Roll damping

The first of the commonly encountered lateral motions that we will consider is not oscillatory in nature at all. This motion takes place when the aircraft is given a disturbance which causes it to have a rate of roll.

In order to simplify the argument we will suppose the motion to be relatively rapid and purely in roll, so that no significant sideslip or rate of turn has time to develop.

The first thing to notice is that the roll itself will produce no restoring moment because of the new position of the aircraft (Chapter 11). The motion is therefore neutrally stable as far as the static stability is concerned. There is however a rolling moment which is caused by the rate of roll, or rolling velocity. This moment arises because the downgoing wing has its angle of attack effectively increased, while the angle of attack of the upgoing wing is correspondingly reduced, as is shown in (Fig. 12.8). This causes a change in lift on the two wings which results in a moment which opposes the rolling motion.

The magnitude of the moment is dependent on the rate of roll and, as it opposes the motion, damps it out. The damping caused by this effect is strong and the ‘roll damping’ of most aircraft is high.

Rolling

Aircraft with a very low aspect ratio, such as Concorde, will, in general, have a much lower roll damping than a conventional aircraft because of the proportionally reduced span.