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If a helicopter only has one lifting propeller, it has a
speed limit imposed on it by the decrease in lift of the
rearward moving blades as the speed increases. This
results in either the left or right side of the vehicle
losing lift compared to the other.
If it has two counter-rotating propellers,
there is no
such speed limit, but if the props are both above the
copter, on the same shaft, the linkage to the upper
prop will be complicated.
This idea is to have one lift propeller above the
helicopter, and another lift propeller below.
The control mechanisms for both of these propellers
would be perfectly ordinary helicopter swashplates and
collective controls.
Before and during takeoff, the lower prop doesn't spin;
only after the landing gear has fully retracted is it
brought up to speed. Similarly, during landing, the
lower prop is slowed to a halt, then locked in a "safe"
position, before the landing gear is retracted.
Another benefit of having counter-rotating propellers is
that the tail prop becomes mostly unnecessary. During
regular flight, yaw control can be accomplished by
adjusting the collective pitch of the top and bottom
propellers, such that one prop provides a larger portion
of the vehicle's lift (and thus, more rotational drag)
than the other prop.
We do still need to prevent the helicopter from
spinning wildly during takeoff and landing, since only
one lifting prop is spinning, but it may be reasonable to
use attitude control jets instead of a tail prop.
Attitude jets would be less efficient, but since they'll
only be used for takeoff and landing, their low weight,
small size, low drag, and (for military vehicles) small
radar profile makes them quite attractive.
The only downside I can think of is that, from some
angles, this helicopter might look a bit like a Star Wars
TIE fighter flying on it's side (especially if we use six
bladed props). Is that a downside?
*Not* the photo I meant. That one had the guy standing above raw blades. Crazy.
http://cf.geekdo-im...es/pic739943_md.jpg
[2 fries shy of a happy meal, Oct 02 2011]
Segfly
Segfly
Includes links to the scary photo [pocmloc, Oct 02 2011]
Sikorsky X2
http://en.wikipedia.org/wiki/Sikorsky_X2
Coaxial dual rotor helicopter - capable of 181kts, apparently they expect to get 265kts from it. [TomP, Oct 02 2011]
That's the one. Thanks [pocmloc].
http://en.wikipedia...kner_HZ-1_Aerocycle
[2 fries shy of a happy meal, Oct 02 2011]
The essay I mentioned (for [Twizz])
http://www.charterh...omas%20Pinnegar.pdf
Describes how rotorcraft don't fall over on page 8. [TomP, Oct 04 2011]
Flettner Wing
http://en.wikipedia...i/Flettner_airplane
"... there is no record of them ever having flown ..." [8th of 7, Oct 04 2011]
The "Cyclogyro"
http://www.aqpl43.d...ogyro/cyclogyro.htm
" ...those that were constructed were completely unsuccessful. ..." [8th of 7, Oct 04 2011]
[link]
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Well, yes. But is the complication of having both
rotors above the fuselage actually that great? |
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Also, is it not preferable to have two rotors, but
mount them fore and aft, as on a Chinook? Such
an arrangement is relatively simple, and has the
advantage that the helicopter can be rapidly
turned in any direction, since the fore and aft
rotors can each provide lateral thrust. Many have
argued that the Chinook arrangement is the
ultimate evolution of the helicopter. |
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The bearing required to support the mass of the helicopter
(or even a
portion thereof) from underneath would
be a beefly chunk of metal, and thus prohibitively heavy.
Also, a bottom-mounted rotor is, for a variety of reasons,
ridiculously inefficient. And lastly, it would chop people's
legs off when they jumped out (disembarking passengers
while hovering is a must for military choppers). |
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Those are the first three 'down sides' that come to mind. |
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Since the lift (and drag) arc of the top prop is on one side, and on the bottom prop it's on the other, and the two are separated by a goodly bit, my imagination is telling me there's going to be a sizable roll-torque couple going on. |
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But I'm wondering how pyrotechnically complex we could make a prop-synchronized man-ejection device to solve the egress issue [Alterother] noted... |
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I have seen a black and white photo of a man flying a dual rotor helicoter where both rotors were beneath him. I remember trying not to picture the hamburger that would result if he slipped.
