Proposal for: Design and Fabrication of a Vibration-Isolated Avionics Box for a Rotary-Wing UAV

Submitted to: Steve Glusman, Boeing Defense and Space Group Helicopter Division

Dr. Ed Smith, Department of Aerospace Engineering at Penn State University

Dr. Joe Horn, Department of Aerospace Engineering at Penn State University

Dr. Eric Mockensturm, Penn State University

Submitted by: Good Vibrations (Group 7, Boeing 5)

Thomas Donahue

Mike Schroeder

Rob Liszewski

Ben Cohen

Date: 02 / 01 / 04

Executive Summary:

Contained in this report are details of the preliminary design and proposal for fabrication of a vibration-isolated avionics box for a rotary-wing UAV. The preliminary design proposed in this report is for a remote control helicopter which is manufactured by Bergen R/C Helicopters and named the Intrepid 60. This project is sponsored by Boeing Defense and Space Group Helicopter Division and directed by the Department of Aerospace Engineering at Penn State University.

This report contains an introduction explaining the recent interests developed in Unmanned Air Vehicles (UAV’s) and the problems involved with electrical sensors and equipment installed in rotary-wing UAV’s. The introduction further explains the attention that has been directed toward the high vibration levels that are produced in rotary-wing UAV’s mostly by the main rotor of the helicopter which is the main cause for most of the vibration related damage to the sensitive electrical equipment added to the aircraft. The high vibration levels are due to high accelerations (G’s) that are produced form the rotary-wing UAV. The introduction further explains the vibration-isolation design problems and states the objective of this design and fabrication project.

This proposal also contains guidelines and design specifications that were given for the preliminary design process for the avionics vibration-isolated box. These specifications were distributed by Ed Smith and Joe Horn from the department of Aerospace Engineering at Penn State University. The engineering guidelines and specifications are mainly directed toward the vibration levels the equipment contained and protected inside the avionics box can withstand and measure.

Three different designs are also shown in this proposal and their details are contained in the Conceptualization section of this paper. This section compares all three individual designs according to cost and ease of manufacturing and further explains the details on the choice of the final preliminary design of the vibration-isolated avionics box.

The last section of this report explains the timeline concerning the design and fabrication of this project. Contained in this section is a Gantt chart created using Microsoft Project to plan the different tasks that will be involved in this project. Schedules are also provided in this section for ordering materials, manufacturing the box, and lab time used to test the final product. A budget for materials is also contained in this section of the report.

Table of Contents:

Page Title

i. Cover Page

ii. Executive Summary

iii. Table of Contents

1 Introduction

2 Design Specifications

3 Frequency Plots

4 Conceptualization

5 Conceptualization Cont. / Planned Activities

6 Planned Activities Cont.

7 Gantt Chart

8 Bibliography

Appendix – SK - 1 Schematic of Design 1 - SK-1

Appendix – SK - 2 Schematic of Design 2 - SK-2

Appendix – SK - 3 Schematic of Design 3 - SK-3

List of Figures

Page Title

1 Figure 1. - Figure of The Intrepid 60

7 Gantt Chart

Appendix – SK - 1 Schematic of Design 1 - SK-1

Appendix – SK - 2 Schematic of Design 2 - SK-2

Appendix – SK - 3 Schematic of Design 3 - SK-3

Introduction:

Recently there has been a considerable interest in the field of Unmanned Air Vehicles (UAV’s), both computer and remote control guided, for achieving a diverse range of tasks for military and commercial applications. UAV’s require highly reliable, lightweight electronic sensors for guidance, navigation, control, communication, and other similar purposes. For applications involving rotary-wing UAV’s protecting this equipment is a major issue due to high vibration levels produced by the engine, the main rotor and the tail rotor of the aircraft. Because a helicopter has three main sources of vibration that produce multiple frequencies of excitations, vibration-isolation and vibration analysis of a helicopter is very difficult.

For this project, sponsored by the Boeing Defense and Space Group Helicopter Division, our group will attempt to design and build a vibration-isolated avionics box for a rotary-wing UAV. The rotary-wing UAV used in this project will be the Intrepid 60 Remote Control Helicopter that was built by Bergen R/C Helicopters and is shown below in Figure 1. This helicopter has been modified in the past by other groups who added inertial / GPS navigation and vibration measurement systems. These systems, however, when subjected to the high vibration levels produced by the RC helicopter where damaged and failed both electronically and physically. The vibration-isolated avionics box designed for this project will attempt to protect the previously installed electrical sensory equipment added to the RC helicopter and any equipment that might be added in the future.

