Mark Ewing

Validation of Panel Damping Loss Factor Estimation Algorithms Using a Computational Model

Panel damping loss factors are estimated in full octave frequency bands with standard 1/3 octave band center frequency using a finite element model of a rectangular plate mechanically exited at a single point. The Impulse Response Decay Method (IRDM) and the Random Decrement (RD) techniques are studied for a range of loss factor from 0.001 to 0.1. The effect of considering varying numbers of response points was the primary focus. The effect of alternative time-domain filtering schemes for the RD method is also considered.

Multichannel sense-and-avoid radar for small UAVs

To unlock the economic and societal benefits of unmanned aerial vehicles (UAVs), they must possess an acceptable level of situation awareness so as not to become a public safety hazard. Implementation of a system to provide this awareness will surely involve a sense-and-avoid radar due to its capability and robustness. This paper focuses on the development of a sense-and-avoid radar system being developed for operation on a 40% scale Yak-54 UAV that will provide range, Doppler, and angle-of-arrival information of targets which might cause a collision. A two-dimensional (2-D) fast-Fourier transform (FFT) is implemented which maps targets of interest to a single range-Doppler cell. Phase differences in the target signal from an array of receiving antennas are used to determine the target echos three-dimensional angle-of-arrival. The complete radar system consumes slightly less than 20 W. While the investigation is still underway, initial results indicate that a frequency-modulated, continuous-wave radar system using a 2-D FFT processing algorithm is one viable solution to improve UAV situation awareness.

Shielding effectiveness of composite and aluminum aircraft, model and measurement comparison

Modern aircraft are subject to a barrage of high intensity radiated fields (HIRF) from a wide variety of man-made sources. Fortunately, the aluminum skin of traditional aircraft provides significant shielding to the sensitive electronics inside the aircraft. However, the drive to create lighter, more fuel efficient aircraft has created a trend in the aerospace industry away from aluminum, to lighter composite materials. These composite materials do not always provide the same level of shielding as their aluminum counterparts and has created an urgent need to characterize and measure the shielding effectiveness of entire airframes in a timely, cost effective manner. This paper presents preliminary results for an airframe analog and a composite Uncrewed Arial Vehicle (UAV) that show the viability of virtual measurements to identify and help correct shielding problems earlier in the design phase of an aircraft than traditional measurements.

Effects of Vibration on a Wing-Mounted Ice-Sounding Antenna Array

Airborne sounding of ice sheets requires large, wing-mounted antenna arrays to effectively filter and suppress the surface clutter that often masks weak bed echoes. However, when a high-sensitivity antenna array is mounted to the wings of an aircraft, the array is subjected to structural dynamics and subsequent deformation. We measured the response of a scaled wing-mounted array when excited at four different vibration frequencies to characterize the effects of airframe vibration on array beamforming and received radar signals. We determined that phase and amplitude errors caused by the expected vibration from the aircraft do not significantly degrade the radiation pattern when the Chebyshev or minimum-variance distortionless response (MVDR) beamformers are used. In the case of the Chebyshev-weighted array, vibrations did not cause pattern sidelobes to vary by more than 1.5 dB. In the case of the minimum-variance-distortionless-response-weighted array, vibrations did cause pattern nulls to shift and decrease in depth, but these pattern distortions were negligible, and did not significantly degrade clutter suppression. In addition, we were able to identify the frequency of vibration as well as the frequency of local structural modes by taking the FFT of the signals phase.

Predicting Damping Loss Factors for Beams and Plates with Constrained Layer Damping

Constrained layer damping (CLD) treatments, wherein a viscoelastic sheet is sandwiched between a structural sheet or plate and a covering or constraining sheet, are widely used to suppress vibration and noise. Techniques for predicting the damping loss factor for such a construction include the Ross-Ungar-Kerwin (RUK) complex modulus technique [1] (which is only applicable when the treatment completely covers the structural sheet), the Modal Strain Energy Method [2] and the Analytical Power Input Method [3].

