Aeromechanical Response of a Distortion-Tolerant Boundary Layer Ingesting Fan

Abstract

Boundary layer ingestion (BLI) is a propulsion technology being investigated at NASA by the Advanced Aircraft Transportation Technology (AATT) Program to facilitate a substantial reduction in aircraft fuel burn. In an attempt to experimentally demonstrate an increase in the propulsive efficiency of a BLI engine, a first-of-its-kind subscale high-bypass ratio 22″ titanium fan, designed to structurally withstand significant unsteady pressure loading caused by a heavily distorted axial air inflow, was built and then tested in the transonic section of the GRC 8′ × 6′ supersonic wind tunnel. The vibratory responses of a subset of fan blades were measured using strain gages placed in four different blade pressure side surface locations. Response highlights include a significant response of the blade's first resonance to engine order excitation below idle as the fan was spooled up and down. The fan fluttered at the design speed under off operating line, low flow conditions. This paper presents the blade vibration response characteristics over the operating range of the fan and compares them to predicted behaviors. It also provides an assessment of this distortion-tolerant fan's (DTF) ability to withstand the harsh dynamic BLI environment over an entire design life of billions of load cycles at design speed.

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

“…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.…”

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

“…7) may occur close to idle speed, and could be excited during descent. The 2EO/M1 crossing had significantly higher response than any other mode crossing, although it remained below the modified Goodman stress limit [6]. However, the high response prompted the development of a fan/tunnel startup/shutdown procedure to limit the duration and magnitude of the resonant response.…”

“…The blades were designed such that the predicted worst case stress level at the Aerodynamic Design Point (ADP) at 100% speed was less than the modified Goodman limit; however, with the distorted flow it was critical to measure blade vibrations at all rotor speeds and aerodynamic conditions throughout the test. Strain gages were placed in several locations on a few of the blades, and those test results are presented in detail by Provenza [6]. Additionally, a Non-contacting Stress Measurement System (NSMS) consisting of laser probes was used to measure the blade tip circumferential displacement during the test.…”