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I kind of assumed that some version of this would be baked but I can find no reference to it on the Hlafbakery or in real life if it does exist, so it might not be as good an idea as I thought... or maybe I just don't know the right word for it.
Dual, opposed, nested Archimedes screws which use counter
rotation for stability have a large diameter shared opening which tapers to create thrust.
If made from light enough material, (aerogel maybe?), vertical take-off could be achieved using the deflection of exhaust to provide directional control.
If there are no rules against height requirements then this concept may be able to claim that human powered helicopter grand prize.
Well first off...
http://www.halfbakery.com/user/archimedes
it's nice that someone rsvp'd this moniker in '01 and then never came to the party. [2 fries shy of a happy meal, Apr 01 2012]
Powers helicopter 1862 - project
http://www.aviastar...ters_eng/powers.php
The helicopter had two rotor units in the form of spiralling screws .... [baconbrain, Apr 01 2012]
Lipkowski's Helicopter -- 1905
http://www.aviastar...s_eng/lipkowski.php
... two gigantic contra-rotating 'screws' ... [baconbrain, Apr 01 2012]
Archimedes screws work better on soggy ground
http://s4.e-monsite...Screw-Amphibian.jpg
[Vernon, Apr 01 2012]
So it's a more complex version of da Vinci's drawing then...
http://www.leonardo...nci-helicopter.html
[RayfordSteele, Apr 02 2012]
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I dunno... is an Archimedes screw more efficient
than a rotor/propellor? |
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There's an episode in the development of the
ship's propellor. It was assumed that a long,
multi-turn screw would be optimal, but then the
guy (I forget who) tested to see how few turns
would still work acceptably. Surprisingly, the
efficiency got better as the number of turns was
reduced, until the optimum was found to be much
less than one complete screw-turn. |
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Thinking about it another way, the first turn of
the screw thread gets the air moving at a certain
velocity (something like the sine of the pitch
angle times the rpm). Thereafter, the rest of the
screw isn't doing much. |
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So, I'm not convinced that a metre-wide, two-
metre long multi-turn Archimedean screw will be
any better than a regular rotor. |
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//Thereafter, the rest of the screw isn't doing
much// What if the pitch increased along its length?
Something like a multistage compressor, then. |
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// What if the pitch increased along its length? Something like a multistage compressor, then. // |
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Possible, but it's only efficient in a very narrow range of fluid densities, flow rates, and most of all rotational speeds. |
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In a conventional chopper, lift is varied by changing blade pitch angle; early designs tried to do it by engine revs, which was not a success, for may reasons - not least, as the lift and therefore revs vary, significant yaw is induced. |
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// Dual, opposed, nested // |
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There's going to be a substantial amount of turbulence at the interface between the two screws, unless they're separately ducted. |
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Consider the mass air flow. In a conventional small helicopter the rotor blades are about 3 to 4 m (for something like and R-22) which gives a swept area of about 40m2. The more the area of the "intake" is reduced, the greater the velocity of the air needed to sustain the downforce ... |
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//What if the pitch increased along its length?// |
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Yes, but then the top and bottom of the screw
are working against eachother. The bottom of
screw is trying to pull air through faster than the
top of the screw is feeding it in, and so you're
constantly trying to pull a vacuum between the
two, I think. |
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I like the general idea, though, even if it's not
practical. It seems to be based on the
attractive notion that if you can just somehow
grab
hold of a big enough mass of air, you can pull
yourself up on it. |
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Ultimately, the logical extension of that idea is to
have as big an area of wing/rotor/whatever as
possible, so that "leakage" around the edges is
relatively little. |
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[edit: I only just noticed that the two screws are
nested, which is pretty. But I don't think it
helps.] |
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// so that "leakage" around the edges is relatively little // |
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Or use a ducted fan, like the Raptor. |
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// Dual, opposed, nested // |
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It does, in terms of torque (yaw) cancelletion - and intrinsic benefit of contrarotating designs. |
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Many high-powered propeller driven aircraft (Like the "Bear") used dual contrarotating props to cancel the huge torque that would otherwise be transferred to the mounting pylons and then to the airframe; and it also counteracts the gyroscope effect. |
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Note that due to their different radii and mass, the inner and outer screws would have to rotate at different velocities ... |
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There's a new aircraft that looks a little like two archimedean screws in ducts, one on each side, laying flat, but I think it's mostly marketing hype. I'll see if I can find. |
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The general rule for aircraft is that you want to move as much air as possible while handling it as little as possible, and a lot of air moving slow is better than a little air moving fast. Which is to say that I don't like this idea--it may be possible, but it isn't better. |
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// and a lot of air moving slow is better than a
little air moving fast. Which is to say that I don't
like this idea// |
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But, as I read it, the idea doesn't specify that the
screws are small and fast - I had assumed that
they would be large and slow (a bit like the spoof
helicopter that da Vinci drew). |
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As I see it, the objection isn't so much over the
mass/velocity of air being moved, but rather over
the efficiency of an Archimedian screw compared
to a regular rotor of similar diameter and speed. |
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//the top and bottom of the screw are working
against eachother. The bottom of screw is trying
to pull air through faster than the top of the
screw is feeding it in// Or, to put it another way,
the bottom half of the screw is assisting the top
half, and the top half is assisting the bottom
half. |
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If one or the other of the top or bottom half
were passive, then it would be a mere
obstruction, as you say. But if power's delivered
to both, then I think they work with each other,
rather than against. |
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A cardiologist would say that the lower half is
reducing the afterload of the top half, while the
top half is increasing the preload of the bottom
half. Reducing afterload, and increasing preload
both increase cardiac output. |
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Sorry, [MaxB], my objection should have been more clearly tied to the handling part. But it does tie in to the screws, as in diameter, because every inch bigger across they make the bad boys is another foot longer--sooner or later they are going to skimp on diameter just to save weight. |
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If you look at early helicopteroids, you will see that many used rotors that had screw-like length to them--chord, it is called. They were un-ducted archimedean screws, if you will. That didn't work well. |
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A modern ducted-fan aircraft is using an archimedean screw of the proper chord. Which is to say as short as possible. |
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(To be clear, "chord" is the length of an airfoil fore-and-aft. If you stick your arm out a car window and fly your hand, the distance from thumb to little finger is the chord. (Generally speaking, shorter chord is better.)) |
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hmmm, many of the objections to the design seem to have been covered. |
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I see the differeing rotational rates of the inner and outer screws meshing seemlessly by either interlocking zipper-like so that they can not turn at seperate rates, or joined at their blade edges so that they can not uncouple.
If I understand correctly, the propper chord for modern aircraft blades are determined by an open system where much of the airflow is lost to the sides and between blades, as in a helicopter, or ducted like a turbine.
This system, if housed within a funnel shaped sleeve, would have the advantage of having no loss of air volume while its ducts were sealed or expelling force laterally as desired by venting the sleeve/duct.
The tapering of the screw shape and reduction of thread size and... (frequency?) would account for continuous pressurization to overcome the whole 'propper chord size and angle for a single revolution' problems I see creeping up. |
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The only obstacles I can see are material weight vs. rpm's. |
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Those links are awesome [baconbrain]. That's the stuff I'd hoped to find in my search.
Neither of those designs taper to increase pressure nor are they encaplsulated to reduce losses to turbulance. It's sad that Lipkowski discontinued research....
I mean, just try saying 'Lipkowski usurped Sikorsky' five times fast. |
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