Shielding Effectiveness of Carbon–Fiber Composite Aircraft Using Large Cavity Theory

This paper extends reverberation chamber theory to include chambers constructed out of non-metallic composite materials. This extension allows reverberation chamber theory to predict the shielding effectiveness (SE) of modern aluminum and composite aircraft. Existing theory is based on a power balance approach for aperture-excited cavities, and this paper extends it to include leakage through the cavity walls. Cavity excitation and power dissipation mechanisms are examined in detail, and the cavity SE is related to cavity energy loss in terms of the “quality factor.” SE measurements were made on a partially assembled Uncrewed Aerial System constructed with a carbon-fiber composite skin. The test-analysis agreement shows a high degree of correlation.

Coupling loss factor estimation for plates joined along a line: Analysis and experiment.

Coupling loss factors for two plates joined at a line (a 90‐deg bend), but otherwise free to respond in lateral vibration, have been studied both analytically and experimentally. Two cases have been studied: one with no added damping and one in which one of the plates has a small amount of damping provided by a constrained layer treatment. Statistical energy analysis (using VA ONE) and the finite element method (using NASTRAN) have been used to predict the coupling loss factors. The finite element predictions are based on the averaged responses for a range of boundary conditions on the plates, essentially a “pseudo‐statistical” approach. Experimentally, one of the plates was excited while the kinetic energy of the two plates was assessed with a scanning laser vibrometer (scanning a dozen or more points) and four accelerometers on each plate. Both impulse and persistent excitations were used. The coupling loss factors were assessed by balancing the power input with the energies of the plates, also using th...

Evaluation of the reverberant environment of carbon-fiber composite airframes

The interior of a metallic aircraft can be modeled as a reverberant chamber to successfully predict the shielding effectiveness of aircraft. However, with the trajectory of aircraft design, planes will soon be constructed not with aluminum skin, which fits well with the reverberation chamber model, but with composite skin. It is not clear that a reverberation chamber constructed from composite materials will in fact reverberate. This study seeks to fill this void by constructing a carbon-fiber composite chamber and evaluating its reverberation characteristics through direct measurement. The results of the study can then be used to establish the validity of the reverberant assumption for carbon-fiber composite airframes. Chamber quality metrics such as the Max-to-Average Ratio (MAR) test and the Coefficient of Variation (CV) test are presented from 3 GHz to 6 GHz along with representative single frequency distributions. Measurement data from the chamber is in good agreement with theoretical values.

On the Effect of Mechanical Excitation Position on Panel Loss Factor Estimation With the Power Input Method

For experimental determination of structural loss factor with mechanical excitation, the excitation location — i.e. “where to excite?” — and sensor placement — i.e. “where to measure?” — are quintessential considerations. For a highly-damped panel a significant portion is not experiencing reverberant field conditions, especially in higher frequency bands. That is, localized disturbances “die-out” before they can reflect off boundaries. As energy flows away from the excitation point, the energy the level in the direct field is higher than elsewhere, thereby resulting in higher response levels. Since the level of response is inversely proportional to damping, the loss factor predicted inside the direct field is underestimated. The size of direct field is proportional to frequency and loss factor. Therefore, for a highly damped and/or small plate, loss factor estimation based on randomly positioned accelerometers—which have a higher probability of being inside the direct field—will tend to underestimate damping.Copyright

Method for Design and Analysis of Externally Mounted Antenna Fairings in Support of Cryospheric Surveying

Snow and ice penetrating radar antenna arrays have been developed and flown on NASA DC-8 and P-3 aircraft as part of the NASA Operation Ice-Bridge program. These arrays required custom, externally-mounted fairings to house the antennas. This paper documents the method for generating project requirements, load development, and subsequent structural design and analysis. Static load and modal tests were also performed to verify the physics based simulations. Involvement in such interdisciplinary work provides the basis for large numbers of graduate and undergraduate students to gain practical experience in designing, fabricating, ground testing and flight testing aerospace structures.