Maintenance Manual STEMME S10-VT
Date of Issue: Jan. 01, 1998
Page 3-24
Amendment Nr. 8
Date: Nov. 11, 1999
A4011121_B23.doc
Doc. No. A40-11-121
3.4.10 Propeller (Fig. 3.4.10.a/b/c)
The articulated propeller consists of a central part and two propeller blades hinged to this central unit. The
articulation axis is aligned so that the propeller blades are movable in the plane of propeller rotation. When
the propeller is not rotating, the blades fold inwards by means of springs. The central part of the propeller is
made of high tempered aluminium. The propeller blades consist of carbon, kevlar and glass composite.
During engine starting, the blades unfold automatically by centrifugal force. Soft rubber stops protect the
blades in case of a possible over-swing. The fully folded position of the propeller blades also has rubber
stops. The propeller blades may be retracted at any possible blade angle.
After engine shut-down in flight, the propeller has to be stopped by the propeller brake to allow the blades to
fold inwards. The brake is operated by the propeller brake handle on the instrument panel. In the folded
position the propeller can be positioned with the propeller positioning handle on the instrument panel to allow
the propeller dome, which forms the front fuselage, retract to the closed position. After closing the propeller
dome with the propeller dome handle in the centre part of the instrument panel, the propeller is completely
enclosed within the contours of the fuselage to achieve optimum soaring performance.
The propeller blade pitch can be changed from takeoff (fine pitch) to cruise (course pitch) position. The pitch
control is electrically actuated and operated by a switch on the centre console behind the throttle assy. The
takeoff position of the propeller blades is indicated by the green light next to the switch.
Design of Variable Pitch Propeller
The numerical positions in the following text refer to the propeller diagram (see figure 3.4.10 a/b).
The propeller blades (1) are hinged in a forked mounting plate (4). The complete assembly, consisting of the
fork, blade and hub, is rotated in order to set the pitch-angle of the blade. The hollow axle (3) houses a spiral
torsion spring (23) to fold the blade by means of a cam lever (22) that fits into the aperture for the buffer-stop
in the propeller blade. Electrically heated expanding servo-elements (15) actuate the blade pitch mechanism.
On achieving their activating temperature, these expanding servo elements drive a piston connected to the
propeller blades. All components of blade angle mechanism are double-redundant and mechanically
interconnected by a coupling ring (12) so that both propeller blades always have identical pitch.
Propeller blades
The propeller blades are manufactured from FRP (fibre reinforced plastic) material in a twin shell
construction. The shells are of hybrid laminate type (glass, carbon, kevlar). PU-tape is affixed to the leading
edges to improve protection against gravel.
Pressure balance is achieved by connecting all cavities in each blade. To balance the pressure in the blades
with the outside atmosphere there is a 1 mm borehole in the blade tips. These holes also drain condensed
moisture by centrifugal force.
Propeller Blade Angle Control Mechanism
When disconnected from the power supply (unheated), the dilation effect actuator (15) is pushed inwards via
the rocking lever (14) by the spring (20), thus moving the propeller blade into T/O position via the pushrod
(13), the synchronising ring (12) and the connector (11).
Heating of the servo-element by the heating element (16) generates piston pressure that pushes the swing-
arm/pushrod system into the almost stretched position, thus rotating the propeller blade against the spring
towards high pitch. With increasing RPM´s, the fly-weight (19) generates higher force in the same direction.
The fly-weight forces are never high enough to exert forces of the spring and the aerodynamic restoring
moment. With this ratio of forces it is assured, that, in case of heating element malfunction, propeller blade
position is automatically in T/O under all operating conditions.
The transmission of the actuating forces to the fork is effected by the drive pin (10) which is attached to the
fork with an eccentric bolt to allow for accurate setting of propeller blades. The available range of adjustment
of the servo-elements is 12.1 mm / 0.476 in. The stop setting for the T/O position is adjusted for the fully
retracted piston. The range of the blade angle change is set to 6°
24’, limited by a
stop switch. A mechanical
stop is adjusted so that only minimum override of the end position switch setting is possible (
6’). The piston
movement, caused by the two-point control system, result in minor changes in propeller blade pitch angle
which have no influence on the propeller behaviour.
