S5C5: Airplanes

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a) The physics of flight

So far in this series, we have tackled moving over solids as well as on top of and within liquids; it is now time to set our sights up and ponder flight, the moving through our gaseous atmosphere. Birds and various other winged species can naturally achieve this physical tour de force, and having looked at animal flight in S2 Section 3.e, we already know planes work differently since there is no significant vertical movement of their wings, no flapping involved. Greek mythology had Daedalus and his son Icarus taking flight but in practice it was not until the mid-19^th century that the first powered flight using a fixed wing took place. The difficulty was twofold: achieving enough speed considering the weight to be lifted off the ground and controlling the flying machine from take off until a successful landing. We will look into the first aspect now and the second in the next section.

If you have read S5 Section 3.b on the forces exerted on the sails of a sailing ship then you should have a fairly good idea of the physical principles involved, if not then then I invite you to do so, even if only to get a visual illustration of the decomposition of forces. As a wing moves through the air at speed (this is the apparent speed), the gaseous molecules spread around the wing that acts as a foil. Spread around means below and above since wings are set on a horizontal plane, and depending on their shape and inclination – their angle of attack in the jargon – the air flowing will be exerting pressure on either the bottom side or the top side of the wing. In turn, this creates drag in the direction opposite to the aircraft movement and lift generated at a perpendicular angle, which is vertically. Provided the wing is oriented with an angle of attack where the air flow is facing a negative slope, then by reaction the lift will be upward. There is an alternative way to think about this, which is as a difference in air pressure between underside and topside of the wing but it comes down to the same actually because, if the pressure was equal on both sides, the up and down vertical reactions produced would neutralize each other.

It is this lift that is pushing both wings of the aircraft up, and the higher the apparent speed of the air flow, the stronger the lift and the drag. Accordingly, the ability of the propulsion system to generate speed for the entire craft is integral to resolving the flying equation. No wonder airports need to have long runways and the engines are pushed hard at take-off. So, what are the main airplane engine technologies in use today?

There are essentially two different principles employed: propellers and jet. We already partially looked at propellers in S5 Section 4.a when describing container ships and the rotation of a crankshaft induced by a combustion engine was detailed in S5 Section 2.b, but I will still provide an overview here. The combustion of the fuel causes an expansion of the air in the chamber, applying pressure on a piston and causing its displacement. This linear displacement is mechanically converted into torque, a rotational force, through bearings and cogs. This force is transmitted to propellers, which is a set of blades or air foils and their asymmetrical shape, like inclined wings, induces a pressure differential between the front and the back of the blade: more pressure behind resulting in forward thrust and less pressure in front generating suction. The choice of the pitch of the blades, a technical term for the angle in a fluid, is a trade-off between wind resistance and thrust: the more pitch the more lift and thrust but the greater the drag as well. In addition, the apparent wind velocity is a function of both the true wind speed and the velocity of the aircraft so, at higher speeds, the effective angle of attack decreases and, to compensate for this, the rotation speed must be increased as the plane travels faster. Furthermore, the part of the blade closer to the centre of rotation of the propeller is significantly slower than at the extreme so the pitch or twist shape will be designed to even out the differential this would normally create with respect to angle of attack. There is more than meets the eye at first glance.

Completely different type of thrust in the case of jet engines. We already looked at jet propulsion in S3 Section 10.d on interstellar travel and the central notion is to exhaust a controlled stream of fluid away from the intended direction of travel to propel the aircraft forward, by reaction – also known as Newton’s third law. There are essentially four stages in the workings of a jet engine: #1 air intake at the front of the engine and its compression, #2 injection of fuel and compressed air in the combustion chamber(s), #3 passing the hot, high-pressure air being thrusted through a turbine to drive the power of the compressor and #4 exhaust the gas through the nozzle to create forward propulsion, by reaction. The prevalent type of jet engines uses turbofan technology where fans accelerate the air intake in front of the engine but not all of it is ducted into the compression chamber and through the turbine.

