Reference Books 

• Multi Engine Airplane Guide. Embry‐Riddle Aeronautical University, 2009.

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• ADMINISTRATION, FEDERAL AVIATION. AIRPLANE FLYING HANDBOOK 2019: Faa-h-8083-3b. INDEPENDENTLY PUBLISHED, 2019.

• ​Flying Light Twins Safely. FAA, 2008.

Reference Media

• Propeller Aircraft Detail - Free Photos on Pixabay. pixabay.com/photos/propeller-aircraft-detail-587059/

• ​.Airplanes- Free Photos on Pixabay. pixabay.com/photos/airplane-detail-587059/

Induced Flow

The propellers of the wing-mounted engines generate an accelerated flow or accelerated slipstream of air over the wings called induced flow. The production of lift on multi engine airplanes looks like this:

Turning Tendencies

• Twin-Engine Airplanes experience the same effects that single-engine airplanes. These effects are greater in twins since they have two engines.

• Twin‐engine airplanes where the propellers for each engine rotate in the same direction are called conventional twins. To combat the torque and P‐factor tendencies counter-rotating engines were created. The effects of torque and p‐factor will cancel each other out, resulting in less rudder needed to oppose these forces..

Conventional Twin 

Counter-Rotating Twin

Engines Failures

Yaw

• The asymmetrical thrust produced by the operative engine will cause a yawing motion around the CG towards the inoperative engine.

Roll

• The yawing moment and the induced flow (accelerated slipstream), will cause the wing with the operating engine to move faster through the air. This will cause the air over that wing to accelerate and produce more lift, causing a roll towards the inoperative engine.

Ground  Effect

• The reduction in induced flow and the disruption of wingtip vortices due to ground interference causes a decrease in induced drag. As a result of the reduction in induced drag, there will be more thrust available on the engines (more power available = higher VMC) .

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• Outside of ground effect, the airplane regains induced drag, which reduces the power produced, and therefore VMC decreases.

Ground Effect

Critical Engine

• The critical engine is the engine that, if it were to fail, would most adversely affect the performance or handling characteristics of the airplane.

• On conventional twins, the critical engine is the left engine. On a counter-rotating twin airplane, there is no critical engine since the yawing and rolling effects of having an engine failure would be the same regardless of which engine failed.

1.

2.

3.

4.

P factor 

• P factor happens when the downward moving prop blade creates more thurst than the ascending blade. To figure out the effect it has on an airplane the formula THURST x ARM = MOMENT should be used. The greater the arm from the CG to the thurst vector the greater the yawing moment.

Since the descending blade on the right-wing engine has a longer arm (A2), than the descending propeller blade on the left-wing engine (A1), the airplane will have a greater yawing moment on the left if the left engine fails than if the right engine fails. This makes the left engine critical.

On a counter-rotating twin, arms (A1 and A2) to the descending blades are the same lengths. The same amount of yaw will be generated without considering which engine failed. No critical engine

Accelerated Slipstream

• The props will accelerate the air over the wings and more lift will be prodcued were more thurst is generated. 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.

• 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.

Example

Text

Large Arm = Large Roll

Small Arm = Small Roll

Torque

• " 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 the 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.

Conventional 

In the counter‐rotating twin, the torque will oppose the yawing and rolling moment caused by an inoperative engine. The resulting yaw will be the same regardless of which engine fails. There would not be a critical engine.

Spiraling Slipstream

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.

VMC

What is it?

• When a multi-engine airplane loses an engine, it experiences a yaw and a roll. The rudder is used to stop the yaw and the subsequent roll.

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• As airspeed decreases, the rudder becomes less effective. Eventually, an airspeed will be reached where full rudder will become necessary to maintain directional control. VMC is the minimum airspeed at which directional control can be maintained with the critical engine inoperative.

VMC

Certifying VMC

• In order to certify a multi-engine airplane, it needs to go through a process that includes calculating a Vmc speed. VMC is only a fixed airspeed only under certain conditions specified in FAR §23.149. VMC must not exceed 1.2 VS1 at maximum takeoff weight.

