Since the pioneering days of the Wright Brothers, aircraft engines have come a remarkably long way. Early piston engines strained just to keep flimsy wood-and-cloth contraptions airborne for minutes at a time. Today, immensely powerful yet efficient jet turbines transport hundreds of passengers across continents and beyond Earth‘s atmosphere.
Join me on an exploratory journey across over a century of aviation history to discover the magic behind aircraft propulsion. We‘ll uncover how innovative minds like Frank Whittle and Hans von Ohain birthed the jet age. And we‘ll compare the strengths and weaknesses of different engine types that keep our skyways filled with flying marvels.
The Origins of Aircraft Engines
While dependent on earlier steam engine roots, lightweight gasoline internal combustion motors purpose-built for flight began appearing in the late 1800s. These primitive piston engines spun propellers using cylinders, crankshafts and cooling radiators that would feel familiar to a modern gearhead.
Attached to precarious wood-and-canvas biplanes, they produced barely enough thrust for short hops. But they launched a new epoch of manned flight.
"The desire to fly is an idea handed down to us by our ancestors who…looked enviously on the birds soaring freely through space…on the infinite highway of the air." – Wilbur Wright
Soon more reliable water-cooled designs allowed barnstormers and pioneers to push limits with risky aerial feats. And by World War One, aircraft technology was advancing in leaps and bounds.
WWI Royal Aircraft Factory SE5a fighter with a HISPANO-SUIZA piston engine [1]
Higher performance fighters demanded ever more grunt as massive dogfights erupted thousands of feet in the clouds. But piston engines were rapidly hitting developmental ceilings as their cooling systems, vibrations, and propeller torque grew troublesome [2].
Visionaries began dreaming of radical alternatives. What if an engine could continually burn fuel without harsh reciprocating motions? And generate direct screaming jet thrust while doing away with delicate propeller blades…
The Dawn of the Jet Age
While air-breathing jet power concepts circulated for decades, it took Frank Whittle‘s relentless persistence to prove their merit.
As an aviation-obsessed British RAF cadet, Whittle penned a revolutionary 1928 thesis paper outlining a "turbo jet" engine. Ridiculed by academics, he persevered independently to secure patents and fine-tune prototypes throughout the 1930s.
"I was the only boy in the neighborhood who thought airplanes were interesting. I was considered rather a freak by everybody." – Frank Whittle
On April 12th 1937, the Whittle turbojet made history by sustaining jet thrust stationary on a test rig [3]. Just four years later, the engine powered the Gloster E28/39 to become the first British jet aircraft.
A replica of Frank Whittle‘s pioneering turbojet on a test rig [4]
Meanwhile in Germany, Hans von Ohain independently produced his first turbojet prototype by 1935. Building on Whittle‘s concept, von Ohain evolved his Heinkel-Hirth design towards flight readiness. It propelled the experimental Heinkel He 178 to perform history‘s first successful jet-powered flight on August 27th 1939 [5].
The Heinkel He 178, first turbojet aircraft [6]
Thrusting the globe into the jet age, engineers kept iterating on Whittle and von Ohain‘s])). Early unreliable turbojets remained temperamental.
But by World War Two they were proving themselves as credible propulsion, leading German Messerschmitt Me 262 fighters to streak far faster than anything propeller-driven. Within years commercial jetliners changed travel forever as turbine efficiency improved.
Today gas turbines power many military jets at triple the speed of sound! And airlines continually squeeze more efficiency from high bypass turbofans cruising comfortably below Mach 1.
Turbojet Engines
Now we‘ve covered some pivotal history, let‘s explore common aircraft engine types in more technical detail, starting with the baseline…
If jet propulsion is boiled down to its essence, the result is a turbojet.
Air rushes into an intake and gets squeezed by a rotating compressor before entering a combustion chamber. Fuel injects and detonates, producing hot expanding gases up to 1000°C that escape rearward through a turbine. This spool-up turbine drives the compressor while its leftover energy produces straight jet thrust.
By the 1950s turbojets offered simplistic reliable power for military fighters. Adding an afterburner injection system made them extremely powerful for supersonic flight.
Turbojet Pros:
- Proven reliable technology
- High exhaust velocity for top speeds
- Lightweight and simple setup
- Low frontal area
Turbojet Cons:
- Inefficient and noisy at lower speeds
- High fuel consumption
Today turbojets with up to 10:1 pressure ratios remain useful for very fast light aircraft. But most jets have evolved…
Turbofan Engines
Seeking better efficiency while retaining jet power, engineers conceived the turbofan or bypass engine. It acts like a turbojet hidden inside a large ducted fan.
Only a fraction of entering air flows into the combustion core. Most gets accelerated around the central turbojet by a multi-bladed frontend fan before rejoining the hot exhaust stream.
Cleverly converting kinetic wind energy for propulsion, large high-bypass turbofans (>10:1) offer excellent thrust with decent fuel economy. This makes them ideal for heavy airliners and military transports flying below Mach 0.85.
Turbofan Pros:
- Efficient across wide speed range
- Quieter due to lower jet velocity
- High thrust with good economy
Turbofan Cons:
- Complex with multiple shafts
- Heavy weight hinders supersonic flight
As bypass ratios rise, clever geared turbofans are being developed to reduce fan and turbine speeds for further gains…
Geared Turbofans
Normally a turbofan ties its frontend fan and rear turbine to the same spinning shaft. So the fan wastes energy spinning much quicker than ideal. On large engines this forces designers to make fans too small.
