h a l f b a k e r yFlaky rehab
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This allows a spin stabilized bullet to have a core
containing a camera and or guidance mechanisms that
aren't spinning thousands of RPM.
There are two riflings to achieve this. One turning one
direction is a standard bore rifling that contacts the
front
part of the bullet in standard fashion
imparting a left
handed twist. The back of the bullet has extended "fins"
that fit into an outer rifling consisting of deep notches
in
the barrel that rotate opposite the standard rifling giving
the articulated back portion of the bullet a right handed
twist, opposite the front part of the bullet.
The body of the back part of the bullet is slightly
narrower
than the front part so it doesn't hit the inner rifling, only
the outer rifling into which its extended fins fit.
Well I'll be darned.
https://www.youtube...watch?v=bzbVwiIeM0M
[doctorremulac3, Jul 19 2016]
Wikipedia's explanation of the dihedral effect
https://en.wikipedi...zes_the_spiral_mode
see my anno [notexactly, Jul 21 2016]
[link]
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The obturation is therefore going to be at the rear of the front portion, where the pivot is. The propellant gases will impinge on the pivot, requiring a high-speed rotating seal. |
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The front portion has to pull the rear portion. That's a lot of tension on the coupling. |
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It would make more sense to have the full width portion at the rear and the contrarotating portion in front, being pushed. A much simpler seal would be required. |
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How is the reliable mating of the splines with the anti-rifling achieved, other than by manual loading ? |
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//It would make more sense to have the full
width portion at the rear and the contrarotating
portion in front, being pushed. A much simpler seal
would be required.// |
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The wider part is at the back if that's what you
mean. |
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//How is the reliable mating of the splines with
the anti-rifling achieved, other than by manual
loading ?// |
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Not sure I catch what you're saying, could you
dumb it down a bit? Are you talking about possible
issues with the gas seal? |
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Again, I think I should probably throw together a
drawing of this so it's clear what I'm proposing. Re-
reading it, it is a bit confusing. |
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I'm liking the idea of this, but am not sure it's the best
approach to achieve your objectives. You are effectively
suggesting three parts to the bullet; one
going clockwise, one anti-clockwise and one non-rotating.
That might be
overkill - I believe there are tank-fired missiles which use
a rotating sabot
to launch a non-rotating projectile from a rifled barrel. |
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A couple of decades or so ago, I read (IIRC in Scientific
American) about a
suggested guided air to air (large caliber) bullet. It was
proposed as
illumination targeted, i.e. the attacker would illuminate
the target plane
with a laser, and the bullet would home in on that. To do
so it had a
camera in the nose, and could flex very slightly in the
middle to change
direction. It would only need to flex in one axis because
the bullet is
rotating, but would have to do so rather quickly.
You might say that's not particularly relevant, but I found it
interesting,
and it does lead me to the following suggestion:
Don't worry about preventing the rotation, just use a very
high-speed
camera (which you'll need anyway, otherwise most things
will be 'warp-
speed' effect blurred), and synchronise the frame rate to
the rotation
rate. |
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There must come a point at which it's easier to
rotate the target and use a non-rotating bullet. |
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concentric - the outer catches the lands and spins, imparting stability; the inner is the camera... doesn't spin. Bearing, bushing, whatever, between. Picture a .17 bullet (the camera) wedged into a ball bearing wedged into a hollowed out .40 . |
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Well, just having bearings isn't going to do much
since just a little friction on an outer part spinning at
180,000 rpm is going to get that camera spinning
really fast. |
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As far as using the spin for power, yea, I guess you
could do that. |
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As a guy who deals with bearings quite often this seems like
asking quite a lot from them. |
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As I said on your other recent stabilized-bullet idea: |
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I guarantee this will not work even a little bit, and the
bullet will be just as unstable as it would be without the
two cylinders spinning (barring some unexpected
stabilization from the Magnus effect or something). Any
two
equal-in-magnitude-(the site tells me I need to put a
space here)-moment-of-inertia counter-rotating
rotors will cancel out each other's momenta of inertia,
leaving you with no stabilization. (I know because I have
looked into this before, for CMGs.) |
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So you're saying a contra-rotating helicopter blade pair has
no
stability because each one cancels the other one out? |
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I know that's not the case, they're extremely stable and
don't even need a tail rotor except one facing backwards for
forward thrust only, so maybe I'm misunderstanding
you. |
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Yes, that's exactly what they do, and indeed the main reason they exist. Copters with contrarotating blades don't need a tail boom and rotor for yaw stability. All directional and attitude control is applied via the swashplate linkages. |
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They're stable because of the active control of the airfoils. A bullet is purely passive and relies on gyroscopic effects. |
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Yea, nothing unstable about two opposing gyroscopes, quite
the contrary. |
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Bearing issues, possibly. Stability issues, no. |
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No. Still wrong. [8th] is right. |
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I'm not claiming that two counter-rotating or
contra-rotating rotors of any kind impart negative stability
to
the thing that holds them. I'm claiming that they don't
impart positive stability like a single rotor does. (As long
as their angular momenta are equal in magnitude; if they
are different, then the effect is be the same as a single
rotor whose angular momentum is equal to the
difference.) |
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A regular helicopter needs some kind of sideways thruster
on its tail only because the one main rotor imparts a
reaction torque to the body, and this needs to be
counteracted or else the whole helicopter will rotate all
the time. This can be counteracted using a tail rotor
(conventional or Fenestron), a NOTAR system (jet exhaust
blowing out one side of the tail boom), or—only in RC
helicopters so far—a fixed drag plate at the end of the
tail boom. The important thing to understand is that THE
TAIL THRUSTER IS NOT A STABILIZER in the sense of
gyroscopic stabilization. However, the main rotor DOES
provide gyroscopic stabilization. It just also produces a
torque that needs to be counteracted. |
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A two-rotor helicopter (counter-rotating or contra-
