“Good evening ladies and gentlemen. We’re pleased to welcome you on board. This is your captain speaking. Today we shall be cruising at an altitude of 40,000 ft and a groundspeed of 900 km/hr. The time to our destination shall be 4hrs. On behalf of the entire crew, I wish you a pleasant and comfortable flight”.
Upon hearing this announcement, have you ever wondered, “Ugh 4hrs? If we can cruise at such high speeds, why stop at 900km/hr? Why can’t we go even faster?”
Let us slowly get to the position to be able to answer this question.
There are three major types of aircraft engines in use today. They are the Turboprop, Turbofan and, Turbojet engines. These engines have respective speed ranges for which they’re most efficient.
Turboprop engines, as the name suggests, are used in propeller aircraft. Most of the thrust in these engines is produced by the propeller. The turbines present in the engine do speed up the air but the exhaust air accounts for less than ten percent of the overall thrust produced. Hence, propeller aircraft usually operate at low speeds and are also inexpensive to purchase and operate. These aircraft can operate to a maximum of 600 km/hr, beyond which it is advisable to switch to a turbofan engine.
Turbofan engines are the most commonly used engines in the commercial aviation sector today. These engines provide high propelling speeds and are also fuel-efficient at the same time. This engine consists of two major segments i.e. the primary (core) and the secondary. The core consists of the multi-stage compressors and turbines whereas the secondary consists of the region between the core and the casing (nacelle) of the engine.
When air enters this engine, a very small fragment (less than 20%) passes through the core. The remaining air passes through the secondary. Within the core, fuel is injected into the air as it passes through the compressor-turbine setup, causing it to heat up and get ejected at a high velocity. The cold air passing through the secondary also speeds up as it leaves the engine due to the reduced cross-section of the nacelle. Thereby the total air (through the primary and secondary) leaves the engine at very high speeds.
As a result, high thrust is produced, which propels the aircraft forward. The ratio of the amount of air passing through the secondary to the primary is known as the ‘Bypass Ratio’. With an increase in the bypass ratio, there is an increase in the performance and efficiency of the engine. This way, a significant amount of fuel is also saved as a very small portion of the air passes through the core. These engines operate efficiently up to 990 km/hr. Beyond these speeds, the aircraft starts venturing into the supersonic zone where Turbojets are used.
(Blue-secondary, Red-primary)
Turbojet engines are used for supersonic aircraft i.e. at speeds beyond Mach (Ma) 1. These engines operate in a fashion similar to the turbofans, except that they are Zero Bypass Engines. This means that they only consist of the core and no secondary. Due to this, all the air passes through the core producing immensely high thrust. But at the same time, these engines burn a huge amount of fuel, thereby limiting their usage.
The supersonic passenger jet Concorde ran on Zero Bypass Engines burning about 18.5 kg of fuel per mile flown whereas the Boeing 787 Dreamliner (with high bypass ratio engines) burns about 8.5 kg per mile. This made flying in the Concorde highly expensive. In addition, the Concorde could accommodate only up to 100 passengers, out of which most paid a lesser fare due to travel upgrades. A flight in the Concorde also wasn’t favourable due to less comfortable seating arrangements and large noise production. With the onset of larger and more comfortable aircraft in the late ’90s, a lot of passengers chose to opt-out of the Concorde option. This meant that the airlines operating this aircraft had to pay immense amounts of money, leading to financial instability, and ultimately to the termination of the Concorde’s operations.
From this, it was pretty evident that in the aviation sector, speed was a small factor of consideration in comparison with the cost of operation. With an increase in speed, the operation costs also increase. In addition, for most passengers, the speed of travel doesn’t really matter as long as they get to their destination. There are more comforts associated with lower travel speeds causing them to choose this option. Hence airlines operate their flights at the most fuel-efficient speeds.
There is another important reason why aircraft don’t exceed a certain speed limit. Most commercial aircraft travel between 500-550 mph which is well below the speed of sound i.e. 767 mph. One might wonder, “Why don’t we fly just below the speed of sound?” It turns out that it is extremely difficult and dangerous for a commercial liner to fly between 0.8 to 1.2 Ma. This speed range is called the ‘Transonic Range’. Within this range, the speed of air past the aircraft is a mix of subsonic and supersonic flow. With the onset of supersonic flow, there is an exponential increase in drag, which tends to destabilize the aircraft. To combat this, a large amount of thrust is required, which in turn burns an immense amount of fuel. As a consequence, a speed limit of 614 mph has been assigned to commercial jets, to prevent them from entering the ‘Transonic’ zone.
To summarize, although flying at supersonic speeds and covering a journey of 7hrs in just 3hrs might seem flashy, it isn’t the best option out there today. The demand for an increase in speed causes excessive fuel consumption (a switch from turbofans to turbojets) which is environmentally unfriendly. In addition, it increases operating costs and travel discomfort. It is also not the safest way to complete a journey. As a consequence, aircraft manufacturers and airlines have a prolonged focus on reducing the cost of travel, not the duration.
In the end, time is the enemy of the privileged few, the cost is the enemy of the masses.
– Article by Soumyabrota Sen, Final Year Department of Mechanical Engineering.