Retractable Landing Gear

Justify the mechanism before building it.

1. Mount a model fuselage in the wind tunnel on the drag balance and measure drag with no gear fitted.
2. Fit representative fixed gear — two struts, two wheels, a cross brace — and measure again.
3. Now fit spatted gear, with streamlined fairings over the wheels, and measure a third time.

Bare struts and wheels add a startling amount of drag; fairings recover a good part of it but never all. A wheel is a bluff body, and a bluff body in an airstream leaves a wide turbulent wake that costs far more than its frontal area suggests.

This linkage is the reason retractable gear is trustworthy, and it uses no friction, no catch and no power.

1. Make a two-part drag strut: two links joined by a central pivot, the upper anchored to the structure and the lower to the leg.
2. Arrange it so that when the gear is down, the central pivot travels slightly PAST the straight line between the two end pivots.
3. Fit a light spring that pulls the linkage toward that over-centre position.
4. Extend the gear and then try to fold it by pushing up on the wheel.

It will not fold, and the harder you push the more firmly it locks. Once the centre pivot is past the line of the end pivots, a compressive load on the leg produces a moment that drives the knee further over-centre rather than collapsing it. The load holds the lock closed.

Notice what this means: the locking force comes from the aircraft’s own weight, not from an actuator, a spring or a latch. The hydraulic system can fail entirely and the gear stays down. Only a deliberate force applied to break the knee back over centre will unlock it.

The wheel has to end up somewhere, and where it ends up drives the whole aircraft layout.

1. Mock up the wing or fuselage bay with the leg attached at its pivot.
2. Swing the leg through its full travel and trace the path the wheel sweeps.
3. Check for interference with spar, skin and structure at every point in the arc — not just the endpoints.
4. Adjust pivot position and leg geometry until the swept path fits.

Checking only the extended and retracted positions is the classic error: the leg can pass through structure at some intermediate angle and be perfectly clear at both ends. Trace the whole arc.

Assume the power system is dead, and make the gear come down anyway.

1. Add a release that disconnects the actuator from the linkage.
2. Arrange the geometry so that with the actuator released, gravity alone swings the leg down.
3. Confirm the over-centre lock engages under gravity, with a spring assisting if needed.
4. Test it repeatedly with the actuator entirely removed.

Free-fall extension is a design requirement, not a nicety, and it constrains the geometry from the beginning. The leg must fold in a direction where gravity helps it extend — which for a wing-mounted leg usually means retracting inboard or rearward, so that dropping it also swings it down and locked.

Aircraft that retract FORWARD exploit this further: with the actuator released, airflow pushes the leg back and down and helps drive it over centre. The airstream becomes the backup actuator.

The indicator must answer the question the pilot is actually asking.

1. Fit a microswitch triggered by the ACTUATOR reaching the end of its travel, and wire an indicator to it.
2. Now fit a second microswitch triggered by the over-centre LINKAGE reaching its locked position.
3. Deliberately jam the linkage just short of over-centre and operate the mechanism.
4. Watch both indicators.

The actuator-driven indicator reports gear down; the lock-driven indicator correctly reports that it is not locked. The first one is measuring whether the command was executed. The second measures whether the gear will hold. Those are different questions and only the second one keeps the aeroplane off its belly.

This is why real aircraft sense the downlock itself, usually with three independent switches, and why the classic three green lights mean three legs individually locked rather than one actuator finished.