Direct-Thrust Moments in Yaw

For a multiengine airplane whose engines are mounted on the wings, when all engines are running and developing about the same power, there is no unbalanced yawing moment due to power. Failure of a wing-mounted engine of course sets up a thrust-caused yawing unbalance that must be counteracted by an equal and opposite aerodynamic yawing moment. The more engines on a multiengine airplane, the less effect will the failure of a single one have on yawing moments. Flight crews of Boeing B-29 and B-50 four-engine airplanes had the strange experience of losing engines during normal cruise flight and being unaware of it for many seconds. RPM for the dead engine would drop very little at first because of propeller windmilling. The directional stability of both airplanes was high enough to keep the ships close to course, initially.

Current Boeing Company design practice requires that a twin-engine jet transport be able to continue a climbout after takeoff, rudder-free, with one engine failed. This accounts for the generously sized vertical tails on the 737, 757, 767, and 777 models.