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rev. 10/2012
Hover Frenzy
Scenario
Your team has been selected as one of the final qualifiers to supply hovercraft
to evacuate personnel from the Australian Antarctic base in the event of an
emergency. Today each team is building a model hovercraft that will be tested
against the other finalists.
Aim
The aim of this full-day activity is to design, build and fly a model hovercraft
that is fast, manoeuvrable and has a good hover height.
What to do
Each team gets a lift fan, two motor/propellers, a battery, controller, saw,
cutting block, scissors, and consumables (balsa wood, styrofoam trays, plastic
sheet, tape, etc) to make a working model hovercraft.
Some things to consider when building the hovercraft are:
•
Should your hovercraft have a skirt? If so, what type?
•
Where and how should the motors and fans be mounted?
•
How much drag does the craft experience? How can this be reduced?
•
How is weight distributed around the hovercraft? What effect does it have?
•
Is speed the most important design goal?
Discuss these questions and build the craft!
Get as much practice as possible ‘flying’ the hovercraft over the 3 courses.
There is one course to test the craft’s speed, another to test hover height, and
the third to test the craft’s manoeuvrability around an obstacle course.
Improve the hovercraft design (and the pilot’s skill!) by practice, trial and error.
Rules
For all the tests, the lift fan and thrust propellers may only be turned on when
the coordinator is watching and indicates that it is time do so.
Steering or braking the hovercraft by pulling on the power cable is not
allowed. There is a 10 point penalty per offence.
In the obstacle course the hovercraft must fly between the red and the white
witch’s hats in the correct order.
The hovercraft must start the hover height test sitting, stationary, on the
rubber matting with the rear of the craft level with the end of the matting.
Scoring
The total score is obtained by adding together the results of 3 individual tests.
Tips
The lift fan only blows air in one direction. This is denoted on the fan by a
coloured dot; placing this dot down will push the air beneath the craft.
A pair of motor/propellers is used to provide forward movement and to steer
the hovercraft. Slowing the right propeller causes the craft to turn right, etc.
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rev. 10/2012
STUDENT ACTIVITY NOTES
Hover Frenzy
The problem
The aim of this activity is to design, build and fly a model hovercraft that is
fast, manoeuvrable and has a good hover height.
Duration
This activity runs for a full day (approximately 4 hours).
Terms
There are some terms used in this activity that you may not be familiar with:
Skirt
Flexible material (either individual ‘fingers’ or a continuous
strip) attached to the edge of the hovercraft that forms a seal
between the base of the hovercraft and the ground. When
inflated the skirt increases the craft’s hover height and allows it
to clear obstacles. It can also improve the craft’s controllability.
Ground effect
The lift fan forces air under the hovercraft forming a cushion of
air. The air also rebounds off the ground causing a force that
lifts the craft. This lift force is known as the ground effect.
Baffle
Sometimes a diffusing panel known as a baffle is mounted
under a hovercraft’s lift fan. It is used to improve the airflow
(particularly with advanced skirt designs) and to reduce the
tendency of the hovercraft to rotate about its axis.
Materials
Each team will be given a lift fan, 2 propulsion motor/propellers, a control box
and a battery. Scissors and a balsa saw are also supplied to cut the materials.
Various other consumable materials are also available including two sizes of
styrofoam trays, balsa wood, plastic sheeting (for the skirt), paper binders,
masking tape and plasticine (used as ballast).
Rules
The rules are straightforward:
1. For all the tests, the lift fan and thrust propellers may only be turned on
when the coordinator is watching and indicates that it is time do so.
2. Steering or braking the hovercraft by pulling on the power cable is not
allowed. There is a 10 point penalty per offence.
3. In the obstacle course the hovercraft must fly between the red and the
white witch’s hats in the correct order. There is a penalty of 5 points for
each tennis ball that is knocked off a witch’s hat.
4. The hovercraft must start the hover height test sitting on the rubber matting
with the rear of the craft level with the end of the matting. When instructed
by the coordinator, the craft must travel over as many steps as possible,
as quickly as possible.
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rev. 10/2012
Scoring
The total score for each team is obtained by adding together the results
of three individual tests:
1. A speed test over 12 metres, worth a maximum of 100 points. This
models the craft’s ability to quickly cross open waters.
2. An obstacle course worth a maximum of 200 points. Points will be
awarded based on the time taken to negotiate the course and the
number of obstacles hit. This test models the craft’s manoeuvrability.
3. A hover height test, worth a maximum of 150 points, simulates the
climb up out of the water. Your team’s score for this test will be a
combination of the number of steps crossed and the time taken.
Tips
Don’t forget to bring these notes with you to the Challenge.
Think carefully about size and materials used. Also, consider the type of
flexible plastic skirt on the hovercraft (if any). The addition of a skirt will greatly
increase the hover height and controllability of the craft but may slow it down.
The lift fan only blows air in one direction. This is denoted on the fan by a
coloured dot; placing this dot down will push the air beneath the craft.
A pair of motor/propellers is used to provide forward movement and to steer
the hovercraft. Slowing the right propeller causes the craft to turn right, etc.
The control box has three parts. The central switch turns the lift fan on or off.