1.  Most Unfavorable Weight and Center of Gravity

2.  Standard Day Conditions at Sea Level (Max Engine Power)

3. Maximum Power on the Operating Engine (Max Yaw)

4. Critical Engine Prop Windmilling (Max Drag)

5. Flaps Takeoff Position, Landing Gear Up, Trimmed for Takeoff (Least Stability)

6. Up to 5° of Bank into the Operating Engine

Conditons for Certification

1. The rudder pedal force required to maintain control must not exceed 150 pounds.

2. It must not be necessary to reduce power of the operative engine(s).

3. The airplane must not assume any dangerous attitude.

4. It must be possible to prevent a heading change of more than 20 degrees.

Conditons for Recovery

Recognizing and recovering from VMC

Recognition

Recovery

1. Loss of directional control

2. Stall warning horn

3. Buffeting before the stall

4. A rapid decay of control effectiveness

1. Reduce power on the operating engine – this will reduce the asymmetrical thrust  causing the VMC in the first place.  

2. Pitch down: Lowering the nose of the airplane will increase the forward airspeed  making the rudder more effective in regaining and maintaining directional control.

Factors  affecting VMC

Published VMC will almost always be different than actual VMC.  Several factors affect this speed and they are listed below:

Power

When the operating engine develops maximum power, adverse yaw is increased towards the inoperative engine. The pilot must overcome this yaw to maintain directional control with the rudder. More airflow will be needed to generate sufficient rudder power, therefore as engine power increases VMC increases.

Density Altitude

As density altitude increases, temperature increases, pressure decreases, and/or humidity increases the output of the engine or thrust created by the engine decreases. This leaves more rudder availabe to the pilot, which decreases VMC.

CG Location

As the center of gravity moves forward, the 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.

Gear Position

Gear retraction shifts the CG aft, which increases VMC, when the gear is extended causes drag to generate an additional force that will oppose the adverse yaw generated by the inoperative engine, which will lower VMC. 

Propeller

A windmilling propeller creates more drag than a feathered propeller. This extra asymmetric drag adds to the asymmetric thurst produced by the failed engine. This situation will require more rudder deflection to maintain directional control, which means that VMC increases.  

Flaps

The drag generated by the flaps will create a turning force away from the yaw generated by the operating engine. This asymmetrical drag will help to reduce the asymmetrical thrust, and thus more rudder will be available to the pilot. VMC will decrease with the use of drag devices.

Weight

When an airplane is banked, an additional force is generated (the horizontal component of lift).  This additional force can be used to oppose the asymmetrical yaw produced by the inoperative engine. The greater the amount of lift produced, the higher the horizontal componen of lift. More rudder will be available to the pilot. Therefore, as weight increases, VMC decreases.

Weight with a 5° of Bank

Bank Angle

• Bank angle can be used to help the rudder oppose the asymmetrical yaw produced by the dead engine. However, certain bank angles will have a more significant effect on VMC and airplane performance.

• The greater the angle of attack of the rudder, the greater the force it will produce. Only certain bank angles will allow the rudder to respond more effectively to the yaw produced by the inoperative engine.

Rudder Effectiveness

SLIPPING VS. COORDINATED

Anytime the relative wind is not parallel to the longitudinal axis of the airplane more drag is created.

Fuselage Lift 

Just like an airfoil, the fuselage produces lift in significantly lower proportions. Fuselage lift is more noticeable when the relative wind is not flowing directly parallel to the longitudinal axis of the airplane.

Bank Angle Demonstrations

0 degress of bank 

Example

Example

2°-3° Bank toward operating engine

8° Bank toward operating engine

• The direction of the relative wind will allow the rudder to become more effective.

• This increased bank angle will allow a larger force to be produced by the horizontal component of lift, which will allow the rudder to maintain heading and opposed the yaw generated by the inoperative engine. This results in a lower VMC.

• Performance is greatly affected due to the angle of the relative wind, which will generate a large amount of drag on the airplane.

Example

Text

5° Bank toward inoperative engine

• Banking toward the inoperative engine will result in the horizontal component of lift to be added to the yaw produced by the inoperative engine.

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• The angle produced by the relative wind will make the rudder ineffective. And the amount of rudder needed to maintain the heading will increase significantly. This will result in the highest VMC, and the slip conditon will reduce performance significantly as well.

Example