12.multiengine-aerodynamics. Multiengine Aerodynamics
Multiengine airplanes introduce aerodynamic considerations that single-engine pilots never encounter. The most critical of these arise when one engine fails and the remaining engine continues to produce asymmetric thrust. Understanding the forces, moments, and control responses involved is fundamental to safe multiengine operation.
Asymmetric Thrust and the Yawing Moment
When an engine fails on a conventional twin, thrust from the operating engine acts at a moment arm from the airplane's center of gravity, creating a powerful yawing moment toward the dead engine. Simultaneously, drag from the failed engine (windmilling propeller, cooling drag) creates a yawing moment in the same direction. The combined yaw produces sideslip, which the vertical stabilizer and rudder must counter. Maximum rudder deflection plus a small bank angle into the operating engine is required to maintain directional control.
Critical Engine
On most U.S.-built twins with both propellers rotating clockwise as viewed from the cockpit, the critical engine is the left engine. The critical engine is defined as the engine whose failure most adversely affects the performance and handling of the airplane. Four factors — remembered as P-A-S-T — explain why:
- P – P-factor: The descending blade of each propeller produces more thrust than the ascending blade. On the right engine, the descending blade is on the outboard (right) side, placing the thrust line farther from centerline. On the left engine, the descending blade is inboard, with a shorter moment arm. Loss of the left engine therefore leaves the right engine producing thrust on a longer arm, resulting in a larger yawing moment.
- A – Accelerated slipstream: The asymmetric, faster slipstream from the descending blade of the right engine strikes more of the wing aft of it, producing more lift on the right wing. Loss of the left engine removes lift asymmetry working in the pilot's favor.
- S – Spiraling slipstream: The slipstream from the left engine spirals to strike the left side of the vertical fin, helping counter yaw. With the left engine inoperative, this beneficial yaw-correcting force disappears.
- T – Torque: Engine torque rolls the airplane left. With the left engine failed and the right engine operating, torque adds to the left-rolling tendency already caused by yaw.
On airplanes with counter-rotating propellers (e.g., many Piper Senecas), no critical engine exists because P-factor effects are symmetrical.
V_MC — Minimum Control Speed
V_MC is the minimum airspeed at which directional control can be maintained with the critical engine inoperative, the operating engine at takeoff power, and the airplane in a specified configuration. It is marked by a red radial line on the airspeed indicator. The certification conditions in 14 CFR Part 23 include:
- Critical engine windmilling (or feathered, depending on certification basis)
- Maximum takeoff power on the operating engine
- Most unfavorable weight (often max gross or aft CG)
- Landing gear retracted, flaps in takeoff position
- Up to 5° bank into the operating engine
- Not more than 150 lb of rudder force
Below V_MC, the rudder cannot generate enough yawing moment to overcome the asymmetric thrust, and the airplane will yaw and roll uncontrollably toward the dead engine. V_MC is not constant — it decreases with altitude (lower power available), with a feathered propeller (less drag), and with proper bank into the good engine.
The Zero-Sideslip Bank Angle
After an engine failure, holding wings level with the ball centered actually produces a sideslip toward the dead engine and significantly degrades climb performance. The optimum technique is to:
- Apply rudder toward the operating engine until the slip-skid ball is displaced approximately one-half ball width toward the operating engine
- Bank approximately 2° to 3° toward the operating engine
This zero-sideslip condition minimizes drag and may improve single-engine climb performance by 100–200 fpm or more compared to wings-level flight.
Single-Engine Climb Performance
Loss of one engine on a twin removes 50% of available power but typically reduces climb performance by 80–90%. Excess power, not excess thrust, determines rate of climb. With one engine out, the operating engine is dragging the dead engine's drag plus the airframe's full drag, so very little excess power remains.
V_YSE and V_XSE
- V_YSE (blue line): Best single-engine rate of climb — gives the most altitude gain per unit time.
- V_XSE: Best single-engine angle of climb — gives the most altitude gain per unit distance, used only to clear obstacles.
V_SSE, the safe single-engine speed, is the minimum airspeed recommended for intentionally rendering an engine inoperative during training. It provides a margin above V_MC.
Drag of a Windmilling Propeller
A windmilling propeller produces drag roughly equivalent to a flat disc of the same diameter. Feathering the propeller — turning the blades parallel to the relative wind — dramatically reduces this drag, often improving single-engine climb by several hundred feet per minute. Prompt identification, verification, and feathering of the failed engine is therefore essential.