How Do Airplanes Fly?

How Do Airplanes Fly?

With air travel being one of the most popular form of transportation after the automobile, how a 41-ton metal contraption flies can still be a mystery for the billions of its annual passengers.

*Static on the intercom* Hello ladies and gentlemen this is your Captain speaking. Welcome aboard The Weekly Learner's flight TWL-167 bound for Knowledgeville. It is a sunny 32 degrees with North-Easterly wind speeds of 8 knots at the time of writing. Our flight today would take us from a brief history of air travel all the way to the basics of flight in about 7 minutes. As this isn't a long-haul flight, there will be no "extensive information" meals served but do not worry as the "essential knowledge" refreshments should suffice just fine. If you're interested in our long-haul routes, you'd be happy to know that future flights would go more in-depth to the different sub-systems and theories touched on below. On behalf of The Weekly Learner and the flight crew, we'd like to wish you a very informative flight.

The Many Milestones of Flight

To truly appreciate how far we have come in terms of conquering the skies, this short section would briefly list the notable events in history that have shaped general aviation into the mode of transportation we know so well today.

1903 - The Wright Brothers made four brief flights with the world's first successful airplane, the Wright Flyer.

1919 - John Alcock and Arthur Brown made the first crossing of the Atlantic in 16 hours from St John’s Newfoundland to Co. Galway, Ireland.

1941 - The first successful test flight of the Gas Turbine Engine invented by Frank Whittle of Great Britain occurred.

1943 - Britain sets up the Brabazon Committee to prevent the aviation industry from collapsing after the war, subsequently laying the foundation for civil aviation.

1947 - First supersonic flight of Mach 1.05 achieved by Chuck Yeagar in a Bell X-1.

1950's - The 'dawn' of the jet age with the earlier introduction of the Comet 1 in 1952 and later replacement of the Boeing 707 in 1958.

1976 - The first supersonic transport Concorde entered service with a maximum cruising speed of Mach 2.04.

1987 - The Airbus A320 pioneered the fly-by-wire airliner, subsequently shifting the human pilot - flying machine interaction from direct mechanical systems to indirect, electronically controlled systems.

1994 - Digital design takes to the skies with the Boeing 777-200 being the first aircraft whose geometry was completely designed on a computer aided design system (CATIA - Computer-Aided Three-dimensional Computer Application).

2000's - Computational Fluid Dynamics becomes a design tool as numerical simulation comes of age.

2010's onward - Usage of air transportation increases dramatically as innovation lowers cost and a larger middle-income population develops.

The Modern Aircraft

Boeing 787-8 Technical Description and Cutaway [1]
Rolls-Royce Trent 500 Turbofan Engine [2]

If Orville and Wilbur Wright were alive today, they probably would never have imagined a world where thousands of giant aluminum tubes carrying hundreds of thousands of passengers are zooming across the heavens every single day at speeds unrivaled by any other forms of transportation apart from space vehicles. From the two diagrams above, it is pretty easy to tell that the engineering behind modern aircrafts is no simple undertaking. The once ingenious flying machine that took the two Wright Brothers four years to make is easily shadowed by today's awfully massive airplanes that are designed by several hundred engineers and constructed by thousands of workers in almost the same time frame.

As with any complicated machinery, an airplane is able to do what it does best due to the myriad of intricately combined sub-systems designed around the mechanics of flight and optimized for efficiency. The modern aircraft comprises of many of these sub-systems hosting within themselves even more individual parts that really makes the mind go crazy when visualizing them.

Take for example the Boeing 787-8 above, one potential and definitely important sub-system would be the turbofan engine which is shown in greater detail on the right (take note that the 787-8 actually uses either the Rolls-Royce Trent 1000 or General Electric GEnx engines and not the Trent 500). Imagine this same level of detail applied to the hydraulic, pressure, air-conditioning, electrical and flight control systems and you can see just how quickly the complexity increases.

Creating an aircraft is so complex in fact that specialized undergraduate courses were made in order to educate aspiring aeronautical engineers (surprise, surprise). In future articles, we will be diving deeper into some of these systems to truly understand the hidden workings of an airplane and how engineers were able to use their knowledge on subjects such as flight mechanics, electronic and electrical systems, thermodynamics, aerodynamics and many more in order to design and construct these magnificent, metal birds. For now however, let us look at the simple science behind how airplanes fly.

