How Ornithopters Fly

Flagge flag vlag pavillon

Ball screw drive
of the ornithopter model EV8

1. Components


2. Driving mechanism

  1. The unit, consisting of the drive unit and the wing centre section (DW), is pivoted in the fuselage near the trailing edge of the wing, in such a way, that it can be turned upward for an additional angle of attack of the wing root (e.g. + 6 deg.).
  2. The roller moves up and down by the drive of the flapping wing.
  3. The lateral motion of the slotted link is used to control the additional angle of attack during wing upstroke. It must be converted into an up and down motion of the DW near the leading edge of the wing, either by a lever, a Bowden cable, or by a string.
  4. The lateral motion of the slotted link must be easy to move and cant-free in a guide which is fixed to the drive frame.
  5. The shape of the guideway corresponds approximately to the course of the angle of attack α in the article Lift during wing upstroke, versie 10.1, Fig. 21, PDF 1.0 MB.
  6. In the area of the stroke end positions, the lateral motion of the slotted link must be supported. When circulating the roller counterclockwise,
    • near the upper final roller position to the left,
      e.g. by a further slotted link with diagonally arranged guideway and a spring or by an angle lever-spring construction
    • near the lower final roller position to the right,
      e. g. by use of the second slotted link mentioned above with diagonally arranged guideway and a spring orby the steady spring tension of the following point 7.
  7. The DW should be pulled by a spring to the zero position of the angle of attack (against the lift force).

It can also be used to control other angles of attack directly on the wing from the drive.


3. Functional description of the drive

When using a ball screw drive the direction of the motor rotation must change with every reversal of the wing flap direction. At first, it seems to be a little unfavourable. But if the reversal of rotation is largely affected by mechanical forces from the outside, losses of the motor are kept within limits.

The advantage of this drive technology lies in a relatively high degree of efficiency in the center part of the stroke.

Also at the previous EV-mechanism the drive motor was not very effective in the range of the final stroke position (no-load operation).


3.1 Functional diagram

functional diagram with text
Functional diagram

At the EV8, the reversal of motion of the rotating drive- and wing masses is mainly affected by mechanical and aerodynamically forces:

  • in the lower final wing stroke position by the lift forces of the wing and
  • in the upper final wing stroke position by the so-called end position spring

While reversing, the electromechanical forces should support the reversal of motion only a little


3.2 Reversal of the stroke movement

3.2.1 Reversal in the lower final wing position
wing structure in cantilever 
upstroke twisting position (EV8b)
Wing structure in upstroke twisting position

As with the EV6 and EV7, the flapping wing spar construction is designed in a way that its internal load always attempts to take the upstroke twist position. For illustrating the internal initial load the wing framework is arranged in cantilever supine position in the marginal photo. This initial load is important for the reversal of the motion in the lower final stroke position. The reversal happens approximately like this: (Numbering, Please look at the functional diagram)

  1. The change-over of the motor happens when the wings reach their lower pre-final position during the downstroke. At first, due to the delay of time the speed controller works like a switch-off.

    The lift at the wing is directed towards the still running downstroke. Thereby, the moved masses become slower. The lift force and the wing twisting become decreased, too.

  2. If the downstroke comes to standstill in the lower final position, theoretically the wing twist equals the one in gliding flight. Also the lift force. Now it accelerates the wing upwards again.

    Approximately at the same time the electrical run-up of the motor in the new rotation direction is getting started. But it still works without loading to a large extent. Via the ball screw drive the acceleration of the motor is mainly affected by the wing forces.

  3. With increasing upstroke speed the lift forces continue to decrease. More and more the wing takes on the maximum upstroke twisting position. It is reached approximately in the lower pre-final position.

    There, the motor has reached the full rotation speed. With its electromechanical force it now takes over the drive for the next upstroke of the wing and the tensioning of the compensating spring.

For the whole procedure the high efficiency of the ball screw drive is very important when converting a linear motion into a rotating motion.

The reversal of motion work - also of the rotating drive components - is primarily affected by the wing lift force.

Among the rotating drive components are the ball screw drive, timing belt pulleys and the motor (rotor). Because of its high rotation speed the angular momentum of the motor is considerable.

Also, you can support the reversal motion of the wing in the lower final position by an end position spring as at the (rack and pinion drive system). This would be without importance for generating thrust, but unfavourable for the lift.

3.2.2 Reversal in the upper final wing position
steel spring
End position steel pressure spring

Instead of the wing lift a so-called end position spring made of steel takes over the positive and negative acceleration of the masses in this range. Else, the motion sequence works analogically to aforesaid.

In the area of the upper final wing position there even during the upstroke decreasing lift always works against the reversal of motion. Hence, the end position spring also must overcome this force. Accordingly strong is its dimension (spring force max. 976 N).

If the energy recirculation of the wing mass momentum via the ball screw drive and the speed control in the accumulator are effective, the end position spring can later be designed frailer.

The flow of energy of the oscillating flapping wing mass is specified in the handbook, chapter 5.3.


4. Test rig

opened test rig for the flapping wing drive
test rig for the flapping wing drive
Test rig for the
EV8 flapping wing drive

To test the drive functions of interacting electronics and mechanism there was built a test rig.

The pendulum hanging downwards has the same mass moment of inertia as the two flapping wing halves. The aerodynamically forces of up- and downstroke are roughly reproduced by adjustable dashpots.

But some questions remain open till the practical testing of power flight.