Flapping Ornithopters

HOW ORNITHOPTERS FLY

A View on Flapping Wings

MOKHTAR ELNOMROSSY

Albert Einstein

“We call theory when we know much about something but nothing work, and practice when everything work but nobody knows why”

With this in mind,It seems to be inappropriate to discuss theory.Thus, we will just observe and analyze flapping flight

How Ornithopters Fly

Definition

Ornithopter is an aircraft heavier than air, that flies like a bird by flapping its wing

Leonardo Da Vinci: ornitotero

Flapping wings generate bothLift

andThrust



Operating principle of the flapping wing

On a stretched wing, lift is generated similar to an inflexible airfoil flown against from the front.

But at the upstroke, the airflow hits the wing rather from above and at downstroke rather from bottom.

These modifications are small in the area of the wing roots and get bigger towards the wing tip.

With permanently changing twisting, the flapping wing must adapt to these alternating incoming flow directions. Nevertheless, the lift distribution is modified along the span.


Operating principle of the flapping wing

At the wing downstroke the lift distribution is bigger altogether than when gliding and more shifted towards the wing tip. Thus, thrust is generated along the whole wing span during stroke motion. This works similar to a propeller blade with a very large pitch – only that the propeller torque force that has to be overcome, is here called lift.

At the wing upstroke circumstances are reversed. Overall, the lift distribution is smaller and more shifted towards the wing root. Moving in the direction of the lift force, the flapping wing now acts as a wind turbine blade. If the lift force is big enough, it presses the wing upwards even without a mechanical drive. Thereby, the wing operates with the operating drag of a wind turbine against the flight direction.

In addition to the forward motion

1. Flapping: motion at shoulder point2. Pulling: dragging of the outboard wing section during upstroke by the inboard wing

section3. Inclination: determined by the inclination of the stroke axis4. Twisting: increases toward wing tip mostly during upstroke in the direction of a positive

angle of incidence and during the downstroke in the direction of a negative angle of incidence

5. Turning: (of the wing root) especially when flying with thrust and mostly in the direction of wing twisting

6. Sweep: of the outboard wing section during upstroke with the backward motion of the wing tip

How Ornithopters Fly Basic motion components of a bird wing

Basic motion components of a bird wing

The flapping motion of the wing is absolutely necessary for thrust generation. In general, also the wing twisting is necessary for aerodynamic reasons.

In contrast, the turning and sweeping of the wing, as well as the pulling of the outboard wing section serve only to increase efficiency.


How Ornithopters Fly Wing axes of a bird

How Ornithopters Fly Gait change of ornithopters

Methods of wing twisting:

a) Passive wing twisting

b) Active wing twisting

c) Aeroelastically wing twisting

d) Aeroelastically controlled wing twisting

e) Controllable wing twisting- radio controlled- automatically controlled


How Ornithopters Fly Wing twist during stroke motion


Requirements

To build a man made bird modeled on a specific bird that could takeoff, fly and land by means of flapping wings alone.

This means excellent aerodynamics, high power density for propulsion and lift, and maximum agility.


Objective

To achieve an overall structure that is efficient in terms of resource and energy consumption, with minimal overall weight, in conjunction with functional integration of propulsion and lift in the wings and a flight control unit in the torso and tail regions


1. Active articulated torsional drive (for the two principal motions)

First, the wings beat up and down, whereby a lever mechanism causes the degree of deflection to increase from torso to the wing tip.

Second,the wing twists in such a way that its leading edge is directed upwards during the upward stroke(positive angle of incidence). If the rotation were solely due to wing’s elasticity passive torsion would result. If the sequencing of the torsion and its magnitude are controlled by an actuator, the wing’s torsion is active.

Design Stages


2. The Wing: Lift and propulsion in birds

The wing should consist of two-part arm wing spar with an axle bearing located on the torso, a trapezoidal joint, and a hand wing spar.

The arm wing generates lift and the hand wing beyond the trapezoidal joint provides propulsion.

Both spars of the inner and outer wing are torsional resistant. The active torsion is achieved by servomotor at the end of the outer wing which twists the wing against the spar via the outmost rib of the wing.

Design Stages


3. Partially linear kinematics for optimal thrust

When the bird lifts its wings, the servomotor for active torsion twists the tips of the hand wings to a positive angle of attack, which is then changed to a negative angle a fraction of a wing beat period.

The angle of torsion remains constant between these phases.

Due to this sequence of motions, the airflow along the wing profile can be optimally used to generate thrust.

Design Stages


4. The torso (the fuselage)

The battery, engine, transmission mechanism, and the control and regulation electronics are housed in the bird torso.

By means of a two-stage helical transmission, the exterior rotor motor causes the wings to beat up and down with the required reduction ratio. This motor is fitted sensors that precisely registers the wing’s position.

Both flapping and bending forces are conveyed from the transmission to the hand wing via flexible links.

To make the bird agile and maneuverable, the opposing motion of the head and torso section is synchronized by means of a system of cables and servomotors. Thus the torso bends aerodynamically with simultaneous weight displacement.

Design Stages


5. The tail section an aid for lift and control

The tail of the bird also produces lift; it functions as both pitch elevator and a yaw rudder.

When the bird flies in a straight line, the V-position of its two flapping wings stabilizes it in a similar way as a conventional vertical stabilizer(Fin) of an aircraft.

To initiate turn to the left or to the right, the tail is tilted; when it is rotated about the longitudinal axis, a yawing moment about the vertical axis is produced.

Design Stages


6. Control and regulation

The on-board electronics allow precise and thus efficient control of wing torsion as a function of wing position.

For this purpose, a powerful microcontroller calculates the optimal setting of the two servomotors which adjust the torsion of each wing.

The flapping motion and torsion are synchronized by three sensors, which determine the absolute position of the motor for the flapping motion.

Since the active joint torsion drive requires precise coordination between the flapping and twisting motions, it is subjected to continuous all round monitoring.

Design Stages


THANK YOU

Flapping Ornithopters

Technology

Transcript of Flapping Ornithopters