Multi Engine Aerodynamics & Vmc Explained

Multi-engine training earns its reputation the moment one engine quits. That’s when the aerodynamics change completely, and pilots must understand that multi-engine aerodynamics are distinctly different from single-engine, as pilots must understand the unique relationship between uneven thrust and control. A very small margin exists, requiring pilots to master the engine-out procedures to maintain control at all times.

Multi-engine flying is a high-stakes endeavor. The second engine ultimately works to improve the performance of the aircraft. But in the event of engine failure, twin-engine aerodynamics quickly shift.

OEI, or one engine inoperative flight training uniquely prepares pilots to navigate engine failure and become familiar with the aerodynamic changes that occur, enabling them to maintain composure and control during an emergency in the air.

Why Two Engines Change Everything

Asymmetrical thrust is the biggest aerodynamic challenge of flying a multi-engine aircraft. Commonly referred to as the P-factor, asymmetrical thrust occurs when one engine on a twin loses power. In the case of an engine failure, the operating engine will immediately yaw toward the failed engine, giving pilots a split-second moment to maintain control of the aircraft.

To effectively do this, multi-engine pilots must have an extensive understanding of the balanced and unbalanced aerodynamic forces at play. At the exact moment the engine fails, the yaw and roll toward the failed engine will begin. Recognizing this, pilots must immediately begin engine-out procedures to maintain directional control.

The zero sideslip technique is commonly used to reduce drag and increase performance in an engine failure. By combining yaw from the rudder with the horizontal component of lift that results from bank with the ailerons, pilots can achieve zero sideslip along with steady climb performance.

As the Federal Aviation Administration (FAA) notes, when a multi-engine aircraft experiences 50% power loss, it immediately loses about 80% of climb performance. Failure-based multi-engine training is critical to preparing pilots for engine-out scenarios in which every moment matters.

The Critical Engine

The critical engine is considered the engine that would have the most detrimental impact on the performance and directional control of the aircraft in the event of an engine failure. On conventional twin-engine aircraft, both propellers turn clockwise, making the left engine the critical engine.

You can remember the left engine as the critical engine using the acronym PAST:

P-factor
Accelerated slipstream
Spiraling slipstream
Torque

P-factor, or asymmetrical thrust, occurs at the time of engine failure. When the engine loses power, the blade on the right side of the aircraft produces more thrust, causing the plane to yaw and roll toward the failed engine. The severity of the P-factor will depend on the moment arm, or the distance between the center of gravity and the rudder, which balances the aerodynamic force.

During aviation training, a V_MC demonstration may be used to help pilots better understand how an aircraft behaves amid an engine failure. A V_MC demonstration is a mandatory component of your multi-engine checkride. Keep in mind that counter-rotating engine trainers do not have a critical engine, but there is still a V_MC demonstration included as part of the multi-engine checkride.

Vmc Explained: The Red Line You Respect

On most airspeed indicators, V_MC is represented with the red line, and it’s vital to pay attention to it. V_MC refers to the minimum control speed with the critical engine out and the other at takeoff power. It is the calibrated airspeed at which it is possible to maintain directional control of the aircraft after a sudden loss of thrust.

You must take notice of the V_MC because, in the event an engine loses power, dipping below the V_MC impedes the ability to counteract the yawing and rolling of the plane toward the inoperative engine. V_MC addresses directional control only, not the climb of the aircraft.

Factors that can influence V_MC include:

Power
Weight
Bank angle
Density altitude
Configuration

During your V_MC demo, remember that the published V_MC is the worst-case scenario. Moving your center of gravity forward will decrease actual V_MC and lend you greater ability to maintain directional control.

One nuance to bear in mind is that the density altitude for normally aspirated engines may impact the V_MC, and a stall-yaw danger exists where V_MC meets stall speed.

Single-Engine Performance and the Blue Line

The performance and control of a multi-engine aircraft depends on both the minimum control speed and the best rate of climb speed.

In the event of engine failure, single-engine performance relies primarily on the single-engine best rate of climb speed (VYse), indicated by the blue line on the airspeed indicator. It’s important to pay attention to it because it indicates the optimum performance of the aircraft. On a warm day, the VYse may be limited or even negative.

During your multi engine training, you will learn drag reduction techniques to improve operational performance in the case of single-engine failure. To reduce drag, retract the landing gear and the flaps, then begin feathering propeller techniques to eliminate the windmilling propeller effect.

Drag and Feathering

When an engine fails, the propellers will continue to rotate due to airflow, but this effect (known as windmilling propellers) creates substantial drag. Essentially, windmilling propellers act as a speed brake, which is not what you want when you have just lost power in an engine.

To counteract the windmilling effect, you should rely on feathering propeller techniques. Through feathering, you can align blades edge-on to cut drag, but you need to direct feathering toward the correct engine.

Throughout your multi-engine training, you will learn to rely on the “identify, verify, feather, secure” mantra, which serves as a valuable guide in the event of an engine failure:

Always identify the problematic engine first.
Pull the throttle to verify that you have properly identified the dead engine.
Start feathering to achieve stability once again.
Finally, secure the aircraft and attempt to land safely.

It may be an emergency scenario, but using the slow-and-deliberate principle is crucial. Be focused and intentional, and act with precision to maintain control. This is why an organized cockpit and access to checklists are essential.

Stability and Directional Control

Rudder authority is the limiting aerodynamic factor that will decay as airspeed decreases, and as airspeed continues to drop, the rudder will no longer be able to counteract the yawing toward the dead engine. To maintain directional control in an engine-out scenario, pilots rely on a two-degree bank toward the operational engine. This creates a zero-sideslip condition that counteracts the asymmetrical thrust.

Elevate Your Flight Training With MzeroA’s Commercial Pilot Course

Twin-engine flying involves an understanding of multi-engine aerodynamics and engine-out procedures; the two are essentially one interconnected system that activates when an engine quits. A solid foundation of advanced aerodynamics, performance, and emergency procedures lets pilots move confidently into high-performance, high-stakes flying.

The MzeroA Commercial Pilot Course features in-depth lessons and scenario-based exercises to prepare pilots in their advanced training. Providing training based on emergency scenarios, this course develops a pilot’s understanding of complex aerodynamics, aircraft performance, and maneuvering techniques.

Learn the principles now, and fly with confidence toward the next level of your aviation career. Enroll in MzeroA’s Commercial Pilot Course today.