DiscoverHover CURRICULUM GUIDE #19
DRAG
© 2005 World Hovercraft Organization

NAME DATE

A hovercraft, such as the DiscoverHover One, is able to glide or slide easily because there is so little contact friction with the surface over which it is hovering. Still, there are forms of friction which come into play, and these frictional forces are usually known as drag. Drag occurs in several forms, the most familiar being wind resistance, or form drag, which is created by the hovercraft having to push aside air as it moves forward. This effect increases more and more as the hovercraft’s speed increases. Streamlining the shape of the hovercraft decreases the wind resistance, resulting in higher top speeds. While wind resistance is always present, it becomes a much greater problem at speeds of 50 km/hr [31 mph] and above. Since DiscoverHover hovercraft usually don't exceed 50 km/hr [31 mph], wind resistance is not as noticeable as it would be on other light hovercraft.


Figure 19-1: Hovercraft experiencing different forms of drag
Image ©2005 DiscoverHover

A hovercraft operating over water is subject to three other forms of drag not experienced on solid surfaces: wave drag, skirt drag, and impact drag. Wave drag (called hump drag at low speeds) occurs when lift air under the hovercraft pushes down on the surface of the water. Some of the water is displaced from under the hovercraft, creating a depression in the water. The total weight of the water displaced is equal to the weight of the hovercraft and pilot, according to Archimedes’ Principle. As the hovercraft starts moving forward, the depression moves with it and forms a small wave in front of the bow. This causes the bow (front of hovercraft) to rise and the stern (back of hovercraft) to sink slightly. The hovercraft is, in effect, trying to fly uphill. As the hovercraft increases speed, the bow wave increases in size. Eventually, the hovercraft will reach a speed where it's moving faster than the wave and will “climb” over it. Technically known as planing speed, this is commonly referred to as “getting over the hump”. At this point the hovercraft will accelerate rapidly. The moment before planing speed is reached, wave drag is at its greatest. When traveling above planing speed, the lift air under the hovercraft doesn't have enough time to depress the surface of the water and the wave drag decreases dramatically.


Figure 19-2: Wave making "hump drag"
Image ©2005 DiscoverHover

Skirt drag occurs when the skirt contacts the surface of the water. Skirt drag is worse when the hovercraft is traveling over small waves.


Figure 19-3: Skirt drag
Image ©2005 DiscoverHover

Impact drag occurs when a hovercraft’s skirt or hull strikes large waves on turbulent water or strikes objects, such as ice flows or ridges.


Figure 19-4: Impact Drag
Image ©2005 DiscoverHover

Quiz Question:

  1. What can be done to reduce the drag from wind or from traveling at high speeds?
    2.  
     
    ©2005 World Hovercraft Organization
    All rights reserved. Copies of this Curriculum Guide may be printed for classroom use exclusively by DiscoverHover registered members. This Curriculum Guide and all materials contained in the DiscoverHover web site are protected by copyright laws and may not be reproduced, republished, distributed, or displayed on any other web site without the express prior written permission of the World Hovercraft Organization.