Why are some fan blades straight while some are curved and what is the difference?

In Technical Bulletin No. 6, SPAL UK MD Matthew Morris discusses the subtleties of the right type of blades for the job.

"The choice of straight blades to curved blades is dependant on the application and at the ‘working point’ of the fan, i.e. at what pressure it has to deliver the correct airflow.

"There is no hard and fast rule, although some manufacturers will claim that the curved blades are quieter when at the ‘working point’ because of the way they move the air."