In my previous “Forgotten Aircraft” article I covered Boeing’s YC-14 and talked about how in the early 1970’s the United States Air Force (USAF) had expressed an interest in acquiring a replacement for the Lockheed C-130 Hercules. For this they initiated the Advanced Medium STOL Transport (AMST) program, and if you want more details on that I suggest you read that previous piece.
But in brief, the USAF decided on two designs to compete against one another; the afore-mentioned Boeing YC-14 and this, the McDonnell Douglas YC-15.
The roots of the YC-15 actually come from a surprising place; a French design, the Breguet 941.
This was a four-engine light-to-medium weight transport that had excellent Short Take-Off and Landing (STOL) characteristics and which McDonnell thought would be ideal for running commuter and freight services into small regional airports.
McDonnell Aircraft obtained a licence to produce this aircraft and attempted to market upgraded versions, ultimately unsuccessfully. When McDonnell merged with Douglas in 1967 the decision soon followed to design a much larger and more powerful STOL aircraft based off the experience gleaned for the -941, which became the McDonnell Douglas 210E-and-G design.
This was a rather interesting concept that was like a supped-up C-130, but nothing followed as it didn’t really offer enough worthwhile improvements over the Hercules to justify further development. But the work that McDonnel Douglas conducted on these STOL designs would have use when they made their bid for the USAF’s afore-mentioned AMST project.
For this McDonnell Douglas created a design that might appear somewhat conventional to modern eyes, but which paved the way for many of the standard features on military transport aircraft that we now take for granted.
The four engines initially selected for the YC-15 were the Pratt & Whitney JT8D turbofan. A well-established engine that was widely used on commercial airliners, including the Douglas DC-9, these each produced 16,000lbf each. This meant the YC-15 had considerably less power available than the competing YC-14 but the choice was thought to make the aircraft both more economically and combat sustainable.
After all, as a rough field, front line transport, the YC-15 would be expected to take damage at some point in its career, and McDonnell Douglas designed the aircraft to be capable of both STOL take off and landings as well as sustained flight should one engine be lost, something that was a potential concern on its twin-engine YC-14 rival.
Additionally, many of the aircraft’s parts were taken from existing aircraft, both reducing the costs of the prototypes and speeding up their production, meaning the first aircraft flew in August 1975, a full eight months ahead of schedule and a year before the Boeing YC-14.
And as said, the YC-15 didn’t lack innovation despite the speed of the design getting airborne. The aircraft was the first to fly with externally-blown flaps (EBF). Blown flaps divert some of the jet exhaust through ducts in the wings or fuselage to provide additional lift when required. But with the YC-15’s EBF’s, the wing flaps were designed to have double slotted bars that diverted the exhaust through and downwards across the flap’s upper surfaces, providing a tremendous amount of lift.
Obviously, this is a hugely simplified explanation and if you want to know more there is an entire paper on JSTOR detailing all the science – suffice to say, it was an extremely clever piece of aeronautic engineering.
Another interesting feature on the YC-15 was the fitting of what was called the Visual Approach Monitor, which today we would call a Heads-Up Display (HUD), the first time such equipment had been fitted to a transport aircraft. A feature more commonly associated with combat aircraft, McDonnell Douglas recognized that trying to land a heavily loaded aircraft onto a tiny dirt strip was also an exercise in intense focus for an aircrew and logically they, like their combat brethren, might appreciate not having to glance around the cockpit to gather critical flight information in such intense circumstances.
In addition, McDonnell Douglas recognized that the two prototype YC-15s were effectively test beds and made both aircraft slightly different. Though they shared the same basic fuselage, the second prototype had wings lengthened from 110 feet (34m) to 132 feet (40m).
Additionally, as McDonnell had time to spare before a fly off was organized between their aircraft and the YC-14 they conducted some experimentation on the engine fits, trialing a more powerful CFM56 on one of the aircraft; tests that give some of the photos of the YC-15s a rather odd appearance.
Flight tests showed the aircraft to have promise, and it proved capable of loading most of the US Army’s standard vehicles, including Huey Cobra attack helicopters.
