Principles Of Flight PHAK Ch. 4-5

Question 1

What are the 2 forces that create drag?

Answer 1

PHAK 4-2

1. Viscosity - when a fluid (air is a fluid) flows it has has a slight resistance
2. Friction - all surface at the micro level have a certain roughness… at the surface of the wing (boundary layer) the air molecules have a relative velocity of zero.

Question 2

What is the average atmospheric pressure in standard conditions?

Answer 2

Standard conditions is at sea level of 15 Celsius (59 F) and a surface pressure of 29.92 inches of mercury (Hg) or 1013.2 millibars (mb) or 14.70 psi.

Question 3

What is the standard temperature lapse rate?

Answer 3

2 C per 1000 ft up to 36000 ft approximate temp is -65 - -55 C.

Question 4

What is a standard pressure lapse rate?

Answer 4

Pressure decreases at a rate approximately 1 Hg (inches of mercury) per 1000 ft of altitude gain to 10,000 ft.

Question 5

What is used for correlating aerodynamic performance in a nonstandard atmosphere?

Answer 5

Density Altitude

Question 6

What affect does less dense air (high density altitude) have on aircraft performance?

Answer 6

Power - because the engine takes in less air
Thrust - because a propeller is less efficient in thin air
Lift - because the thin air exerts less force on the airfoils

Question 7

What is air density affected by?

Answer 7

Affected by changes in altitude, temperature, and humidity.

Question 8

What conditions result in high density altitude?

Answer 8

High elevation
High pressure altitude
High temperatures
High humidity

Question 9

Newtons 1st law

Answer 9

Note: not exact definition…

Nothing starts or stops moving until an outside force causes it to do so.

Question 10

Newtons 2nd law

Answer 10

“Force is equal to the change in momentum per change in time. For a constant mass, Force equals mass times acceleration “

Question 11

Newtons 3rd law

Answer 11

“For every action, there is an equal and opposite reaction.”

Question 12

Bernoullis principle

Answer 12

“As velocity of a moving fluid increases, the pressure within the fluid decreases”

My explanation…
This is what creates faster moving air over an airfoil causing low pressure over the curved part of the airfoil because the air pressure must equal to the beginning part of the airfoil to the aft of the airfoil.

Question 13

Where does the chord line begin and end.

Answer 13

The leading edge to the trailing edge.
This is a straight line drawn.
With flaps extended the straight line drawn changes angle of attack.

Question 14

What is Center of Pressure or CP?

Answer 14

The average pressure variation of any given AOA…
Aerodynamic forces acts through this CP.
High AOA CP moves forward.
Low AOA CP moves aft.
An airplanes aerodynamic balance and controllability are governed by changes in the CP.

Question 15

What is a tip vortex?

Answer 15

At the tip or edge of the wing, the high pressure below tries to reach the low pressure on top.
This creates a downwash resulting in a reduction of lift.
Winglets or tapering the airfoil tip reduces the pressure differential and smoothing the airflow around the tip.

Question 16

What are the four forces acting on an aircraft?

Answer 16

Thrust
Drag
Lift
Weight

Question 17

When are the opposing forces equal to each other?

Thrust=Drag)(Lift=weight

Answer 17

In straight, level, unaccelerated flight. To include the previous statement while ascending/descending.

Question 18

Definition of Angle of Attack (AOA)

Answer 18

The acute angle between the chord line of the air foil and the direction of the relative wind.

Question 19

What are the different types of drag?

Answer 19

Drag is resist movement of an aircraft through the air.
1. Parasite drag - does not help flight
Form drag - due to aircraft shape and airflow around it (cowling, antennas.)
Interference drag - 2 surfaces meet at perpendicular angles, (fuselage/wing)
Skin friction drag - all surface are rough at microscopic layers.
2. Induced drag - is the result of an airfoil developing lift.

Question 20

Understanding skin friction in layers of air flow.

Answer 20

1. Boundary layer - air is virtually motionless (as thick as a playing card)
   - creates an “effective” shape that can be slightly different than the physical shape. This causes a dramatic decrease in lift and in increase in drag.
2. Free-stream velocity- each layer above the molecules move faster until the molecules are moving at a velocity of the air moving around the aircraft.

Question 21

Induced drag is?

Answer 21

Is inherit whenever an airfoil is producing lift.

1. Whenever you have more downwash (vortices at the tips) your lift vector points back more causing induced drag.

Question 22

What is ground effect?

Answer 22

The air that is trapped in between the wing and the surface acting as a cushion of air.
This is because high pressure (below wing) reaching low pressure (above wing) is stopped by hitting the ground.

Question 23

What are the axes of the aircraft and what do they pass through?

Answer 23

Lateral - pitching - elevators
Longitudinal - rolling - ailerons
Vertical - yawing - rudder

The lines pass through the Center of Gravity (CG).

Question 24

What is stability and what are the two types?

Answer 24

Stability is the inherit quality of an aircraft to correct for conditions that may disturb it equilibrium and to return to or to continue on its original flight path.

1. Static
2. Dynamic

Question 25

What is static stability?

Answer 25

Refers to the the initial tendency, or direction of movement, back to equilibrium… this is the initial response when disturbed from a given pitch, yaw, or bank.

Question 26

What is dynamic stability?

Answer 26

Refers to the aircrafts response over time when disturbed from a given pitch, yaw, or bank.

Question 27

What are the subtypes of static and dynamic stability.

Answer 27

Positive
Neutral
Negative
Look at PHAK 5-15 for visual reference and definition of above subtypes.

Question 28

What is longitudinal stability? And which axes does it affect?

Answer 28

Is the quality that makes the aircraft stable about its lateral (pitch) axes.

Question 29

What is lateral stability?

Answer 29

Stability about its longitudinal axes (roll).
Positive lateral stability is when the wing that is lower creates more lift and the high wing decreases lift which helps it return to level flight.
This is also due to gravity pulling the aircraft down acting like a pendulum- keel affect

Question 30

What are the 4 main design factors that make an aircraft stable?

Answer 30

Dihedral - wings angled upward.
Sweepback - about 10 degree swept back provides 1 degree of dihedral.
Keel effect - tendency to turn the longitudinal axes of aircraft into relative wind
(High wing aircraft always has this tendency)
Weight distribution - the greater portion of keel area is above the CG.

Question 31

What is directional stability?

Answer 31

Stability about the vertical axes is called yawing or directional stability.

Question 32

How does an aircraft turn? What are the forces in play?

Answer 32

The horizontal component and vertical component make up the total lift to turn in the direction of bank.
Vertical component needs to be equal to the weight in order to maintain altitude which means the AOA must be increased (pitch up). However, an increase in AOA means an increase in drag thus requires more thrust (power) to maintain airspeed.

Question 33

What are the left turning tendencies?

Answer 33

1. Torque
2. Corkscrew effect
3. Gyroscopic reaction
4. Asymmetric load (p-factor)