OTHER ~ Flight Dynamics - Rotor Hub - Constant Velocity Rotor w/ Hub Spring

• Faster cyclic response than that of a teetering rotor. This is due to the ability to use hub springs [see; OTHER ~ Flight Dynamics - Rotor Hub - Hub Spring (Hinge Spring)] to place a moment on the craft when the disk tips from being normal to the mast.
• Greater control in low G's than that of a teetering rotor due to the same reason as the above line.
• Reduced vibration compared to that of an articulated rotor. This is due to the ability of a constant velocity joint to minimize the Coriolis effect.
• Reduced rotor complexity than that of an articulated rotor due to the minimization of the Coriolis effect. This should eliminate the need to compensate for the in-plane movement of the blades (lead/lag hinges).

To see designed unit go to Product Development links lower down on this page.

• Doman patent: US 2,648,387
• Doman LZ-1A: This rotor incorporates the Constant Velocity Rotor feature, but not the Hub Spring.
• Picture and description of crafts rotor,

• Picture of rotorhub and two sketches of the hub's method of operation.[Source ~ MDD p.162].
• Chuck Beaty said on the Rotary Wing Forum ~ "Some floating hub helicopters have been the Doblhoff tip jet helicopter, the Doman helicopter and the McDonnell (St. Louis) series of helicopters." The Doblhoff and the McDonnell XV-1 were driven by tip jets and therefore did not connect to a driving mast.
• Remark about a teetering rotor with more than two blades; [Source ~ MDD p.38]
• The Firestone XR 9 (begun in 1943) had "a single rotor with three blades, installed on the mast with a 'floating head' a sort of universal joint." [Source ~ HOTH p.118]

Search for Related Patents: OTHER: Flight Dynamics - Rotor - Hub - Elastomeric CVJ - Patents Those of Bell and Sikorsky are intended to give similar results, but their designs are quite different from this invention.

• A double universal joint (Cardan joint, Hooke's joint) consists of a driver shaft, a universal joint, an intermediate shaft, a second universal joint, and a driven shaft. [see drawing below] [An example of this is the drive shaft of most cars and trucks.] If the angles of the two universal joints are equal and the two forks of this intermediate shaft lie in the same plane, the variation of angular motion between driver and driven can be entirely avoided. More information on this subject.
• By bringing the two universal joints together, so that they are concentric [see drawing below], the intermediate shaft will be shortened to the point where it becomes a simple housing. The two pair of bearings, which were on parallel axes, will now be on a single common axis.

• A device located in the hub of a teetering rotor that exerts a force between the mast and the hub at all azimuth angles. It has two (maybe three) functions in this CVJ+HS rotorhead.
• It attempts to return the rotor disk to its mean position (normal to the mast) when the disk is tipped. Functionally, it will offer advantages similar to that of the centrifugal effect of blades on an articulated rotor, with its flapping hinge offset. Supplementary information on hub springs 
• It is responsible for making the angles of the two universal joints always equal each other, no mater what the teetering angle of the rotor is.
• Perhaps negate the need for droop stops.

