One-Engine Inoperative Drag
Mandatory requirements by certifying agencies (e. g., FAA and CAA) specify that multiengine commercial aircraft must be able to climb at a minimum specified gradient with one engine inoperative at “dirty” configuration. This immediately safeguards an aircraft in the rare event of an engine failure; and, in certain cases, after liftoff. Certifying agencies require backup for mission-critical failures to provide safety regardless of the probability of an event occurring.
Asymmetric drag produced by the loss of an engine would make an aircraft yaw, requiring a rudder to fly straight by compensating for the yawing moment caused by the inoperative engine. Both the failed engine and rudder deflection substantially increase drag, expressed by ACD_engine out+ruddert. Typical values for coursework are in Table 9.7.
9.12 Propeller-Driven Aircraft Drag
Drag estimation of propeller-driven aircraft involves additional considerations. The slipstream of a tractor propeller blows over the nacelle, which blocks the resisting flow. Also, the faster flowing slipstream causes a higher level of skin friction over the downstream bodies. This is accounted for as a loss of thrust, thereby keeping
Table 9.7. One-engine inoperative drag
the drag polar unchanged. The following two factors arrest the propeller effects with piston engines (see Chapter 10 for calculating propeller thrust):
1. Blockage factor, fb, for tractor-type propeller: 0.96 to 0.98 applied to thrust (for the pusher type, there is no blockage; therefore, this factor is not required – i. e., fb = 1.0)
2. A factor, fh, as an additional profile drag of a nacelle: 0.96 to 0.98 applied to thrust (this is the slipstream effect applicable to both types of propellers)
Turboprop nacelles have a slightly higher value of fb than piston-engine types because of a more streamlined shape. However, the slipstream from a turboprop is higher and therefore has a lower value of fh.