AeroClub-Module 4 – Make a Hovercraft

Transcription

Aero Club
Presents
Introduction to RC Modeling
Module 4
Make a Hovercraft
Centre for Innovation
IIT Madras
Aero Club - Intro To RC Modelling – Module 4 - Make a Hovercraft
Table of Contents
Introduction ...................................................................................................................................................................... 3
System of the hovercraft: ................................................................................................................................................. 3
Lift System: .................................................................................................................................................................... 3
Thrust system:............................................................................................................................................................... 4
Steering system: ............................................................................................................................................................ 4
Design: Mechanical Calculations....................................................................................................................................... 4
Lift Calculation .............................................................................................................................................................. 4
Thrust Calculation ......................................................................................................................................................... 5
Propeller Selection ........................................................................................................................................................ 5
PARTS OF A RC HOVERCRAFT AND THEIR PURPOSE ......................................................................................................... 5
List of Materials................................................................................................................................................................. 6
Structure: .......................................................................................................................................................................... 7
Design-1 .................................................................................................................................................................... 7
Design 2 ....................................................................................................................................................................... 10
Electronic Connections: .................................................................................................................................................. 11
Exercises: ......................................................................................................................................................................... 12
Acknowledgements:........................................................................................................................................................ 12
Centre For Innovation – cfi-iitm.org
Indian Institute of Technology Madras
INTRODUCTION:
Hovercraft can be defined as a self-driven vehicle, dynamically holed by a self-created cushion of slow
moving, elevated pressure air which is ejected against the surface below and contained within a flexible
skirt such that it is totally amphibious and has some ability to travel over less than perfect surface (Fig1).
Air propellers, water propellers,or water jets is usually provide forward propulsion whereas air-cushion
vehicles may be able to attain higher speeds than the ships or most land vehicles of same size. Owing to
lower frictional resistances and exploit a comparatively fewer power than helicopters of the similar weight.
System of the hovercraft:
For understanding the correct running and operation of a hovercraft, it required to know three basic
systems that are mainly responsible for lifting and moving of the entire vehicle: Lift System, Thrust System,
and Steering System.
Lift System:
The hovercraft relies on a constant cushion of air to sustain adequate lift. The air evicted from the
propeller was alienated by a horizontal divider into pressurized air exploited for the air cushion and
momentum taken for thrust. The weight distribution on top of the deck was given so that the air was
disseminated from the rear of the deck to throughout the cushion volume in an approximately even
fashion to provide the necessary support. The skirt extending under the deck given containment, better
balance, and permit the craft to traverse more assorted terrain.
Thrust system:
The air is not directed to the cushion and skirt is propelled backwards, providing forward thrust to the
craft. The size of the propeller, rpm output of the engine, and height of the lift/thrust divider are the
formative parameters for the thrust force. A thrust duct channeling the air into the propeller can offer up
to a 15% boost the efficiency [Universal Hovercraft]. The limiting factor for the thrust is the air flow existing
to direct backwards as our prime concern is providing pressurized air for air cushion and lift. Consequently,
our forward speed is limited but maintainable.
Steering system:
Since a hovercraft required the similar frictional and drag effects as boats or cars, steering must be
approached without precise control in mind. This is especially true in our case as the power supply is
