AERODYNAMICALLY-INDUCED WING-FLAPPING FOR THE PROPULSION OF ORNITHOPTERS

Abstract: Ornithopters are, by definition, flying machines which imitate natural avians in flapping their wings to generate the requisite thrust and lift necessary for their flight. Most exist­ ing designs of ornithopters employ direct mechanical means in one form or another to flap their wings. Such arrangements place a high demand on the pilot/engine in view of the large inertia of the virtual air mass set in motion by the flapping wings. In one exceptional design, recorded in French Patent No 1 228 320, an outboard segment of the wing is induced into a flapping mode by a reciprocating forward-rearward sweeping motion of the whole wing. Along these lines, a greater reduction in the power require­ ment can be achieved by essentially reversing the order of priorities, i.e. flapping of the whole wing is sought and effected through the action of a reciprocating sweeping motion imparted only to the outboard segment of the wing. Observation of birds in flight lend support to this view.

Description: Aeorodynamically-induced wing-flapping for the propulsion of ornithopters This invention relates to an ornithopter employing aerodynamicallyinduced wing-flapping as a means to reduce the input power required for its propulsion. Pilots readily recognize that an aircraft undergoing a yawing motion develops roll. The reason is that the advancing wing generates a greater lift than does the receding wing. Thus, it can be argued that by imparting a reciprocating yawing motion to a wing, appropriately restrained in the flapping direction by spring action, it would be possible to excite said wing into a flapping mode of oscillation. To some extent this principle is employed in the design of M. Lanoe -French Patent No 1 228 320 (Cl. Int. B 64c) published August 29, 1960- where an outboard segment of the wing is excited into a flapping mode as the whole wing is subjected to a reciprocating yawing motion. Observation of birds in flight, however, indicate strongly that the emphasis should be reversed, i.e. the excitor should be the outboard wing segment while the excited should be the whole wing. The intent of this invention is to employ the increase in the lift force generated on the outboard wing segment, when the latter is yawed forward, to lever the whole wing upwards away from its spring-restrained initial position. The downstroke is achieved by a reverse torque which develops as the impulsive lift force on the outboard wing segment diminishes when the latter's forward sweeping motion is slowed down or halted. The primary objective of this invention is to reduce the required input power as a result of the smaller inertia of the outboard wing segment being actuated as compared with that when the whole wing is actuated. Another objective is to reduce the excursions of the overall lift developped on the wing since the inboard segment of the wing does not recede. A further objective is that the spring restraining the wing can be conveniently located out of the air stream in a streamlined auxilliary strut, or even inside the fuselage, to reduce aerodynamic drag. A still further objective is to provide adequate and effective means for controlling the ornithopter. One way of carrying out the invention is described in detail below with reference to drawings which illustrate embodiments of the invention, Figure 1 is a front elevation of one embodiment, Figure 2 is a section of the line A-A and line B-B of Figure 1, Figure 3 is a perspective view of the infra structure of the port wing segments and assembly thereof to each other and to fuselage, Figure 4 is a section view of the restraining-spring-pack unit showing a cluster arrangement of constant force springs, Figure 5 is a perspective view of the general arrangement of the input-power/control stick and its attachment to cables transmitting the input power and control functions to the outboard wing segments Figure 6 is an elevation view of another embodiment having restraining- spring-pack unit and shock-absorber combination located inside fuselage. The ornithopter illustrated has each of its wings (1) made-up in two segments (2J & (j) connected by a yawing hinge (4) whose axis is generally perpendicular to the plane of the inboard wing segment (2). The outboard wing segment (3) can be yawed relative to the inboard wing segment (2) about the axis of hinge (ii). The inboard wing (2) is connected to the fuselage (5) via a flapping hinge (6) whose axis lies generally parallel to the longitudinal axis of the fuselage (5). The inboard wing segment (2) is spring-restrained in the flapping direction by a cable (7) and spring (8) located in an auxilliary streamlined strut (9). An idler pulley (10) is provided to allow the restraining cable (7) to operate freely. Forward or rearward yawing movement of the outboard wing (3) relative to the inboard wing (2) is effected through the arrangement of sprocket wheel (11), chain (12), cable (1it, idler pulleys (14) and input-power/control stick (15) by the action of pulling back or pushing forward the stick (15), respectively. A spring (16) keeps the chain (12) taught at all times and a guide (17) retains the stick (15) in proper alignment with cable (13) and idler pulleys (14). The restraining-spring-pack (8) is made-up of a number of constant load springs (18) clustered together to guard against catastrophic failure that might be occasioned by the use of a single spring. The constant load springs (18) offer the desirable feature of compactness, a nearly constant step load and a large working deflection. in operational use the ornithopter is foot launched, towed or driven via pedalled or powered wheels (19) into the oncoming wind. When a suitable speed is reached the stick (15) is pulled back swiftly. This action causes the outboard wing segment (3) to be flicked forward in a yawing motion generating impulsive lift in the process. If the resulting impulsive torque is sufficiently high it would force the whole wing upwards against the pull of the restraining cable (7) and spring (8). As the motion of the stick (15) is slowed down, towards the end of the stroke, the excess lift is lost and the process reverses, i.e. the wing is pulled downwards by the spring (8). At the bottom of the downstroke the outboard wing (j) is swept rearwards, by pushing the stick (15) forward, to generate additional thrust and to prepare the wing for its next beat. Control of the ornithopter is available through the stick (15) -longitudinally by a symmetrical forward/rearward movement of the stick and laterally by an antisymmetrical, yawing, movement of the stick (15) as indicated by the broad arrows in Figure 5. In accordance with another embodiment the restraining-spring-pack unit (8) is located inside the fuselage (5) with the optional single wheel (19) partially concealed in said fuselage to reduce aerodynamic drag. Buffeting, if a problem, may be damped by the installation of a shock absorber unit (20).

Claims: Claims: 1. An ornithopter characterised b aerodynamically-induced wing-flapping as a means to reduce the input power required for its propulsion. 2. An ornithopter according to Claim 1 comprising at least one pair of wings (1), a restraining-spring (8)-and-cable (7) arrangement located in the fuselage (5) or in an auxilliary streamlined strut (9). 3. An ornithopter according to Claims I L 2 wherein each wing (1) is made-up in two segments as follows: an outboard wing segment (3) connected at its inboard extremity to the inboard segment (2) by means of a yawing hinge (4) whose axis lies generally perpendicular to the plane of the inboard wing segment (2), said inboard wing segment is connected to the fuselage (5) by means of a flapping hinge (6) whose axis lies generally parallel to the longitudinal axis of the fuselage (5). 4. An ornithopter according to Claims 1 & 2 wherein the restraining spring (8) is made up of a number of constant force springs (18) clustered together delivering a net force to each wing (1) commensurate with keeping the wing at its lowest position when no yawing of the outboard wing (s) is employed. 5. An ornithopter according to Claims 1, 2 & 3 wherein yawing of the outer wing segment (s) relative to the inner wing segment (2) is effected through an arrangement of sprocket wheel (11), chain (12), cable (13), idler pulleys (14) and stick (15).

Inventors: ATRAGHJI, EDWARD
Applicants: ATRAGHJI, EDWARD
Application year: 1983
Main classification: B64C33
IPC codes: B64C33