rotating) does not need the tail thruster because the two
rotors counteract each other's torque directly. However,
they also counteract each other's angular momenta,
removing the gyroscopic stabilization effect on the
helicopter as a whole. (However, each one still attempts
to stabilize itself (but in opposite directions, obviously),
and this results in torques and forces being transmitted
through the helicopter's frame or rotor axle assembly,
where they cancel each other out as they meet.) |
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However, such helicopters are still stable because
helicopters have another source of stability: active
stabilization by the pilot or autopilot. Helicopters deviate
from upright quite slowly (unless doing so deliberately),
making it easy for the pilot to maintain stability, and
their rotors are powerful enough to correct even quite
large deviations from upright under the control of the
pilot. (On the other hand, a quadcopter or Harrier
deviates very rapidly (and a Harrier's stabilization jets
are much less thrusty relative to its moment of inertia),
so those require much more rapid reactions to remain
stable, which only an autopilot can handle.) |
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Another possible source of stability in helicopters is the
upward flex of the rotor blades under load. This, I would
think, acts like dihedral of an airplane's wings. (The
mechanism of the dihedral producing roll stability of a
plane (or roll/pitch stability of a helicopter rotor, if I'm
right) is quite unintuitive and I can't remember how it
works, but it does.) |
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So I guess counter rotating gyroscopes DO cancel each other
out. (see link) |
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Thanks NotEx, learned something. |
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That's actually quite a good demonstration. I'll have to
remember to show it next time someone doesn't believe it. |
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// Thanks NotEx, learned something. // |
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Oh good... you can still have the sadistically violent beating if you want, as an aide-memoire perhaps ? |
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// upward flex of the rotor blades under load.... is quite unintuitive and I can't remember how it works, but it does. // |
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It's very intricate. The rotor tips follow a complex sinusoidal path with respect to the hub, different on the advancing and retreating segments of the cycle because of the change in relative velocity. It's one of those little quirks of rotary-wing aerodynamics that attracts a very particular breed of pointy-headed twisty-brained Übernerd to the profession.... which is a good thing, because without helicopters to work on, people like that might design anything... |
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Only because it's not possible for us to be wrong. |
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Pretty sure gyroscopes use black magic or some kind
of voodoo to do their thing. |
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I'll leave this post up just because the science
discussion is interesting. |
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Basically angular momentum is a vector quantity, and so
vector addition applies. |
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//mechanism... dihedral.. unintuitive// I was going to say Wikipedia, but their article is rather unintuitive. Look at an aircraft, flying level, fom the front. Both wings have the same amount of lift so it flies level. But, if you roll it a bit to one side or the other then the wing that's closest to being horizontal has the most lift... so it rolls back. |
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That effect can escalate into Dutch roll. |
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Unfortunately, it's an extremely complex problem not really amenable to simple analogies; and extrapolating from a (relatively) stiff fixed wing to a flexible rotary wing really isn't possible - no criticism intended [FT]. |
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// But, if you roll it a bit to one side or the other then
the wing that's closest to being horizontal has the
most
lift... so it rolls back. // |
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Nope (if you don't mean anything more complicated
by
that). That's the #1 dihedral misconception, IIRC.
Looking at the Wikipedia article now, though, it
doesn't
seem to clarify that well. |
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Lift doesn't know or care which way is 'up' relative to
the outside world; it only pushes the plane up along
the
plane's own vertical axis, which may be tilted relative
to
the ground. If the plane is rolled to one side, the lift
vector rolls with it, to still point straight up in the
plane's coordinate system. |
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According to Wikipedia, with a dihedral, this
somehow
causes a sideslip, which causes the plane to yaw in the
other direction, which increases the angle of attack
on
the down-rolled wing and decreases it on the up-
rolled
wing, causing differential lift, which restores roll angle
toward zero. |
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The net effect is that the wing that is rolled down
does
experience more lift, but this is not due to it being
closer to horizontal. (In fact, it should work just as
well
if that wing is below horizontal.) It is instead due to it
being yawed such that it leads the other wing and
points
more in the direction of travel, resulting in higher
angle
of attack. |
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Interestingly, I think that would mean that if you
were
flying with wings and/or in a regime where increased
angle of attack reduced lift, then dihedral would
destabilize and anhedral would stabilize. Maybe. |
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And I'm still not clear on why the sideslip causes yaw.
But I'm pretty sure it won't work for a helicopter
rotor, which is always yawing anyway. |
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While //lift doesn't know or care which way is 'up'//, gravity still does, and is a constant force pulling the airplane straight down. If the plane had wings at 90deg to each other, which way would it roll if one was straight out and the other straight up ? At what angle would it end up balanced at ? (For simplicity's sake let's say the CG is right at the wingroot). |
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It's reasonably obvious that sideslip happens... no clue how that would affect roll except maybe something like the wing in the lee of the wind (which is the up-pointing wing) gets less air because it's blocked by the fuselage, or because it sticks up more and is pushed back down by the crosswind. |
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It makes no sense to me whatsoever that the plane yaws in the opposite direction, ie: the left wingtip dips down and the plane yaws right ? Since there's a crosswind happening (from sideslip), the plane should yaw left, that being what the tailfin is for. |
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Helicopters ? What breed of chicken they sacrifice before each flight probably has more to do with stability than dihedral, but the same principle should apply. |
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Chicken ? You could ritually sacrifice a whole flock of sacred
virgin ostriches on top of a Ziggurat built entirely from flight
safety test documents and dedicated to the Gods of Aeronautics,
and the bloody helicopter still wouldn't be safe, or even a little
bit less lethally dangerous. |
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Sp. "helicopter", Pr. "Assisted Suicide". |
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