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									Electric Tug for a Stearman Airplane:

 A final design report for an electric tug for a

                  Stearman airplane.

                        Prepared For:

 The University of Idaho Capstone Faculty and Dan Holmes

                        Prepared By:

                        Team Airhaul

Ryan Burns, Juan Barajas, Vaughn Schiweck, Aaron Rawlings

                      University of Idaho

      Departments of Mechanical and Electrical Engineering

                        May 5, 2006

DATE: 05/05/2006

TO: Dan Holmes; Project Sponsor

FROM: Team Airhaul: Ryan Burns, Juan Barajas, Aaron Rawlings, Vaughn Schiweck

SUBJECT: Final Report for “Optimization of Aircraft Tug”

Mr. Dan Holmes,

Attached is the Final Report entitled “Electric Tug for Stearman Airplane.”

This report explores the entire design and development for the Optimization of an
Aircraft Tug. It begins with the problem definition, outlining your needs and our
constraints. We continue by investigating mechanical and electrical concepts. Finally, we
conclude with some recommendations and future considerations for the design and
development of the aircraft tug.

We hope to have provided a design that best meets the needs of the end user and yourself.
The report explains our decisions and presents a design that will outperform competing
products at an equal price to the consumer.


Juan Barajas, Aaron Rawlings, Ryan Burns, and Vaughn Schiweck

Executive Summary

        This document is a final report containing the design and development of an
Aircraft Tug. Dan Holmes wants to market an airplane tug that is superior to existing
products. He would also like to market the tug at a price that is competitive to existing
tugs. The report states the details and definition of our client’s request to design an
aircraft tug. We then define the specifications and constraints that the final design must
        We’ve explained all of the concepts that we considered for the final design. We
divided the “Concepts Considered” section into mechanical and electrical concepts. The
team decided on a design that was recommended but our client and our ability to produce
an acceptable product. This report provides a detailed explanation of the concepts that
we have decided to incorporate into our final design.
        A product description of the end design is included to give you a detailed idea of
its functionally. All of the electrical and mechanical components are described along
with their operation. We have documented all of the design testing in the “Product
Evaluation” section of the report. Described are all of the tests and design changes
throughout the duration of this phase.
        A detailed economic analysis of the materials and components are listed. A
materials list for the mechanical assembly and a product list for the electrical are included
with costs. The overall cost of our design is higher then the cost per tug, based on
development. More material was purchased for our design due to changes and
        Lastly, conclusions and recommendations are made from the team based on
experience and design development.



Letter of Transmittal............................................................................................................i

Executive Summary............................................................................................................ii



Problem Definition………………………………………………………………………...2

Concept Development………………………………………..............................................4

Product Description……………………………………………………………………...10

Product Evaluation……………………………………………………………………….13

Economic Analysis……………………………………………………………………....16

Conclusions and Recommendations……………………………………………………..17

Appendix A: Drawing Package

Appendix B: Bill of Materials

Appendix C: Economic Analysis

Appendix D: Instruction/Maintenance Manual

Appendix E: Electrical Training and Assembly

Appendix F: Calculations

Appendix G: DFMEA Chart

Appendix H: Gantt Chart

Appendix I: Resumes

Appendix J: Business Plan

1.0 Background
        Our client’s father designed an aircraft tug prototype about a year ago to
maneuver a Stearman Airplane. The current prototype is still a rough design, allowing
for many areas of improvement. Our goals for the project are to modify and improve
several aspects of a current aircraft tug design by: (1) developing a control scheme to
accommodate multiple aircraft, (2) incorporating a more efficient and cost effective
motor, and (3) engineering a structurally sound chassis, while optimizing weight and
        The incentive behind the design is our client’s need for improvement of the
prototype and possibly a business opportunity. The initial investment from Dan Holmes
is expected to be about four thousand dollars. Two-thousand dollars will go entirely
toward materials, research, and development. There are other costs for this project that
will not be paid for by the sponsor because of the University setting. These include
approximately ninety thousand dollars for student time (assuming fifteen hours per week
per student at fifty dollars/hour), nine thousand dollars for faculty time (assuming two
hours per week at one-hundred fifty dollars/hour), and two thousand three hundred
dollars for shop time (assuming three hours per week at twenty five dollars/hour).
        This project will allow people who own small aircraft to move them without
having to physically push them. The benefit to the client, Dan Holmes, is his family can
use our prototype at their home and that he can market our prototype or a similar version.
The capstone design program and the University of Idaho will continue to grow and gain
advertisement from our project.
        Although the costs above represent what this project would cost in an industrial
setting, the large majority of the cost will not exist because the students are working for
credits toward graduation. The costs for faculty time and shop time still exist. The
University of Idaho receives public exposure and if the tug is put into production. Dan
Holmes receives a good return on investment because he gets a working prototype and
plans for production.

