1230

Item 1230

OTHER ~ Flight Dynamics - Rotor Hub - Hub Spring (Hinge Spring) (K_β (β - β_p) )

Overview:

A pair of springs are located at the teetering hinge. They produce a restoring moment on the blades. It might be used to augment rotor control power.

The hub springs on a 2-blade rotor will produce a vibration of 2P when the rotor teeters from being normal to the mast.

Reduces the phase lag to less than 90º.

Slightly reduces the amplitude of the flap response to cyclic.

The hub spring will increase the loads on the pitch links (pilot's cyclic forces), plus on the mast, rotorhub, and blades. Why will it increase the loads on the pitch links?

"Bell has long known that its teeter-rotor system is susceptible to mast bumping. The physics of helicopter flight requires Bell's see-saw design to have free-floating blades. Vigorous maneuvers, adverse weather, and mechanical problems can all trigger the excess "flapping' of the blades that slams the underside of the rotor hub into the mast, almost always snapping the mast and the blades off."

This hub spring assembly that is discussed on this web page rotates about the mast with the blades. It can therefor apply a restoring at any azimuth about the mast.

Sketch:

Uses:

360º Hub Spring:

To augment the rotor control power at all azimuths.

Fixed Azimuth Hub Spring:

Could be located at 0º and 180º to increase longitudinal moments
Could be located at one specific azimuth to prevent the clashing of closing intermeshing rotor blades
Perhaps located at 0º and 180º to improve yaw by opposed longitudinal cyclic on intermeshing rotors.

Related Patents:

From rec.aviation.rotorcraft ~ Yes, I reviewed a US patent on that system from years ago. ~ Dennis Fetters. I cannot find the patent.

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6,296,444 ~ Prop rotor hub ~ Bell Helicopter Textron Inc. ~ October 2, 2001

4,333,728 ~ Compound hub spring system for helicopters

4,115,031 ~ Hub spring moment isolation in underslung two-bladed teetering rotor ~ Textron, Inc.

3,807,896 ~ CONCENTRIC TUBE SPRING ROTOR MOUNT

3,804,552 ~ FOUR BLADE MAIN ROTOR CONTROL POWER COUPLING

Attempt to eliminate hub spring induced vibration:

4,115,031 ~ Hub spring moment isolation in underslung two-bladed teetering rotor. Have hard copy.

A method and system which isolates two-per-rev hub spring moment vibrations while providing control power during zero-g flight in a two-bladed teetering rotor helicopter. Hub spring structure is connected between the rotor and the pylon to resist rotor flapping about the teeter axis and thus provide control during zero-g flight. To isolate from the fusilage the hub spring moment two-per-rev vibrations, the rotor is coupled to the pylon below the rotor center of gravity at a distance related to both the hub spring rates and the in-plane frequency of the helicopter blades.

Patents Referencing 4,115,031;

