Effects of ice accretions on aircraft aerodynamics

The separation bubble formed on an airfoil at low Reynolds number behind a simulated leading-edge glaze ice accretion is studied experimentally. Surface pressure and split hot-film measurements as well as flow visualization studies of the bubble reattachment point are reported. The simulated ice generates an adverse pressure gradient that causes a laminar separation bubble of the long bubble type to form. The boundary layer separates at a location on the ice accretion that is independent of angle of attack and reattaches at a downstream location 5-40 percent chord behind the leading edge, depending on the angle of attack. Velocity profiles show a large region of reverse flow that extends up from the airfoil surface as much as 2.5 percent chord. After reattachment, a thick distorted turbulent boundary layer exists. The separation bubble growth and reattachment are clearly seen in the plots of boundary-layer momentum thickness vs surface distance. Local minima and maxima in the boundary-layer momentum thickness development compare well with the shear layer transition point as indicated by the surface pressures and the reattachment point as measured from surface oil flow, respectively.

Measurements in a leading-edge separation bubble due to a simulated airfoil ice accretion

The effect of large-droplet ice accretion on aircraft control and in particular lateral control is examined. Supercooled large droplet icing conditions can result in the formation of a ridge of ice aft of the upper surface boot. By comparing this ice shape to data acquired with a spanwise protuberance on a different airfoil, it is clear that a ridge of ice aft of the boot can lead to large losses in lift, increases in drag and changes in the pitching moment. This effect is most likely due to the formation of a large separation bubble aft of the ice accretion which grows with angle of attack and eventually fails to reattach, leading to premature airfoil stall. The bubble alters the pressure distribution about the airfoil resulting in a more trailing edge up (negative) hinge moment on the aileron and the resulting change in aileron stick force. This can lead to aileron hinge moment reversal and aileron snatch. In aileron snatch the hinge moments are altered to the extent that the aileron is pulled up by the low pressure over the upper surface of the aileron with sufficient force to induce a rapid roll if a large stick force is not immediately exerted to oppose it. There is evidence in the literature which shows that similar lateral control problems are possible with other types of ice accretions and airfoil types.

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Aircraft aerodynamic effects due to large droplet ice accretions

Experimental measurements of the effect of simulated glaze ice on a three-dimensional wing are presented. A semispan wing of effective aspect ratio five was mounted from a splitter plate in the OSU subsonic wind tunnel. The model has a straight, untwisted rectangular platform, and uses a NACA 0012 airfoil section. Surface pressures were measured at 5 semispan locations and a total-pressure wake-survey probe was used on the model centerline. The section lift and drag data from the model centerline compared well to earlier two-dimensional data. These data show a large drag and maximum lift penalty due to the simulated glaze ice. Three-dimensional span-load data compare well to computational results.

Effect of simulated glaze ice on a rectangular wing

The effect of large-droplet ice accretion on aircraft control is examined. Supercooled-large-droplet icing conditions can result in the formation of a ridge of ice aft of the upper surface boot. By comparing this ice shape to data acquired with a spanwise protuberance on an airfoil, it is clear that a ridge of ice aft of the boot can lead to large losses in lift, increases in drag and changes in the pitching moment. This effect is most likely due to the formation of a large separation bubble aft of the ice accretion which grows with angle of attack and eventually fails to reattach, leading to premature airfoil stall. The bubble alters the pressure distribution about the airfoil resulting in a more trailing edge up (negative) hinge moment on the aileron and the resulting change in aileron stick force. This can lead to aileron hinge moment reversal and aileron snatch. The fundamental aerodynamic cause of this lateral control problem is the same as that experienced when elevator control is lost due to horizontal tail stall.

/pdf/aerodynamics-of-supercooled-large-droplet-ice-accretions-and-2vhriz8xsh.pdf

Aerodynamics of supercooled-large-droplet ice accretions and the effect on aircraft control

An experimental study has been conducted in the OSU subsonic tunnel to measure the detailed aerodynamic characteristics of an airfoil with simulated glaze ice. A special model was built with interchangeable leading edges to be used in this study. One leading edge is that of a NACA 0012, while the other is a simulation of a glaze ice accretion measured in the NACA Lewis Icing Research Tunnel. The model was instrumented with a dense distribution of surface pressure taps to provide excellent detail around the ice shapes and reattachment point. A traversing total pressure probe was used to document the boundary layer characteristics on the NACA 0012 section. The ice shape caused a severe lift and drag penalty, reducing the maximum lift by over 50 percent and causing a 300 percent increase in drag. Surface pressure distributions revealed a large lower surface separation in addition to the expected large upper surface separation.

M. B. Bragg

Papers

Aerodynamic measurements of an airfoil with simulated glaze ice