The Physics of Flight, Simplified

Everything in this Universe changes motion due to Newton's First Law of Motion - which states that every object will remain in motion or be at rest unless acted upon by an external force. We can view this force as a push or pull upon an object resulting from its interaction with another object (sort of like how impending deadlines "pushes" you to complete your work). There are many types of forces such as electromagnetic, frictional, tension and normal but what we will be looking at today are the gravitational, applied and air resistance forces.

Forces Acting On An Aircraft [3]

Fundamentally, there are only four forces acting on an aircraft. The many sub-systems employed by an aircraft is mainly used to control the balance of the Thrust, Lift, Drag and Weight forces. Simplifying the operation of an airplane in this manner may be sufficient in understanding the physical phenomenon of flight but it gives no credit to the ingenuity and complexity of its design. It would however serve as a good starting point for future, more comprehensive articles.

In flight, based on Newton's First Law, an airplane would stay in motion (travelling in a straight line at a constant speed) unless acted upon by an external force. From the diagram, we can see that there are four forces acting on the plane. However, we would have to apply a 'sum of forces' analysis in order to actually figure out their effects on it.

Though a real aircraft would have 6 degrees-of-freedom - the number of directions in which independent motion can occur (in this case up/down, left/right, forward/backward, roll, yaw, and pitch), viewing its motion in this manner constraints the analysis to 2 DOF which are up/down and forward/backward. As such, if the thrust and drag forces are equal, the plane has no external force in the forward/backward direction and does not speed up or slow down. Adding on to the hypotheticals, if the lift force is greater than the weight force, there will be a subsequent external 'up' force on the airplane which causes it to go up and increase in altitude.

Degrees-of-freedom of an Aircraft [4]

Now that we know how the interaction of these forces allows an aircraft to move, let us take a look at how they were created in the first place. The easiest of the forces to comprehend is the weight of an airplane. All objects have gravity, a natural phenomenon in which things with mass or energy are brought towards one another. The more massive an object is (etc. the Earth), the more pronounced the gravitation force is. Therefore, an airplane flying on Earth would experience a downward weight force, W = mg where m is mass of the plane and g is the gravitational acceleration of Earth (the steady gain in speed an object experiences solely by the force of Earth's gravitational attraction, 9.81 m/s²).

Secondly, the next force we'll be looking at is the thrust force which propels the plane forward. We won't be dissecting a jet engine in this article but instead we'll simply understand how it is used to accelerate an aircraft forward. Most modern airplanes use a turbofan engine which intakes, compresses and combust air to produce a hot, high-velocity gas jet which is exhausted in the opposite direction of travel to produce thrust. This makes use of Newton's Third Law which states that for every action there is an equal and opposite reaction (or in other words, ejecting a jet in one direction causes an equal force in the opposite direction which we perceive as thrust).

Visual Representation of Airflow Over an Airfoil [5]

The final two forces we'll be investigating are the lift and drag forces. We view them together as a pair simply because of their relatively similar looking equations. If you were to cut the wing of an aircraft in half, you would notice that its cross-section would look something like the teardrop shape seen in the figure above. This special shape is called an "airfoil" and it is the reason why airplanes are able to generate lift. Looking from left to right and taking a single column of black dots as the relative position of air molecules at a given time, we can see that as this front of air particles passes the airfoil, it splits into two distinct halves.

Furthermore, the top half of the original front can be seen to be moving quicker than its lower counterpart. This is due to the unique shape of an airfoil creating a lower pressure at the top of the airfoil which sucks air molecules towards it making them move faster while the bottom of the airfoil exerts a higher pressure thus slowing air particles down. This process can be akin to a roller coaster ride where air molecules below the airfoil are slowly ascending the roller coaster and those above the airfoil are rushing down it. As air particles decelerate and group together below the airfoil, there is an imbalance in the number of particles surrounding the airfoil. More air molecules below the wing than above it causes an applied, upward 'push' force that we associate with lift.

To determine the amount of lift force a wing generates, engineers use the equation L = 1/2ρv²SCl (we'll explore this equation further in a future article as this one has already long passed its ideal length). Drag on the other hand is an inherent property of anything that moves and it can be further broken down into parasitic and induced drag (which will also be elaborated upon in another article). It shares a similarly looking equation with the lift force, that being the drag force experienced by an aircraft, D = 1/2ρv²SCd. Cl and Cd are called the lift and drag coefficients respectively and we'll gain a deeper understanding of them alongside the intimidating array of equations in the upcoming mechanics of flight series.


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