I haven't found it yet but I'm going to give it another look. |
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//And lastly, it would chop people's legs off when they jumped out // |
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Only? I think that theory doesn't have a leg to stand on. |
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Just as a machine gun is timed to fire through a propeller, so too may a person be timed to jump through a rotor. All that would be necessary is the pilot to use a stopwatch and give the signal to jump by lowering his raised arm at the appropriate moment. Making sure his cell phone is turned off first. |
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[2 fries] I know the photo you mean because we posted it as a link on an idea here before. Which suggests to me that we may have done this idea before... |
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Not a great idea, even for autonomous aircraft. [Alt...] said it best. It is better to have top mounted rotors for the same reason it is better to pull something with a rope, instead of pushing it with the same rope. |
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Then there is the whole "center of gravity" thingy, for stable systems... |
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// Many have argued that the Chinook arrangement is the
ultimate evolution of the helicopter. // |
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And many others have argued that if one of those rotors
fails you're utterly fucked because the Chinook can't
autorotate properly. People like my buddy Tom, for
instance, whose legs don't fully straighten anymore. |
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Thanks for the props, all. (no pun intended. No, really, I
mean it. No, seriously, people... ahh, nevermind.) |
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//if one of those rotors fails you're utterly fucked
because the Chinook can't autorotate properly.// |
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Well, yes. On the other hand, any form of aircraft
is a precarious arrangement. Helicopters are
particularly so - only the brave or foolhardy
voluntarily take to the air by means of a glorified
egg-whisk. Complaining that a Chinook can't land
nicely with a failed rotor completely overlooks the
fact that, by rights, helicopters shouldn't get off
the ground in the first place. |
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A (single-main-rotor) helicopter, on the other hand, is
capable of making a semi-controlled landing without
power, a significant advantage over those fixed-wing
deathtraps everybody insists on flying around in. |
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I was also thinking of the rolling problem. |
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//the props are both above the copter, on the
same shaft, the linkage to the upper prop will be
complicated.// |
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Err... if we can do counter-rotating props on
planes, why not on a chopper? It should be easier
in a helicopter because the engine does not need
to be so close to the rotor, therefore a hollow
shaft to one with a solid shaft to the other should
be feasible. |
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Perhaps another way to do this would be to have
the rotors arranged side by side, counter rotating
so that the tips went forwards in the direction of
flight, thus doubling the potential top speed. I
think Fokker did something like that, but I don't
know which direction the rotors went in. |
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I just found [link], by the way which seems similar
to this. It sounds as though the speed limit for
this comes when the rotor tips start going
supersonic. |
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I think [Altoerother] needs an autogyro. |
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How about having both sets of rotors on the ground,
but inverted. Payloads could then be wafted. |
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Sadly, autogyros are on the list of things that The Good
Fairy Jenny has banned [The Alterother] from attempting
to build. Or, if they aren't, they will be when she learns
what they are. |
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The big problem here is pendular stability. If the centre of lift is above the centre of mass, the craft will tend to be stable ie the mass will "swing" below the lift point. Small variations/perturbations in alignment will tend to stabilise. |
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In your design with the lift below the mass, any misalignment will result in a force acting to increase the misalignment. It's analogout to torsionally divergent wings. It could still work but would need lots of active control and perhaps additional control surfaces/mechanisms not already present on a helicoper. |
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TL/DR : it's more stable to "hang" a mass rather than prop it. |
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//The big problem here is pendular stability.// Unfortunately, [Custardguts], you're laboring under a misconception. If you google "pendulum rocket fallacy", you'll find some good explanations for why that doesn't work as you expect. |
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(You'll also find that you're not the first to get caught out by this one - there was this one guy, named Robert Goddard...) |
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Here in San Diego we have seen a bunch of these plan helicopters with dual rotors left and right, which pivot. Straight up for helicopter hovering, right in front for dual prop plane speed and everywhere in between. |
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Thanks for the tip, that's pretty cool. I'm a little caught up on the "no torque as the mass vector is aligned witht the COM" issue and what that means - perhaps I'm thinking of a two-body problem when I should be thinking about a rigid mass.... I think this calls for a series of escalating and increasingly extravagant experiments, but I won't let that stand in the way of a good rant, however: |
