Lift-induced drag

About: Lift-induced drag is a research topic. Over the lifetime, 2861 publications have been published within this topic receiving 41094 citations.

Drag Reduction by Cooling in Hydrogen-Fueled Aircraft

Abstract: Drag reductions are possible for cryo-fueled aircraft by using fuel to cool selected aerodynamic surfaces on its way to the engines. This is because cooled laminar boundary layers in air at subsonic and low supersonic speeds are more stable than adiabatic boundary layers and therefore more resistant to transition to turbulent flow. Calculations for A/=0.85 hydrogen-fueled transport show that drag reductions in cruise of about 20% are within reason. The weight of the fuel saved is well in excess of the weight of the required cooling system. These results suggest that the hydrogen fueled aircraft employing surface cooling is quite attractive as an energy conservative aircraft and warrants more detailed study.

Aerodynamic corrections for the flight of birds and bats in wind tunnels

Abstract: Few wind tunnel studies of animal flight have controlled or corrected for distortions to behaviour, physiology or flight aerodynamics representing the difference between flight in the tunnel and flight in free air. Aerodynamic correction factors are derived based on lifting-line theory and the method of images for an animal flying freely within closed- and open-section wind tunnels; the method is very similar to that used to model flight in ground effect, and as in ground effect the corrections to induced drag may be substantial. These correction factors are used to estimate bound wing circulation, drag and mechanical power for comparison with free flight, and to derive testable predictions of optimum flight strategies for an animal in a tunnel. In an open-section tunnel, mechanical power is increased compared to free flight, and the animal should fly at the tunnel centre. In a closed tunnel mechanical power is usually reduced, and substantial savings are available, particularly at low speeds, if the animal flies close to the tunnel roof. Anecdotal observations confirm that birds and bats adopt this strategy. The mechanical power-speed curve in a closed tunnel is flatter than the curve for free flight, and this may explain the flat metabolic power-speed curves for birds and bats obtained in some measurements.

Lifting surface design using multidisciplinary optimization

Abstract: Lifting surface design is affected by many considerations; drag, weight, and high-lift are particularly important. These effects place different and often opposite requirements on wing shape, complicating the selection of a best configuration. To assist this selection, a preliminary design method using nonlinear optimization has been developed. An isolated lifting surface design problem is formulated from aircraft mission parameters, typically to calculate the platform and twist minimizing cruise drag or maximizing range, subject to constraints such as structural weight and maximum section lift. Solving this with optimization requires very fast analyses that are capable of capturing the effects of detailed changes in wing shape. This motivated significant improvements that were made to structural calculations and maximum-lift prediction methods for preliminary design. A method was developed to evaluate wing weight and stiffness considering bending and buckling strength. A critical section method was modified to enable the prediction of flaps-down maximum lift, correcting for induced camber near the flap edge. The lifting surface optimization method performs platform design while accounting for many effects: static aeroelasticity, weight evaluated from multiple structural design conditions, induced drag, profile drag, compressibility drag, maximum lift, static stability, and control power constraints. The method was used to explore the influence of these effects on optimal wings, demonstrating how strongly lifting surface design is influenced by maximum-lift constraints. The method was also applied to studies of wing tip shape and optimal wing-tail configurations. In many cases, the optimizer exploits physical effects, creating design features that are easy to interpret in hindsight but difficult to predict in advance. In creating these designs, the method has demonstrated that optimization can be a valuable tool for lifting surface design.

Computational Fluid Dynamics Study of the Effect of Leg Position on Cyclist Aerodynamic Drag

Abstract: An experimental and numerical analysis of cycling aerodynamics is presented The cyclist is modeled experimentally by a mannequin at static crank angle; numerically, the cyclist is modeled using a computer aided design (CAD) reproduction of the geometry Wind tunnel observation of the flow reveals a large variation of drag force and associated downstream flow structure with crank angle; at a crank angle of 15 deg, where the two thighs of the rider are aligned, a minimum in drag is observed At a crank angle of 75 deg, where one leg is at full extension and the other is raised close to the torso, a maximum in drag is observed Simulation of the flow using computational fluid dynamics (CFD) reproduces the observed variation of drag with crank angle, but underpredicts the experimental drag measurements by approximately 15%, probably at least partially due to simplification of the geometry of the cyclist and bicycle Inspection of the wake flow for the two sets of results reveals a good match in the downstream flow structure Numerical simulation also reveals the transient nature of the entire flow field in greater detail In particular, it shows how the flow separates from the body of the cyclist, which can be related to changes in the overall drag

Theoretical determination of the minimum drag of airfoils at supersonic speeds

Abstract: A method is reported for determining mathematically the combined disturbance field, and in certain cases the minimum drag, of wings at supersonic speeds. The simplest analytic example is provided by the wing of elliptic planform, which achieves its minimum drag when the lift is distributed uniformly over the surface. With a symmetrical distribution of thickness, the requirement of minimum drag for a given total volume is found to lead to profiles of constant curvature.