Performance and Limitations (ACS)

Question 1

(Performance and Limitations)
What are the four forces of flight?

Answer 1

-Lift: keeps the aircraft in the sky. its a force that act perpendicular to the relative airflow and is concentrated at the center of pressure. The center of pressure is where we say the sum of the aircrafts lift is located. in straight and level flight then lift will be straight up through the wings.
-Weight: always pointed straight towards the center of the earth regardless of aircraft orientation. Acts through the aircrafts center of gravity. Force equals mass times acceleration. In level flight lift will be equal to weight.
-Thrust: the force that points in the direction of the engine. makes the aircraft move forward. pilots control thrust.
-Drag: comes in two types. induced and parasite drag.
induced drag: product of lift
parasite drag: form drag, interference drag, skin friction.

Lift and weight oppose each other
Thrust opposes drag

Question 2

(Performance and Limitations)
What is a force?

Answer 2

A force is a vector. a vector has both a direction and a magnitude. like an invisible arrow, the direction of the arrow is where its pointing and the length of the arrow represents its magnitude or how strong the force is.

Magnitude of the force is equal to mass times acceleration. Mass(Acceleration)=Magnitude
Newtons second law

Question 3

(Performance Limitations)
What is relative airflow/wind?

Answer 3

The direction of air in relation to the aircrafts wing. its opposite of the direction of movement of the aircraft.

Question 4

(Performance Limitations)
what is Interference Drag?

Answer 4

Interference drag is caused by intersecting airstreams from different parts of the aircraft. For example, eddies and currents are caused where the wing and the fuselage meet. The turbulence from these two intersecting air flows causes drag.

Question 5

(Performance Limitations)
What is camber?

Answer 5

The curve of an airfoil. The upper and lower cambers are measured based on how much their respective surface moves away from the chord line. The combination of upper and lower camber distance gives the thickness of an airfoil.

More camber means more lift. It also means more drag and a higher stall speed.

Question 6

(Performance Limitations)
Can the camber be changed?

Answer 6

yes.

The aircraft can use flaps to manipulate the camber. Extend or retract the flaps to do this.

Question 7

(Performance Limitations)
What is the Angle of Attack and can we change it?

Answer 7

the angle of attack is the angle created between the chord line and the relative wind.

The AOA changes every time the aircraft changes pitch, add or retract flaps, change speed, or increase load factor.

If we exceed the critical AOA the wing will stall.
The critical angle of attack is the largest angle an airfoil can be at before airflow is disrupted moving over the wing.

Question 8

(Performance Limitations)
What is the angle of incidence? Can we change it?

Answer 8

The angle between the chord line and the longitudinal axis of the aircraft.

Question 9

(Performance Limitations)
Describe the basics of Bernoulli’s principle and explain how that applies to the wing of an aircraft.

Answer 9

The pressure of a fluid is inversely proportional to the speed of the fluid. In other words the faster the water or air flows the lower the pressure of that fluid.

Question 10

(Performance Limitations)
How is lift created?

Answer 10

newtons third law and Bernoulli’s principle.

Airfoils are designed so that the air moving over the top is faster than the air moving over the bottom. This creates a higher pressure pushing up on the wing and a low pressure overlying the wing resulting from the faster flowing air. Newton’s third law states that for every action there is an equal and opposite reaction.

Question 12

(Performance Limitations)
What is Skin Friction Drag?

Answer 12

Drag that results from air moving over a surface that is not perfectly smooth. Additionally the air at what is known as the boundary layer is still.

Question 13

(Performance Limitations)
What is Form Drag?

Answer 13

Form Drag, which is the result of the aerodynamic resistance to motion due to the shape of the aircraft.

Question 14

(Performance Limitations)
What is induced drag? Why do wingtip vortices form? Why do they matter?

Answer 14

A byproduct of lift; and is caused by the tendency of high pressure air hitting the bottom of the wing to overflow to the top side of the wing. This flow from bottom to top happens at the tips and creates what are called wingtip vortices.

Increasing lift by increasing the angle of attack causes the pressure differential between the wing surfaces to increase thus increasing induced drag. this is why induced drag increases as airspeed decreases. In other words induced drag varies inversely with the square of the airspeed.

Wingtip vortices are lateral tiny tornadoes that rotate towards the fuselage and extend behind the aircraft. They create downwash which points the relative wind downward. This brings the lift vector backwards and creates a horizontal component of lift.

