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Oct 03, 2017, 10:02 AM
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aspect ratio in thermal birds

i am into thermaling. 2 meters electric powered. and i wonder why some birds-that are very good at thermaling-have short wide wings, and others, that are not thermal birds, have long narrow. examples of short are buzzards, eagles, haws, and long are albatross. perhaps because the short fly on the ground, and albatross over sea?
a link to short:
and here albatross:

seems that, after all, it is the way each flies. 1 using dynamic soaring, the other thermals.
read the comments; especially the latest.
Last edited by phil alvirez; Oct 11, 2017 at 12:27 AM.
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Oct 03, 2017, 10:26 AM
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eagle60's Avatar
Albatros do not must come down in a vertical dive at 200 mph and brake the last 5-6 yards to catch a pray. I am not sure, but that could be a reason.
Oct 03, 2017, 11:10 AM
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low aspect ratio

that makes sense. thanks for the coment.
but, thinking on 2 meters limit, a low aspect ratio could mean larger area as a trade-in for more lift and higher reynolds?
just speculating.
Last edited by phil alvirez; Oct 03, 2017 at 05:57 PM.
Oct 04, 2017, 12:55 AM
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eagle60's Avatar
Longher and narrower means more efficiency (same lift with less air friction). At the end there is a reason if a glider and the F22 have so much different wings. Everything starts from the weight you need to carry ( the total glider weight), then you consider the specific weight, that for a glider should not be over 6 onces per square inch. ( the lower the better) and you get the needed wing area. You multiply it for the "aspect ratio" (long divided narrow) which for glider is in the range of 20 to 30 (the higher the better) and you get the square of the wing span. The wing span divided for the choosen aspect ratio gives the average cord of the wing. Now depends from where you want to start : from the weight to fix the geometry or from the geometry to fix the max weight.
This is the theory, now depends on how much efficient you want your wings to be (but I am afraid that for a thermal glider should be as higher as possible).
Oct 04, 2017, 05:10 AM
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straight ahead

thank you for the detailed data. i also found this:
which brings that as they dont fly circles into thermals and they just fly facing the wind, their purpose in entirely different from the goal of thermaling that is achieved by short winged birds. that explains their long thin wings.
Last edited by phil alvirez; Oct 04, 2017 at 09:18 AM.
Oct 04, 2017, 05:24 AM
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the way i see it

what happens with albatross is that there is the common idea that they are the highest efficiency and best for soaring due to their long thin wings, but as we see on the link, they are the way they are because are an adaptation for the way they fly near the surface and in straight line using the wind, not in circles for climbing with up going air (thermals). so it depends on which way they all fly. therefore short wings do better for circling, even if the low aspect ratio does not good on wind tunnel tests. this is the conclusion am reaching
Oct 04, 2017, 10:06 AM
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eagle60's Avatar
I see your point, large wingspan will give too much inertia for circling, but in my last comment I just suggested general headlines. Anyway thermals could be weak, in which case low wing load and high wing efficiency will for sure help, up to you to select the point where you want your glider to be. I am fond of speed and I never had a glider, but I know that for soaring they often have a tank to be filled with water or left empty according to the wind condotions.
Good luck for your project.
Oct 04, 2017, 11:05 AM
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1 or the other

thank you sir. the thing is that efficiency and thermaling conflicts. even if at same wing loading, if looking for long glides with large diameter circles and no thermals, long wings stay longer.
but if looking for thermals, i think short wide wings allow the plane to circle on a way smaller diameter and that makes the difference. anyway, i will try a lower aspect ratio of same weight and see.
you can not eat the cake and have it.
Oct 04, 2017, 12:22 PM
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the lower ar the better

i remember from my days of free flight, that the best for catching thermals were those hand launched gliders that had a wing of 18" wingspan and 4" chord. anything with narrower chords didnt do it. 4" chord was the way. and they did the smallest diameter turns.
Oct 05, 2017, 01:11 AM
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eagle60's Avatar
You are adding another variable : size. Pls consider that if you want to keep the same aspect ratio on a 2 meter wing (80") you should have a 36" cord and on 1 meter (40") a 18" cord. Do not forget another very (very very) important variable : which airfoil ?
Oct 05, 2017, 05:27 AM
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scale factor

thank you for your comment.
i am not trying to tell that i intend to use same ar for larger planes. just provided the data for small gliders that works the best for those. the idea is that lower aspect ratio (ar) works better for thermaling. and of course that ar has to be adjusted; is sort of scale factor. just as the smaller a plane the larger the tail has to be, here the larger the plane the larger the ar too. but still lower ar seems to make a plane more able to thermal. and am using the example of birds that i hope works similar for planes.
Last edited by phil alvirez; Oct 10, 2017 at 02:54 AM.
Oct 06, 2017, 11:54 AM
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more data

here is another article regarding birds soaring:

again, it does not make clear that thermaling birds fly in clircles, hence the shorter wings makes them more efficient for staying in small diameter thermals, whilst albatross fly facing the wind in a more straight line, thats why the need for narrow wings that generate smaller tip vortex.
a completely different approach for each situation.
so. long wings for straight flying, short for circular flying.
and, yes, full size sailplanes have long narrow wings because they fly distance and use large thermals (with large diameter, where the plane fits) for climbing only, and want (need) the most efficient wing for long straight flying towards the next cumulus for climbing again.
remember the original sailplanes that were flown from slope and returned to the same field for landing? those had shorter wings.
for instance:
have you been at elmira?
Last edited by phil alvirez; Oct 06, 2017 at 12:13 PM.
Oct 06, 2017, 02:08 PM
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dynamic soaring

this video shows how albatross fly without flapping their wings.
it is called dynamic soaring. not thermal soaring. 2 different things.
therefore the great difference between their wings. and the aspect ratio.
long thin wings for dynamic soaring.
short wide for thermaling turning into small diameter circles.

Dynamic Soaring: How the Wandering Albatross Can Fly for Free (2 min 35 sec)

Oct 06, 2017, 09:57 PM
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Birds are so much more complex than our models. They have an onboard computer that allows them to constantly vary their camber, angle of attack and even center of balance as they feel conditions change instantly. I see hawks and buzzards flare out their trailing edge feathers when going slow or thermalling in light lift but running much cleaner in the big lift in the afternoon. Frigate birds have very high aspect wings and thermal well in the humid coastal air. Our little planes are so limited by comparison. Since there are no low aspect competition planes I would stick with higher aspect designs.
Oct 07, 2017, 05:16 AM
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pioneers get all the arrows

all that you say makes sense. still, i will try a low ar wing and see how it compares. it is 1 of those things that we have to try to reach conclusions. there have been so many things that did look wrong and at the end worked even better than the supposed better so why not. the fact that nobody uses it or has tried does not prove that something else may work, even work better. it is a matter of trying.
but will take time for me to reach those conclusions, as bad weather is the trend and will take until spring comes and then lots of testing. it is with the things of the air, that is necessary to perform as many tests as possible before reaching conclusions.
thanks for all your thoughts
Last edited by phil alvirez; Oct 07, 2017 at 06:31 AM.

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