I believe Phillip is correct, in that the fully articulated system simply accommodates individual balde acceleration and deacceleration that is induced by flapping, an thus prevents transmitting those forces to the rest of the rotor system. Consider now, in retrospect, what happens in a semi-rigid system. The advancing blade flaps up, and tries to accelerate, while at the same time the retreating blade is doing just the opposite. Of course this all reverses every 180 degrees of rotation. Without drag hinges the rotor system has to flex back and forth to accommodate the forces. Please note that I am not an authority in this field, and would welcome any feedback from an industry expert.

Thanks for adding information on those advanced designs.

Tim

I would now like to call your, and your readers, attention to some promising ideas that I hope will be put into regular production by the helicopter industry.

First, the Sikorsky X2. We all know how the majority of the world’s helicopters have a main rotor for propulsion and lift, and a tail rotor to balance out the main rotor’s drive torque. The X2 uses two main rotors turning in opposite directions, whereby drive torque is divided equally between the two so you don’t need a tail rotor to counteract main rotor torque like you do if you only have one main rotor. This is the same basic principle used by Piasecki/Boeing Vertol tandem rotor helicopters, notably the CH-47 Chinook. The difference between the Chinook and the X-2, is that with the latter the two rotors are coaxial (one above the other).

This means that in addition to not needing a tail rotor, you have advancing blades on both port and starboard sides. This offers the advantage of eliminating the retreating rotor blade stall problem. Also, instead of a tail rotor in back, there is a propeller (think of the pusher props on some homebuilt airplanes) which can propel the X2 at speeds well past 200 knots. (Here, an aside: You need to be just as careful about not being hit by that auxiliary prop as has always been necessary with conventional tail rotors).

For those who are interested, see Aerospace Testing International, March 2009 edition.

There is another concept that also shows promise: Piasecki Aircraft Corporation has developed a technology demonstrator, the X-49A. This machine has one single main rotor. As with the Sikorsky X-2, there is a pusher prop in the rear instead of a conventional tail rotor. Since the X-49A does not have counter-rotating main rotors, it utilizes control surfaces to deflect the pusher prop’s slipstream sideways, so as to produce as great a yawing effect as is necessary to counteract main rotor torque. Piasecki calls their basic idea a Vectored Thrust Ducted Propeller. The X-49A also features auxiliary fixed wings, to provide lift in high-speed forward flight.

Let us all wish both Sikorsky and Piasecki (the latter was founded by the late Frank Piasecki, father of the tandem rotor configuration) all the best with their respective concepts.

Nice article. Didn’t realize that the robi hinge is called a coning hinge and primarily there to reduce reinforcement requirements.

Also, some key benefits to hinge-less rotor systems are 1. less parts to break and replace 2. lower operating costs (though higher initial costs due to more costly materials) as most rotor system components, including composite rotors, are moving from being “life limited” to “on demand”.

-avi