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What are the aerodynamic effects of an engine failure?

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Asymmetric thrust and drag cause the following effects on the aircraft's axes of rotation:

Pitch Down (Lateral Axis)

Loss of accelerated slipstream over the horizontal stablilzer causes it to produce less negative lift, causing the aircraft to pitch down. To compensate for the pitch down effect, additional back pressure is required.

Roll Toward the Failed Engine (Longitudinal Axis)

The wing produces less lift on the side of the failed engine due to the loss of accelerated slipstream. Reduced lift causes a roll toward the failed engine and requires additional aileron deflection into the operating engine.

Yaw Toward the Dead Engine (Vertical Axis)

Loss of thrust and increased drag from the windmilling propeller cause the aircraft to yaw toward the failed engine. This requires additional rudder pressure on the side of the operating engine. "Dead foot, dead engine."

1

Explain engine inoperative climb performance.

Climb performance depends on the excess power needed to overcome drag. When a multi-engine airplane loses an engine, the airplane loses 50% of its available power. This power loss results in a loss of approximately 80% of the aircraft's excess power and climb performance. Drag is a major factor relative to the amount of excess power available. An increase in drag (such as the loss of one engine) must be offset by additional power. This additional power is now taken from the excess power, making it unavailable to aid the aircraft in climb. When an engine is lost, maximize thrust (full power) and minimize drag (flaps and gear up, prop feathered, ect.) in order to achieve optimum single-engine climb performance.

2

Explain the Sideslip Condition and the Zero Sideslip Condition.

Sideslip Condition (Undesirable)

When an engine failure occurs, thrust from the operating engine yaws the aircraft. To maintain aircraft heading with the wings level, rudder must be applied toward the operating engine. This rudder force results in the sideslip condition by moving the nose of the aircraft in a direction resulting in the misalignment of the fuselage and the relative wind. This condition usually allows the pilot to maintain aircraft heading; however, it produces a high drag condition that significantly reduces aircraft performance.

Zero Sideslip Condition (Best Performance)

The solution to maintaining aircraft heading and reducing drag to improve performance is the Zero Sideslip Condition. When the aircraft is banked into the operating engine (usually 2 to 5 degrees), the bank angle creates a horizontal component of life. The horizontal lift component aids in counteracting the turning moment of the operating engine, minimizing the rudder deflection required to align the longitudinal axis of the aircraft to the relative wind. In addition to banking into the operative engine, the appropriate amount of rudder required is indicated by the inclinometer ball being "split" toward the operating engine side. The Zero Sideslip Condition aligns the fuselage with the relative wind to minimize drag and must be flown for optimum aircraft performance.

3

What is the single-engine service ceiling?

The single-engine service ceiling is the maximum density altitude at which the single-engine best rate of climb airspeed (Vyse) will produce a 50 FPM rate of climb with the critical engine inoperative.

4

What is the single-engine absolute ceiling?

The single-engine absolute ceiling is the maximum density altitude that an aircraft can attain or maintain with the critical engine inoperative. Vyse and Vxse are equal at this altitude. The aircraft drifts down to this altitude when an engine fails.

5

What four factors does climb performance depend on?

  • Airspeed: Too little or too much will decrease climb performance.
  • Drag: Gear, Flaps, Cowl Flaps, Flight Control Deflection, Prop and Sideslip.
  • Power: Amount available in excess of that needed for level flight. (Engines may require leaning due to altitude for max engine performance.)
  • Weight: Passengers, baggage, and fuel load greatly affect climb performance.

6

What is the definition of the critical engine?

The critical engine is the engine that, when it fails, most adversely affects the performance and handling qualities of the airplane.

7

Explain how four factors determine which engine is critical.

The clockwise rotation of the props contributes to the following factors that cause the left engine to be critical:

P — P-Factor

A — Accelerated Slipstream

S — Spiraling Slipstream

T — Torque 

 

P-Factor (Yaw)

Both propellers turn clockwise as viewed from the cockpit. At low airspeeds and high angles of attack, the descending blade produces more thrust than the ascending blade due to its increased angle of attack. Though both propellers produce the same overall thrust, the descending blade on the right engine has a longer arm from the CG (or grater leverage) than the descending blade on the left engine. The left engine produces the thrust closest to center line. The yaw produced by the loss of the left engine will be greater than the yaw produced by the loss of the right engine, making the left engine critical.

