The objective of this report is to design, build and enhance a
light, low cost all terrain hovercraft. There are many factors one should
consider when designing a hovercraft. Some calculations that must be considered
are the lift, skirt design and material, stability, thrust, hump drag,
dynamic effects and hull structure. The effects of the drag through air
and water are very important and you will see later on in this report that
why this is so. This report will show that the drag through air and especially
the hump drag through water will not play a major role in this model design,
However, when considering full size crafts this drag is extremely important.
To improve the performance of this hovercraft, these calculations must
be considered. The calculations will be shown later on in this report.
Another major part of this report is the type of internal combustion engine
or motor that will be used and why, also the wireless remote control of
this hovercraft will also be explained. The total payload of our model
hovercraft is approximately 25 lbs.
This report will also introduce a newly designed breaking system for the hovercraft. The rudders were redesigned to allow the hovercraft to stop in a much shorter distance and to move in reverse.
A hovercraft, being an all terrain vehicle, slides along while balancing on top of an air cushion bubble. This bubble is generated by an air pump (fan) while a flexible skirt helps retain the bubble beneath the vehicle by limiting the air loss. The air acts as a lubricant which can enable the craft to slide across many relatively flat and smooth surfaces such as grass, snow, ice, water, mud and sand. This craft can hover over these surfaces with virtually very little or no friction. However, the smoother the surface is the better the performance of the hovercraft. The advantages of this craft are many, especially when dealing with emergency rescues over water.
In order to keep this project in manageable proportions we have been restricted to light amphibious air cushion vehicles. However, some scaling laws have been included in this report in order to tackle the bigger full size hovercrafts. These laws will help in determining the size and performance of larger versions of the test model and vice versa.
Most frames designed today have a square shape, which increases
the drag force acting on it. A major portion of this project is to design
an aerodynamic frame for the hovercraft. This frame must reduce the drag
caused by the air and water. The drag force is an important part of the
design of a hovercraft or any other vehicle because it can reduce its speed
and affect its stability. It also causes the vehicle to lose power since
more thrust is required to move this vehicle to overcome the drag force.
Another important consideration is the pressure of high-speed vehicles. This pressure distribution is directly related to the shape of the vehicle and will vary from point to point along the vehicle shape. The pressure will be at a maximum at the base of the windshield due to streamline curvature. A detailed analysis of our aerodynamic shape will be shown later using the finite element package Cosmos/M.
The design of a hovercraft can be broken up into the following aspects: .a) the dynamic effects and hump drag, b) lift system, c) controls, d) engine, e) propulsion, f) skirt system and g) stability.
THE DYNAMIC EFFECTS AND HUMP DRAG
In the situation where a hovercraft begins to hover over water, its cushion pressure will displace the water under the craft.. The total weight of water displaced equals the gross weight of the hovercraft. As the hovercraft accelerates, the cushion pressure continues to depress the water under the skirt as it moves along. As the speed of the hovercraft increases so does its need for more energy. This is because the water under the craft is depressed downward by the cushion pressure. This will also cause the cushion to depress the water forward, which results in the tendency for the water to move in the direction of the craft’s cushion, resulting in the generation of a standing wave under the craft. This causes the nose to rise up and the stern to sink, which is equivalent in principle to climbing a hill. If more thrust is applied the craft will begin to climb this self -created hill whereupon it will eventually reach the top and begin to hover above the water’s surface. At that point that will be a drastic reduce in drag and the craft accelerates quickly. This phenomenon is common for all watercraft, but for a hovercraft it is called “hump drag”. It is named this way because of the shape of the drag curve shown in the figure 2 on the next page.
When a hovercraft is moving over water there is a drag caused by the air and also a much larger drag (hump drag) caused by the water. The drag caused by a moving vehicle is directly related to the frontal area, cushion area, speed, density of the fluid and cushion pressure. The cushion pressure will affect the hump drag. It is very important to keep the cushion pressure, area and the weight of the craft as low as possible because that will lower the aerodynamic and the hump drag.
The hump drag of a vehicle is proportional to its width. In fact, the least hump drag is achieved with pointed nose streamlined structure. This is also the best design for high speed-race vehicles because it minimizes the area that is affected by the drag.
