Article
Experimental Investigation of the Wake and the
Wingtip Vortices of a UAV Model
Pericles Panagiotou, George Ioannidis
ID
, Ioannis Tzivinikos and Kyros Yakinthos *
ID
Laboratory of Fluid Mechanics and Turbomachinery, Department of Mechanical Engineering,
Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece; peripan@eng.auth.gr (P.P.);
georioan.meng@gmail.com (G.I.); ioannistzivinikos@gmail.com (I.T.)
* Correspondence: kyak@auth.gr; Tel.: +30-231-099-6411
Received: 5 October 2017; Accepted: 30 October 2017; Published: 1 November 2017
Abstract:
An experimental investigation of the wake of an Unmanned Aerial Vehicle (UAV) model
using flow visualization techniques and a 3D Laser Doppler Anemometry (LDA) system is presented
in this work. Emphasis is given on the flow field at the wingtip and the investigation of the tip
vortices. A comparison of the velocity field is made with and without winglet devices installed at
the wingtips. The experiments are carried out in a closed-circuit subsonic wind tunnel. The flow
visualization techniques include smoke-wire and smoke-probe experiments to identify the flow
phenomena, whereas for accurately measuring the velocity field point measurements are conducted
using the LDA system. Apart from the measured velocities, vorticity and circulation quantities
are also calculated and compared for the two cases. The results help to provide a more detailed
view of the flow field around the UAV and indicate the winglets’ significant contribution to the
deconstruction of wing-tip vortex structures.
Keywords: UAV; measurements; LDA; vortex; winglets
1. Introduction
In recent years, there is an increasing trend in the development and use of fixed-wing Unmanned
Aerial Vehicles (UAVs) by both military forces and civilian organizations [
1
]. They are ideal solutions
for a wide range of operations, such as fire detection, search and rescue, coastline and sea-lane
monitoring, and security surveillance [
2
]. Due to the absence of crew on-board, they present several
advantages, such as the reduced operational cost, the ability to operate under hazardous conditions,
and the increased flight endurance, which is one the most important characteristics when it comes to
the aforementioned missions. In the case of a UAV, where the endurance is only limited by the available
fuel on-board, it is essentially up to the aerodynamic efficiency of the configuration to maximize the
flight time. For a given mission, the required lift force is defined, therefore the optimization process is
essentially keeping the corresponding drag force as low as possible.
For a UAV that operates in the low subsonic, incompressible regime, as is the case with most
Medium-Altitude-Long-Endurance (MALE) UAVs [
3
–
6
], a drag breakdown analysis was conducted
in [
7
], which shows that the part that contributes the most to the total drag force of the aerial vehicle is
the main wing. More specifically, when exposed in subsonic, incompressible flow, a finite wing’s drag
is a combination of profile drag and induced drag. The profile drag is the drag due to the shape of
the body (skin friction + flow separation effects), whereas the induced drag is a result of the pressure
imbalance at the tip of a finite wing between its upper (suction side) and lower (pressure side) surfaces.
At the tip though, this imbalance causes the high-pressure air from the lower side to move upwards,
where the pressure is lower, leading to the formation of a vortex, i.e., the wingtip vortex (Figure 1).
This three-dimensional motion alters the flow field above the entire wing as well, thus resulting in the
Aerospace 2017, 4, 53; doi:10.3390/aerospace4040053 www.mdpi.com/journal/aerospace
