Convection Propellers are not easily available from lab suppliers but are easy to make in the preproom. Teachers may ask for propellers made from different metals or of different shapes or may even ask for the raw materials so that pupils can make their own.
Propeller shapes can be cut out of thin but moderately sturdy sheets of copper, (available through lab suppliers in rolls or sheets) or other metals. Thin Zinc will break down in a strong flame so is probably best avoided. light grade tin foil is sometimes used in conjunction with a candle. Tiny propellers can be made to turn with little convection heating.
Pupils can experiment with different shapes sizes and amount of fins. Generally the lighter propellers with large flat fins work well.
The propeller needs to be attached to a metal shaft that can withstand being heated. Large hat pins are perfect for this as they have a large head which will hold the propeller in place nicely.
The free end of the pin should be clamped securely to a stand and held directly above a naked flame. Some teachers will ask for candles if they are being used by pupils although the rotating effect is better shown by using a Bunsen burner. The propeller needs to be clamped securely above the flame no closer than about 50cm until it turns.
Convection fans are commonly used in the construction of electric fires to create a moving flame effect. The small propellers sit above a lamp of element and turn creating the effect.