How Ornithopters Fly

1.1 Pictures of the flying EV7a

Inflating the airpring
Before the start the gas pressure spring is pumped up. By the pressure you may control the flapping time comparison between the upstroke and downstroke. At the EV-models mostly was aimed for ratio 1:1.

High start
With the EV7a ornithopter high starts with a rubber cord were carried out for the first time. A height of about 10 meters was enough. To do this the mechanism must be locked in glide position (please see Rack and pinion drive system).

While gliding the wing twisting was OK.

Twisting of the wing during the downstroke
On the inner arm wing the twisting is approximately correct. But from today's view wing twisting is inadequate near the wing tips. Refer to the following:

• - Report about The development of the EV- flapping wing models,  figure 26 (in German, PDF 1.4 MB)
• - Handbook, chapter 6.5 (without photos, in German, PDF 3.3 MB)

Twisting of the wing during the upstroke

Overshoot of the angle of attack
(Annotation like corresponded picture of EV6 )
At the end of the strokes in the hand wing the twisting increases too much. This is mainly caused by the inertia of the trailing edge when slowing down the stroke motion and when accelerating in the opposite direction.

When the wing comes to a standstill at the end of the stroke, in the interest of lift and thrust the angle of attack should be like gliding. But there are also small advantages to the overshooting (see handbook, chapter 6.8 and 6.9).

Long distance flight
This is one of the original photos whose display details with the model were composed to a strip. All display details was composed in correct positional arrangement by land marks.

Flight pictures in large size (136 KB)

1.2 Flight stability of ornithopters

With all EV-ornithopters the powered flights often had to be interrupted prematurely because the ornithopters lost their flight stability. Causes include the following:

• Loss of stabilizing force due to changes in the air speed or the angle of climb.
• Lacking practice und nervousness of the pilot.
• Visibility problems because far distance or large turning radius.

Probably the constantly changing longitudinal dihedral by the wing pitching and twisting has a part of this. In any case, in nearly all EV ornithopter models the horizontal stabilizers were too small after a new design was built.

The unstable forces of a flapping wing by variation of the speed could also duplicated by calculation (please look at the handbook, chapter 8.6).

The stability problems are apparent from the varying presentation of the fuselage in some of the following pictures of a circle flight.

In the flight tests I often have made the experience that the model changeover to descent with the increase of the flapping frequency. At that time, I had always only led this back to an insufficient wing twist and a stall in the outer wing area, especially during wing downstroke. This also can happen during wing upstroke. With a stall, thrust and lift are simultaneously reduced. But there are also other possible causes for this flight behaviour.

One of them is the shift of the centre of lift during wing upstroke. If during upstroke the lift near the wing tip is perhaps only slightly negative at first, it becomes even more negative when the stroke speed increases. Since the lift near the wing root remains nearly the same, this corresponds to a shift of lift towards the wing root.This increases the thrust, but reduces the lift. In this way, the model goes into descent flight, despite the increase in thrust. Only when the flapping frequency is further increased, it comes to a stall on upstroke.

The reduction in lift during upstroke due to the displacement of lift can be compensated by larger lift during downstroke or by height control. For this purpose, however, the wing profile must have sufficient lift reserves (e.g. by large wing depth). This was obviously not the case with my models.

In addition, when the flapping frequency is increased, the inclination of the lift force along the wing becomes greater. On upstroke it inclines more backwards and on downstroke more forwards. This increases the thrust, but reduces the lift. If the greater thrust is used to increase airspeed, lift can increase again. However, the increase in speed is not abrupt. It takes a certain amount of time, especially with heavy models. During this acceleration time the lift is still too small and the model loses height. To avoid this effect, the flapping frequency should only be increased in small steps or very slowly. However, corresponding tests were not carried out at that time.

Therefore, an increase in the flapping frequency does not guarantee a climb flight. Unfortunately, in my models also a reduction in the flapping frequency did not lead to more climb. The window of the optimal flapping frequency simply was too small. According to more recent findings, the EV7 model probably flew more in cruising flight than in distance flight mode. Thereby, the distribution of lift during upstroke is completely different than originally intended (please see the diagram of lift distributions on the site Gait change).

1.3 Videos of the EV7a (1990)

Scene 1:

It shows a short powered flight with a changing to gliding flight. Thereby, a rack-and-pinion drive system with an electronic switch was used.

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640 x 480 mp4 (0.8MB)

480 x 360 mp4 (0.5MB)

320 x 240 mp4 (0.2MB)

Scene 2:

In this powered flight the cyclical changes of the wing twisting is clearly to be seen.

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480 x 360 mp4 (0.3MB)

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Scene 3:

With a length of about one minute it shows one of the few longer flights.
In particular one can see:

• a high start to a height of about 10 meters,
• a changing from gliding flight to powered flight with a rack and pinion drive unit,
• the relatively slow stroke motion with a stroke cycle of about 1.1 s,
• nevertheless there is only to be seen a little bobbing up and down of the fuselage,
• a horizontal flight over a longer distance without upcurrent due to a slope or thermal and
• the result of the breakdown of lift and thrust due to stalling after a short climb. For details please look above to the chapter Flight stability of ornithopters.

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640 x 480 mp4 (1.3MB)

480 x 360 mp4 (0.7MB)

320 x 240 mp4 (0.4MB)

2.1 Three meter bird on approach

- an aero modellers highlight -

Following pictures show the EV7b model (with feathers at the wing tip) during approach. With a few wing strokes it overcomes large distances (in this case nine wing beats). The different twisting of the wing during up- and downstroke can be well detected.