In this post, we will be discussing the following topics; how the aircraft operate, How lift is generated in an aircraft?, what is thrust and how does it affect aircraft operations?, how an aircraft Combats/overcomes weight, Different phases of aircraft flight,
Aircraft are, simply put, one of the greatest inventions mankind has ever made. The idea is that a vehicle can transport people (and load) over thousands of miles in a matter of hours rather than the days, or even weeks, that it would take to traverse land and sea.
The idea is that, almost like birds, humans too can take on the skies and make their way through. And that is exactly how early aircraft were first developed: through the observation of birds.
What seemed impossible, like a myth, was proven to be possible through observation and the realization that dreams could also follow simple scientific principles. This article will address the simple scientific principles by which aircraft are able to fly.
Aircraft move through the air on the basis of four forces: weight, drag, thrust and LIFT. The latter is arguably the most relevant force in aviation, so much thought is often given to how to generate and maintain it.
Weight is the earth’s gravitational pull on a body. Drag is the resistance to motion a body experiences as it moves through air – sort of like friction for air. Thrust is the force that propels a body forward through the air; it is generated by the body’s powerplant (engine).
Lift is the upward acting force that the air exerts on an aircraft that keeps it in the air or raises it above ground. Lift acts against the aircraft’s weight which pulls it down, while thrust acts against drag which resists the aircraft’s moving forward.
Drag and weight exist on their own, while lift and thrust are forces that must be generated by the aircraft’s design. Lift and drag are called aerodynamic forces, “aero” meaning air and “dynamic” meaning changing. This means they are forces that have to do with the way the airflow interacts with the aerofoil change.
How Aircraft lift is generated
In an aircraft, the wings are the main generators of lift. The wings have a special cross-sectional shape called an aerofoil. Imagine viewing the wing of an aircraft from the side of the aircraft. The shape outline that you see is the aerofoil. It is the same consistent shape you get when you cut slices through the wing from the side.
The aerofoils of planes are positively cambered, meaning the top of the aerofoil is more convex (curved outwards) and therefore has a larger surface area than the bottom of the aerofoil. When a stream of air meets the tip of the aerofoil, it separates the top part and bottom part.
The airstream at the top has a longer distance to go to get to the other side of the aerofoil than the airstream at the bottom, however, they both have to get there at the same time. This means the airstream at the top has to move faster than the one at the bottom and consequently, there is lower pressure at the top than at the bottom.
The pressure-speed-camber relationship can be explained further with a simple analogy. Imagine two friends meet at the end of a road that breaks into two paths and then connects again in front. The first path is curved while the second is fairly straight but they are both to get to the front meeting point at the same time.
Friend X takes the first path while friend Y takes the second path. Friend X knows she has to run faster if she wants to meet up with Friend Y at the same time. If pressure is seen as the footprints they both leave on the sand paths as they run, we will see that Friend X’s footprints are shallower than Friend Y’s footprints.
This is because Friend X did not stay on any single point long enough for her footprint to deepen while Friend Y took his time, so his impact on points along the path would be more.
Back to the aerofoil. The higher pressure at the bottom means there is a more intense push by the air there than at the top and that causes the aerofoil to rise.
This is the basic principle of the wing. There are other aerofoils on aircraft such as the horizontal stabilizer, the rudder, and so on, but the wing is the main aerofoil. The idea of wings is clearly gotten from birds. The shape of their wings was observed and used as a guide on how airflow could be used to generate lift.
What is Thrust and How Does it Affect Aircraft Operations?
Thrust, as I said earlier, is generated by the aircraft’s powerplant AKA engine. There are different types of aircraft engines, but they all work on the same basic principle: using airflow to propel them forward.
For propeller- and turboprop-driven aircraft, a piston engine drives the propeller which spins the air as it moves and creates a sort of horizontal lift which moves the aircraft forward. Propeller blades are also aerofoils so imagine the principle of the wings working when they are placed vertically instead.
There is a higher pressure behind the propeller than in front so the lift force pushes it forward. For gas turbine engines, the air is taken into the engines, combusted with fuel, and then released through an exhaust nozzle.
The air leaving the nozzle exerts a force on the aircraft, but it also has its Newton Third Law force pair which acts forward. That force is the thrust that propels the aircraft forward. The air leaving the nozzle is of a (typically much) higher velocity, called the jet velocity, than the one entering the engine. Generally, the higher the jet velocity, the larger the thrust produced.
How an Aircraft Combats/overcomes weight
The weight of the aircraft pulls it down, while the main goal of the aircraft is to go or stay up. Consequently, a lot of thought is given to how to reduce the weight of aircraft.
Everything from using lightweight materials for the construction of the aircraft structure to minding the weight of the load carried aboard an aircraft – ergo, the luggage restrictions on the flight – acts to reduce the overall weight of the aircraft and make the flight more efficient.
One way weight affects the efficiency of flight is this:
• The greater the weight of the aircraft, the more lift the aircraft has to generate to fly.
• There is always a component of the lift generated by the aircraft that causes drag: lift-induced drag.
• The greater the drag (lift-induced), the more thrust the engine needs to produce to combat the drag and propel forward.
More and more research is being put into how to make aircraft lighter, especially as aircraft systems are becoming more sophisticated; sophistication comes with more systems and possibly more weight.
The lightweight structures used to make aircraft must not be too light that they reduce the structural integrity of the aircraft. The aerodynamic forces acting on aircraft are very strong so the materials used must be able to withstand them.
The most common material used for constructing aircraft is Aluminium, and composites are being increasingly used in modern aircraft. These materials are lightweight, without compromising too much on strength.
Before arriving at this point, materials such as wood and steel were tried and tested. The problem with wood was, though extremely light, it was not strong enough. The problem with steel was the opposite: it was extremely strong but too heavy.
When the aircraft’s lift matches its weight, the plane maintains a level altitude.
Different phases of Aircraft Flight
There are three main phases of flight that I will address in this post:
• Ascent (take-off and climb): During this phase of flight, the aircraft needs to go up so the lift generated needs to be higher than its weight. Aircraft usually climb gradually. They don’t get to their cruise altitude immediately.
During ascent, there is a lot of drag being generated. This is because of the combination of the attitude (pitch) of the aircraft, the large lift being generated, and the large air density (air density decreases with altitude).
There must be a lot more thrust generated by the aircraft to combat the drug. Take-off thrust is the largest amount of thrust the aircraft will produce during flight.
• Cruise (straight and level flight): Most commercial aircraft cruise at an altitude of 35, 000 ft. At this altitude, there is a sweet spot between required air pressure and density and the speed that is attainable.
Once aircraft get to this altitude, the job of the pilots and aircraft is to maintain that altitude. The weight of the aircraft is basically constant, so it means care must be taken to ensure the lift generated matches the weight.
At this phase of flight, the thrust is also made to be equal to the drag, so there is no acceleration. The aircraft moves at a constant speed. Cruise is usually the longest phase of flight.
• Descent (approach and landing): During this phase of flight, the aircraft needs to go down and so reduce its lift. Through the aid of flight control surfaces on the wing, the aircraft is able to generate negative lift which, together with the weight, helps to bring it down.
As you can see, the composition of the four main forces acting on an aircraft is tailored to meet the requirements pertaining to the different phases of flight. They are also tailored accordingly for different maneuvers pilots might want to perform with aircraft.
I hope this article has demystified the working of aircraft for you. It is a majestic vehicle, the actualization of ancient dreams, but ultimately, it is just another invention of science that was developed based on simple scientific principles.