1.1 Stake Holders

          Customer: Dan Holmes – Assistant Project Manager (Siemens, Inc)
          Sponsor: Dan Holmes
          Client: Dan Holmes
          Advisor: Joe D. Law – Ph.D., P.E. – Electrical Engineering Professor
          Capstone Instructor: Joe D. Law
          Mentor: Phil Arpke, Mike Severance – Graduate Students
          Consultants / Staff: Joe D. Law, Don Elger, Edwin Odom, Karen Den
           Braven, Russ Porter, Steve Beyerlein, Molly Murphy, Joe Plummer, John
           Jacksha, Dorota Wilk, and Greg Klemesrud
          Others: Airport Runway Assistants

2.0 Problem Definition
         The purpose of an airplane tug is to allow a person to move a plane using minimal
effort. There are times when a plane is not able to move itself, such as when the pilot
needs to back the plane into a hangar after a flight. It takes about two hundred pounds of
force to push a small airplane. An airplane tug minimizes human labor. For a small
plane, a typical airplane tug is about the same size as a push lawnmower.
         Mr. Holmes would like to market his own version of an aircraft tug for a
Stearman airplane. Currently, there are several airplane tugs on the market, but none are
user-friendly. Today’s products are expensive costing airplane owners about two
thousand dollars each. Mr. Holmes believes that it is possible to design an airplane tug
that is user-friendly at the same price as existing tugs.
         Our team goal is: To build, fabricate, and assemble an airplane tug that meets the
needs of the client and the end user by satisfying the target specifications and constraints
for the project.

2.1 Project Constraints

We have discovered many constraints for the project.

 The tug is safe to operate.
 The tug is user-friendly.
 Electric motors drive the tug.
 The tug secures/ lifts the rear wheel of the plane.

Target Specifications
 The tug costs less than seven hundred dollars in materials.
 The tug is aesthetically pleasing to more than seventy five percent of potential end-
 The life of the tug is greater than two years.
 The tug has continuously variable speeds ranging from zero to four miles per hour.
 The tug weighs less than one hundred fifty pounds.

2.2 Client Needs

We prioritized the needs where the first bullet represents highest priority. The client and
end user’s needs are the following:

   The tug is inexpensive to manufacture.
   The tug is aesthetically pleasing.
   The tug is durable.
   The tug has continuously variable speeds.
   The tug is lightweight.

2.3 Project Deliverables

Prior to project completion the team will meet the following deliverables:

   A fully functional tug that satisfies the constraints, needs, and target specifications for
    the project.
   A complete drawing package of the tug.
   A bill of materials.
   An operator’s manual.

The primary client for this project is Mr. Holmes, who is sponsoring the project. After
completion he has plans for production the final design. The secondary clients for this
project are the end users.

3.0 Concept Development
3.1 Electrical

3.1.1 Motors
        The original prototype has two wheelchair motors, which work very well. The
only problem is that wheel chair motors are expensive due to health and safety codes.
We can purchase wheel chair motors, but this would greatly increase the overall cost of
the aircraft tug. For this project, we must take into account the cost of purchasing a pair
of motors and their future availability.
        After performing hands-on experiments and numerical calculations, we concluded
that we need a pair of motors that will operate at 12/24VDC, 120-125 RMP, ½ HP, and
have dual directional operation. For the concepts that we have, the motors must also
have a right angle gearbox. The first option is to purchase the motor and gearbox
separately. Our second option is to buy the motor and the gearbox as one piece (a right-
angle gear motor.

