The modern airplane is an amazing piece of technology, and its development over the last 50 years has been nothing but remarkable. That said, it is also fair to say they haven’t changed dramatically if you only look skin-deep.
Why is that? Every single component and aspect of an aircraft, including the placement of the engine, is designed with one thing in mind; how it functions and performs. The engines on most modern passenger jets are below the wing for several reasons such as safety, ease of maintenance, aerodynamics and engine noise. For instance, an engine below the wing is easier to reach for members of the maintenance staff and has a safer position in the event of an engine fire.
Given that there seem to be so many reasons why engines are placed under the wings, why then do some aircraft not have engines under the wings?
Well, there are pros and cons with every configuration and, we will outline the reasons, as well as the various configurations, below.
We’ll begin by looking at the most common engine locations!
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Where are engines mounted on an airplane?
Engineers have thousands of considerations when designing an aircraft, and the engine placement is chief among those. Engines can be placed in a number of locations on an airplane, and each has its own advantages and disadvantages that have to be weighed up to determine what is ideal.
1. In the nose
Small single-propellor-engined aircraft almost always have their engines mounted in the nose, and it makes sense because it allows for better weight distribution when passengers and cargo are loaded, and access is easy for maintenance. Some examples include the Cessna 172 and Pilatus PC-12.
2. The rear fuselage
Normally seen on small business jets and some larger passenger airliners, engines mounted at the rear means that:
- the landing gear can be shorter,
- a cabin can be bigger because the wing spar can be lower to the ground
- engines are less susceptible to damage from sucking in any debris.
However, fuselage-mounted engines can not be as big as engines fitted under the wings, and they need a lot of strengthening through the fuselage due to the engine weight.
The classic Boeing 727, business jets like the Learjet and the still-popular Canadian Regional Jet have rear-mounted engines (and the classic DC-10/MD-11 and L-1011 TriStar.
3. Above the wings
This rare and seldom-used layout has the advantage of being well away from the ground, and taking advantage of the Coandă effect which improves STOL performance.
STOL simply stands for short takeoff and landing and refers to the runway length that’s needed for take-offs and landings. An airplane can be classified as a STOL aircraft if it manages to clear a 50-foot (15 meters) high obstruction using a runway distance of no more than 1500 feet (457 meters).
The greater noise heard in the cabin and the difficulty in accessing them for inspections and maintenance makes this layout less desirable. The Antonov An-72 and the HA-420 HondaJet utilize this layout.
4. Blended into the wings
The world’s first passenger jetliner, the de Havilland DH 106 Comet, featured such a design, which is great for aerodynamics but makes maintenance difficult. Also, being so much closer to the fuel tanks in the wings makes an engine fire potentially catastrophic.
5. Under the wings
This is the most common layout we see today, and is used on most modern aircraft including the Boeing 737, Airbus A380, and Embraer E-Jet.
Why then has this become essentially the default choice for aircraft design? Let’s discuss the reasons below!
The benefits of mounting jet engines below the wings
As we have already determined, the vast majority of jet-powered passenger airplanes have their engines mounted under the wing. Even as the modern high-bypass turbofan has gotten bigger and bigger, they haven’t moved elsewhere. The following advantages highlight why this continues to be the case;
1. Aerodynamics
As aircraft have become bigger and more efficient, the wing has evolved to create more lift and less drag.
When the engines are under the wing, airflow over the wing is left to work. A wing is able to create lift because when it is at a certain angle of attack, the air passing over the top is accelerated faster than the air traveling below it. This creates an area of low pressure above the wing, which in turn creates lift.
2. Safety
In the rare event of an engine fire, the engine is kept away from the cabin and fuel tanks. The wing also protects the cabin from engine fragments in the event of uncontained engine failure.
3. Structural strength
Wings want to bend upwards as lift increases, and to counteract this wing bending relief, fuel tanks are in the wings and engines are mounted under the wings. Due to this wing bending relief, much larger and heavier engines can therefore also be used.
Another benefit of mounting the engines under the wings is reduced wing flutter, or high-frequency vibrations (the engine has a damping effect).
4. Maintenance
An engine that is closer to the ground is naturally going to be easier to access by mechanics and inspectors. No need for a crane, or high lifts to reach the engine, it means basic work and inspections can be conducted more or less from the ground.
For heavy maintenance, or engine removal and installation, it is also easier as you can lower the engine onto a cradle, instead of needing to lift it up and clear of the wing. And besides, you wouldn’t want to be the mechanic who drops a wrench onto a wing, would you?
5. Cabin Noise
Engine exhaust is the noisiest problem for airplanes, and with engines mounted low and away from the cabin, the noise is further away from passengers and also shielded by the wings.
Airplanes with rear-mounted engines like the MD-80/717 twins and the Canadian Regional Jet (CRJ) are notably very loud for passengers unfortunate enough to sit at the back (but for those lucky enough to be in the front it is whisper quiet!)
Are there disadvantages when mounting jet engines under the wings?
Mounting engines under the wings does come with a few drawbacks, unfortunately (some of which other layouts avoid). But to take advantage of the overall benefits of an under-wing layout engineers and pilots must overcome them. The major disadvantages are listed below:
1. Asymmetric power
In the event of engine problems or engine failure, there is an imbalance of power. This creates a yaw effect as the engines are mounted away from the center, causing the aircraft to turn into the direction of the power-reduced or dead engine. This is counteracted with rudder (which is why the rudders on aircraft like A350s and 777s are huge!)
2. Low-speed handling characteristics
This is generally a problem on final approach, and means that if a pilot needs to apply power to increase speed, the airplane will want to pitch up. This is something the pilot will correct with elevator input to push the nose down. It is worth noting that the asymmetric power delivery above is a greater problem at lower speeds, too.
3. Ground clearance
This isn’t such an issue on the large, widebodies with the long, heavy-duty undercarriage required to support the large landing weights. However, for the smaller 737 engineers had to design its engines with a flat engine cowl to clear the ground!
Ultimately, even Boeing had to redesign the overall concept for the new 737-MAX, building it with longer landing gear so they could fit larger, high-bypass turbofans for greater efficiency.
Will we see other engine layouts in the future?
Since the Boeing 707’s introduction in 1957, passenger jets have remained largely the same in terms of overall design; low-wing monoplane construction, one or two aisles, and, yes, engines mounted under the wings.
However, in 2001 Boeing released a revolutionary design, a design that sent shockwaves through the industry, with its Sonic Cruiser concept.
A high-speed, delta wing airliner with canards, capable of cruising at just under the speed of sound (Mach 0.98), the Sonic Cruiser bucked most of the known design trends of the modern-day passenger jet. With its two engines integrated neatly into the wing, the Sonic Cruiser looked every inch the modern replacement of the supersonic Concorde.
However, with the 2000s came the demands for greater fuel efficiency and the market prioritized fuel burn over sector times.
The Sonic Cruiser gave way to the 7E7 concept which evolved into the popular and successful, yet more conventional-looking, 787 Dreamliner.
With its massive and efficient high-bypass turbofans mounted, you guessed it, under the wings, the 787 ticks most of the same boxes aesthetically as most other jetliners over the last 60 years. It is hard to imagine any great changes to this tried and tested layout in the coming decades!
Conclusion
There are many reasons why airplane engines tend to be mounted under the wing. Some include safety in the event of fires, ease of maintenance, cabin noise, and aerodynamics. However, this doesn’t mean that it doesn’t come without disadvantages. Still, these advantages are minor enough to not stand in the way for wing-mounted engines to dominate not only for now, but also for the foreseeable future.
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