Only Three Factors Affect Vmc - FAA
OK - let's set this straight once and for all. There are only three factors that the FAA states that affect Vmc. CG, Density Altitude and Angle of Bank. The laundry list of factors that supposedly "affect Vmc" may or may not depending on the aircraft, and is a list of the
configuration that a multiengine aircraft (not centerline thrust) must be in when it is originally certified - they must state to the general public what Vmc is without being able to "cheat."
This is directly from the FAA's Flight Training Handbook in Chapter 16, and is actually pretty clear:
Vmc Demonstrations
Every light twin engine airplane checkout should include a demonstration of the airplane's engine out minimum control speed. The engine out minimum control speed given in the FAA approved Airplane Flight Manual, Pilot's Operating Handbook, or other manufacturer's published limitations is determined during original airplane certification under conditions specified in the Federal Aviation Regulations. These conditions normally are not duplicated during pilot training or testing because they consist of the most adverse situations for airplane type certification purposes. Prior to a pilot checkout, a thorough discussion of the factors affecting engine out minimum control speed is essential.
Basically, when one engine fails the pilot must overcome the asymmetrical thrust (except on airplanes with center line thrust) created by the operating engine by setting up a counteracting moment with the rudder. When the rudder is fully deflected, its yawing power will depend on the velocity of airflow across the rudder - which in turn is dependent on the airspeed. As the airplane decelerates it will reach a speed below which the rudder moment will no longer balance the thrust moment and directional control will be lost.
During engine out flight the large rudder deflection required to counteract the asymmetric thrust also results in a "lateral lift" force on the vertical fin. This lateral "lift" represents an unbalanced side force on the airplane which must be counteracted either by allowing the airplane to accelerate sideways until the lateral drag caused by the sideslip equals the rudder "lift" force or by banking into the operative engine and using a component of the airplane weight to counteract the rudder induced side force.
In the first case, the wings will be level, the ball in the turn and slip indicator will be centered and the airplane will be in a moderate sideslip toward the inoperative engine. In the second case, the wings will be banked 3 to 5 degrees into the good engine, the ball will be deflected one diameter toward the operative engine, and the airplane will be a zero sideslip.
The sideslipping method has several major disadvantages: (1) the relative wind blowing on the inoperative engine side of the vertical fin tends to increase the asymmetric moment caused by the failure of one engine; (2) the resulting sideslip severely degrades stall characteristics; and (3) the greater rudder deflection required to balance the extra moment and the sideslip drag cause a significant reduction in climb and/or acceleration capability.
Flight tests have shown that holding the ball of the turn and slip indicator in the center while maintaining heading with wings level drastically increases Vme as much as 20 knots in some airplanes. (Remember, the value of Vmc given in the FAA approved flight manual for the airplane is based on a maximum 5 degree bank into the operative engine.)
Banking into the operative engine reduces Vmc, whereas decreasing the bank angle away from the operative engine increases Vmc at the rate of approximately 3 knots per degree of bank angle.
Flight tests have also shown that the high drag caused by the wings level, ball centered configuration can reduce single engine climb performance by as much as 300 FPM, which is just about all that is available at sea level in a nonturbocharged light twin.
Banking at least 5 degrees into the good engine ensures that the airplane will be controllable at any speed above the certificated Vmc, that the airplane will be in a minimum drag configuration for best climb performance, and that the stall characteristics will not be degraded. Engine out flight with the ball centered is never correct.
The magnitude of these effects will vary from airplane to airplane, but the principles are applicable in all cases.
NOTE - A bank limitation of up to 5 degrees during Vmc demonstration is applicable only to certification tests of the airplane and is not intended as a limit in training or testing a pilot's ability to extract maximum performance from the airplane.
For an airplane with nonsupercharged engines,
Vmc decreases as altitude is increased. Consequently, directional control can be maintained at a lower airspeed than at sea level. The reason for this is that since power decreases with altitude the thrust moment of the operating engine becomes less, thereby lessening the need for the rudder's yawing force. Since Vmc is a function of power (which decreases with altitude), it is possible for the airplane to reach a stall speed prior to loss of directional control.
It must be understood, therefore, that there is a certain density altitude above which the stalling speed is higher than the engine out minimum control speed. When this density altitude exists close to the ground because of high elevations or temperatures, an effective flight demonstration is impossible and should not be attempted. When a flight demonstration is impossible, the check pilot should emphasize orally the significance of the engine out minimum control speed, including the results of attempting flight below this speed with one engine inoperative, the recognition of the imminent loss of control, and the recovery techniques involved.
Vmc is greater when the center of gravity is at the rearmost allowable position. Since the airplane rotates around its center of gravity, the moments are measured using that point as a reference. A rearward CG would not affect the thrust moment, but would shorten the arm to the center of the rudder's horizontal "lift" which would mean that a higher force (airspeed) would be required to counteract the engine out yaw. Figure 16-8 shows an exaggerated view of the effects of a rearward CG.
http://www.faatest.com/books/FLT/Chapter16/Vmc%20Demonstrations_files/imageI1M.jpg Questions?
