Ornicopter Multidisciplinary Analyses and Conceptual Design

Abstract

The tail rotor of conventional helicopters has always been considered a necessary 'evil'. It is necessary to counteract the reaction torque of the engine and to control the helicopter in yaw but it consumes substantial power, has only marginal control authority under unfavourable wind conditions, and it is noisy, vulnerable and dangerous. A solution to all these problems would be a helicopter concept that eliminates the need for a tail rotor. The so-called 'Ornicopter', a helicopter with flapping blades, is such a concept. The mechanism of the Ornicopter is inspired by bird flight. When birds flap their wings they are able to generate both a lifting force and a propelling force from this single movement. Instead of propelling a helicopter blade by spinning it around and deriving lift from this rotating movement, as is done in conventional helicopter configurations, the Ornicopter flaps its blades like a bird and derives both lift and a propulsive force from this movement. In this case the blades propel themselves and there is no longer a need for a direct torque supplied by the engine to rotate the blades. The Ornicopter's rotor, therefore, will not cause a reaction torque on the fuselage, which makes the tail rotor's anti-torque function redundant. The goal of the present thesis is to develop a thorough understanding of the Ornicopter concept and its feasibility throughout a realistic flight envelope. The first part presents the analysis of the Ornicopter's main characteristics regarding performance, stability, controllability, handling qualities, as well as an exploratory vibratory analysis. In the second part a preliminary design and sizing thereof are presented based on the conclusions obtained in the first part. The basis for the Ornicopter's study is an analytically-derived flight mechanics model. The model is based on the blade element theory (BET) and considers 6 rigid body degrees of freedom (DoF), 3 DoF blade flapping dynamics and 3 DoF Pitt-Peters inflow dynamics. Previous mathematical models developed as a proof of the Ornicopter concept have concentrated mainly on hovering flight. The model developed in this thesis is capable of representing the Ornicopter's dynamics well within its entire operational flight envelope. As a benchmark for the Ornicopter's specifications, the Bolkow Bo-105 helicopter is used. The Bo-105 helicopter is a light twin-engine, multi-purpose helicopter developed in Germany in the 1970s. For the initial values of the design parameters of the Ornicopter (such as rotor radius, blade loading, rotor tip velocity, vertical fin size) the Bo-105 helicopter design is used. A 2X2 anti-symmetric rotor flapping configuration (with two opposite blades flapping in the same direction) is used for the Ornicopter in order to eliminate the necessity for a tail rotor. Using these design parameters it is demonstrated that the Ornicopter rotor can generate enough propulsive torque to rotate the blades with modest flapping amplitudes (maximum less than 9 deg). The present thesis demonstrates that, compared to the Bo-105 helicopter, the Ornicopter concept suffers from higher required power, a smaller flight envelope (mainly due to the larger rotor stall area) and lower yaw stability. All these drawbacks are attributed to the large rotor blade angle of attack variation introduced by the forced flapping mechanism and the absence of a tail rotor. In the second part of this thesis, the Ornicopter concept is optimized for performance. Keeping the performance specifications of the Bo-105 as the objectives, the design values (blade radius, blade loading etc) are turned to fit the Ornicopter concept and no longer use the Bo-105 design values. The design optimization is formulated through minimization of the required power, while satisfying the stall area requirement. The thesis proves that the optimal design for the Ornicopter as compared to the Bo-105 benchmark is characterized by a lower blade loading, increased rotor tip velocity and larger vertical fin size. This optimal design results in an enlarged flight envelope due to the reduced rotor stall area and improved yaw stability in forward flight. Nevertheless, despite these improvements in the Ornicopter's flight envelope, there is a slight increase in required power when compared with the Bo-105 specification (approximately 5% at 150 knots). To compensate for the higher profile power needed for the Ornicopter's optimal design, a larger rotor radius is required in order to reduce the induced power and keep the increase in the total required power to a minimum. This thesis may be considered as a first step in rationalizing the expectations regarding the Ornicopter's tailless helicopter design. The thesis proved that this new concept shows a slightly poorer performance than that of conventional helicopters regarding power consumption in forward flight and service ceiling. This is disappointing since one of the assumptions was that the elimination of the tail rotor would also eliminate the power consumption associated with a tail rotor. Further analyses of the Ornicopter's performance (such as endurance, payloads, climbing performance, and environmental performance related to safety and noise), costs or maintenance should be performed for a comprehensive understanding of the advantages and disadvantages of this helicopter concept.