Spin direction

[Phoinix]

A question for those, who have seen a lot of cargo and people spining (human cargo) on the end of the hoist line. Long line, not connected to a hoist doesn't count.

In what direction is the cargo spining? The same as the main rotor or the other direction? Supposing that the rescuer doesn't push himself from the ground.

I have seen some videos of helicopter hoist rescue training in our country, and the direction of spining was in most cases the same as the one of main rotor. It's those other few ocassions, that make me doubt the theory, that is spins the same direction as main rotor.

What's your experience on that?

[Vfrpilotpb]

Possibly it follows the same reason why water goes down the plug in a sink one way in the N hemisphere and in the oposite way in the southern Hemis,

or is it the other way round?


Vfr:8

[sandy helmet]

Maybe its to do with if the winchman 'dresses ' to the left or to the right.

[Phoinix]

I don't think coriolis has anything to do with the spin, couse, it spins in different ways on the same hemisphere. :p

[SASless]

Would not the angle of the dangle as modified by the shift of the drift...and aggravated by the weight of the freight...mitigate the drag of the shag as altered by the dip of the ship enter into the formula needed to calculate the calculus of this for us.

[Tony Chambers]

mmm well as an experienced external load operator i would say it is all to do with how the load/person on the end of the line leaves the ground + any twist tension in the line. :ok:

[rotorcraig]

Wonder whether fat people spin faster (more tension on twisted line) or slower (balerina effect) :confused:

RC

[Phoinix]

Sure, the jokers come out right when needed... :yuk:

Tony, what if there is no push when leaving the ground? Plus, i mentioned the hoist, couse the wire can rotate freely around its axis on the point where the hook is atached to the wire.

[boomerangben]

Almost all wire ropes will tend to twist when put under load. I have seen a swivel on a large diameter wire rope generate sparks when unloaded. I was under the impression that hoist wire was a "non rotating wire" ie doesn't twist when put under tension so in theory would not cause a load to spin. However I doubt that a non rotating wire is perfect and therefore, a winchman will always tend to spin. I guess the direction of spin would be a characteristic of the wire manufacturer, much the same as main rotor blades.

The spin can be worse if a stretcher is being recovered, but I suspect that more of an aerodynamic effect.

Hope this explains it.

[pilotwolf]

Plus, i mentioned the hoist, couse the wire can rotate freely around its axis on the point where the hook is atached to the wire.

No experience of sling loading etc, but worked extensively with wire rope at sea. Whilst my experience was with using wire mainly for towing as opposed to lifting, I never found any swivel device that actually worked well under pressure. Most would rotate with the assistance of a bar but not on their own - those that did weren't under much strain and even then the movement was very jerky.

Seem to recall that as wire is wound onto a drum, as in a winch, the coiling will induce turns to the wire... IE. if you try to pull it off without any strain on it bights form... when pulled tight the wire will try to twist to straighten out the lay of the strands= spinning if you re dangling on the end of it!

PW

[Phoinix]

Right, this will make you wonder...

on more than one occasion, there is a rescuer and a bag connected on a hoist, and the lift begins (no push from the ground). Everything is stable (no rotation) for the first few feet, and than suddenly it starts to spin, but not slow, with a kick start.

That is why, i think it's all in aerodynamics and the downwash.

[tecpilot]

Hi Phoinix,
seems you are really engaged since some months with the spinproblem :)

As described in other threads the spinning during a stable lift is caused only by aerodynamics. Of course it's also possible due to load and gravity changes during the lift off from ground. (Base jump ;) )
But to aerodynamics: The downwash is turning into the same direction as the main rotor, but as known not absolutely vertically also in hovering. The accurate motion vector depends from many factors as wind, air speed, rotor diameter, RPM, pitch, shadow of the fuselage, distance to helicopter,... but the air stream is turning in all cases.

Anyway, during the lift-off the load must be lifted through the different phases of the downwash with different and changing vectors. Any load, human or cargo has it's own aerodynamics, it's own levers and it's own air drag. Therefore the load beginns a motion during the hoist ops with coming into the turning air stream produced by the main rotor and in nearly all cases into the main rotor direction . (Other direction may be provocated by the pilot or hoist operator) Sometimes during the motion, with the changing factors (levers and mass potential,...) the motion could be grown stronger and stronger. With any kind of air speed (any direction) the hoist cable and the load comes out of the main downwash vector and the load comes into a more horizontally air flow and turns into a more stable position.

