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Coffee House => Coffee House Boards => CH / Science and Technology => Topic started by: blobrana on August 13, 2004, 05:45:11 PM
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Hum,
A Human-powered helicopter doesn`t get off the ground...
(http://mysite.wanadoo-members.co.uk/blobrana/nmonth/clickhere.gif) for more info (http://batman.mech.ubc.ca/~hph/)
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If someone would actually have read up on the science, they could have saved their time and effort. The human body doesn't have the muscle or energy it needs to supply lift to its own weight. Our power to weight ratio is too feeble.
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Hum,
Luckily Dr Paul B MacCready didn`t read that...
The Gossamer Albatross was a human-powered aircraft, and on June 12, 1979 it completed a successful crossing of the English Channel ...
"that`s easy" - Icarus
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This may at first sound callous, but their best bet is to get an amputee. His upper body strength will be formidable and he will weigh less than a walker. Also the action of winding wheelchair wheels may be convertible to the gearing system of the chopper.
I'm not saying they can do it, but if they want to keep the weight down and power up, that's an idea.
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KennyR wrote:
If someone would actually have read up on the science, they could have saved their time and effort. The human body doesn't have the muscle or energy it needs to supply lift to its own weight. Our power to weight ratio is too feeble.
Power to weight isn't the issue here. Aeronautical engineering is. After all, whats the power to weight ratio of your average glider?
A human powered aircraft has already flown the channel, as blob points out...
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Winged aircraft ie. all aeroplanes, gliders as well as the Gossamer Albatross (GA) get lift from the passage of air over their aerofoiled surfaces. This is achieved using a propellor or a jet engine to create forward motion through the atmosphere. A reltively slow airspeed can produce quite substantial lift, as evidenced by the peddling man powering GA's large but very light propellor. Helicopters on the other hand get lift from an equal and opposite reaction to downthrust.
Downthrust is created by the rotors (which have a small aerofoil section themselves) compressing air downwards into a relatively small column below the craft. In effect, the rotors press down and the craft rises. I suppose a small amount of lift is generated via the aerofoiled blades but nowhere near that of a formal wing.
As has been said, no man has enough energy to generate a downthrust equal to the combined weight of the craft and himself. No amount of gearing or body weight reduction will change that. Interesting thing is a bumble bee can do it with ease even though the principles of aerodynamics say it shouldn't.
Cheers,
JaX
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Hum,
i don't think that driving an aerofoil around, and around, in a circle would be any less powerful, than driving it forward, in one direction (AKA Gossamer)
Though the material stresses on it would , i imagine, be formidable to overcome
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JaXanim wrote:
As has been said, no man has enough energy to generate a downthrust equal to the combined weight of the craft and himself
No, Kenny said that no human has the power to weight ratio to achieve powered flight, which is not true since it has been done. The method was unspecified. Although its fair to assume downthrust, given the thread title, I suppose :-)
No amount of gearing or body weight reduction will change that. Interesting thing is a bumble bee can do it with ease even though the principles of aerodynamics say it shouldn't.
Yeah, the humble bumble bee. Our A level maths/mechanics teacher (a former aeronautical engineer) used to bring that one up from time to time.
It basically demonstrates that there is more to aerodynamics than we fully appreciate (simply because we can't make the numbers fit), so perhaps there is hope for the helicopter yet...
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@blobrana
Not sure I understand your point.
Anyway, horizontal flight embraces three major forces:
Mass (gravity) which is down.
Lift (via the aerofoil or reaction to a down thruster) which is up.
Drag (air friction) which is backwards in the case of 'forward' motion through the air.
Provided lift exceeds mass, the craft will hover.
Provided the motive force exceeds the drag, the craft will move horizontally, ie: it will fly.