Figure 1. The Intrepid 60

To protect the electrical equipment installed in the RC helicopter our group will need to reference data analyzing the vibrational frequencies produced by the engine, main rotor and tail rotor of the aircraft. The data that will be used in this project will be obtained from a report written by Ronald A. Davis that is titled “The Development of Experimental Teaching Facilities for Rotorcraft Aerodynamics and Dynamics.” This report contains data and graphs pertaining to the multiple vibration levels produced by the Intrepid 60 RC helicopter. Included in this report are Fast Fourier Transforms plots that analyze the multiple frequencies of excitations produced by the helicopter at different modes of flight (hovering, low-speed and high speed flight). These plots and along with other technical information included in this report will aid in the design and fabrication of the vibration-isolated avionics box.

Design Specifications:

As mentioned in the Introduction section of this report a helicopter has three main vibrational sources which are: the engine, main rotor and tail rotor. For the design of the vibration-isolated avionics box the vibrational frequencies from the engine and the tail rotor are secondary to low frequency main rotor frequencies. According to Figures 2, 3, and 4 on the following page which are Fast Fourier Transform plots, the operating frequency of the engine can be interpreted to be around 250 Hz and the operating frequency of the tail rotor can be interpreted to be a frequency of 130 Hz. These frequencies of vibration are very fast as compared to the natural frequency of the avionics box, approximately 30 rad/sec. This gives frequency ratios of approximately 8 and 4 respectively. Theses ratios are large enough to provide sufficient reduction in transmissibility.

Vibrational frequencies generated by the blades of the main rotor occur at frequencies that are integer multiples of the rotor speed. The hub of the helicopter, however, acts as a filter only allowing frequencies that are multiples of the number of blades times the rotor speed to pass through to the fuselage of the aircraft. These frequencies are shown on Figures 2, 3, and 4 and are labeled 1/rev, 2/rev and 4/rev respectively. These frequencies were interpreted to be 28 Hz, 56 Hz, and 112 Hz respectively. These frequencies are extremely important in the design the vibration-isolated avionics box. The frequency of the box must not operate in the range of any of these frequencies or resonance will occur. If resonance is allowed to occur the avionics box and the electronic equipment inside will undergo severe damages.

Preliminary design specifications have been provided to us by advisors Ed Smith and Joe Horn from the department of Aerospace Engineering at Penn State University concerning the design and fabrication of the avionics box and they are as follows:

· The weight of the components will be between 3-5 lbs

· Maximum space allowed for the Avionics is 10” x 6” x 6”

· Vibrations inside the box will be minimized to less than 0.5 g’s

Figure 2. – Frequency Content of Hover Flight for the Intrepid 60 RC Helicopter

Ronald A. Davis, 2003

Figure 3. – Frequency Content of Low-Speed Flight for the Intrepid 60 RC Helicopter

Ronald A. Davis, 2003

Figure 4. – Frequency Content of High-Speed Flight for the Intrepid 60 RC Helicopter

Ronald A. Davis, 2003

Conceptualization:

Design 1: (see schematic SK-1 for reference)

Design one will place the avionics box inside a second box. The outer box will be constructed from aluminum. All of the outside corners of the avionics box will have springs connected to the inside corners of the outer aluminum box. The outer aluminum box will be rigidly connected to the bottom of the helicopter with screws. Neoprene isolators will be installed between the avionics box and the helicopter and will be chosen according to the k-value of the material. The neoprene isolators have a damping ratio of 0.05 and are ideal for decoupling rotational and translational modes in all axes of rotation and have a very low cost.

This design will allow for the avionics box to undergo 3D movement while minimizing vibrations in all three axes of rotation. One shortcoming to this design is that the volume of the outer box may exceed the area underneath the helicopter. To adjust to this problem the idea or concept of having an outer box was discarded and the helicopter skids replaced the outer box. This is shown in drawing labeled SK -2 located at the end of this report. A major concern with this design is the use of the neoprene isolators. The problem with using the neoprene isolators is that under resonance they have an amplification factor of 10:1. This factor could affect this design because the box will be exposed to many different frequencies of vibrations causing the possibility of resonance to be high. This is a favorable design because of the ease of design and the accessibility to the internal components of the avionics box while the box is still attached to the helicopter.

