h a l f b a k e r yI think this would be a great thing to not do.
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Coaxial rotor helicopters have two rotors: one above the
other spinning in opposite directions. These have the
advantage of not needing a tail rotor to counter the
torque
of the main rotor, but have a much more complicated
linkage to control collective and cyclic for both
rotors. They also
tend to be tall because there needs to
be
significant separation between the two to avoid rotor
collisions.
What I propose is that rather than having two similar
rotors
spinning in opposite directions, make the top rotor
significantly smaller, and have it spin faster. It might
look
very much like a fixed pitch propeller for a fixed wing
aircraft. The lower rotor would be a fairly standard
helicopter rotor, but of course have a hollow shaft to
allow
for the shaft to the propeller.
The helicopter would be controlled with the main
(lower)
rotor just like a normal helicopter. The speed of the
propeller (upper) would be adjusted to counter the
torque
to control rotation, but would have the beneficial side
effect of providing additional lift.
https://www.google....safe=active&ssui=on
[hippo, Sep 01 2017]
Tailwheel Takeoff
https://www.youtube...watch?v=qbIQR02y3zo
[FlyingToaster, Sep 07 2017]
[link]
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Might work. How about curving the upper blades upwards?
An egg beater would therefore be more apt name. |
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But still my subconscious, directs me towards a state change
rather than just an aerial double screw. |
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Or have one rotor and make it rotate alternately
clockwise and counter-clockwise.
Or give the
helicopter one rotor and little stubby wings with
little propeller engines on, one pointing down and
forward and the other pointing down and backwards, to
counter the rotation of the main rotor |
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Maybe have the surfaces spark up. Pretty in the night sky. |
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It's been tried, nearly a century ago. Didn't end well. |
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There are many reasons, some obvious (downwash from upper blade destabilizes airflow over lower blade) and some not so obvious (asymmetric impingement of airflow on top of fuselage, Coanda effect). |
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But coaxial helicopters exist; why would a smaller upper rotor be worse? |
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If we give you a coaxial helicopter and a hacksaw, will you promise to try and find out by practical testing ? |
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No, because I suspect that the speed of the upper rotor cannot be adjusted as necessary. I was hoping for a more reasoned argument. |
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[+] though challenging : a lifting blade going supersonic would be A Bad Thing. |
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// I was hoping for a more reasoned argument. // |
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In the face of all previous experience ? |
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//// I was hoping for a more reasoned argument.
//
In the face of all previous experience ?//
He didn't say he was hoping for it from
*here* |
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So, my thinking is this. Start with a normal helicopter, and modify the blades so that the innermost, say, 25% of them is just a rod with no aerodynamics to it. Lengthen the blades if necessary. Keep the outer 75% the same, so that it provides lift. |
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So far, I see no reason why it won't work. |
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Now chop off the tail rotor. The helicopter will spin round and it will be bad. |
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Now add the smaller coaxial rotor - 25% the diameter of the main - either above or below the main rotor. All it has to do is to provide drag to counter the main rotor; if it provides lift as well, all well and good. It won't interfere with the main since it's over (or under) the aerodynamically null part of the main rotor. |
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So, such a helicopter should fly OK. |
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But what's wrong with all these designs? (see link) |
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^ well, there's the large gap between the rotors. Looks awkward. |
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If the counter-rotor were small enough it could just be a propeller without articulation (though possibly with variable pitch). |
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Yeah, right ..... damn-fool way to commit suicide .... |
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And then there's the stealth version which looks akin to a B-2 bomber but simply spins the entire aircraft. |
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//But what's wrong with all these designs? (see link)// |
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The sales figures are the biggest problem. Helicopters are
already maintenance nightmares, noisy and very crashy.