Question 15

(Performance Limitations)
True of False: parasite drag increases with airspeed.

Answer 15

True

Question 16

(Performance Limitations)
What is the airspeed where parasite and induced drag meet?

Answer 16

L/D Max

The point at which the wing creates the greatest lift with the lowest drag. Another name for this speed is the best glide speed. It will keep an airplane in the air for longest in a power off glide.

Glide speed cannot be stretched by going faster or slower. Best glide is as good as it gets.

Question 17

(Performance Limitations)
Which elements are changeable in flight and which are not?

Answer 17

Lift Equation: Lift equals the coefficient of lift times 1/2 the density of air times velocity squared times the surface area of the wing.

The coefficient of lift is changeable by altering the angle of attack of the wing. There is a point at which the coefficient cannot go beyond known as the critical angle of attack. CLmax.

Extending flaps can change the camber and chord line of a wing.

The density of air is not changeable by the pilot. However, it does change with temperature and altitude.

Velocity is a changeable factor. Lifting or lowering the nose or increase/decrease the thrust. Velocity has a major effect on overall lift.

Wing surface area can be changed with fowler flaps. Otherwise this is a static factor.

Question 18

(Performance Limitations)
What causes a wing to stall?

Answer 18

Exceeding the critical angle of attack. When the angle of the leading edge of the airplane disrupts the smooth airflow over the wings. It becomes turbulent like a burbling river over rocks.

Recovery is as simple as lowering the angle of attack. Drop the nose.

Question 19

(Performance Limitations)
What is an aircrafts center of gravity

Answer 19

The point at which the aircraft would balance if set on a fulcrum.

Question 20

(Performance Limitations)
Where is the CG located?

Answer 20

Forward of the center of lift. To counter the weight of the engine, cockpit instrumentation, and pilot weight. The horizontal stabilizer group (elevator) imparts a tail down force to counter act the nose downforce the CG creates.

Question 21

(Performance Limitations)
How is CG calculated?

Answer 21

Weight times Arm length equals moment.

Then for the CG it is total moment divided by total weight.

Question 22

(Performance Limitations)
How does a forward CG affect flight?

Answer 22

It would cause the nose to drop which would require additional tail downforce to counter it. This requires a higher angle of attack to counteract the artificial aerodynamic weight that has been added.

this increases the stall speed.
Decreases the efficiency of cruise
increases stability
Increases the stall recovery

Question 23

(Performance Limitation)
How does rearward center of gravity affect how the aircraft flies?

Answer 23

since the distance between the CG and the center of lift becomes smaller the elevator forces become lighter. The aircraft becomes pitchy and more difficult to control.

Decreases the stall speed.
Increases the efficiency and speed of cruise
Decreases stability
Decreases stall recovery

Watch out for the unrecoverable flat spin

Question 24

(Performance Limitations)
Why do we need to add backpressure on the yoke in turning flight?

Answer 24

There are two types of lift components. Vertical component of lift that most directly counters the weight that gravity has on an aircraft and a Horizontal component of lift that is a result of changing the bank angle or pitch of the aircraft (this is what causes induced drag).

Backpressure on the yoke during a turn increases the angle of attack of the wings to increase the total lift that was lost when the vertical lift was lost due to an increased horizontal lift. The steeper the bank the more the angle of attack needs to be increased so as to keep the same amount of vertical lift force to counter the weight being exerted by gravity.

Question 25

(Performance Limitations)
What is load factor?

Answer 25

The ratio of lift and weight. The unit of measurement is g. It is dimensionless and is meant measure the acceleration of gravity.

In steady unaccelerated flight the load factor is 1 g. When in a bank and want to maintain altitude we need more lift since some of the lift is pointing sideways (horizontal lift). The load factor changes based on the bank angle and pitch of the airplane.

Note: it is the horizontal component of lift that makes the airplane turn.

Question 26

(Performance Limitations)
How does load factor affect stall speed?

Answer 26

As angle of bank increases so does stall speed. This is due to the fact that an increased angle of attack is needed to maintain lift in a bank. A necessary increased bank angle brings the aircraft closer to breaking the critical angle of attack.

Question 27

(Performance Limitations)
What is an accelerated stall? Why are they dangerous?