Design 2: (see schematic SK-2 for reference)

The second design will have two compliant connecting arms rigidly attached to the helicopter by means of nuts and bolts. The use of a second set of connecting arms that will connect to the first pair of connecting arms will be used to model the physical characteristics of a spring. Neoprene washers will then connect the avionics box to the second set of connecting arms to help isolate vibrations. These brackets will aid in the damping of vibrational forces and frequencies. A schematic of this design is shown in drawing SK-2 located at the end of this report.

Again, a problem with this design is the neoprene isolators. As mentioned above, the amplification factor of 10:1 may cause problems. Another problem with this design is that the box will most likely sit below the helicopter skids. This will cause problems and the helicopter skids will have to be either modified or changed to encase the avionics box. Also access to the internal components while the avionics box is attached to the helicopter is limited.

Design 3: (see schematic SK-3 for reference)

The third design will be similar to design #2 except the two sets of connecting arms that were connected to the helicopter that were assumed to be rigid will now take on the characteristics of a spring and will not be rigid. The neoprene isolators will also be relocated (refer to schematic SK-3).

The problems with this design are basically the same as the problems in design 2. However this design has one main advantage over design two and that is the boxes potential displacement is higher, depending on the k-values of the connecting rods. The disadvantage with this design is the relocation of the neoprene isolators may cause a problem with vibration isolation.

The design we chose to further analyze is number one. This design was chosen over design two and three because of its ease in design and testing. The four springs can be configured to allow optimal movement of the avionics box while minimizing the vibration effects. The springs on top and bottom of the avionics box should provide substantial dampening. The cost to construct this design should be fairly low. The only additional parts along with the avionics box are four springs and some sort of fasteners to connect the springs to the helicopter and helicopter skids.

Planned Activities:

Included in this section of the report is a Gantt chart which provides a timeline for the remainder of the semester. This timeline indicates different tasks for both class criteria (ME 415W) and project design and fabrication. It also includes a plan for doing detailed design, data analysis, fabrication, design refinement, and final testing. The Gantt chart for this proposal can be found on the following page.

- A Gantt chart providing a timeline for the remainder of the semester

- A plan for doing detailed design and analysis

• Weekly meetings on Thursdays with Ed Smith and Joe Horn to discuss previous weeks analysis
• Week 1: Discuss location of avionics box and design
• Week 2: Perform vibration isolation analysis
• Week 3: Verify calculations and begin fabrication process

- A plan for choosing suppliers and manufacturing processes

• Use online sources to find a local manufacturer of custom springs
• Use local hardware stores to purchase raw materials needed for fabrication of avionics box
• Use the Learning Factory’s resources to fabricate the avionics box

- A schedule for ordering materials and building a prototype

• Design phase must be completed by __/__/____
• Allowing 6 weeks for order processing, spring fabrication and shipping; order must be placed by __/__/____
• During allowed 6 weeks, raw materials will be purchased at local hardware stores
• Also, fabrication of avionics box will be completed using resources of the Learning Factory
• Any modifications to helicopter must also be completed before delivery of springs

• Once springs are delivered they will be attached to avionics box
• Completed avionics box will be then verified for specifications

- A schedule for verification of your design and subsequent refinement

• Once avionics box fabrication is completed a benchmark test will be used
• Any bugs in design will be worked out then, another benchmark test will be preformed
• The avionics box must be compliant before any test flights will be preformed
• Upon successful completion the avionics box will be mounted on the helicopter with “dummy” equipment and then test flown to check for compliance with given specifications
• If first test flight completely successful then a test flight may be scheduled with the real avionics equipment
• Successful completion of either test flight will mark the end of the design project fabrication phase

-A budget for materials and other expenses

Material or Service	Quantity	Cost /Item ($)	Total Cost ($)
Sheet aluminum	1	50	50
Neoprene washers	4	10	40
Hinges	2	2	4
Screws	1 box	2	2
Custom springs	8	5	40
Test Pilot	1 day	200	200
		Total	$336.00

Bibliography:

Davis, Ronald A. “The Development of Experimental Teaching Facilities for Rotorcraft Aerodynamics and Dynamics”. The Pennsylvania State University: The Department of Aerospace Engineering, 2003.

Rao, Singiresu S. Mechanical Vibrations, Third Edition. New York: Addison-Wesley Publishing Company, 1995.