1140

Item 1140

DESIGN: Dragonfly ~ Rotor - Hub - Assembly w/ Inline Arm and Spindle

Objective:

A 3-blade (tri-teetering), thereby eliminate the vibration that is caused by the difference in the rotor's tilting moment when a 2-blade (bi-teetering) rotor has is blades pointed for-aft versus side-to-side. See: DESIGN: SynchroLite - Rotor - Hub - Bi-Teetering W/ Offset ~ Disadvantages Since a 3-blade rotor will be subjected to lead-lag, see the solution in the following section.

Ref: The scale multiplier for 2 to 3 blades while maintaining the same volume is: 1 * (0.87358)³ = 2/3

Drawing:

Features of Tri-Teetering Rotor (3-blades) vs. Bi-Teetering Rotor (2-blades):

Pros:

Less rotor induced vibration.

The moment caused by tilting the tip path plane of a tri-teetering rotor will be less then that of a bi-teetering rotor.

The above moment will be constant. It will not vary at 2P as it will with a bi-teetering rotor.

The weight of the tri-teetering will probably be the same as the bi-teetering. The total blade weight will be the same. The tri-hub will weigh a little more but the drive frame, final housings and mast bearings will be less because the moment from the rotors is less and there is less vibration (torque) in the assemblage.

The gearing etc. is lighter because the speeds are faster.

It is closer to the UniCopter's Absolutely Rigid Rotor, and this eases the transition to the UniCopter

Cons:

If the rotor sizes are such that the weight of the three 3-blades equals the weight of the two 2-blades⁽¹⁾, 6-7% more power will be required. If the tri-teetering disk is made slightly greater than above, the power will be the same.

Possibly slightly more expensive.

Greater 'V' angle and/or greater stagger. This is not necessarily so.

___________________________

(1) The radius of the 3-blade rotor is 7/8s that of the 2-blade rotor. Therefore the mass and weight of a blade on the 3-blade rotor will be ([7/8s the length] * [7/8s the chord] * [7/8s the thickness]) = [67% the volume] of a blade on a 2-blade rotor. This means that the total weight of the blades will be the same on both rotors [3 * 0.67 = 2 * 1]. The 3-blade rotor will have the disadvantages of greater tip loss, smaller Reynolds number and greater disk loading.

Production Costs:

The amount of composite material required for the blades of the 2-blade and the 3-blade rotor will be the same but the 3-blade rotor will have the advantage of a 50% higher volume production. This will be offset by the 50% increase in hub machining.

Helicopter Specification Comparison with 2-blade Rotors:

The radius decreases from 8'-8" to 7'-8"

The root chord decreases by 1"

The required power to hover OGE increases from 32.6 hp to 33.3 hp.

All other inputs have not been changed.

Handling Lead-Lag:

The in-plane component of the delta-3, by flap hinge geometry, may absorb the lead/lag.

The angle of the delta-3 has to be calculated to suit the Coriolis effect. 0433.html ~ teetering

Some Calculations:

The undersling on the drawing is 3 degrees. If the flap on the blade at azimuth 90º is 10º then the movement at the tie-bar bearing below is 0.5569". The delta-3 is 15º so the lateral movement is cosine (15) * 0.5569 = 0.5379" The lateral movement at the tie-bar bearings for the blades at azimuth 210º and 330º should therefor be sine (30) * 0.5379" = 0.2689".

Toward Absolutely Rigid Rotors:

If the lateral movement of the 'Y' connection at the junction of the 3 tie-bars is given an exponential resistance (rubber grommet, springs, etc) to movement in the mast plane and the blades, pitch bearings etc. are strengthened, then the rotors should start to act closer to that of an Absolutly Rigid Rotor. This will also allow for a reduced stagger and mast 'V' angle. Reference; DESIGN: SynchroLite - Rotor - Hub - Centering Teetering Hinge (Hub Spring)

This strength of the blades and pitch bearings etc. should not have to be as great as the ARR since this rotor incorporates the cone resulting from the rotor thrust. In addition, a certian mount of blade flex in this situation may not be exceptionally detrimental.

Notes:

The bearings in the center of the tie-bars rotate in one plane only and must be rigid in the other two.

The 'Y' joint can only be allowed to move in the mast plane. It must not be allowed to move along the mast axis,

Supplemental Information:

For initial idea see: DESIGN: Dragonfly - Rotor - Hub - Bi-Teetering W/ Offset

For basis of this initial idea see: DESIGN: SynchroLite - Rotor - Hub - Bi-Teetering W/ Offset

Potential Problems:

The delta-3 angle means that the other two teetering yokes are not being pulled equally by the triangular tie-frame.

Potential Solution:

Mount the tie-bar to the yoke on an axle that is normal to the center of the mast, not to the teetering axis. This axle is a screwball so that the horizontal location of the tie-bar moves laterally as the yoke swings.

It may not be possible to link the three yokes together so that the flapping amount is as desired.

For Bi- and Quad-Teetering Rotor see; DESIGN: SynchroLite - Rotor - Hub - Bi-Teetering & Quad-Teetering

Introduction Page | Dragonfly Home Page| SynchroLite Home Page | UniCopter Home Page | Nemesis Home Page

Initially displayed (& posted on PPRuNe): December 20, 2002 ~ Last Revised: October 6, 2007

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.