Hovercrafts

MAE 442: Automotive EngineeringSpring 2009

Justin BridgesAmexia Harris

Jonathan (Drew) Blackwood

History

• Considered as the brainchild of (Sir) Christopher Cockerell (6/4/1910 – 6/1/1999)

• Resulted from developments of John Thornycroft's initial discovery in the late 1800's that air trapped beneath a ship's hull would reduce the friction imposed on the ship as it moved through water .

Early uses of the air cushion effect 1920 - 1949

• The German-built Dornier Do-X flying boat proved the reality of Thornycroft's theory in 1929.

• During an Atlantic crossing, it flew much closer to the ocean's surface.

• The trip time was significantly reduced as a result and the aircraft's performance was that much greater.

The Do-X Flying Boat loading and unloading.

(c) Aviation Posters.

History

• In the 1950s Sir Christopher Cockerell, a British engineer, discovered that by creating a wall of high pressure air forced beneath a hull, to create an air cushion.

The First Test

• Sir Christopher Cockerell first tested his idea by using household items.• His intentions were to produce a cushion of air between the two coffee

tins using a hair dryer.

The first real hovercraft - 1950 to 1960

• Cockerell successfully tested his theory and filed his first patent in 1955. The year after he formed a company called Hovercraft Ltd.

• Soon after, in 1956, the air cushion vehicle was classified as "secret" and a construction contract was placed with a British aircraft and seaplane manufacturer. The result was the SR.N1 in 1959.

Christopher Cockerell's SR.N1 hovercraft - 1959.

The first real hovercraft - 1950 to 1960

• The first SR.N1 weighed four tons and could carry three men. Its maximum speed was 25 knots on calm water.

• It had a 6-inch (15 cm) rubberized skirt to make it easier to contain the air cushion on uneven ground.

• Due to significant wear and tear of the skirt through friction with the water at high speeds, durable material and a rubber and plastic mixture was developed by 1963.

• The length of the skirt had also been extended to about 4 feet (1.2 m).

Hovercraft and Ram-wing Craft in the 1960's and beyond

• The air cushion principle found an application in the form of a sidewall craft in 1963, reusing the plenum chamber principle of Thornycroft.

• Advantages of a sidewall air cushion vehicle are:

– The use water propellers– Effecting in greater steering control, especially at low speeds. – Greater stability of the craft as opposed to the circular skirt.

• The problems with the air cushion vehicle included:

– Steering control – Noise, – Salt and skirt erosion.

The Russian ekranoplan Aquaglide.

The Russian Icarus WIG effect vehicle designed for use on the ice.

The remote controlled RC-StarCruiser model hovercraft

Hovercraft Uses

• Hovercraft are being used as passenger ferries, where they transport passengers, goods and vehicles.

• Police and Fire Department use

• Ice and water transportation

• Oil spill clean up

• Rescue and surveillance companies and institutions.

• Environmentalist organizations using hovercraft in areas, especially wetlands, swamps and other sensitive water systems such as rivers.

Military Use

• The first hovercraft to see military action was the “SK-5” Hovercraft in the Vietnam War.

• The gun boat, also known as the PVAC (Patrol Air Cushion Vehicle) was used greatly for interdiction missions and patrolling in the Mekong Delta.

US ARMY SK5 Hovercraft, Assault Craft Air Cushion

• Military and Rescue hovercraft are exceptionally suited to various tasks as they are not dependent on road conditions and can quickly move across the various terrains in pursuit of their goal.

U.S. Marine Corps: "Boats that fly" - Operation "Iraqi Freedom" proved the hovercraft's worth as military assault vehicles. (2003)

                                     

U.S. Marine Corps: "Boats that fly" - Operation "Iraqi Freedom" proved the hovercraft's worth as military assault vehicles. (2003)

Recreational Use

• Small homebuilt and kit-built hovercraft are increasingly being used for recreational and racing purposes.

• mainly on inland lakes and rivers but also in marshy areas.

• Hovercraft Club of Great Britain regularly organizes inland and coastal cruising hovercraft races in various venues across the United Kingdom. Similar events are also held in the U.S.

Hovercraft Video Clips

Special Features

• 1. Propellers

• 2. Air

• 3. Fan

• 4. Flexible Skirt

Thrust Propellers

• The thrust propellers are used to drive the hovercraft.

