My Science School

 

               Helicopters are lifted into the air by their large rotating propellers, or rotors. These work like long narrow wings, generating lift as they spin rapidly through the air. The tail rotor generates enough thrust to stop the fuselage spinning.

               A helicopter turns by increasing or decreasing the thrust of its tail rotor. It climbs by increasing the angle of attack of the rotor blades. It moves forwards by increasing the angle of the blade moving back on every rotation so that it pushes against the air.

Picture credit: google

               The hovercraft is an ingenious machine that rides on a cushion of air. It looks like a flat-bottomed ship, and is usually powered with huge propellers like those of an aeroplane. Engines draw in large amounts of air, which is pumped out under the hovercraft and kept in by flexible rubber skirts. The craft rises up, supported on the column of air, and is then able to travel quickly over the water or over land. There is little friction because the bottom of the hovercraft does not actually touch the water.

               Hovercrafts have a number of uses, but have never replaced ordinary ships because they are expensive to run, very noisy and are unable to operate in stormy seas.

 

 

               A ship, or any other floating object, displaces (pushes away) water. You can see this for yourself if you fill a bowl with water right up to the brim, and then float a smaller dish inside it. Water will spill over, and the amount spilled will be the same as the volume of the smaller dish. A vessel will float if it is lighter than the amount of water it displaces.

               A very small boat built of steel would probably sink like a stone. However, a very large ship built of steel would be so buoyant that it could also carry huge amounts of oil or other cargo. The capacity of a cargo ship is measured by the weight of water that it displaces.

            Engines that burn fuel inside them are called internal combustion engines. A petrol engine injects fuel into a cylinder and ignites it with a spark. The mixture of fuel and air explodes and drives a piston down the cylinder. The piston is connected to a crankshaft, which turns as the piston moves down. As the crankshaft continues to revolve, it pushes the piston up again.

            A diesel engine works in the same way, except that a spark is not needed to ignite the fuel. Instead, the engine is spun over by a starter. As the piston comes up the cylinder it compresses the mixture of diesel oil and air. Compressing the mixture heats it up and it explodes, driving the piston down.

Picture credit: google

            Like rockets, jet engines propel an aeroplane with a stream of hot gases. Unlike rockets, which produce hot gas as a result of burning an explosive mixture, jet engines burn fuel in the air that they draw in at the front of the engine?

            Air enters the front of the engine and is compressor wheel. Fuel is injected into the compressed air, burns fiercely and expands. The hot gas rushes out of the jet pipe at the rear of the engine. The hot gas passes through a turbine, which drives the compressor at the front of the engine.

Picture credit: google

               As an aeroplane moves though the air, the air passes over the surface of the wings. These are shaped with a curved top surface and a flatter lower surface, which means that air passing over the top of the wing, has to travel a little faster than that below the wing. This lowers the pressure above the wing, while the air pressure below pushes up, and the end result is the lift that keeps the aeroplane in the air.

               The tail surfaces keep the wing at the proper angle to provide the right amount of lift. The power to propel the aeroplane can come from the engines or, in the case of gliders, from rising air currents.