How Airplanes Fly (Private Pilot)

So how do airplanes fly? With a little magic…

Not really!

Structure of the Atmosphere

The atmosphere contains air, which has mass, weight, and shape; it surrounds the Earth and is made-up of a mixture of gases. Earth’s atmosphere is made-up of several different gases. Most of the oxygen in the atmosphere is contained below 35,000 feet.

Air:

Air and other gases are fluids, just like a liquid. Fluids have the ability to flow and fill containers as well as having the ability to fill an available volume of containers. With that being said, we need to understand the properties of fluids (air) to understand the principles of flight.

Fluids create friction when they flow around an object.  As air flows around a wing, it has friction even though the surface appears very smooth.

Viscosity:

Viscosity is when a fluid begins to resist flowing, in another words the thicker the fluid is the more resist to flowing it becomes.

An example of this is oil and water. Water flows freely in one direction while the oil flow much slower in the other direction.

Friction:

Friction plays another factor on the flow of fluids over and around an object. When any two materials come in to contact with each other, friction exists.  If n object or surface is rough and the other is smooth, the flow differs significantly.

A good example is the surface of an aircraft wing. If you look at the surface of a wing with the naked eye or rub your hand along the surface, it appears to be smooth. However, if you look under a microscope magnified to 1,500x the surface of the metal wing is rough which causes resistance in the air flowing over the wing.

Air molecules will adhere to the rough surface of the wing, causing even more resists in the airflow over the wing. When this two factors occur, that is known as drag.

Atmospheric pressure:

Atmospheric pressure affects weather changes, allows aircraft to lift and gives information to important flight instruments. As mentioned earlier, air has weight. At sea level under standard conditions the atmosphere weighs approximately 14.7 pounds per square inch (psi). As you go higher in altitude the atmospheres weight at 18,000 feet is one-half than sea level.  

The atmospheric pressure varies with location. The reference for the standard atmospheric temperature at sea level is 15 degrees Celsius and the standard pressure at sea level is 29.92 inches of mercury.

Pressure altitude:

Is the altitude shown on the altimeter when the pressure is set to 29.92” This can be done at airports that do not have an ATIS or any other weather observation.

  • Simple set the pressure to 29.92" and ready

the altitude.

Density Altitude (DA):

We know that pressure altitude is 29.92” (1,013.2 mb) when measured with a barometer.

Density altitude is the vertical distance above sea level in a standard atmosphere. Density altitudeis pressure altitude corrected for nonstandard temperature. In calculating aircraft performance we use density altitude.

As the density of the air increases (low density altitude), the aircraft performance increases. The same goes for high density altitude, as the air density decreases (thin air), the aircraft performance decreases.

  • Let’s simplify it:

Low density air means thick air, found near sea level. If the DA is 5,000 feet at sea level, the aircraft will perform great, as if it was flying at 5,000 feet.

 

High density air means thin air, found on hot summer days. When the density altitude is high, on a hot summer day the aircraft performs poorly, as if it is at a higher altitude.

 

An example of poor aircraft performance on a high-density altitude day is, if we are taking off from Las Vegas (KLAS) in August and the altimeter is 29.76”, and the OAT (outside air temperature) is 43⁰C, this would give us a DA of 7312 feet. With that said, the airplane would perform as if it was flying at 7312 feet MSL.

Effect of Humidity on Density:

Humidity, also known as relative humidity is the amount of water vapor that can be held in the air, which is represented in percentage. As temperature increases the air can hold more water vapor. Saturated air can no longer hold any more water vapor, is known as 100% relative humidity.

Aircraft performance is effected by humidity, temperature, and pressures.

Humidity on Destiny altitude Example:

If the pressure altitude is 22.22” at 8,000 feet, and the temperature is 26°C (80°F) and dew point is 24°C (75°F), the density altitude (DA) would be 11,564 feet. If there was no humidity the DA would be 500 feet lower.

 

Theories of Producing Lift

Newton’s Laws:

Sir Isaac Newton formulated the law of universal gravitation and three basic laws of motion.

  • Newton’s First Law of motion –an object at rest will remain at rest unless a force strong enough to overcome it.

  • Second Law –“Force is equal to the change in momentum per change in time. For a constant mass, force equals mass times acceleration.” Simply put, a force applied to an object at rest, will cause the object accelerate in the same direction of that force.

  • Third Law –“For every action, there is an equal and opposite reaction.”

Bernoulli’s Principle:

Bernoulli’s Principle states that the velocity of a fluid increases, the pressure within that fluid decreases. With that said, Bernoulli’s Principle explains what happens to fluid (air) when it passes over the curved top of an aircraft wing.

Bernoulli’s Principle is applied to a venture tube. The tube has large inlet that narrows to a constricted point and returns back to a large outlet. For air to pass through the tube, the mass of air entering must equal the mass of air exiting.  When the passes through the constricted point, the air must increase in speed to allow the same amount of to pass through.

 

Airfoil design

An airfoil profile is a cross section of a wing. By looking at the cross section of an airfoil, it shows different curvatures called cambers.

The upper camber is more curved than the lower camber. There are two other features, the rounded end called the leading edge, which is the part that faces forward. The back end of the airfoil tapers down to an edge is called the trailing edge.

The chord line is an imaginer reference line drawn from the trailing edge to the leading edge that denoted the upper and lower cambers.

Airfoils are designed to force air flow underneath and over the top. If airfoils are design in a way that cause a lift actin that is greater than the weight of an aircraft, the aircraft will fly.

Pressures:

Remember Bernoulli’s Principle of Pressure, the airflow over the top of the airfoil increases in speed which drops the pressure, this is the component of lift. But does not account for the total lift. We need to apply Newton’s third law, when the air flows backward over the upper surface of the airfoil it travels downward, which produces an upward forward force on the airfoil.

The air flowing underneath the airfoil also produces a certain amount of lift, which results in a high pressure. When the pressure differences between the upper and lower surfaces of an airfoil increase total lift increases.

Third Dimension:

We know that most of the lift is produced by air flowing in two dimensions across the upper and lower surfaces of an airfoil. To make it even more confusing, a third dimension has an aerodynamic effect.

The third dimension of the airfoil is the tip. As air flows from the bottom (high pressure) around the tip to the top (low pressure), this creates a rotating air flow called a tip vortex.

Tip vortex create a downwash resulting in a reduction of lift.

 

For an example the heavier and slower the aircraft is, the greater the angle of attack (AOA) it is during flight, the stronger the vortices. When an aircraft is in the takeoff, climb or even in the landing phase it produces stronger vortices, which leads to dangerous wake turbulence.

To help reduce the tip vortex winglets can be added to the top or bottom of an airfoil tip. Winglets act like a dam to prevent the vortex from forming.

 

References

Private Pilot ACS PA.I.F.K1 - PA.I.F.K2a, PA.I.F.K3

PHAK Ch. 4

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