• The rotational speed must remain fixed to that of the lift fan and engine.

• The engine speed is dictated by the amount of lift air required.

• The amount of propulsion is obtained by varying the propellers pitch.

The Momentum Curtain/Air• The air from the lift fan is forced into a plenum chamber and is guided

around the edges of the craft.

• Contact friction is reduced to zero enabling the craft to travel over any surface.

• The amount of escaping air is reduced and thus has a greater hovering height and more stability compared to the open plenum .

The Lifting Fan• The lifting fan is designed to force air into the chamber below the craft to

create a pressure underneath.

• The lifting fan used on most hovercrafts uses a centrifugal fan.

• At high speeds, air sucked into the fan and the slated blades force the air out.• Cons:

1. Without lift, hovercrafts would be useless.2. Too much airflow directed under the craft will cause it to hover too high

and tip.3. Not enough lift will cause the craft to remain on the ground

• Pros:1. Will move larger volumes of air compared to a propeller2. Operate efficiently when backpressure is high

The actual amount of lift depends on how much weight the craft is carrying. A fan of a given power will create a certain amount of pressure

under the craft. Now since:

pressure = force / area

it follows that a bigger hovercraft (one with a bigger overall area) can carry more weight than a smaller hovercraft with a fan the same size.

The Lifting Fan

The Skirt

• The skirt is made out of rubber and is designed to be placed around the vents of the plenum chamber.

• The skirt increases the height that the hovercraft hovers enabling it to perform in any weather condition.

• There are three main skirts: 1. The Bag skirt2. Segmented skirt3. Jupe skirt

The Bag Skirt

The Juped Skirt The Segmented Skirt

BAG SKIRT

• Consists of a tube that encircles the perimeter of the craft.

• The bag is inflated which serves to lift the craft off the ground and more importantly, to contain the air cushion.

• There are two methods of inflating the bag skirt, the first being serial feed and the second being parallel feed.

• The serial feed method requires that air be directed from the lift fan _through_ the skirt and then out into the cushion. The parallel feed method requires that a certain amount of air be split off of the lift fan into the skirt (about 10%) and the rest into the cushion.

THE SEGMENTED SKIRT

• Called a 'finger' skirt because it consists of several separate nylon segments that, when inflated, press together to form a shape that looks like fingers of a hand that are pressed tightly together

• Offers much less resistance to obstacles and much more ease of repair when damaged since you only need to replace one or two damaged fingers instead of an entire skirt.

• Less stable and more complex to manufacture than bag skirted craft

JUPED SKIRT

• Consists of several cells that look like cones with their tops cut off and have their bases attached to the bottom of the craft.

• When inflated, these cones readily support the weight of the craft upon a stable cushion.

• Very stable but will experience difficulty when attempting to inflate the jupes on a rough terrain such a tall grass or deep gravel.

• Tend to scoop water in rough conditions and drag on grass.

Bag segment Jupe

Cost low high low

Drag (smooth water/rough water

Same/high Same/low Same/very high

Stability good poor excellent

Roll ability for turning slight excellent none

Hump performance moderate good Poor moderate

High speed good moderate moderate

Bounce poor good good

Durability good poor moderate

Life good moderate good

Movement

• How do you make a vehicle move when it doesn’t touch the surface?

• The answer is thrust from propellers that generate a lift force as they move in a circle.

• The air being pushed by the propeller exerts a force carried through and into the hovercrafts structure propelling it forward.

• There are several ways to steer a hovercraft:– Rudders behind the propeller or at the back of the hovercraft– Moving the propellers themselves to change the direction of the

thrust– Moveable air ducts

Hovercraft Operation

• Piloting the hovercraft is accomplished with movable rudders mounted behind the thrust propellers.

• There is very little friction between the hovercraft and surface, making it very difficult to steer.

• There is not a break on a hovercraft. To stop the pilot must reduce power and slow down.

Appendix• http://www.jameshovercraft.co.uk/hover/interest/concept.htm• http://www.quicktechhobby.com/Hovercrafts/what_are_hovercrafts.htm• http://www.hovercraft-museum.org/images/scc11.jpg• http://www.discoverhover.org/abouthovercraft/faq.htm• http://www.britannica.com/EBchecked/topic/10660/air-cushion-machine