1127

Item 1127

OTHER: Flight Dynamics - Rotor Hub - Offset Bi-teetering

Overview: (Abstract)

A rotorhub that integrates advantages from the teetering rotor and from the fully articulated rotor.

A teetering hub tends to keep the sum of blade masses at the teetering hinge, and therefor on the centerline of the mast. This reduces vibration when the rotor teeters.

A hub with offset flapping hinges has better cyclic authority and less chance of tailboom incursion then that of a teetering hub. In addition, on the intermeshing configuration, the offset teetering hinge will allow for a reduced obliqueness and/or a reduced stagger.

This concept can be incorporated into rotors with two, three, four, or more blades. However, a two-blade offset teetering rotor will perform similar to a two-blade teetering rotor with a hub spring, and thereby have a 2/rev. vibration.

Drawings:

The following figures are of a two-blade rotor, for simplicity of explanation.

Fig. 1 is a simple conventional teetering hinge.
Fig. 2 is a bi-teetering hinge with offset.
Fig. 3 shows that the virtual teetering center (geometric center) does not move during teetering (flapping).

Fig. 1
Fig. 2
Fig. 3

This is the drawing of Figure 3 in dwg format

The above dimensions are to the center of (gravity/aerodynamic/percussion/etc.) of the blades, not their tips.

Note that teetering (flapping) reduces the horizontal component of the arm length of both blades equilly. It does not move the virtual center.

For a clearer description see the Boeing patent below.

Teetering Rotor

Articulated Rotor

Offset Teetering Rotor

Pros and Cons:

As compared to a conventional teetering rotor.

Advantages:

The response of the helicopter to cyclic inputs will be faster. This is because the rotor will tend to act as a fully articulated hub. Note;- It appears that the offset teetering rotor will present smaller moments to the fuselage than that of a comparable fully articulated rotor. See DESIGN: Dragonfly ~ Rotor - Hub - Load Distribution:

There will be a reduced tendency for a blade to make an incursion into the tail boom, when the rotor disk is unloaded.

On an intermeshing helicopter, there will be less chance of blade-blade contact, therefore it should be possible to reduce the stagger between the two rotorhubs or reduce the angle between the masts.

The mast(s) will be shorter.

On an intermeshing helicopter the control rods could be inside the mast head and exit just below the location of the virtual hinge. This will have ramifications for delta3.

Disadvantages:

Increased Weight over a Comparable Two-blade Teetering Rotor: The increased control moments will necessitate stronger and thus heavier hubs, bearings and masts etc. In addition, going from two blades to three or four blades per rotor will increase the craft's weight. The shorter masts will save a little weight.
The load on both of the two teetering bearings will be greater than the force on the single teetering bearing. This is because the centrifugal force of the two blades on the single teetering bearing neutralize each other.
The parasitic drag of the rotor hub will increase.
It might result in slightly stronger forces on the pitch links and cyclic control.

Direct Hub Moment:

Fig. 3 in the above drawings, has a coning angle is 4º and the flapping angle of 10º. This is used to see if the moment of the Offset Teetering Hinge is different from that of a comparable Offset Flapping Hinge. The following force arm measurements can be found on the original .dc drawing.

	Offset Teetering Hinge:		Offset Flapping Hinge:
	2.097 CW		0.484 CW
	1.386 CCW		0.209 CW
Sum:	0.693 CW	=	0.693 CW

If my thinking is correct, the Offset Teetering Hinge moment is no less than that of the Offset Flapping Hinge.

Blades Arraignments:

Bi-Teetering Hub: (2-blades per rotor)

See; DESIGN: SynchroLite ~ Rotor - Hub - 2-blade Teetering - Offset Bi-teetering Hub

Concern: Vibration (on 2-blade rotors only): If the cyclic was to be pushed forward it will cause the tip path plane of the disk to tip down at the front, in respect to the mast plane. This must mean that when the blades are at 0º & 180 º azimuth, the centrifugal force and hub offsets are attempting to 'torque' the mast plane into alignment with the tip path plane. This is in addition to the thrust from the rotor, which is attempting to 'drag' it into alignment. When the blades are at 90º & 270 º azimuth, only the previously mentioned 'dragging' activity is taking place. This 'torque ~ no torque' will probably create a 2P vibration about the roll axis, caused by the combined lift of the rear two blades as they alternate their crossing in the aft two quadrants. See: DESIGN: Dragonfly ~ Rotor - Disk - Lift Distribution re: Vibration. The force-moments must be eliminated or neutralized. See Related Patents below.

Possible Solution: Linking the swashplate (actually a link from the cyclic stick) and an airspeed indicator to a movable horizontal stabilizer may significantly reduce the vibration, by assisting the fuselage and its mast to realign with the rotor disk. This will only be applicable during forward flight.

