Pilot's Handbook of Aeronautical Knowledge
Chapter 4: Principles of Flight
To control an aircraft, the pilot must understand fundamental physical laws governing the forces acting on an aircraft in flight and learn to use or counteract these principles.
Air, like liquids, is a fluid. Fluids take on the shape of their containers and generally do not resist deformation when even the smallest stress is applied, or they resist stress only slightly. This resistance is viscosity — the property of a fluid to resist flowing. Air has low viscosity. However, all fluids have some viscosity, including air.
Friction is the resistance that one surface or object encounters when moving over another. The roughness of a surface increases friction. When molecules of air pass over the surface of the wing, they adhere to the surface because of friction. The layer of molecules that adhere to the wing surface is referred to as the boundary layer. Air beyond the boundary layer tends to adhere, because of viscosity (the air tends to stick to itself). Both of these forces create drag — the resistance of air to pass over (and under) the wing.
Air has mass, is affected by gravity, and can have force. Since it's a fluid, the force is exerted in all directions. The ICAO has established a standard atmosphere, due to variable temperature and pressure at different locations. Any temperature or pressure that differs from the standard lapse rates is considered nonstandard temperature and pressure. Pressure altitude is the height above the Standard Datum Plane (SDP) — a theoretical level where the weight of the atmosphere is 29.92" Hg. If an altimeter is set for 29.92", the altitude indicated is the pressure altitude.
Pressure is the force applied in a perpendicular direction to the surface of an object. An immersed object will experience pressure uniformly. When pressure is asymmetrical, the object will move in the direction of the lower pressure. Because of its weight and fluidity, air exerts a force equally in all directions. Its effect on bodies within the air is called atmospheric pressure.
Atmospheric pressure actuates the altimeter, airspeed indicator, vertical speed indicator, and manifold pressure gauge.
Air has mass and is affected by gravity. Under standard conditions at sea level, the average pressure exerted by the weight of the atmosphere is approximately 14.70 pounds per square inch (psi) of surface (1,013.2 mb). The thickness of the atmosphere is limited. The weight of the atmosphere at 18,000 feet is one-half what it is at sea level.
The International Standard Atmosphere (ISA, or ICAO Standard Atmosphere) establishes a standard temperature lapse rate with a decrease of 2º C (3.5º F) per thousand feet up to 36,000 feet. The standard pressure lapse rate establishes a pressure decrease of approximately 1" Hg per 1,000 feet up to 10,000 feet.
The Standard Datum Plane (SDP) is a theoretical level where the weight of the atmosphere is 29.92" Hg. The SDP may be below, at, or above sea level. When an altimeter is set for 29.92" , the altitude indicated is the pressure altitude. Pressure altitude also can be determined by applying a correction factor to the indicated altitude according to the reported altimeter setting.
Density altitude is the vertical distance above sea level in the standard atmosphere at which a given density is to be found. Density altitude is pressure altitude corrected for nonstandard temperature. As the density of the air increases (lower density altitude), aircraft performance increases; conversely as air density decreases (higher density altitude), aircraft performance decreases. A decrease in air density means a high density altitude; an increase in air density means a lower density altitude.
The computation of density altitude involves consideration of pressure (pressure altitude) and temperature. Density altitude is determined by first finding pressure altitude, and then correcting this altitude for nonstandard temperature variations. Regardless of the actual altitude, aircraft will perform as though operating at the existing density altitude.
Air density is affected by changes in altitude, temperature, and humidity. High density altitude refers to thin air, while low density altitude refers to dense air. (Note: "high density" does not mean "more dense".) High elevations, low atmospheric pressures, high temperatures, and high humidity contribute to high density altitude.
A greater or lesser amount of air can occupy a given volume, since air is a gas that can be compressed or expanded. A column of air will contain less mass at lower atmospheric pressure because it expands and is less compressed — density is decreased, since density is directly proportional to pressure (thus, it is "low density" at "high density altitude"). This remains true when temperature is constant.
Increasing temperature will decrease density, and decreasing temperature will increase density. This remains true when pressure is constant. Thus, the air will be more dense when the temperature is lower.
Because temperature and pressure decrease with a gain in altitude, they have conflicting effects upon each other. The reduced temperature will make the air more dense, but the reduced pressure will make the air less dense. However, the rapid drop in pressure is more dominant. Typically, density decreases at higher altitudes, despite the lower temperatures.
As the humidity (water content) of the air increases, the air becomes less dense, increasing density altitude and decreasing performance. Relative humidity refers to the amount of water vapor contained in the atmosphere, expressed as a percentage of the maximum amount of water vapor the air can hold. Warmer air can hold more water vapor compared to cold air. High humidity increases density altitude, but it is not a dominant factor. Various formulas can be used to measure humidity's effect on density altitude and aircraft performance.
Lift
Lift from the wing that is greater than the force of gravity, directed opposite to the direction of gravity, enables an aircraft to fly.
Bernoulli‘s Principle states that as the velocity of a moving fluid (liquid or gas) increases, the pressure within the fluid decreases. Air sent through a venturi tube will decrease in pressure as it passes through the narrowest part of the tube.
An airfoil is shaped to cause an action on the air, and forces air downward, which provides an equal reaction from the air, forcing the airfoil upward.
Increased velocity reduces the pressure above the airfoil. However, the pressure difference between the upper and lower surface of a wing alone does not account for the total lift force produced. The downward backward flow from the top surface of an airfoil creates a downwash, meeting the flow from the bottom of the airfoil at the trailing edge. Applying Newton‘s third law, the reaction of this downward backward flow results in an upward forward force on the airfoil.
The average of the pressure variation for any given AOA is referred to as the Center of Pressure (CP). Aerodynamic force acts through this CP. At high angles of attack, the CP moves forward, while at low angles of attack the CP moves aft.
The production of lift is much more complex than a simple differential pressure between upper and lower airfoil surfaces. As an airfoil moves through air, the airfoil is inclined against the airflow, producing a different flow caused by the airfoil‘s relationship to the oncoming air.
The high- pressure area on the bottom of an airfoil pushes around the tip to the low-pressure area on the top. This action 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 results in an overall reduction in lift for the affected portion of the airfoil.
Commercial Pilot & Flight Instructor Test Questions
Which statement relates to Bernoulli's principle? Air traveling faster over the curved upper surface of an airfoil causes lower pressure on the top surface.
An aircraft's wing is designed to produce lift resulting from positive air pressure below the wing's surface and negative air pressure above the wing's surface.
— In this example, the lower pressure is referred to a "positive" pressure, i.e. "high" pressure or "more" pressure.
The point of an airfoil through which lift acts is the center of pressure.
Why does increasing speed also increase lift? The increased impact of the relative wind on an airfoil's lower surface creates a greater amount of air being deflected downward.