• Bisector:
• The above mentioned simple housing will now be referred to as the Bisector. It and the other components of this Constant Velocity Rotor with Hub Moment can be see on this this drawing below.
• The Bisector is a part, which has a yoke configuration at the top and a 360º ridged skirt at the bottom. It functions the same as the intermediate shaft in a conventional double universal joint arrangement. Each of the two upper halves of the yoke interface with two bearing. The inner bearings interface with the Universal X-joint #1. The outer bearings interface with the universal X-joint #2. This Bisector must rotate in concert with both the universal X-joints when the mast rotates. This Bisector can rotate independent from either or both of the universal X-joints when there is rotation about (a fictitious axis that is horizontal on this page) i.e. teetering.
• The rigid skirt at the bottom is located between a pair of hub spring units, which encircle the outer side and the inner side of this skirt.
• Hub Spring:
• The Bisector is positioned by the two hub spring units. One unit is located between the mast (formerly the driver shaft) and the Bisector. The other unit is located between the rotorhub, which hold the blades (formerly the driven shaft) and the Bisector.
• Both hub spring units may consist of elastomeric rings, or a set of individual springs that are located at various azimuths around the axis of the mast and rotor hub. Both hub spring units have equal force-length(compression) values.
• The springs exert a strong force when compressed. They are also under compression after being installed in the assembly. Therefore, when the rotorhub tips (teeters) down at a specific azimuth, the outer and inner springs in proximity to this azimuth will compress and resist the tipping.
• From Double Universal Joint to Constant Velocity Joint:
• It has previously been mentioned that the Bisector skirt is located between the outer and the inner ring of hub springs. It has also been mentioned that the outer and inner springs have the same force-movement ratio. It should be noted that the Bisector offer no resistance to teetering movement. Therefore, the inner and the outer springs in proximity to the azimuth of tipping will compress exactly equal amounts.
• This means that the angle that the Bisector tips will always be exactly half of the angle that the rotorhub tips. In addition, both the hub and the Bisector tip at the same azimuth
• This, in turn, means that the angle of deflection at the universal X-joint #1 and at the universal X-joint #2 will always be equal. The equality of deflections results in the Concentric Double Universal Joint becoming a Constant Velocity Joint.
• For a 3D reference, for pictorial clarification, an exploded view of a Concentric Double Universal Joint be seen at Thompson Coupling. Note that the Thompson coupling converts the double universal joint into a Constant Velocity Joint by the use of linkages to maintain the correct position of the simple housing. The Thompson coupling may be an ideal solution for conventional CVJ applications, since it offers no resistance to angular change and can except axial loading. However, for a helicopter rotor ,a resistance to changes in the axial angle between the driver shaft (mast) and the driven shaft (rotorhub) is desired, since it acts as a hub spring.
• Mitigation of Flap Induced Lead/Lag:
• See; Rotor - Disk - Hub Spring Induced Lead/Lag Mitigation
• A potential problem with the above is that of pitch change on a 'bowed' blade
• Summation:
• The hub spring assemblies cause the rotor to provide a moment to the mast and on to the fuselage.
• The constant velocity joint eliminates the cyclical Coriolis effect (Hooke's joint effect) and thereby it eliminates the primary cause for lead/lag hinges, which are mandatory in an articulated rotor.

1. The mast and the rotor hub will be rotating at a constant velocity however the bisector, which represents the intermediate shaft, is rotationally accelerating & decelerating. Its mass is insignificant however it's rotational oscillation will interact with the inner and outer elastomeric bearings.
2. The two opposing elastomeric bearings act as springs that position the bisector. Could the springs get in to a resonance and thereby cause the angles of the two 'universal joints' to differ? Perhaps a damper may overcome any problem. Perhaps a mechanical linkage can be used to provide a constant equality of angles.

• [Source ~ RW p.155] states that "A velocity ratio of unity may be obtained ...... as long as the input and the final output shaft are parallel."
• [Source ~ MH p.2240] states "the shafts must make the same angle with the intermediate shaft".
• http://www-2.cs.cmu.edu/People/rapidproto/mechanisms/chpt8.html#universal3fig ~ By using a double joint ..... the variation of angular motion between driver and follower can be entirely avoided. This compensating arrangement is to place an intermediate shaft 3 between the driver and follower shafts. The two forks of this intermediate shaft must lie in the same plane, and the angle between the first shaft and the intermediate shaft must exactly be the same with that between the intermediate shaft and the last shaft. If the first shaft rotates uniformly, the angular motion of the intermediate shaft will vary according to the result deduced above. This variation is exactly the same as if the last shaft rotated uniformly, driving the intermediate shaft. Therefore, the variable motion transmitted to the intermediate shaft by the uniform rotation of the first shaft is exactly compensated for by the motion transmitted from the intermediate to the last shaft, the uniform motion of either of these shafts will impart, through the intermediate shaft, uniform motion to the other."

• In addition, the following appears to confirm the above statements. See e-mail folder Thompson Glenn ~ CV Joint

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