Anyone Ever Use A "Project Manager" on a Renovation?

My husband and I have had lots of experience over the years renovating the rental income apartments we’ve owned in Detroit and Asbury Park, NJ, as well as doing a combination of two small apartments to create our former home on West 72nd Street, so we’re very familiar with the process of renovating. But now that we own a 3-story town house in Williamsburg and are planning to add a 4th floor and create a ground floor rental unit and 3-floor owners triplex, this will easily be the biggest and most expensive project we’ve ever taken on, and frankly, it’s rather daunting. An architect friend suggested that one possible solution to consider is hiring an experienced project manager to work with us a few hours a week to oversee the general contractor, the schedule, the billing and do regular site visits, since his/her experience in this type of project would be much greater than ours, and would help expedite and keep costs under control. I’ve never heard of such a position and am intrigued by the idea. Have any of you ever had someone like this on your renovations? If so, what are their typical duties, what do they cost when working part time for you, and has it been beneficial to your schedule and budget? Any advice would be welcome. Thanks! Brad In Brooklyn