The McDonnell Douglas MD-11 is well-known among enthusiasts for its striking tri-jet configuration and popularity with cargo operators. It was an improved derivative of the McDonnell Douglas DC-10, which was the world’s second-ever widebody airliner, and it was designed specifically for long-haul flights.

To make it more capable and more efficient than the prior generation of widebodies, McDonnell Douglas introduced several minor improvements to the plane’s aerodynamics, in addition to the updated engines and stretched fuselage. Unfortunately, these changes did not transform the MD-11 into a plane fully competitive with clean-sheet designs like the Airbus A340 and Boeing 777.

However, the bigger issue was that it missed initial fuel burn specifications. The MD-11’s higher fuel burn reduced its range as a result, and this meant that McDonnell Douglas had to make further improvements to the MD-11. Still, this wouldn’t be enough, as airlines ultimately went with rivals from Boeing and Airbus, while the MD-11 became a failure.

Split Tip Winglets

The DC-10 did not have wingtip devices

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Wingtip devices were explored throughout the 20th century and were first put on a production jet aircraft in 1977 with the Learjet 28. The Airbus A300, A310, and A320 got wingtip fences in the 1980s, while the Boeing 747-400 introduced canted winglets in 1989. The MD-11 continued this trend when it debuted in 1990, featuring relatively small winglets that extend upwards with little angle.

It also had a secondary, smaller device that extends downwards, similar to split scimitar winglets on a modern 737. It’s generally been reported that the MD-11 sees as much as a 2.5% reduction in fuel burn from the implementation of the winglets. They work by weakening swirling wingtip vortices while also acting as a ‘wall’ to stop high-pressure air below the wing from bleeding to the low-pressure air above the wing.

In addition, the winglets slightly increase the aircraft’s wingspan and also increase its effective span even more, resulting in superior lift performance. Combined with the drag reduction, this reduces fuel burn. The main downside of wingtip devices is that they add weight, but generally speaking, they’ve become standard in the industry, and the MD-11 was one of the first types to widely adopt them.

The later Boeing 777 did not initially feature wingtip devices (raked wingtips were added with the second-generation 777 in the 2000s), and the 737NG also initially debuted without its signature blended winglets. Meanwhile, the winglets that you find on the 757 and 767 were developed as aftermarket options.

The Smaller Horizontal Stabilizer

Redesigned to reduce drag

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The MD-11 is based on, and is highly similar to, the older McDonnell Douglas DC-10. There are several common design elements between the two, which is expected, as the purpose of iterating on an existing design is to save development costs by reusing as many elements as possible and only revising what is necessary.

One of the major changes with the MD-11 was that McDonnell Douglas stretched the fuselage significantly compared to the DC-10, which also led to a new horizontal stabilizer. Stretching the fuselage resulted in a longer moment arm, increasing the effectiveness of the stabilizer and elevators.

In an aim to decrease drag during cruise, McDonnell Douglas therefore saw fit to decrease the size of the horizontal stabilizer, shrinking it by about 30% compared to the DC-10. The improved aerodynamics of the smaller stabilizer during cruise cut fuel burn, although there were side effects of the decision. The stabilizer was managed by a longitudinal stability augmentation system (LSAS) to ensure the aircraft’s stability through elevator deflections and trim.

It also featured ballast fuel tanks to further manage the plane’s center of gravity. However, the comparatively small horizontal stabilizer made the MD-11 tricky to fly on approach, and the aircraft had one of the highest landing speeds of any airliner. McDonnell Douglas subsequently updated the MD-11’s handling software to make it feel more similar to the DC-10.

Horizontal Elevator Bias Software

Optimizing for lower drag

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The MD-11 is not a fly-by-wire aircraft like the Airbus A320, A330, A340, or Boeing 777, which means that the cockpit controls are physically connected to the control surfaces via a system of cables and pulleys. The autopilot, meanwhile, uses mechanical servos to manipulate the flight control system.

However, the MD-11 does extensively feature advanced computer systems for its time, and that includes the LSAS, which not only functions to improve handling but also to decrease drag. The LSAS software was always designed to make elevator deflections and trim adjustments to cut fuel burn, and McDonnell Douglas further iterated on it.

After the MD-11 failed to meet its fuel burn specification, the manufacturer returned to the drawing board to figure out how to cut drag even more. The main selling point of the MD-11 was that it was meant to have even more range than a Boeing 747-200 while burning less fuel and seating fewer passengers, so fixing the fuel burn issues was critical. The major Phase IV package in 1996 was a thorough overhaul of the aircraft to recover the lost range and improve fuel burn.

With this, McDonnell Douglas was able to shave off 0.4% of the aircraft’s fuel burn by altering the elevator deflections and bias. While the figure seems small, it still has a major impact considering the size and mission of the MD-11, and the Phase IV package also incorporated other benefits to cut fuel burn even more. However, the Phase IV package didn’t do much to boost MD-11 sales.

Improvements To The Flap-Hinge Fairings And Seals

Part of a crucial performance improvement package

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The MD-11, before entering service, was meant to have a range of roughly 7,000 NM (13,000 km) with a payload of 28 tons, although the exact payload-range depends on the engine model equipped. However, the excessively high fuel burn meant that its range was far more restrictive, and even after the first Phase I drag improvement project, it could only fly the advertised range with 22 tons of payload.

McDonnell Douglas therefore continued to work on improving the MD-11 to catch up to its original promises. The Phase IV package was among the most thorough set of improvements made to the MD-11. There were the previously mentioned changes to the LSAS software, but McDonnell Douglas also modified the design of the flap-hinge fairings (essentially the large fins underneath the wings).

By improving the fairings and the seals, McDonnell Douglas was able to achieve a 0.5% reduction in fuel burn, which, when paired with the LSAS improvement and other refinements, helped the Phase IV package to deliver a 1.2% reduction in fuel burn. Not only were the Phase IV improvements rolled out to the standard MD-11, but they were also incorporated on the MD-11ER.

Combined with the aerodynamic improvements, an extra fuel tank and a new Maximum Takeoff Weight (MTOW) of 286 tons, the aircraft had a range of 7,240 NM (13,410 km), which is more than the missed target for the MD-11. However, only a handful of MD-11ERs were ever built, although most of the MD-11ER’s features were rolled out onto the standard MD-11 variants.

Wing-Pylon Fillet Modifications

Crucial to reducing drag

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As part of an earlier improvement program, McDonnell Douglas began working on a modification to the wing-pylon fillet, intended to smooth airflow where the pylon meets the wing. Further improvements to the modifications were found to yield as much as a 1% saving in fuel burn, which is a big number in commercial aviation.

With this and other improvements, McDonnell Douglas engineers clearly put significant effort into enhancing the MD-11’s aerodynamics to minimize fuel burn, but this didn’t save the aircraft. No amount of aerodynamic modifications and improvements would change the fact that the MD-11 was a three-engined aircraft based on a design that entered service in 1971 and was first developed in the 1960s.

The aircraft also entered service heavier than expected, and the wing proved undersized for the missions that the MD-11 was designed for. On its own, these issues would have been less problematic, except for the fact that the MD-11 was also competing against the Airbus A340 (which proved more fuel-efficient) and the twin-engine Boeing 777. In the cargo market, the MD-11F succeeded in large part because it entered service before the Boeing 747-400F and 767-300F and was sized in between the two.

The similarly sized 777F entered service almost 18 years later and is a fundamentally heavier aircraft designed for much longer missions, which is why it’s remained in service for so long after production ended. Passenger airlines, however, simply had much better choices available, and the aerodynamic fixes were simply a band-aid solution.