Is it legal to use a prop with a cowling (ducted fan)

[Reno_Steve]

At the prop tip speeds the aircraft are reaching it would probably be more efficient if the prop was in a duct.

Is this legal under the current rules and do you think it would be of benefit ?

[JayReno]

Re: Is it legal to use a prop with a cowling (ducted fan)

At the prop tip speeds the aircraft are reaching it would probably be more efficient if the prop was in a duct.

Is this legal under the current rules and do you think it would be of benefit ?

Legal: yes. Internal combustion engine and a prop.
See link to Jim Millers Formula 1 pusher with a duct for first few years.

http://simanaitissays.com/2013/11/23/miller-texas-gem/

Benefit: Jim removed the duct after a couple years & did not go back to it suggesting he got better performance without the duct for his application.

A duct creates a large amount of internal & external wetted area, some frontal area, & weight that must be overcome before a benefit is realized. The fan blade tip clearance is critical to fan performance on Jet engines.