OH-58DModal Analysis Results

By: Nicholas Noell, John Macnamara, John Targonski, Khushboo Patel, Davendra Chatterpaul,Raja Akif Zahirudin.

IntroductionModal analysis is the study of dynamic properties of structure under

vibrational excitation

Analysis of vibrational modes is a critical component of design

SpecificationsCruise Speed with weapons - 176 kphRange - 260 kmHover Ceiling - 2286 m Seating - 2 crew seats located in the cockpitEngine - (1) Rolls-Royce 250-C30R3Typical Flight Weights Include:

Combat Mission Weight - 5,200 lbsUseful Load - 2,004 lbsMaximum Gross Weight - 5,500 lbs

Weapons Include:M3P.50 Cal. Machine Gun2.75” Rocket Pods Laser Guided hellfire missiles

SignificanceStructural elements are prone to perceptible vibration, affecting sensitive

equipment and can ultimately shorten the life of the designated structure

Detailed modal analysis can determine the structural mode shapes and their pivotal frequencies

It is of the utmost importance to ensure that the operational modes do not coincide with the structural modes because this phenomena can lead to catastrophic failure

History of OH-58D● The Bell OH - 58 kiowa “Warrior” was first introduced in 1969 and is still in

continuous use by the US Military today● Its designated function was for observational utilities and direct fire support

but has adapted other roles over the years ● Primarily operated in an armed reconnaissance role in support of ground

troops today● It has been utilized in all of the wars since the Vietnam era● There are 4 variants or models to date (A, B, C, D, F) With the F model still

under development

Modal Analysis Background● In the field of Engineering, a Modal Analysis has been an effective way to

measure and analyze the dynamic response of structures● The input for simulated on the structure have usually been achieved with a

hammer and/or shaker● The response have usually been measured with sensors (accelerometers,

load cells, and/or laser vibrometers ● Strain gauges mounted perpendicular to each other were used to measure

vertical and horizontal strain across the structure.● Approaches that can be taken:

○ SIMO (Single Input Multiple Output)○ MISO (Multiple Input Single Output)○ MIMO (Multiple Input Multiple Output)

Modal Analysis Background● Finds various periods where the structure will naturally resonate

● These structural modes are then compared to additional systems that the structure will interface with and determined if the two have any of the same modes

● Locating these modes are critical to ensure that the structure won’t see a high or extended response that could ultimately damage or lead to a structural failure

EquipmentLocation of accelerometers along the tailboom

point 16 point 25Point 61

Vertical and horizontal stabilizers

Point 100 point 58

point 89Vertical Stabilizer Horizontal Stabilizer

Shaker

Location of shaker on the tailboom of OH-58 D

LabVIEWGraphical User InterfaceLABVIEW - a data acquisition software was used to retrieve output from the

accelerometers and vibrational frequencies as well as modal loss factorsfe PostProc was used to animate mode shapes through the frequenciesOther programs like matlab were used

Results

First bending

Second bending

Torsional HorizontalVertical bending

Mode Characteristics

Main rotor frequency drive point accelerance

4

f1=6.6 Hzshaft rate

f2=26.4 HzBPF

f3f3=52.8 Hzf4=79.2 Hz

f1=6.6HzShaft rate f2=26.4 Hz

BPF

Tail rotor frequency drive point accelerance

f1=39.7 Hzshaft rate f2=79.4 Hz

BPF

f3=158.8 Hz f4=238.2 Hz

Main Rotor frequency with strain gauge

Tail Rotor frequency with strain gauge

Modal loss factorsLess Damping was observed since modal loss factors did not exceed 0.10

Animations: Along the tail structure itself

Mode Vibration at 91.028 Hz

Mode Vibration at 81.67 Hz

Animations:Shell modes

Shell modes along the cross section of axis

Shell mdes align to the axis of the geometry

DiscussionAs previously shown, there are multiple occurrences where both structural

and operational modes coincide The main rotor blade passage frequency, 1st harmonic as well as

second were found to be dangerously close to the structural mode frequencies of the tailboom (26.4, 52.8, and 79.2 Hz)

Similarly, the tail rotor encountered the same issues at the shaft rate, 1st harmonic and 2nd harmonic (39.7, 79.4, and 238.2 Hz)

ConclusionThere clearly is a structural issue with the tailboom of the OH - 58 that arises

from vibrationThe findings clearly show that there are numerous instances where both

structural and operational modes resideOur research findings suggest that Bell should develop a new geometric

shape for the tailboom, thus changing the frequencies of the structural modes

But at the end of the day it’s all about the $$$$$ and these problems will persist if the Helicopters operate with their current configuration