What it shows
A propeller pushes air backwards and is pushed forwards in return. The size of the push depends on how many blades there are and how they are angled to the air.
The science
The blades act like small wings that deflect air backwards. By Newton’s third law, the air pushes the propeller forwards with an equal force called thrust. A blade at a small angle to the air produces little push, at about 20 to 30 degrees it is more effective, and at a very steep angle the blades stall and the motor is overloaded and slows. More blades push more air until they crowd one another.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A DC toy motor with a small shaft adapter
- Cut-out plastic bottle blades in sets of two, three and four
- A 2 x AA battery holder with two cells and a switch
- A cardboard platform on a kitchen scale (reads grams)
- Cardboard to make a hub, glue and a protractor
- A notebook
Safety
For this project: Cells only. Cut plastic blades with an adult and sand the edges. Keep your face and hair away from the spinning propeller, and always put a card guard around it.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Fix the motor to the platform of a kitchen scale so it pushes the platform sideways or upwards, not against the table.
Attach a two-blade propeller set at 15 degrees to the motor shaft using the cardboard hub.
Switch the scale on and zero it, then power the motor for five seconds and note the change in the reading in grams.
Repeat three times and take the average.
Change the blade angle to 30 degrees and then 45 degrees, repeating the readings each time.
Now fix the best angle and test three and four blades.
Plot a bar chart of average thrust against angle and number of blades.
Report which combination gave the best push and which uses more current (measure with a multimeter if available).
What you should see
The reading changes with blade angle and number, with a best angle somewhere around 20 to 30 degrees, and extra blades give limited extra push.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is thrust?
The forward force produced by pushing air backwards.
2Which law explains thrust?
Newton’s third law: action and reaction.
3Why does a very steep blade angle not help?
The air flow separates from the blade and the motor slows because of high load.
4How is thrust measured here?
By the change in reading of a kitchen scale in grams.
Take it further
Use the best propeller to drive a lightweight cardboard hovercraft or a toy fan-boat.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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