3.1.2 Control System
        For the electrical design, we need to determine a method for controlling the two
drive motors. We chose to use two permanent magnet direct current motors. Two 12-
volt batteries will supply power to the two series-connected motors. Using this
configuration, the control system will be required to handle 24-volts. For the system
described, we will need a controller that will allow either variable speed control or a way
of turning the motors on or off. In addition, we will also need a way of allowing both
forward and reverse direction. There are several ways of achieving these desired
        The method used on the existing prototype is a common three-way switch. This
allows for forward, reverse, and stop modes. To limit the starting current, solenoid relays
were used. This is an inexpensive design that does all of the required functions.
The second way of controlling the motor is by Pulse Width Modulation (PWM).
Modern control systems commonly use PWM. The idea behind PWM is that we can
program a microcontroller that will send on/off switches to the motor. These switches
are called pulses which are time varying. By varying the time, we can precisely control
the speed of the motor. The longer the pulses are turned on high, the more current goes
to the motor.

3.1.3 Batteries
       The original prototype uses a 12-volt car battery as the primary DC source. Our
concepts will use either a 12-volt car battery or two smaller 12-volt batteries. If we use
two 12-volt motors, then we will use a car battery as the main DC source. If we use two
24-volt motors, then we will use two smaller motorcycle batteries.
       A car battery can range from thirty five to one hundred eighty five dollars, while
motorcycle batteries range from twenty to one hundred twenty dollars. Our project

budget for the batteries is about fifty dollars. The amount of power that each battery
carries is about twenty to forty amp hours.

3.1.4 Chargers
        The user can charge the existing tug overnight using a small 12VDC charger. We
plan to use a similar charging device that is compact and fixed to the tug. The tug
operator charges the tug by plugging it in to an extension cord. We will use one or two
chargers depending on the number of batteries we use. The charger must have reverse
polarity and short circuit protection to protect the tug operator and the tug from an
accident. We found several chargers online ranging from $29-$50.

3.2 Mechanical
3.2.1 Frame
        Frame 1, illustrated in figure 1, is similar to the frame of the tug built by Dave
Holmes. This frame optimizes material cost as well as overall weight of the tug. This
design also optimizes time spent fabricating the frame due to minimal parts in its
assembly. The open-ended handle bar will enable us to use the motorcycle-type throttle
to control the operating speed of the tug. The handle bar will also allow us to use
motorcycle handgrips for user comfort.
        Frame 1 may have issues associated with some of our designs such as the push
down tug and the pull up tug. Pushing or pulling the tug handle downward or upward
respectively, may force the tug to tip over. All mechanical linkages will have to run
down one side of the tug, decreasing simplicity of the designs. Users may not view frame
1 to be aesthetically pleasing.
        Frame 2, illustrated in figure 2 supports downward and upward forces applied to
the handle. We can negotiate mechanical linkages down both sides of the frame.
        We will not be able to use motorcycle handgrips or the motorcycle-type throttle
without installing them prior to mounting the handle bar the remainder of the frame. It
uses more material than Frame 1, increasing the cost.
        Frame 3, shown in figure 3 accounts for possible downward and/or upward forces
that may be applied to the handle. We can design mechanical linkages for both sides of
the frame. We will be able to able to use handgrips and a motorcycle-type throttle to
control operational speeds of the tug.
        The frame uses the most material of the three designs, increasing project cost.

Figure 1: Frame 1                 Figure 2: Frame 2              Figure 3: Frame 3

3.2.2 Swinging Mechanism
        Swinging Mechanism 1 is shown in orange in figure 4. We could put a simple
mechanical linkage along the side of the tug to open and close the mechanism about the
origin of the yellow rod. This mechanism uses one beam supported at both ends to
reduce bending. We developed this concept to improve the original tug design, which
uses two cantilever beams that yielded and bent due to the load of the airplane.
        This concept supports a bending load at both endpoints of the beam. The load
that the mechanism will see is absorbed by the frame of the tug due to the low tolerances
between the inside height of the frame and outside height of the mechanism. The
mechanical linkage, used to close the mechanism and lock the tire, will see a high load.
        Swinging Mechanism 2 is illustrated in figure 5 serves as a ramp, securing the
airplane tire once the tire locks in position. The ramp prevents the plane from falling out
of the tug on an inclined surface.
        This mechanism supports the bending load that the plane will create. The design
supports the torsional load that the plane tire creates (i.e. the close tolerances between the
frame and the mechanism).
        There is a ground clearance issue with this concept. In order for the ramp to serve
as such, it will have to be close to the ground, which limits the diversity of terrain that the
tug can effectively operate on.