Commercial and military planes operating at high altitude and elevated speeds adopt jet engines rather than propellers whereas the later provides more power-to-weight ratio, which makes taking off on short runways much easier, and they are also more fuel efficient. Most jet fuels are based on a middle distillate of crude oil called kerosene with a low freezing/melting point of -47°C and a high flash point of 38°C (the higher the flash point the less flammable a compound is). Increasingly, a small percentage of sustainable aviation fuel (SAF), produced from biomass rather than hydrocarbons, is blended to aviation fuel in a bid to reduce the carbon footprint of commercial flights.

b) From cockpit to tail

Ropes and pulley for the sailing ship, steering wheel and brakes for cars and trucks, but what about airplanes, how do they control direction and speed? The answer is actually very similar to a submarine, without the ballast. The speed is dictated by the amount of thrust of the engine, itself a function of the amount of compressed air and fuel injected in the combustion chamber, and the direction is managed with a fin-like element called rudder at the back of the fuselage, in the tail area known as empennage, which creates a lift/drag when moved off the longitudinal plane of the craft. The rudder thus controls the yaw, turning to the right or to the left, nearby horizontal stabilizers prevents it from tilting up or down (pitch) and the wings stabilize against sideways roll while retaining control over the roll by actioning ailerons at the outer, trailing edge of the wings.

As for vertical motion, that is deciding whether to create upward or downward lift, this is a function of gravity versus lift with lift being a function of the wing’s angle of attack and of the wind’s apparent speed. In order to help with take-off and landing, using flaps at the trailing edge of the wings helps increase lift and drag. These can clearly be seen in action if you look out of the window from your passenger seat and their operation is sometimes accompanied by the deployment of flap-like surface in front of the wing. These are called leading-edge slat and they allow for a more aggressive angle of attack to be maintained, thus allowing an aircraft to remain airborne at lower speeds. I include a link to the relevant Wikipedia entry at the end of this chapter if you wish to read more about this. Airbrakes can also be positioned on the wing to increase drag without creating much lift when extended and come in useful to reduce the speed of the aircraft after touch down.

The above captures most of the flight control surfaces, nonetheless there is plenty more than this in an aircraft flight control system. The interface with this system is primarily concentrated at the front of the plane, in the cockpit, which offers the best visual vantage point for the pilot and first officer if there is one. The yaw is controlled by a pair of rubber pedals and the pitch and roll by a joystick. Just like in a car, there is also an accelerator in the form of a thrust lever. Most of the remaining instrumentation and indicators have to do with sensors providing information such as the amount of remaining fuel, the three-dimensional position and the velocity, including the rate of altitude loss or gain. Headsets with microphones are also used to communicate with air traffic control and get instructions regarding taxi, take-off, and runway availability for landing. Also, for aircraft with retractable landing gear, there is an up-down lever for this.

A landing gear can be conceptually simplified as the pairing of tires with a shock absorber providing the aircraft with an interface exhibiting much less friction that the fuselage would. There is no traction involved there, the shock absorbers dissipate a lot of the downward force upon landing and the wheels provide an efficient way to move the plane on land. For seaplanes, the landing gear may consist of a pair of floats or the fuselage itself acting as a ship’s hull.

Finally, in passenger planes, the seats are located within the fuselage, extending between the cockpit and the tail, with some cargo space for the luggage located in the curved area of the cylinder-shaped body below the cabin floor. In small and medium-sized aircraft there are two rows of seats extending radially away from a single aisle and in a wide-body there are two aisles separated by a central row of seats. In cargo planes, nearly the entire interior of the fuselage can be used to transport goods.

c) Supersonic flight

As speed increases, there are considerations other than lift and drag to take into account and these relate to fluid dynamics. When approaching the speed of sound, a threshold called Mach 1 (in homage to the physicist Ernst Mach), the asymmetry of the various parts of the plane, and not just its wings, will result in some air flowing at faster than the speed of sound and other areas where the air will be flowing slower than this speed. This disparity creates significant perturbations with consequences in terms of handling as well as a dramatic increase in drag. This phenomenon takes place in the range called “transonic”, between Mach 0.8 and Mach 1.2, because above Mach 0.8 there will be some air flow crossing Mach 1 and below Mach 1.2 there will be some air travelling at subsonic speed. For reference, the speed of sound changes depending on the nature of the conducting medium and temperature; at mean sea level and 20°C it is 343m/s (1,235km/h) and this drops significantly at altitude where the air temperature is colder – for example at -40°C it is down to 307m/s (1,105km/h). One way to reduce this unsettling effect of fluid dynamics is to design an aircraft with swept wings, meaning they do not project away from the fuselage perpendicularly but diagonally with a pronounced angle (think Concorde), usually backward but forward versions also exist.