	5,853,145	Rotor head for rotary wing aircraft
	5,145,321	Helicopter rotors with elastomeric bearings
	4,897,073	Shaft coupling for rotating elements
	4,695,227	All composite, constant speed universal joint for use in a shaft driven tiltable main rotor for a helicopter. A universal joint for use in a pitch cone rotor system in a helicopter is comprised of crossed lift beams, one beam of which is coupled to the rotor hub and the other beam of which is coupled to the driven rotor shaft. The two crossed lift beams are coupled to each other through a cruciform case. The cruciform case in turn is coupled to each of the crossed lift beams by means of a plurality of flexures. The flexures and cruciform case are made of composite fiber materials and the flexures are soft enough to permit angular and translational deflections of the crossbeams with respect to each other thereby tending to smooth out and reduce sudden changes in rotor speed between the shaft and rotor system and thereby tending to make rotor speeds more uniform. The crossed lift beams are further coupled together at the center of their crossing by a flexible elastomeric tension link which conducts rotor loads between the two beams when the rotor develops negative lift loads on the ground and in flight. A hollow laminated elastomeric compression fitting is placed between the beams at their center to transmit the rotor lift force between them.
	4,580,945	Helicopter gimbal rotor
	4,569,629	Helicopter gimbal rotor
	4,566,856	Helicopter gimbal rotor
	4,522,563	Elastomeric system for mounting a helicopter rotor
	4,466,773	Countertorque rotor for helicopters. A countertorque rotor for helicopters in which a transmission shaft keyed transversely onto a drive shaft and disposed obliquely within a hub has each of its ends coupled in a slidable manner within an inner bush of an associated elastomeric coupling this latter carrying screw actuator means which can act axially on the said shaft to displace the said hub with respect to the shaft (3) itself in the direction of the axis of this latter.
	4,333,728	Compound hub spring system for helicopters ~ Textron Inc. A system for opposing helicopter blade flapping applies restraining forces at a low spring rate at low flap angles and at a high spring rate at flap angles greater than a predetermined angle. More particularly, bearings rotatably support the blade yoke on rotor hub trunnions which form a flapping axis. A resilient mechanical coupling between the blade yokes and the hub trunnion applies a restraining force having a characteristic linear with flapping angle. A second restraint mounted to rotate about the flapping axis with the rotor, applies a nonlinear restraining force which preferably rapidly increases beginning at a predetermined flap angle, and over the range short of hard flapping contact between the rotor and the rotor mast.

Remarks by Others:

[ Patent 4,115,031]

"Another source of two-per-rev excitation is a hub spring. Two-bladed teetering rotors tend to have zero control power in zero-g flight. Control power can be provided with a hub spring which provides a spring force acting between the rotor and the pylon. However, spring rates should be kept moderate to prevent them from causing a rotor/pylon instability. It can be shown that about 25% of the one-g control power is adequate for zero-g flight. Such hub spring rates are acceptable as far as rotor/pylon stability is concerned, but the hub spring introduces a two-per-rev hub moment and therefore a two-per-rev fuselage vibration."

[ 10 March 2002: by: LZ ]

The original Bell rotorheads had metal bearings to allow teetering. The first underslung heads also had metal bearings and as the heads evolved, the metal bearings were replaced with elastomeric bearings. They do not provide a restoring force and do not function as a spring. The aerodynamic and mechanical forces far outweigh any type of resistive or restorative force exhibited by the elastomeric bearing. You might feel resistance if you were to teeter a static head but this resistance is minimal compared to the other forces involved.

The only exception to this is the rotorhead used on the Bell AH-63 which competed against the AH-64. The elastomeric elements in the teeter hinge were so strong as to allow a man to stand on a blade several feet from the teeter hinge without deflecting the blade. The purpose of this spring was to minimize the excessive flapping of the blades during Zero G, which could lead to mast bumping. The spring was designed to meet the nap of the earth flight characteristics required by the design spec.

NB. The Bell design lost the competition.

[ 10 March 2002: by: NL ]

Those are known as "Hub Springs" which hold the teetering head at 90 degrees to the mast. When cyclic is added, and the disk tilts away from 90 degrees, the spring imparts a force on the mast that adds control moment, like an articulated head. An elastomeric device on some teetering rotorheads does not qualify as a hub spring, since the rubber elastomer force is almost non-existent in comparison to rotor forces. Three things are bad about adding a powerful hub spring:

1) the spring introduces a dynamic mode with little damping, so a flap damper might have to be added to avoid resonant conditions that might make real problems. Articulated rotor blades don't need a flap damper because the aerodynamic damping of a flapping blade is significant, but with a teetering head, that damping is canceled by the other blade flapping in the opposite direction.

2) The spring has to be a hum-dinger to make much control power. For a 10,000 pound machine like the S-76, an articulated rotor delivers about 1500 foot-pounds per degree of flapping for use as control. That is 10% of the weight of the machine on a 1 foot lever arm(!). A spring that can deliver 10% of the weight of the machine per degree would be a packaging and structural problem. ~ My thoughts ~ I doubt that this 'structural problem' will be any different for either rotor configuration, once the moments get to the mast; and beyond.