Tri-Teetering Hub: (3-blades per rotor)

Application: This arrangement is to be used on the Dragonfly intermeshing helicopter, see: Dragonfly - Rotor - Hub

DESIGN: Dragonfly ~ Rotor - Hub - Layout

Quad-Teetering Hub: (4-blades per rotor)

Application: Single rotor, side-by-side or tandem configurations. It is not applicable for the intermeshing configuration because of the large number of blades.

Arraignment: The rotorhead will support four yokes. These four yokes will be identical to those used on the 2-blade [offset bi-teetering hub] rotor. The tie-bars will also be the same as those on the 2-blade [offset bi-teetering hub] rotor in that the two opposing blades are connected to each other. These two tie-bars will be independent of each other and one will cross over the other. This will allow the blades at 90º azimuth to flap up and the blade at 270º azimuth to flap down, an equal amount, while the blades at 0º and 180º azimuth are not flapping. The mast must drive both hubs and yet allow for a dampened rotational oscillation between them.

Very Related Patent:

See; OTHER: Flight Dynamics - Rotor Hub - Hinge Spring (Hub Spring)

September 26, 2007 ~ Maybe this idea ain't new. See; US 4,681,511 ~ Low Vibration Helicopter Rotor ~ Boeing ~ July 21, 1987. It was referenced in Dennis Fetter's patent 5,163,815.

The vertical distance between 18 and 62 is the undersling.

The 'center point' 32 is a virtual center (geometric center).

The tilt angle of the rotor disk in Fig. 2 is 0-degrees and in Fig. 4 it is 10-degrees, but, what ever the tilt angle is, a straight line between the 26 (CofG) on the two blades will always (1) go through 32.
(1) Like all rotors, the last sentence will vary slightly with different operating conditions.

When a teetering rotor tilts, in respect to the mast, the Coriolis Effect causes the blades to accelerate and decelerate. This is because the blades' CGs are drawn in toward the center of rotation, at a rate of 2/rev. Centrifugal force will be attempting to remove this tilt and align the rotor disk and the mast perpendicular to each other.

Random Notes:

Rotorhead Loads:

Most of the loads will be handled by the composite thread tie-bars and not by the frame of the rotorhead.

Variable Pre-cone:

If the length of the tie-bar's arms are slightly longer than the yoke hinge to mast centerline distance, the pre-cone angle will automatically increase when the rotor teeters. Alternatively, if the length of the tie-bar is slightly shorter than two-times the offset length, the pre-cone angle will automatically decrease when the rotor teeters.

Rotor Governor by Cone-Pitch Coupling:

If the rotor was to have delta-3 there is another 'theoretical' advantage, that of a cone-pitch-coupling governor.

If the Tie-bar was to have an elastic property then as the rotor RPM slows, the cone of the disk will increase, by stretching the Tie-bar. This causes the cone-pitch-coupling (delta-3) to reduce the pitch of the blades and thereby help maintain rotor RPM. This must not be excessive to the point that it is a problem when flaring in autorotation and wanting to give up RRPM for thrust.

Intermeshing Configuration - Reducing Stagger or V angle when going from SynchroLite 2-blade teetering to Dragonfly 3-blade w/ offset: These notes will eventually be removed.

Half the V angle is 12.5º

The coning angle is given as 3º

The lateral downward flapping is -8º

This means the least angle of the blade is (12.5 + 3 - 8) = 7.5º

From drawing DESIGN: SynchroLite ~ Rotor - Disk - Blade to Hub Clearance, it appears that for a 4" of offset the stagger can be reduced by 2.67", from 27" to 24.33, or the angle be reduced by 2 * 1 = 2º, from 25º to 23º. The greater the offset the better. Actually as the offset gets longer the clearance can be less because the flexing of the blade (blade to hub clearance) becomes less of a concern.

A reduction in stagger should reduce the vibration about the X-axis and increase the higher frequency vibration about the Y-axis. A reduction in the V angle will reduce the crafts pitch - torque coupling. Consider an 8" offset, or more.

For additional information see thread 'Dynamics ~ 2 Blade Rotor w/ Offset Flapping Hinges', on PPRuNe, started Dec 7 2002

~ http://www.pprune.org/forums/archive/index.php/t-74679.html

Initially displayed (& posted on PPRuNe): December 7, 2002 ~ rec.aviation.rotorcraft & www.rotorcraft.com: January 17, 2003: Last Revised: September 27, 2011

The above utility invention is openly and publicly disclosed on the Internet to negate an entity from patenting it, to the exclusion of all others whom may wish to use it. ~ Reference patent law 35 U.S.C. 102 A person shall be entitled to a patent unless - (a) the invention was known ... by others in this country, ..., before the invention thereof by the applicant for patent.