In aviation, a star-shaped cylinder engine is an internal combustion engine where the cylinders are placed next to each other around an axis. This type of engine can be fixed or rotary. In the latter case, the cylinders (photo) and the crankcase rotate around a crankshaft attached to the frame. This is the type of engine that was most common in the early aeroplanes (Gnome and Le Rhône engines). A single star engine must have an odd number of cylinders (3 to 9), for reasons of balancing and cyclic regularity. In 1917, 80% of aircraft were equipped with such an engine.