Koji Isogai

Transonic dip mechanism of flutter of a sweptback wing. II

Conclusions An experimental investigation of the RR—-MR and MR-*RR transitions over concave and convex, smooth and rough wedges revealed that 1) as the surface roughness increases, the wedge angle at which transition takes place (for a given Mach number) decreases and 2) for a mesh 40 sand paper type surface roughness, it seems that the transition angle becomes independent of the incident shock wave Mach number, i.e., a) MR—>RR transition occurs at 6W =^54 deg and b) RR-MR transition takes place at Ow-21 deg. These results should be of great importance to those dealing with reflections of blast waves, since this type of problem involves nonstationary flows and high degrees of surface (ground) roughness.

Direct Measurement of Unsteady Fluid Dynamic Forces for a Hovering Dragonfly

An experimental study of the aerodynamics of a dragonfly in hovering flight is conducted. Measurements are made on a mechanical flapping wing apparatus that simulates, in water, the Reynolds number and reduced frequency of the tandem wing configuration on a dragonfly. The length scale of the flapping wing model is four times larger than on a real dragonfly. The phase difference between the flapping motions of the fore- and hindwings is independently varied in the range 0-90 deg to examine the flow interaction between the wings when the dragonfly is in hovering flight. The time histories and time average values of the fluid dynamic forces and the rate of work show that, in hovering flight, there is only a small interaction between the flows over the fore- and hindwings

Experimental and Numerical study of forward flight aerodynamics of insect flapping wing

Experimental and numerical studies are conducted on the aerodynamic characteristics of a flapping wing of an insect in forward flight. Unsteady aerodynamic forces and flow patterns are measured using a dynamically scaled mechanical model in a water tunnel. The design of the model is based on the flapping wing of a bumblebee. The forces and flow patterns are also computed using a three-dimensional Navier-Stokes code. Comparisons between the experimental and numerical results show good agreement in the time histories of aerodynamic forces and flow patterns in both hovering and forward flight. Aerodynamic mechanisms of a flapping wing in forward flight, such as delayed stall, rotational effect, and wake capture are examined in detail. The results indicate that these aerodynamic mechanisms had an effect on the aerodynamic characteristics of the flapping wing in forward flight; however, these mechanisms function differently during the up- and downstroke, for different stroke plane angles, and for different advance ratios.

Optimum Aeroelastic Design of a Flapping Wing

A method is presented for the optimum aeroelastic design of a flapping wing employing a lifting-surface theory as an aerodynamic tool and the complex method as the optimization algorithm. The method is applied to the optimum design of a flapping wing of a Kite Hawk (Milvus migrans) unmanned air vehicle and the optimum thickness distribution of the main spar is determined. As a result of the optimization, a high propulsive efficiency of 75% is attained considering only dihedral flapping of the main spar. By evaluating the viscous effect for this optimum design using a three-dimensional Navier-Stokes code, the effectiveness of the design is confirmed.

Unsteady Three -Dimensional Viscous Flow Simulation of a Dragonfly Hovering

In order to clarify the basic aerodynamic mechanisms of the hovering flight of the dragonfly, numerical simulations of the unsteady viscous flow around a tandem wing configuration have been performed using a three -dimensional Navier -Stokes code. The flow simulations are conducted for Anax parthenope julius as a typical dragonfly model. The total lifting force and specific necessary power predicted by the present simulation show close agreement with those observed experimentally for the present dragonfly model. The present code is furth er validated by comparing the results of the simulation with the experimental values of total lift and stroke -plane angle obtained using a robot.

Application of Genetic Algorithm for Aeroelastic Tailoring of a Cranked-Arrow Wing

A computer code for aeroelastic tailoring of a cranked-arrow wing for a supersonic transport is developed. The code includes static strength, local buckling, and aeroelastic analyses. The original finite element code is used for the static strength and local buckling analyses, and the original code is used for the vibration analysis of the aeroelastic analysis. In the optimization process of this code, a genetic algorithm is employed to find the optimum laminate construction of the wing box for which the structural weight is minimum under the static strength, local buckling, and aeroelastic constraints. These codes are applied to the preliminary design of a cranked-arrow wing. The optimum design satisfying only the static strength and local buckling constraints does not satisfy the aeroelastic constraint. Therefore, the flutter characteristics are optimized, and the optimum laminate construction that satisfies the static strength, local buckling, and aeroelastic constraints is obtained.

