Gurney flap—Lift enhancement, mechanisms and applications

Abstract

Since its invention by a race car driver Dan Gurney in 1960s, the Gurney flap has been used to enhance the aerodynamics performance of subsonic and supercritical airfoils, high-lift devices and delta wings. In order to take stock of recent research and development of Gurney flap, we have carried out a review of the characteristics and mechanisms of lift enhancement by the Gurney flap and its applications. Optimum design of the Gurney flap is also summarized in this paper. For the Gurney flap to be effective, it should be mounted at the trailing edge perpendicular to the chord line of airfoil or wing. The flap height must be of the order of local boundary layer thickness. For subsonic airfoils, an additional Gurney flap increases the pressure on the upstream surface of the Gurney flap, which increases the total pressure of the lower surface. At the same time, a long wake downstream of the flap containing a pair of counter-rotating vortices can delay or eliminate the flow separation near the trailing edge on the upper surface. Correspondingly, the total suction on the airfoil is increased. For supercritical airfoils, the lift enhancement of the Gurney flap mainly comes from its ability to shift the shock on the upper surface in the downstream. Applications of the Gurney flap to modern aircraft design are also discussed in this review.

Introduction

Over the years, high-lift devices have attracted attention of aircraft designers and engineers as they can improve takeoff and landing performance of aircraft through aerodynamic enhancement. Flaps are such aircraft devices for lift enhancement, where leading-edge slotted flaps and trailing-edge flaps are commonly found in many aircrafts (see Fig. 1). Flow over flaps is very complicated, as it involves boundary layers, main wing wakes, potential flow outside the boundary layer and flow in the flap slot, which result in recirculating boundary layer on the flap surface. This makes the design of high-lift device very difficult. Due to the design complexity, manufacturing difficulty and maintenance cost, high performance airfoils can not be easily adopted in both commercial and military aircraft. Even if this could be done, high-lift devices cannot meet the expected performance quite often [1], [2], [3], [4], [5]. Therefore, it is a common practice to add a simple device such as the Gurney flap (GF) to improve the aerodynamic performance of airfoils. The GF is simply a short flat plate attached to the trailing edge perpendicular to the chord line on the pressure side of the airfoil (Fig. 2). Race car driver Dan Gurney used this flap on an inverted wing to increase the downward force, therefore increasing the traction during acceleration, braking and cornering. The GFs have attracted much attention over decades due to its effectiveness from a simple configuration [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20].