Adding the complexity of a second counter-rotating rotor
makes all those worse. |
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//If the counter-rotor were small enough it could just be
a propeller without articulation (though possibly with
variable pitch).// |
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This makes a lot more sense. Standard tail rotors provide
all the counter torque but no lift. A counter-rotating lift
propeller would provide torque and lift. Some of which
would be both free and counter to the main rotor in lift
asymmetry. You'd still need the whole tail arrangement,
but it could be a lot smaller. |
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So, Mr Clever, why do 'copters typically have long, thin blades ? |
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Why not just have two contrarotating variable-pitch props ? |
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In a normal helicopter, the rudder pedals control the pitch of the tail rotor to balance the torque. |
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In a coaxial helicopter, both sets of blades are the same, with that complicated double swashplate meaning they balance each other automatically, right? |
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With one tiny rotor and one big, to have torque balanced and full control over the collective pitch, I think you'd need to vary both the pitch of the blades *and* the RPM. So, you'd end up needing the same double swashplate and also something like a CVT. |
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// With one tiny rotor and one big, to have torque balanced and full control over the collective pitch, I think you'd need to vary both the pitch of the blades *and* the RPM. // |
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// So, you'd end up needing the same double swashplate and also something like a CVT. // |
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... which could not only handle a shedload of power, but do it with cyclic asymmetry and get it right every time ... |
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Consider yourself promoted, [mit]. |
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Now, there's another critical problem no-one has mentioned yet. Clue: it depends on whether the small rotor is above or below the large one. |
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We'll leave that as an exercise for the class. |
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<addresses padded envelope to [IT], starts to fill it will selected moist morsels of animal excrement > |
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I know the answer, but I'm waiting to see if anyone else does. |
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You won't get a gold star on the wallchart if that's your attitude. |
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Not that we doubt you; it's not rocket science (just aeronautical engineering) and we know you have the critical intellectual faculties, but sitting back with your arms folded and sulking won't score any points. |
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Lack of rudder pedals ... |
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Aside: In a Fourier transformation is the addition of oscillations communitative? |
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// critical problem ... it depends on whether the small
rotor is above or below the large one. // |
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[8th] I'm interested to hear what your critical problem is,
though perhaps I'm missing it if it only occurs when the
prop is below the rotor because I've only considered
having the small prop on top. |
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In my original idea description, it does say that the small
one will be on top with the purpose of avoiding the need
for the more complex linkage (double swash plate). I
specified a fixed pitch prop as well for the same reason.
I was about to say that if a fixed pitch prop wouldn't work
then the idea was toast, but I just looked up how variable
pitch propellers work. Hydraulic control using a tube up
the center of the shaft is common, and shouldn't add
excessive complication. I am still concerned that a
variable pitch prop might be more susceptible to fatigue
from the flapping forces. With a fixed pitch prop, I'm
hoping fatigue can be dealt with by simply making it a bit
stronger. I'm also hoping that since the prop is small, the
roll force from unequal lift will be easily countered with a
little cyclic on the large main rotor. |
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And yes, some sort of independent speed control might
be needed between the two rotors. A CVT would be one
way to accomplish this. That might doom this idea,
though it seems that CVT technology has improved in
recent years. |
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One possibility for not needing independent speed
control: What if you use a fixed gearbox to set the
relative rotor speeds for the situation where you want the
most lift and/or highest efficiency. In other situations
you'd need to prevent rotation by a somewhat non-ideal
angle of attack on the main rotor to increase or decrease
torque and compensate with more or less rotor speed.
