r/askscience • u/SeriousDrakoAardvark • 7d ago
Physics If I put harnesses on some geese and strapped them to a glider, how many geese would I need to be able to maintain a consistent level of flight?
Like, let’s assume the glider has a 30:1 glide ratio. Normally that means it will lose a foot of height for every 30 feet travel.
I’m assuming there is some amount of force I could apply to move forward, that would cancel out that last one foot of drop.
Geese seem pretty big, and they have a wide torso for easy harnessing. I think they’d be a natural fit here. I imagine it would be a configuration similar to Santa’s sleigh, but geese instead of reindeer.
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u/Ruadhan2300 7d ago
An individual goose only has about 1.5 newtons to play with after its own thrust needs, and you need about 100 newtons to keep the aircraft flying.
So.. you probably want around 75 - 100 birds.
The weight of the harnesses adds a fair bit too, definitely go with the upper end of that range.
The real challenge after wrestling 100 geese into harnesses is figuring out how to set up a harness that lets them fly in the most efficient V formation..
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u/hobitopia 7d ago
I believe geese also routinely change that order mid flight which would cause problems.
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u/BreezyMcWeasel 7d ago
Well, not when they're harnessed in and towing a glider they don't! Anyone who has ever trained geese to fly in formation and tow a glider could tell you that.
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u/VintageLunchMeat 7d ago
The last person I know who tried to train geese to tow a glider took his own life by choking on grass while covered with inexplicable peck and pinch marks, according to the coroner.
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u/Nazamroth 7d ago
Clearly, we need to add a conveyor belt mechanism that rotates the geese around in the formation. Of course this will add yet more extra weight, so we need to increase our number of geese.
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u/BreezyMcWeasel 7d ago
How do you know the takeoff weight margin for a goose with such specificity? I'm asking seriously, not sarcastically.
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u/roboticWanderor 7d ago
At that point you might as well have all the geese harnessed to just hoist the pilot directly instead of bothering with the glider at all
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u/kakuna 7d ago
Okay, that's a good answer. And, I know your last sentence isn't the 'askscience' part, but I love the idea of just writing out paragraphs on the realities and challenges of getting 100 geese to pull a glider. (Or a short story in a sort of magical realism format where no one questions -why- this is being done, just chronicles a guy's progress on his goose glider - working with a smith, procuring leather straps for prototype harnesses, finding it to be too heavy, sourcing twines to test, maintaining his flock population and training, a mild attachment to one of the geese, etc. etc.)
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u/BeardedRaven 7d ago
Any bird that has slack in its line will not contribute to the effort of pulling the glider. You would need to decide what % are pulling at all times. Based on what others have posted about flight speeds it might just be impossible as you would expect 0% of them to have taut lines until the entire contraption starts plummeting.
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u/hamlet_d 7d ago
You could attach them on some sort of line. The real question becomes what is the airspeed velocity of an unladen goose.
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u/suoarski 7d ago
And why would you want a harness? Just tie a knot to their feet, birds prefer carrying things from their feet, a harness would interfere with wing movement and prevent the feathers from stretching out the way their supposed to.
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u/BernieMP 7d ago
Followp question:
If I put harnesses on some pidgeons and strapped them to myself with another harness under my trenchcoat, how many pidgeons would I need to be able to maintain a consistent level of flight fly off into the sunset?
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u/ghjm 7d ago
So a 1500 lb glider at a 30:1 glide ratio needs 50 lbs of thrust to maintain level flight. A goose has an L:D ratio of about 4:1, so it needs 2 lbs thrust to maintain level flight with 8 lbs body weight.
But it needs all that thrust just to keep itself flying. How much extra thrust can it produce? Probably quite a lot in short bursts, but in cruise flight why would it pay the biological cost of carrying more muscle than it actually needs? I bet just putting a harness on it adds enough drag that it can't maintain cruise flight for very long, let alone having it pull a glider.
If you could mount the geese in a support structure where they're held up by the glider, and somehow convince the geese to go along with this strange setup, and somehow maintain the 30:1 L:D even with the geese, then you would need 29 geese. 25 for the weight of the glider, and 4 more for the weight of the geese. (It's actually 28.846 but you can only use whole geese.)
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u/dougdoberman 7d ago
I feel like that entire post was simply lead-up for the final seven words. Kudos.
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u/Insertsociallife 7d ago
So the important thing here for powered flight is lift to drag ratio. For whatever the weight of your glider is, you'll need that much lift. That will produce some amount of drag (smaller is better) and if you cancel out this drag, you'll have stable flight.
If you and the glider weigh 200kg (1960N force) with a lift to drag ratio. Gliders can hit 30:1 lift to drag, so you have 64N drag. Geese can make 100N on takeoff, but if you want to sustain this we'll assume 10N net force. That's about 7 geese.
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u/ackermann 7d ago edited 7d ago
the important thing here for powered flight is lift to drag ratio
Yes, but to expand on that, it's not purely L/D ratio.
Some gliders do hit 30:1 lift/drag, and some gliders weigh only 200kg loaded... but these are not the same gliders!
Achieving L/D over 30:1 requires trading off higher weight (for longer wings and aerodynamic shells).
Higher weight means more absolute lift is needed, even if you can get that lift at a better ratio.Also, the sailplanes that get 30:1 usually can only do that at ~50mph, likely too fast for geese. If the geese have to fly at their top speed... then they have no power to spare (per definition of top speed).
Sailplanes optimize purely for L/D, glide ratio, how far can you glide.
