Tip vortex reduction?

[Dave_Jackson]

Has anyone ever heard of a rotorcraft where the tip of the blade had a twist about its chord axis?

This twist would be in the opposite direction from that of the tip vortex and its purpose would be to minimize the tip vortex.

Thanks

Dave

[bugdevheli]

Show me yours and ill show you mine:D :D

[Dave_Jackson]

OK. You go first. :E
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Seriously,

We may have different objectives. Mine is an attempt to reduce rotor induced vibration.

Item C on this page explains the idea (wish) in a little more detail; DESIGN: UniCopter ~ Vibration - Rotor Induced - Control - High Frequency Modifiable Tip Control (http://www.unicopter.com/1544.html)

Your turn. :)

Dave

[bugdevheli]

I will respond when i have had time to dijest the info. Thanks Bug

[Gael Warning]

Dave,

I recall reading a paper written some years ago by the RAF at Bedford regarding the BERP and trailing edge swept down technique. The results if memory serves were pretty conclusive regarding control loading, and do seem to be borne out by in service operations, at least on EH101. Basically during the trial, various abrasives were attached to the outer 25% on the blade leading edge, everything from course sand to sea salt etc. The results showed that the finer the abrasive, the more pronounced the effect. The effect was that with abrasive build up, e.g. flying over the sea, the effect on the blade tip increased load on the pitch control mechanism to the extent that in some cases, the pitch control rods were at maximum stress loading at only 40% of aircraft max forward speed. This resulted in the aircraft chewing pitch change rod bearings like you can't believe.

Anyway, not sure if this helps.. food for thought hopefully.

GW

[Graviman]

Some wind turbines have adjustable tips, so the technique is viable. Complexity might count against it, depending on packaging - bear in mind FBW systems still use Brushless DC motors to actuate a mechanical swash plate. In theory if there was an accelerometer at the tip the tip could be continuously trimmed to null out any vibration. This would totally damp out any blade flexural modes.

Comanche has shown that careful design using the basics also works well. BERP is old hat now, but would be very sensitive to tip vortex variation caused by surface features affecting laminar to turbulent transition. Again most of the pitch loading would be caused in the retreating portion, since this is where the vortex is used to generate extra lift at low speeds.

On balance, i would go with Bug's idea. ;)

Mart

[Dave_Jackson]

GW,

Thanks for the specific information on the trailing edge swept down technique.

Dave

[NickLappos]

The abrasive tests for the EH-101 prove more about the behavior of the airfoil used than they do about anything else. Those tests showed that the airfoil is particularlt sensitive to roughness, and that they therefore stall earlier as a result. Those control forces are the direct sign of early stall and high pitching moment due to the stall. A laminar flow airfoil is particularly sensitive to roughness. Most helo airfoil sections are not laminar flow because roughness, bugs and icing all are unavoidable.

The downward anhedral tip on the Sikorsky 3rd gen (S-92) and 4th gen blades are specifically to control the tip vortex and create extra lift at low speed without adding drag at high speed. They add between 5 and 7% more hover performance to the aircraft.

I flew the first flight on the Black Hawk with the S-92 blades ( the combination that is now on the UH-60M) and the blades allowed at least 1500 pounds of increased performance at the same engine power (even though they had 16% more chord, which should have reduced hover performance if other factors were equal), as well as adding about 15 knts to Vne (mostly due to the extra chord).

[Graviman]

Nick, i am still struggling a little to understand how the downward anhedral tips control the tip vortex to create extra lift at low speed. Does the centrifugally accelerated outwash force the vortices above the tip path plane, to push up local foil velocity? Alternately does the shape just better match the inflow (like a bell mouth)? At 5 to 7% FM increase they clearly work.

When looking at images of a static Comanche it always seems that there is some blade droop, yet these are very rigid blades. Is this actually the anhedral i am seeing?

Mart

[NickLappos]

Grav,
The drooped tip adds downward speed to the shed vortex so that it pushes the vortex away from interfering with the next blade, the aero guys said. It also minimizes the vortex energy to some degree. It is akin to the upswept tips of fixed wing jets.

One aerodynamicist told me not to hurt my brain understanding why these things work, there is no rule nor reason. It is nearly trial and error, but at least now the trials are relatively cheap CFD runs of various shapes.

[Dave_Jackson]

Nick,
Thanks for the S-92 information. Found some pictures of the here (http://www.aerospace-technology.com/projects/s92/)


This is the wild idea that I am thinking of.
http://www.unicopter.com/1544-A.gif

Dave

[bugdevheli]

DAVE



http://img.photobucket.com/albums/v28/eboki/not%20sophs%20stuff/sappho1.jpg

[Shawn Coyle]

At the risk of giving it away, look in a future issue of Professional Pilot magazine for an article on Morphing Rotor Blades (sorry if this sounds like an advertisement...). I know the author well enough that I used to let him sleep with my ex-wife.

Interesting and may be a very good application of the technology for Dave's idea.

The other thought for reducing blade tip vortices is to use very small, powerful, Wheeler-type vortex generators to reduce spanwise flow in the first place. But that's a whole other story…

[NickLappos]

The trick with all these ideas (they all seem o be good ones, BTW) is that they must also work at high speed, where the drag from the device must not kill long range cruise.
Variable geometry is a way to go, with piezoelectric devices that reconfigure the blade as it transitions from low speed hover to high speed flight.

[Graviman]

Bug, thought you were refering to the ring rotor - this is likely the overall winner. Is that sketch a upwards wing tip?

Nick, think i understand the anhedral tips now - CFD visualisation helps though. The downwash pushes the vortex outwards spanwise. This affects the inflow, so that the wake contraction occurs more above the blade than at the blade. The way i understand wing tips is you take a long wing, snap the end off and stick it back on at 90 degrees, so it's lift doesn't cause bending moment.

Dave, this would work but bear in mind control actuation lag. Worse case is twist in the tip spar delays accelerometer correction by 180 degrees. Tip piezo actuation may be the solution.

Shawn, how effective would a Wheeler-type vortex generator be at reducing blade spanwise flow? I'm thinking about the continuous change in airspeed.

Mart

[Dave_Jackson]

bugdevheli,

OK, how does it work. http://www.unicopter.com/Thinking.gif

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Thanks for the discussion.

I wonder if anhedral and conventional vortex generators will work on future rotorcraft. These craft will have faster forward speeds and slower turning rotors, during cruise.

On these craft, when a blade is pointing forward or aft, the airflow angle will be up to 45º from being normal to the blade's span.

Dave

[Gael Warning]

I hear what you are saying Nick, I think the point I ought to have made was that I believe the abrasive test results were amplified due to BERP extended chord and not necessarily anhedral sweep in isolation.

[Graviman]

Not an exhaustive seach, but happened to come across this:

http://www.iop.org/EJ/abstract/0964-1726/10/1/302

Mart

[quadrirotor]

http://www.onera.fr/coupdezoom/17-helicopteres.php




http://www.onera.fr/coupdezoom/images/17-helicoptere-actif-rpa-s3.gif

[Graviman]

Good site. It is fair to comment that helicopters are undergoing a period of development, not unlike the pace which produced supersonic fixed wings in the 60s. Interesting to see how far the technology can be pushed.

I would imagine that once the performance envelope is opened up, efficiency will become the next drive. This may well produce numbers which make helicopters look more competetive for short-haul passenger flights, focusing on point-to-point.

Interesting times.

Mart