Aeroplanes get lift from the differential pressure above (low) and below the relatively vast wing aerofoil area. A very slow forward motion through the air will create a lot of lift if the aerofoil is pronounced (as in GA and typical gliders). A man has enough energy to achieve this lift if the total mass is low enough (as in GA). The craft also has to be designed with sufficient streamlining to allow drag to be minimised. Even so, GA flies very slowly, perhaps 10mph.
In the case of hovering, which only helicopters and vectored thrust planes can achieve, there is no drag because there's no motion. All the power of the engine is directed in opposition to the mass. In other words, the downthrust must equal the mass of the craft. The air is simply used as the down thrusting medium in the helicopter. In the Harrier, it's the high pressure exhaust stream.
So, no man has the energy to counterbalance the mass and can never power a hover. He can, however, power just sufficient forward motion, by overcoming the drag, and thereby get a hand from Mother Nature's lift as the air passes over the aerofoil.
As a matter of interest, the tips of a helicoper's rotor break the sound barrier. So yes, the stresses are immense.
Cheers,
JaX
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@JaXanim
hum,
what i mean is that it must be possible...
If the `gossamer` flew in a circle then by your logic it wouldn't fly...
The rotor blade is like an aerofoil wing and gives lift , just like an aeroplane; except it's moving in a circular path.
lift is lift - the motion that created it doesn't depend on it being in a straight or circular path...
And if you say that someperson moved the `gossamer ` with sufficient forward motion, overcoming the drag, to get it to fly the by that logic it must be easier to fly a helicopter since there is no drag to overcome...
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"i should have used epoxy resin" - Daedalus
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"Never assume the obvious is true" ;-)
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Hum,
tnx.
@Dr Watson
I have no data yet. It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts.
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When I first saw the title of this thread I immediately thought...
Yes! we can finally burn some neds and get use out of them :-P
Pity no one's thought of that yet :lol:
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Vincent, your argument has one logical flaw: sh!te doesn't burn.
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Ahh but it does, and its good for so many other things too. More here. (http://www.guardian.co.uk/netnotes/article/0,6729,1261812,00.html)
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Hum,
most impressive bit of research (google?) i`ve seen...
i like the bit about `dung is also an organic recreational aerodynamic device...`
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@ Blob
Don't go making your own frisbees now :-o
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What I couldn't get my head around was the 'dung smoked cheese'
Can't smell too good, can it?
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@blobrana
I see your argument, but it's a fallacy.
When you propose GA flying in a circle, I guess you mean to simulate a 'copter. The ultimate analogy is GA rotating about its centre.
If GA's wings were given 'opposite' pitch akin to helicopter blades and it was made to rotate about the wings' centre point and with a relative wind speed of say 10mph, it wouldn't hover. Hovering requires downthrust, and there is none in the case of aeroplanes. The wings create lift. The equal and opposite force (as required by Newton's first law) is the mass of the craft.
You could think of the 'copter rotor as an air pump, sucking in air from above and compressing it by altering the pitch of the blades. It's not their aerofoil section which creates lift, it's the downward jetstream against which the rotors react and thereby lift the mass.
When planes turn in too tight a circle they become unstable because the relative airspeed across the wings goes down. The airspeed becomes a vector determined by the angle of turn. At a hypothetical 90 degree turn, airpseed is zero and the plane will fall out of the sky. You've probably seen videos of US and Russian fighters getting into such predicaments.
[EDIT: Flying a plane in circles requires more and more power as the radius decreases. In the GA case, I doubt if it could be made to turn in anything but very big circles, maybe a mile across. The peddling man wouldn't have enough extra energy to do otherwise and remain aloft.]
Now it just might be possible to power a helicopter on dung, but never by a man.
Cheers,
JaX
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KennyR wrote:
Vincent, your argument has one logical flaw: sh!te doesn't burn.
Now that's true, but we could harness the methane gasses and use that to power the chopper :-D
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@Vincent
Yep. Remember Mad Max's mate with the autogyro? That was powered by methane derived from fermented pigshit wasn't it? I'm sure it was something like that.
