This post follows the other I just sent on v-formation flying.

>Date: Wed, 9 Oct 1996 09:12:37 -0500
>Sender: The scientific discussion of Ornithology
> <ORNITH-L at UAFSYSB.UARK.EDU>
>From: Keith Kimmerle <progne at iland.net>
>Subject: Formation Flight
>To: Multiple recipients of list ORNITH-L <ORNITH-L at UAFSYSB.UARK.EDU>
>
>Okay, I'm an (former) AF flight instructor, and part of what was said is
>correct. But first I'll address the idea that formation flight results in
>flight efficiencies to all individuals but the leader.
>
>I distinctly recall a similar discussion on another list. Part of the
>discussion entailed whether there actually was ANY gain, and based upon my
>flight instructing experience, I would tend to say there is not.
>
>During instruction in formation flight, students went through a logical
>progress of fear (flying far away), to limited proficiency (able to hang in
>there), to a certain level of comfort (too close), which varied from
>individual to individual.
>
>In the T-37, formation position was defined as approx. 10 feet aft, with
>approx. 3 feet of lateral and horizontal wing tip clearance. This position
>was flown at speeds ranging from approx. 100 knots (115 mph) to 250 knots
>(287.5 mph). Wing span on the T-37 was about 33 feet, if memory serves me
>correctly.
>
>When the students got to the "too close" level of proficiency, one of the
>"instructional techniques" was to instruct the student how the aircraft
>began to react differently once wing-tip overlap began to occur (ie. if you
>moved forward 10 feet and up 3 feet, you would have just had a mid-air
>collision). There was a slight, but noticeable effect, of being pushed away
>and down from the lead aircraft. This would require increasingly larger
>control inputs to move in closer, at which point you were WAY too close and
>the instructor had already taken the aircraft from the student. The point
>to be made here, is that wing-tip overlap resulted in an aerodynamic force
>that pushed you away from, and DOWN, not up.
>
>Additionally, if we look at basic aerodynamics, the production of lift
>results in the production of wing-tip vortices - the equivalent of a
>horizontally rotating airshaft, that rotates inward toward the centerline of
>the aircraft. This occurs because of two reasons; the production of lift
>(Bernoulli's Law) results in a lower surface pressure on the upper wing
>surface, and the AOA (angle of attack) results in a higher surface pressure
>under the wing due to "kite effect". What happens, is that some of the high
>pressure air under the wing tries to escape around the end of the wing-tip,
>toward the area of lower pressure on the upper wing surface. So as the
>aircraft is traveling forward, this results in a counter-clockwise vortex
>from the left wing-tip and clockwise vortex from the right wing-tip (aft
>view). When drawn, this clearly shows a down-draft flow as the trailing
>wing-tip moves into the leading aircraft's wing-tip vortices. And while
>soaring birds have many adaptations for over coming some of these
>short-falls in life production, they cannot change the basic dynamics of the
>vortex.
>
>Some additional points to emphasize. We flew with only 3 feet of wing-tip
>spacing, which represents 1/11th (9%) of the overall wing-span. I think you
>can see how close together avian species would have to fly, even if there
>were a energy benefit to formation flight.
>
>As far as the Brown Pelican extremely low altitude flight; this is caused by
>"Ground Effect". Essentially when ever the wing gets within 1/2 of the
>wing-span of the ground, changes occur in the fluid mechanics of the
>airflow, which begins to greatly reduce induced drag (drag resulting from
>the production of lift). That's why for anyone who's flown even a little
>puddle-jumper (aka. bug smasher), the plane tends to float as you attempt to
>land (provided you have sufficient flying airspeed). While it is partially
>due to some compressional effects of angle of attack, these effects are
>present at all altitudes. It is the reduction of drag which makes it
>advantageous to fly at such low altitudes. For those of you who know what a
>B-52 is, once you get down below 100 feet AGL, you actually have to push
>forward on the stick to fly any lower.
>
>I apologize for anyone who I've bored with this long post.
>
>Keith Kimmerle
>Central Missouri State University
>Department of Biology
>WCM 306
>Warrensburg, MO 64093

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