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

I believe that what really counts in determining the pivotal altitude is groundspeed rather than airspeed. So your equation to determine the pivotal altitude using airspeed applies only if there is no wind. There is a maneuver "eights on pylons" in power planes, where you stay at exactly the pivotal altitude around 2 pylons on the ground during the maneuver. If done correctly, the pivot points or pylons on the ground will appear to remain motionless in relation to the extended centerline of your wingtip. If not, and you are either above or below the pivotal altitude, the pivot point on the ground will appear to be moving, either forward or backward relative to your wingtip, depending on whether you are above or below the pivotal altitude. If there is any wind, it is necessary to actually change the altitude of the aircraft continuously as you go around the pylons in order to stay at the pivotal altitude. This is because your groundspeed changes due to the wind, hence the pivotal altitude constantly changes. The faster the groundspeed, the higher the pivotal altitude. There is a pretty good explanation of it here.:
http://m.aopa.org/asf/publications/i...fm?article=226

On Monday, February 3, 2014 10:39:57 AM UTC-8, wrote:
I believe that what really counts in determining the pivotal altitude is groundspeed rather than airspeed. So your equation to determine the pivotal altitude using airspeed applies only if there is no wind. There is a maneuver "eights on pylons" in power planes, where you stay at exactly the pivotal altitude around 2 pylons on the ground during the maneuver. If done correctly, the pivot points or pylons on the ground will appear to remain motionless in relation to the extended centerline of your wingtip. If not, and you are either above or below the pivotal altitude, the pivot point on the ground will appear to be moving, either forward or backward relative to your wingtip, depending on whether you are above or below the pivotal altitude. If there is any wind, it is necessary to actually change the altitude of the aircraft continuously as you go around the pylons in order to stay at the pivotal altitude. This is because your groundspeed changes due to the wind, hence the pivotal altitude constantly changes. The faster the groundspeed, the higher the pivotal altitude. There is a pretty good explanation of it here.:

http://m.aopa.org/asf/publications/i...fm?article=226

Yes - good point. If you are making a crosswind to upwind turn to final the stiffer the breeze the lower the crossover altitude.

9B