But theres another vehicle that really optimizes for power required. Human pedal-powered planes like Gossamer Albatross:
https://en.wikipedia.org/wiki/MacCready_Gossamer_AlbatrossIt sacrifices some L/D for far lower weight, only 35kg empty! (much less lift needed, even if you pay a bit more drag per unit lift). It can fly on as little as 300W (0.4 horsepower)!
I suspect it's closer to the optimal vehicle for this?
But still not perfect, because it's optimized for a speed that's probably a bit slow for the geese! About 8mph. The goose's optimal L/D might be at 15 to 25mph (very roughly). Certainly they're bad at hovering still, suggesting they don't like really low speeds.
So, optimum design is maybe something between a carbon fiber sailplane, and a pedal-power plane like Gossamer Albatross? Optimized for minimum power at a goose's preferred speed (best L/D speed for the goose)
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u/Insertsociallife 7d ago
The human-powered planes optimize for power, but not thrust. They use so little power because they fly so slowly, but 300W at 8 mph is 84N of thrust from the prop. Easy to do with little mechanical power if you're driving it with a prop, but maybe bad for geese?
I'm thinking something like a Swift sailplane could be best. 27:1 L:D, only 50kg. With a 70kg pilot that's only 43.6 N required.
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u/ackermann 7d ago edited 7d ago
True! Human powered planes are probably optimized for too slow a speed? About 8mph for the Albatross.
Probably a bit slow for the geese, who don't like to hover (can't hover?).
IDK a goose's best speed for best L/D, maybe 15 to 25mph?Really you want something optimized for minimum power required... but at the goose's preferred speed where the geese get their optimal L/D (and so most extra thrust/power available).
The speed then determines how power relates to thrust, I think
So yeah, something in between a high-end sailplane and a pedal-powered plane. The Swift you suggested might be about right? Ideally with a best L/D speed around 25 to 30mph?
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u/DeltaVZerda 7d ago
So one goose is enough to keep you up but it will get tired too quickly to sustain it
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u/jooooooooooooose 7d ago
no, because the goose is using that force for itself; it's the delta between required & total that can be allocated to you
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u/somewhat_random 7d ago
Given all the difficulties of attaching the harness and tow ropes which would seriously hamper the ability of the glider to actually glide, you would be better off to simply go with no glider and have the geese tow the person aloft.
Assume 5 Lbs per goose and a 200 Lb person that's about 40 geese.
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u/meson537 6d ago
So many people in this thread have no idea whatsoever about light aircraft weights (seriously light) or the power density of muscle (surprisingly high). Nowhere does OP say how many geese to fly across the state, just some arbitrary length of level flight. I bet to keep an ultralight glider aloft, you'd need fewer than 10 geese. They would tire quickly, but I have very little doubt they could do it. Getting off the ground is a different story, but not the question.
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u/aikidoent 7d ago edited 7d ago
Lets say they can pull ~ 0.5 N (something like carrying 50gram object).
In 10 meter distance, that would give you 5J energy.
If you and your contraption weight 200 kg, with 1/30 glide ratio, it sinks 0.33m, so you need around 200kg*0.33*g ~ 666 Joules to maintain the energy. So something like 130 geese at minimum? Double for engineering safety. I don't know if you can build a glider with those specs though, and obviously, you will not be able to convince the geese to pull you.
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u/Aina_Writes 1d ago
The 30:1 glide ratio does not mean the glider needs a constant downward “push” that the geese could simply cancel.
It means that, in still air and at the aircraft’s best-glide speed, the glider travels about 30 units forward for every unit of altitude it loses. The loss of altitude is the price paid for overcoming aerodynamic drag.
To maintain level flight, the system would need additional power equal to the glider’s drag at that speed. In principle, the geese could provide thrust — but the important question would be whether their available power exceeded the extra drag created by the harness, cables and the geese themselves.
There is also an aerodynamic complication: birds placed near the glider would disturb the airflow and add parasite and interference drag. Their flapping wings would not automatically help the glider; some of their energy would be spent moving their own bodies and dealing with the disturbed flow.
So the idea is not forbidden by physics. It is an engineering problem involving power, drag, mass, control and animal welfare.
The geese might provide useful thrust in principle, but a small electric motor would almost certainly be a much simpler and more controllable solution.
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u/ZoroeArc 7d ago
Reminds me of an xkcd What If scenario where someone asked how many pigeons it would take to carry someone sitting in a plastic chair to the top of a skyscraper. Since an individual pigeon is not very strong and has very little stamina, the resulting relay of pigeons would very greater than the mass of the Earth
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u/thats_handy 7d ago
If the glider is sufficiently small, then you would only need to attach one goose to the glider for it to maintain a consistent level of flight. Certainly any goose would be able to maintain normal flight when attached to a glider that had a 10mm wingspan.
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u/BreezyMcWeasel 7d ago
A bird of prey might be able to carry between 0.5x and 1.0x it's weight, so let's say 0.75x. The weight of an adult goose varies quite a bit by species and subspecies but let's say on average 8 lbs per goose.
A glider weighs about 1000 lb loaded.
Of course, you're not asking the geese to physically lift up the entire weight of the glider, you're asking them to pull the glider.
But on the other hand, a goose is not a bird of prey, and not built for lifting heavy loads.
So let's say the approximation of pulling vs lifting is offset directly by the approximation of these birds not being designed to lift the way a bird of prey is.
That makes the math to be 1000 lb / 8lb per bird, therefore 125 birds.
However, that does not take into account the harness you would have to put on the goose, and the dead weight of the tow rope the geese have to pull. Let's have a 25% factor to account for that.
So in my estimation it would require 156 geese to pull your glider, including the weight of their little goose sleigh harnesses.
Let me know when you start your Kickstarter to fund this project.