Aerodynamic drag, is the component of a force on a body acting parallel to the direction of motion (ref 1). The equation for calculating the drag is
D= ?V2ACD (1)
Where D= drag lb
?= density of air slug/ft3
V= velocity of the craft ft/s
A= wetted area in contact with air while craft is moving ft2
CD= coefficient of drag
In the case of the hovercraft this is the force that the thrust must overcome. Based on equation 1, the drag can be reduced significantly by reducing the frontal area of the craft.
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 = L? W (2)
Based on equation 2, it is clear that for a better performing hovercraft,
the surface area should be kept as large as possible, therefore less pressure
will be needed.
When designing a system to provide thrust, there are some criteria that must be considered. When using a fan, its diameter plays a very important role in the effect of the thrust output. Large diameters can not be incorporated into small hovercraft. The next best alternative is the propeller. Its major drawbacks are the difficulty of guarding such large devices and the high thrust line. Ducted fans are safer, more compact, have lower thrust lines, and, consequently, are favored by the majority of hovercraft customers despite their lower thrust/HP capability (ref 1).
The selection of the engine and the fan/propeller depends on many
considerations. After the decision has been made as to what to use, a power
output curve for the selected engine and an absorption curve for the fan
or the propeller is needed. This information is usually, provided by the
respective manufacturers. In most cases the engine will have to be geared
down since commonly available engines generate power, at higher speeds
than either fans or propellers can absorb. Therefore, a reduction is required
which generally will be a chain, sprocket, gearbox, or belt and pulley.
In the case of a hovercraft, the thrust from the propellers must overcome the aerodynamic drag only which can be calculated by equation 1.
There are typically three kinds of skirt systems, a) jupe b) bag and c) segmented. The most popular one is the bag because it uses the least amount of material compared to the other skirt types. There is usually little or no wastage. The pressure in the bag should be slightly higher than in the cushion. Jupe skirts are difficult to inflate when sitting on porous surfaces like grass and each jupe should be fed directly from the fan to obtain maximum cushion stability. Segment skirts offer less resistance when passing over grass or rough ground. They also have good sealing properties, which means less dust, less noise and less horsepower, However, they are not as stable as the bag or jupe skirts.
Stability is very important when dealing with hovercrafts. If there is more weight on one side of the hovercraft than the other, the hovercraft will sink towards the heavier side and the lighter side will tend to lift. Once this happens all the air will escape from the side with the least weight and cause an undesirable effect as shown below.
Hovercrafts are usually twice as long as they are wide. Like any other
vehicle, a hovercraft is more stable about its length because it is longer
(ref 2).. The most common cause for instability is when passengers shift
their weight around inside the craft, referred to as the trimming effect.
Trim means that drivers weight at the front should be balanced by the weight
of the engine in the back. The tank for the engine should be placed as
close to the craft’s center of gravity as possible.
Stability also depends on the type of skirt used. For example, bag skirts are more stable than segments because the more the bag touches the surface, the more it is depressed and the more the bag flattens out along the surface. Typical stability curves for the three common skirt systems are shown in the on the next page.
The best method to avoid instability is to balance out the forces in the hovercraft. That is to have the center of mass located in the center of the hovercraft. Therefore, the engine, propellers and gas tank must be strategically placed. The shape of the hovercraft must also be carefully designed to overcome the forces made by these components.
Being virtually a zero friction craft, it is essential that a braking system is implemented in the hovercraft. Although, the drag force will slow the hovercraft down after the propellers hove been turned off, the speed the hovercraft attains will be too high and the craft will not come to a complete stop for many feet. Therefore, thrust-reversers were built into the hovercraft to stop it in a shorter time, avoiding accidents and dangerous situation for the driver. Many airplanes today use thrust reversers to slow the airplane down for landing. Thrust reversers are also used to move the airplane in reverse. This system has never been implemented into a hovercraft but the design fits well with any propeller driven vehicle. The way the thrust reversers work in the hovercraft is by blocking the thrust produced by the propellers and rerouting the thrust out of the front of the hovercraft through ducts. Therefore, the same thrust that was used to push the hovercraft forward is now used to stop the hovercraft and allow it to move in reverse.
DESIGNED AND BUILT BY ABRAHAM ALGABYALI AND RAFAEL GONZALES, POLYTECHNIC
ANY QUESTIONS OR COMMENTS CONTACT POLYTECHINC UNIVERSITY, 6 METROTECH CENTER, BROOKLYN NY 11201