1186

Item 1186

DESIGN: Dragonfly ~ Rotor - Hub - Bearings - Tie-Rod

Forces:

See; OTHER: Flight Dynamics - General ~ Lead/Lag

The following forces are;

From one blade.
Located in the yoke, at the intersection of the feathering axis of the blade and the vertical axis of the drag and flapping hinges.
At 100% RRPM (no overspeed).
From GW at hover.

The values below all originate at; DESIGN: Dragonfly ~ Rotor - Hub - Blade Forces - Preliminary

Centrifugal:

The centrifugal force is 3,577 lbf [15.911 N].

Moment from Thrust:

The thrust is GW / 6 blades = (550 / 6) = 92 lbf

The center of thrust is at 75% of disk radius = 0.75 * 96" = 72"

The distance from the yoke to the center of thrust is 72' - 13.5' = 58.5"

The moment is therefore 92 * 58.5 = 5,382 lbf-in = 608 N-m.

Moment from Engine Torque:* (u / (u + d)) / 2 = 15,911 * ((1 / (1 + 1.58)) / 2

The maximum torque from the engine, per blade is 63 lbf-ft = 756 lbs-in = 85 N-m.

The center of drag is at 75% of disk radius = 0.75 * 96" = 72"

Therefore the drag is 756 lbs-in / 72" = 10.5 lbf

The distance from the yoke to the center of drag is 72' - 13.5' = 58.5"

The moment is therefore 10.5 * 58.5 = 614 lbf-in = 69 N-m.

The Combined Moment from Thrust and Engine Torque:
√((608)² + (69)² ) = 612 N-m

The values for the symbols below come from DESIGN: Dragonfly ~ Rotor - Hub - Load Distribution

n is the number of tir rod bearings at the yoke,; which is 2

Centrifugal:
3,577 lbf * (u / (u + d)) / n = 3,577 * ((1 / (1 + 1.58)) / 2 = 693 lbf = 3,082 N
Thrust:
5,382 lbf-in. / ((u + d) / n) = (5,382 / (1 + 1.58)) / 2 = 1,043 lbf = 4,639 N
Torque:

614 lbf-in / (2 * r2) = 307 / (2 * 5) = 31 lbf This value will be added to the leading bearing and subtracted from the trailing bearing.

Loads on Bearing:

The Radial Load on the Leading Bearing: [F_r]:

693 lbf + 1,043 lbf + 31 lbf = 1767 lbf
Ultimate load:

1767 lbf * 4 [LLF] * 1.5 [SF] = 10,602 lbf = 47,160 N

See Note (1) below.

The Axial Load on the Leading Bearing: [F_a]:

31 lbf
Ultimate load:

31 lbf * 4 [LLF] * 1.5 [SF] = 186 lbf

States:

High Thrust:

([High thrust - Blade weight] * [Moment arm] / [Leverage]) + [Centrifugal force]⁽¹⁾

Hover:

([Thrust - Blade weight] * [Moment arm] / [Leverage]) + [Centrifugal force]

No Thrust:

([Blade weight] * [Moment arm] / [Leverage]) + [Centrifugal force]⁽²

Parked:

([Blade weight] * [Moment arm] / [Leverage])

Negative Thrust:

([Blade weight] * [Moment arm] / [Leverage]) + [Centrifugal force]⁽³⁾

Notes:

At high thrust the blade's CG will be above the teetering hinge, therefore the centrifugal force will have a downward component and will want to reduce the tension on the tie-bar.

At no thrust the blade's CG will be below the teetering hinge, therefore the centrifugal force will have an upward component and will want to reduce the compression on the tie-bar.

At negative thrust the blade's CG will be below the teetering hinge, therefore the centrifugal force will have an upward component and will want to reduce the compression on the tie-bar.

Thoughts:

The greatest loading, by far, is that of tension. During No Thrust the centrifugal effect might keep the blades in approximate position and at low rpm and when parked, droop stops could be employed.

Consider tying the three yokes together with many strands of a thread. This would be somewhat like a tension strap pack. The thread could be 'piano wire' or prepreg fiberglass thread, which is then bound and cured.

If the tie-bars do not have sufficient flexibility in the horizontal plane, change their cross-sectional shape from round to a vertical oval or vertical flat bar.

Bearing Selection:

Rod End Bearing: (Option A)

The female rod end bearing HF7 has a 0.4375" bore, a 7/16"-20 thread and a maximum static load rating of 5,356 lbs. Weight is 0.18 lbs. This value may not have a high enough safety factor. Consider the following rod end bearing or increasing the Leverage.

Rod End Bearing: (Option B)

The female rod end bearing HFX-7G has a 0. 4375" bore, a 7/16"-20 thread and a maximum static load rating of 11,055 lbs. Weight is 0.18 lbs.

Rod End Bearing: (Option C)

Better yet seriously consider Rexnord Tuflite composite bearings with unique journal bearing bores on; OTHER: Mechanical - Bearing ~ Teetering Bearing

Additional Information:

One ore more versions of the Alouette, the Lama and possibly the Sud-Est had lead-lag cables between their three rotors. The 3 cables made a triangle with a point of the triangle at each blade. For one example see; Aerospatiale - Alouette II Note that the three lead-lag hinges are probably not concentric with the center of the disk and this will mean that as the blades lead and lag the cables will goes slack and tighten. Perhaps, as Shawn Coyle says. "Most rotors could go without the damper in flight, as they are only needed to prevent ground resonance. The new bearingless rotors need the damper to prevent air resonance."

OTHER: Mechanical - Bearings

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Last Revised: October 19, 2004

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