Geared turbofans insert an epicyclic gearbox between the core and fan. Letting them each operate at optimum speeds boosts propulsive efficiency through larger fans. Noise also falls 5-10 decibels as tip speed reduces.
Pratt & Whitney‘s PW1000G brought this technology to airliners in 2016 with IndiGo A320neos showing 13% fuel burn improvement [7]. Expect geared turbofans to become widespread once reliability grows.
Ramjet Engines
Before looking at turboprops and turboshafts, we‘ll make a quick detour to simpler ramjet technology used mainly for missiles…
Ramjets entirely eliminate rotating compressors and turbines for utter simplicity. Instead they rely on the craft‘s rapid forward motion ram-pressurizing air through specially shaped intake diffusers.
Fuel then combusts with this high-pressure air emerging from the exhaust nozzle for thrust. Used mainly on missiles above Mach 2, ramjets produce zero static thrust. So launch boosters are required until sufficient airflow builds up.
Ramjet Pros:
- Extremely simple and lightweight
Ramjet Cons:
- No static thrust, needs speed
- Inlets inefficient below Mach 2
NASA‘s experimental Blackbird turbo-ramjet plane attained blistering Mach 3.3 speeds back in the late 1950s using this technology [8]. Recent scramjet developments push hypersonic flight frontiers…
Turboprop Engines
Where turbofans occupy the middle ground between efficiency and speed, turboprop engines sit at the economic end of this spectrum, sipping minimal fuel while moving modestly.
Very similar to a turbofan, additional turbine power instead drives a gearbox to spin an open propeller. Generating thrust more slowly and quietly, turboprops work well for small passenger and transport planes typically below 500 mph.
Turboprop Pros:
– Excellent efficiency and economy
– Proven reliable technology
– Well suited for short flights
Turboprop Cons:
– Slow compared to turbofans
– Gearbox introduces complexity
Modern designs with scimitar-edged propeller blades counter torque issues on single engine planes. When shouted about as Propfans or Unducted Fan Engines they can fly near Mach 1…
Turboshaft Engines
Very similar inside to a turbojet, a turboshaft instead exhausts turbine energy through a shaft rather than making jet thrust. This spins helicopter rotors or drives tanks and ships.
Turboshaft Pros:
– Rotor speed independent of turbine RPM
Turboshaft Cons:
– Wasted energy not producing thrust
By varying the fuel flow, helicopter pilots can modulate lifting power as needed without impacting rotation speed. Turboshafts also give tanks more efficient and reliable variable speed instead of direct gear linkage…
The Future of Aircraft Engines
So those are the most common propulsion drivers keeping our planes soaring! Jet turbine technology has advanced enormously since Frank Whittle‘s first prototypes. But what does the future hold as aviation reaches for ever more ambitious heights?
Expect a growing drive towards energy diversity to temper volatile fuel security and costs. Electric planes may one day ply the skies using batteries, solar, hydrogen fuel cells or even microwave beams [9]. And sustainable biofuels can cut aviation emissions substantially until these breakthroughs mature [10].
NASA‘s experimental Helios solar-powered unmanned aircraft [11]
Additionally as electrification matures, hybrid gas-electric architectures will bridge gaps at the performance fringes. Small planes may fly primarily on batteries with turbine generators charging them and boosting range minimally. While airliner cores could stay jet fueled with electric fans enhancing efficiency [12].
And further out, spaceplanes projected by ESA and NASA may realize horizontal takeoff access to orbit. Air-breathing SABRE engines could initially lift vehicles beyond Mach 5 before rocket boosters ignite! Combining turbopumps, precoolers and revolutionary heat exchangers, they‘ll surf the atmosphere‘s frontier with our dreams [13].
The most exciting times in aviation still lie ahead…
So I hope you‘ve enjoyed this whistle-stop tour through aircraft engine history! Let me know if you have any other questions in the comments.
References:
[1] https://en.wikipedia.org/wiki/Royal_Aircraft_Factory_S.E.5#/media/File:SE5a_s06-8743.jpg
[2] https://archives.newport.ac.uk/articles/from-piston-engine-to-gas-turbine-a-new-autobiography
[3] https://www.rafmuseum.org.uk/research/online-exhibitions/taking-flight/the-first-jet-engine/testing-whittles-engine.aspx
[4] https://commons.wikimedia.org/wiki/File:Turbojet_engine_Replica_(7103013463).jpg
[5] https://www.britannica.com/technology/turbojet
[6] https://en.wikipedia.org/wiki/Heinkel_He_178#/media/File:Bundesarchiv_Bild_146-1987-026-03,_Erprobungsstelle_Rechlin,_D%C3%BCsenj%C3%A4ger_Heinkel_He_178.jpg
[7] https://simpleflying.com/pratt-whitney-geared-turbofan/
[8] https://www.nasa.gov/centers/armstrong/news/FactSheets/FS-030-DFRC.html
[9] https://www.telegraph.co.uk/travel/comment/future-of-flight-power/
[10] https://www.icao.int/environmental-protection/GFAAF/Pages/default.aspx
[11] https://www.nasa.gov/centers/dryden/images/content/707208main_EC01-0058-23.jpg
[12] https://www.bbc.com/future/article/20220321-the-electric-planes-and-hybrid-aircraft-of-the-future
[13] https://www.esa.int/Enabling_Support/Space_Engineering_Technology/World-first_firing_of_air-breathing_SABRE_rocket_engine
