The J-3 is distinguished from its successors by having a cowl that exposes its engine's cylinder heads — the exposed cylinders of any J-3's engine were usually fitted with sheet metal "eyebrow" air scoops to direct air over the cylinder's fins for more effective engine cooling in flight. Very few other examples exist of "flat" aircraft engine installations (as opposed to radial engines) in which the cylinder heads are exposed. From the PA-11 on through the present Super Cub models, the cowling surrounds the cylinder heads.