OTHER: - Rotor Concept - Active Blade Twist (ABT) (θ1A)

'The Helicopter Law; "Whatever helps hover, hurts forward flight, and whatever helps forward flight hurts hover."' ~ Prouty, in Rotor & Wing, Jan 99

It's time to change the law.

Advantages of Active Blade Twist:

Heavier payloads, faster flight, slower descent in autorotation, and reduced vibration.

Range of Pitch Amplitude and Frequency:





< 1P

Transitions between hover, forward flight and autorotation. They are infrequent, since their occurrence is dictated by a change in the forward velocity or collective setting. (steady state active twist)


= 1P

To utilize the reverse velocity experienced by the retreating root in fast forward flight,


> 1P

Additional improvements in vibration control will require pitch changes at frequencies greater than 1P. This will necessitate the inclusion of some method like High Frequency Leading + Trail Edge Flaps. In addition, Higher Harmonic Control or Individual Blade Control may provide some improvement, but it will probably be nominal in a rotor that already includes Independent Tip & Root Control.

Efficient Lift/drag ~ The reason for Active Blade Twist:

"The most efficient angle of attack for an airfoil is where its lift-to-drag ratio is the highest. For most rotor airfoils, this angle is in the neighborhood of 8.

This is part of the explanation of why a helicopter can never be as efficient as an airplane. An airplane designer can arrange things so that the entire wing is flying near the optimum angle of attack at cruise speed. The other part of the story is that we burden our aircraft with rotor masts and hubs-aerodynamically dirty things not imposed on airplanes. The overall airplane lift-to-drag ratio can be 10 to 30, depending on the configuration, whereas the maximum a helicopter can do is 4 to 6. ~ Prouty, Rotor & Wing Jan 99

It would appear, IMO, that the optimum L/D ratio could be achieved if in this 2D polar coordinate system the optimum l/d could be achieved at all radii - azimuth positions about the area of the disk. The radius coordinate will be taken care of by independent root & tip pitch control. This is because the variation in airspeed is a linear one between the root and the tip, therefore optimizing the root pitch and the tip pitch, for a particular azimuth, will optimize the angle of attack at all locations along the span. Alternatively, perhaps the twist must be exponential because the lift is determined by the square of the air velocity. Whatever, linear or exponential, the twist will be a function of the blade design (filament winding angle etc).)

The azimuth coordinate will be taken care of by higher frequency Independent Blade Control.

Some might argue that technology has not advanced sufficiently for rotorcraft manufactures to consider ABT. They may argue that they must wait for improvements in piezoelectric etc. to be done by outside companies. It has been said that Igor did not produce an operational helicopter earlier because he had to wait for outside sources to develop a satisfactory engine. However, while he was waiting in America for somebody else to develop the engine, others in Germany were building and flying helicopters.

Active blade twist, in one form or another, will come about. IMO, any research and development in this field that is done by the rotorcraft industry today will advance and improve the eventual implementation of ABT. It's time for rotorcraft's Generation II.

Potential Going from no twist to ideal twist can raise the figure of merit about 5%. Most of the benefit of twist is realized in the first 10 of twisting. [Source ~ RWP1 p.41]

Related Articles and Patent:

Airfoil Design and Rotorcraft Performance Excellent paper covering L/D ratio at points (ψ, r) on rotor disk. I have hard copy.

An interesting and related subject to review will be 'Active Aeroelastic Wing'. Perhaps search NASA and Google including the word 'piezo'.




5,505,589 ~ Controllable variable twist rotor blade assembly ~ April 9, 1996

5,730,581 ~ Rotor blade ~ March 24, 1998

Means of Improving the Lift to Drag Ratio of Helicopter Rotors, plus Reducing Vibration.








Rotor Concept - Active Blade Twist - Overview



Rotor Concept - Active Blade Twist - Electromechanical






Rotor Concept - Independent Root & Tip (IRAT) (1)


(1) The Independent Root & Tip concept is actually a subset of Active Blade Twist, however, since the Independent Root & Tip has its own group of subsets it has been located at the same hierarchical level as Active Blade Twist.

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Last Revised: January 5, 2009

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