Main rotor systems

June 29, 2009 by Tim McAdams

There are several different main rotor system designs that are used on modern helicopters. The three basic designs that have traditionally been taught to students are semi-rigid, fully articulated, and rigid. Today there are versions that make extensive use of composite materials and are known as hinge less systems.

A fully articulated system normally has more than two blades. In this design each blade is attached to a hub with hinges that allow it to move independently of the others. A feathering hinge is used to change the pitch of each blade. A flapping hinge allows each blade to move up and down to compensate for dissymmetry of lift. Blades are able to move fore and aft or lead-lag, (called hunting) by use of a drag hinge. Normally a damper is attached to the blade and hub to restrict excessive movement. The drag hinge is used because, when a rotor blade flaps up, its center of mass moves closer to the axis of rotation. This causes the rotor system to spin faster, much like a spinning ice skater speeds up when pulling her arms in closer. Allowing the blades to lead-lag reduces this tendency.

A semi-rigid system refers to a two-blade system where each blade is mounted to a hub that has a center teetering hinge. In this configuration, when one blade flaps up the other one flaps down – like a see saw. As with the fully articulated system, each blade has a feathering hinge. The two blades are mounted in an under-slung position, that is where the teetering hinge is mounted above the plane of rotation. The geometry of this arrangement minimizes the change in distance between the center of mass and the axis of rotation during flapping. This allows a semi-rigid system to not need a drag hinge.

In a slight departure from the traditional semi-rigid design, Frank Robinson used a coning hinge on each blade (some refer to this as a flapping hinge, but it is used for blade coning). When rotor blades produce lift (especially under high load or low rotor rpm) they flex upward (coning). This places a high stress load at the blade’s root, so in order to relieve this stress Robinson’s design allows the blade root to cone about a hinge. This reduced the amount of reinforcing required at the blade root making for a lighter easier to manufacture rotor blade.

Rigid rotor systems do not use hinges and limited movement is absorbed through the hub and rotor blades. Many of the modern composite rotor systems also do not use traditional hinges, but have elastomeric and specially designed composites structures (flextures) that allow the blades to flap, feather, and hunt. Manufactures do not use the term rigid rotor system, opting instead to describe these systems as a fully articulated hinge less rotor system. These systems do not require lubrication and are less maintenance intensive. The extensive use of composite materials also increases reliability and helps absorb vibration.


  • Avi Weiss


    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”.


  • Phillip Peterson

    In the second paragraph, how does “allowing the blades to lead-lag reduce[s] this tendency”? Doesn’t a lead-lag hinge just accommodate this tendency?

  • Joe Connell

    The H-43 series by Kaman had still a different rotor head architecture. It consisted of two rotor drive shafts driven through a common transmission. Each shaft had a head with two rotor blades and the shafts rotated opposite the other to conteract torque. The head had a teeter hinge with droop stops to control blade vertical travel at power-up and shutdown. Both blades had lead-lag hinges. There was no “feather” hinge. The blades were made of wood with a pitch control servo flap. The blades physically twisted for pitch change. The wood characteristics could change somewhat due to weather conditions. This would cause the tip path plane of the blades to get somewhat out of track causing a vertical vibration. The pilot could adjust the blade tracking in flight via a servo tuning function. Less than 200 H-43s were manufactured…

  • Alex Kovnat

    Thanks Tim for describing the basics of helicopter operation for those who are not familiar with this topic.

    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.

  • Tim McAdams


    Thanks for adding information on those advanced designs.


  • Bob Kuriger

    Phillip Peterson had a great qustion that I didn’t see an answer for.

    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.

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