L/D Ratio of Birds
Thomas Clarke
(condors?) in Africa found they had L/D ratios of 30 or more. This is better
than all but the best gliders despite the low aspect ratios of the bird wings.
How is this?
Could it have something to do with the fan of feathers that soaring birds
typically have at the ends of their wings? Perhaps these feathers have the
effect of making the wing behave as if it were several individual high aspect
ratio wings rather than one low aspect wing.
If this is the case, could signficant drag reduction be obtained on aircraft by
fanning the wingtip into multiple winglets?
Al Bowers
>I recall a Scientific American article wherein a glider soaring with
>buzzards (condors?) in Africa found they had L/D ratios of 30 or more.
>This is better than all but the best gliders despite the low aspect
>ratios of the bird wings. How is this?
I believe that data is quite old. Several refernces of more recent
vintage show that these large soaring birds actually only achieve L/Ds
of 12 (vultures like the California Condor and Andean Condor) to 18
(Wandering Albatross). Ref - Chen and McMasters, Soaring magazine
5/83, "From Paleoaeronautics to Altostratus".
>Could it have something to do with the fan of feathers that soaring
>birds typically have at the ends of their wings? Perhaps these
>feathers have the effect of making the wing behave as if it were
>several individual high aspect ratio wings rather than one low aspect
>wing.
Tip vortex control was the original idea behind winglets (Dr. Richard
Whitcomb's development).
>If this is the case, could signficant drag reduction be obtained on
>aircraft by fanning the wingtip into multiple winglets?
This research goes on today, the most recent technical develpments are
by the Germans (Dornier and the entire German sailplane industry). A
good look at recent sailplane designs shows the advantages of certain
bird-like charateristics in wingtip design. The problem so far with
slotted tips (like vultures, large land soaring birds) is that the
tactile feel and individual control of the spread primaries has not
been achieved in a human mechanical system. Maybe something for
controls engineers to try and work on?
See NASA TM 75711, H. Zimmer, "The significance of wing and
configurations in airfoil design for civil aviatoion aircraft" for
more info. Also worth looking up are:
Soaring magazine:
2/83, Wil Schuemann, A new wing planform with improved low speed
performance
7/84, Doug Jacobs, A contest pilot reports on flying the Discus
2/86, Richard Johnson, A flight test evaluation of the Discus
--
"Forward, those sick of overadvertised and nastily-made tin
boxes! Viva Italia!" -Henry N. Manney III
Albion Hideto Bowers bow...@rigel.dfrf.nasa.gov Al Bowers
Muso Shinden Ryu Iaido Alfa Romeo GTV6 Ducati MHR (DOD #900)
Steve Bussolari x5956 g41
> I recall a Scientific American article wherein a glider soaring with buzzards
> (condors?) in Africa found they had L/D ratios of 30 or more. This is
> better than all but the best gliders despite the low aspect ratios of the
> bird wings. How is this?
I would like to get the reference for this article if you can supply
it. Actually, an L/D ratio of 30 is rather modest for a modern
glider. The best 15m span gliders approach 40 and a typical mid-range
ship is in the mid to upper 30's.
I recall that Gus Raspet did some work on soaring bird aerodynamics in
the 60's and concluded that they typically had an L/D in the low to
mid 20's. I am skeptical of the measurements taken from a glider.
First of all, it is difficult to determine the actual L/D achieved by
the glider in still air and, if they were flying with buzzards, it was
likely not still air. Second, even given accurate measurements, all
that can be demonstrated by this type of measurement is that the bird
is _achieving_ a given glide ratio. Soaring birds are very adept at
flying in rising air, even between thermals. At 40kt, the difference
in sink rate between glide ratios of 20 and 30 is about 1 foot per
second or 60 feet per minute. This is a very mild vertical airmass
movement, particularly for the portions of Africa where gliders and
buzzards fly. In addition, birds can vary their wing geometry to take
good advantage of changes in local conditions, even when they appear
to be simply gliding.
For some reason, we glider pilots often assume that there must be some
teleological reason for soaring birds to have a high L/D. The factors
that determine survival for a buzzard or condor and thus provide the
evolutionary pressure are in conflict. High L/D helps in traveling
long distances in still air, but the same can be achieved by taking
advantage of micrometeorological features. The ability to stay aloft
in weak conditions is at least as important and that requires low wing
loading and maneuverability - things that are generally in conflict
with high L/D. The need to take off from ground level by flapping
imposes yet another conflicting constraint.
Weekend Pilot
>(condors?) in Africa found they had L/D ratios of 30 or more. This is better
>than all but the best gliders despite the low aspect ratios of the bird wings.
