Why (almost) only business jets have their engines behind

Engines at the stern and T-tail: New passenger aircraft hardly look like this today. Why is that?

The Comac ARJ21 is special in two ways. The regional jet is the first commercial aircraft developed in-house by China, and it has a T-tail and carries its engines at the rear. Such planes are not unusual or even exotic, and even the norm for private flights, but as a passenger you are rarely on the move in planes of this type.

Every aviation enthusiast can immediately list several types with this engine arrangement. What is special about the ARJ 21, however, is that a new commercial passenger aircraft is being built. Otherwise, T-tail units and tail engines have long since disappeared from modern commercial aircraft.

Learning through old construction

The fact that China’s state aircraft manufacturer Comac opted for the design for the ARJ21 is due to the inspiration that has its roots in the USA. The aircraft is a shortened modification of the McDonnell Douglas MD-90 in its construction. Components of the American short and medium-haul aircraft were once manufactured under license in the People’s Republic. While imitations are frowned upon in western spheres, imitation is an efficient learning method in China, which is rapidly catching up with technology.

Undisturbed wings

China’s civil aircraft construction goes through a development in a short time that dragged on in western aircraft construction over several decades. At the beginning of the jet age, stern engines under American, European and Soviet designs were widespread. Corresponding models from the 1950s to 1960s such as the Sud Aviation Caravelle, Boeing 727, BAC 1-11, Douglas DC-9 or Tupolev Tu-134 enjoy legendary status today. One advantage of engines at the rear is that they do not disrupt the aerodynamics of the wings. For aeronautical engineers, the change from propeller to jet drives means a large increase in the speed at which the air flows around the wings. The already complicated aerodynamics suddenly became even trickier.

Aerodynamic law gets in the way

Not only did engines at the stern allow wings to be held more easily. Jet planes also rushed into the so-called transonic speed range. From around 80 percent of the speed of sound, the so-called area rule increases in importance. This aerodynamic calculation method describes an optimal course of the fuselage cross-section from bow to stern. According to the area rule, the fuselage must be a little narrower where the wings attach to the fuselage. In the worst case, an air wave could occur that slows the aircraft down.

Boeing 707 paved the way

Some earlier supersonic fighter jets, such as the Northrop F-5, have a fuselage that is shaped like a wasp waist for this reason. Just like the wings, engines also have a significant influence on the area rule. If they sit on the wings, they, together with the wing and its wing box on the fuselage, ensure a fairly high cross-section. Engineers were able to get this problem under control. This was proven by the Boeing 707, which revolutionized long-haul traffic with this design from the 1950s. But in times when no standard computer programs helped the designers, the development of an aircraft with tail engines was simply less complicated.

Dangerous slipstream

In terms of aerodynamics, however, aircraft with rear engines have a disadvantage. And this can be critical. Aircraft with tail motors often have a T-tail unit: The horizontal tail units are not attached to the fuselage, but rather to the tail unit – often high up. Aircraft with such tail units can get into a so-called deep stall. This occurs when the air coming from the front flows around the aircraft at a very oblique angle. The wings can then create a slipstream in which the elevators get.

Manufacturers had to make adjustments

In this state the elevators lose their control effect. Pilots can hardly control the angle of the nose. And this is exactly what has to be changed in order to get out of the Deep Stall. A life-threatening vicious circle. Preventive warnings from cockpit systems, and later the use of flight control software, countered the problem. Many manufacturers also adapted the aerodynamics of their aircraft. But if the engines are under the wings, it is easier for designers in other ways too.

Good balance

If the engines are suspended under the wings, they are also closer to the aircraft’s center of gravity and lift than if they are attached to the tail. This makes it much easier for designers to design the architecture of an aircraft to be aerodynamically stable. Stretched versions of an airplane are also easier to implement. The longer a fuselage, the further away the stern usually is from the wings. If the motors are then suspended, there is also a greater torque which also affects the balance of an aircraft. The problem of mass distribution is exacerbated by the fact that engines have tended to get bigger and heavier for decades – they have to do more and more.

Private jets keep tail engines alive

Even with modern regional jets, current models like the Airbus A220, Suchoi Superjet or Embraers E-Jets ring in the era of planes that carry their engines under the wings. But T-tail units and rear motors are by no means completely obsolete. This design is still the standard for private jets, the most famous exception being the HondaJet. The reasons for this can be found in their locations. Unlike commercial passenger jets, small private jets often land outside large airports, where there are no boarding bridges.

Closer to the ground

If there are no engines under the wings, the fuselage requires less ground clearance. Indeed, the HondaJet has the engines perched on supports on the fenders. In order to be able to board a conventional private plane, only a small ladder is needed, which can be unfolded out of the on-board door. The design of flight stability is also less tricky because private jets are more compact. Combined with this, the distribution of the centers of buoyancy and weight is less complicated. Likewise, high-bypass rate engines, which make engines heavier and heavier for passenger planes, barely found their way into private planes.

More safety in the event of an engine failure

When used on much smaller runways, rear engine private jets also have a safety advantage. In the event of an engine failure, the asymmetry of the uneven thrust has less effect on the control of the aircraft with the engines close together.