Landing gear must absorb the energy of a landing, support the aircraft on rough ground and let it be steered, and for a simple aircraft that means fixed gear, either a taildragger with two main wheels forward and a tail wheel or skid, or a tricycle with a nose wheel. A taildragger is lighter and simpler but tends to swerve (ground loop) in a crosswind, while a tricycle is easier to handle and slightly heavier. The gear is a structure to be calculated like a spar, because a rough landing is the highest load many airframes ever meet.
Part of Aerodynamics and Flight
The wheels, legs and shock absorbers that carry an aircraft on the ground, and the handling behaviour each layout produces.
The Landing Is An Energy Problem
An aircraft touching down has vertical speed. Its kinetic energy, ½ × mass × speed², is absorbed by the tyre, the leg and its spring over a short travel. Worked example with illustrative inputs: 400 kg landing at a vertical speed of 2 m/s carries ½ × 400 × 2² = 800 J. If the gear compresses by 0.2 m, the average force is 800 J ÷ 0.2 m = 4,000 N, about the aircraft’s weight (3,924 N) again. That is a gentle landing; at 4 m/s the energy is four times larger, and the same travel gives 16,000 N. The numbers show why a hard arrival can break gear, and wing roots with it, though the real vertical speeds, travel and peak forces of your aircraft are for you to measure and calculate.
Layouts
- Taildragger: two main wheels ahead of the centre of gravity and a small tail wheel or skid. Lighter, with better propeller clearance, but the aircraft is unstable on the ground: a swerve builds on itself because the tail swings behind the centre of gravity. It also needs a nose-up landing attitude, which is closer to the stall.
- Tricycle: nose wheel plus two main wheels behind the centre of gravity. More stable, easier to taxi and to land, a little heavier, and the nose leg is a weak point on rough fields.
The carrier lists main gear legs of spring-steel flat bar or steel tube with rubber bungee, wheels of 30-40 cm diameter with solid rubber tyres as a fallback (Aerodynamics and Flight). Rubber needs checking for age and for fuel contact (Rubber and Polymers).
Method: Checking A Gear Design
Step 1: Estimate the landing energy from weight and a sink speed you can justify, and the force from the travel, as above.
Step 2: Check the legs in bending and the attachments in shear (Force Calculation). Side loads from a crosswind landing matter as much as vertical ones.
Step 3: Place the main wheels so that the aircraft will not tip onto its nose or tail on the ground. The track (width between mains) should be wide enough to resist tipping in a turn.
Step 4: Test the gear by dropping. Drop the loaded aircraft or a loaded gear leg from a small height onto the wheels and inspect.
Step 5: Choose the wheels. Wheel Hub Bearings covers bearings; wheel strength under landing and braking loads is a separate calculation.
Common Mistakes
- Stiff gear with no travel — peak force is energy over travel; no travel means a very large force.
- Narrow track — it invites a tip-over in a crosswind.
- Brakes grabbing — a taildragger braked hard can nose over.
- Not inspecting after a hard landing — a cracked leg or hidden fitting failure.
Safety
Ground loops and gear collapse
A swerve at landing speed can take a wing into the ground, and a gear that folds can break the fuselage or start a fire. Practise ground handling slowly, with an experienced person, before any fast taxi (Ground Testing).
What’s Next
- Learn next: Tail Assembly, stability behind the wing
- Combine with: Ground Testing, the first use of the gear