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I propose however that there's a significant difference between a hovering helicopter, and a rocket accelerating - and that's inertial effects. Here's where I wish I could show you sketch. |
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In the case of the rocket that is accelerating, you have three force vector components (ignoring aerodynamic drag, coriolis, etc). That is "inertial drag" acting exactly opposite to the vector of acceleration, thrust, and weight. Now as per the "pendulum rocket falacy" theory, the inertial drag is opposing the thrust in an approximate (but not exact) alignment. The weight will still point straight to the centre of the earth, and will, especially at low accelerations, still act to stabilise a mass-under rocket, and destabilise a mass-over rocket. |
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I posit that in the helicopter example, vertical acceleration, and thus "inertial drag" is zero while hovering, that the weight vector is not aligned with the thrust vector, will generate stabilising torque and the pendular effect is in place. I can draw pretty pictures if there's a way to post them. As my lecturer used to say - if in doubt, draw a Free Body Diagram. |
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To take an extreme example, it is absolutely undeniable that a paraglider, where the COM is somewhere within the pilot's body, and the COL is at or about the wing centroid (some 4 or so metres above), has massive pendular stability effects. I secretly suspect that the paraglider example is a bad one, due to differential wing loading and damping causing regression. |
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...or maybe I'm just not a rocket scientist. |
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The pendulum thingy only works when there's no aero or gravity elements. Common sense says that if you're flying a rocket sideways then if you have the engine at the back it's going to tilt down and if... if... hmm... crap. |
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So why is an airplane a "pendulum" ? |
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If you look at "high wing" designs like the classic Cessna 152 and its descendants, the lift generated by the mainplane supports the mass of the fuselage and passengers below it; in fact, the occupants are pretty close to being under the mainspar, their weight counterbalanced by the moment of the engine. |
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This makes high wing aircraft inherently much more stable, due to the "pendulum effect". |
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[8th]! I'm amazed at you! Follow that logic out: if you're stabilizing via pendulum effect, then you should get the very best stability when you're overloaded just shy of breaking the wings. Doesn't happen. You do get stability from dihedral wings: each wing slightly raised. Then, when you tip to the side, the low wing has a longer length (more lift) measured perpendicular to the vertical; that force imbalance corrects the roll. |
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Helicopter blades do a dihedral angle as well, which will work either under or over the fuselage. Or, at least, if the blades have room to flex up without interfering with, uhh, things... |
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The fore-and-aft (pitching moment) stability also doesn't come from a pendulum effect. If it did, that would mean the aircraft would fight you when you try to change the pitch. It does, but indirectly: push the stick forward, craft noses down, airspeed increases, center of lift moves forward, craft responds by pitching up - now it's fighting you. |
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//So why is an airplane a "pendulum" ?// Notice that most passenger planes are low wing? High wing aircraft exist, certainly, but generally to meet one or more of the following requirements:
I need downward visibility
I'm likely to operate from an un-paved surface, and want the engines to avoid ingesting rocks / sticks / gooney birds
I'm carrying ukoo huge weights, and don't want any interruptions in my main wing spars
My plane has so much excess lateral stability I can't turn it easily, and need some anhedral |
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k, the dihedral thing is pretty straightforward: nothing to do with pendulo: that's aerodynamics. But Cessnas are non-dihedral high-wings so they should tip right over: the two forces, engine thrust and lift are always in the same directions in regards the aircraft. |
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//Cessnas are non-dihedral high-wings// Most Cessnas have some actual dihedral (1 degree on a 152, or a 170B; the 170A had none); but they all have a positive "effective dihedral" (q.G.) |
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<placeholder for long-winded discourse on why [lurch] needs a
little refresher in basic Euclidean geometry> |
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Ahem. Sorry for the delay, we had to clean all the whiteboards and sharpen the pool cue. |
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// overloaded just shy of breaking the wings. Doesn't happen // |
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Does too. You've clearly never flown with Aeroflot. |
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// when you tip to the side, the low wing has a longer length (more lift) measured perpendicular to the vertical; that force imbalance corrects the roll // |
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It's nothing to do with length. In level flight, the lift vector from the two airfoils acts vertically, and hopefully coincident with the weight balance point (centre of gravity). When you bank, each airfoil is still producing exactly the same amount of force normal to its primary axis; however, this force is no longer acting in direct opposition to the weight of the aircraft, but is now an offset vector. This vector can be resolved into two components, a horizontal and a vertical one. It's the horizontal component that tends to restore level flight. |
