spinning sling loads?
 

I've been doing some work on and off over the last few years on the aerodynamics and dynamics of slung loads, and one thing that no-one is really sure of is why some loads spin on a single strop (in forward flight) and others don't.

There does seem to be a stability boundary for box-like loads, based on the length/height and width/height ratios - so for example the US Army CONEX container spins, but the TRIO doesn't. (Geometry effects on autorotation of rectangular prisms)

However, most of the available data is from wind tunnel tests (basically boxes on a bit of string), so it would be really useful to know what actually happens in the real world.

I'd therefore appreciate any information on peoples experiences with box-like loads (containers, blade boxes, ACUs etc) on a single strop or longline ... which ones spin, which ones don't?

Thanks

Can't tell you exactly which spin and why but when I did some jungle long line a few years ago, we used to chop a small tree branch and attach it to the load. It acts as a drogue and stablises the load. There are other things you can use for a similar effect.

There's an excellent thread on this already. It hasn't surfaced for a few years.

I can tell you that there is a device in development for crane loads that can turn the load to the orientation required using wireless remote control. It is anticipated that eventually a version with remote input or automatic control of a selected or sensed heading can be produced.

The principal target market is offshore oil and gas and loads like containers and drill pipe bundles. Prototypes have produced good results when tested with containers. It is the subject of patent activity.

The weight of the device makes it a problem for the usual range of utility helicopters since the principal of operation will always involve a moving mass. However, as development progresses, the overall weight of the assembly might be trimmed to the extent that it is a reasonable device for heavier machines. helicopters are not currently a target market.

Sir Korsky

thanks - I've found several very interesting threads on sling loads and sling load 'incidents', but there tends not to be too much in the way of detail on the loads themselves, so I thought I'd ask a slightly different question!

jimf671

I've heard of such devices, but I guess the issue for a slung load (other than cost, weigh and complexity) might be that any control system that applies a torque to the load has got to react that torque against the helicopter.

I believe the US Army have looked at active fins/flaps on container loads, but these concepts never seem to be too successful. A new idea I've seen recently is a sort of cup anemometer device on the strop to increase damping.

That is like asking how long is apiece of string !
Flown 17 hours lifting this week in The Peak District, have flown the following this week
Dumpy bags of brash each weighs around 125 kgs taking 4 at a time on a carousel (3 hooks ) on a 40 ft line under Hughes 500
Result
3 bags are put on one to a hook with a 6 to 8 ft strop the load will spin a like a bugger
4 bags stropped in pairs one pair to a hook ( 4 bags in total) load normally stable
5 bags 2 pairs and a single then load will sometimes spin seems to depend on the single bag if it gets into the airflow on one side

Pallets of fence posts ( 2 m ones load 2 choke stoops going to single hook) in 450 kg loads, they all seemed to spin could have something to do with 40 kts of wind !!!!
Planks ( 4m )with gate posts, spins all over the place !

For James May, Lada car spins so need a windsock to keep it straight ! Also helps tip car over when dropped form 500ft so it hits nose first !

Hope that helps

Hughes500

thanks - I appreciate that it's a pretty open-ended question ... but anything that helps connects model scale results to the real world will be useful.

I'd never heard of a carousel for slung loads ... I'm not surprised it spins!

With the fence posts, how would they be stacked on the pallet? I'm trying to get a rough picture of the height/width/length of the load.

"Blue Rooms" spin when full due to the un-even weight distribution at the bottom. Make sure you tape down the toilet lid.....

http://i76.photobucket.com/albums/j3...t/Slinging.jpg

Glamorous Life ain't it, Gordy!

aerodoug,

That reaction only occurs with these if you did not put a swivel in the line and therefore a portion of the torque applied by the device may reach the crane/aircraft. There are two methods for creating a torque at a suspended load independently of the carrying machine that have been used in prototypes. The simpler and safer of the two is being developed at the moment.

[An associate company has also been work with helicopter-suspended instrumentation during the same period. Again, simple wins. I love the one about sticking a branch in the side of the load and their eventual solution ends up being a little bit like a built-in branch.]

Mmmm ....


...I've been doing some work on and off over the last few years on the aerodynamics and dynamics of slung loads, and one thing that no-one is really sure of is why some loads spin on a single strop (in forward flight) and others don't....

