Multi Engine Aerodynamics Flashcards
What the total list of factors that affect Vmc & are used for its certification?
S - Standard atmostphere @ sea level
M - Most unfavorable weight: Light
A - Aft CG
C - Critical Engine inop
P - Propeller windmilling
P - Power Full
L - Landing gear up (or down)
F - Flaps Up
B - Bank 3-5 degrees
O - Out of Ground Effect
What is the difference between a Conventional Twin & a Non-Conventional Twin engine aircraft?
A conventional Twins engines will rotate the same way (clockwise)
A Non-Conventional Twins engines will be counter rotating
What are the pros and cons of a Conventional Twin vs Non-Conventional?
Conventional Twin
* Simple design & cheaper to repair
* Create a Critical Engine situation that negatively impacts single engine ops
Non-Conventional
* Iliminate the Critical engine which positively impacts single engine ops
* More Complex & Expensive to repair
What are the aerodynamic factors that would determine which engine is the Critical Engine?
P - P-Factor (Yaw)
A - Accelerated Slipstream (Roll)
S - Spiraling Slip Stream (Yaw)
T - Torque
- These factors will cause turning tendencies to the left, making the left engine our Critical Engine
Explain what the “P” stands for in the P.A.S.T. acronym used to describe the aerodynamic factors that determine the Critical Engine
P-Factor (Yaw)
* The P-Factor created by the RH Operative engine, has a longer Arm (distance) from the CG than the left engine.
* This creates a YAW motion to the Left
Remember: “Thrust Arm”
Because of this, it is more detrimental to performance for the Left engine to become inoperative
What is P-Factor?
P-Factor
- The descending blade has a greater AOA (bite of air) than the ascending blade
- This occurs when at low airspeeds & high AOAs or durring a climb or T/O
- The result is that the descending blade creates more thrust. Assymetrical Thrust.
What is the difference between Vmca & Vmc?
Vmca - The PUBLISHED speed on the airspeed indicator (red line) that we can maintain DIRECTIONAL control of the aircraft while airborn
Vmc - The actual speed that directional control can be maintained, as it will vary depending on the conditions (SMACPPLFBO).
Explain what the “A” stands for in the P.A.S.T. acronym used to describe the aerodynamic factors that determine the Critical Engine
Accelerated Slipstream (Roll)
* Also created by P-Factor
* Greater thrust on the descending blade of the operative RH engine creates more prop wash
* More lift is able to be produced behind the RH engine giving us asymmetrical lift on the right wing, inducing a roll to the left
Remember: “Lift Arm”
Because of this, it is more detrimental to performance for the Left engine to become inoperative
Explain what the “S” stands for in the P.A.S.T. acronym used to describe the aerodynamic factors that determine the Critical Engine
Spiraling Slipstream (Yaw)
Principle:
* Due to P-Factor, the descending blade creates more thrust and therefore lower pressure than the ascending blade
* The faster moving air behind the descending blade creates Lower pressure than the air behind the ascending blade (Bernoullis Principle)
* Since air likes to flow from High to Low, this causes the slipstream to be deflected to the RIGHT on BOTH engines.
Affect
* When the Left engine is inoperative the, the slipstream from the Right operative engine drifts away from the airplane, therefore there is less rudder effectiveness than if the slipstream were to flow back and hit the rudder
* With the Right engine inoperative & the Left engine operative, the slipstream from the left engine deflects to the right and hits the rudder, giving us MORE rudder effectiveness & helping to yaw us into the operative engine
Because of this, it is more detrimental to performance for the Left engine to become inoperative
Explain what the “T” stands for in the P.A.S.T. acronym used to describe the aerodynamic factors that determine the Critical Engine
Torque (Roll)
Principle
* Newtons 3rd law requires a clockwise rotating engine to create a torque effect in the oposite direction
* This causes the aircraft to having a rolling tendency towards the left
Affect
* If the Left engine is inoperative, our asymetrical thrust is exasperated by the torque from the Right operative engine, rolling us even more into the left inoperative engine.
Because of this, it is more detrimental to performance for the Left engine to become inoperative
Explain how the “S” in SMACPPLFBO effects Vmc
Standard Atmosphere at Sea level
1. Low DA, temps, & humidity increase engine performance, increasing Asymmetrical Thrust resulting in the need for more Rudder to counteract this = Increases Vmc
2. Conversely, high DA, temps, & humidity will lower Vmc
Explain how the “M” in SMACPPLFBO effects Vmc
Most Unfavorable Weight: Light
* 1. A light weight aircraft will have less forward momentum or inertia to counteract Asymetrical Thrust, requiring more Rudder to maintain straight flight = Increases Vmc
Explain how the “A” in SMACPPLFBO effects Vmc
Aft most CG
* 1. The further back the CG is placed, the shorter the distance it is to the Rudder, this decreases the Rudder Arm length and therefore the rudder produces less Moment or force, requiring more Rudder to compensate = Increase in Vmc
Explain how the “C” in SMACPPLFBO effects Vmc
Critical Engine Inoperative
1. When the critical engine is made inoperative, aerodynamic factors such as P-Factor & drag (PAST factors) will increase the yawing and rolling motion to the left, requiring more rudder to counteract = Increasing Vmc
Explain how the first “P” in SMACPPLFBO effects Vmc
Propeller Windmilling
* 1. If the propeller on the inoperative engine is windmilling (unfeathered, low pitch) then it will produce more drag, this asymetrical drag will incease the yawing motion & require more Rudder to counteract = Increases Vmc
