What is meant by the “minimum control speed” of a multi-engined aircraft?
Let’s consider a twin-engined aircraft that has wing-mounted power-plants: one on each wing. It’s easy to see that if the thrust output of an engine on the one side of the aircraft is different to the thrust output of the engine on the other side, a yawing moment around the aircraft’s vertical axis will be created.
The aircraft will tend to yaw in the direction of the engine that is producing the lesser thrust, either because it’s malfunctioning, producing only partial thrust or has failed.
This yaw is stopped by applying rudder on the side of the engine with greater thrust. By so doing, the aircraft is able to maintain its direction.
The greater the thrust differential between the two engines, the greater the yawing moment will be and the greater the amount of rudder that will be required to keep the aircraft straight.
If the speed of the aircraft was then to fall, either because of a necessity to climb, or simply because there isn’t sufficient total power available, the lower airspeed would result in the rudder and fin combination becoming less effective. There would be a need for greater deflection of the rudder to maintain the desired direction.
With a further reduction in speed, even more rudder would be required until eventually full rudder would need to be applied.
If the speed was to fall any further, it would be impossible to keep the aircraft straight. It would yaw away from the required direction towards the failed engine.
Since the further effect of rudder is roll and then a side-slip, control would be lost and the aircraft would enter a spiral dive.
The speed at which this phenomenon occurs − where directional and then lateral control can no longer be maintained − is known as the ‘minimum control speed’ and is abbreviated as Vmc.
Aircraft engines are mounted in many different configurations
The phenomena associated with minimum control speeds apply, to lesser and greater extents, to all sorts of twin, three-engined and four-engined aircraft.
Some twin-engined planes have wing-mounted engines that face backwards and “push” the aircraft through the air, but these are few and far between.
The Cessna 337 Skymaster
An aircraft that has engines that are unusually and almost uniquely configured is the Cessna 337 Skymaster, commonly known as the “Push-Pull”. Both of its engines are mounted in line with the aircraft’s longitudinal axis, one being behind the aircraft’s cabin that ‘pushes’ and the other, which ‘pulls’, located at the front.
Cessna’s marketing campaign for its C 337 hinged on the fact that the aircraft had ‘centre line thrust’. In the event of an engine failure, the aircraft would not be affected by the usual yawing effects associated with asymmetrical thrust.
Jet transport aircraft, from the smallest six-seater general aviation types, right up to the mighty Airbus A350s and 380s and Boeing 777s and 747s are also subjected to a variety of different minimum control airspeed restrictions.
Many of these jets have tail-mounted engines close to the stabilisers and the fin of the aircraft, with one mounted per side. Since they are close to the aircraft’s longitudinal axis, very little yaw occurs as a result of thrust asymmetry. Such aircraft can almost be regarded as having centre line thrust.
The majority of light twin engine aircraft have wing mounted engines
The subject of minimum control speeds for multi-engined aircraft is extremely vast and becomes increasingly complicated due to various technical, design, environmental and procedural factors.
Further discussion on the subject of minimum control speeds will now be reduced and confined to propeller-driven twin-engined aircraft that have wing mounted engines.
These aircraft are those from the light, four to twelve-seater piston-engined machines to the ten to twelve-seater executive class turbopeller aircraft. All of these aircraft exhibit significant yawing moments when any thrust asymmetry occurs.
The Vmc speed may be marked on the airspeed indicator by a red line.
This red line tells the pilot that in the event of an engine having failed and, with the remaining engine operating at full power, flight below the Vmc speed is impossible.
In the world of sophisticated airline operations, minimum control speeds are published or derived for many different regimes of flight, aircraft weights and environmental conditions.
However, in our world of light twin operations, the rule is very simple. When operating with an engine inoperative, the pilot must never let the speed decay to the minimum control speed. This speed is usually indicated by a red line that is drawn on the dial of the airspeed indicator.
How is the Vmc determined?
The Vmc is determined during the test-flying process that is required for the eventual certification of the aircraft.
Remember that the Vmc is determined and published for the results achieved at sea level with the temperature at 15°C and the C of G on the aft limit so that the rudder moment arm is shortest