Displaying stream lines in downwash

[Phoinix]

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.

[Phoinix]

Not one single guy? Darn, then it's another first... NOT.

[Heliport]

It would indeed be a first. :eek:

Be patient. You posted on a Saturday. Rotorheads (and Pprune generally) is usually quieter at weekends - especially in the summer months. You may still get some responses.

Heliport

[SEL]

Hi Phoinix

I must admit that I have no experience of flow visualisation with full-scale helicopters.

There have been tests done with smoke generators suspended above the hovering helicopter. The smoke is then drawn through the rotor and 'should' make the wake visible. You're right about the type of smoke, or flow seeding, used (One thing you wont have to worry about is it filling up a closed return tunnel!). That may be trial and error.

Thinking more about it, you could try asking the folks at Boscombe Down (QinetiQ) as they have conducted tests similar to this , I believe.

I would definately suggest posting this in the Flight Testing Forum too, there's a good chance that would lead you to folk who've actually done this.

Steve

[Phoinix]

Thank you SEL. I posted it there too.

The test will be done IGE and OGE, but there is a little thing bothering me about IGE. Can the smoke in the downwash block the pilots ground reference? If I'll activate it on one side it shouldn't circulate on the other side too, or does it? Sure hope that smoke grenade won't make a mess out of it all.

[SEL]

Dont know if a smoke grenade would do the job properly, maybe look into a more controllable source, if possible.

As to where the smoke would go IGE, I'd expect it to go anywhere and everywhere! The tip vortices should move outwards and would be expected to form a circulation just outboard of the rotor tip, hence shouldn't be of to much interest in your tests. Most of the 'inner flow' should move outboard as well, some feeding into the tip vortices circulation. However, some of the inner flow will be drawn up through the centre of the rotor, which is then drawn back through the rotor again; the so-called 'fountain effect'. There is also the issue of the root vortices. In Ray Prouty's books, he suggests these could come together to form one big one that then wanders about unpredictably. Finally, of course, all these effects also affect all other parts of the rotor wake!

All of this could mean that the pilot's view could very well be obscured adding to the idea of being able to control the smoke (even if its being able to bat a smoke grenade away!!).

As a rule of thumb though, the ground layer is around 10-15% of rotor diameter at its thickest point, thinning as it moves outward.

Interestingly, as you'll be using hoists etc., you may be out of any appreciable wake effects. I'm guessing that is what you're trying to find out. Just make sure you've got plenty of good video gear filming! Try a few different azimuth positions, especially if there is wind. High-speed cameras are great but damn expensive.

End of ramble.....

One thing's for sure though, you'll have a lot of fun doing it!

[Phoinix]

So, that smoke will be all over the place... Should look spectacular :D

Actually, 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. That is why i need OGE, IGE is only to find out, how much the spin decreases or increases during the lift/drop. The one technique to get it stopped is to go into forward flight or to lower the rescuer on the ground - sometimes bone breaking.

I've got one normal camera and two digital cameras, and off course the guys, that'll be running around taking pictures and taping the whole thing.

Controling that smoke grenade will be a whole different thing. I only hope we won't set something on fire... again :D

[SEL]

From what I know of this topic, which, admittedly isn't a lot, I do not think it is a wake problem.

Swirl in the rotor wake comes from two sources. Firstly, the air adheres to the blades which then drag it around. This gives a swirl velocity. Secondly, the root and tip vortices move down below the rotor plane in a helical path. Due to this angle, the rotational velocity has a component in the direction of rotor rotation. I grabbed 'Helicopter Performance, Stability and Control' by Ray Prouty and there on p.53 is some data from a US Army test. This showed that at a tenth of the rotor radius below the rotor plane, the average swirl velocity was at most 5ft/sec while the vertical was nearly ten times this. Also, the swirl effect reduces with distance below the rotor.

I remember seeing a film from the US somewhere (a Bob Tur film I think) which showed a crewman and stretcher at the end of a line spinning wildly. However, at the distance shown, I wouldn't have thought the wake could have an effect there.

I think the most probable cause is the many vibration sources in the machine itself. Lots from the rotor (flapping, lagging, damping problems, imbalance in the blades etc.) and also from the transmission, with all the rotating components.

As a weight on a line is, in effect, a pendulum, you would expect some oscillation. This could lead to swinging and/or rotation. In looking into this, such parameters are the weights suspended, the load mass distribution, line length and certainly the vibration in the machine itself.

Now, as said, I'm not an expert on this, so could have this wrong. So, if there are guns turning in my direction, put me right and help the bloke out! I would certainly expect there to be literature on this subject which would be invaluable to you. Plus, where are all the folk out there that have practical experience of this??

End of Rant2: The Rant Strikes Back