landing-gear-design

name	landing-gear-design
description	Aircraft landing gear design — tricycle vs. tailwheel, load cases (FAR/CS-25), oleo strut sizing, brake energy, shimmy, retraction kinematics, tire selection, weight estimation.
metadata	{"priority":7,"promptSignals":{"phrases":["landing gear design","landing gear load","oleo strut","nose gear","main gear","tire aircraft","landing gear retraction"],"minScore":3}}

Aircraft Landing Gear Design — Complete Skill

Gear Configuration

Tricycle (Nose Wheel)

Most common: 2 main gear + 1 nose gear CG location requirement: CG aft of main gear contact point (prevents nose-first tip) Typical: CG at 10–20% ahead of main gear; nose gear 10–15% of aircraft weight

Advantages: ground stability (directional); pilot forward visibility; anti-ground-loop Disadvantage: nose gear drag; weight; nose gear structural loads on rotation

Tailwheel (Conventional)

CG behind main gear → tendency to ground loop Lighter; lower drag; for propeller clearance on tailwheel aircraft Rarely used in modern aircraft (aerobatics, bush planes)

Load Cases (FAR/CS-25 Simplified)

Level landing (FAR 25.473): Vertical load per main gear: V_mg = n_mg × W_max / 2 n_mg = design load factor: n = 2.0–3.0 (limit); n × 1.5 = ultimate

Spin-up and spring-back (landing): Horizontal friction load: F_spin = μ × V_mg (μ = 0.8 braking; 0.5 static) Dynamic amplification for oleo: apply dynamic factor C_d = 1.5–2.0 for spin-up

Three-point landing (tailwheel): Main gear + tailwheel simultaneous; different load distribution

Braked roll: Brake torque reaction; gear leg in bending + torsion

Turning: Side loads from taxiway turns; steering moments

Limit side load (FAR 25.485): 0.8W (static); dynamic side load ground turn

Oleo Pneumatic Strut Design

Oleo strut = pneumatic spring + hydraulic damper in one unit

Strut sizing (load capacity): F_max = P_air × A_piston + F_hydraulic A_piston = F_max / (n × P_max_air) P_max_air: typically 1000–3000 psi (7–21 MPa)

Energy absorption: KE = ½ m_sink × V_sink² + W × S_stroke [J; S_stroke = oleo stroke length; W = weight on gear] E_stroke = F_avg × S_stroke = η × P_max × A × S_stroke η = load-deflection efficiency (0.50–0.65 for oleo)

Required S_stroke = KE / (η × P_max × A) V_sink = 3.05 m/s (10 fps) → certification design sink rate V_sink,reserve = 3.66 m/s (12 fps) → reserve energy case

Strut dimensions (typical main gear, medium aircraft, MTOW 50,000 kg): A_piston ≈ 200–400 cm²; S_stroke = 250–500 mm; extension stroke ≈ 400–600 mm

Brake System (Energy)

Kinetic energy at max braking: KE = ½ W V² / g (stopping from V_ref = V_LOF on rejected take-off) V_LOF = lift-off speed (typically 1.1–1.2 V_stall)

Brake energy per wheel: E_brake = KE / N_braked_wheels

Brake heating: ΔT_rotor = E_brake / (m_rotor × c_p) Must not exceed fusible plug melt temperature (≈ 180°C) or excessive brake heat criteria Carbon brakes: c_p ≈ 720 J/kg·K; steel: c_p ≈ 460 J/kg·K

Brake assemblies: hydraulically actuated pucks on carbon-carbon rotors (modern commercial) Anti-skid: maintains max μ_tire at all speeds

Shimmy (Nose Gear Stability)

Shimmy: self-excited lateral oscillation of nose gear (typically 10–30 Hz)

Cause: pneumatic trail unstable coupling between tire yaw and lateral forces Prevention: Shimmy damper: hydraulic or elastomeric; minimum damping coefficient c_min Geometry: forward trail distance (stability if trail > 0) Tire pressure: affects pneumatic trail and stability; proper inflation critical

Dynamic criterion: gear natural frequency must be decoupled from tire-road coupling frequency

Retraction Kinematics

Requirements: All loads (limit) must be sustained in extended AND retracted positions (locked) Retraction mechanism must work from 0 to limit load (partial retraction under load) Manual extension: backup system; gravity or emergency extension

Four-bar linkage or trunnion pivot: typical for single-pivot retraction Kinematics: check clear of fuselage/wing structure in all positions; door timing

Tire Selection

Tire loads: rated load per tire from gear load distribution Ply rating: pressure × ply rating → strength limit Bottoming clearance: deflected radius at maximum load must clear structure

Tire specifications (TSO-C62 / EUROCAE ED-79): Size format: 40 × 14.0-14 (OD × width - rim diameter in inches) Speed rating: must exceed V_max_ground + margin Load rating: rated load > max dynamic load with factor of safety

Weight Estimation

Total gear weight ≈ 4–5% of MTOW (tricycle, commercial transport) Main gear: ~60% of gear weight; nose gear: ~15%; structure/actuation: ~25%

Output

Provide: gear configuration, main and nose gear loads at limit [kN], oleo strut A_piston [cm²] and S_stroke [mm], brake energy per wheel [MJ], rotor temperature estimate [°C], tire size, retraction torque requirement [N·m], total gear weight estimate [kg].