A cardan gear mechanism is a way of converting rotary motion into straight line motion. She was invented in the 16th century by the Italian mathematician Girolamo Cardano.

A cardan gear mechanism with a crank pin especially consists of an internal gear and a planetary gear with a crank pin. The internal gear has a diameter exactly twice the size as the planetary gear. Every crank pin on the planetary gear pitch diameter moves on a straight line of an internal gear diameter.

Only the main wheel in the center of the unit is driven by the motor. The axle of the planetary gear is connected to this wheel.

To generate the flap moving of the wing only one crank pin is needed.

To actively control the twisting or pitching of a flapping wing two staggered crank pins are needed. The main crank pin (blue, next picture) generates the flapping and the phase displaced movement of the control crank pin (lilac) the twisting or pitching of the wing.

Here, in power flight the control crank pin always runs ahead of the main crank pin - like the leading edge of the wing compared to the main spare while flapping.

The vertical movement of both crank pins are transmitted by scotch yokes to the flapping wing (linkages please see the drive mechanism of the EV4).

In the gliding flight position both scotch yokes are in their stroke center and according to this also the flapping angle and the angle of setting of the wing.

When the drive unit in the gliding flight position is stopped in a crank dead point it is able to take any wing forces in vertical direction. Therefore a brake is not necessary.

There are three versions of the cardan gear mechanism for ornithotpers:

• Transition between gliding and power flight
  by reversing the motor
• Transition between gliding and power flight
  by actuating an interlock device with a servo
• Direct adjustment of the
  flapping angle with an auxiliary drive

A. Transition between gliding and power flight

by reversing the motor

With eatch reversal of the rotatoin the internal gear gets rotated by 90 degrees between two corresponding blocks. It can move freely between the stops.

Prerequisite for the switching and keeping of the power flight position of the internal gear is a continuous braking force on the main crank pin and on the scotch yoke, respectively.

The path line of the smaller control crank pin being slanted in gliding flight is good to bee seen by its scotch yoke.

Such a drive unit was used in the ornithopter models EV1 to EV4

B. Transition between gliding and power flight

by actuating an interlock device with a servo

The driving motor has only one direction of rotation.

The interlock device of the internal gear was actuated by a simple radio-controlled servo (please look at picture 15 of the report).

Only while switching between gliding and power flight a continuous braking force is needed on the main crank pin and on the scotch yoke, respectively. Subsequently the setting of the internal gear will be locked.

Otherwise as variation A above.

Such a drive unit was used in the EV5 and EV6.

C. Direct adjusting of the

flapping angle with an auxilliary drive

In addition to the main drive with only one rotation direction an auxiliary drive with a reversing rotation direction is needed.

The performance of the auxiliary drive depends on the occuring crank forces and the desired speed control. Designed for short term use is sufficient.

The flapping angle of the wings can be adjusted directly.

For the single steps the path line of the main crank pin is represented as a white dotted line.

In this case an internal-combustion engine was designed as the main drive and an electric motor as the auxiliary drive. This drive concept for ornithopters is very reliable, because the transition to gliding flight can also be used in case of a failure of the main drive.

Such a drive unit with changeable stroke has not been built yet.