JaX
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JaXanim said:
"Now it just might be possible to power a helicopter on dung, but never by a man."
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Fine, then we'll ask Blobrana to do it :-)
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Hehe,
according to the original link:
"A helicopter rotor is notoriously inefficient in producing lift compared to a fixed wing aircraft. Consider the large portion of the rotor (the inner portion) that sees a low free stream velocity. Lift depends on how fast a local wing section is moving. Airfoil sections on a fixed wing aircraft see a constant free stream velocity (the speed of the aircraft"
The rotors arn`t used for `thrust` but for lift...
The more i think about it, the more obvious it seems ( that it is possible)...
I hope your not an engineer JaXanim :-)
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@Blob,
Yep, spot on with the observation comparing a helicopter rotor to a conventional aerofoil.
One thing though re the Gossamer Condor human powered channel crossing:
Although the Gossamer was propelled by human effort, the aerofoil was partly supported by thermals from the body of seawater. You only have to see how a sea bird skims the surface of the water for some distance without actually flapping to see that enough lift is being generated to keep the bird aloft without requiring the bird to flap constantly.
You might also notice that the wings of seabirds tend to be longer and narrower than their land bound cousins.
A human powered helicopter would be under constant power to remain airborne, the pilot couldn't stop pedalling and coast because the rotor would cease to generate sufficient lift (constant movement is required), plus the balancing fan (tail rotor) needs it's power modulated to compensate for torque.
I'd love to see the thing fly though...
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Interesting thing is a bumble bee can do it with ease even though the principles of aerodynamics say it shouldn't.
New research has made clear that bumble bees can fly! (DUH! :-)).
Their wings are shaped in a way that gives much better dynamics compared to that of a ordinary bee, or a wasp.
Their wings are simply shaped in a way that creates tiny air whirls around them. Theese whirls... well... do stuff... (I'm no expert on aerodynamics).
Anyway, the old saying that "Bumble bees can't fly, they just aren't aware of it" isn't true.
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As for the topic:
I'm sure, that with advances in rotor technology, and a very well trained pilot, like Lance Armstron, could get a human powered helicopter up in the air for short periods of time. However, I seriously doubt that it is possible to travel anywhere.
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I remember reading a book all about Lamina air flows and how insects can use Vorticies on the top surface of the wing increase the speed of the air, many years ago (probably about 15!).
when you think about it like that, it's hard to see why any one would think a Bumble Bee can't fly!
The real tricky one, is the butterfly, but that takes advantage of the large wing area and the distinctive flapping motion to lower the pressure above the wing.
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bloodline wrote:
I remember reading a book all about Lamina air flows and how insects can use Vorticies on the top surface of the wing increase the speed of the air, many years ago (probably about 15!).
when you think about it like that, it's hard to see why any one would think a Bumble Bee can't fly!
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The American P51 Mustang fighter of WW2 was revolutionary because it utilised a laminar flow wing. This afforded great manoueverability without sacrficing speed and fuel economy through drag because the wing was excessively thick in chord.
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Until the advent of slow motion photography, physics was unable to explain how a four winged dragonfly remained aloft. Wings that occur in nature have the ability to change shape to suit the conditions. Insect wings flex to ensure both lift and thrust are delivered on up and down strokes. Bird and mammal wings are more advanced and can physically change in shape to match the prevailing winds, thermals etc. This is why a Peregrine Falcon can soar to a respectible altitude which requires an efficient lifting wing and still dive at speeds in excess of 200Mph, simply by folding the wing a certain way.
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hum,
yeah,
>>Bumble Bees :
i believe that their wings are crinkly and bumpy which helps , also i imagine that the air would take on different properties (become `stickier`) when you get smaller in size...
Though i do remember reading about dragonflies that were 1.5 meters in span that existed during the cretaceous (Earths atmosphere was denser)
update See! (http://www.kansan.com/GetStory.aspx?id=7117c30b8ee44c1c8091dee9f70401eb)