Vladimir KULESH, Dr. Sc. (Tech.), head of Department of Optico-Physics Research, Zhukovsky Central Institute of Aerohydrodynamics (TsAGI);
Vladimir MOSHAROV, Cand. Sc. (Phys. & Math.), senior researcher of the same institute
In developing new flying machines, experts focus their attention, among other things, on their aerodynamic shape and configuration. They build a number of scale models of future aircraft, choppers and space probes which are then submitted to tests in wind tunnels. Such tests help assess the strain and stress to which these craft will be exposed in general and their sections and segments in particular. For measuring these integrated forces and momentums (lift and lateral force, drag and moments of forces around three orthogonal axes) a model is positioned in a stream of air upon wind-tunnel balance for aerodynamic load tests.
The pattern of local loads distribution on the surface of a model is of key importance for understanding the air-How physics around an aircraft and for working out a configuration with minimal drag and maximum strength.
In the traditional approach hundreds and thousands vents are drilled through the model skin, or surface, which are attached to pressure sensors by tubes. This procedure is known as venting, or drainage, and such tests are usually few and far between because of their cost and complexity, although they are a must for every new type of aircraft. For example, according to the US company Lockheed Martin the cost of a model for weight tests approaches 570 thousand dollars and for drainage tests it exceeds 950 thousand. And if weight models could also be used for other tests, stress distribution patterns could be obtained at early stages of new aircraft design which could expedite the whole development process of new aircraft.
The search for alternative methods of stress distribution measurements has been going on for quite
some time, but none of them warranted practical application. In the late 1970s the attention of our Institute experts, Georgi Pervushin and Lev Nevsky, was drawn by studies conducted by Illarion Zakharov of the Leningrad Institute of Technology. He investigated the quenching of luminescent glow of organic dyes (luminophores) by oxygen molecules. And our researchers decided to try and use this effect for measuring aerodynamic pressure distribution on the surface of aircraft models in wind tunnels. The experiment was a success and the idea was patented in 1980. The first pressure-sensitive coating (a layer of silica gel with absorbed molecules of organic luminophore) was patented in 1981. The authors of the invention called this substance baroindicator and it was far from perfect because it could only help visualize the stress pattern upon a model, but not to measure these stresses.
The first results of such visual observations of pressure distribution upon a sphere and a plate were published in 1982. And even with all of the drawbacks of such tests, they demonstrated the promising potential of such studies. Drawn to the new research project were the Institute of Chemical Physics of the USSR Academy of Sciences, the Institute of Physical Chemistry of the Ukrainian Academy and chemistry departments of the Lomonosov Moscow State University and of the State Universities of Leningrad and Karaganda. Several different types of coatings were developed by late 1989 which were sensitive to pressure (they were called luminescent pressure converters - LPC). Also developed at that time were means of excitation and digital registration of luminescence intensity and methods of digital processing of the obtained images. Several wind tunnels at TsAGI were equipped with LPCs.
The new technique consists in coating the surface of an aircraft model with a thin layer consisting of a binding substance, such as some polymer, and a luminophore. The coated model is placed into a wind tunnel and irradiated with excitation light. The picture of distribution of luminescence intensity upon the model is observed by digital cameras and registered by computers. This picture is taken twice-first without stress and then in an airflow at given aerodynamic flow regimes. A reduction of pressure produces more intense luminescence and its increase causes the opposite effect. The exact dependence of luminescence intensity on pressure is determined in advance through a procedure of LPC-coating calibration. An important stage of this technique is the digital processing of images obtained as a result of which the picture of intensity distribution is translated into a picture of pressure distribution under investigation.
At the present time the LPC technology is having its heyday, so to say, being applied by practically each and every aerodynamic center and/or aircraft company or lab. Specialized conferences are held on an annual basis in the United States and leading West European countries at which LPC methods are discussed and debated.
As for Russia's TsAGI, it can probably boast the largest store of experience and practical achievements in this field in tests covering a whole number of domestic and foreign types of aircraft. Russian experts were the first to apply LPC for studies of hypersonic Hows in shock tunnels at time intervals of only 20 ms. A special coating was developed with this aim in mind which rapidly responds to pressure changes and such coating is indispensable in dealing with problems of unsteady aerodynamics and in studies of high-speed processes.
TsAGI has also been the first to use LPC techniques in studies of
pressure distribution on rapidly rotating objects such as propellers, helicopter rotor blades and compressor vanes. In future this method can be very promising in the development of not only turbojets, but also of turbines for power stations and compressors for gas pumping stations. This, of course, will call for the development of LPC coatings with new properties matching a higher range of temperatures and pressures. It will also be necessary to improve the measuring gear and the image processing program support. Studies in all of these areas are now underway in many countries.
Over the past few years luminescent conversion technologies have been finding their way into the region of lower subsonic velocities which apply not only to aircraft landings and take-offs but also to automobile engineering, industrial aerodynamics and the aerodynamics of buildings and structures. Pressure fields measurements by the above methods are a difficult proposition and their attraction lies in the fact that such measurements, same as optical ones, are noncontact ones which do not add noticeable distortions into the processes under investigation. US scientists have already succeeded in obtaining authentic results at airflow velocities of about 30 m/s (100 km/h) and attempts continue to conduct such measurements at velocities of 10 m/s which calls for much more sensitive coatings.
In a word, achievements of TsAGI researchers in this promising field have already received international recognition and studies in this area now continue in many countries.
Prepared by Arkady MALTSEV
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