In A Nutshell
In recent years the phrase “do a barrel roll” has secured its place as a part of our pop-culture lexicon. Usually, the phrase is used to mistakenly refer to an aileron roll. A barrel roll is an aerial maneuver wherein an airplane completes a longitudinal revolution (what we usually think of as a barrel roll) while also following a helical path (think of a plane going around the outside of a barrel, demonstrated in the above photo). An aileron roll, on the other hand, is achieved when a plane completes this revolution without changing altitude (think of a plane flying through a barrel while rolling, as seen in the straight-line path above).
The Whole Bushel
Although the term “barrel roll” has been used since at least 1917, it became known to many more people through its inclusion in the Star Fox video game series. In Star Fox 64, one of the titular character’s allies can be heard delivering the line that would later become a meme: “Do a barrel roll!” In the game, this refers to a maneuver that allows Star Fox to evade attacks by spinning his jet along its longitudinal axis.
This phrase began making its rounds on the Internet in late 2011 and the Google search engine even performs a trick when you use it to search for the phrase. The problem is that none of these examples are actually barrel rolls. They are aileron rolls.
The two maneuvers are similar in that they both require an aircraft to complete a 360° revolution horizontally. The difference between the two lies in vertical movement. In a properly executed aileron roll no change in altitude will take place and the aircraft will exit the maneuver on the same heading as it entered. A barrel roll requires the aircraft to follow a helical path while completing its sideways revolution, and it is sometimes described as a combination of a loop and a roll.
The maneuvers seen in the Star Fox series are all, despite the dialogue, aileron rolls. This is because the jets stay in a relatively fixed vertical position while spinning sideways. Even the Google web browser’s “do a barrel roll” Easter egg does not follow the correct helical path of a barrel roll. Though it has become popular visually, it seems unlikely that the aileron roll will ever become as popular in name as the often misattributed title of the barrel roll.