Engine thrust line

Many older aircraft had the thrust line passing close to the centre of gravity position, so that it had little effect on the trim or stability, but on jet airliners with pylon-mounted engines, the thrust line is well below the centre of gravity and the line of action of the drag. The trim of such aircraft is, therefore, quite sensitive to thrust changes. On full thrust, the nose-up tendency must be cor­rected by down-elevator, so the effective longitudinal dihedral is reduced. This in turn means that the static stability is reduced. The considerable advantages obtained by mounting the engines in this way, however, outweigh the dis­advantages arising from stability considerations. Mounting the engines above the centre of gravity is less common, but a notable example is the A-10 Thunderbolt shown in Fig. 11.7.

In the event of an engine failure on multi-engined aircraft, the yawing torque produced can have a strong lateral destabilising effect, which is one reason why tail or fuselage mounting of the engines is sometimes preferred to wing­mounting. The SR-71 Blackbird (Fig. 6.40) was susceptible to intermittent

engine failures in supersonic flight, due to instabilities of the intake shock waves. The resulting yawing moments are known to have been violent enough to cause crew members to smash their helmet visors by impact with the canopy.