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// that would mean the aircraft would fight you when you try to change the pitch. // |
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Some do; they're designed with significant positive stability. |
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// It does, but indirectly: push the stick forward, craft noses down, airspeed increases, center of lift moves forward, craft responds by pitching up - now it's fighting you // |
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The change in pitch with airspeed is a complex interaction between the mainplane, the fuselage, the tailplane and the elevators, hence the need for trim tabs. Not all aircraft have the sort of negative feedback response to a dive that you describe. |
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A significant proportion of STOL airframes are high wing. |
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//Euclidean geometry// I've ridden on a Euclid before. They don't fly, period. Worse than a Caterpillar. |
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Clearly the cliff you were traversing was simply not high enough. |
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Isn't there a fundamental issue with moving a rotating wing at such speed that the retreating surface airspeed drops below zero (i.e the section is moving backwards relative to the surrounding air)? |
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How is the stability of the rotor bowl maintained when lift is generated for only half the rotation? |
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// Here in San Diego we have seen a bunch of these plan
helicopters with dual rotors left and right, which pivot.
Straight up for helicopter hovering, right in front for dual
prop plane speed and everywhere in between. // |
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How is this any different from a tilt-wing (or tilt-rotor)
like the undeservedly-maligned Osprey? |
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As a compromise, mount the two rotors with their
axes horizontal, on either side of the cabin. |
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Lift will be compromised, but the taxiing will be
awesome. |
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// As a compromise, mount the two rotors with
their axes horizontal, on either side of the cabin.
// |
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Baked, if I understand your description properly. Side-by-
side dual-rotor choppers were very popular
with the Russians for a time. |
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//taxiing will be awesome// riding in that taxi would be awesome, getting in and out would require astronaut diapers. |
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//taxiing will be awesome// |
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Yeah, because taxiing is my favorite part of flying and I've
always dreamed of a way to enhance the experience. |
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//getting in and out would require astronaut diapers.// |
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//if I understand your description properly.// Alas
not. The axes of the two rotors are horizontal,
achieving the effect of a high-speed ground-based
Mississippi-paddle- steamer-style contrivance. |
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//How is the stability of the rotor bowl maintained when lift is generated for only half the rotation?// |
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There are a few methods, but they all alter the lift of each blade individually so that the advancing one is made to create less lift than if it were unchanged and the retreating blade makes more. |
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Normally this is done with flapping hinges, a central hinge, or by altering the blade's pitch as it goes around. Juan de Cierva discovered the need to flap by finding that his solidly mounted blades were prone to fatigue at the root. This was caused by the blade bending to flap rather than remaining rigid as was intended. The way it works... actually, stuff it, if you are intersted, I wrote about it in an essay [link] and you want to look at page 8. |
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// The axes of the two rotors are horizontal, achieving the
effect of a high-speed ground-based Mississippi-paddle-
steamer-style contrivance. // |
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Oh, I see. So what you're proposing is not so much an
aircraft as it is a novel approach to the creation and
deployment of high-velocity shrapnel. Outstanding. |
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// high-speed ground-based Mississippi-paddle- steamer-style contrivance // |
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No, he means a Flettner wing <link>. |
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Well, it would definitely be ground-based. Only the
inevitable explosion could propell such a marvelous
contraption into the air. |
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//high-velocity shrapnel// Arturas Zuokas will buy one. Do not, however, take his parking spot. |
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//No, he means a Flettner wing // |
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True, in all respects except the broad concept and
the details. |
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[MaxwellBuchanan]'s //Mississippi-paddle- steamer-style contrivance// could actually fly, provided the paddles are in the form of Gyromill-like impellors. |
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[lurch], [8th], a more rigorous explanation of dihedral stability is that roll induces sideslip (due to the horizontal component of lift mentioned by [8th]), which causes the angle of attack of the lower wing to be greater, which (under non-stall conditions) increases its lift, which tends to counteract the roll. |
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Surpassed by quadcopter technology. |
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