Are you serious ?

The answer is quite simple ... you don't know what is going to happen till you pick up the load ...

There are so many variables ... aerodynamics, GoG, type of rig, length of line, speed of flight etc etc

That is why you get the 'big bucks' to fly this kinda stuff ...

Find a good external load instructor and get him to explain it to you ...

It took a lot of time for me to learn how it was done (and a lot of that time was in New Guinea with a 330 J Puma flying 4500Lbs of drill pipe and some awkward pre-fab buildings) so I do know how to do it ... alas with some things there is no substitute for experience ...

Anyhow I am in Texas with a 'big boobed blonde' and drinkin Tequila shots so I really don't give a **** Y'all ... :p



Cheers

Give me the slammers every time

Aerodoug

Pallet of fence posts . Posts 2 m long, height about 80cm width about 80cm weight approx 450kgs.
Line length 35 ft to a swivel to a single hook. The posts were choked at each end ( that is 1 point at each end not each corner ) about 30 cm in from the end, going up to a single point. The load would not always be totally level, didn't spin that much but certainly rotated. It will depend on how it is stropped, just think of picking up a load on a single point but that point is not in the middle. You will then have different levers in terms of drag. If drag is equal on both sides of the load then in theory it won't spin !!!However !!!!!!!!!

jimf671

good points - I hadn't though about onboard torque reaction devices.

Still, I'm with you on simple ... putting sensors, actuators and control systems on a load looks good in theory, but I can see all sorts of practical problems in the field.

There are so many variables ... aerodynamics, GoG, type of rig, length of line, speed of flight etc etc

Sure - but most of these are physics, so it should be possible to predict the effects of them. Possible, but not easy - and it's not obvious up front which are the most important variables ... which is where experience is critical, and kind of the point in asking the question in the first place.

Gordy,
thanks for the picture ... I remember having the 'flying brick ****house' tactical approach demonstrated to me by an ex-Herc pilot, but I don't think this is quite what he had in mind.

My five cents:

As mentioned there is a lot of things that influence how a load will behave, and it is often hard to predict in advance, but in general the more symmetrical a load is, the more stable it is.
A cylindrical bucket for concrete will not spin at all.
A bigbag will also usually not spin, but if it is loaded a bit bulky or a flap of the bag catches air, it could start to spin. This can also start suddenly after it has been flying stable for a while.

For square shaped loads, the lower the length to width ratio is, the more stable it will be.
The weight to drag ratio should also be as high as possible. A pallet weighing 800-1000 kg can fly quite nice, but a barack measuring 2,5x2,5x2,5m with the same weight is more prone to start spinning.
I guess it has to do with inertia. The larger the mass, the larger force you need to move it, and the airflow is more powerful if it can "grab" a larger area.

Rectangular loads like lumber and planks might not spin to bad, but as it slowly rotates, the vary in drag depending on if the short or long side is facing the airflow will make it vary between dragging behind and "falling" forward, and therefore dance around in all directions. You must therefore adjust the speed to where it is safe and comfortable enough. A more heavy and compact load of iron reinforcement bars will on the other hand fly smooth an you can fly as fast as the helicopter can.

How you strap up the load can also influence how it will fly. A pallet of bricks will probably fly quite nice and stable if you use two straps and choke them so the two lifting points are over the centerline of the load and at equal distance from the center. If you instead choose to choke them at opposite corners for better choking around the load it is more likely it will start to spin.

Some loads can be stabilized by horizontal airflow and fly nice, but start to spin once you reduce airspeed on the approach to delivery and vertical downwash start to influence it.

I hope this can help you a bit, but as other have said it is not possible to make a textbook about this, and it is more about having the experience so you can best predict what will happen.

I really don't see how you can produce a product to put on a load to stop it spinning. Like others have said, each load is different and how do you attach something to each load? On fires, I could fly 30 different loads throughout the day and we have a hard time keeping up with backhaul of empty nets let alone spin arresting devices.

And then of course, how do you prepare for doubles and triples?

http://i76.photobucket.com/albums/j3...ps36158f8d.jpg

I've spent quite a bit of time flying "10 ft" ISO containers ( which I think are 8 ft x 9ft10) and they always spin. (Up to 4000lbs loaded, 8ft strop, under an S61)