• Question: How does a plane take off and fly when it is so heavy?

    Asked by edrienepadua to Christina, Colin, Jess, Samaneh, Steve on 23 Jun 2013.
    • Photo: Colin Dick

      Colin Dick answered on 23 Jun 2013:

      Hi there!

      The main forces on an aeroplane are ‘lift’ and ‘weight’ (in the z-direction, or up-to-down) and ‘thrust’ and ‘drag’ in (in the x-direction, or left-to-right).

      For an aeroplane to get in the air, it needs to create a lift force which is greater than the weight. This upwards force comes from the wings, which create lift using a formula:

      Lift = 0.59 x lift coefficient x wing area x (speed)^2

      It starts to get a bit complicated (let’s ignore the ‘lift coefficient’) but we’re basically saying that if you want to increase the lift then you need to
      a) increase the size of the wings or
      b) increase the flight speed.

      Both of these things also increase the drag of the aircraft (this is the force that pulls it backwards) so now you need to increase the thrust of the engines to balance this. If you increase the thrust from the engines then you burn more fuel!

      So, we’re saying that heavier aeroplanes need to have bigger wings or fly faster to take off, and this means they use more fuel!

      Now it’s clear why we have to pay more money to put our heavy suitcases on the plane =) also, did you know that some airlines now charge people depending on how much they weigh? I’m not sure I like this idea…

      http://www.bbc.co.uk/news/world-22001256

      Cheers,

      Colin.

    • Photo: Christina Pagel

      Christina Pagel answered on 23 Jun 2013:

      Gravity pulls downwards on a plane with a force equal to its weight (gravity doesn’t know or care if it the plane is in the air or on the ground, it pulls the same!).

      For a plane to take off, the wings must push upwards more than gravity pulls downwards. A big plane (like a 747 or 777) weighs about 200 tonnes. That means the wings must push upwards more than 200 tonnes!

      So how do the wings push upwards? The wings are flat on the bottom and curved on the top. As air rushes over them, the air on the top has to go further over the curved bit (the top) than the air that goes past the flat bit (the bottom). This thins out the air on top of the wing and make the air there have a lower pressure than under the wing. So the air then pushes up from high to low pressure (just like the wind blows from high to low pressure on a weather forecast map).

      So it’s air that lifts a massive plane into the sky!

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