the glider is turning (clockwise or counter-clockwise as the case may be). Above the pivotal altitude the extended line along the wingspan will intersect the ground at a point on the opposite side of the center of the circle and will appear to trace a circle on the ground that is anti-clockwise to the turning direction of the glider.

Second- how was equation described developed.

Two equations:
1) Radius of the turn= R=V^2/(32xsin(alpha)), where V is TAS in ft/sec and alpha is the bank angle.

2) Determine pivotal height (H) by setting the ratio of AGL height (H) to R equal to TAN(alpha) so the wingtip points at the turning center on the ground. For a 45 degree bank TAN(alpha)=1 so the radius equals the pivotal height.

It's a subtle peripheral vision effect that can lead the pilot to think they are over-banked or under-ruddered on a low turn to final because the inside wing isn't retreating against the background they way it does up higher.

9B