Titelbild

How Ornithopters Fly

Flagge flag vlag pavillon

Ornithoptermodel EV7

1. Ornithopter model EV7a

key

1.1 Pictures of the flying EV7a

key

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 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).

key

1.3 Videos of the EV7a (1990)

key

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)
key

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
key

3. Ornithopter model EV7c