Dynamics - Taming Rotor Induced Vibration

[Dave_Jackson]

An article by Ray Prouty entitled 'Calming the Rotor' has been published in the latest AHS Vertiflite magazine. In this article, he identifies the four competing systems for smoothing the rotorcraft ride. The four systems are Higher Harmonic Control (HHC), Individual Blade Control (IBC), Active Controlled Flap (ACF), and Active Twist Rotor (ATR).

With bloody little humility :uhoh: , I would like to use this thread to place in the public domain another methodology. One which should be an improvement over the first three and compliment an Active Twist Rotor.
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The concept is that of having two adjustable tabs on each blade. One tab would be located on the leading edge and the second tab would be located on the trailing edge. Activation of the tabs provides a short-term spike in the blade's lift. The leading tab will bend up and the retreating tab will bend down. This method will negate the time lag required for a single trailing flap to change the pitch of the blade. In other words, a section of the blade will have sealed leading and trailing flaps and this section will quickly morph between the two different profiles.

The major advantage of this method is the very high speed at which the profile (and lift) can change. This high speed results from having to reposition very little mass. The major parts of the blade (and its mass) will not have to rotate to effect the change in lift.

Another advantage is that the temporary morphing of the blade section from one profile to the other will cause minimal, if any, additional loading on the pitch links. This is because the leading and trailing moments, about the pitch axis, should cancel each other.

This means of vibration reduction may be of a significant value to a high speed intermeshing helicopter, since there is a high rate of aerodynamic interaction action between the many counterrotating blades.
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More boring information can be found on the web page;
DESIGN: UniCopter ~ Vibration - Rotor Induced - Control - Leading & Trailing Edge Flaps (http://www.unicopter.com/1087.html)

[Chiplight]

Dave,
What about using trailing edge flaps alone, as described in this article.
http://www.ingenta.com/isis/searching/ExpandTOC/ingenta?issue=pubinfobike://aiaa/ja/2004/00000041/00000002&index=3
The complete pdf file is also available if you google on "swashplateless helicopter"

Also, there is piezo electric "smart" rotor
http://www.isd.uni-stuttgart.de/sfb409/projectA5.html

See other thread on smart materials.

I wonder if some form of "active" gurney flaps might be useful? A gurney flap has the advantage of being very short(about 2% of chord.) A small strip could be easily rotated on its axis.

[Lu Zuckerman]

If I remember correctly (as it was a long time ago) helicopters that had NACA 0012 airfoils did not have these problems. Granted there were some vibrations due to a traveling wave passing down the blade from the tip to the root but they did not cause problems as long as the blades were tracked properly. On those helicopters that did have problems with traveling wave induced vibration the problems were solved with "bob weights" or in the case of some Bell rotors they placed weight at the nodal points on the blade damping out the traveling wave.

But then again maybe I'm talking about another problem and not what is being discussed on this thread.

I personally believe that many of the problems are because of the pitching and diving tendencies of cambered blades.


:E :E

[Dave_Jackson]

Chiplight,

Thanks for the lead to the full article. It was an interesting report but its results are based on a computer simulation. With today's technology, it appears that the single flap (be it used as a Kaman trailing flap, or an aileron) can not provide a high enough amplitude (for collective and cyclic control) and at the same time a high enough frequency (for vibration elimination).

The Gurney flap is an interesting possibility, particularly since it's small size would allow for fast response rates. It appears that it works best when the airfoil is operating with trailing edge separation. The stalled tip of a retreating blade might benefit from the Gurney flap, as long as this flap did not impart a large moment about the pitch axis of the blade.

The 'rotor' is definitely the place for significant improvements in rotorcraft performance. Just providing separate root and tip controls (active blade twist with a frequency of 1/rev) will result in large benefits, such as much greater thrust, less vibration and lower noise. Then, the addition of a separate small amplitude but high frequency pitch change device should result in the proverbial jet-smooth ride.

The proposed leading and trailing edge tabs is a means of providing this small amplitude high frequency pitch change device. In addition, it should reduce 'the pitching and diving tendencies' mentioned by Lu.


Any critiques and comments are appreciated.