>How is this?
>
I wonder about this... Did the article specifically state that the
comparison was made in straight line cruise? Was the glider 30:1
at the speed of comparison?
I have some experience with flying with red tailed hawks and bald
eagles, most of which was acquired in an SGS 1-34 (best real world
L/D of 29 - 30 @ 50 mph). I have found the following to be true
over the course of perhaps ten or twelve encounters close enough to
make meaningful comparisons in performance: 1) Eagles thermal
at higher airspeed than red-tails. The larger bald eagles appear to
circle at 30 mph or so whereas the hwaks tend to fly slower, tighter
radius turns (probably 20 mph, but hard to estimate). 2) Red-tails
in cruise at 40 mph are pretty poor L/D wise. I'd estimate 10:1 or
less based on the heavy use of the 1-34's (very effective) spoilers
required to match sink rate. 3) Eagles do somewhat better than this
however I've never had a chance to get close enough to an eagle in
cruise to get an idea of how much. The one time I *was* getting close
the eagle was leading me down wind, away from home on a scratchy day,
and into a control zone I had no wish to enter, oh well.
On a somewhat more qualitative basis, I've found these encounters
really pretty neat. The birds are generally somewhat inquisitive
and (within limits) do not seem to be particularly concerned about
a glider flying formation 50 feet away, and frequently they will
alter their course to approach a bit closer when thermalling.
Getting close to a bird in cruise is much less common (seems to happen
only during the migratory seasons).
As far as winglets and tip feathers... AvLeak had a story many months
ago about Burt Rutan et al experimenting with a variable geometry, dual
tip winglet, but I am not aware of any further developments (one *more*
thing to add to that pre-landing checklist, sigh ;-)).
--
Evan Ludeman "To err is human, to forgive is
ludeman%astroa...@cs.wisc.edu Not Company Policy" -- anon
{...}!uwvax!astroatc!ludeman
Thomas Clarke
Clarke) writes:
|
| I recall a Scientific American article wherein a glider soaring with
| buzzards (condors?) in Africa found they had L/D ratios of 30 or more.
There have been some requests for references.
The most recent and the one that put me in mind of this subject is
"Drag Recuction in Nature" by D. M. Bushnell and K. J. Moore, _Annual Review of
Fluid Mechanics_, vol 23, 1991, p 65-79.
They reference
C. D. Cove in _Scientific American_, vol 50, 1962, pp 130-140 and 180-209.
[Could it have been that long ago that I read the Sci Am article? I was in
junior high school!]
At any rate, Bushnell and Moore report the albatross as having a 17:1 aspect
ratio - practical for soaring at sea, but probably not for scavenging on land.
They also assert that some birds "utilize the tip flow via angled slots to
pruduce thrust". That is they reduce drag by clever use of the wing tip flow.
Benton Holzwarth suggested that some RC modelers have tried this technique.
Has it been used on any large planes?
Tim Axelrod
GOSLOW GE; DIAL KP; JENKINS FA.
BIRD FLIGHT - INSIGHTS AND COMPLICATIONS.
BIOSCIENCE, 1990 FEB, V40 N2:108-115.
MCMAHON TA.
BIRD FLIGHT PERFORMANCE - A PRACTICAL CALCULATION MANUAL - PENNYCUICK,CJ.
Pub type: Book Review.
NATURE, 1989 DEC 7, V342 N6250:628-628.
VIDELER J; GROENEWOLD A.
FIELD MEASUREMENTS OF HANGING FLIGHT AERODYNAMICS IN THE KESTREL
FALCO-TINNUNCULUS.
JOURNAL OF EXPERIMENTAL BIOLOGY, 1991 JAN, V155 JAN:519-530.
TUCKER VA; HEINE C.
AERODYNAMICS OF GLIDING FLIGHT IN A HARRIS HAWK, PARABUTEO-UNICINCTUS.
JOURNAL OF EXPERIMENTAL BIOLOGY, 1990 MAR, V149 MAR:469-489.
I spent a morning in the library following up some of these. The J. of Exp. Biology articles
reference a number of others that are useful also. Some of the experiments have been very
carefully done with trained raptors "soaring" in wind tunnels. The results I've seen so far
indicate L/D's are fairly low. Where birds seem to excel is in the use of variable geometry
to optimize their aerodynamics for various tasks.
The best single reference would appear to be the book "Bird Flight Performance..." by
Pennycuick, but I haven't yet found a copy.
Tim Axelrod
Mt. Stromlo Observatory
Canberra, Australia
t...@merlin.anu.edu.au