     Figure 4: Swinging Mechanism 1                             Figure 5: Swinging Mechanism 2

3.2.3 Locking Mechanism
        Locking Mechanism 1, shown in figure 5 illustrates this design concept, which is
shown in red. This concept pivots about a hinge that is fixed to the frame. It serves as a
locker, preventing the swinging mechanism from opening up once the plane tire comes to
static equilibrium. A mechanical linkage, running from the handle down the side of the
tug, would open and close the locking mechanism.
        This device is simple and user-friendly to operate. The concept may be difficult
to fabricate. The mechanical linkage will see a load for the entire time the plane is
secured, potentially causing short linkage life.
        Locking Mechanism 2, shown in figure 6 depicts locking mechanism 2 (red). The
inclined surface of this design allows it to lift up as the swinging mechanism passes by.
Once the swinging mechanism passes by, it drops behind it due to gravity, securing the
swinging mechanism. The user would operate this concept using a motorcycle brake /

clutch lever and cable. Pulling on the lever would pivot the mechanism about its hinge
and releasing the lever would allow the mechanism to fall.
        This design is simple, user-friendly, and easy to fabricate. The hinge for the
mechanism will see the bending load created by the weight of the plane; a hinge designed
to support a large load would need to be used.

3.2.4 Tire Size Adjustment Mechanism
        Figure 7 illustrates the tire size adjustment mechanism. The concept consists of
two channels (blue), a cross bar (green), and two cotter pins (not shown). The cross bar
(square tubing) moves to different positions along the two channels, accounting for
different plane tire sizes.
        This concept is user-friendly, durable, cost effective, and simple. There are no
negative aspects to this design.

                           Figure 6: Tire Size Adjustment Mechanism

3.2.5 Tire Lifting Concepts
        Tire Lifting Concept 1, figures 8, 9, 10, and 11 portray tire lifting concept 1. This
concept lifts the airplane tire off the ground. Figure 8 illustrates the initial and final
positions of the tug. To operate this design, the user would pull up on the tug handle,
relieving the weight from the pivoting plate, shown in Figure 10 (purple), and then pull a
motorcycle-type hand lever, which would lift the pivoting plate. The operator would then
lower the tug back on its rear castor wheels, shown in Figures 8 and 11. The castor wheel
plate rests against the frame rails, enabling the castor wheels to continue their purpose.
After the operator secures the plane tire, he/she would pull up on the handle until the
pivoting plate falls into position, shown in Figure 10.
        This design is simple, cost effective, durable, maneuverable, and has minimal
fabricated components. There is an issue with pulling up on the tug handle to lift the
plane resulting in operator injury due the lifting motion.

Figure 7: Tire Lifting Concept 1                             Figure 8: Tire Lifting Concept 1
          (Lowered Position)                                           (Lifted position)

Figure 9: Tire Lifting Concept 1                             Figure 10: Tire Lifting Concept 1
          (Lifted Position)                                           (Lowered Position)

        Tire Lifting Concept 2, figures 12 and 13 illustrate this concept. A mechanical
linkage runs along the side of the frame to the handle. The design consists of two jacks
(orange), a motor (purple), sprocket (yellow), drive shaft (dark green), and a roller-ram
(light blue). The motor uses a chain (not shown) to rotate the sprocket and drive shaft.
To operate this design, the user pushes forward on the lever, opening the swinging
mechanism (green) to the open position, shown in Figure 12. The user would then move
the tug forward, until the plane tire touches the roller-ram (Figure 12: light blue). Next,
the operator pulls on the mechanical linkage, closing the swinging mechanism (position
shown in Figure 13). The operator then turns on the electric motor which, using the
chain, turns the drive shaft, pushing the roller-ram toward the ramp. The roller-ram rolls
the plane tire up the ramp.
        This concept has few machined parts, few moving parts, is user-friendly, and is
safe. The motor durability is questionable.

