Chapter 3: Landing Gear Kinematics & Door Sequencing

Created by Sarah Choi (prompt writer using ChatGPT)

Landing Gear Kinematics & Door Sequencing — Suspension, Steering & Locomotion Details (for Vehicle Concept Artists)

Why Landing Gear Kinematics Matter for Believability

Landing gear are short‑stroke, high‑energy suspensions that also fold into tight bays without cutting the vehicle’s structural rings. The choreography—leg motion, lock engagement, door timing—decides safety, drag, stealth, and maintenance cost. Concept‑side, clean kinematic reads make silhouettes feel airworthy and grounded. Production‑side, clear callouts for pivots, locks, door drives, and sensors let engineers translate sketches into mechanisms that pass loads, clear structure, and service easily.

The Anatomy of a Gear Leg

A typical strut combines: an oleo (gas over oil) to absorb impact; trunnions that attach to structure and define the retraction axis; drag/side braces that lock the leg in extension; torque links to prevent strut rotation and manage caster; a truck or fork that carries the wheel(s); axles and brakes; and uplock/downlock devices for stowed/extended retention. Around it: actuators for extend/retract; WOW (weight‑on‑wheels) switches, proximity sensors, and hydraulic/electric lines; and doors whose hinges and latches must preserve structural continuity when closed.

Kinematic Families: How Legs Fold

Landing gear fold by rotating about one or more axes to minimize bay volume while keeping wheels oriented for stowage.

Single‑axis trunnion fold rotates the entire leg inward or outward into a wheel well. It’s compact for nose gear and small mains.

Knee‑fold (articulated) legs add a “knee” joint to shorten stowage length, common on long main gear or tall vehicles. A side brace coordinates knee timing so the leg shortens as it retracts.

Truck‑tilt bogies (two‑, four‑, or six‑wheel) rotate the wheel truck relative to the strut so large wheels fit shallow bays and toe angles ground evenly at touchdown.

Rotary retraction with wheel yaw turns the wheel ninety degrees to lie flat; often used on space‑constrained nose gear. Include a centering cam that straightens the wheel before uplock engagement.

Telescoping legs shorten stroke height before folding, useful on VTOL or rover designs where long extension is needed on rough terrain.

For tracked or hybrid vehicles with deployable gear, a swing‑down skid or belly ski can share a trunnion with a smaller wheel leg, each with its own lock but a shared bay.

Structural Interfaces: Where Loads Go

Gear loads must pass into bulkheads, keel beams, and frames without severing torsion paths. Put trunnions in reinforced nodes (castings or built‑up ribs) that bridge left‑right sills. Drag braces should tie forward into a drag frame so landing loads loop into multiple members rather than a single web. Nose gear benefit from a steering bay beam linking the two sidewalls; mains prefer wheel‑well crossbeams and side‑of‑body ribs. Show doubler plates around pivot bores, generous fillets, and removable liners for wear surfaces.

Extension vs. Retraction Loads

Extension/landing demands high vertical energy absorption and anti‑shimmy control. Retraction demands compact packaging and positive uplock. Struts are designed for peak sink rates; valves control orifice flow to square the pulse. During retraction, hydraulic loads are modest but structural moments at the trunnion can be high due to wheel weight times lever arm—call out safety factors and actuator stall forces.

Steering, Centering, and Anti‑Shimmy

Nose gear typically steer via a rack‑and‑pinion or dual actuators on a torque link. A centering cam recenters the wheel as weight lifts off; once centered, a downlock holds geometry for retraction. Anti‑shimmy dampers (hydraulic or friction) suppress high‑frequency oscillations at rollout and taxi. For concept reads, show torque links forward on nose gear and aft on mains, with damper cylinders placed where they can be serviced without removing wheels.

Door Taxonomy: Types and Tradeoffs

No doors (open wells): simple, drag/stealth penalty, good for rugged or low‑speed craft.

Partial doors (truck doors): small flaps close the opening left by wheel/strut after retraction.

Clamshell doors: two halves meet along a seam; hinges on opposite sides. Great sealing, more actuators.

Plug‑type doors: close into a rabbet for flush outer skin and good load transfer; need robust latches.

Pantograph or link‑assisted doors: translate outward then rotate to clear thick skins or stealth faceting.

Door structure should be non‑structural when open, structural when closed—use stepped laps, shear ties, and latch pins that recreate the outer skin’s shear path. Show hinge lines parallel to major frames to ease sealing and avoid cutting ring paths.

Sequencing Logic: The Choreography

A typical sequence from flight to landing:

  1. Gear down command → doors unlock and open (if needed).
  2. Strut extends under hydraulic/electric power, assisted by gravity/over‑center locks.
  3. Downlock engages (mechanical over‑center with uplock hook released). WOW sensors arm ground systems.
  4. Doors close if they’re only for retraction clearance (truck doors remain open).

