The below article is courtesy of our resident IntoScientist and passionate educator, Dave Canavan.
A Jumbo Jet (Boeing 747) can weigh up to 390 metric tonnes when it is ready to take off.
It holds about 400 people on average, all of their luggage and massive quantities of fuel, yet despite all of this mass, it can still get off the ground and fly over 7000 nautical miles!
They do however, require a run up of over 3 km in order to become airborne, and this is part of the secret as to how planes fly. Forces must play a role … But which ones? Dave explores these elements below and discusses a few experiments to try at home too!
Aeronautics is the science of flight and has fascinated scientists for centuries. At its basic level, it is concerned with four main forces; thrust, drag, weight and lift. Thrust is the forward force that must be exerted by an airplane in order to overcome drag. Many planes, like the 747, use a jet engine to propel forward. Based on Newton’s third law of motion which states “For every action there is an equal and opposite reaction”, the heat from a chemical reaction forces gases out the back of the engine at high speeds which allows an equal force to be exerted on the engine propelling the plane forwards. Drag is the force that resists the motion of an object in a fluid. In science, air is classed as a fluid, as is syrup, and the ways in which they behave are the same. The reason why flaps on wings and the landing gear is stowed during cruising speed is to reduce drag. Drag is the undesirable force in aeronautics, although without drag, lift could not be achieved.
Weight is the force of gravity acting on an object. The force of gravity is acting equally on all objects at 9.8N/kg and it is this force that must be overcome if a plane is to get off the ground. Lift is the key to flight and how it is produced brings about a whole new and complex area of science. In order to produce lift, we need to understand the properties of wings and the pressures acting on them.
An aerofoil is the shape of a wing, blade or sail viewed in cross-section. The shape of an aerofoil is seen in many examples in nature with the obvious one being birds wings. Similar in shape to a plane’s wings they are the perfect shape for flight. Many fish are also in the shape of an aerofoil so as to reduce drag in water. The shape of a wing determines how air travels over it. The classic aerofoil shape has a flattened bottom, a rounded front and a curved top that tapers to meet the flat bottom at the end of the wing. This shape ensures pressure differences when air travels over it. The Bernoulli Principle Daniel Bernoulli was a Swiss scientist who lived in the 1700’s. He found that when a fluid moves over an object quickly, the resulting pressure is less than a fluid moving over an object at a slower rate. When applied to wings, air moving over a longer path has less pressure than the same volume of air moving over a shorter path.
Looking at a wing, as the bottom is flat and the top is curved, the distance over the top of the wing is longer than the distance over the bottom of the wing. As the air is moving at the same speed when it hits the wing, and if we imagine the air meets at the same point at the end of the wing at the same time, the air on top has a greater distance to travel, therefore the air moving over the top must be moving faster.
Based on Bernoulli’s principle, as the air over the top of the wing is moving faster, the resulting pressure on the top of the wing is less than the pressure on the bottom of the wing, and this forces or sucks the wing upwards in the effect known as lift. This is the Bernoulli Effect.
The amount of lift produced by the wings is determined by the speed at which the air is travelling over the wings. The greater the speed, the greater the lift. If a 747 is to get airborne, the amount of lift must be greater than the weight of the plane, therefore the speed at which the plane must reach in order to have that effect is the reason why the runway is so long.
Planes also use many adaptations in order to take off and land, such as flaps at the rear of wings, and slats at the front of wings. These take into account the Coandă effect, where fluids follow an object on a curved path. This allows air to be forced towards the ground which increases lift. Flaps and slats do increase drag though, and therefore are not used when cruising. In order for a plane to climb or fall, speed is increased or reduced which, according to the Bernoulli Effect, will increase or decrease pressure and therefore change the plane’s altitude.
You can observe the Bernoulli Effect at home, with a few simple experiments:
1- Take a piece of paper (a thin strip works best) and hold it width ways with one end against your chin. Let the strip hang freely, and then blow on the paper and watch what happens. The paper should lift. This is because you are forcing the air on top of the paper to move quicker, resulting in less pressure. The air underneath is now forcing the paper upwards.
People tend to forget that air is pushing on us all the time from all directions. In an airplane, when the air pressure above the wing or below the wing become suddenly different, we rise or fall dramatically in an effect we know as turbulence. You are forced up or down due to the air around you pushing you one way or another.
2- Experiment number two is a little more complicated. Get a piece of A4 paper and fold it widthwise leaving an overlap of 2-3 cm. Tape the ends together and then observe that one side is flat whilst the other is curved. This is your aerofoil. Then, carefully make a hole in the top and bottom and put a straw through your aerofoil and attach with tape. You can now run string through this straw. Hold your aerofoil in front of an upright fan and observe it rising!
3- If you place a table tennis ball on a table and place a funnel over the top, blow, and lift the funnel away from the table. The ball will stay inside the funnel and defy gravity! The speed of the air rushing over the ball by the funnel, compared to underneath the ball is quicker, therefore making the pressure less and pushing the ball into the funnel. The magic of Science!
4- A final experiment on air pressure is to simply put your hand out of the window when in a moving car. Having a flat palm will make you experience the pressure of air, whereas putting it level with the wind, and then angling up or down will show you how pressure can make your hand lift or fall.
The Bernoulli Effect is not only used for planes, but also for race cars. Spoilers are basically an upside-down aerofoil. Instead of the air moving faster over the top of the aerofoil, as in planes, the air moves faster over the bottom of the spoiler. This creates a greater pressure on top of the spoiler, forcing the car down and therefore giving the car more traction on the road. Isn’t physics clever?! But do remember, the birds have been employing Bernoulli’s effect well before Bernoulli, or any human, was ever born so biology still rules the sciences!
Read more of Dave Canavan’s articles here : www.thechingchokhunter.com