Several mechanisms provide variable / adjustable pitch to propellers / rotors.
In applications like unmanned drones, air-taxis, toys, personal flying devices, small airplanes, helicopters etc, the pitch of the propellers / rotors is critical, because it defines the power required at some operational conditions (like hovering and cruising) and because it limits the top speed.
In this Portable Flyer:
the propellers / rotors have fixed (constant) pitch.
The fixed-pitch propellers / rotors is a compromise for a flying device .
For vertical take off, vertical landing and hovering, a small pitch may offer the necessary thrust without consuming too much power, which in turn increases the hovering duration; however the small pitch limits the maximum cruising speed.
The blades move slightly outwards when the propeller spins faster.
Click here for slow motion.
A large pitch, on the other hand, may allow high cruising speeds, but requires too much power by the engine(s) during hovering, take off and landing.
For instance, with propellers having 43" diameter / 15" pitch and spinning at 3,500rpm, the total static thrust is more than 1000N (100Kg), while the required power by the engine(s) is about 30kW; however even with the propellers spinning at 4,500rpm, the maximum speed is limited at only 100Km/h.
In order the Portable Flyer to be able to cruise at 250Km/h, the required propeller pitch is nearly 40".
With 40" pitch, instead of 15" pitch, the power required by the engine(s) at hovering, take off and landing is overdoubled.
Each red wire-frame "forms" a tetrahedron.
Click here for slow motion.
Tetrahedron
Starting with a regular tetrahedron (Fig. 1), a wire-frame is formed following the four, from its six, edges (Fig. 2):
A characteristic of the tetrahedron is that each pair of "opposite" (non intersecting) edges are skew lines.
Looking from a viewpoint on the line connecting the midpoints of the AC and BD line segments, the angle between the edges AC and BD appears orthogonal (90 degrees, Fig. 3):
In Fig. 4 two collinear forces F and -F load the pair of skew edges / arms AC and BD of the wire frame of Figs. 2 and 3; the wire frame is considered inflexible.
With the edges / arms AB and CD of the wire frame being flexible, Fig. 5 shows the deformation of the wire-frame due to the forces F and -F of Fig. 4.
The previous applied to make a variable pitch (PatPitch) propeller:
wherein the centrifugal forces on the blades cause the deformation of the tetrahedron and vary the pitch.
With adjustable pitch propellers, the power demand for hovering / take off / landing is low, without limiting the top speed.
If the propeller pitch could be properly, and widely, adjusted to the operational conditions then, among others, the hovering duration, the top speed and the maximum take-off weight would substantially increase.
The hub 2 comprises a center 3, two pins 4 and two pairs of flexible "skew" arms 5.
Each blade 9 has a hole / bearing 10 through which it is mounted on its respective pin 4 of the hub 2; each blade 9 has a pair of supports (like 11, 12 and 13), through which it is held by a pair of flexible "skew" arms 5.
The hub also comprises two stops 7, one per blade 9; the stop 7, with its bolt 8 prevents the respective blade from going to a pitch below a minimum; the stop also allows the preloading of the flexible arms.
At operation a shaft (or a sprocket, or a gearwheel etc) forces the propeller 1 to spin about a rotation axis. The blades 9 supported on respective pins 4 of the hub 2 are forced to follow the rotation of the hub about the rotation axis.
The centrifugal forces on the blades push them away from the rotation axis of the propeller; however the hooks 6 at the ends of the flexible arms 5 hold the blades 9 from moving away the rotation axis of the propeller.
Until an angular speed, the preloading of the flexible arms in combination with the stops (7, 8) keep the blades at a minimum pitch.
Above an angular speed, the centrifugal force acting on each blade gets so strong that it pulls the blade slightly outwards (away from the propeller rotation axis), causing the bending of the flexible skew arms, which in turn causes the angular displacement of the blade about its long axis, which varies the pitch.
The higher the angular speed, the heavier the centrifugal force pulling each blade outwards, the larger the bending of the pair of flexible arms that hold the blade, the larger the angular displacement of the blade about its long axis (which, here, is the axis of the pin), and the bigger the pitch of the propeller.
Among the advantages of the PatPitch are the simple, lightweight, compact, robust and cheap structure of the hub / blades of "automatically variable pitch" propellers / rotors.
