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.
Sea Level Conditions
Bad for Vmc
At sea level the dense air allows to the operating engine and prop to produce maximum thrust. Since there is more thrust, there is a greater force towards the dead engine for the rudder to overcome. Therefore, Vmc is higher.
Most Unfavorable Weight
Bad for Vmc
Vmc increase as weight is reduced, so the lightest legal weight is the most unfavorable. The lightest weight provides the aircraft the least momentum. The heavier the aircraft the more likely its inertia will carry it forward and help prevent the yaw and roll associated with a dead engine.
Most Unfavorable CG
Bad for Vmc
Vmc increases as the center of gravity moves rearward. The further aft the CG is, the shorter the rudder’s arm is. The shorter the arm, the less effective the rudder. Vmc will be higher since the rudder produces less force at any given speed compared to a forward CG.
Out of Ground Effect
Bad for Vmc
Vmc decreases while in ground effect. Ground effect reduces induced drag and “increases” lift, reducing Vmc.
Gear Retracted
Bad for Vmc
When the gear is down it acts as a keel (Like on a boat) which aids in directional stability and decreases Vmc. With the gear up the keel effect is removed and it cannot help keep the aircraft straight.
Cowl Flaps Open
Good for Vmc
With the cowl flaps open the operating engine’s prop will push air into the cowl flaps resulting in increased drag. Increased drag on the operating engine decreases Vmc since it assists in counteracting the yaw towards the dead engine.
5° Bank Towards the Good Engine
Good for Vmc
The horizontal component of lift generated by the bank assists the rudder in counteracting the yaw. Vmc is reduced considerably with bank angle, so the FAA limits the bank during testing to 5 degrees. Approximately 3 knots gained per degree of bank.
Flaps in Takeoff Configuration
Good/Bad for Vmc
Most light twins takeoff without flaps, therefore there will be no effect. However, with many different flap sizes, types, and settings, having the flaps down could help or hurt your Vmc. Having the flaps down could produce more drag on the operating engine and help stabilize the airplane through keel effect. Both reducing Vmc. But it could also increase lift on the operating engine’s wing which would increase the roll towards the dead engine.
