The objective of this research was to measure the
effect of pressure-sensitive paint (PSP) on the aerodynamic
performance of high-aspect-ratio, semi-span
wings at transonic cruise and landing conditions. The
PSP technique for measuring pressure distributions on
wind-tunnel models requires coating the surface of
the model with special paint that luminesces when
illuminated by light of appropriate frequency. The
technique has the potential to eliminate the need for
pressure taps in wind tunnel models while yielding
pressure information over entire surfaces rather than
just at discrete points. The presence of paint on a
model, however, can alter the flow (that is, it can
become "intrusive") by adding thickness to the model
or by changing the roughness of the model and thus
altering the development of the boundary layer.
Changes in surface roughness are likely to be most
critical at high Reynolds numbers where boundary
layers are thinner.
Two models were tested: (1) a single-element,
supercritical wing at transonic cruise conditions in
High Reynolds Number Channel 2 (HRC-2); and (2) a
multi-element wing-body model complete with slats,
flaps, and engine pylon and nacelle at landing
conditions in the Ames 12-Foot Pressure Wind
Tunnel. The effect of the paint was determined by
comparing pressure-tap data (both models) and
balance data (high-lift model only) from runs with
and without paint on the models.
Paint intrusiveness was measured on both
models. The shock wave on the cruise model was
displaced slightly upstream when the model was
painted relative to when it was not painted. This
occurred at all Reynolds numbers (7.3 million to
13.6 million) even after the paint had been polished
to a "hydraulically smooth" finish. The stall angle of
the high-lift model at the highest Reynolds number
(6.7 million) was nearly 4 lower when there was
unpolished paint on the leading-edge slats compared
to when the model was unpainted (figure 1). Polishing
the paint on the slats restored the stall to its
paint-off behavior. Applying paint to other parts of
the wing had very little effect. Even before being
polished, the paint was hydraulically smooth at all
Reynolds numbers (3.4 to 6.7 million).
These experiments demonstrated that pressure
paints applied to wind tunnel models must be very
smooth. The roughness of paint along the leading
edges of high-lift models is especially important.
Accepted roughness criteria developed for simplified
geometries may not apply to complex, three-dimensional
configurations. This research shows the
importance of assessing the intrusiveness of pressure
paint whenever it is used.
Point of Contact: E. Schairer
(650) 604-6925
eschairer@mail.arc.nasa.gov
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