Asymmetric flight and multi-engine aerodynamics
Overview
Asymmetric flight and multi-engine aerodynamics tests the critical engine, V_MC, asymmetric thrust and the handling of an engine failure.
Core rules
- The critical engine is the one whose failure most adversely affects performance and control. On a conventional US twin, the left engine is critical because of P-factor, spiralling slipstream, accelerated slipstream and torque.
- V_MC is the minimum control speed in the air: the speed below which full rudder cannot counter asymmetric yaw. V_MCG is the ground equivalent.
- Asymmetric thrust yaws the aircraft toward the failed engine; rudder opposes it. Drag from a windmilling propeller adds to the problem - feather it.
- V_MC increases with power and aft CG, and falls with altitude as available thrust falls.
- Counter-rotating propellers remove the critical-engine asymmetry.
Worked example
An engine fails at 90 kt with V_MC = 85 kt: full rudder can hold heading, but the aircraft is close to the limit. If the failed engine is feathered and power on the live engine reduced, V_MC falls and control margin improves.
Traps
- V_MC is not a performance speed; it is a control speed, marked as a red radial line.
- Below V_MC, reducing power is the priority, not pulling harder.
- Best single-engine climb speed (V_YSE) is not V_MC.