limited. Rudders are a main source of steering and are attached to the rear of the duct to direct the flow of
air and the direction of the subsequent momentum transfer from the air to the craft. The driver controls
the movement of the rudders through a joystick located in the front of the craft. A throttle on the engine
situated next to the driver allows him to vary the Speed of the craft.
Design: Mechanical Calculations
Lift Calculation
Hovercrafts are supported by a fluid air, which allows the hovering with little or no friction. The amount of
air pressure that is needed is directly related to the weight of the craft. Therefore, the less the weight is for
a hovercraft, the less the air pressure required, which in turn results in energy savings. The cushion
pressure (Pc) multiplied by surface area (A) of the craft equals its lift. Once the lift is equal to the weight of
the hovercraft (C), the craft will lift off and hover.
Pc*A = C
Where A = Length* Width
According to Bernoulli’s equation, the existing velocity of the air from under the skirt through the gap
made from the craft hovering can be calculated by:
Ve =2 𝑃𝑐 𝜌 𝜌 =density of air
The lift design was based on obtaining a hover gap of “1”, which is typical for hovercraft such as this. This
gives us the total area through which air would be escaping, as defined by Ah=L*H
Where, Ahis the area of the space between the ground and the hovering craft in respective unit, L is the
perimeter in same unit and H is the hover gap height, as previously given. The proper design pressures for
a skirt require at least 20% more pressure inside the skirt than the pressure directly lifting the craft over
the area of the underside. If these aforementioned pressures were equal, it is unlikely that the craft would
hover since escaping air would not provide any lift.
The flow rate of air escaping through the hover gap is calculated using the general formula for flow rate
Q=Ah* Ve
Power of the motor turns out to be P=Q*𝑉e2 /2
Thrust Calculation
Thrust is the force applied by the volume of air passed at the discharge of the fan. For propeller which
pitch to diameter ratio less than 0.6 the formula for thrust is
Thrust = P ∗ 𝐷3 ∗ 𝑅𝑃𝑀2 ∗ 10−10 𝑜𝑧
Propeller Selection
These calculations will give the idea about the selection of the fan or propeller required for the proper
operation of the vehicle
Diameter of the required propeller = 632.7 ∗ 𝑆𝐻𝑃0.2 ∗ 𝑅𝑃𝑀−0.6
Here “RPM” is the propeller shaft turning speed,“SHP” is propeller shaft horsepower, and “D” is propeller
diameter in inches.
PARTS OF A RC HOVERCRAFT AND THEIR PURPOSE
The Common parts of the hovercraft that we are going to build are:
a) Hull: It is a structural member. It is a flat base. All other parts are mounted over the hull, except the
skirt. In our case the flat wooden base along with the thick foam sheet form the structure of the hull.
b) Lift Motor: It is DC motor. It supplies huge amount of air to the skirt. It helps in lifting the hull and the
other components on the hull. The lift motor operates in the range of 9-11V. The RPM of this motor cannot
be controlled i.e. it varies with the voltage only. The controller does not have direct control over this
motor.
c) Thrust Motor: It is a brushless AC motor.. The RPM varies with the voltage supplied. It helps in moving
the hovercraft forward by pushing huge amount of air backward. We can control the RPM of this motor
directly using the transmitter, thus the speed of the hovercraft. Brushless motors are very popular in RC
projects because of their durability and efficiency. But they are expensive.
d) Skirt: It a flexible chamber made of nylon cloth. It forms a cushion of air by allowing continuous flow of
air. There are three main types of skirts: Bag skirt, Wall skirt and Finger skirt. Because of the complexity
involved in the fabrication of the latter two, we will make a bag skirt.
e) Duct: It is a channel that encloses the thrust motor. It makes the thrust of the motor efficient by
reducing the losses that might occur due to divergence of the air flow.
f) ESC: ESC stands for Electronic Speed Controller. It determines how much current has to be sent to the
motor depending upon the signal sent by transmitter. ESC’s are specified by their current ratings. For
example, we will use a 6Amp ESC. The rating of the ESC is decided by the power of the motor. The rating of
the ESC will be given in the data sheets of the motors.
g) Transmitter: It is a kind of remote. You can control what the hovercraft does at a distance away from it
wirelessly. The transmitter comes coupled with a receiver. The transmitter is specified by the frequency of
the signal and the number of channels. We will use transmitter operating at 240GHz and it has four
channels.
h) Receiver: It receives the signal sent by the transmitter and reads it. It acts as an antenna. It is connected
to the ESC and the servo. It is powered by the battery. Servo receives power through the receiver.
i) Servo: It is a motor that rotates through only certain angular positions. For example the one which we
will use can rotate both clockwise and anti-clockwise through an angle of 90 degrees and all the
intermediate angles. Generally the servos are used to control the angular positions of the ‘control
surfaces’.
j) Rudder: It is a control surface. It helps hovercraft take turns. It is attached in front of the thrust motor.
Technically speaking rudder controls the yawing of an aircraft or a hovercraft. It directs air in directions
such that the hovercraft takes turn in desired direction. The rudder is coupled to servo i.e. servo controls
the rudder.
k) LiPo battery: It stands for Lithium-Polymer. It is a high power battery. It is most popular among RC