Answer 27

A stall due to an increased load factor. Accelerated part of accelerated stall refers to a change in velocity. since velocity is a vector it has magnitude, speed, and direction. So anytime the aircraft is changing direction its accelerating. increased acceleration increases load factor which in turn increases the stall speed.

The most common places that pilots find themselves in accelerated stalls are in the traffic pattern, tight turns, and pulling out of a dive, or recovering from a stall.

Question 28

(Performance Limitations)
What is maneuvering speed?

Answer 28

the speed below which you can move a single flight control one time to its full deflection for one axis of an airplanes rotation only (pitch, roll, or yaw) in smooth air without risk of damage to the airplane.

Below Va (maneuvering speed) the airplane will stall before being damaged or encountering stresses that will break a wing.

Question 29

(Performance Limitations)
Why does the wing stall before damaging itself when flying below maneuvering speed?

Question 30

(Performance Limitations)
How do you determine maneuvering speed in the aircraft?

Answer 30

Listed in the POH of an aircraft. It is based off of different weights. The heavier the plane the higher the angle of attack for a given speed. Thus you will be closer to a stall when heavier than when lighter.

every 2% reduction in weight reduced maneuvering speed by 1%

Question 31

(Performance Limitations)
What is a spin?

Answer 31

An aggravated stall. The stall is destabilized by being uncoordinated at the point of stall. In a yaw for example one of the wings would be swinging forward relative to the other which would cause the outside wing to have a lower angle of attack and be less stalled than the other wing. Both wings are stalled but the outside wing is less stalled. This imbalance of lift causes the aircraft to roll towards the inside wing.

Question 32

(Performance Limitations)
What are common situations in which a spin occurs? what factors come into play?

Answer 32

Spins are most likely to occur when the pilot is distracted. Stretching the glide, flying a too tight traffic pattern, a sudden pitch up during a go around, not being outside the plane.

Weight
Turbulence
Gusts
CG loading
Icing

Question 33

(Performance Limitations)
How do you recover from a spin?

Answer 33

recover at the first sign of a spin do not wait. PARE; power to idle, ailerons neutral, rudder opposite the spin, and elevator back pressure to recover from the dive.

Power can exacerbate the spin due to the left turning tendencies that come with it.
Neutralizing the ailerons will reduce the angle of attack differential
Opposite rudder is the primary method in recovering from a spin

Question 34

(Performance Limitations)
Explain the general aerodynamic principles of ground effect.

Answer 34

When an airplane is less than half its wingspan above the ground the airplane is in ground effect.

When the airplane is close to the ground the surface of the earth interferes with the wingtip vortices and downwash created by the wing. Wingtip vortices and downwash are reduced due to the interference from the ground which reduces drag and increases lift.

Question 35

(Performance Limitations)
What are some of the hazards of ground effect?

Answer 35

on takeoff the airplane will want to leave the ground before the rotation speed it needs to reach true flight.

on landing the aircraft will float on high power settings which will increase the amount of runway needed for a landing.

Question 36

(Performance Limitations)
What are the different types of altitudes and airspeeds?

Answer 36

Airspeeds:
-Indicated airspeed: The speed shown directly on the airspeed indicator instrument. Calculated based on the pressure derived from the pitot static system. Or in other words the speed of the air that the wing is experiencing.
-Calibrated airspeed: true airspeed corrected for instrument and positional errors.
-True airspeed: Calibrated airspeed corrected for non standard temperature and altitude. True airspeed represents the speed at which you’re flying through the air.
-Ground speed: the speed at which you are flying over the ground. head wind and tail wind will affect this speed greatly. A headwind will subtract from your ground speed and a tail wind will make you go faster.

Altitudes:
-Indicated Altitude: the altitude as read off the altimeter.
-Pressure Altitude: the vertical distance above the standard datum plane. Altitude corrected for nonstandard pressures.
-Density Altitude: pressure altitude corrected for nonstandard temperature. high density altitude is the equivalent of high altitude (it reduces the performance).
-True Altitude: the height above the mean sea level or MSL.
-Absolute Altitude: the heigh above ground level AGL.

Question 37

(Performance Limitations)
When temperature, altitude, and or humidity are high what happens to the density altitude?

Answer 37

it increases

Question 38

(Performance Limitations)
Why are performance calculations not representative of actual experience?

Answer 38

Aircraft and engines age which results in a decrease in effectiveness and efficiency plus the chart data was created by expert test pilots.

