Why are most props unducted?
 

Hi guys.

I've been wondering why most props are unducted? Having a duct around the prop will increase safety of bystanders, decrease noise, and increase efficiency by reducing blade tip losses.

Thanks

At usual propeller speeds, with a usual number of blades (5 or less), the airflow efficiency saving of ducting is less than the extra weight, drag and complexity of adding the duct.

As the number of blades and tip speeds increase the advantage of ducting is clearer. However even here the advantages of removing the duct and instead using clever blade design have been thought about (google "NASA/GE Unducted Fan").

how many bystanders are there in the air?

Wonder what became of the unducted fan (UDF)? I'm Sure I remember experiments involving such engines being fitted to a B717 a number of years ago.

It was fitted on one side of a 727 and an MD-80. It was too noisy and then the price of fuel dropped so the effiency gains were no longer worth the cost and effort in trying to solve the noise problem.

MD80 UDF was demo'd at Farnborough in 1988.

I'm not at all sure that a ducted propellor would increase efficiency as drag losses in and around the duct would be much larger than the relatively small improvements at the blade tips. Not to mention the extra weight of the duct and supporting structure.
If they were more efficient we'd see them, as it is we see virtually none and for good reasons.

I'm not sure that bystander safety is relevant. What would bystanders be doing close to running aircraft?

Ducted props are pretty pointless in 99.9% of applications.