So we've discussed the aerodynamic factors of a conventional twin and which engine would be the critical engine. However, not all multi-engine aircrafts are conventional. Aircrafts such as the Seminoles are non-conventional twins. The engines are counter rotating, meaning they rotate into each other. The left engine rotates clockwise, while the right engine rotates counter clockwise. The same aerodynamic factors apply to these aircrafts, but they have different effects, so lets take a look at each one.
After seeing how these aerodynamic forces act on a counter rotating multi-engine aircraft, we can see that there is no critical engine. It doesn't matter which engine we lose, the effects will be the same whether the left or right engine is inoperative.
We've already talked about the definition of Vmc. We know it is the minimum controllable airspeed at which directional control can be maintained with the critical engine inoperative. Just like other V speeds, we know that the actual speed will vary with different factors. So how do aircraft manufacturers come up with this speed and what are the factors that change it?The acronym I like to use to remember this is SMACFUM.First lets talk about the factors used by manufacturers to determine the aircrafts published Vmc speed, then we will break them down and talk about how each of those factors affects the actual Vmc speed.
Standard Day at Sea Level
Most Unfavorable Weight
Aft CG
Critical Engine Windmilling
Flaps Takeoff Position/Landing Gear Up
Up to 5 Degrees Bank
Max Power in Operating Engine
Standard day at Sea Level
As we already know from previous knowledge of density altitude and single engine aircrafts, engine performance decreases as density altitude increases. When the operating engine has more performance, it will have more asymmetrical thrust into the inoperative engine. This causes you to need more rudder input to counteract the yaw from that asymmetric thrust. Resulting in an increase in airflow (airspeed) over the needed to maintain directional control. As density altitude increases, performance will decrease and Vmc will decrease. Making a standard day at seal level be your worst case scenario where Vmc will be the highest.
Most unfavorable weight
Unfavorable weight is light. The heavier the airplane, the lower the aircraft’s Vmc. The lighter the airplane, the higher the aircraft’s Vmc.
Aft CG
When you have an aft CG,the arm between the CG and the rudder is shorter, which means the rudder will be less effective. A higher airspeed would be required to counteract the yaw into the inoperative engine. So with an aft CG,the Vmc speed increases and a forward CG would decrease Vmc.
Critical engine wind milling
When the critical engine is windmilling, it will create more drag than if it was feathered. This will raise your Vmc speed. In turn, once you feather that propeller, you decrease the Vmc.
Flaps takeoff position/Landing gear up
We will break this down further into the two sections of flaps and gear. When flaps are out, they help stabilize the aircraft, which helps reduce Vmc. On the reverse side of that, we have less stabilization when flaps are up (in takeoff position), which increases our Vmc. Looking at the landing gear now, by lowering the gear we creating the “keel effect” which helps keep the aircraft straight. The accelerated slipstream behind the engine at full power encounters the gear and creates excess drag and that drag helps to counter the turning tendency. So both flaps up and gear up will negatively affect your Vmc speed.
Up to 5degrees Bank
When one engine is failed and you have wings level with the ball centered, you will actually be in a mild side-slip because the failed engine is causing drag and a loss of lift. By turning to up to a 5 degree bank toward the operating engine, you will decrease the drag causing a better climb performance and improve performance, as well as decrease Vmc.
Max power
This goes along with the same idea we discuss when we talked about standard day at sea level. The more power you have in your operating engine, will give you more performance in your operating engine causing more yaw into the inoperative engine. This will again, as explained before, require more rudder input to counteract that yaw. We will need to have more airflow (airspeed)to have more effectiveness of the rudder. We can now say that with more power, we will both increase our engine performance and increase our Vmc speed. Making max power our worst case scenario and Vmc will be at its highest.
How to do the Vmc Demo in the Piper Seminole
The objective of the Vmc Demo is to teach the student how to recognize and recover from a loss of directional control.You know you have lost directional control when you have full rudder deflection into the operating engine and the aircraft begins to yaw toward the inoperative engine.
Clearing turns
Flaps-up, gear-up
Slowly close left throttle while maintaining heading and altitude.
Mixtures–Enrichen, Props–Fwd, Fuel Pumps–On
Slow to 100 KIAS (approx. 10 KIAS above VYSE)
Slowly increase right throttle (operating engine) to full power. Use rudder to maintain directional control and bank up to 5 ̊ towards the operating engine.
Increase pitch attitude slowly, decrease airspeed at approximately 1 knot per second until full rudder is applied to maintain directional control.
Recover at 1st sign of:
Loss of directional control.
First indication of stall (stall horn or buffet)
Recover promptly by simultaneously reducing power sufficiently on the operating engine while decreasing the angle of attack as necessary to regain directional control within 20 ̊ of entry heading.
Continue recovery by increasing power slowly on operating engine while maintaining an AOA that allows for airspeed to increase to a point where directional control can be maintained with a minimum loss of altitude
Accelerate to 82-88 KIAS
Bring throttles slowly together to 20" MP
“Cruise Checklist.”