Direct search method to aeroelastic tailoring of a composite wing under multiple constraints

In order to avoid the possible breakdown of usual optimization methods using gradient information, which is caused by a discontinuous behavior of the flutter velocity as a function of design variables, a feasibility study is made using a direct search method that does not depend on the derivatives of objective/constraint functions. The complex method, as one candidate for such a method, is applied to the minimum weight design of high-aspect-ratio forward/aft swept wings under strength and aeroelastic constraints. It is shown that the complex method is very effective and robust in finding the optimum fiber orientations and the thickness distributions of the upper/lower skin panels of the wing box, especially when the flutter velocity is one of the constraint functions. The deficiency of the complex method is that the rate of convergence rapidly degrades with increasing number of design variables.

Numerical calculation of unsteady transonic potential flow over three-dimensional wings with oscillating control surfaces

Numerical calculations of the unsteady transonic potential flow over three-dimensional wings with oscillating control surfaces are performed. For this purpose, a new grid system, which is appropriate for solving the control surface problems, is introduced into the computer code USTF3, which solves the unsteady three-dimensional full potential equation by a two-step semi-implicit time-marching technique. To validate the code, the unsteady pressure distributions on the NLR swept tapered wing with an inboard control surface and on the RAE swept tapered wing with a part-span control surface are calculated and compared with those of existing theories and experimental data. The qualitative behaviors of the unsteady pressure distributions due to the control surface oscillation are well predicted by the present method, but some quantitative discrepancies due to the viscous effects in the experiment are observed for supercritical Mach numbers.

Effects of flapping wing kinematics on hovering and forward flight aerodynamics

The effects of wing kinematics on the aerodynamic characteristics of a flapping insect wing are investigated experimentally. The time-varying aerodynamic forces acting on the flapping wing are measured in hovering and forward flight using a dynamically scaledmechanicalmodel in awater tunnel, which simulates a bumblebee flapping wing in hovering and forward flight. Wing kinematics can be categorized as trapezoidal or sinusoidal types. The trapezoidal flapping motion is divided into translational and reversal phases, and the trapezoidal feathering motion is divided into fixed-angle and rotational phases. In the sinusoidal type, the time histories of angles for the flapping and feathering motions are represented as sinusoidal functions. The feathering rotation during the flapping translation causes an increase in aerodynamic power rather than lift and thrust for hovering and forward flight. Therefore, it is preferable for the feathering rotation to be conducted during the flapping reversal phase for high efficiency. The trapezoidal flapping motion and trapezoidal feathering motion with a shorter duration of rotation should be selected for a higher efficiency in hovering and forward flight. This result indicates thatmany insects using the trapezoidal type attach importance to a good efficiency in selecting the wing kinematics. For a larger lift in hovering and slower forward flight, the sinusoidal flappingmotion and trapezoidal featheringmotion with a shorter duration of rotation should be selected.

Experimental and Theoretical Study of Attitude Control of Flapping Wing Micro Aerial Vehicle

The stability and control capability of a dragonfly-type Micro Aerial Vehicle (MAV) which employs resonance-type flapping wings has been studied using an experimental model and a flight simulation technique. The experimental model is designed to be supported at its CG position with pitch and roll freedoms. The control forces needed to keep the pitch and roll motions stable are generated by changing the frequency of each wing (four wings, namely, right- and left-, and fore- and hind-wings) that are activated by four motors. We employed two different types of the attitude sensors, that are an ultra-sound sensor and a G-sensor (acceleration sensor). A PID control law is employed for the attitude control. It has been demonstrated that the attitude (pitch and roll) of the present experimental model can be successfully controlled by changing the frequency of each wing. In addition to the experimental study, the theoretical study using the flight simulation technique has also been conducted to examine the sensitivity of the present control method to the various parameters, such as moment of inertia and the arrangement of the flapping wings. As a result, the several points which must be improved towards the development of a free-flight model are clarified.