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Minimum control speeds for light twin aircraft – Part 2

How minimum control speeds are determined 

Part One of “Minimum Control Speeds for Light Twins”, concluded with the statement that operating with an engine inoperative, the pilot should never let the speed decay to below the “red line” minimum control speed.

HAVE YOU READ: Minimum Control Speeds for Light Twins Part 1

Key factors when determining the Vmc for a particular light twin-engine aircraft 

First, the power output of the live engine…

The atmospheric conditions that exist should be those of a “standard day” at sea level. This means an air temperature of 15°C and an air pressure of 1013 hectopascals. The claimed power output of an engine is what it should produce at sea level on a standard day.

If higher than normal temperatures and lower than normal air pressure conditions exist, then the density altitude would be higher than it would be at sea level. The full maximum output of the engine would not be achievable in these conditions.

With an engine failure or shut down, one of the factors that affects the magnitude or intensity of the resulting yaw is the power output of the live engine. The higher the power is, the greater the yaw will be.

It follows then that Vmc will be higher at sea level than it would be in areas where the air is thinner.

Secondly, the drag created by the failed engine and its propeller contributes greatly towards increasing the yawing moment.

The drag created by a propeller that is windmilling is unbelievably high. In general, it equates to the same amount of extra drag that is created when flying with the landing gear extended and flaps set at about half extension.

If, on the other hand, the propeller is feathered, it “cuts” through the air like a knife and creates very little drag in the process. 

The greater the drag from the dead engine, the higher the Vmc will be. The test is therefore accomplished with the propeller of the failed engine windmilling.

Thirdly, the aircraft is loaded to its maximum allowable weight, with its centre of gravity on the maximum aft limit.   

The power and effectiveness of the rudder is affected by its distance from the centre of gravity, this being the same point through which the vertical axis of the aircraft passes.

At its aft limit, the rudder moment arm is at its shortest and the effectiveness of the rudder will be least. A higher Vmc will result from this.

With a centre of gravity that is further forward, the rudder moment arm is longer and therefore the rudder is more effective. It would then be possible to maintain directional control down to a lower speed.

It is more “CRITICAL” to have the left engine fail than the right.         

Barring certain light twins that have counter-rotating propellers, the vast majority of these aircraft  have propellers that turn clockwise when viewed from behind. Therefore, on the left engine, the down-going propeller blade passes very close to the aircraft’s fuselage.  On the right engine, the down-going propeller blade is well away from the aircraft’s fuselage.

If, in flight, the body angle of the aircraft is perfectly level, all four propeller blades will cut through the air at the same angle of attack. However, the instant that the aircraft’s body angle is increased, the down-going blades, on both the left and the right engines, meet the air at a greater angle of attack than the up-going blades do. They therefore generate more thrust than the up-going blades do.

However, the down-going blade on the right side is furthest from the aircraft’s vertical axis and it will have a far greater thrust moment arm than the down-going blade on the left side has.

This phenomenon is known as “asymmetric blade thrust”.

With the right engine running and the left engine failed, the aircraft is going to have a higher Vmc than with the left engine running and the right engine failed. In the latter case the aircraft would be controllable down to a lower speed.

This is why the left engine is viewed as being the “critical engine”.              

Asymmetric blade thrust plays another part in why, for determination of the Vmc, the aircraft must be loaded to its maximum weight.

For the test, the aircraft is flown at its maximum gross weight. This means that for any required speed, the aircraft will need to fly at a higher angle of attack than if it was lighter. The higher the attitude of the aircraft, the more the asymmetric blade effect increases. The overall result is an increase in the Vmc.

An infinite number of Vmc speeds can exist.

Most of the factors that were adhered to during certification are relaxed, disregarded, or simply not present in everyday operations. The result is that on these occasions, the minimum control speed would be below the red-line speed on the ASI.

This would happen at altitudes higher than sea level where the engine power output is less. The left engine (critical) could be the one that is operating. The right hand, “non critical” engine would have its propeller feathered instead of leaving it windmilling. Then too, the aircraft could be only lightly loaded and its centre of gravity could be well forward and a fair distance away from the aft limit.

All of these differences, in part or fully, give rise to the fact that an infinite number of minimum control speeds actually exist, all of which that have come about because of a departure from the rigorous rules and datums used during certification.

In training, flight on one engine at speeds that are below the red-line speed is often demonstrated. However, the very big danger here is that the aircraft might arrive at its stalling speed earlier than expected and when most of the rudder capability has already been utilised. This would very definitely not be a phase of flight that you ever wish to be in!   Work it out for yourself!




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