Displaying stream lines in downwash
 

Does anyone have experience on displaying stream lines in the downwash of a helicopter? I will be trying to do that late this summer on a AB-412 for stream lines study around the hoist and hoist cargo (human). Will a normal smoke grenade do the trick? Downwash is too turbulent to use a conventional smoke from wind tunnel testing, i think.

I'll be very grateful for any suggestions and tips.

Good luck!
If there were a good way to do it, I'm sure we'd all know it. Smoke grenades will dissipate quite quickly, and how and where are you going to place them?
To what purpose are you trying to show the streamlines?

My idol to the rescue :D

So, that smoke will be all over the place...

I want to explore the spin, a rescuer with a rescue bag encounters sometimes, without any warning. And it doesn't stop on it's own.

The purpose of finding streamlines is, to find out, if there is any effect of the downwash on the spining cargo, and if maybe that air flow is the reason for spinning in the first place.

Controling that smoke grenade will be very difficult. I was thinking about mounting a beam with two grenades (some distance apart) just under the hoist (mounting on ceiling).

There can hardly be any doubt that the spin you refer to is a product of the downwash. After all if the survivor was similarly suspended from a hangar roof with the doors shut you would not expect much spinning to arise.

Clearly any rotor downwash is not a very steady stream of air and will vary with helo type, local wind and ground effects. To suspend anything in such an airflow and expect it not to rotate is pretty optimistic.

Since the velocities involved are modest could I suggest that what you wish to study might be looked at usefully in a pretty simple ‘wind tunnel’/lab experiment with a simple ceiling mounted fan where you could well come up with some useful data on the way various shapes of suspended loads react to a known airflow asymmetry? I say known because you will be able to measure, control (and even visualise) the airflow you provide relatively easily in the lab.

Wind tunnel is expensive compared to one hour flight time. And the air current coming from a fan is not the same as a downwash. What i want to do, is to figure out how to stop the spin or how to avoid it. There is no wind tunnel that can simulate a downwash. I'll try my luck on the real thing.

After a while in flight test you will find that people take no notice of data that are not repeatable. There are so many variables in what you are trying to do that one hour of anything will nor scratch the surface of the issues involved.

But good luck - trying anything is the only way to learn.

PS I did put wind tunnel in quotes....I had in mind a simple blow down open circuit rig specially fabricated for your purpose.

You have a point about variable data. But my mentor is insisting on flight testing, so i'll probably stick to that and hope for the best. Thanks for your advice.

The easiest way to stabilse an underslung load is to endow it with a little Nv and then keep a few knots on. But I realise this may not always be possible with SAR ops. But if your concern about the spin is stopping it so as you can get the load inboard then a drogue plus a few knots would work wonders.

No stopping isn't such a problem when working above level ground, the problem is, when working in the Alps (close to rock walls), where often you don't have any way to go, but up. Gaining a few knots is standard procedure only when there is room to manouver.

When a spin has developed, winching up only makes things worse, plus the skids are not such a soft spot to hit. Been there done that - never again. So that is out of the question.

I was thinking about mounting a stabilizer on one end of the rescue bag and work from there.

If you are looking for a method to stop spinning...

Use the hoist as on short haul. Means, after catching the rescuer/ rescue bag, climb up vertically with the whole ship+cable+rescuer. If you are out of obstacles go up to a few knots and start hoisting up. Same procedure on hoisting downwards. And try to use cable lengths greater than 20m.
You are right not to underestimate the spinning. Some operators use a "antispinning-line" from ground to rescue bag, but that needs ground forces, the right terrain and the right angle between rescue bag and ground troops.

Have rarely seen a spinning on short haul operations but really impressive spinning with hoists.

Measuring Downwash flow - Poor Man's Solution
 

You might try hovering over a concrete area covered with water. This will show the flow out from under the rotor. If you can add water while hovering you may be able to see rotor circulation. Video the test for review and let us know how it goes.

I don't know what your budget is, but go to the following website:
http://sandersaircraft.com/
and you will find smoke generating systems that work quite well in some applications. The US FAA used these systems on an S-76 to visual wake turbulence effects. Then there is always the "smoke oil in the exhaust" should you be on a shoestring.

The fact of the winchman spinning has never been a great problem in my experience. Most winchman manage to get themselves facing forwards when it matters by leg movement. The purpose of trawling the winchman in the water on the run in to a sea rescue is so that he is stabilised facing forwards when he makes contact. Normally the winchman and rescuee are stopped just below the door on the way up so that any spin can be damped before cabin re entry. In my own experience on the cable, whether you spin or not is subject to so many variables of winching height/wind etc that trials as John Farley says would not produce repeatable results. However the worst case scenario always seemed to be no wind and high hover. Personally I was more worried about getting a static electricity jolt if the guy on the ground/water reached out before I was earthed.

I presume that the rotation is caused by the swirl component of the induced velocity. That being the case, do suspended loads below co-axial rotor machines rotate? Apologies for thread creep. (Swirl velocity can be reasonably easily approximated though)

There was many factors. The rotation of the rotor, the fuselage beeing in the way of the downwash and the downwash separation close to the ground.

I'm sure there are many factors which induce the spin, but the most simple model that I can imagine is where you have a surface with a center of drag that is not coaxial with the pivot point. The aerodynamic forces push the mass away from the vertical. When gravity tries to restore the mass to the vertical, the drag force in the lateral plane couples with the lateral component of the restoring force to create a spinnning moment. Initially, this couple is unstable in direction of rotation, but once the spinning starts this couple becomes very stable and can rapidly increase the rate of spinning. A picture is worth a thousand words here.

If you're not convinced, go fishing. Spinners and spoons do the same thing when pulled through the water. As an experiment, tie any object from flat plates, to army men onto a piece of string and pull them through water in your bath tub. When moving slowly, there is a little dance that results in movement lateral to the string and some rotation. When pulled quickly enough, a spin can easily develop.

Since this model only requires linear flow it implies that the same can happen under a coaxial.

To stop the spinning, you could proactively get rid of the cause or reactively get rid of the spin. Balancing the drag would be difficult, but may result in some success. Active surfaces may help but would increase mass, complexity and could aggravate a spin. A two line system with both lines coming from the helicopter would work to an extent, but would increase the risk of snagging in trees and comes with a dangerous result from a line failure (potentially less dangerous than using a single line, but something to consider).

I've seen many people develop very fast spin rates in short times. Putting them back on the ground, back in the water, or holding them just underneath the helicopter tends to fix the problem. However, if there is a critical casualty also on the line, the spinning is an additional trauma that could make a huge difference in the success of the rescue.

Matthew.

Hi, very interesting thread.
Im not a flight tester of any sort (can only dream of eventually being one).
I might be well off the point here (probably) but would using a small remote controlled petrol helicopter in a big hanger not work .
It would be quite cheap and the downwash would have more of an affect on the smoke generated from a smoke grenade.

You could even tie the helo to a stick or something so its easier to control?
Anyway, just a thought would like to know if/ why this wouldn't work.
Thanks very much guys.

Ghostie

For my diploma work, that was completed more than a year ago, i used a AB-412 for a test flight. First test was the smoke grenade, the second was measuring the direction and velocity of the air stream while on the hoist. Here are is a photo from that day.

http://makete.net/galerija/albums/al...0856.sized.jpg

And the solution from my point of view, a little big, but later i corrected it to about half it's size, to still perform its function as a pasive counteract against spinning. Active stearing system was proven by french, with a movable flap on one end of the strecher. They had proven that it can be done, even more... very controlable.

http://makete.net/galerija/albums/al...reca.sized.jpg