Laser Displacement Measurements of Fan Blades in Resonance and Flutter During the Boundary Layer Ingesting Inlet and Distortion-Tolerant Fan Test

Abstract

NASA's Advanced Air Transport Technology Project is investigating boundary layer ingesting propulsors for future subsonic commercial aircraft to improve aircraft efficiency, thereby reducing fuel burn. To that end, a boundary layer ingesting inlet and distortiontolerant fan stage were designed, fabricated, and tested within the 8'x6' Supersonic Wind Tunnel at NASA Glenn Research Center. Because of the distortion in the air flow ingested by the fan, the blades were designed to withstand a much higher aerodynamic forcing than for a typical clean flow. The blade response for several resonance modes was measured during start-up and shutdown, as well as at near 85% design speed. Flutter in the first bending mode was also observed in the fan at the design speed, at an off-design condition, although instabilities were difficult to instigate with this fan in general. Blade vibrations were monitored through twelve laser displacement probes that were placed around the inner circumference of the casing, at the blade leading and trailing edges. These probes captured the movement of all the blades during the entire test. In addition to blade vibration results, benefits and disadvantages of laser displacement probe measurements versus strain gage measurements are discussed.

Aeromechanics Analysis of a Distortion-Tolerant Fan with Boundary Layer Ingestion

“…This location corresponds to the tip probe location of the optical measurement system in the wind tunnel test, as described in Ref. 22. The calculated response amplitude is 16.2 mils.…”

“…Further work is required to understand how flutter computations can be performed near the operating conditions where flutter occurred during the test. In addition, the observed structural mode 21,22 during flutter was mode 1 and a 2 ND pattern was identified through post-test data analysis. In contrast, the TURBO computations showed the lowest levels of aerodynamic damping for mode 2 and a 1 ND pattern.…”

“…The fan blade forced response near a resonance is dominated by a single mode unless the modes are closely spaced. Since the lower modes of the DTF fan blade are well separated from each other [20][21][22] , it is not necessary to include other modes in the resonant response analysis. Further, the BLI distortion produces aerodynamic excitations that act as (backward) traveling waves on the fan blades and excite only specific blade modes with particular nodal diameter patterns when the dynamics of blade to blade differences in structural characteristics (mistuning) is not included in the formulation.…”

“…During the wind tunnel test, data was acquired [21][22] at this resonant crossing for aeromechanics code validation by increasing the fan rotational speed at a constant rate through this crossing. Analysis of the data provides the following information at resonance for each of 18 blades as measured by each of 7 probes: rotational speed, vibration frequency, response amplitude, and damping.…”

“…The measurements were acquired in three separate test runs at approximately the same nominal test and operating conditions. The analysis results are compared to all these measured values 22 in Fig. 23.…”