Retraction is the reverse with safeguards:

  1. Takeoff + WOW false → steering centers and locks, brakes release from autobrake/spin‑down.
  2. Doors open, uplock ready.
  3. Strut retracts, truck tilts if required, uplock engages.
  4. Doors close and latch with proximity sensors confirming. Interlocks prevent door closure on a partially extended leg.

Call out sensors (prox, WOW), locks (uplock hook, downlock over‑center), latches, and control valves. Provide a one‑line emergency free‑fall sequence (bypass valves, uplock release, doors pinned open).

Hydraulics vs. Electromechanical Actuation

Hydraulics deliver high power density and graceful compliance; accumulators enable emergency extension. Electromechanical actuators simplify plumbing and leak risks but require brake/spin‑down management to avoid back‑driving. For production notes, state actuation loads, stroke, and end‑stop damping. For concept reads, place reservoirs and pumps outside pressure cabins and route lines through grommeted, strain‑relieved pass‑throughs.

Tires, Brakes, and Spin‑Down

A retracted spinning wheel can shred bays. Use snub brakes on takeoff to stop rotation before uplock, or air scoops/brushes at door lips to windmill down. Hot brakes need thermal standoff, reflective shields, and cooling scoops. In callouts, show temperature limits for nearby composites, vent paths for brake dust, and heat sensors that inhibit retraction when thresholds are exceeded.

Anti‑Skid, WOW, and Interlocks

Anti‑skid modulates brake pressure to stay near peak μ; tie logic to WOW so braking authority transitions smoothly at touchdown and liftoff. Interlocks inhibit retraction on WOW true, unlock steering only on WOW true and low speed, and block door closure unless uplock proximity is good. Place sensors where they avoid tire spray and FOD.

Bay Design: Volume, Cooling, and Cleanliness

Bays must house wheels, strut, actuators, and doors with clearance envelopes for bump, steering, and truck tilt. Add liners for FOD protection, drains at low points, and venting to purge ozone and brake gases. For stealth or high‑speed craft, inner faces should continue shear paths and include acoustic damping. Show removable panels for actuator access that don’t sever ring beams.

Rough‑Field and VTOL Considerations

Bush strips and unprepared fields need large‑diameter, low‑pressure tires, long stroke, and generous fork clearance for debris. Add mud guards and stone deflectors to protect doors and bay lips. VTOL/tiltrotor craft require tall gear for rotor/duct clearance; consider kneeling functions for loading. For amphibious concepts, add spray rails and waterproof seals on doors.

Maintenance Logic and Modularity

Design wheel change access with jack points on structures near trunnions. Make brake packs removable without disconnecting lines (quick disconnects with drip trays). Use captive fasteners on doors and hinge pins that pull outward for service. Provide rigging points for lock adjustment and gauge holes for alignment. In drawings, include a mini service storyboard: pin doors, pull uplock safety, depressurize, swap, re‑rig, leak‑check.

Failure Modes & Safeguards

Common issues: shimmy from worn joints or soft tires; door flutter from weak latches; uplock hang‑ups from mis‑rigged hooks; over‑center lock incomplete due to tolerance stack; hot brake retraction damaging composites; seal water traps leading to corrosion. Safeguards: dual sensors, redundant actuators or manual release, door gust locks, downlock springs, and spin‑down inhibit based on temp/speed.

Visual Language for Artists

Use transparent cutaways to show pivot axes and lock geometry. Draw envelope sweeps (dashed) for wheels through retraction. Place sequence frames along the page edge (1–4) with arrows. Give hinge and latch callouts identical in style to structural fasteners. Indicate structural rings around the bay with a soft color underlay so reviewers see what the doors must restore when closed.

Concept‑to‑Production Handshake

Close with numbers: sink rate (m/s) and design touchdown load factor, strut stroke and gas pre‑charge, actuator forces, door open/close time, clearances to bay walls, tire/wheel sizes, brake energy per landing, steering angles, and truck tilt degrees. Provide tolerance stacks for locks and door latches, sensor locations, and emergency extension steps. These convert art into a credible test plan.

Case Studies in a Paragraph

A tiltrotor’s tall nose gear folds forward about a transverse trunnion; a centering cam locks the fork before retraction, and a plug door closes into a stepped lap restoring the stealth skin. Main gear are knee‑fold bogies that tilt the truck 12° for stowage, with clamshell doors that open before motion and close after uplock. A bush‑plane concept uses fixed main gear with fairing doors only on the nose to protect avionics; anti‑shimmy and large tundra tires allow soft‑field ops. A sci‑fi rover deploys telescoping legs with skids for uneven terrain; doors are pantograph types that move outward then rotate, avoiding thick thermal tiles and preserving the outer shear path when latched.

Final Encouragement

Think of landing gear as a compact ballet under load. When your pages show where loads travel, how locks engage, and how doors restore the skin, your vehicles feel inevitable. Draw the choreography—kinematics first, sequencing second—and the structure will make sense to both art directors and engineers.