The University of Arizona Micro OrnithoptersD. Silin*, B. Malladi*, and S. Shkarayev The University of Arizona, Tucson, AZ, 85721 The research and development project outlined in the paper addresses the aerodynamic design of flapping-wing micro air vehicles (also called ornithopters). Rigorous wind tunnel testing was conducted over the wide range of geometric, elastic, and kinematic parameters of flapping wings. Specifically, effects of a wings bending stiffness on the generated thrust force and power required were investigated at different flapping frequencies. The lift and drag forces on flapping wings were studied with the stroke plane angle varied from horizontal to vertical. The major result of this study is that no abrupt stall was found for the full range of angles at all test speeds. Three ornithopters were designed utilizing the results of aerodynamics studies. The smallest ornithopter has a 15 cm wingspan, weighs only 9 grams, and has a flight endurance of 3 min. The 20-cm ornithopter with a thrust-to-weight ratio in excess of 1.2 is capable of hovering, as well as of sustained steady flight. The 53-cm ornithopter is equipped with an autopilot, and several fully autonomous flights have been performed to date. I. Introduction Because of their small size, micro air vehicles are often considered for applications ranging from military to scientific, and their versatility allows them to perform in conditions that might otherwise endanger human life. Their reconnaissance capabilities were the driving factor for the first generation of micro air vehicles (MAVs). These developments concerned fixed wing MAVs. Flapping-wing micro air vehicles generate lift and thrust for forward motion using their flapping wings, emulating birds and insects. However, just mimicking the flight of birds and insects is insufficient in designing flapping-wing vehicles. Here is how this viewpoint was elucidated by Mueller and DeLaurier:1 The primary motivation for studying animal flight is to explain the physics for a creature that is known to fly An ornithopter designer, in contrast, is trying to develop a flying aircraft, and its ability to achieve this is no given fact. And conversely, a successful micro air vehicle design provides a verifiable physical model of flight in nature. Aerovironment pioneered the designing of radio-controlled micro ornithopters called Microbats.2 The most successful vehicle of this type has a half-ellipse wing planform with a 20cm wingspan flapping at 22 Hz. The project proved to be challenging because of the limited knowledge on unsteady aerodynamics of flexible flapping wings of this small size.
Graduate Student, Department of Aerospace and Mechanical Engineering, University of Arizona, 1130 N. Mountain, Tucson AZ, 85721. Associate Professor, Department of Aerospace and Mechanical Engineering, University of Arizona, 1130 N. Mountain, Tucson AZ, 85721, Senior Member of AIAA.
DeLaurier and his group developed a 35-cm radio-controlled ornithopter capable of hovering.3 The kinematics of the 4 wings (X-wing) mimic the cling-flip mechanism employed by some insects and birds. This mechanism balances flapping forces and decreases vibrations. Hovering flights in excess of one minute were achieved with a flapping frequency of 28 Hz. It was noted that transition to forward flight remains a problem, but that it may be overcame by an intelligent flight stabilization system. Ellington4 summarized the flight kinematics of insects, which could be useful for prospective insect-sized MAV designs. The motion of the flapping wing is described with respect to the flapping plane, also called a stroke plane. Insects have been observed to perform gentle maneuvers by tilting the stroke plane of their wings, just like a helicopter. Lateral direction changes can be accomplished by a roll of the stroke plane (often by increasing flapping amplitude and/or angle of attack of the outside wing). Angle of attack changes also initiate lowspeed acceleration. For slower flight and hovering, the body hangs below the wing bases, and the insect benefits from a passive pendulum-like stability. An ornithopter competition was added to the 8th International MAV Competition in Tucson, Arizona in 2004. This competition involves building the smallest radio-controlled ornithopter that can fly the most laps around a pylon course in 2 min. The pylons were spaced 40 feet apart and the ornithopters flew either an elliptical course around them or a figure-8 through them. The University of Arizona (UA) won the 2004,5 2005,6 and 20067 competitions with 28-cm, 20-cm, and 15-cm ornithopters, respectively. For the 2006 US-European MAV technology demonstration, we are planning to present the three ornithopter designs shown in Fig. 1. The smallest known ornithopter has a 15 cm wingspan, weighs only 9 grams and has flight endurance of 3 min. Birds and insects are unstable flyers, yet they have evolved very sophisticated methods of maneuvering capabilities, including vertical takeoff, landing and hovering.8,9 The 20-cm ornithopter with a thrust-toweight ratio in excess of 1.2 is capable of hovering, as well as of sustained steady flight. Our team has extensive experience in the successful integration of an autopilot into fixedwing micro air vehicles. We have developed two fully autonomous air vehicles, Dragonfly and Zagi, that have demonstrated the ability to complete practical surveillance missions.10 In the present project, the 53-cm UA ornithopter was outfitted with a Paparazzi autopilot11 for fully autonomous operation, and several fully autonomous flights have been performed to date.
