PHAK 4: Principles of Flight Flashcards
Most of the atmosphere’s oxygen is contained below…
35,000 feet
Air is a…
Fluid
Fluids take on the shape of…
Their containers
What is viscosity?
The measure of a fluid’s resistance to flow
What determines how much a fluid resists flow.
The way individual molecules of the fluid tend to adhere, or stick, to each other.
Higher viscosity is…
Thicker
Lower viscosity is…
Thinner
What is friction?
The resistance that one surface or object encounters when moving over another.
What is the boundary layer?
The thin layer of air that flows directly over a wing’s surface, where air speed changes from zero (at the surface) to the free stream velocity of the surrounding airflow.
What is pressure?
The force applied in a perpendicular direction to the surface of an object.
How is pressure often measured?
Pounds of force exerted per square inch of an object, or PSI.
An object completely immersed in a fluid will feel…
Pressure uniformly around the entire surface of the object.
If the pressure on one surface of the object becomes less than the pressure exerted on the other surfaces, the object will…
Move in the direction of the lower pressure.
Which flight instruments are actuated by atmospheric pressure?
- Altimeter
- Airspeed indicator
- Vertical speed indicator
- Manifold pressure gauge
Under standard conditions at sea level, what is the approximate average pressure exerted by the weight of the atmosphere?
- 14.7 psi
- 1,013.2 mb
The weight of the atmosphere at 18,000 feet is…
One-half what it is at sea level.
What is the standard atmosphere at sea level?
- 29.92 “Hg at 59 °F
- 1013.2 mb at 15 °C
Lapse Rate
Change in temperature over altitude.
Standard Temperature Lapse Rate
2 °C or 3.5 °F per 1,000 feet
up to 36,000’
What is the temperature from 36,000’ to 80,000’
Approximately –65 °F or –55 °C.
What is the standard decrease of Hg with altitude?
Approximately 1 “Hg per 1,000 feet up to 10,000’
What does ISA stand for?
International Standard Atmosphere
Any temperature or pressure that differs from the standard lapse rates is considered…
Nonstandard temperature and pressure.
What does SDP stand for?
Standard Datum Plane
What is pressure altitude?
The height above an SDP, which is a theoretical level where the weight of the atmosphere is 29.92 “Hg (1,013.2 mb) as measured by a barometer.
The pressure altitude can be determined by one of the following two methods:
- Setting the barometric scale of the altimeter to 29.92 and reading the indicated altitude.
- Applying a correction factor to the indicated altitude according to the reported altimeter setting.
What is density altitude?
The vertical distance above sea level in the standard atmosphere at which a given density is to be found.
How do you find density altitude?
Pressure altitude corrected for nonstandard temperature.
The density of air has significant effects on the aircraft’s performance because as air becomes less dense, it reduces: (3 things)
- 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.
As the density of the air increases (lower density altitude), aircraft performance…
Increases
As air density decreases (higher density altitude), aircraft performance…
Decreases
Is high density altitude more or less dense?
Less dense
Is low density altitude more or less dense?
More dense
What type of altitude does the airplane altimeter display?
Pressure Altitude
Air density is affected by changes in
- Altitude
- Temperature
- Humidity
Density varies ______ with pressure and ______ with temperature,
directly, inversely
If the pressure is doubled, the density is doubled; if the pressure is lowered, the density is lowered. This statement is true only at a constant temperature.
Increasing the temperature of a substance decreases its density. Conversely, decreasing the temperature increases the density. Thus, the density of air varies inversely with temperature. This statement is true only at a constant pressure.
Increasing the temperature of a substance ______ its density.
Decreases
Decreasing the temperature of a substance ______ its density.
Increases
In the atmosphere, both temperature and pressure decrease with altitude and have conflicting effects upon density. Which is the more dominant effect?
Pressure
As the water content of the air increases, the air becomes ______ dense, ______ density altitude and ______ performance.
Less, increasing, decreasing
What 3 things have a great influence on aircraft performance because of their effect upon density.
- Pressure
- Temperature
- Humidity
Newton’s First Law:
Inertia
“Every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed on it.”
Newton’s Second Law:
Newton’s Second Law of Motion states:
The acceleration of an object is directly proportional to the net force acting on it, inversely proportional to its mass, and occurs in the direction of the net force.
Mathematically, it is expressed as:

Where:
•  is the net force applied to the object (in newtons, ),
•  is the mass of the object (in kilograms, ),
•  is the acceleration of the object (in meters per second squared, ).
This law explains how the motion of an object changes when a force is applied. For example:
• If you push a lightweight object (small ) with the same force as a heavy object (large ), the lightweight object will accelerate more.
• The greater the force applied to an object, the greater its acceleration, assuming mass is constant.
Newton’s Third Law:
Newton’s Third Law of Motion states:
For every action, there is an equal and opposite reaction.
This means that whenever one object exerts a force on a second object, the second object exerts a force of equal magnitude and in the opposite direction on the first object.
Key Points:
• These forces always come in pairs, known as action-reaction pairs.
• The forces act on different objects, not the same object, so they do not cancel each other out.
Examples:
1. Walking: When you push backward on the ground with your foot, the ground pushes forward on you, allowing you to move forward.
2. Rocket propulsion: The rocket expels gas downward (action), and the gas pushes the rocket upward (reaction).
3. Jumping off a boat: When you jump forward off a boat, the boat moves backward due to the reaction force.
Bernoulli’s Principle
A principle that explains how the pressure of a moving fluid varies with its speed of motion. An increase in the speed of movement causes a decrease in the fluid’s pressure.
What is the chord line?
An imaginary straight line drawn through an airfoil from the leading edge to the trailing edge.
An imaginary straight line drawn through an airfoil from the leading edge to the trailing edge.
Chord line
Line drawn from the leading edge to the trailing edge of a wing.
Equidistant at all points from the upper and lower surfaces.
Mean camber line
Mean camber line
Line drawn from the leading edge to the trailing edge of a wing.
Equidistant at all points from the upper and lower surfaces.
What is 1?
Mean camber line
What is 2?
Trailing edge
What is 3?
Leading edge
What is 4?
Chord line
What is 1?
Early airfoil
What is 2?
Later airfoil
What is 3?
Clark ‘Y’ airfoil
(Subsonic)
What is 4?
Laminar flow airfoil
(Subsonic)
What is 5?
Circular arc airfoil
(Supersonic)
What is 6?
Double wedge airfoil
(Supersonic)
Is the velocity of air over the top of an airfoil faster or slower than the bottom?
Faster
Is the velocity of air under the bottom of an airfoil faster or slower than the top?
Slower
Is the pressure of air over the top of an airfoil higher or lower than the bottom?
Lower
Is the pressure of air under the bottom of an airfoil higher or lower than the top?
Higher
What does CP stand for?
Center of Pressure
What is the CP?
A point along the wing chord line where lift is considered to be concentrated. For this reason, the center of pressure is commonly referred to as the center of lift.
At high angles of attack, the CP moves ______, while at low angles of attack the CP moves ______.
forward, aft
What does an airplanes CP govern?
Aerodynamic balance and controllability
What is a wing tip vortex?
The high pressure area on the bottom of an airfoil pushes around the tip to the low-pressure area on the top. This creates a rotating flow called a tip vortex. The vortex flows behind the airfoil creating a downwash that extends back to the trailing edge of the airfoil. This downwash results in an overall reduction in lift for the affected portion of the airfoil.
