AEROLAB

Background

The optimum location for subsonic lift control is at the trailing edge of an airfoil since small changes in the flow field near the trailing edge can result in large changes in the overall flow field. The aileron is a typical aerodynamic control device used to change the lift and drag properties of the airfoil. Despite their lift enhancement properties, aileron devices tend to be bulky, complex to design and expensive to maintain. Typically sized around 25% of the chord, they require a significant amount of actuation power to deflect fast enough to control loads under operating conditions. These so called Gurney-flaps enhance lift in the linear range; however they may also cause a significant drag penalty especially at low lift conditions, such as cruise flight. This drag penalty is the main reason why Gurney flaps are used on only a few aircraft configurations for which high maximum lift is more important than low cruise drag. To avoid the drag penalty, miniature split flaps hinged to the airfoil lower surface have been conceptualized.

Test Rig

Experiments are conducted using three, 12-inch chord, 33.5 inch span test airfoil models. One airfoil is used to perform validation tests and to develop a consistent data set for comparison and correction data. The other two GU-like airfoils will be fitted with the microtabs. All experiments are conducted in the UC Davis Wind Tunnel Facility.

The translational devices are fabricated in silicon using chemical etching techniques and are readily produced in predefined arrays of arbitrary geometry. The dovetail structure provides a natural interlock to prevent the slider from falling out. Completed microtabs have total dimensions of 2 cm × 5 mm × 1.2 mm (l×w×h). The wind tunnel airfoil models are fabricated using foam, fiberglass and epoxy resin. For microtab installation, a recess is routed in the trailing edge. Based on computational results and volume constraints, the tabs will be installed and tested at 5% chord from the trailing edge. This location allows for sufficient room for retracting the tabs without loosing the lift enhancement benefit. Fully retracted the tabs are nearly flush with the surface of the airfoil. Fully extended the tabs extend approximately 3 mm (1% of chord) perpendicular to the surface. This design allows for minimum changes to current wing design and manufacturing techniques. Over 90% of the airfoil would remain unchanged with only modifications to the trailing edge region.

Results

Wind tunnel experimental results are in good agreement with numerical design data from INS2D. The lift at zero angle of attack and lift-curve slope characteristics are matched within ±1%. For a range of angles of attack, an increase in the lift coefficient by 30-50% has been numerically and experimentally validated.



Some experimental results for varying tab heights and tab locations are presented. These tests show the consistency and accuracy in which experimental results can be duplicated in the UC Davis facility and the validity of using numerical design data. Dynamic testing of microtabs continues at Re=1.0 million with fixed transition.