a) 15-cm, smallest
b) 20-cm, hovering 2
The University of Arizona Micro Ornithopters
c) 53-cm, autonomous Fig. 1. UA ornithopters. II. Wind Tunnel Measurements on Flapping Wings Studies to date on the aerodynamics of flapping flight, although beneficial to an understanding of the subject, have not taken into account all the details that are necessary to obtain a complete and thorough understanding (and more accurate representation) of the true aeromechanics of flapping flight. For the same reasons, no design methods for flapping wings are readily available and, therefore, the first phase of the present project was focused on the aerodynamics of flapping wings currently used for UA ornithopters. Wind tunnel measurements were performed on flapping wings to complete the following tasks: the investigation of the effect of a wings bending stiffness on the thrust force and power required at different frequencies; the determination of the lift and drag forces with the stroke plane angle varying from horizontal to vertical. Testing was performed using the UA Low Speed Wind Tunnel (Fig. 2a). The test section is 3 4 ft and has a velocity range from 2 to 50 m/s. The flow is laminarized in a settling chamber to less than 0.3% turbulence in the axial direction. 3The University of Arizona Micro Ornithopters
a) Model in the wind tunnel
b) Model schematics Fig. 2. Wind tunnel testing of flapping wings. 4The University of Arizona Micro Ornithopters
The force balance is capable of accurate measurements of lift and drag. Force measurements are made using precision strain gages. Data from these strain gages are logged using two National Instruments SCXI-1321 terminal blocks in a low-noise SCXI-1000 chassis capable of sampling at 330,000 Hz. The flapping wing model consists of a mounting rib, wing, and flapping transmission (Fig. 2b). The wing is a half-ellipse planform with a 250 mm wingspan and 70 mm root chord, resulting in a 13,700 mm2 wing area and aspect ratio of 4.56. The wing structure consists of a membrane bonded to the front and radial spars with rubber cement. The membrane is 0.015-mm Mylar. Front and radial spars are pultruded carbon rods. In the basic flapping wing model (model A), carbon rods are used for the spars, T315-412 of diameter 0.8 mm for front spars, and T305-412 of diameter 0.5 mm for radial spars. Two radial spars are placed in each wing, as shown in Fig. 2b. The total weight of the wing is 1.1 g. The front spar flapping motion sweeps through 72 of travel with a 19 dihedral offset (Fig. 2b). Since the main goal of this testing series was to obtain wing-only aerodynamic data, the flapping-wing models were supported as close to the trailing edge as possible with an aerodynamically clean mount system. An aluminum mount was constructed with minimum frontal area and a smooth aerodynamic leading edge. The mount was reinforced by patches along the mid-chord from the leading to the trailing edges. These parts also functioned as the mounting points to the wind tunnel pylon. In order to measure the flapping frequency of the wings, an experimental setup was equipped with a built-in optical tachometer. The tachometer consists of a CP-36 photodiode and ECG3038 phototransistor connected to the data acquisition board.
Fig. 3. Thrust vs flapping frequency.
5The University of Arizona Micro Ornithopters
Fig. 4. Thrust-to-power vs flapping frequency. Wing design trade-offs are their weight and stiffness. Since the center of pressure and the shear center do not typically coincide in the actual wings of insects and birds, a resultant torque causes the wing to twist. Hence, the one very key element in the production of the lift and thrust by flapping wings lies in the flexibility or stiffness of the wing structure, and not only in the flapping motion. Stiffer wings allow for higher maximum flapping frequencies and, therefore, higher thrust force. However, having a more rigid wing increases the wing structure weight. Increased weight requires more energy to induce flapping. Is there an optimum? In order to investigate the effects of wing stiffness on the developed thrust and required power, in addition to the base wing (model A), described above, two more wing models were built