What Are Those Spinning Things on Top of UPS Trucks?

For millions of people, the sight of a UPS truck brings as much excitement as an ice cream truck. Drivers in their basic brown attire are usually carrying something you need (or want) from Amazon or other mail-order services.

But the trucks have some interesting quirks. For one, drivers don’t like to turn left in them. (The company discourages left turns because it goes against the flow of traffic, wasting time and fuel.) Many of them also frequently sport a peculiar spinning device on the roof.

That strange contraption is called a Flettner ventilator, and its purpose is simple: It’s meant to keep air circulating inside a vehicle. UPS trucks—which the company refers to as package cars—can’t really operate air conditioning efficiently because the doors are left open so drivers can quickly move in and out.

Having the Flettner ventilator installed to circulate air keeps the cabin and package area from becoming too hot, especially during the summer months. The ventilators are wind-powered, so when the vehicle is in motion—or on a particularly windy day—it'll spin and move air. Flettner claims the fan can reduce the interior temperature of the truck by as much as 10 to 15 degrees. Flettner ventilators aren't exclusive to UPS trucks; you can often spot them on Amazon delivery vehicles and other cargo trucks.

If you dream of one day owning a UPS package car that’s been retired, Flettner ventilator and all, you’re probably out of luck. The company doesn’t allow for the resale of vehicles in their fleet. Old models are either retired to lighter work in internal service or scrapped.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.