ETW is the most advanced aerodynamic test facility in the world. By applying low temperature operation, ETW is capable of accurately simulating actual high-lift and high-speed flight conditions of modern transport aircraft, defined by the Mach number and the Reynolds number. What this tunnel stands out for is its ability to match the respective high Reynolds number, a key feature which cannot be done in conventional wind tunnels at ambient temperature.

Fundamentally, if the temperature of the flow is decreased, the viscosity of the gas and the velocity of sound decrease and the density increases. The overall effect of cooling is that the Reynolds number increases rapidly. Thus a pressurized tunnel at very low “cryogenic” temperatures can provide real-flight Reynolds numbers by virtue of both increased pressure and decreased temperature.

In ETW the models are not tested in an air stream, as is the case in conventional wind tunnels. Instead, a pure nitrogen flow with a temperature down to 110 Kelvin (= -163°C = -261°F) is driven through the closed aerodynamic circuit of the wind tunnel, subjected to pressures up to 4.5 bars and passes through the test section at speeds up to the low supersonic range (M = 1.35).

A further remarkable advantage of the cryogenic concept applied to a pressurized tunnel is that Mach number, Reynolds number and dynamic pressure can each be varied, keeping the others constant so that their effects can be studied independently (i.e. compressibility effects, friction effects and deformation effects).

Fuel is one of the highest cost items of an airline operation and oil prices are volatile. Therefore, when an airline decides to buy new equipment, fuel consumption is one of the first things it looks at, and the aerodynamic performance of an aircraft is essential (see ATAG Beginner's Guide to Aviation Efficiency). For optimizing such performance with appropriate risk mitigation, ETW provides a key capability. As shown in various cases, the excellent, unmatched accuracy of the ETW test data with flight test results more than compensates the extra efforts by cryogenic testing making it the most effective wind-tunnel testing method.

In many cases, ETW testing at flight-Reynolds numbers, or separation of Reynolds-number and aeroelastic effects revealed serious performance-critical shortcomings of intermediate aircraft designs which were not detectable either by numerical simulation or by preceding conventional wind-tunnel testing. ETW enabled clients to detect and overcome these deficiencies early enough and avoid follow-on costs due to late design changes. Such experiences foster the trend from conventional to flight-Reynolds number testing.