The sliders on either side of the control box are connected to the thrust motors
via a long, flat power cable. When centred, these sliders provide no power to
the motors. However, when they have been moved forward or backward, they
power the motors in the forward or reverse direction. The direction of the fan
rotation can be reversed by flipping the switch immediately next to the slider.
A ‘skirt’ increases the craft’s hover height and, because a skirt can fold and
deform, allows the craft to negotiate rough surfaces and obstacles. There are
three main types of skirts.
1. A bag skirt (shown below in figure 1) consists of one sheet of material that
covers the base of the craft and has holes in it to allow the air to escape.
This type of skirt is simple to manufacture and reduces friction with the
ground. Unfortunately it may have poor hover height.
Air intake for lift propeller
Skirt
Skirt
Air flow
Air cushion
Holes in skirt
Holes in skirt
Figure 1: LEFT: Cross-section through a hovercraft. RIGHT: View underneath a bag skirt.
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rev. 10/2012
2. A tube skirt (shown below in figure 2) is similar to a bag skirt except that
the material forms a loop around the edge of the craft. The top edge of the
loop is attached to the rim of the craft, the bottom edge of the loop is
attached to the edge of the baffle mounted below the lift fan. Holes are
placed on the inside wall of the tube, allowing the air to escape and form
the air cushion.
Air intake for lift propeller
Baffle
Skirt
Holes
in
skirt
Skirt
Holes in skirt
Figure 2: LEFT: Cross-section through a hovercraft. RIGHT: View of underneath a tube skirt.
3. A segmented skirt is similar to a tube skirt but is composed of a large
number of individual segments. Each segment inflates to give a good
hover height and excellent obstacle clearance.
This type of skirt is most commonly used in real hovercraft but
should not be used on your model hovercraft as it is extremely
difficult to make and install.
Air intake for lift propeller
Baffle
Skirt
Skirt
Individual
segments
with holes
on inside
Segments of skirt
Figure 3: LEFT: Cross-section through a hovercraft. RIGHT: View of underneath a segmented skirt.
The tube and segmented skirt designs use a baffle underneath the lift fan to
improve air flow, cushion formation, and hover height. Setting up a baffle can
be very tricky. If your team uses a baffle and the performance of the
hovercraft doesn’t improve, it may be better to remove it.
This material may not be reproduced without permission
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rev. 10/2012
SCORE SHEET
Hover Frenzy
School name: ___________________________________________
1. Speed test
Attempt
Time
Score = 100 - time
(seconds)
1
2
3
Best score (A)
2. Obstacle course
Attempt
Time
(seconds)
Score (B) = 200 - time
(C) = Tennis balls x 5
Total score
Number of balls knocked off witch’s hats x 5
=B-C
1
2
3
Best score (D)
3. Hover height test
Attempt
Time
(seconds)
Step number
Score
from lookup table on back of this sheet
1
2
3
Best score (E)
Additional 10 points for disassembling and returning parts (F) ____________
Penalties (G) _______ Subtract 10 points for each time the power cord was used to steer or brake
FINAL SCORE = ( A + D + E + F ) - G = __________
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rev. 10/2012
Lookup table for the hover height test
Time (s)
Step 1
Step 2
Step 3
Step 4
Step 5
Step 6
Step 7
Step 8
Step 9
Step 10
2
34
44
54
64
74
84
94
110
130
150
3
33
43
53
63
73
83
93
109
129
149
4
32
42
52
62
72
82
92
108
128
148
5
31
41
51
61
71
81
91
107
127
147
6
30
40
50
60
70
80
90
106
126
146
7
29
39
49
59
69
79
89
105
125
145
8
28
38
48
58
68
78
88
104
124
144
9
27
37
47
57
67
77
87
103
123
143
10
26
36
46
56
66
76
86
102
122
142
12
25
35
45
55
65
75
85
101
121
141
14
24
34
44
54
64
74
84
100
120
140
16
23
33
43
53
63
73
83
99
119
139
18
22
32
42
52
62
72
82
98
118
138
20
21
31
41
51
61
71
81
97
117
137
22
20
30
40
50
60
70
80
96
116
136
24
19
29
39
49
59
69
79
95
115
135
26
18
28
38
48
58
68
78
94
114
134
28
17
27
37
47
57
67
77
93
113
133
30
16
26
36
46
56
66
76
92
112
132
32
15
25
35
45
55
65
75
91
111
131
34
14
24
34
44
54
64
74
90
110
130
36
13
23
33
43
53
63
73
89
109
129
38
12
22
32
42
52
62
72
88
108
128
40
11
21
31
41
51
61
71
87
107
127
42
10
20
30
40
50
60
70
86
106
126
44
9
19
29
39
49
59
69
85
105
125
46
8
18
28
38
48
58
68
84
104
124
48
7
17
27
37
47
57
67
83
103
123
50
6
16
26
36
46
56
66
82
102
122
52
5
15
25
35
45
55
65
81
101
121
54
4
14
24
34
44
54
64
80
100
110
56
3
13
23
33
43
53
63
75
90
100
58
2
12
22
32
42
52
62
70
80
90
60 or
more
1
11
21
31
41
51
61
65
70
80
This material may not be reproduced without permission
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