| The second is an aileron roll done purely with the ailerons, engine and tail, not using the lift generated by the wings. Not all planes can do it because the engine needs to be very powerful to support horizontal flight, and the wings need a neutral airfoil shape to provide lift when upside down. Trying to come up with a better explanation for the first. EDIT: Try this on for size: Given - the force imparted by the wings is 1G (enough to cancel the force of gravity), and will always be pointed straight through the roof of the aircraft. The pilot takes no action to increase the amount of lift. The G's are measured from the frame of reference of the passengers in respect to the aircraft. Rotational forces are not considered - most people are unable to kinesthetically perceive any rotation which occurs at less than 5 degrees per second, and don't realize that they could be upside down and still feeling like they're right side up. It's why instrument training involves so much instruction and reminders to trust the instruments, not your body. The maneuver is initiated by rolling the plane (clockwise, with respect to the pilot) with the ailerons. Since no effort is made to change the amount of force being generated by the wings, the downward component of that thrust (as measured from an external reference point) will lessen, allowing the aircraft to start accelerating downwards while also accelerating to the right. The force felt by the passengers is still exactly 1G - the force created by the wings, and it is still pointed vertically through the plane. Since the downward component affects the passengers and plane equally, there is no measurable effect on the G forces with respect to the passenger's frame of reference. The aircraft reaches 90 degrees, and is accelerating to the right at 1G, and downwards at 1G. The downward force is not felt by the passengers, again because their entire frame of reference is accelerating at the same speed, only the force pushing them into their seats. The aircraft reaches 180 degrees, and is now accelerating at 2Gs downwards. Passengers are still feeling only 1G of pressure from the seat. 270 degrees - the acceleration to the left cancels out the previous acceleration to the right, passengers are still being just pushed into their seats. 360 degrees - the maneuver is complete. The plane is significantly lower and some distance to the "right" of its original position. Their velocity now includes a significant downwards component. Now here's where things perhaps become a matter of semantics - pilots would consider the barrel roll to be completed at this point, and they simply need to recover from their new orientation. Of course, this will probably require little to no input from the pilot, it will simply happen naturally due to the changed angle of attack induced by the downward motion increasing the lift generated by the wing. |