“What makes aeroplanes fly?” asked C(10) a few weeks ago. We’ve had fun finding out.
An aircraft flying in steady forward motion is subject to four forces:thrust, drag, lift and weight. Aviation for Kids: A Mini Course For Students in Grades 2-5 (free online) contains dozens of ideas for experimenting with these forces.
Thrust
Thrust is the force that moves a plane forward through the air. To investigate thrust, we made three different “aircraft”, each powered by a different thrust mechanism.
1. Elastic band thrust
First, we made elastic band-powered planes. We experimented with thrust by observing the distance our planes flew when we changed how far we pulled our elastic bands before we released them. {Instructions are on p3 of Aviation for Kids.}
In real aircraft, of course, thrust is created by a jet engine or propeller.
2. Air pressure thrust (blowing through a straw)
Our second aircraft was powered by blowing through straws, one inside the other. {For instructions see p7 of Aviation for Kids.}
We experimented with placing the wings at different points along the straw and observing the effect on our aeroplanes’ flight.
C(10) suggested a competition to knock down unifix cubes with the planes. It was more difficult than she’d expected!
3. High air pressure thrust (balloon power)
We made a balloon-powered rocket when we learned about space travel last year. The kids were delighted to recreate it as part of our investigation into thrust and drag. {See p 10 of Aviation for Kids for instructions.}
Newton’s third law states that for every action, there is an equal and opposite reaction – as the balloon deflates, it whizzes along the wire. It’s very cool!
Drag
Next, we investigated drag – the air resistance that slows an aircraft as it moves forward. If drag is greater than thrust, the aircraft cannot move forward.
To see the effects of drag, we attached a foam plate to the front of our balloon aircraft.
When it was subject to the increased drag of the plate, the balloon only managed to travel halfway across the room before it ran out of air.
Apparently it was hilarious to lie underneath the wire as the balloon zoomed to a halt.
Lift
The other pair of forces that operate on an aircraft are lift and weight. Lift is provided by the airfoil shape of an aircraft’s wings.
If you look up how planes fly, you find many references to Bernoulli’s principle. But some scientists think that lift can be explained perfectly well by Newton’s laws – that aircraft are forced up by the vast amount of air that the wings throw down.
Bernoulli’s principle
Whatever role it plays in helping planes fly, Bernoulli’s principle is fun to demonstrate, partly because it’s counter-intuitive. We demonstrated Bernoulli’s principle in two ways.
First we placed a sheet of paper between two books. We blew through the gap.
The children had predicted that the paper would lift up as they blew underneath it. Instead, the paper sagged down. Bernoulli’s principle says that within a stream of fluid (such as air), pressure goes down as speed of flow goes up. So when we blew, the air pressure under the paper decreased and atmospheric pressure from above pushed the paper down.
We also demonstrated with ping pong balls and a hairdryer. J(8) especially liked this one as he got to use a gadget.
When the hairdryer blows fast-moving air between the ping pong balls, air pressure between the balls decreases. Atmospheric pressure outside the balls pushes them together. (This doesn’t happen when you aim directly at the ping pong balls, but J(8) had fun doing it anyway.)
How do wings produce lift?
Lift is created by the curved shape of an aeroplane’s wing. This “airfoil” shape causes air to move faster over the top of the wing than the bottom. When lift is greater than the weight of the plane, the plane will move upwards.
We made paper airfoils and attached them to thread(see p25 ofAviation for Kids).
When we ran with our airfoils, they moved up the thread.
The airfoil also lifts when you spin around very quickly. This one was repeated many times!
(Aviation for Kids also suggests pointing a hairdryer at the airfoil. We did this but found it difficult not to blow the paper wing directly up or down.)
Weight
To experiment with weight, we made the world’s best paper aeroplane. This plane has great lift!
Then we increased the plane’s weight by attaching paperclips. It was easy to see that the extra weight reduced the plane’s lift. One paperclip actually had a stabilising effect, making the plane easier to direct. Two or three clips seriously impacted our planes’ ability to stay in the air.
What next?
C(10) has been making notebook pages about the science of flying. She’s come up with lots more questions as she’s been writing!
Our science this month is going to focus around the excitingengineering box we’ve just received on loan from the James Dyson Foundation, but we’ll definitely be continuing with the science of flying after that.
Further resources on the science of flying
Up, Up and Away: The Science of Flight(favourite)
Flight aerodynamics video Explains how the four forces of flight operate (turn up the volume to compensate for the narrator’s monotone)
Bernoulli’s paper airplane experiment
Airfoil lifting force misconception
BBC Bang Goes the TheoryBernoulli’s principle and wings
Science kids – Flight lesson plans
A physical description of flight; revisitedTechnical article on how aeroplanes actually generate lift, and the Newton v. Bernoulli debate
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