VMC FACTORS

2 Answers

1. 
   

   
   

   Robert Jankowski on Mar 08, 2016

   Why we want trim for takeoff or neutral?
   Actually, I’m not entirely sure on this one, the regulation calls for takeoff trim and the only thing I can think of off hand is that it sets you up for something close to Vxse or Vyse versus being trimmed more nose up. Maybe a standardization?

   How do the gear up helps to increase the Vmc?
   The landing gear in the extended state creates a keeling factor which helps stabilize the aircraft and helps work against the yawing into the dead engine.

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2. 
   

   
   

   Skyfox on Mar 18, 2016

   To answer the question about trim first, I did some digging. FAR 23.149 only says “trimmed for takeoff” but gives no explanation as to why. The only real explanation I could find comes from the AirlinePilotForums and it basically says trimming the aircraft is to reduce excessive control loads and keep those control loads in a normal range. Here’s the source:
   http://www.airlinepilotforums.com/archive/index.php/t-15717.html

   There are 7 main factors to be followed/maintained when an airplane is certified for Vmc. They’re commonly known as the 7 Factors of Vmc…who’d have thought? (There’s actually more than 7 factors in the list.) In no particular order:

   1. Most unfavorable CG location, usually at the aft limit–this results in the shortest arm (referring to leverage) between the rudder and the CG, giving the rudder the least ability to maintain heading.

   2. Maximum takeoff weight–this requires the greatest amount of lift for climb or maintaining altitude, which creates the greatest horizontal component of lift for combating the yaw of asymmetric thrust. Increased weight = decreased Vmc

   3. Maximum takeoff power on the good engine–this creates maximum asymmetric thrust and therefore the most yaw, and the demonstration for certification must show that the airplane in the single-engine configuration can counter the adverse yaw at maximum power. Also, in an actual engine-out situation if it requires less than full power on the good engine to maintain altitude and safe airspeed, a reduction in power on the good engine will help relieve some of the load to counter the yaw.

   4. The bad engine prop is windmilling, and it’s the critical engine that’s failed–This means the prop is not feathered and therefore is creating the most amount of asymmetric drag to fight against. The critical engine is only a factor for an airplane where both engines turn the same direction, and it’s typically the left engine in that case since they usually turn clockwise, placing the maximum asymmetric prop thrust on the right side prop with the greatest arm to yaw the airplane around the vertical axis. When the engines are counter-rotating, each engine/prop creates a mirror image of thrust and therefore they are equally critical to lose an engine. A tandem engine airplane (one in the front, one in the back) may or may not have a critical engine, depending on the configuration of the aircraft.

   5. Gear up, cowl flaps closed, flaps in takeoff position, trimmed for takeoff–Gear down can create a keel effect that helps counter the yaw, so requiring gear up for certification will eliminate that benefit. Gear down may help in a real engine out situation but the added drag would have to be considered. Also, putting gear down can potentially move the CG in a favorable direction that places the CG farther from the rudder, increasing rudder effectiveness. Cowl flaps can also add a tiny amount of keel effect so they’re closed for certification. Flaps down will increase lift and therefore increase the horizontal component of lift, and increases drag behind the good engine which helps counter the yaw. If takeoff configuration has them down then they have to be in that position for certification. And trim is discussed above.

   6. Standard temperature and pressure at sea level–Density altitude should be 0 for certification because an increase in altitude or temperature or decrease in pressure will increase the density altitude. Higher density altitude affects the engine and prop more than the control surfaces, so that will lead to less control force being required to fight the yawing tendencies.

   7. No more than 5° of bank–certification requires a limit of 5° of bank towards the good engine because if there was no limit then airplane manufacturers could advertise unsafe claims about how well their airplane does in a single engine situation. If Vmc requires more than 5° of bank to maintain heading then it can potentially lead to a dangerous situation.

   One more factor is that the airplane must be out of ground effect because ground effect helps control effectiveness which reduces Vmc, so out of ground effect will make Vmc the highest.

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