Maximum payload was 150 paratroopers or a total of 35,000kgs (78,000lb) of cargo and top speed was recorded at 590mph (950km/h). The aircraft demonstrated its ability to refuel while flying and performed an air drop of 28,243lb (12,810kg) of cargo during one of the hundreds of test flights it performed.
Naturally, there were issues, but surprisingly few; the troop doors were really too narrow for paratroop deployment and the thrust reversers didn’t work as well as would be hoped in “hot-and-high” environment testing. But overall, the YC-15, and indeed the YC-14, were considered good aircraft and both suitable for potential development and deployment with the USAF. As a result the Air Force Systems Command (AFSC) initiated a selection process in September 1977, intending to award the production contract in April 1978 for whichever aircraft it decided best met the AMST requirement.
But in fact, the writing was already on wall for the YC-15, and indeed for the entire AMST program. Focus had begun to shift in the 1970s as it was recognized that the United States military didn’t need to worry so much about their tactical airlift capacity but more on expanding on their heavy lift, long-range cargo capabilities.
Incidents like the sudden need to reinforce Israel during the 1973 Yom Kippur War had shown that the existing aircraft, though they performed well, were hard stretched to meet sudden urgent need.
In 1976 the idea was even mulled as to whether a derivative of the AMST, whichever of the designs chosen, could be built as a strategic airlifter, dropping the STOL capability. But it was soon recognized that this wasn’t really a very efficient idea, plus other stakeholders began to make vocal their concerns and objections.
Lockheed, obviously with a heavy vested interest as the producer of the C-130 Hercules, lobbied hard for a modernized version of their aircraft to be adopted and that the AMST to be dropped on the grounds that the much greater fuel usage of the jet-powered aircraft was now far more expensive because of the oil crisis that was besetting the world at the time.
The USAF meanwhile hoped to get the support of the US Army for the AMST, but found that the Army was rather cautious of voicing in favour of the Air Force because of the poor management that service had displayed getting the C-5 Galaxy into service and the fact that a previous project that the Air Force had convinced the Army to throw their weight behind, the Light Intratheater Transport, had been abandoned at the whim of the Air Force.
In August 1977 the Army released their own study on what they needed in terms of airlift capacity, and that concluded that the C-130 was adequate for tactical usage but that they needed far more heavy lift capability from the USAF, principally more aircraft that could carry Main Battle Tanks if necessary.
In early 1978 the AMST program was suspended and in late 1979 cancelled completely, with a fresh requirement issued for a new strategic airlifter, the C-X.
Here the AMSTs, and especially the YC-15, played their final role. Boeing offered an enlarged version of the YC-14, fitted with a third engine.
Lockheed proposed a design that looked a lot like a beefed-up version of their C-141 Starlifter.
McDonnell Douglas created an aircraft that showed a definite family connection with the YC-15, an aircraft that would become the C-17 Globemaster III.
In fact, though it is easy to simply assert that the C-17 is a YC-15 with swept wings, the Globemaster is almost twice the size of its predecessor, an increase that required a massive amount of work to accomplish.
However, with much of the development team working on both programs there can be no denying that the YC-15 played a critical part in giving McDonnell Douglas experience in the complexities of heavy STOL aircraft design, helping to make the C-17 the success it is today.
Plus, the YC-15 did play a more direct part in the development of its giant successor. Though both YC-15s were placed in storage at the end of their flight-testing regime, with one unfortunately being broken up in 2012, the other was restored to flying order in 1996. It was then used to conduct testing of self-defense systems for use on the C-17.
And this aircraft, the only remaining YC-15, is currently on display at Edwards Air Force Base in California.
Sources/Related:
The AMST Program’s Lasting Legacy
External Blowing Flap Technology on the USAF McDonnell Douglas YC-15
The C-17: Modern Airlift Technology
https://en.wikipedia.org/wiki/McDonnell_Douglas_YC-15
https://thelexicans.wordpress.com/2013/04/23/mcdonnell-douglas-yc-15-part-1/
https://www.youtube.com/watch?v=rxjZ6HrUrmA
The Boeing YC-14; Taking on the King
