Aerodynamics Flashcards
Non-Counter Rotating (Conventional)
Both props rotate clockwise
Counter Rotating
Left prop rotates clockwise where the right prop rotates counterclockwise
Contra Rotating
Left prop rotates counterclockwise, and the right prop rotates clockwise
Center Line
Both engines are inline with the longitudinal (Centerline) axis
Asymmetric Thrust
With one engine inoperative the thrust of the good engine is displaced from the centerline of the airplane and creates a yawing and rolling moment around the airplane’s CG toward the dead engine.
Asymmetric Drag
Increased due to propeller windmilling.
Asymmetric Thrust (P-Factor)
Causes the aircraft to yaw towards the dead engine.
Roll
Torque
Accelerated Slipstream
Pitch
Due to the loss of the accelerated slipstream over the horizontal stabilizer, downward lift being produced by the tail is reduced and thus the nose of the aircraft falls.
On the DA 42 the nose does not immediately go down because of it having a T-tail. Instead, as the airspeed decreases, so does the airflow over the tail and eventually the nose will lower.
Critical Engine on NON-Counter rotating Aircraft
The critical engine is the engine whose failure had the most adverse effect on directional control. On twins with each engine rotating in conventional, clockwise rotation as viewed from the pilot’s seat, the critical engine will be the left engine.
Operational critical engine
Certain aircraft have accessories run by only one engine (i.e. engine driven pump that is installed only on the right engine)
Factors that determine the critical engine
P – Factor
Accelerated slipstream
Spiraling Slipstream
Torque
P-Factor (yaw)
The descending blade produces more thrust than the ascending blade. The arm from the CG to the descending blade is longer on the right engine than the left
Both engines produce a yawing motion towards the dead engine, but is less pronounced on the left engine due to the force being closer to the CG
Accelerated Slipstream (Roll)
20-30% of lift generated is a result of
propeller slipstream accelerated over
the wings. With one engine dead, that
wing loses 20-30% of its lift. Both
engines produce a rolling moment
around the CG (due to the extra lift on
that wing associated with the working
engine) but is more pronounced on the
right engine due that extra lift being
applied at a greater distance or “Arm” from the CG
Spiraling Slipstream (Yaw)
On conventional rotating props, the slip stream from the right engine spirals away from the rudder having little effect on the vertical stabilizer. Whereas on the left engine the slip stream spirals into the vertical stabilizer. This helps apply a force to counteract the left turning forces towards the dead engine.
Torque Effect (Roll)
The operating engine will cause a rolling moment in the opposite direction of rotation due to Newton’s third law of motion.
If the left (Critical) engine failed
Three of the four forces turn the aircraft towards the dead engine. This decreases controllability and has much more adverse affects on the airplane than if the right engine had failed. Due to this, the left engine is considered the critical engine.
BAD:
• P-Factor (BLUE) yaws the airplane around the CG towards the dead engine
• Accelerated slipstream (YELLOW) creates more lift that rolls the airplane around the CG towards the dead engine
• Spiraling slipstream (PURPLE) spirals away from the vertical stabilizer and gives no help at all to the control of the airplane
• Torque (ORANGE) creates a left rolling tendencies towards the dead engine.
If the right engine failed
Two of the four forces turn the aircraft towards the dead engine. The forces are less pronounced than the right engine because the ARM is shorter between the forces and the CG. This still decreases controllability and has an adverse affect on the airplane. Also, two of the forces are helping with control of the airplane because they create turning tendencies AWAY from the dead engine
BAD:
• P-Factor (BLUE) yaws the airplane around the CG towards the dead engine
• Accelerated slipstream (YELLOW) creates more lift that rolls the airplane around the CG towards the dead engine
GOOD:
• Spiraling slipstream (PURPLE) spirals towards the vertical stabilizer and gives help to the control of the airplane
• Torque (ORANGE) creates a left rolling tendencies away from the dead engine.
What is VMC?
VMC is the airspeed at which directional control can no longer be maintained following the loss of the critical engine’s thrust.
▪Red line on the airspeed indicator.
▪Not enough rudder effectiveness to counter the forces created from the now asymmetric thrust.
How Is Vmc Certified?
Set of conditions used by the manufacturer to determine Vmc
▪Maximum Takeoff Power
▪Critical Engine Inoperative ▪Inoperative Engine Windmilling
▪Sea Level Conditions
▪Most Adverse Legal Weight
▪Most Adverse Legal Center of Gravity ▪5° of Bank Into the Operative Engine
▪Gear Up
▪Flaps in the Takeoff Position
▪Cowl Flaps Open
▪Out of Ground Effect
Vmc is not a fixed airspeed. It is only fixed for the specific set of circumstances under which it was tested for certification (Previous Slide).
▪We want to maintain control at the slowest possible speed, so a lower Vmc is GOOD and a higher Vmc is BAD.
▪Anything increasing the force to the dead engine will increase Vmc, and vice versa.
▪Anything increasing the amount of force the rudder can produce will decrease Vmc, and vice versa.
Factors Affecting Vmc Part 23
▪Maximum Takeoff Power
▪Critical Engine Inoperative
▪Inoperative Engine Windmilling
▪Sea Level Conditions
▪Most Unfavorable Weight (Light) ▪Most Unfavorable CG (Aft)
▪Out of Ground Effect
▪Cowl Flaps Open
▪5° Bank Towards Good Engine
▪Flaps in Takeoff Position
Maximum Takeoff Power
Bad for Vmc
The more power on the operating engine means the greater force pushing towards the dead engine. The greater the force, the more rudder force required to counter it and the earlier the rudder will lose control. Vmc will be higher with a greater power.
Critical Engine Inop
Bad for Vmc
The critical engine is the engine that has the most adverse affect on control of the airplane. By failing this engine, the rudder has more force to overcome than if the non-critical engine were to fail. Therefore, Vmc will be at a higher speed. Remember the “PAST” acronym for determine the critical engine.
Inoperative Engine Windmilling
Bad for Vmc
A windmilling propeller creates more drag than a feathered propeller. Increased drag on the inoperative engine will create a stronger yaw toward the dead engine. Therefore, the rudder must overcome more force which raises Vmc.
