Merits (if any) of free-tip rotor blades?

[piolenc]

While going through a big pile of as-yet-unfiled NASA Technical Support Packages, I came upon ARC-12151, about a kind of helicopter rotor blade with a blade tip that is free to feather with respect to the rest of the blade, subject to a calibrated pitching moment. Many virtues are claimed for this system, including much-reduced rotor drag at cruise, but I had never heard of it before. It turns out that there is literature going back the the Eighties, perhaps earlier. What little of it I have read so far seems equally applicable to gyroplanes and autogyros. Does anybody on the forum have any knowledge of this?

The file is too large for the forum settings, so if anybody wants it please email or message me privately.

[DennisFetters]

I would appreciate seeing that documentation. Thank you for offering.

fettersbuiltco@sbcglobal.net

[Rotor Rooter]

Me too. Thanks for your offer.

jackson.dave@shaw.ca

Perhaps there can be a discussion on this thread about the concept.

Dave

[choppergabor]

I'd love to see it if it's possible.

applehood@gmail.com

Thank you so much.

[piolenc]

The amount of interest this has generated is gratifying. I've sent several copies of the NASA document to interested parties, and am looking forward to their comments.

One worry of mine - namely, that the free tip would interfere with autorotation - seems to be groundless. Apparently, most of the lift-vector tilt that brings about autorotation occurs inboard on the blade, so the free tip should not cause problems. Higher cruise speed/less vibration, anyone?

Marc

[autogyro]

I am very curious whether this free-tip technology is applied to a real helicopter?
If not, why?

[Rotor Rooter]

The following are some random thoughts related to the free-tip concept.

The document does not mention the effects during hover.

It lists benefits of; reduced out-of-plane bending moments, reduced oscillatory loads going to the control system and lower gust sensitivity. Could this also imply a slower response to pilot created control inputs?

It mentions 12% less power required and a 16% increase in lift at cruise speeds. However, conventional tip loss is only 3%.

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This appears to be the future of 'The Rotor'

"Active control blades: a technology breakthrough in the pipeline. This could well be the disruptive technology in helicopters for the coming decades. Onera is working on the development of rotors with active control blades, in which the shape of the blade changes according to its position in each revolution, thus ensuring maximum efficiency.Two main concepts are being studied: active flaps, and active twist blades. Wind tunnel tests to date have generated very positive results."

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What about combining "the best of both' to give an aerodynamically active blade twist that has a 'clean' transition from root to tip?

Will it work for; helicopters?, gyrocopters?, anything??



Produce a composite blade where;

• The spar runs from 25% of chord at the root end to 13% of chord at the tip end.
• The blade is given a built-in twist of, perhaps, minus 9-degrees.
• Unidirectional carbon cloth is used in the spar and in the skin and the tows runs span-wise, aligned with the axis of the spar.
• Bidirectional fiberglass cloth is laid with it's tows at approximately +&- 45-degrees off of the spar's axis.
• The above will result in a blade that resists in-plane and out-of-plane bending but will allow twisting about the spar's axis.
• The bias of the fiberglass cloth (angle of the tows in respect to the blade's pitch axis) at each sector along the span of the blade will set the amount of resistance to twisting under give aerodynamic loads.

The above idea has not been thought out. It will require the criticism and/or the contributions of many.

Dave

[Luc De Keyser]

On the old forum there was a discussion on the "free wing" concept for rotors. Craig Wall animated part of it. It also included references to the rotor control of the Kaman crane and using elastomeric properties for a model of a rotor consisting of an infinitesimal number of free wing segments ...

Would this be accessible somewhere?

Luc

[Rotor Rooter]

Luc,

It sure would be interesting to hear, if and what, conclusions were reached on the old form's discussion.
This is some information on the subject by Craig Wall

Your mentioning of "infinitesimal number of free wing segments" might relate to the US patent Constant lift rotor for a heavier than air craft This patent is by Robert Stroub, who appears to be the principle person behind the 'free-tip' concept.

The following contains a simplified description of the Active Blade Tip in section 2.2.3.
A Combined Piezoelectric Composite Actuator and its application to Wing/Blade Tips

Another source of information is;
An analytical investigation of the free-tip rotor for helicopters

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I wonder if the use of aerodynamically active blade twist might;

1. Eliminate the need for a teetering hinge on gyrocopter rotors?
2. Allow for larger disks on a weight-shift coaxial helicopter?

Dave

[Rotor Rooter]

The web page Rotor Hub - Aerodynamically Active Blade Twist is a rough and preliminary work-up for a rotor that utilizes a modified version of the 'free-tip' concept.

It looks very promising ~ so far.

Dave