In our Ask a Pilot series, pilot Spencer Marker answers one of your aviation-related questions each week. See past installments here and submit your own to Whitney@johnnyjet.com.
Hey Spencer. Why do some airplanes have the engines on the wings while others have the engines on the body of the plane?
Hey, William. Thank you so much for the excellent question! I’m sure many readers out there have wondered why some airplanes mount the engines on the wing, while others mount them on the aft fuselage or body of the airplane. There are several reasons aircraft designers choose one configuration over another. Things like ease of ground servicing and structural weight, among other things, drive this decision. Additionally, for pilots, there are benefits and disadvantages to the operation of each. Let’s take a look.
At the dawn of the jet age, when airplanes like the Boeing 707, De Havilland Comet and Douglas DC-8 were just beginning to leave their smoky trails across the skies, passengers in smaller communities yearned for the speed and comfort these high-tech jets could offer. The problem was that these smaller airports lacked the infrastructure of their larger counterparts. Their small airfields simply did not have stairs or baggage loaders tall enough to service these large jets.
So taking the task to heart, airplane designers on both sides of the pond began pondering what an airplane would need to service such rudimentary airports. First, the airplane had to be low to the ground. This way, baggage could be loaded without baggage loading equipment. It also had to be low to facilitate the use of integral air-stairs and have an APU so the airplane could be completely self-sufficient (and not need ground equipment).
Positioning the engines on the wings made the plane too tall to sufficiently service without equipment, so designers moved the engines to the back of the fuselage or body and lifted the horizontal tail up to a T-tail position. The result was two similarly designed smaller aircraft: the Douglas DC-9 and the British Aerospace BAC-111. Boeing would design the mid-sized 727 with a similar aim.
Aerodynamic benefits of this configuration included a simpler wing structure and a lower likelihood of the engines ingesting debris from the ground. Positioning the engines on the tail also made a jet easier to control in the event of an engine failure, as the thrust line of both engines was then closer to the centerline of the airplane.
Such a unique design was not without drawbacks. To begin, this configuration made the tail of the airplane very heavy, as a strong tail structure was required to support both the engines and vertical and horizontal tail. The wing would have to be moved aft to accommodate the heavy tail. Additionally, the T-tail placement made for undesirable stall characteristics. This would eventually be remedied.
Today, airplanes like the MD-80, the 717, and CRJ and ERJ aircraft employ this configuration. All of these aircraft—with the exception of the 717—can employ internal air-stairs and be serviced without ground support equipment, although in the modern air transport system, this rarely occurs.
So with the good qualities of tail-mounted jets, why would anyone put the engines on the wings? That’s a good question. Wing-mounted engines have a lot of aerodynamic benefits that tail-mounted jets don’t enjoy.
Firstly, the weight of the engines on the wing relieves some of its bend. By providing the weight of the podded engines, designers can control how much the wings bend, and thereby reduce some structural weight. The engine pylons, or fairings where the engine pod connects to the wing, also reduce the flow of air laterally on the wing, which reduces drag. In addition, having the engine mounted below the wing helps mechanics performing inspections or servicing the jet.
When Boeing designed the 707, the engines were mounted in pods below the wing just as they’d been placed on the B-47 and B-52 jet bombers. Douglas took a similar approach with its DC-8 series aircraft while De Havilland of England mounted its jets in the root of the wing.
The downside of wing-mounted engines is pretty much the opposite of the benefit of tail-mounted engines. Wing-mounted jets require landing gear tall enough to make them fit, which often makes the aircraft too tall for servicing without ground support equipment (early 737s are a rare exception, although now this is proving troubling for Boeing!). Additionally, with engines being so low to the ground, they tend to act like vacuum cleaners and suck up dirt and debris on the runway and taxiways.
Big jets without ground support
There are instances in which large airplanes are flown into airports without proper servicing equipment. What are pilots of these jets to do?
Well, in the west, most large airplanes like the L-1011 and 747 fly into airports with jetways, air-stairs and modern support equipment. However, behind the Iron Curtain, airplanes had to be totally self-sufficient as many airports had a landing strip, a tarmac, a terminal, and little else.
So when the Soviet Union set about building its first “Aerobus” wide-body aircraft, wing-mounted engines were a requirement, but it also had to make each jet self-contained. The result of this requirement was the Ilyushin IL-86. It was made self-sufficient with features like an APU and integral air-stairs that led passengers into the airplane’s belly. Here, the “luggage in hand” system allowed passengers to deposit their own baggage in the jet’s hold, eliminating the need for belt loaders. Passengers then walked up stairs to the main deck of the jet.
It doesn’t take a PhD to see that allowing passengers to stow and collect their own baggage would be a lengthy affair! Luckily the system was rarely used.
Western operators would occasionally find similar challenges. British leisure airline Court Line found this out when it ordered a pair of L-1011 jets in the 1970s. These large aircraft were used in leisure destinations where ground support equipment was sparse, so it needed built-in belt loaders and air-stairs. Lockheed engineers were happy to oblige and built the jets a huge set of hydraulically operated stairs and integral belt loaders. A unique feature in the airline industry.
Skip ahead to the 8:18 mark in the video below from our friends at Classic Airliners & Vintage Pop Culture to see these unique features in action.
To sum up
Thanks again for the question, William. As a pilot that has flown jets with both wing-mounted and tail-mounted engines, I can say unequivocally that both have their strengths and drawbacks. The original tail-mounted engine design is still employed by airlines today, but is becoming rarer as airports improve and ground servicing equipment becomes more abundant. So enjoy those T-tailed jets before they become extinct!
Remember, if anyone has a burning aviation question or something you would like cleared up, drop us a line at Whitney@johnnyjet.com to get your question featured in an upcoming Ask a Pilot column.