• Question: explain how bernolies principle can keep a bird in air?

    Asked by josieb123 to Amy, Karen, Sarah, Vijay, Will on 18 Jun 2012. This question was also asked by anon-22966.
    • Photo: Will Reynolds

      Will Reynolds answered on 18 Jun 2012:

      Hi Josie. Birds (and other things like aeroplanes and helicopters) fly because their wings are curved. The top of a wing is curved and the bottom is straight. This means that the surface area of the top is bigger than the surface area of the bottom so air takes longer to travel over the top.

      If you imagine two air molecules at the front of the wing one goes over and the other goes under, the one that goes underneath the wing will get to the back first. This means that there is a space above the wing where the other molecule should be so the one at the bottom rises to fill the space and as it rises it pulls the wing up with it.

    • Photo: Sarah Martin

      Sarah Martin answered on 20 Jun 2012:

      Hi Josieb123!

      We chatted about this one yesterday, right? Well I’ll go for a more detailed explanation this time.
      Bernoulli’s principle states that for smooth flowing air or fluid, an increase in the speed of the air or fluid occurs simultaneously with a decrease in pressure.
      So if you think of two parts of the bird’s wing, above (A) and below (B) the wing, you can write Bernouille’s principle out as an equation:

      Pressure A x Speed A = Pressure B x Speed B
      (A abover the wing) = (B below the wing)

      So that’s the rule that’s Bernouilli’s principle.

      Now bird wings (and airoplane wings, and sailing boat sails) are curved a bit like an up-side-down spoon, so the air has to travel further around the top, but can just zoom across flat past the bottom. This makes the speed of air above the wing faster than that below, because it has more distance to cover in the same time (and speed is distance divided by time, right?).

      So now, if we check back with Bernouille, Speed A is larger than Speed B – so the rule will only balance if Pressure A is smaller than Pressure B!
      And higher pressure on one side of something than on the other is something we usually call a push! So the wing is being pushed up by a force that results from the pressure difference that results from the difference in air speed that results from the curved surface where that makes one surface longer than the other.

      And that’s all Mr Bernouilli had to say about birds – or the only thing he wrote down about them anyway! 🙂

      Sarah

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