AeroClub-Module 4 – Make a Hovercraft

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

Centre For Innovation – cfi-iitm.org Indian Institute of Technology Madras

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



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 ∗ 10−10𝑜𝑧

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

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

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 –

[email protected], [email protected] .

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. Anil Kumar

2. Hanut Vemulapalli

3. Dheepak N Khatri

4. Sanjesh Hoskopple

5. Nikhil Gupta

6. GuruPrasad Kallanje

mailto:[email protected]

mailto:[email protected]

https://plus.google.com/u/0/101736879033096320463?prsrc=4

Documents

AeroClub-Module 4 – Make a Hovercraft