Figure 11: Tire Lifting Concept 2                    Figure 12: Tire Lifting Concept 2

                Tire Lifting Concept 3, figures 14, 15, and 16 displays the concept. After
        securing the tire, the user pushes down on the tug’s handle; the pivoting lever arm falls
        into a slot due to gravity (Figure 15), raising the front-end of the tug, thus lifting the
        plane tire. To lower the plane, the user pushes down on the handle and pulls a hand lever
        that swings the pivoting lever out of the notch as shown in Figure 14. The user then
        eases the downward force to lower the tug.
                This concept is user friendly and safe. This concept has many machined parts, is
        not cost effective, and is not simple.

Figure 13: Tire Lifting Concept 3 (Lowered Position)                 Figure 14: Tire Lifting Concept 3 (Raised Position)

                                      Figure 15: Tire Lifting Concept 3 (isometric)

4.0 Project Description
         Aircraft tugs are comparable in size to a lawn mower. You would walk behind
the aircraft tug, as one would walk behind a lawn mower when cutting the grass. The tug
has a steel frame that is welded together. Handlebars, similar to that of motorcycle, are
used to control and maneuver the tug. The base of the tug houses the mechanical and
electrical components. There are two front wheels and two small rear wheels. The two
front wheels are driven by two electric motors. The motors have a right angle shaft and
are mounted to the frame of the tug. Controlling the tug and lifting the airplane is done
from the handlebars. The mechanical and electrical components are described in greater
detail later this document.

                                   Figure 16. Aircraft Tug

        The electrical system is comprised of all the parts needed to move and operate the
tug. The electrical system consists of two batteries, a microprocessor based motor
controller, and two permanent magnet direct current motors. The frame and lifting
components are mechanical. They are (1) the frame, (2) wheels and hubs, and (3)
linkages and lever arms.
        The tug is approximately forty five inches tall from the ground to the handlebars.
The top of the motors / batteries to the ground is about ten inches. The overall length of
the tug from the front wheel to the handlebars is seventy-two inches. The handlebars are
angled at fifty-two degrees from the ground and have an overall length of thirty-two
inches. The width of the tug at the handlebars is twenty-two inches. The width at the
front wheels is thirty-five inches from outside of each tire. The overall weigh of the tug
is roughly two hundred pounds.

4.1 Electrical

        All of the electrical components act like the tugs engine. Similar to an engine,
they are integrated into a system to allow the tug to maneuver itself and the airplane. The
batteries supply the power to the motor controller, which is then fed a signal to the
motors. The motor receives the signal and depending on the function, turns the motor at
the desired speed and direction. Let’s begin by describing the batteries.

4.1.1 Batteries
        The aircraft tug uses two 12-volt direct current (DC) batteries. Each battery
supplies each motor with 12-volts and has its own battery charger. The batteries are
located at the base of the tug. They are lawn and garden batteries, which are used for a
riding lawn mower. These batteries are lighter and smaller then regular car batteries but
have lower starting current. The batteries are physically connected to the microprocessor
based motor controller.

4.1.2 Motor Controller
        The motor controller is mounted between the two batteries shown in figure 1.
The unit is small and a hard cover protects all of its electronics. The unit has one set of
input terminals and one set of output terminals. All of the functions are configured using
a pin connector, which is also on the outside of the unit. Each pin is wired to a different
function. The speed throttle is wired directly to three pins and is used to vary the speed
of the tug. The forward/reverse switch is wired to a pin and allows you to select the
direction of the tug. Lastly, an on/off switch allows you to turn the controller off for

4.1.3 Motors
         The last electrical component is the motors. Theses motors are specifically
designed for wheelchairs and robots. They are mounted parallel with the frame, allowing
for a right-angle shaft to support a wheel. Shown below you can see the wheel mounted
to the motors shaft.

                             Figure 17. Motor with wheel mounted

4.2 Mechanical

         The mechanical components make up the physical structure of the aircraft tug.
The tug has two positions. First, the tug can maneuver without an airplane, shown in
figure 3. Second, the tug can have and airplane on its frame, and therefore be moving an
airplane shown in figure 4. By observing figures 3 and 4 you can see the airplane wheel
just to the right of the front wheels. The mechanical components consist of a frame,
wheels and hubs, and lifting mechanisms.

                 Figure 18. Tug without Airplane   Figure 19. Tug with Airplane

4.2.1 Frame
        The first mechanical component is the frame. The frame is the body of the
aircraft tug and supports both mechanical and electrical parts. The frame is fabricated
from one and three-quarter inch square steel. For maneuvering the airplane a handlebar
is mounted to the top of the frame allowing you to have a good grip on the tug. At the
base of the frame there is a place to mount the batteries and electrical components, along
with an angled surface for the motors. Attached to the frame are the necessary
mechanical components to lift and hold the airplane wheel.

