Pressure measurement on the surface of aircraft models plays an important role in wind tunnel testing. The cost of this testing is very large because a specially designed model with numerous pressure taps is required.

New, non intrusive optical pressure measurement method -Luminescent Pressure Sensor (LPS) method — can lead to essential reduction in the cost of wind tunnel tests and in aircraft design cycle. Method is based on a phenomenon of deactivation of photoexcited molecules of organic luminophore by oxygen molecules (quenching). Aircraft model surface is covered by Luminescent Pressure Sensor — thin polymer layer, penetrable for oxygen, containing probe luminophore molecules. Exited by appropriate light source the probe molecules may emit light (luminescence) or may lose energy by interaction with oxygen molecules (luminescence quenching). The part of the lost energy is directly proportional to the oxygen concentration and oxygen mobility in the polymer layer. According to Henry’s law the concentration of oxygen in polymer layer is directly proportional to oxygen partial pressure on its external boundary. As a consequence the luminescence output is inversely proportional to oxygen partial pressure above polymer surface. If oxygen concentration in the air is constant, the effect of luminescence quenching may be used for measurement of the air pressure.

In LPS-method the model is covered by LPS paint and no other special preparation of the model is necessary (e.g. manufacturing of numerous pressure taps and installation of any transducers inside the model). Thus, optical pressure measurements can be performed on any model designated for an ordinary balance measurements and, thus, are faster and cheaper than usual pressure taps measurements. Non-intrusive nature of LPS-method also allows to perform pressure measurements on extremely thin and movable objects, e.g. wing edges, control surfaces, rotating machinery, propeller blades, etc..

LPS method of wind tunnel measurements consists of illumination of the model covered by LPS paint by excitation light and acquisition of luminescence intensity value. Two images (one with airflow — «wind-on» — and another without -«wind-off») should be acquired and «wind-on» image should be divided on «wind-off» image. This rationing allows to exclude an effect of LPS thickness and excitation light intensity variations on the model surface, but both images should be perfectly aligned and excitation light intensity distributions during acquisition of both images should be the same or carefully measured.

Temperature Sensitive Paint technique is very similar to LPS except that the coating is temperature sensitive rather than pressure sensitive. The luminescence of TSP is decreases, as the temperature increases.

LPS method was invented in Central Aero-Hydrodynamic Institute (TsAGI), Russia and now is also developed in USA (Boeing, NASA-Ames, NASA-Langly, AEDC, Purdue University, ...), in Germany (DLR), in France (ONERA), in Great Britain (BAS, DERA), in Japan (NAL) and in other aerodynamic centers and universities worldwide. In 1994 LPS technology was awarded by Gold medal on ’43rd Word Exhibition of Invention, Research and Industrial Innovation’ (EUREKA’94).

TsAGI has a many years experiance of LPS method application for aerodynamic investigations. Numerous tests (including commercial) was fulfilled in TsAGI’s wind tunnels T-128, T-109, T-112, UT-1 etc..