hobbyists mostly because its power to weight ratio is high compared to lead acid batteries. Because of this
high power to weight ratio, the weight of the crafts we build reduces drastically. Also LiPo batteries are
available in various configurations. These are specified by number of cells (Xs) and mAh. All the electronic
components of the hovercraft are powered by this battery.
In this module we will show you how to make a hovercraft of Dimensions 30*25 cm, required thrust and
lift motors and propellers have been selected as explained before. You can accordingly change the
dimensions and come up with the suitable motors and propellers.
List of Materials
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Lift motor 18,000 RPM, 12v
Thrust motor, 1400 kV
Propeller 6030 3-bladed(2)
Battery-3S LiPo, 1300 mAh
L-Clamps
Wooden frame
High density foam sheet
Skirt cloth (Nylon or tarpaulin)
RC Transmitter, 2.4 GHz
RC receiver
Servo, 9 gm
Coroplast
Electronic Speed Controller, 30 A
Foam glue
Structure:
First let’s build the wooden frame. Gather all the wooden frames (7 in number). They are designed to meet
our requirements. Follow these steps:
Here we have two designs for the hovercraft. First one is relatively easier when compared to the second.
Dimensions of the parts are as shown below.
Design-1
Attach the frames numbered 2 and 3 to the frame numbered 1.You just have to insert the frame 1 into the
lot provided on the frames 2 and 3. Refer to the figures below
Fig (2) Take the frames 1, 2 and 3 and join the way as shown in the adjacent figure (in the direction of
arrow)
Attach this setup to the frame numbered 4 using L-clamps, nuts and bolts. Do not forget to use the
washers while fixing. Dimensions are in centimetre
Insert the frame 5 into the slot provided in the frame 4.
Here you can see lot of parts cut out from the wooden base. This is basically to reduce the weight as much
as possible by removing the unwanted materials.
Lift and thrust motors are attached as shown in the figure below.
Fixing the rudder: Two rectangular pieces of Coroplast and two cylindrical rods are being used. The common base of
both the rudders in mounted on a 9gm servo motor.
The place the entire system on the foam and stick it using Foam Glue.
For the skirt, cut a sheet of nylon or tarpaulin of dimension 54*49 and wrap it around the foam so that on all sides 2
cm goes inside the foam ,5cm sticks to the side of the foam as the foam is 5cm thick and remaining 5 cm will be left
hanging .
After everything is done, your hovercraft will pretty much like this.
Design 2
Sidemounts
Base
Motor Mounts
Assembling follows the same procedure as design-1. You can follow the design which is convenient.
Electronic Connections:
The transmitter we are using operates on 2.40GHz frequency. It runs on eight 1.5V batteries
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Insert the batteries into the transmitter to power it.
Switch the transmitter on
Take a receiver which is binded to that transmitter.
Connect the ESC terminal to the receiver. ESC has three sets of terminals. Two sets go to the AC
motor and the Battery. The third one goes to the receiver. The third terminal in turn has three
wires. When connecting to the receiver, you should ensure that the lightest of three wires (usually
white) should face the body of the receiver. Connect the ESC to the battery. After the connection is
made. A red light appears on the receiver if the connection is successful.
Now see that the channels of the transmitter are in zero position. Connect the motor to the ESC.
Note that both motor and esc have three wires by interchanging the direction, direction of rotation
of the motor changes. Here the motor has to push the air so make the connection accordingly
Connect the Servo to the receiver. Its connection is same as connecting Esc to the receiver
To see if the communication module is successful, try operating the motor using the transmitter
through that particular channel to which ESC is connected to.
Now power the lift motor by connecting it to the Li-Po battery. You can use two batteries
separately for lift and thrust or can use one battery for both by connecting in parallel.
After all these connections are done you can run both Lift and thrust motor simultaneously and control the
hovercraft using the Transmitter.
You are now ready with your Hovercraft. Hover On!
Exercises:
1. How does a Hovercraft hover? What is the mechanism behind it?
2. What are the three main systems of Hovercraft?
3. How do you calculate the thrust provided by a motor given its diameter and pitch?
4. Why do you think brushed motor is used for lift?
5. What is the reason for making the front part of the Hovercraft in curved shape?
6. How do you think the Receiver is powered?
Do try searching for answers to these questions.
We hope you have liked the module, and have gained a good deal of factual material from it.
For any queries/feedback please send an email to any one of the following addresses –
dheepaknkhatri@gmail.com, anil316619@gmail.com .
Do send your answers to the above mentioned addresses too!
To join aero club, go to the Clubs tab -> Aero Club on the CFI website, and click on Join Aero Club.
Acknowledgements:
This course on Intro to RC Modeling was formulated, prepared and compiled by the following members of
Aero Club:
(In the order of the module of the courses)
1.
2.
3.
4.
5.
6.
Anil Kumar
Hanut Vemulapalli
Dheepak N Khatri
Sanjesh Hoskopple
Nikhil Gupta
GuruPrasad Kallanje

AeroClub-Module 4 – Make a Hovercraft

Transcription

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