3) That large moment would play havoc with the dainty rotor mast, transmission mounts and airframe structure of the typical teetering rotor helicopter. Note the "simple" job of installing a 4 bladed moment-generating rotor on the previously teetering Y Huey and Z Cobra, a project that threatens to sink Bell helicopter as the entire helicopter (except the data plate) is beefed up. .

[ 12 March 2002: by: NL ]

This whole subject underscores the neat idea behind the articulated rotor. Its strong head moment is a product of the centripetal force, which creates high head moment with no weight penalty of a spring (although the increased moment requires stronger masts, transmissions and airframe structure). ~ My thoughts ~ But, the articulated rotor is subjected to a greater problems from the Coriolis effect than a teetering rotor is.

NACA ~ RECON

Index RECON select contains the following 6 items relevant to '"hub spring"'. The first figure for each entry is its relative score,

1000 -1 Flight test evaluation of a nonlinear hub spring on a UH-1H helicopter, AD-A098794, Apr 01, 1981

860 -1 Combined preliminary airworthiness evaluation and airworthiness and flight characteristics evaluation of the UH-1H with preproduction hub spring and composite main rotor blades installed, AD-A202316, Jun 01, 1988

194 -1 Development of linear and non-linear hub springs for two-bladed rotors, Jan 01, 1978

194 -1 The application of elastomeric products on the V-22 tiltrotor aircraft, Sep 01, 1989

156 -1 BHTI's technical assessment of advanced rotor and control concepts, Jan 01, 1990

Hub moment springs on two-bladed teetering rotors
Sonneborn, W. (Bell Helicopter Co., Fort Worth, TX, United States); Yen, J. (Bell Helicopter Co., Fort Worth, TX, United States)
NASA Center for AeroSpace Information (CASI)
1974
Two-bladed teetering rotors with elastic flapping hinge restraint are shown to be suitable for zero-g flight. The alternating moment component introduced into the fuselage by the hinge spring can be balanced about the aircraft center of gravity by alternating hub shears. Such shears can be produced in proper magnitude, frequency, and phase by additional underslinging of the hub and by judicious choice of the location of the first inplane cantilevered natural frequency. Trends of theoretical results agree with test results from a small scale model and a modified OH-58A helicopter.
No Digital Version Available - Order This Document
Updated/Added to NTRS: 2003-05-08

Other Outside Stuff:

EXPLORATORY FLIGHT INVESTIGATION AND ANALYSIS OF STRUCTURAL LOADS ENCOUNTERED BY A HELICOPTER HINGELESS ROTOR SYSTEM;- http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660030151_1966030151.pdf Read the report. It may be more applicable to "Absolutely' rigid Rotors.

Related Information:

The hub spring is used in; OTHER: Flight Dynamics - Rotor Hub - Constant Velocity Rotor w/ Hub Spring

DESIGN: SynchroLite - Rotor - Hub - 3-blade - CVJ & HS - Hub Spring

For mathematics on hub spring see; [HT, Section 5-13 p.222]

OTHER: Flight Dynamic - General - Phase Lag

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

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Re: use on 2-blade rotor:

Idea: 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 mast to realign with the rotor disk. This will only be applicable during forward flight.

Possibly Better Idea: Only applicable to an electric helicopter, and not applicable to the Intermeshing or Coaxial configurations. See OTHER: Helicopter - Inside - Principal Assembly - Electrotor-Plus

A Thought (concern):

If the blades on a rotor have a very large twist then the blade that is teetering (or flapping) up may want to rotate forward while the blade at 180º is teetering (or flapping) down and may want to rotate back. In other words, opposing flap-lag couplings, which may create stresses.

Last Revised: April 11, 2012