Manfolk.net - Lesson 2.2 Airfoils, Lift & Angle of Attack

Lesson 2.2 – Airfoils, Lift & Angle of Attack

In Lesson 2.1 we learned that the wings create the upward force called lift. In this lesson we look more closely at how the wing does that, and why angle of attack is the most important concept in aerodynamics.

Reference:

FAA Pilot’s Handbook of Aeronautical Knowledge (PHAK), Chapter 5 – Aerodynamics of Flight

https://www.faa.gov/sites/faa.gov/files/07_phak_ch5_0.pdf

What is an Airfoil?

An airfoil is any surface designed to produce lift when it moves through the air. The wing of your airplane is an airfoil. The propeller blades are also airfoils.

The most important parts of an airfoil are:

Leading edge – the front of the wing
Trailing edge – the back of the wing
Chord line – an imaginary straight line from the leading edge to the trailing edge
Camber – the curve of the upper and lower surfaces of the wing

How the Wing Creates Lift

Two things work together to create lift:

The angle at which the wing meets the oncoming air.

As the wing moves through the air, it pushes air downward. By Newton’s third law, the air pushes the wing upward. This upward push is lift. Air is smoothly directed in a downward flow and air pressure incleases beneath the wing.

2. Pressure differential created by the shape of the wing (airfoil).

A Short History of Bernoulli’s Principle:

In 1738 the Swiss mathematician Daniel Bernoulli published Hydrodynamica. He showed that in a moving fluid, speed and pressure trade places: where the fluid speeds up, the pressure drops. That statement is now called Bernoulli’s principle (sometimes Bernoulli’s law). Giovanni Battista Venturi later built the tube that makes the idea visible—a wide inlet, a narrow throat, a wide outlet. Air must speed up in the throat to get the same mass through the pinch. Pressure in the throat falls. That low pressure is what pulls fuel from a carburetor jet, what an old wing-mounted venturi used to drive gyro instruments, and part of what a cambered wing does to the air on top.

Weight, Load Factor, and Angle of Attack

To maintain level flight, the wings must produce lift equal to the weight of the airplane. If the airplane is heavier, or if you increase the load factor (for example by turning), the wings must produce more lift. The pilot usually does this by increasing the angle of attack. This is one reason stall speed becomes higher when the airplane is heavy or when it is under higher G-loading.

Critical Angle of Attack

Every wing has a critical angle of attack (usually around 16°–20° for a typical light airplane).

When the wing reaches its critical angle of attack:

The smooth airflow over the top of the wing breaks down and separates
Lift decreases sharply
The wing stalls

Very important:

A wing always stalls at the same critical angle of attack, regardless of airspeed, attitude, or weight.

Angle of Attack

Angle of Attack (AOA) is the angle between the chord line of the wing and the relative wind (the direction of the air flowing toward the wing).

This is the single most important idea in this lesson:

Lift is controlled primarily by angle of attack and airspeed.
The pilot controls angle of attack mainly with the elevator.

The following chart is taken directly from the FAA Private Pilot written exam:

FAA Pilot’s Handbook of Aeronautical Knowledge, Chapter 5 https://www.faa.gov/sites/faa.gov/files/07_phak_ch5_0.pdf

Key Points to Remember

The wings generate lift.
Angle of attack is the angle between the chord line and the relative wind.
Raising the nose increases angle of attack.
Exceeding the critical angle of attack causes a stall.
You can stall the airplane at any airspeed and any attitude if you exceed the critical angle of attack.

Quick Facts About Aircraft Venturis

External venturi tubes.

Fast air in the throat = low pressure.

The suction is used to drive the vacuum gyros.

A wing creates lift the same way a venturi creates suction

I chose to use the venturi example as it applies to your job as Pilot in Command. It is critical to understand your instrumentation. Many classic aircraft do nto have an artificial horizon. Those that do are often equipped with a venturi rather than a modern vacuum pump. A venturi requires airspeed to create vacuum. A gyro needs time to spool up.

Departing in an aircraft without an operational attitude indicator without a pronounced horizon outside (city lights etc) is highly dangerous. A hazy take-off or over water can be dangerous. Some years after I trained a good pilot, he bought a nice Piper Tri-Pacer. He departed off of a nice hay field far enough from town to not have an adequate horizon. His gyro's had not spooled up. He and his family died on that hazy summer night.

The Shape: It features a wide opening at the front, a pinched center section (the throat), and a flared rear exhaust.
The Purpose: It generates a vacuum using Bernoulli's principle. As the airplane flies, fast-moving air passes through the pinched neck, creating a low-pressure suction.
The System: This suction pulls air directly out of the cockpit's gyroscopic flight instruments (like the attitude indicator or directional gyro), spinning up internal rotors without requiring an engine-driven pump or electrical system.
The Limitation: The primary drawback is that the aircraft must be airborne and moving fast enough before the vacuum is strong enough to make the dashboard instruments function reliably. Also, it takes a bit of time for the gyro's to spool up to speed.