Item 0799

DESIGN: SynchroLite ~ Rotor - Hub - 3-blade - CVJ & HS - Hub Spring (Hinge Spring)

Overview:

This centering device can only impart a small to medium moment, but it may result in;

Advantages:

  1. The response of the helicopter to cyclic inputs will be slightly faster. This is because this rotor will have a slight tendency to act as an Absolutely Rigid Rotor.
  2. There will be a reduced tendency for a blade to make in incursion into the tail boom, when the rotor disk is unloaded.
  3. During heavier, or lighter, loading on the rotor, the plane of the blades' CGs will deviate less from the teetering hinge. This might reduce 2P vibration caused by differing accelerations of the blades during teetering.
  4. On an intermeshing helicopter, there will be less chance of blade-blade contact, therefore it should be possible to shorten the masts slightly, reduce the V-angle, and reduce the stagger.
  5. There may be no need to include droop stops.
  6. There may be a reduced need for blade tie-downs.

Disadvantages:

  1. The additional forces will necessitate stronger and thus heavier blades, hub, bearings, mast etc.
  2. Some form of damping may be required.
  3. Will it place additional loading on the pitch links and cyclic control?
  4. On a helicopter with two blades per rotor, the hub springs will cause a 2P vibration. See: DESIGN: Dragonfly ~ Rotor - Disk - Lift Distribution re: Vibration with 2-blades Three blades should solve this but this will add weight and probably require more stiffness. See Related Patents below.
  5. The Hub Spring concept will place an out-of-plane loading on the blades because they are attempting to 'pry' the craft to the desired orientation. The bi, tri, quad teetering rotor with offset hinges concept puts a 'prying' action on the hub but not on the blade. I.e. the blades on the Hub Spring method will have to be stronger and heavier.
  6. Airworthiness Directive Go to page 33 of 133 or search 'hub spring'.

Re-think about the possible effect regarding interaction with the delta-3

Possible Modifications to Existing SynchroLite:

  1. Shorter mast plus larger diameter or stronger.
  2. More unidirectional spanwise carbon in blades.
  3. Make sure hub, feathering and upper mast bearings can handle the increased load.

Consideration Regarding Moments of Hub Springs: I hope that this is the correct thinking.

The forces must be equal at all azimuths for a specific amount of movement.

At each spring pair, the outer spring force per amount of movement and the inner spring force per amount of movement must always be equal to each other. This is so that the 'Bisector' bisects the teetering angle exactly, at each azimuth were the springs are located. In turn, this is necessary so that the double universal joint will operate correctly.

Hub Spring Moment: Consider as 10 pound force on each blade at 75% of rotor radius is (10 * 8.67 * .75) = 65 ft-lb. The two blades per rotor will result in a moment of 130 ft-lb at the individual hub spring and a total moment of roughly 260 ft.lb. Can the blades, hub, bearings, mast and frame etc. take this additional bending moment?

Helicopter Moment: The SynchroLite GW - weight of rotors is (550 - 42) = 508 lbs. and the arm from the CG to the rotor hub is 5 feet. Therefor the moment is 508 * 5 = 2540 ft-lbs.

Rotor Disks' Moment: [ref. DESIGN: SynchroLite ~ Rotor - Disk - Coning Angle] 185.3 ft-lb * 4 blades = 741.2 ft-lb.

The GW to rotor ratio is 3.4 to 1. The rotor to force ratio is 2.9 to 1.

Phase Lag: The incorporation of hub springs may require a slight reduction in the phase lag angle. This reduction will be in addition to any delta-3 phase lag adjustment.

Cross-Coupling: Acceleration crosscoupling will require a small amount of off-axis positioning be put into the cyclic stick by the pilot. It would appear that if the hub spring was incorporated then during the following rate crosscoupling, the hub spring will act a little like a offset hinge and this will involve some off-axis positioning also. In other words, the positioning of the cyclic stick might be the same for the acceleration and the rate crosscoupling. [Source ~ RWP4 p.41]

Additional Consideration Regarding Hub Springs:

The 'springs' must have a 3:1 ratio between uncompressed and totally compressed. When the disk is normal to the mast, all the springs will be 2/3 of there uncompressed length. When the disk is teetered, the springs will be shorter or longer than this value, depending on if the spring is on the low side or the high side of the disk.

The current arraignment shows an inner and an outer ring of elastomeric. All things being equal, the elastomeric outer ring will have less resistance per increment of movement than the inner one will. This is because the smaller inner ring must compress more for a given increment of movement. A couple of possible solutions are;
    1. Have the durometer at the outer diameter of the outer spring higher than the durometer at the inner diameter of this ring and a gradual transition between the outer and inner diameters.
    2. Insert a ring of harder durometer against the outer diameter of the outer ring. This ring would have a triangular cross-section with one flat against the bell and the opposing edge pointing toward the bisector.

I think that 3 spring sets (inner and outer) at 120º may give the same constant moment through the rotation as a single set of single 'ring' springs.

Notes re Design:

Since the volume of the outer ring is greater than the inner ring consider;
    1. Increasing the durometer of the outer ring.
    2. Decrease the thickness of the outer ring.

Since the elastomer may be poured into the cavities, consider having them retained by simply machining a rim in the mast ring, bisector skirt and bell.

The moment arm length on the hub springs is 3.33"

Potential Suppliers:

From Graviman. & GregLocock on Eng-Tips

Greg and Mart,

Trelleborg (VERY slow to respond): dermot.fitzgerald@trelleborg.com
Silvertown (Seem to be pretty good): Eddie.Deeming@silvertown.co.uk
Maskew: darryl@allanmaskew.co.za
Boge
Avon
Peradin
Cooper
Lord
Freudenberg
___________________
Once you come up with a compliance and max load definition for each axis (ie 6 numbers) then you can get serious.

Related Information:

MAKE: SynchroLite ~ Rotor - Hub - 3-blade - CVJ+HS - Hub Spring

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

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Last Revised: December 16, 2004