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How Ornithopters Fly

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Ornithoptermodel EV7

1. Ornithopter model EV7a

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1.1 Pictures of the flying EV7a

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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.

wing downstroke
wing downstroke
wing upstroke
wing upstroke
upper final wing stroke position
upper final wing stroke position
lower final wing stroke position
lower final wing stroke position
gliding flight
gliding flight
middle of wing downstroke
middle of
downstroke
in banking flight
in banking flight
unstable flapping flight
unstable
flapping flight
unstable flapping flight on downstroke
unstable flapping flight
gliding flight
gliding flight
unstable flapping flight on upstroke
unstable
flapping flight
in the middle of upstroke
middle of
upstroke

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 lead this back to an insufficient wing twist and a stall in the outer wing area, especially during wing downstroke. Also this can happen during wing upstroke. In a stall, thrust and lift are simultaneously reduced.

Furthermore, however, also a shift of the lift on upstroke is considered as a cause for the descent. If on upstroke the lift near the wing tip is still positive, it becomes smaller and then negative when the flapping speed increases. If nothing changes at the wing root, in this way the center of lift is displaced to the wing root. This indeed increases the thrust, but necessarily reduces the lift. This can be well imagined also without calculation. In this way, the model goes into descent despite the increased thrust. Only when the flapping frequency is increased further, the airflow brakes during upstroke.

The lift reduction during upstroke, as a result of a displacement of lift in the direction of the wing root, is unavoidable without countermeasures. However, it can be counterbalanced by higher lift during downstroke or by the elevator. For this purpose, however, the wing profile must have enough lift reserves (e.g. by large wing chord). In my models this was obviously not the case. There has also not been a sufficient lift increase due to a possible increase in the flight velocity.

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 and was only for the level flight.

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1.3 Videos of the EV7a (1990)

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2. EV7b with primary feathers

Flight in dawn
The pixel-aligned adjustment of the picture series only can be made by using landmarks. Therefore the last two pictures of the flapping wing vehicle with feathers are only approximately correct.

Sequence of a flapping flight
Sequence in large size (321 KB)

Fly-by of an ornithopter
These pictures of a photo series of an ornithopter in flight are composed in an arbitrary manner.

some flight pictures of the wing-beat cycle
Sequence in large size (335 KB)
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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.

wing upstroke far away
wing upstroke
wing upstroke far
wing upstroke
wing downstroke in distance
wing downstroke
wing upstroke more nearly
wing upstroke
wing downstroke more near
wing downstroke
wing upstroke nearly
wing upstroke
wing upstroke nearby
wing upstroke
wing downstroke nearby
wing downstroke
wing downstroke close
wing downstroke
wing upstroke close
wing upstroke
wing downstroke from below
wing downstroke
wing upstroke frome below
wing upstroke
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3. Ornithopter model EV7c