Tuesday, February 22, 2011

Jet Straw Engine

Science in a Van is on a road trip at the moment visiting schools in Northland. As we crawl up some of the steep hills around here we often wish our van had a little more horsepower!

Hans von Ohain from Germany and an English man, Frank Whittle, are both recognised as inventors of the jet engine. Each worked separately at the same time. Hans von Ohain's jet was the first to fly in 1939. Frank Whittle's jet first flew in 1941.

In the following blog Science in a Van teaches you how to make a very simple and cheap engine using the same scientific principal as the original inventors.

For this experiment you will need:

Two bendy straws
  1. Fit the straws together by sliding the longer ends into each other (furthest away from the bendy bit)
  2.  You should now have a really long straw with bendy bits at both ends.
  3.  Put one end in your mouth and bend it down at right angles.
  4.  The other end, bend to point to the left or right.
  5.  Now just blow and watch it spin.
The science is quite simple. As you blow, the air comes out of the straw at a greater pressure than the air normal around us. This applies a force.
As we know from Sir Isaac Newton’s 3rd Law of Motion, if you apply a force in one direction you apply the same amount of force in the opposite direction. This is what makes our straw spin. Jet engines do the same by compressing the air, then mixing in fuel. A spark ignites the fuel. The air and fuel expands quickly causing a large force.

Maybe the next blog will detail how we have modified our van….     

1 comment:

  1. There was another explanation in which the force came from the change in momentum of the air at the bend. I think your explanation sounds better, but then we'd have to consider the straw and the air inside it as a whole and look at the force at the boundary with the outside air. Also quite counter-intuitively that the pressure in the stream is low as explained by Bernoulli's theorem and demonstrated by blowing it between two pieces of paper.

    For either explanation why does it not work when we suck instead of blow? If it's the change in momentum then I'd expect an accelerating force from the straw to the air. If it's force across that boundary then atmospheric pressure is pushing in, though the air is moving. Hmmm...


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