Topic
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.
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15 Dec 2010TL;DR: The ground effect is the aerodynamic phenomenon whereby the flow field around a vehicle, either an aircraft or a car, is constrained and altered by the presence of the ground.
Abstract: The ‘ground effect’ is the aerodynamic phenomenon whereby the flow field around a vehicle, either an aircraft or a car, is constrained and altered by the presence of the ground. For an aircraft operating in ground effect, it is the creation of an effective air cushion between the lower surface of the wing and the ground that modifies the physics of the flow resulting in positive lift enhancement. For a car utilizing ground effect, the dominant feature is a low-pressure field between the vehicle and the ground caused by significant flow acceleration, similar to the ‘Venturi effect’, leading to downforce (negative lift) enhancement. Components such as wings and diffusers operate within this regime. The impact of ground effect is generally an increase in aerodynamic efficiency, that is, an increase in lift-to-drag ratio for aircraft or the equivalent downforce-to-drag ratio for cars. When a vehicle operates too close to the ground, the beneficial response diminishes. The ground effect can adversely affect the flight stability of an aircraft during take-off and landing as well as under cruise conditions in close proximity to the ground.
This chapter provides a brief summary concerning the history of ground effect vehicles. Various methods employed to study the phenomenon are introduced; these include empirical approximations, analytical methods, numerical simulations, and wind tunnel tests. Applications of ground effect to aircraft and vehicle design are also discussed. Advantages and risks are further examined with an emphasis on control and stability.
Keywords:
aerodynamics;
ground effect;
induced drag reduction;
wing-in-ground effect;
diffuser-in-ground effect;
wing-in-ground effect vehicles
25 citations
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TL;DR: In this article, the rear downward flap was designed to have an optimum downward angle to increase the cabin back surface pressure coefficient, which increased the downwash of the bed flow to be inclined on the tailgate.
Abstract: The drag reduction of a pickup truck by a rear flap add-on was examined through CFD simulations and wind tunnel experiments. When installed at the rear edge of the roof, the flap increased the cabin back surface pressure coefficient, causing the downwash of the bed flow to be inclined on the tailgate. Thus, the attachment of the bed flow to the tailgate was eliminated; consequently, the drag coefficient was reduced with increasing flap length and downward angle despite the enlarged reverse flow in the wake. However, the drag coefficient did not decrease any further after a specific downward angle was reached because the bed flow increased the drag force at the tailgate and the flap lowered the pressure field above the flap. To maximize the drag reduction effect, the rear downward flap should be designed to have an optimum downward angle.
25 citations
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TL;DR: It is numerically verified that a quasi-closed C-wing presents the same optimal induced drag and circulation of the corresponding closed system, and it is shown that there are an infinite number of equivalent solutions obtained by adding an arbitrary constant to a reference optimal circulation.
Abstract: An invariant procedure for the minimization of induced drag of generic biwings and closed systems (Joined Wings) was presented in the companion paper (minimum induced drag theorems for Joined Wings, closed systems, and generic biwings: theory) and is now adopted to study several theoretical open questions regarding these configurations. It is numerically verified that a quasi-closed C-wing presents the same optimal induced drag and circulation of the corresponding closed system. It is also verified that when the two wings of a biwing are brought close to each other so that the lifting lines identify a closed path, the minimum induced drag of the biwing is identical to the optimal induced drag of the corresponding closed system. The optimal circulation of this case differs from the quasi-closed C-wing one by an additive constant. The non-uniqueness of the optimal circulation for a closed wing system is also addressed, and it is shown that there are an infinite number of equivalent solutions obtained by adding an arbitrary constant to a reference optimal circulation. This property has direct positive impact in the design of Joined Wings as far as the wing load repartition is concerned: The percentage of aerodynamic lift supported by each wing can be modified to satisfy other design constraints, and without induced drag penalty. Finally, the theoretical open question regarding the asymptotic induced drag behavior of Joined Wings, when the vertical aspect ratio approaches infinity, has been resolved. It has been shown that for equally loaded wings indefinitely distant from each other, the boxwing minimum induced drag tends to zero. In that condition, the upper and lower wings present a constant aerodynamic load. Prandtl's approximated formula for the minimum induced drag of a boxwing (Best Wing System) cannot be used to describe the asymptotic behavior. This work also shows that the optimal distribution over the equally loaded horizontal wings of a boxwing is not the superposition of a constant and an elliptical functions. This is an acceptable approximation only for small vertical aspect ratios (of aeronautical interest).
25 citations
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25 citations