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From: watson@intermec.com (Bob Watson)
Newsgroups: rec.aviation
Subject: Re: Aerial Photography
Summary: On-Pylon Turns about a point
Message-ID: <1580@intermec.UUCP>
Date: Wed, 26 Jun 91 17:50:35 PDT
References: <1991Jun25.205414.17982@convex.com>
Reply-To: watson@intermec.com (Bob Watson)
Organization: Intermec Corporation, Everett WA
Lines: 32

The "on-pylon" turn about a point is one of the useful maneuvers taught 
for the commercial ticket and is especially suited to what you have
described. If you're not familiar with it, an hour of dual should be all
you need to learn it.  

The "on-pylon turn is similar to the "turn about a point" maneuver
taught to private pilots, but corrects for wind using altitude as opposed
to bank. The purpose of the manuever is to maintain the "pivotal altitude"
for your ground speed. If there's any wind, then as your ground speed changes
so does the pivotal altitude. The net result is that you maintain the pylon
(i.e. the subject being photographed) at a constant point relative to the
airplane (e.g. off the wing tip) which is what you want for the photo.

What unfortunately happens to circling pilots not familiar with the concept
of the pivotal altitude is that if they are not at the right altitude,
the spot on the ground will move (relative to the plane's view) as they turn 
and they may try to correct using only the rudder (since changing the bank 
would obscure the object on the ground)
potentially resulting in a cross-controlled stall if things get too
slow. (bad news close to the ground :-().

At 100 kts, the pivital altitude is 900' AGL. If you are going faster then the
altitude is higher (120 kts is 1300' AGL). (for the mathematically inclined,
the formula for pivotal altitude (from William Kershner's "Flight Instructor's
Manual") is A (feet) = V*V (fps) / g (32.2 fpsps) or roughly
A (feet AGL) = V*V (knots)/11.3.) 

The interesting thing is that the pivotal altitude is a function of SPEED and
is the same regardless of bank angle (though the RADIUS is certainly a 
function of bank angle and speed).

Bob Watson, CFI-AGI