Notes on soaring birds:  Aerodynamics and flying skills

Summary:
The ultimate soaring birds are raptors.  A sailplane pilot's view is that their skills are superior to those of human pilots. An aerodynamicists view is that their "airframe" design is highly optimized for soaring flight.

Photos below to illustrate these points show turkey vultures in El Dorado Hills.  All photos were taken by Paul Raveling (Paul.Raveling@sierrafoot.org) and are released to the public domain:  These are explicitly not copyrighted, even the implicit copyright for web-published materials is disclaimed.

The bird on the ground:  Two of a group of 5, both looking at the photographer.


Turkey vultures on ground, in El Dorado Hills


Raptors' visual acuity is much better than that of humans. Their retinas have two foveas instead of the humans' one. Some sources quote up to 10 times higher resolution. They also perceive four color channels instead of the human's three, with the fourth in the ultraviolet range.

Many (human) sailplane pilots believe that raptors can literally see thermal lift, the rising convective currents used to climb by both soaring birds and human pilots. One conjecture is that birds are superior to humans in seeing fine dust picked up and carried aloft by thermals. Another is that they may be able to perceive refractive distortions due to thermal turbulence. [To do:  Check index of refraction for air as a function of temperature, pressure, and humidity.] [To do:  Look for research into sensitivity of both conjectured factors as a function of ultraviolet vision.]

Placement of the eyes enables binocular vision, being used by the bird on the right. Monocular vision, being used by the bird on the left, provides vision through a much wider range of angles than for humans. Raptors have only a small blind zone to the rear.


Turkey vulture, wings extended

The photo above is a turkey vulture with wings extended for low speed flight, near minimum sink speed. Minimum sink speed is that which minimizes the rate of sink (vertical speed) in gliding flight. Sailplane pilots use this speed to minimize rate of descent in still air and to maximize rate of climb in rising air (air mass lift such as thermal or orographic lift), usually just called "lift"), such as in thermal lift or ridge lift (orographic lift). (A few types of soaring birds can sustain flight at this speed by using dynamic lift, from horizontal gusts -- the best known  example is the Laysan albatross.)

Several factors visible in this photo are standard aeronautical engineering wing design measures to reduce, or even to minimize induced drag.  Induced drag is produced as a result of the wing's generation of aerodynamic lift. Wingtip vortices are the main physical process causing induced drag. Induced drag is proportional to the square of the wing's coefficient of lift (CL), and CL is directly proportional to the wing's angle of attack.

Elements of wing geometry in the photo above that contribute to minimizing induced drag are:
Birds and aircraft both achieve very low parasite drag in three main ways:
Birds may have an additional advantage (to be investigated):  Their small wing chord and relatively low flight speed results in flight at Reynolds numbers far smaller than those experienced by aircraft. Conserquently, their wings may produce laminar flow. If birds' wings were as smooth as aircraft iwings they would easily achieve laminar flow across their entire chord. The only issue is whether feathers are smooth enough to avoid tripping the boundary layer into turbulent flow.

Conjecture to investigate:  Can birds use flight feathers to control their wing boundary layer, switching at will between laminar and turbulent? A ltroaminar boundary layer benefits lift to drag ratio in flight, a turbulent boundary layer benefits controllability at high angleris of attack and in trurbulence. Birds experience high AOA aft least at every landing and every takeoff. The turbulent boundary layer would also benefit acrobatic flight, such as is seen among many birds (including hawks).

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This photo shows the same turkey vulture in either cruise flight near the speed for maximum lift to drag ratio or in higher speed flight, used to minimize altitude loss when penetrating areas of sink or when flying into a headwind. In still air the lift to drag ratio is numerically identical to the glide ratio of an bird or an gliding aircraft. Maximum L/D produces the flattest glide slope.

The turkey vulture above is optimizing its cruise performance in these ways:

In aerodynamics for stability and control, most birds of all types demonstrate very good stability.

Turkey vultures are the main exception, with limited stability and controlability in the roll axis. The photos above suggest the main reason:  Small tail area, an optimization for efficiency in cruise flight.

Like the North American Aviation X-15, birds use their tail for both pitch and roll control, mainly twisting their tails to control roll. Where most other soaring birds have tails that fan out into a larger control surface, the turkey vulture's center of gravity and tail area are optimized to minimize pitch trim drag in cruise flight. The tail area is small and the aft-pointing feathers are nearly on the roll axis, minimizing their ability to produce roll moments. The turkey vulture makes up for this deficit by holding its wings at a high dihedral angle. In short, it uses dihedral for static roll stability and sacrifices dynamic roll stability for reduced trim drag.

Most raptors and other soaring birds are highly maneuverable. Hawks in particular sometimes perform acrobatic flight. If approached to closely, ravens use an evasion technique of folding their wings and dropping to create vertical separation. The main exceptions appear to be the largest birds, California condors and turkey vultures, whose mass limits their capabilities for both translational and rotational acceleration. Eagles probably are intermediate between hawks and vultures.

Raptors are exceptionally adept in terms of flying skills.  In marginal lift conditions, especially in blue sky soaring, they are more adept than human pilots at finding and using good lift. Studen pilot instruction for soaring includes techniques for finding lift.  The most reliable technique is to look for soaring birds and to join them.

Most sailplanes' thermaling speeds are in a range of about 35 to 45 knots, only a few knots faster than raptors. Sailplanes carrying a full load of water ballast when thermals are narrow often need to use a 60-degree bank to stay close to a thermal's core, and consequently need to thermal at speeds up to about 60 knots. Soaring birds's slower airspeed allows them to remain in the core as much as they want. This, combined with their low wing loading, allows them to outclimb sailplanes.

In practice, soaring birds' ability to sustain [gliding flight] on even very weak lift makes them less likely than sailplane pilots to spend substantial time in any given thermal, except in very weak soaring conditions. They are more likely than sailplane pilots to spend limited time in any given thermal and to leave early, cruising to a different area or to a different thermal.

In my personal experience, when soaring birds and sailplanes share the same thermal, the birds normally stay well clear of the sailplanes, usually maneuvering to keep minimum separation of at least 100 to 200 feet. On very rare occasions a soaring bird will approach a sailplane on its own terms, apparently when it accepts the sailplane as another bird.  My own closest approach occurred when for the first time in my soaring experience a red-tailed hawk joined up with me in the thermal I had been working, over the hills just west of El Mirage Field. The hawk passed 8 feet above my canopy, briefly throwing the entire cockpit of the sailplane into shadow.

Even rarer evidence is of hawks considering sailplanes as rivals. There has been at least one report in Soaring Magazine of a hawk attacking the wing of a sailplane in flight. Soaring also once published photos of a red-tailed hawk attacking a radio controlled model sailplane.