4.2.2 Wheels and Hubs
        Both the wheels and hubs support the tug and allow it to move freely. The two
front wheels drive the tug and the back wheels support the rear weight. The hubs mount
the front wheels directly to the motor shaft. The tires are similar to that of a wheelbarrow
or garden cart. They have enough tread to move on slick or rocky surfaces.

4.2.3 Lifting Mechanisms
        Lastly, the main mechanical components are the lifting mechanisms. The system
works in three-steps. The first mechanism is a lever arm that remains open until the tug is
in position to move the airplane. Next, the lever closes by pulling a hand lever next to the
handlebar. This hand lever moves mechanical linkages that close the lever arm into a
locked position, not allowing the airplane wheel to move from the tug. Next, you
physically put up on the handlebars until the pivoting locker drops into place. The wheel
is now an inch off the ground and locked onto the aircraft tug. You can now maneuver
the plane at will.

        5.0 Product Evaluation
                An aircraft tugs main purpose is to move an airplane. Our design is to specifically
        move a tail-dragger airplane. This type of airplane has two front wheels that support
        most of the airplanes weigh and a rear wheel that supports the weigh of the tail. Our
        aircraft tug is designed to move the airplane by lifting the rear wheel of the airplane.

        5.1 Performance Requirements
        The tug is design to meet these requirements:

               1.   Electric motors drive the tug.
               2.   The tug secures/ lifts the rear wheel of the plane.
               3.   Pushes the tug in the forward and reverse directions.
               4.   Moves the airplane safely from point A to point B.
               5.   It does not damage the environment or the airplane.

        5.1.1 Target Specifications
               Here is a look at are design proposal metrics from first semester.

                                   Table 1. Metrics for Aircraft Tug

            Metric              Importance        Units         Acceptable         Ideal              Need #
Torque on motor/s                   1             In*lb           600-700           640                 3, 4
Speeds of aircraft tug              1           Miles/hour             3         1<x <4                  11
Stiffness of chassis                3             In/lbf        0.25/100         0.25/ 200                6
Current for motor                   2               A                 25             20               6, 2-4
Voltage: series or parallel         2               V              Parallel        Series              4, 11
Force of wheel locking lever        3              Lbf                20             10                 1, 9
Force to lift rear wheel            2              Lbf          Up force 50     Down of 50              1, 2
Weight of tug                       4               Lb               150         Under 100               12
Traction on wheels                  3               %                 75            >90                  12
Cost                                1               $$               500            400                   7
Manufacturability                   2             Hours                4              3                   7
Durability                          2             Years                2            4-5                   6
Non - Marking tires                 4            Binary              No             Yes                 1, 7
Quiet                               1              DB                 60             40                 1, 2
Charger                             3          Hours/ charge           8              4                   5
Compatible                          4              List           Stearman      Other aircraft           7,
PWM control                         2               A            Unknown         unknown                2, 6
Compact size                        4               Ft            3x11/2x8         2x1x6                 10
Battery                             1               V                  9             12                 4, 5
Aesthetically Pleasing              2               %                 75            >90                   8

                    Shown below is the same chart from above with individual evaluations based on
            specific criterion.

                                 Table 2. Evaluated Metrics for Aircraft Tug
            Metric             Importance     Acceptable          Ideal           RESULT              Comments
Torque on motor/s                  1           600-700             640                681
Speeds of aircraft tug             1                3           1<x <4            0-2.2 mph
Current for motor                  2               25               20               27-25
Voltage: series or parallel        2            Parallel          Series             Series
Force of wheel locking lever       3               20               10                 10
Force to lift rear wheel           2          Up force 50      Down of 50        Up force of 50
Weight of tug                      4              150           Under 100             200
Traction on wheels                 3               75              >90                 80
Cost                               1              500              400               1800                (1)
Manufacturability                  2                4                3               +100                (2)
Durability                         2                2              4-5            Unknown
Non - Marking tires                4              No               Yes                No
Quiet                              1               60               40            Very small
Charger                            3                8                4                  8
Compatible                         4           Stearman        Other aircraft    Other aircraft
PWM control                        2              Yes              Yes                Yes
Compact size                       4           3x11/2x8           2x1x6            3x1.5x6
Battery                            1                9               12              12-volt
Aesthetically Pleasing             2               75              >90                >90

