Item 1157

DESIGN: Dragonfly ~ Rotor - Hub - Tie-rod - Layout

Which is optimal ~ One central tie-bar or two (to control lead/lag)?

Picture of Mockup: (preliminary design - partially completed)

Option C

Option D

Drawing:

Option B:

Steel - 4340 ~ Tensile strength - Ultimate = 176,900 lb/in2, ~ Tensile strength - Yield = 113,970 lb/in2, ~ Modulus of elasticity = 29,725 lb/in2, ~ Shear modulus = 11,600 lb/in2,

Using Steel - 4340 and the Yield tensile strength, the cross-sectional area of the rod must be (18,342 / 113,970) = 0.161 in2.

Diameter of 3/8" = area of 0.1105 in2.

Diameter of 7/16" = area of 0.1504 in2.

This appears to include the limit load and safety factors. See DESIGN: Dragonfly ~ Rotor - Hub - Load Distribution:

What improvement will a stronger material make? What loss in strength results from the thread?

Option A, C, & D:

The rods must flex therefore, they must be made out of fiberglass and not carbon. This will increase the weight and size.

Option C:

Consider making the center tube of stainless steel and keep the wall very thin. It can be strengthened by 1) having the ID slightly smaller in the center, because the mast (cylinder) will be angled over to one side, and 2/ have the flanges at both ends part of the tube. I.e. 1 piece part.

Composite Strength:

It appears that carbon thread has roughly twice the tensile strength of fiberglass. The use of carbon will therefore decrease the weight and the size of the assembly. Carbon will not provide any of the 'stretch' that might be desired, if using a pair of assemblies and wanting to allow limited blade-to-blade lead/lag.

Make sure that the carbon thread is not required to wrap around too tight a radius. Carbon will possibly not flex enough.

Pin Strength:

Steel - Quenched / tempered ~ Shear Modulus 11.5e-6 psi ??

Steel - Stainless (302) ~ Shear Modulus 10.6e-6 psi ??

Consider ~ Hollow pin.

Notes:

Increasing the vertical distance between the teetering hinge and the tie-bar (I.e. lowering the tie-bar) has two advantages.
    1. The tie-bar related components can be less strong and lighter.
    2. The horizontal component of the loading between the teetering hinge and the tie-bar is less visa-vie the vertical component. I.e. the angle is closer to vertical.

A disadvantage is that the motions of the tie-bar's center and the control stick are larger.

Pivot Locations:

The only locations requiring the ability to pivot are;
    1. The attachment to the yoke. This must have approximately ±10º rotation about the X-axis and a very small amount of rotation about the Z-axis.
    2. The 'attachment to the central "Y". This requires no rotation about the X-axis and a very small amount of rotation about the Z-axis. A simple bushing with an Z-axis may very well do.

A rod-end can be used at the yoke. If the 'Y' had something similar, then;
    1. The rods can act as turnbuckles; and give fine adjustment.
    2. The assembly can accept compression; and this, in conjunction with a hub spring may negate the need for droop stops.

Initially construct from threaded rod and then later from pultruded carbon thread. Perhaps fiberglass so that there is some elasticity.

Basically, the two tie-bars take the centrifugal force and the thrust. One side tie-bar takes any lead force and the other takes any lag force. The hub frame takes all the torque and some compression during high thrust.

Arraignment:

Have the leading tie-bar assembly located above the trailing tie-bar assembly where they cross at the mast. This is so that the vertical forces, even though they are very small, will oppose the twist about the upper yoke bearings, which is caused by the in-plane drag of the blades.

Damper:
    1. Locate a friction pad between the centers of the two tie-bar assemblies, to act as a damper. This is in addition to the friction of the drag bushings. See; OTHER: Flight Dynamics - General ~ Ground Resonance
    2. An alternative might be to locate an elastomeric bearing between the centers of the two tie-bar assemblies. This elastomeric would consist of a sandwich of a number of metallic and elastomer disks, with one tie bar attached to the top metallic disk and the other tie bar attached to the lower metallic disk.

Adjustment:

Also:

See: OTHER: Flight Dynamics - General - Lead/Lag ~ Offset Tri-Teetering Rotor

Tie-bar / Control- Flight / Mast Clearances:

DESIGN: Dragonfly ~ Rotor - Hub - Clearance - Tie-bar / Control / Mast

Manufacturing:

Make from Carbon thread or more flexible E-glass.

Consider wrapping at slightly under length, then stretch to desired length, then cure.

Increased Rotor Control:

The inclusion of a hub-spring will give the rotors additional rigidity. This will negate the possibility of a blade to blade clash. Unlike the tri-teetering, the hub-spring will place additional out-of-plane loads on the blades. The inclusion of the hub spring will add tensional loading to the tie-rods

See: OTHER: Flight Dynamics - Rotor - Hub - Teetering w/ Hinge Spring (Hub Spring)

The increased forces caused by the hub-spring have not yet been incorporated into the 'Forces:' section, which is located immediately below.

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Last Revised: December 30, 2004