Therefore: beginn the lift-off with a longer cable (outside the downwash) and if the load is out of obstacles get some knots of air speed. That will stabilising the load.

Do you know the REGA video? I estimate they have reached a "Spin" during the hoist ops of more than 70-90 RPM!!! And stopped it only by getting some airspeed in a few seconds!!!

[NickLappos]

Close to the fuselage, the spin is caused by the local downwash, which is often spilled off the cabin top and the side obstructions, and perhaps concentrated and accelerated by any natural funnels in that location. In a heavy helicopter, the downwash near the door might be 40 knots of so, enough to cause the air to rotate as it whips around obstructions. This spin can be in any direction, and is usually the most bothersome kind, as the basket/victim is getting close to the door.

At the very bottom of the hoist line, perhaps 1 rotor diameter or more below, the downwash has the same rotation as the rotor. This is quite a bit slower than the wash near the door, as it averages the general downwash speed. The rotation is due to the drag of the blades, which creates a natural airstream behind each blade that follows along. If you can pcture the air behind an airplane, it would be a core of air that has been accelerated by the airplane's drag so that it follows along behind the plane. This is where some of the engine's energy has gone. Similarly, the blades cause the air they act upon to follow them. Think of a high drag spoon in a cup of tea.

[Phoinix]

Thanks Nick and tecpilot! :)

[Disguise Delimit]

Seen them spin both ways. Also know of Stokes litters that spun so fast that a crewman broke his arm trying to stop it, and another containing a body bag which had body fluids spraying out of it from the spin.

Stokes seems to be worse than a person hanging vertically. Spreading the arms and/or legs slows it down, and makes it more interesting for those on the ground when the hoistee is wearing a dress...

[Vfrpilotpb]

A normal multi strand winch wire, should not have any manufactured tendency to spin when unwound or in re-winding, if it did what would happen would be the total unwind of the manufactured wire, this would then snap or jam in operation, the hook should be fastened to a carrier that encompasses a thrust bearing at the point of joining the wire, the wire is for lifting, but the hook has to be able to rotate, if any spin is there or starts whilst under load that is due to conditions caused when the load either leaves the ground or the air flow from wherever causes movement to start, by affecting the load.

I have multi strand wires on my overhead cranes which can single lift up to 50000 kgs if any wire started to spin in lifting we would have the devil of a job controlling 50t lumps of machinery, so unless the rule book has been re-written winch wires dont spin in operation.

Vfr

[helmet fire]

I am pretty sure it is not the wire or longline. Spins appear to be perfectly aerodynamic in my experience. As for winching, the higher the downwash the patient is travelling through, the more likely the spin. The Black Hawk was easy to see, there was considerable tendancy for spin at very high winches (120ft plus), then reduced tendencey as you got lower until an increase again near the door. I always wondered about this until I rode the winch a couple of times and found that the Black Hawk seemd to have quite a large null (like eye of the cyclone) in it's downwash pattern when less than 100 or so feet. That null can be felt on the ground after getting winched down, everything seems fine, a bit of light turbulance is all, then you unhook and go to walk out from under the aircraft and in a couple of paces you are in a hurricane of downwash propelling you away from the aircraft.

I then tested this against the Huey derivatives and found a similar thing until I got to the 412. That did not seem to have much of a calm area to play with, but I dont have much time winching on these. The BK117 on the other hand, has almost no calm area and almost always tries to spin the winch load at some point. But remember, any wind will shift the downwash downwind, so there are times when the downwash effect on the winch load is minimal until near the top. Adn that is why some small forward speed can sometimes help stabilise the spin, because it effectively moves the load just out of the downwash area.

Assymetric loads also spin more. If you are winching with a bag, try and put it between your legs rather that out to one side. As for stokes litters, I would suggest that they NEVER be attempted without a tag line unless the aircraft has very little downwash effect.

As for external loads (shape aside), the longer the line, the less the tendancy to spi in the hover: again a downwash factor. But forward flight is the real danger for spinning, and there are many loads that will spin up to enormous RPM once you get them up to forward speeds, regardless of what length line they are on. A simple drogue line or chute acting as a tail sorts this out easily.

Happy gyrations.

[Phoinix]

Thanks to you all. I just hope the problem stays aerodynamical. I have to do some downwash speed testing to see how the airstream does the trick :}