Example of a hovercraft thrust duct
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So far we’ve discussed how pressurized air beneath the hovercraft causes
it to hover, and we investigated what happens when hovering over water. We also
looked into some of the drag forces encountered when moving through water. Let’s
examine how a hovercraft’s forward motion is produced. A hovercraft moves
by using air to create forward thrust. The propeller
on the back of the Discover Hover One forces air towards the rear. How does
forcing air behind the hovercraft produce forward thrust? Remember Newton’s
third law: Every action has an opposite and equal reaction. The propeller
exerts a force on the air when it pushes it behind the hovercraft. The air,
in turn, exerts a force back on the propeller in the opposite direction. This
causes the propeller, as well as the rest of the hovercraft, to be accelerated
forward.
When designing propeller systems for hovercraft, efficiency is a big concern.
Efficiency is the ratio of how much work is produced
divided by how much work is put into the system. In order to produce forward
thrust (the output), we must power the propeller with a fuel-driven engine (the
input). Engineers try to get as much output work as possible for the least amount
of input work. Unfortunately, you can never get out as much work as you put
in. The heat produced by the engine, the noise produced by both the engine and
the propeller, and the vibrations you can feel in the hull are just some examples
of wasted energy that isn’t being used to produce thrust.
One way to increase the efficiency of a propeller is by surrounding it with
a circular enclosure called a thrust duct. In a small
hovercraft, a properly built thrust duct can add a 10%-15% increase to the total
thrust output, compared to a non-ducted propeller.
According to Newton's second law, thrust can be calculated by multiplying the mass of the hovercraft by its acceleration. We can determine the acceleration of the hovercraft using the formulas we learned in Curriculum Guide #2. Remember that a hovercraft is affected by different drag forces depending on the surface it's flying over. When you calculate a particular thrust, it is for that particular situation you tested the craft in. Remember that for a fan or propeller, the thrust force decreases as forward speed increases! This is because as the speed of the hovercraft increases, so does the speed of the air that enters the thrust duct. It’s harder for the propeller to accelerate air that’s already moving fast, so it is unable to produce as much thrust at higher speeds.