A cyclic pitch linkage changes each rotor blade’s feathering angle as it travels around the mast. A tilted swashplate drives the pitch links through a repeating height cycle. This model shows a three-blade mechanism and solves the horn-and-link geometry at each rotor position.
Cyclic Pitch Control Linkage Interactive Calculator
Adjust the tilted swashplate’s link-height amplitude, pitch horn and rigid pitch-link length. Watch each blade feather as it turns around the mast.
Equation Used
- Three identical blade stations, rigid horns and pitch links, spherical pin joints.
- Bearing ring rotates in a fixed tilted plane; drive synchronization is idealized.
- No collective pitch, flapping, lead-lag, aerodynamic loads or clearance model.
- The lag input moves a marker only; rotor response is not predicted.
Educational mechanism geometry, not aircraft rigging or aerodynamic performance. Result tiles use the selected azimuth; animation adds rotor rotation.
A tilted swashplate produces changing blade pitch
The bronze ring and its control supports remain stationary in azimuth. The blue ring turns in the same tilted plane, and its pitch links connect to the blade horns. Each blade rotates about its own radial feathering axis as its link joint moves around the tilted ring.
The model holds every horn and pitch-link length constant. The three-dimensional distance between the ring joint and horn pin determines blade angle while keeping the pitch link at its entered length. A separate enlarged end view makes the feathering motion visible.
Explore a generic cyclic linkage
Use the calculator to see how link-height amplitude, horn length and pitch-link length influence feathering in the illustrated geometry. Zero amplitude gives a level ring and zero feathering variation.
The geometry is generic and does not correspond to a particular aircraft. Blade aerodynamics, flapping hinges, bearings, clearances and actual control mixing require separate analysis.
Calculating blade pitch from rigid-link geometry
Let F be the blade feathering-axis point, v the horizontal tangential unit direction, and ẑ the vertical unit direction. A horn of length L has pin H=F+L(cos θ v+sin θ ẑ). The ring joint S must satisfy |H−S|²=l² for pitch-link length l.
The continuous near-neutral solution determines θ. Ring tilt is asin(s/r), where s is the selected vertical amplitude and r is ring radius. The illustrative feathering-axis radius is 1.4L, with ring radius √((1.4L)²+L²), so a level ring has vertical pitch links at neutral.
The first result is half the peak-to-peak pitch range sampled over one turn. The pitch-at-azimuth result uses the entered angle. Peak-to-peak ring-joint height is 2s. The lag marker is simply selected azimuth plus the user-entered lag, wrapped into one revolution.
Change one dimension at a time
Start with a 100 mm horn, 300 mm pitch link and 17.4 mm link-height amplitude. The ring radius is about 172.05 mm, giving a tilt of about 5.80 degrees.
Set amplitude to zero: the tilted ring becomes level and all three blades remain at neutral pitch. Change pitch-link length: the exact pin closure changes the feathering result while preserving the rod length. Change the lag marker: only that illustrative marker changes, because it is not a mechanical dimension.
Geometry and flight response are separate
Pitch-link stroke, pitch-horn motion and aerodynamic rotor response are not interchangeable. This model calculates the first two. It does not infer a phase lag from rotor stiffness or predict the direction or magnitude of a thrust response.
The rotating ring follows an idealized synchronized angular relationship with the rotor. Real drive scissors, spherical-bearing offsets, structural deflection and aircraft-specific rigging are not represented. Aircraft tolerances, rigging procedures and maintenance intervals must come from the documentation for the specific rotorcraft.
Cyclic-pitch model questions
Why are the three blade pitches different?
They occupy different positions around the tilted ring.
Why is blade pitch not a perfect sine wave?
The ring-joint height is sinusoidal, but finite-length horns and pitch links introduce geometric nonlinearity.
Does the lag input affect blade pitch?
No. It shifts an illustrative angular marker. Aerodynamic response needs a separate rotor model.
Can I use these values to rig an aircraft?
No. This is a generic educational mechanism; use the applicable aircraft maintenance and flight documentation.
Reference
FAA Helicopter Flying Handbook, Chapter 4, page 4-7, figure 4-11, swashplate assembly photograph and description. The dimensions and exact pin-closure equations here are an independently specified educational geometry.
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