For example, increase the main rotor angle of attack and
slow both rotors. This would increase torque and lift
from the main rotor and decrease torque and lift from
the prop. Requiring changing the rotor speed to increase
or decrease lift would unfortunately make it very sluggish
for changing vertical speed. A rapid change in collective
would result in spinning if engine power was increased to
keep the rotors from slowing. To start climbing, rotor
speed would need to be increased as collective was
increased, which would be slow. The situation could be
improved with variable pitch prop, though I'd like to avoid
that. |
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My original main concern was that we couldn't get enough
torque from the prop to fully cancel the torque from the
main rotor without the tips being supersonic, but [Max]'s
comment made me realize that we could actually use a
non-ideal prop design to get more torque. So maybe use
a fixed pitch prop design with a steeper than normal
angle of attack. |
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// Requiring changing the rotor speed to increase or decrease lift would unfortunately make it very sluggish for changing vertical speed. // |
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Sikorsky tried that on his early designs and soon abandoned it as impractical. Subsequently a lot of effort went into the development of constant-speed systems and FADEC, with lift modulated by the collective pitch control. |
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The hub and blade assembly is specified so as to operate efficiently in a fairly narrow speed range. Since you have to have cyclic control for direction, collective control is more or less "free". |
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You get points for getting an inkling of what the critical problem is that we mentioned, but constant-speed variable-pitch props are a red herring. As you note, many variable-pitch props are hydraulic - now, ask yourself why there are no hydraulic swashplate couplings. |
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Hint: take a look at pictures of contrarotating 'copter designs. Look closely at the relative positions of the blades at the points where they intersect, and where those points are with respect to the fuselage. Then, consider the likely interaction of a simple two-blade hub with a much smaller sub- or superposed two bladed prop running at a much higher speed and not coupled to the main hub. |
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Your question about a hydraulic swashplate coupling
sounds to me like a stupid question, and I don't think
you're stupid. Therefore I must be missing something or
be making some wildly different assumptions than you. |
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As to your hint, I don't see anything that hasn't been
mentioned already. Also I don't understand what you
mean by // Not coupled to the main hub. // Of course
they aren't connected since they are turning in opposite
directions, but they are very closely coupled since the
shaft for one goes through the middle of the shaft of the
other, and there would be bearings that prevent vertical
movement of either rotor hub in relation to the fuselage. |
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So again I think we're missing each other and it would be
more effective if you just stated the problem. Thanks. |
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My uneducated guess would be that to work best, the blades need clear air. An upper contra rotating blade is going to mess up air dynamics with it's downward thrust. Preferably the blade should mess up the other blade over the cabin because that when downward force is obstructed anyway. A small rotating blade has to move faster so will cross over more times at unwanted places. If the smaller blade is closer to the cabin it's effect maybe more pronounced. |
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// they aren't connected since they are turning in opposite directions, but they are very closely coupled since the shaft for one goes through the middle of the shaft of the other, // |
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By "connected", we mean that the relative rotational speeds can be different. Think in terms of gyroscopic force, where changing the rotational speed of the rotating mass changes the couple which is transmitted to the shaft, and it's working in the reverse direction to the couple induced by the main constant-speed rotor. It makes the whole system aerodynamically unstable and extremely sensitive to the tiniest change in conditions or control forces, making it effectively uncontrollable. |
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The smaller prop needn't spin faster, if it can be draggier. |
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Drag = turbulence, which is a Bad Thing in this circumstance. |
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Yes but (a) The main rotor can have its central portion as just rods, and hence not be very affected by turbulence (b) The small rotor could be below the main, having less imact on the latter. |
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I think what [Max] is saying the main rotor could be paddle like which would loose a lot of wing area and place a load more stress on the an already very complex mechanism. Maybe just wait for material science to hand out the next gen. in super strong lightweight materials. |
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[8th], I don't buy your assertion that unbalanced gyroscopic
effects are a serious issue. Conventional single rotor
helicopters deal with large gyroscopic precession. A tail
dragger airplane has to use the rudder in addition to
elevators/flippers during takeoff to compensate when the
tail comes off the runway. Put them together in opposite
directions and they will partially cancel. The remaining
precession will be dealt with in the normal way. |
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// Conventional single rotor helicopters deal with large gyroscopic
precession. // |
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Yes, because it's a predictable single force and can be factored
into the airframe and control design. |
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// A tail dragger airplane has to use the rudder in addition to
elevators/flippers during takeoff to compensate when the tail
comes off the runway. // |
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All aircraft have that problem, not just tail-draggers.It's more
pronounced because the empennage is deeper into the ground
effect. The fin's usually offset to compensate,or the rudder has
built-in bias. |
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// Put them together in opposite directions and they will partially
