ActiveBlade

Hover

In still air, every blade does the same work.

Alta X carries its load on four large, slow-turning props. A large prop moves more air more gently, which is why the aircraft can lift a cinema camera or a LiDAR and still fly for a long time.

The arrows show lift along each blade. In a hover a blade meets the air at the same speed all the way around, so the two sides match.

Forward flight

Fly forward and the two sides stop matching.

Airspeed adds to the blade on one side and subtracts from it on the other. The advancing blade sees rotation plus airspeed. The retreating blade sees rotation minus airspeed.

Lift goes with the square of that speed, so a modest difference in speed becomes a large difference in lift. Helicopter engineers call it dissymmetry of lift.

Rigid prop · 75 mph

A rigid prop turns that imbalance into a hammer.

Bolt the blades solidly to the motor and the strong side levers the weak side over. In our model of an Alta X at 75 mph, it is like dropping a 4.2 kg (9 lb) weight on the retreating edge of the disc: a peak hub moment of about 17 N·m.

The hub and motor shaft take that bending load. It peaks each time the blades line up across the airflow, then lets go.

Twice per revolution

4,000 hits a minute.

Two blades, so the weight lands twice every turn. At 2,000 RPM that is 4,000 hits a minute. Since you opened this page, one rotor would have taken 0.

This is not particular to one aircraft. Any drone flying forward on rigid two-blade props is hammered at twice its rotor speed: 2,000 to 20,000 times a minute across the usual range of prop speeds.

The hits go into motor bearings, booms, electronics and payload. Over enough hours, that cycling is what fatigues the system.

ActiveBlade · 1 of 4

The advancing blade makes more lift.

The blade swinging into the airflow sees rotation plus airspeed. It meets faster air than its partner swinging away, so it lifts harder.

Here the hub is still locked. Watch the orange arrows grow as each blade comes round the advancing side.

ActiveBlade · 2 of 4

The extra lift flaps it up.

Now the hub is released. The blade pair is free to rock on one shaft, so the harder-lifting advancing blade rises and the retreating blade sinks.

Nothing senses or commands this. The lift imbalance itself does the pushing.

ActiveBlade · 3 of 4

Flapping up takes pitch off.

The shaft is set 45° to the blades, so rising and twisting are one motion. Each degree the advancing blade flaps up removes a degree of its pitch. The retreating blade gains the same amount.

The white outlines are blade cross-sections. Follow one round and watch its nose drop as it rises.

ActiveBlade · 4 of 4

Lift largely evens out across the disc.

Less pitch on the fast side, more pitch on the slow side. The two halves of the disc now lift nearly the same, and the weight landing on the hub shrinks.

Freefly measures peak vibration at one fifth of the level without ActiveBlade.

Try it

Switch it off. Switch it on.

Switching by itself. Pick a side to take over.

Watch the weight, the lift arrows and the vibration trace. Turn on sound to hear the difference, or change the airspeed to see how the load scales.

Inside the hub

One pivot, 45° of delta-3.

The blade pair rocks on one steel shaft through the mast, set 45° to the blades. Rock the pair about a skewed axis and a blade cannot flap without also rolling about its own span.

The tangent of 45° is 1, so each degree a blade flaps up takes one degree of pitch off it and puts one degree on the opposite blade. It is all mechanical, and it acts inside every revolution.

Exploded view

The whole mechanism is a shaft and two bearings.

A steel shaft crosses the mast. Two flanged bearings ride on its ends, clamped between the upper and lower mounts that carry the blades. Two bumpers sit between the mast and the upper mount.

Each blade swings on its own pivot bolt between PTFE washers, so the pair folds for transport and lines itself up in flight.