It’s an enormous aeroplane, designed to carry up to almost 400 passengers. With the weight of the aircraft, the passengers and their luggage, how does this machine manage to lift off the ground and into the air at all? Observant Scientist explains…

Engine power

Even though the the plane is extremely heavy at around 250 metric tons (!), it is important to remember that it also carries phenomenally powerful jet engines. These engines provide the thrust – or push – that is necessary for the machine to move forward fast enough to eventually overcome the downward (pulling) force of gravity.

The thrust that is required by this specific aircraft is 360,000 newtons (newton is the universal unit for force). This means that between the two jet engines it has to provide this amount of thrust – or push force – for the plane to be able to ascend. There are a few types of branded engine that were designed for the 777, the Rolls-Royce Trent 800 being one of them.

So, how do these jet engines work? It’s all to do with air particles and pressure.

The air around us is at a normal pressure. We don’t feel this pressure (usually) because it is, of course, stabilised at sea level. However, the jet engine is designed to draw in air from its surroundings by spinning the fan very fast. Once the air has been sucked in, it’s then placed into a much smaller area that is also spinning. The air is now inside a part called the compressor and due to the spinning AND the restriction in volume, the pressure increases enormously – up to 12x in some cases! This compressed air is then sprayed with fuel in the combustor and so a spark lights the mixture (this addition of fuel explains why you can see that hazy, heated effect coming out of the rear of the engines).

When you add heat to any state of matter (in this case, gas), the particles will move around much faster because they have more energy. Imagine a game of “hot potato” – you’d want to hop around and jiggle about to get away from the heat if you became hot! This increase in particle speed is better known as gas expansion – and there’s only one direction for the gas to go: through yet another turbine that will spin (causing the compressor to spin too) and create even MORE push force. The final thing to occur is for the gases to now exit through the nozzle at the back of the jet, blasting the plane forward. I like to think of it as dragging a wind up car backwards on carpet: the force has built up and so the car will shoot forward. It’s not exactly the same thing in a jet, but it’s a nice way to remember that thrust going backwards in the plane is actually making it go forwards!

As seen in Figure 1, most of the hot gas will exit via the nozzle and produce thrust. However, there’s another way to produce thrust and this is around the core of the engine area in the middle of the jet. Some gases will just whiz around the fan as cool(er) air and then mix (in the mixer) with the hot air at the back to finally exit the engine. This process won’t produce nowhere near as much thrust as the hot gases that have gone through the compressor and turbine.

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Figure 1: The inner workings of a jet engine, with labelled parts. Image credit: http://www.grc.nasa.gov

Speed

The 777 has to travel along the runway at a speed of 170 mph in order to ascend. This data is based upon an Airbus A320. An Airbus A340 will need to move at 180 mph. At these speeds, the pilot will control the plane to begin to lift it off the ground.

Wing shape and pressure

Things start to get a bit tricky here but do read on!

There are a few misconceptions that have been floating about as to how the shape of the wings actually provide lift but this article will give you the truth!

As shown in Figure 2, the upper surface of the wing is curved. The air that travels towards the plane will split with some going towards the top, and some towards the bottom. Air would naturally travel in a straight line but the upper curved surface pulls it around and back down again. As you can see, there is a larger volume surrounding the upper surface of the wing and this will produce a decrease in pressure. This pressure decrease will occur as the air particles have more room to spread out instead of being below the wing – where the volume is lower thereby creating high pressure.

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Figure 2: A cross-section of the wing of an aeroplane. Arrow on top denotes upward force on plane. Arrow on bottom denotes a downward force on air particles. Image drawn by Observant Scientist.

There is now a pressure difference between the top and the bottom of the wing. This difference in air pressure naturally causes a big difference in the speed of the air rushing around the wing. If we were to transform ourselves and a friend into 2 air molecules, you (air molecule No. 1) would travel along the top of the wing much faster and arrive at the tail end quicker, but your friend (air molecule No. 2) would arrive at the end a little bit later underneath the wing.

It’s important to note that both air molecules 1 & 2 will be travelling in a downward direction (albeit going at different speeds) and it is this downward drive that will force the plane into the air.

Things to remember:

  • An aircraft of this size needs to have 2 powerful jet engines that provide enough thrust in the form of around 360,000 newtons.
  • The aircraft has to travel along the runway at a speed of at least 170 mph.
  • The curve of the aircraft’s wings is crucial in order to provide lift.
  • Air molecules travel faster along the top of the wing due to a decrease in pressure.
  • Air molecules travel a little slower along the bottom of the wing due to an increase in pressure.
  • Air molecules travelling along the top and bottom of the wing both hurtle forwards in a downward direction, creating lift.

 

Featured image credit: http://www.store.x-plane.org

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