                (1) Cost specification was based on clients request, actual cost was not reasonable
                    based on specification.
                (2) Includes design changes

            5.1.2 DFMEA
                   Refer to Appendix E.

            5.1.3 Design Calculations
                    All of the mechanical design calculations were simulated using Solid Works. We
            build the frame of the tug and then added individual mechanical components to the
            frame. The design originated in Solid Works, built at the University of Idaho Shop, and
            tested at the Moscow-Pullman Airport. We used a build and test method to perfect the
            mechanical mechanisms. Initially, we did an experiment by pushing an airplane with a
            scale. This gave us a rough estimate of the initial amount of force needed to move a three
            thousand pound airplane. Using MathCad we then ran through some calculations and
            were able to find the amount of torque need to push an airplane. For a detailed look at
            our calculations refer to Appendix D.

5.1.4 Results from Testing
        After several weeks in the shop, we began testing our design. In order to meet are
target specifications we need to perform experiments on an airplane. This tug is designed
to move a Stearman airplane, but due to the lack of Stearman airplanes in the area we
used a couple different planes at the Moscow-Pullman Airport.

First Test: Our goal was to determine what areas of the design worked, how well they
worked, and what we could do to fix failures. Our first failure occurred when we failed
to lift the plane off the ground. We were unable to test the electrical system based on our
failure to lift the wheel of the ground. The swinging bar mechanism that holds the wheel
secure to the tug was not staying closed. The force on the wheel was too great and the
lever arm in the linkage was not able to support that load.

Plan of Action: Add a latch to keep the swinging bar mechanism from opening. The
latch is a small hook that lifts up after squeezing a brake lever that pulls a cable
connected to the latch. The latch is designed to lift up as the swinging bar closes and fall
down after the swinging bar is in the locked position.

Second Test: With the new latch installed we went back out to the airport to test the
design changes. This time we were able to keep the swinging bar from opening. The
new latch holds the force of the wheel. However, our continuing failure was not lifting
the wheel off the ground. With the swing bar closed we were able to lift up the tug to the
upward position. When in the upward position the wheel was still making contact with
the ground, not allowing us to move the plane. We noticed the wheel was not receiving
enough upward motion.

Plan of Action: Buy a bigger caster wheel. This will increase the angle created when
tug is put in the upward position, increasing the vertical travel of the wheel.

Third Test: With the bigger caster wheel on the tug we headed back out the airport to
test our change. When the tug is lifted with the new caster wheel we were able to see a
little move travel, but not enough. The wheel is still barely touching the ground. Our last
option is to move the entire placement of the wheel back away from the wheel to allow
for maximum vertical travel.

Plan of Action: Move the swinging bar mechanism back. Add a front plate to allow the
swinging bar to get further under the wheel when lifting.

Final Test: The tug has all the necessary design changes and we are now hoping to do
our final test. Success, the tug now lifts the rear wheel of a plane approximately ¾ of an
inch off the ground. With the tug in the upward position we can use our variable speed
motor controller to move the plane. The plane accelerates to a 2.2 mph max speed when
in forward and reverse. With the larger caster wheel the tug is easy to steer and
maneuver in tight areas.

6.0 Economic Analysis
        The initial investment from Dan Holmes is expected to be about four thousand
dollars. Two thousand is for research and development, and materials to manufacture the
design. Other costs will include a sponsor fee and shop time for manufacturing. There
are other costs for this project that will not be paid for by the sponsor because of the
University setting, such as labor costs. Refer to the Appendix B for a detailed cost

