All you MEI's out there, i need a little help in understanding the difference as i am currently working towards my CMEL.
I have scoured Airplane flying handbook and walked away with confusion. If any one can recommend any further reading, that will be appreciated. If no references available, kindly enter your explanation on the topic in the response.
Regards,
I have never heard it referenced as "static" or "dynamic" Vmc, I have always simply said "actual" or "published" Vmc.
As for published Vmc, this is the configuration that the manufacturer has to use to get the red line that is
published on your airspeed indicator. The configuration is: Gear up, flaps takeoff, trim takeoff, critical engine failed, full sea level ISA power on the good engine, prop windmilling, max gross weight, aft CG, and no more than 5* of bank and 150lbs of rudder force. When set to this configuration the aircraft will Vmc at the red line published speed. It is important to note that the set up for this is essentially takeoff right after gear retraction. There are some exceptions, such as for aircraft with auto feather in which case they can allow the prop to be feathered for the demonstration.
Actual Vmc though is almost always different. The most important factor is power, if the power is at idle the aircraft virtually cannot and will not ever Vmc, it doesn't matter how slow you go there is no asymetrical thrust causing rotation, this is why the response to a loss of directional control on one engine is to reduce power. The second biggest factor is the position of the controls, if you don't go fully to the limits or don't move the controls at all Vmc will be substantially higher. Third biggest factor is a feathered prop, if you feather the engine the drag is decreased tremendously and therefore Vmc is much lower than published.
Other factors are, zero sideslip, which not only increases performance (not something to tie to Vmc) but also increases the AOA of the rudder and increases controllability. If you draw the vertical stabilizer with the rudder off to one side and remember that AOA (ie lift) is the leading edge to the trailing edge you will see that in a slipping condition with the wind hitting the stabilizer at an angle the AOA is reduced, when you get that 2-3* of bank and half ball to the good engine zero sideslip the relative wind flows straight down the fuselage and the AOA of the tail is increased creating greater controllability and a lower actual Vmc.
Gear position and its effects vary. Imagine throwing a dart with the feathers first, the dart will flip around because the drag wants to be behind the CG. Same goes for an airplane and for tricycle gear aircraft the two main gear down behind the CG create stablity and therefore reduce the rudder required and decrease Vmc. If you take a DC3 on the other hand, the drag is in front of the CG which is a negative. Same goes for the flaps! The drag of the flaps is aft of the CG and therefore good for Vmc.
Aft CG shortens the arm that the rudder has to work through (shorter crowbar) and increases Vmc. The critical engine causes greater turning forces and therefore an increased Vmc.
Lastly is density altitude (assuming a normally aspirated engine) as you climb there is reduced power output and therefore a lower Vmc speed.
, rather I used to teach alot of Commerical Multi students and thad to teach it a lot. Failures were not uncommon for students who couldn't explain just one item of actual Vmc :/ so I put alot of emphasis on it with my students and never had one fail.