Above the transonic range, we enter supersonic speeds and designs become much more complex to preserve good handling because they need to cater both to the subsonic and supersonic ranges. Upward of Mach 5, the aerodynamics regime is called hypersonic and with it comes new variables and constraints. This is however beyond the scope of this chapter so I will only provide a link to the relevant Wikipedia entry in section g) for those interested to learn more.

The ability to fly supersonic commercially has been hampered by another physical phenomenon, one much more difficult to manage than air turbulences across wings, engines and fuselage. I am of course referring to the famous sonic boom, the startling noise made by an aircraft breaking the sound barrier. The physics behind the boom aren’t overly complicated: as an aircraft approaches the speed of sound, the air displacement it creates, i.e. the sound waves, becomes closely spaced together – this is akin to the doppler effect for an approaching object and the blueshift seen with electromagnetic waves – and at exactly Mach 1.0 the air is being pushed at the very same speed it can travel at, thus creating a build-up and the merging into a single shock wave. The speed of travel of the aircraft is such that when plotted from its reference frame the shock wave adopts a conical shape starting from the tip of the aircraft called the shock cone. This shock wave embodies a massive pressure differential which our sense of hearing perceives via mechanoreceptors as a very brief and loud noise, something we looked at in S2 Section 9.a. If you pay close attention to it, and it is not that easy because the military aircraft responsible for the bang traverses the sky above us in apparent silence so the experience can be somewhat unexpected, there are actually two booms. The first is from the pressure differential between the high pressure at the front and the low pressure until the tail, and the second is between this low pressure and the return to a normal pressure level beyond the tail.

d) Evolutionary pressure

Back in the early days of supersonic travel it seems that faster was the name of the game, yet the sonic boom problematic over inhabited areas has never gone away and while it seems technological progress can reduce it to some extent, theoretically it can’t be avoided. Still, there are several companies working on supersonic prototypes.

This issue has been further compounded by a host of factors including the need for an extended runway length for aircraft with such slim angles of attack and a very high fuel-related cost on a per passenger-kilometre basis, an issue exacerbated by the rising cost of petroleum products and efforts to reduce the carbon footprint of flights.

If not higher speed then, what can we expect to revolutionize plane travel? With the exclusion of vertical take-off and landing (VTOL), something we will cover in the next chapter (S5 Section 6.c) and which is not a substitute for traditional mid and long-range air travel, there doesn’t seem to be an obvious prospect for a game changing development. Instead, it seems it is going to be incremental evolution and refinements. Below is a list of the areas where we can see ongoing meaningful improvements:

Advanced materials with better heat resistance and lighter weight, the latter translating into fuel savings. These include new thermoplastics, high-performance metallic alloys and composite materials such as carbon-fibre-reinforced polymers and ceramic matrix composites that do not fracture easily under mechanical stress like standard ceramics.
The use of alternative fuels such as hydrogen (using fuel cells or jet engines), SAF or electrical batteries and improvements in the fuel efficiency of engines. Open-fan technologies (where the fan is exposed rather than encased) is a serious option being tested for example.
The future is almost already there for autonomous flight technology, at least for military purpose where unmanned crafts such as drones are already not only taking to the sky but waging war or striking at terrorist targets. For commercial aviation, we could expect cargo flights might end up being fully autonomous at one stage though for passenger flights it will be a while and, if I had to wager, I would say a captain will remain in the cockpit for at least another couple of decades but the first officer seat might become vacant in the near future.
New wing shapes may result in improved aerodynamics as well as more space (though it could also well be more seats) and overall passenger comfort in terms of soundproofing, lighting and quality of the in-flight entertainment system can be expected to undergo marginal enhancements over time.
Not directly related to the aircraft itself, yet absolutely part of the passenger experience, tracking (RFID), bio-identification and imaging technologies could improve the check-in, security clearance, immigration and boarding processes. In that area, some early adopters stand in stark contrast to older airports and the immigration and custom interactions one needs to go through in some countries…

e) The business of airlines

The last two paragraphs on passenger experience provides the perfect segue to shed some light on the nature of an airline operations. After all, we fly with them, not just in an aircraft. The choice of an Airbus or Boeing made plane does matter and so does the choice of the airline, and you are never given the option to choose which model you will be transported in, or at least not directly.