cancel. The remaining precession will be dealt with in the normal
way. // |
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Only under fixed conditions. Make the relationship independent
and dynamic and it's a whole different can of worms ... |
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Spinning the smaller blade faster limits the total achievable
airspeed, for the reason that FT mentioned above. |
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It would be very hard to tune engine exhaust to the
dynamic demands of flight. |
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//All aircraft have that problem, not just tail-draggers.It's more pronounced because the empennage is deeper into the ground effect.// |
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Aaand, you're just making stuff up. |
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IIRC, tail-draggers don't generally do 3-point takeoffs: pilots lift the tail first bringing it up onto 2 wheels, which is when (propeller) precession kicks in and tries to turn (yaw) the aircraft. Unlike nosewheel birds which are in that orientation to begin with. |
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"Ground effect" is caused by reduced compressibility of air between a wing and ground. I doubt an horizontal stabilizer has any worth mentioning. |
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// I doubt an horizontal stabilizer has any worth
mentioning.// |
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Oh no. It's noticeable, measurable and an important
design consideration. It's particularly prominent on longer
aircraft with low mounted horizontal stabilizers. Imagine
something like an A340-600 where at the limit of 9.5%
rotation, the wing is 20+ feet further up out of ground
effect. The ground effect vs. height above ground is very
much non linear so those few feet make a big difference. |
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The other consideration is angle of attack, wing in ground
effect lift drops off in a non linear relationship with this
too. A rotating aircraft will increase the angle of attack
of its wing while raising it off the ground, at the same
time, the horizontal stabilizer will move closer to the
ground and decrease it's angle of attack, because it's
likely to be all-flying, so you get a pitch down tendency
as the aircraft lifts off. |
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This is one of several reasons why you may want to go
with a high T-tail during design. An example of this is the
Handley-Page Victor. During approach, it's low wing
entered ground effect earlier than the high T-tail giving a
kind of automatic pitch up flare, solid British design.
Conversely, the F/A-18 has very low horizontal stabilizers
and had tremendous problems getting off the flight deck.
The fix involved canting both rudders inward to sort of
lever the nose up. Some may blame shoddy design work
that should have been spotted at the scale model stage, I
think we should give them the benefit of the doubt. Twin
outward-canted rudders look cool, and by actuating them
independently you sort of invent the ruddervator.
Besides, hasty aerodynamic bodge-jobs to fix flawed
airframe design is as close as the US gets to a naval
tradition. |
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// Aaand, you're just making stuff up.// |
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<points smugly at what [bs] said > |
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// IIRC, tail-draggers don't generally do 3-point takeoffs: pilots lift
the tail first bringing it up onto 2 wheels, // |
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Lift the tail ? But... how ? Does the pilot yell at Tinkerbell and
Peaseblossom to lift it up ?No, amazingly the tail generates lift as
airspeed increases ... |
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// which is when (propeller) precession kicks in and tries to turn
(yaw) the aircraft. Unlike nosewheel birds which are in that
orientation to begin with. // |
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Yaw on climbout is unrelated to ground effect. |
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// "Ground effect" is caused by reduced compressibility of air
between a wing and ground. // |
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Airfoil. Any airfoil, not just the wing. Even lifting-body designs
encounter ground effect. |
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// I doubt an horizontal stabilizer has any worth mentioning. // |
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If you have a small plane and a big runway, try it - as ground
speed picks up, you can't keep the tailwheel on the runway. With a Stampe SV.4, there's no way you can do it, long before flying speed is reached - even with the stick right back. Then you have to be very gentle releasing the pressure or you'll put the nose in. |
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//<points smugly at what [bs] said>//
<points smugly at <link> which is not at odds with what [bs] said> |
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Ground effect has nothing to do with rudder control. Lifting the tailwheel to make the aircraft level, prior to attaining rotation speed, causes precession which creates yaw which requires rudder to compensate. Nosewheelers don't have this particular issue since they're already level. |
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That's not to say they don't require rudder handling: since the engine is at full throttle, and the built-in wing and tailfin offsets are made to compensate for cruise throttle, some extra rudder is required. |
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If a tail-dragger aircraft is handled gently - and particularly, if the throttle opening is slow and smooth - then the yaw when the tail lifts is barely noticeable, given that other forces, particularly if there's any crosswind, can predominate. |
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On twins, the effect can be completely "hidden" by simply advancing one throttle a little more than the other. |
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But you're right about the difference between full-power and cruise power - the rudder/fin bias is always a compromise. |
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There's another problem with tail-draggers. Before the tail lifts, the fin and rudder are in the turbulent "shadow" of the fuselage (unless you're lucky enough to have a Lancaster) so the rudder authority is reduced. So initially, disproportionate rudder input is required until the fin lifts into smoother air and starts to do its job properly, |
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