The duct is 1M in diameter, quite large for an F3 but I figured it would be about right for this craft. The duct was made by my old friend Dan who was doing it as a plug for a duct he was building and it was no longer required.
I collected it on a filthy night, strapped to the roof of the car as you can see. You can make out the Kevlar containment area that should hold everything together if a blade lets go and tries to leave through the side.
There is also a flat “plate” running all round the duct. This is to help to keep it circular, particularly where the blades run. It’s quite hard to stop a duct from flexing and as long as the part with the blades in stays circular, the rest can move around a little.