The Aeroplane Speaks. Fifth Edition

In order to secure all the above described advantages, a combination is sometimes effected, thus: BANKING.--An aeroplane turned off its course to right or left does not at once proceed along its new course. Its momentum in the direction of its first course causes it to travel along a line the resultant of such momentum and the thrust. In other words, it more or less skids sideways and away from the centre of the turn. Its lifting surfaces do not then meet the air in their correct attitude, and the lift may fall to such an extent as to become less than the weight, in which case the aeroplane must fall. This bad effect is minimized by "banking," i.e., tilting the aeroplane sideways. The bottom of the lifting surface is in that way opposed to the air through which it is moving in the direction of the momentum and receives an opposite air pressure. The rarefied area over the top of the surface is rendered still more rare, and this, of course, assists the air pressure in opposing the momentum. The velocity of the "skid," or sideways movement, is then only such as is necessary to secure an air pressure equal and opposite to the centrifugal force of the turn. The sharper the turn, the greater the effect of the centrifugal force, and therefore the steeper should be the "bank." Experientia docet. The position of the centre of gravity affects banking. A low C.G. will tend to swing outward from the centre of the turn, and will cause the aeroplane to bank--perhaps too much, in which case the pilot must remedy matters by operating the ailerons.