When the engine is started, centrifugal forces open the propeller at once and propeller is immediately operational. Without any other additional necessary activity in the propeller control for the pilot in the cabin. AM-FF( )-( )/( )-( ) type propeller hub section mechanism design follows good and proven solutions of forward-folding, two-bladed propeller AM-F3-1A/140-( ) of the Carat touring motorglider.
AM-F3-1A/140-( ) is the forward-folding, two-bladed propeller with fixed pitch and with the diameter 140 cm. Its hub section mechanism design is a good and proven solution of forward-folding. It has been developed for the powered glider Carat with a tractor mounted engine in order to reduce drag when the powerplant is switched off.
With further development ground different adjustable propellers, for engines with powers between 40 hp and 100 hp and with propeller diameters from 100 cm to 180 cm, were added.
At ground adjustable forward-folding propeller systems blades have aluminium alloy holders for fixating the propeller blades where the blades angle is adjustable on the ground, between pre-set limits of fine and coarse pitch.
The propeller has an aluminium alloy hub containing the specially coated blade journal bearings. Spring retainers and the spinner mounting plate are also attached to the hub. The spinner cover is screwed to the spinner mounting plate.
Folding shafts which are attached to the blade roots and on which the outer pinion wheels are mounted, are bedded in the blade journal bearings. The gas-loaded springs engage into the pinion wheels pushing the blades forward and thus folding the propeller when it feathers at an appropriately low airspeed. The propeller unfolds using the centrifugal force of the blades which, even at the lowest rpm, is strong enough to compress the springs.
The pinion wheels are designed to fit tightly into each other with virtually no play when the propeller is folded but to have much play when unfolded so that, when in operation, each blade can compensate centrifugal force, thrust and spring tension appropriately and individually. No bending moment is transferred from the blades to the hub thus protecting the crank-shaft of the engine which, in turn, results in smoother running of the engine.
The pinion wheels ensure that the blades unfold / fold together. Unintentional unfolding due to oblique airflow or mass forces during recovery manoeuvres, for example, is thus impossible.
The blades are glass-fibre and carbon-fibre with a hard foam core. The propeller PU varnish with fine finish and the polyurethane leading edge protection tape protect the blades from the effects of weathering, particularly that of rain. This construction is a light-weight allowing constantly high efficiency at a low noise level.