March 23, 2018, ©. Leeham News: In the last Corner, we finished our series about aircraft drag, by studying an airliner flying a mission and noting how the drag changed.
Before we leave the subject of airliner aerodynamics, we shall recap how lift is produced.
The best way to describe how lift is generated on an aircraft and its wings has been debated over the years. There are two ways to explain it and they are the two sides of the same coin.
One can explain lift as the sum of the differences in pressure on an aircraft or a wing. Figure 1 shows the pressure distribution of a Boeing 787 during cruise.
This is a practical method, as static pressure working on an aircraft’s surface is easy to measure. Drill a small hole and connect a tube leading to a pressure sensor and you have the static pressure (if the flow is passing the hole and not blowing into it).
We will come to why the air creates different pressures on an airliner, but first the alternative method to explain lift.
We learned about Newton’s three laws in school. They rule how objects are reacting to force and are not complicated:
First law: An object either remains at rest or continues to move at a constant velocity unless acted upon by a force.
Second law: The force on an object is equal to the mass of that object multiplied by the acceleration of the object: Force = Mass * Acceleration
Third law: When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.
Now we have our bodies: the aircraft and the air molecules. We think of air as light. It’s not really, it weighs 1.2kg per m3 at ground level (assuming we are close to sea level).
When the air is passing the aircraft, it’s forced to curve around the fuselage and the wings top and bottom surfaces. The curving means the air is accelerated, meaning the air’s mass plus the acceleration is exerting a force on the aircraft and the aircraft is exerting a force on the air molecules (Newton’s second and third law).
When the air is accelerated downwards over the wing, this acceleration is caused by the wings curvature forcing the air to curve downwards. The opposing force is the wings lift.
You can easily check this yourself. Put out your hand through the side window of the car at speed and angle it so it curves the air down or up. The mass of the air forces you hand up or down. Newton’s second and third law is at play.
In practice, it’s not easy to measure the minute acceleration the air’s molecules are subject to when curving around an aircraft. Therefore, the use of Newton’s laws and the fact one is flying on the downwash of air is seldom used as an explanation. As pressure and the speed of air is simpler to measure, one works with this method to explain and measure lift.
The pressure and speed of air are coupled. Air which is moving has kinetic energy (manifested by the dynamic pressure) and potential energy (manifested by the static pressure). The sum of these is constant (called Total pressure) if the aircraft is flying in non-compressible air flow (below M0.5).
So we can either measure the static pressure with the simple method I gave or we can measure the airflow’s speed and through:
static pressure + dynamic pressure = total pressure
we can calculate the other values. But the details of this is for the experts.
The simple rules we have to remember of all this are:
Let’s now use this simple method to understand what is happening at the nose of the 787 in Figure 1. It’s a Computer Fluid Dynamic output picture with the colours showing different static pressures. Green shows the lowest static pressure and yellow the highest.
First, the air is hitting the nose head on. This slows the air down and the static pressure increases. We have yellow and gradually, as the air is more diverted along the nose, we have a lower orange and red pressure changing to violet regions before the crest of the nose.
As the air is forced to curve over the convex top of the nose, leading to the fuselage’s cylindrical part, it increases the speed as the curving is convex. We get a lower pressure region (blue and light blue). We have lift created by the aircraft’s nose, in the region where the curving of the air speeds up the airflow.
The same happens over the wings. Here the curving over-the-top is so strong we get a green colour, showing a very low pressure.
In the next Corner, we will dig a bit deeper into lift explained with pressure.