7.0 Conclusions and Recommendations
7.1 Conclusions
        As you can observe the aircraft tug is a unique design that has mechanical
advantages and electric power to perform its task. We were able to incorporate the
existing prototype into a more user-friendly, economical, and stronger aircraft tug.
Throughout the design and development of the aircraft tug we discussed multiple
concepts. We chose a design that was requested by our client and our ability to
        We were able to manufacture a tug with a controlled on/off switch,
forward/reverse switch, and a variable speed throttle controlling two motors. Both of the
motors receive electric power from two 12-volt batteries. The tug has a cover that
protects the user from the batteries and any high current during operation. The tug lifts
the plane using the mechanical advantage of the lever arm of the tug. The new design is
stronger then the existing prototype due to the cantilever beam supporting the weight of
the plane. The new design has a movable beam to allow for multiple sizes of airplane
tires. Our design successfully lifted the front wheel of tricycle airplane and as well as our
constraint to lift a rear wheel. This was not in our design specifications, but proved our
new design applicable to multiple aircraft. At the design expo many people gave us
positive feedback. Many said they like the overall look of the tug. We were able to
incorporate our client’s airplane into the color of the tug. The end cost of the tug is
roughly around fifteen hundred dollars in materials.

7.2 Improvements
      The motors cost less than wheelchair motors.
      The tug has variable speed control.
      The tug has onboard chargers.
      The swinging bar mechanism is much stronger than the previous design
      The wheel capturing mechanism secures the airplane tire better.
      The tug is adjustable for different sized airplane tires.

7.3 Recommendations
       Even though our tug was a significant improvement over its predecessor, we
found some things that we would improve on for the production model. Some of these
recommendations are results of testing, while others are suggestions that we got at the
Design Expo.

      Include a warning that says “Do not lift the tug while it is in Reverse”. A girl
       tried to do this at Expo and was pushed backward by the tug until she tripped over
       a bench. She could have been hurt.
      Include a warning that says “Do not charge the tug while the Power switch is set
       to ON”. This can damage the controller or the motors.
      Instead of using two 12-volt batteries, we could use one 24-volt battery from an
       airplane. This would enable the tug to be thinner and lighter. It would also

    decrease the number of available amp-hours, but we think it would be fine
    because the tug can last for about an hour of plane moving in its current
   Add a vertical wall on the top of the swinging bar ramp to keep the tire from
    rolling out the front. The airplane tire only rolled out the front a couple of times,
    and both were under hard deceleration. However, a vertical wall would
    completely prevent the airplane tire from rolling off the front of the tug.
   Change the programming for the microcontroller. This was the biggest cause for
    the airplane tire rolling out of the front of the tug. It was only a problem when
    moving a tail-dragger, because of the large inertia of the plane and the relatively
    small weight on the tail wheel. When the user lets go of the throttle when the
    plane and tug are moving at full speed, the microcontroller tries to stop the tug too
    fast. It actually gives the motors current in the reverse direction. This results in a
    violent stop that makes the tail wheel jump out of the tug. We suspect that this is
    because the microcontroller was programmed for a handicapped person’s scooter,
    which is much lighter than the airplane. You could have the company program
    the controllers at the factory, or buy their programming device and do it yourself.
   Move the “channels” which house the bridge and swinging bar forward 1 inch.
    We moved them back in order to get more lift on the airplane tire, but moved
    them too far back. When we tried to move a nose wheel plane, the tire was too
    big and hit the electrical parts cover. This is not really a big problem because
    most nose wheel planes are too heavy for the tug’s tires anyway.
   Put bigger tires on the tug. This will allow a higher load carrying capacity for the
    same tire pressure because there is a bigger contact area. The bigger tires will
    mean you need a lower-geared gearbox. NPC robotics sells a motor that is the
    same as the W81 and W82 motors, except with a gearbox that produces an output
    speed of 135 rpm. The motors that the tug uses have an output speed of 190rpm
    at 24 volts. The lower speed will also give more torque to compensate for the
    bigger tires. Bigger tires will mean you have to mount the motors higher on the
    frame. Instead of bigger tires, you could possibly get solid-core tires that have a
    higher load carrying capacity than pneumatic tires.
   Put an electric solenoid on the large caster wheel so the operator does not have to
    lift the tug and airplane. The tug is pretty heavy with two batteries.
   Make the handle length adjustable for people of different heights.
   Reduce the size of the square tubing from 1.75” to 1.5” to reduce the cost from
    approximately $3.54/ft to $1.27/ft
   Use a hinge that has more sections. The hinge that we used had only one section
    on one side of the hinge. This section was loaded in bending and failed. We had
    to beef it up by welding an extra piece of metal to it. A hinge that had more
    sections would not be loaded in bending so much.


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