Optimization of the Stall Characteristics of an Unmanned Aerial Vehicle Using Wing Fences

ECCOMAS Congress 2016

Optimization of the Stall Characteristics of an Unmanned Aerial Vehicle using Wing Fences

Jolan Wauters*, Jan Vierendeels, Joris Degroote

Department of Flow, Heat and Combustion Mechanics Ghent University Sint-Pietersnieuwstraat 1, 9000 Ghent, Belgium {Jolan.Wauters, Jan.Vierendeels, Joris.Degroote}@UGent.be

ABSTRACT The widespread use of unmanned aerial vehicles (UAV) has become clear over recent years. These aircraft are often tailless to be compact and are able to take off and land in confined spaces by operating at high angles of attack. Tailless aircrafts, planes noted for the absence of a horizontal tail plane, require sweeping of the wings in order to obtain longitudinal static stability, i.e. the natural desire of the airplane to remain in its equilibrium position.

The consequence of the swept wing is a thicker boundary layer, an increased downwash and, also due to the presence of tapering, an increased adverse pressure gradient at the tip. These different aspects contribute to the increased likelihood of flow separation starting to occur at that location, resulting in tip-stall. The consequence is a nose-up pitching moment which pulls the plane further into stall.

The presence of tip-stall has led to a renewed interest in the use of devices to counter it, such as wing fences. These aerodynamic devices can be seen as planes placed on top of the wing aligned with the flow and developed from the idea of stopping the transverse component of the boundary layer flow. The straightforward design and application has led to an extensive usage from the ‘50s to ‘80s.

By means of a CFD analysis using the k-omega SST model at Re = 5E5, the working of the wing fence as well as the influence of its design parameters are examined on a UAV. The parameters considered are the size, position, length on the upper surface and length on the lower surface.

The research has led to the conclusion that the working of the wing fences extends past its ability to stop the boundary layer flow: the fence moves the position where separation starts to occur from the tip to the inner side of the fence and alters the lift distribution across the wingspan. From the analysis of the design parameters it could be deduced that a certain size and length on the upper surface is required for the fence to be operative. Extending the dimensions of the fence past this point will undermine the effectiveness of the fence, thus predicting the existence of an ideal geometry. Moving the fence towards the tip decreases the likelihood of the occurrence of separation at the tip but at the same time decreases the longitudinal static stability. This also indicates the presence of an ideal location.