Question 39

(Performance Limitations)
Why do we perform takeoffs into the wind?

Answer 39

A head wind will increase the airflow over the wing and result in the aircraft getting to indicated rotation speed earlier than if the headwind was not there. Necessitating less runway to take off.

Question 40

(Performance Limitations)
Why do we land into the wind?

Answer 40

A headwind reduces the groundspeed while increasing the lift generated by the wings. It allows for a steadier approach and slower descent. It puts less pressure on the landing gear and airframe on landing.

Question 41

(Performance Limitations)
How does increased aircraft weight affect the takeoff distance?

Answer 41

higher weight increases takeoff distance. With increased weight the lift force must also increase to compensate. Also a larger amount of thrust must be present to overcome the amount of weight present and give the proper inertia. Newton’s first and second laws as well as the lift equation are good references.

Question 42

(Performance Limitations)
Which V speeds are marked on the airspeed indicator? Which are not?

Answer 42

The start of the White block nearest to zero is Vs0. Landing configuration stall speed at max weight.
The end of the white block above that is Vfe. Maximum flaps extension speed.

start of the green band is Vs1 which is the clean configuration stall speed at maximum weight
the end of the green band is Vno. Maximum structural cruising speed.

Yellow band beginning means the caution speed for the aircraft and should only be entered in smooth air.

Red line is the never exceed speed. Vne

Va, Vx, Vy, Vr, Vle, and Vlo are not displayed on the airspeed indicator.

Question 43

(Performance Limitations)
Does the aircrafts stall speed change as it climbs?

Answer 43

the airspeed is really just a pressure indicator that tells the pressure of air as it enters the front of the pitot tube minus the ambient static pressure. this is called static pressure. As the aircraft climbs higher the density of the air decreases. This decreases the air pressure entering the pitot tube unless the speed of the aircraft through the air is increased.

60 kts of air over the wing feels the same at any altitude to the wing. What does change while climbing at a fixed indicated airspeed is true airspeed.

As the aircraft climbs the indicated stall speed stays the same as the true airspeed increases.

True airspeed is the speed of the aircraft through the air.

Question 44

(Performance Limitations)
Explain aircraft stability

Answer 44

Static stability and dynamic stability are the two categories which can be further broken down into positive, neutral, or negative stability.

Static stability: the initial tendency of the aircraft when the flight path is disturbed.
Positive static stability means that if the aircraft is in a turn and you release the controls than it will come out of the turn. it won’t continue to bank steeper or stay in the bank that was set.

Dynamic stability: describes how the aircraft behaves over time after a disturbance. Must have positive static stability in order to have dynamic stability. Since it needs to at least make an attempt to return to its initial state of equilibrium. If the plane is statically unstable it will make no attempt to return to equilibrium.
Positive dynamic stability means that over time as the statically stable aircraft attempts to return to equilibrium any undershoots or overshoots will dampen out over time before finally returning to the aircrafts initial state.

Question 45

(Performance Limitations)
Explain the aircrafts left turning tendencies

Answer 45

Acronym: PASTG
P-factor:
Accelerated Slipstream
Spiraling slipstream
Torque
Gyroscopic precession

P-Factor:
caused by the descending part of the propeller having a higher thrust factor than the ascending blade.
asymmetric thrust in particular. One side of the propeller is getting higher thrust or a bigger bite of air in a climb/high angle of attack.
causes left yaw and is countered by right rudder in a climb.

Accelerated Slipstream:
more air is being pushed through the right side of the propeller caused by P-Factor. This creates a left rolling tendency due to the artificial increase of lift on the right hand wing thus increasing lift only on one side of the plane.
Rolling tendency

Spiraling Slipstream: (PHAK Ch.5 5-31 the Corkscrew Effect)
causes left yaw and right roll.
generated by the spiraling effect of the propellers backwash. The air spirals around the fuselage of the aircraft starting at the propeller.
low speeds and high power settings.

Torque:
Most commonly experienced on the ground.
The propeller spins clockwise. Newton’s third law. The left spinning of the propeller makes the engine want to turn right.
Causes left roll in the air
on the ground it causes left yaw.

Gyroscopic precession:
Causes right yaw in a climb, left yaw in a descent.

Gyroscopic principles:
Rigidity in space
Precession: any force applied is felt 90 degrees downstream and in the same direction.