Communication
Effect of Leading-Edge Slats at Low
Reynolds Numbers
Lance W. Traub *
,†
and Mashaan P. Kaula
†
Aerospace Engineering Department, Embry Riddle Aeronautical University, Prescott, AZ 86301, USA;
mashaank@hotmail.com
* Correspondence: traubl@erau.edu; Tel.: +1-928-777-6683
† These authors contributed equally to this work.
Academic Editor: Ning Qin
Received: 5 August 2016; Accepted: 13 November 2016; Published: 17 November 2016
Abstract:
One of the most commonly implemented devices for stall control on wings and airfoils is a
leading-edge slat. While functioning of slats at high Reynolds number is well documented, this is not
the case at the low Reynolds numbers common for small unmanned aerial vehicles. Consequently,
a low-speed wind tunnel investigation was undertaken to elucidate the performance of a slat at
Re = 250,000. Force balance measurements accompanied by surface flow visualization images are
presented. The slat extension and rotation was varied and documented. The results indicate that
for small slat extensions, slat rotation is deleterious to performance, but is required for larger slat
extensions for effective lift augmentation. Deployment of the slat was accompanied by a significant
drag penalty due to premature localized flow separation.
Keywords: slat; slot; low Reynolds number; stall control; flow control; unmanned aerial vehicle
1. Introduction
The stall of an airfoil or wing is a viscous phenomenon attributed to flow separation [
1
]. Stall may
be encountered whenever high-lift is desirous, e.g., during take-off, landing or during maneuvering.
Early studies indicated that devices placed at the leading-edge that attenuated the suction peak and
moved loading aft were effective in delaying stall [
2
–
10
]. Representative devices include leading-edge
slats, fixed slots and auxiliary airfoils ahead and above the leading-edge, as well as flaps [
2
–
17
]. A slat
(or slot) differs from a flap in having a gap through which windward surface fluid is vented to the
leeward surface. An auxiliary airfoil [
2
,
17
] uses a low drag “vane” (typically an airfoil) placed ahead of
and above the airfoil/wing. These devices commonly suffer from excessive drag at low angles of attack,
but avoid the complication of a retraction mechanism required for a slat. Leading-edge stall control
devices work by attenuating the loading over the main wing section and moving the loading peak aft.
They also serve to “refresh” the boundary layer in that a new boundary layer forms on the wing panel.
Studies have explored the effect of the defining variables for a slat, i.e., the forward extension of the
slat and its rotation, the downward “droop” and the gap size at the slot outlet [
2
]. Data indicates that
at low-speed, the slat should be rotated below the chord plane of the airfoil (droop) [
8
]. Suggested
design parameters for a slat are [
2
] a length greater than 12% of the chord, a forward extension greater
than or equal to 60% of the slat chord and a slot gap of approximately 3% of the chord.
The utility of unmanned aerial vehicles (UAVs) has promulgated a large research effort to develop
and characterize supporting technologies to enhance efficiency. A complication of slats employed
at low-speed is the resulting Reynolds number. As an example, assuming a chord of 125 mm for
a small UAV with a slat of 15% of the chord yields a Re of 44,000 (sea level) at a flight velocity of
35 m/s. This value is extremely low and is well within the domain of laminar separation, compounded
by difficulty in transitioning the boundary layer. Another low Reynolds number complication is an
Aerospace 2016, 3, 39; doi:10.3390/aerospace3040039 www.mdpi.com/journal/aerospace
