Lift-to-drag ratio | EPFL Graph Search

In aerodynamics, the lift-to-drag ratio (or L/D ratio) is the lift generated by an aerodynamic body such as an aerofoil or aircraft, divided by the aerodynamic drag caused by moving through air. It describes the aerodynamic efficiency under given flight conditions. The L/D ratio for any given body will vary according to these flight conditions. For an aerofoil wing or powered aircraft, the L/D is specified when in straight and level flight. For a glider it determines the glide ratio, of distance travelled against loss of height. The term is calculated for any particular airspeed by measuring the lift generated, then dividing by the drag at that speed. These vary with speed, so the results are typically plotted on a 2-dimensional graph. In almost all cases the graph forms a U-shape, due to the two main components of drag. The L/D may be calculated using computational fluid dynamics or computer simulation. It is measured empirically by testing in a wind tunnel or in free flight test.

Effects of wing kinematics, corrugation, and clap-and-fling on aerodynamic efficiency of a hovering insect-inspired flapping-wing micro air vehicle

Hoang Vu Phan

A flapping-wing micro air vehicle (FW-MAV) operating with aerodynamically optimal wing configuration and kinematics may save energy and thus prolong flight time. In this work, we use a computational-fluid dynamic method to investigate the effects of wing kinematics, corrugation structures, and clap-and-fling on the aerodynamic efficiency of our hovering two-winged FW-MAV (KUBeetle). From the measured reference wing kinematics, we generated several different wing kinematics, considering the effect of spanwise twist and chordwise camber that produce high lift-to-drag ratio (L/D). Among the investigated cases, the modified wing kinematics version 3, which includes both camber and twist with an average angle of attack of about 37 degrees, was selected, because of its similar to 24% improvement of L/D, while maintaining similar lift to the measured reference wing kinematics. The results also showed that the camber plays a role in the improvement of both lift and L/D, which improvements are approximately (16.7 and 10.6)%, respectively. We then used wing kinematics version 3 to investigate the effects of various leading-edge corrugation structures. Based on the results of lift and L/D, we proposed a wing with distributed wing corrugations along the wingspan, which slightly augments the L/D by 2%. In addition, to see how the clap-and-fling behavior contributes to the aerodynamic efficiency, its effects on lift and drag generation were examined. We found that the clap-and-fling enhanced lift by 5%, but increased drag by 9%, resulting in a 4% reduction of the L/D for both the measured and the modified wing kinematics. Thus, the lift-augmented clap-and-fling is inefficient for FW-MAVs. Finally, the study confirmed that the wing with distributed wing corrugations using wing kinematics version 3 without clap-and-fling presented at the stroke reversals is preferable for the high aerodynamic efficiency of the KUBeetle robot, with 31% improvement in L/D. (C) 2021 Elsevier Masson SAS. All rights reserved.

ELSEVIER FRANCE-EDITIONS SCIENTIFIQUES MEDICALES ELSEVIER2021

Predicting unsteady flow separation in response to a flow disturbance

Julien Dominique Claude Deparday, Guosheng He, Sabrina Henne, Karen Ann J Mulleners

We present here a summary of the activities and results of dynamically stalling airfoils from the UNFoLD lab at EPFL that have been presented at the meetings of the NATO AVT-282 discussion group on Unsteady aerodynamic response of rigid wings in gust encounters during the past three years. The results are based on experimental data for a sinusoidally pitching OA209 airfoil in a wind tunnel at

\Rey=\num{9.2e5}

, a sinusoidally pitching NACA0015 airfoil profile with a trailing edge flap in a wind tunnel at

\Rey= \num{5.5e5}

, and a sharp-edged flat plate undergoing a ramp-up motion in a recirculating water channel at

\Rey= \num{77 500}

. The first two data sets are used to study the onset of dynamic stall and the third one is used to analyse post-stall load fluctuations. Based on the experimental data, we derived a new model of the leading edge suction parameter that accurately predicts the value and the timing of the maximum leading edge suction and dynamic stall onset on a pitching airfoil. The model is based on thin airfoil theory and links the evolution of the leading edge suction and the shear layer during stall development. By including an oscillating trailing edge flap, the lift polars can be significantly altered but the dynamic stall time delay is only marginally affected by the kinematics of the flap. The maximum lift coefficient is strongly affected by both the main wing and the trailing edge flap kinematics. For a flat plate without flap, the maximum lift coefficient and the subsequent post stall peak values increase with increasing pitch rate up to a critical pitch rate beyond which the entire lift response become independent of the pitch rate. The convective time delay to reach the primary lift peak decreases with increasing pitch rate up to the critical pitch rate and the time delay between subsequent peaks slightly decreases until the limit cycle oscillation period is reached.

American Institute of Aeronautics and Astronautics, Inc2020