Questions and Answers
What is viscosity?
What are 2 unique fluid properties?
Viscosity and the ability to flow
What is the viscosity of air?
Air has a low viscosity, which means it flows easily.
What is friction?
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What causes resistance of airflow over the wing?
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What is the boundary layer and what does it cause?
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What is pressure?
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Why are pilots concerned about atmospheric pressure?
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What is the standard pressure at sea level under standard conditions?
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What is the standard temperature lapse rate?
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What is pressure altitude (P.A.)?
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How can pressure altitude be calculated?
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What is density altitude (D.A.)?
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How does density altitude (D.A.) affect aircraft performance?
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How can density altitude (D.A.) be calculated?
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How can atmospheric conditions affect density altitude?
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What is humidity?
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How is relative humidity expressed?
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What does Bernoulli's Principle state?
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What is a chord line?
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What causes lift?
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Study Notes
Viscosity
- Represents the resistance of a fluid (including air) to deformation when stress is applied.
- Viscosity is crucial for understanding fluid dynamics and how air interacts with surfaces.
Unique Fluid Properties
- Viscosity: Resistance to flow.
- Ability to Flow: Characterized by how easily a fluid can move.
Viscosity of Air
- Air is categorized as having low viscosity, described as "thin," allowing it to flow easily.
Friction
- Refers to the resistance encountered when one surface moves over another.
- Exists at the interface of any two contacting materials.
Resistance of Airflow Over Wings
- Surface roughness of the wing contributes to airflow resistance and decreases the air velocity around it.
Boundary Layer
- A layer of air molecules adhesion to the wing surface due to friction.
- Increased resistance occurs due to viscosity; when combined, they create drag, affecting flight performance.
Pressure
- Defined as the force exerted perpendicularly to a surface.
Importance of Atmospheric Pressure for Pilots
- Influences weather changes and aircraft lift.
- Vital for the operation of key flight instruments.
Standard Atmospheric Pressure
- Standard sea-level pressure is 14.7 PSI, 1013.2 mb, 29.92 inches of Hg at 15 °C (59 °F).
Standard Temperature Lapse Rate
- Temperature decreases by 2 °C or 3.5 °F per 1,000 ft up to 36,000 ft; remains constant from 36,000 ft to 80,000 ft.
- This is formally referred to as the International Standard Atmosphere (ISA).
Pressure Altitude (P.A.)
- Defined as the height above a standard datum plane (SDP) with an atmospheric pressure of 29.92 inches of Hg.
- Varies depending on changes in atmospheric pressure.
Calculating Pressure Altitude
- Achieved by setting the altimeter to 29.92 and reading the indicated altitude.
- Alternatively, P.A. can be calculated with the formula: ( P.A. = (29.92 - \text{altimeter setting}) \times 1000 + \text{altitude} ).
Density Altitude (D.A.)
- Calculated by correcting pressure altitude (P.A.) for temperature variations from standard conditions.
Effects of Density Altitude on Aircraft Performance
- Reduced air density leads to decreased engine power, thrust, and lift:
- Power: Engines take in less air.
- Thrust: Propeller efficiency diminishes.
- Lift: Lower air density decreases the force exerted on wings.
Calculating Density Altitude
- D.A. can be computed using the formula: ( D.A. = P.A. + [(OAT - ISA \text{ Temperature}) \times 120] ).
Atmospheric Conditions Influencing Density Altitude
- High Density (thin air) scenarios: elevated elevation, high temperature, high humidity, and low atmospheric pressure.
- Low Density (dense air) scenarios: lower elevation, low temperature, low humidity, and high atmospheric pressure.
Humidity
- Refers to the amount of water vapor present in the atmosphere.
Relative Humidity
- Expressed as a percentage of the maximum water vapor the air can hold; varies with temperature.
- Warm air can hold more moisture, whereas cold air holds less.
Bernoulli's Principle
- States that an increase in the velocity of a moving fluid results in a decrease in pressure within the fluid.
Chord Line
- A straight reference line extending from the wing's trailing edge to its leading edge, crucial for aerodynamic analysis.
Causes of Lift
- Generated through both Bernoulli's Principle and Newton's Laws.
- The shape of the airfoil causes air to accelerate over the top and decelerate underneath, creating pressure differentials that generate lift.
- Newton's third law also applies as the downward flow from the wing's top generates an upward force by interacting with the airflow beneath the wing.
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Description
Test your understanding of key concepts in fluid properties and viscosity as covered in PHAK Chapter 4. This quiz focuses on the essential terms and definitions that relate to the principles of flight. Prepare yourself with these flashcards to enhance your aviation knowledge.