Running an airline is a complex affair and, you may or may not know, they do not always own the aircraft they operate. Often enough, considering the massive cost of purchasing an aircraft, these are being leased – a type of rental. Also, they do not always ensure the maintenance themselves and several groups provide these services, including some run by large airlines. This means the maintenance division of Airline X might be carrying out the maintenance of Airline Y planes, including the engines.

What they do however, is operate the aircraft branded with their livery, which includes the on-board service, deciding the food menu (though the food itself is catered by a separate division or company), and the selection of the cabin outfitting and entertainment system. This is for passenger transport and some airlines also operate separate cargo planes but you don’t come across these because they use different terminals. Indeed, the loading and unloading equipment for freight differs materially and the flow of merchandise is not handled in the same manner as the flow of human passengers. This makes air freight a different business altogether and some of the largest cargo plane operators are logistics companies such as the DHL or Amazon of this world.

I mentioned the livery of a plane and, even if you bought your ticket from Airline X, you may end up travelling in a flight operated by Airline Y. This is called code-sharing and it allows airlines to market more flights than only those they operate. This is not meant to trick passengers, it makes for more optionality, a wider network and better connectivity with potentially less lay over between flights if you are not on a direct route. Sometimes you might be disappointed, and at other times it might feel like a small upgrade but the price you paid might have reflected this in the first place, albeit without you realizing.

Interlining is a slightly different concept and it most frequently occurs within a same marketing alliance – the three major ones being SkyTeam, Oneworld and Star Alliance. It is the arrangement whereby you need not collect your bags between flights even if this involves a change of airline and it entitles you to be rebooked on another flight at no extra cost if the second flight is missed because of the first one – again, even if this involves two different airlines. So interlining works as a service and insurance, and this has a price, therefore it is not always provided by the airlines selling you a ticket. Travel agencies however, might be able to sell this option.

Some airlines only sell their tickets directly, that would be mostly the low-cost no-perks carriers, but most have historically been distributing their tickets via travel agencies. The rise of online transactions has had a significant effect on these practices and saw the rise of online travel agencies (OTAs) and of airline selling tickets directly through their websites. The way pricing and inventory is managed is through global distribution systems (GDS) and three of them still dominate the market: Amadeus, Sabre and Galileo (owned by Travelport).

Finally, it is worth talking shortly about the airline industry as a whole. One of the first thoughts that comes to mind is the dismal profitability over the decades, with rare exceptions mostly concentrated in the private sector. A key reason for this is the overlay of strategic reasons and economic considerations beyond the pure profit and loss outcome of running an airline. This is where national ownership of airline and the notion of national flag carrier comes in. There are two drivers to this: the first is the opening up of remote regions and linking them with the rest of a territory via air transport, and the second is providing options for inbound tourism with all the in-country spending that comes with it. Put together, this creates loss-making routes and distortions in pricing impacting the entire industry. Furthermore, the whole competitive dynamic is further disturbed by the negotiation of temporary subsidies, taking the form of reduced airport fees for example, between some airlines and regions to open new routes or maintain them.

f) Trivia – Airmail and the Aéropostale

Once the proof of concept of flying machines over long distances had been established, it was not long before the aeroplanes were put to use for commercial and strategic purposes. Cargo and passengers were a step too far however, for the engines, frames and wings of the time. So, what is light and valuable?

Information of course. Data.

And thus airmail was the first business carried out by airplanes outside of the military and the most iconic of the operators was certainly the Aéropostale founded in 1918. From Toulouse, in the south of France, it serviced Barcelona, then Casablanca and Dakar before making the jump over the Atlantic Ocean to Natal and Rio de Janeiro in Brazil, eventually even crossing the Andes to Santiago de Chile and reaching the southern latitudes of Tierra de Fuego. It was adventurous, with frequent delays and even casualties, and larger-than-life pilots. Among those were the well-known French figures Jean Mermoz and Antoine de Saint-Exupery, of Le Petit Prince fame, who perished in 1943 during a reconnaissance mission after having joined the Free French Force during the second part of WW2.