Science in the News: Planes slow down to cut costs Title: Planes slow down to cut costs Description: This activity contains an article about planes flying slower in order to cut fuel consumption. The article is followed by questions to test students’ comprehension, linking it to their knowledge of forces and motion. Finally there is a challenge to encourage discussion about how this relates to other issues such as lifestyle and the environment. Links to the curriculum: This activity looks at the factors affecting fuel consumption of an aeroplane, linking to: KS3 Sc4 Physical processes – forces and motion 2c that unbalanced forces change the speed or direction of movement of objects and that balanced forces produce no change in the movement of an object 2d ways in which frictional forces, including air resistance, affect motion [for example, streamlining cars, friction between tyre and road]. GCSE Sc4 Physical processes – forces and motion 2e that balanced forces do not alter the velocity of a moving object. Science in the News: Planes slow down to cut costs Planes slow down to cut costs Brussels Airlines in Belgium has announced that some of its planes are flying slower to reduce costs, and that reducing the speed of their planes by just 10 km/h will cut their annual fuel bill by £800 000, adding only one or two minutes to the flying time. An airline spokesperson said that only the 30 planes flying on short routes within Europe are affected. The 19 planes that fly longer routes to Russia, the Middle East and Africa fly at pre-set speeds so the speed cannot be reduced. The airline points out that using less aviation fuel is good for (iStockphoto) the environment, as well as being good for the airline. And using less fuel means that the planes emit less global warming greenhouse gases and less pollution of other kinds. The airline is also looking at nearly 100 other ways to reduce fuel use, for example, using the fuel more efficiently, reducing the weight of their aeroplanes and angling the tip of the aircraft wings so that they have less drag on take-off. Some of the ways of reducing weight include carrying less fuel, carrying less water, and removing the built-in stairways on the planes. The airline also wants the European Union to change the way flights around Europe are organised. This is because planes often end up flying a zig-zag route from one country to another in order to avoid flying over other countries’ airspace. QUESTIONS 1 Describe the main forces that act on an aeroplane when it is flying. You may wish to do this using a clearly labelled diagram. 2 Use your knowledge of forces to explain as fully as you can why flying at a slower speed uses less fuel. 3 Explain, again as fully as you can, why reducing the weight of the aircraft means less fuel is needed. 4 You are probably aware that the passenger cabins of aircraft are pressurised. This is because aircraft, especially those flying long-haul routes, usually fly very high. Use forces to explain the main benefit of flying at a higher altitude. Discussion and challenge Environmentalists criticise air travel, saying that air travel is one of the areas where emissions of greenhouse gases have increased hugely in recent years. Split into a few groups, representing different people who benefit from air travel, and ask each group to think of ways in which their use of air travel could be changed so that it has less impact on the environment. Hold a debate. Science in the News: Planes slow down to cut costs Answers and useful information Answers to questions 1 Main vertical forces: gravity acting downwards, upthrust or lift acting upwards. 2 Main horizontal forces: thrust from the engine acting forwards, air resistance acting backwards, wind acting in any direction. 3 When the aircraft is flying at a constant speed the forward and backward forces on it are balanced. As the speed of the aircraft decreases, the air resistance decreases (as the aircraft does not have to push so many air molecules out of the way each second), therefore the forward force needed to balance the air resistance and so ensure the constant speed also decreases. It takes less energy to provide a smaller forward force, so the engine does not have to use as much fuel. 4 If the weight of the aircraft is less, the upthrust or lift from the aircraft wings needed to prevent it falling downwards is also less. The lift from the wings is provided by airflow over the wings. The faster the airflow, the greater the lift, so if less lift is needed the aircraft can fly at a lower speed, reducing air resistance and so reducing fuel consumption. 5 As the altitude increases, the air density decreases (thus the pressure falls, causing the need for the passenger cabin to be pressurised). This means that at any given speed the number of air molecules to be pushed out of the way each second is less, so the air resistance is less. Less air resistance requires a smaller forward force to maintain a given constant speed. Therefore it is more economical to fly at a higher altitude. Discussion and challenge Groups benefiting from air travel could include, for example, holidaymakers, business travellers, food producers transporting perishable goods. Ways to reduce the impact on the environment could include: reducing the weight of luggage each passenger is allowed (thus reducing the weight of the aircraft), ensuring that aircraft fly with a full complement of passengers instead of with empty seats, ensuring that cargo aircraft are fully loaded in each direction, flying at higher altitudes.