Accelerated Slipstream (Roll and Pitch)

P-Factor causes more thrust to be produced on the right side of the propeller. This yields a center of lift that is closer to the aircraft's longitudinal axis on the left engine and further from the longitudinal axis on the right engine and also results in less negative lift on the tail. Because of this, the roll produced by the loss of the left engine will be greater than the roll produced by the loss of the right engine, making the left engine critical.

Spiraling Slipstream (Yaw)

A spiraling slipstream from the left engine hits the vertical stabilizer from the left, helping to counteract the yaw produced by the loss of the right engine. However, with a left engine failure, slipstream from the right engine does not counteract the yaw toward the dead engine because it spirals away from the tail, making the left engine critical.

Torque (Roll)

Newton's Third Law of Motion is that for every action, there is an equal and opposite reaction. Since the propellers rotate clockwise, the aircraft will tend to roll counterclockwise. When the right engine is lost, the aircraft will roll to the right. The right rolling tendency, however, is reduced by tthe torque created by the left engine. When the left engine is lost, the aircraft will roll to the left, and the torque produced by the right engine will add to the left rolling tendency requiring more aileron input, which increases drag, making the left engine critical. In summary, on most light multi-engine aircraft when the critical engine is inoperative, both directional control and performance suffer more than when the non-critical engine is inoperative.

8

What is Vmc?

Vmc is the minimum airspeed at which directional control can be maintained with the critical engine inoperative. Vmc speed is marked on the airspeed indicator by a red radial line. Aircraft manufacturers determine Vmc speed based on conditions set by the FAA under FAR 23.149.

9

Under what conditions does the FAA require aircraft manufactures to use when determining Vmc?

S – Standard Day Conditions at Seal Level (Max Engine Power)

Standard conditions yield high air density that allows the engine to develop maximum power. An increase in altitude or temperature (a decrease in air density) will result in reduced engine performance and prop efficiency. This decreases the adverse yaw effect. Vmc speed decreases as altitude increases.

M – Maximum Power on the Operating Engine (Max Yaw)

When the operating engine develops maximum power, adverse yaw is increased toward the inoperative engine. The pilot must overcome this yaw to maintain directional control. Any condition that increases power on the operating engine will increase Vmc speed. Any condition that decreases power on the operating engine (such as power reduction by the pilot, an increase in altitude, temperature, low density, or aging engine) will decrease Vmc.

A – Aft CG 

A more aft CG shortens the moment arm from the rudder to the CG; thus, rudder imputs become less effective.

C – Critical Engine Prop Windmilling (Max Drag)

When the propeller is in a low pitch position (unfeathered), it presents a large area of resistance to the relative wind. This resistance causes the engine to "windwill." The windmilling creates a large amount of drag and results in a yawing moment into the dead engine. When the propeller is "feathered," the blades are in a high pitch position, which aligns them with the relative wind, minimizing drag. A feathered prop will decrease drag and lower Vmc.

F – Flaps Takeoff Position, Landing Gear Up, Trimmed for Takeoff (Least Stability)

When the gear is extended, the gear and gear doors have a keel effect, reducing the yawing tendency and decreasing the Vmc speed. Extended flaps have a stabilizing effect that may reduce Vmc speed.

U – Up to 5 degree of Bank into the Operating Engine When the wings and level, only the rudder is used to stop the yaw produced by the operating engine (sideslip condition). Banking into the operating engine creates a horizontal component of lift which aids the rudder force. With this horizontal component of lift and full rudder deflection, Vmc is at the lowest speed. Vmc increases with decreasing bank by a factor of approximately 3 knots per degree of bank angle.

M – Most Unfavorable Weight and Center of Gravity The certification test allows up to 5 degree bank into the operating engine. In a given bank, the heavier the aircraft, the greater the horizontal component of lift that adds to the rudder force. As weight increases, the horizontal component of lift increases, which added to the rudder, decreases Vmc. As the center of gravity moves forward, the moment arm between the rudder and the CG is lengthened, increasing the leverage of the rudder. This increased leverage increases the rudder's effectiveness and results in a lower Vmc speed.