Longtime readers know I’m not at all a fan of the Marine Corps MV-22 Osprey program. But most of my objections to the program center on its costs, not on the aircraft itself. Personally, I think it is a pretty neat bird. And I enjoy watching them fly by en route from MCAS Miramar to 29 Palms. But every time someone tells me how revolutionary it is, I feel a strong urge to remind them that it is hardly new concept.
Almost from the first time helicopters flew, engineers started tinkering with ways to combine the speed characteristics of an airplane with the vertical take off and landing of a helicopter. After all, the rotors that lift a helicopter look an awful lot like the propellers that move planes forward. Was there a way to use one set of blades for both jobs? Taking off and landing a propeller driven plane from a tail sitting position was tried, but was soon found to be impractical, mostly because the pilot would have to fly looking over his shoulder.
Pretty soon, the concept of rotating either the entire wing, or just the rotors, from the vertical to the horizontal was tested. A variety of test aircraft were designed, built and tested throughout the 1950s. Most were little more than test-beds to explore the concept of a convertible plane.
By 1959, enough experience had been gained with tilt-rotor and tilt-wing test beds that the DoD actually began to consider designing a plane that could eventually enter service. After a couple more years of effort, the Tri-Service Assault Transport Program began in 1961, with the Navy as the lead agency for DoD. Ling-Temco-Vought (LTV) in partnership with the Hiller and Ryan companies, was awarded a contract to design and build a prototype tilt-wing transport that would have better range and speed than existing helicopters.
The resulting aircraft, the XC-142A would be the closest a convertiplane would come to entering service until the MV-22 joined squadron service with the Marines 40 years later.
One interesting administrative note, the XC-142A was numbered in the regular tri-service designation system under the conventional transport series, and not in the convertible aircraft series. It really should have had a designation of XCV-XXX. One can only guess, but perhaps the program managers felt the XC-142A was so much more likely to be bought in numbers than previous aircraft, that it should not share a series of what had heretofore been strictly test beds. And, rather annoyingly for your author, the XC-142A does not appear to have even been given a nickname or popular name.
On a conventional helicopter, each blade of the rotor is independently articulated. That is, it changes its angle of attack, or pitch, continuously throughout its rotation around the hub, and does so independently from the other blades. At any given time, each blade of a rotor is at a different pitch. Conventional propellers, even though they have variable pitch, do not do this. Instead, all blades of a conventional propeller change pitch simultaneously, and maintain that pitch setting throughout their journey around the hub. Helicopter rotors use this articulation to tilt the rotor disc forward or backward or side to side to provide thrust in the desired direction of flight, in addition to providing the lift to keep the helicopter in the air. But this articulation is also rather complex. That’s why when you look at the hub of a helicopter rotor, there’s all sorts of fiddly bits.
Previous convertible planes had suffered from excessive vibration and complexity, and LTV was at pains to avoid this. And so they came up with a pretty simple solution. By vastly overpowering the aircraft with four T64 turboprops, and using conventional propellers only slightly larger than normal, they had more than enough lifting force to meet the requirements. What was needed was a way of controlling the aircraft in hovering flight without adding the complexity of full articulation to the props. Since the entire wing rotated, the ailerons, normally used to control roll, could be used instead to control yaw in hovering flight. The airflow from the propellers would be sufficient to make the ailerons effective. Roll control in the hover would be provided by differential clutching of the outboard propellers. Pitch control in the hover would be by means of a small horizontal rotor at the very tail of the aircraft. In conventional flight modes, conventional control surfaces would be used.
One additional layer of complexity (and thus weight and cost and maintenance) that could not be avoided was crosslinking all four engines to a common drive shaft. Imagine the XC-142A in a hover. Should one of the engines fail, particularly an outboard engine, the loss of lift on one side would cause an uncontrollable roll and loss of the aircraft. The answer was to have all four engines driving a common shaft, so even if a quarter of the total power was lost, the thrust would still be delivered symmetrically.
Aside from the whole tilt-wing thing, the XC-142 was a fairly conventional transport design. A boxy fuselage with a split ramp at the rear, with tricycle landing gear, with main mounts retracting into blisters along the fuselage side. In fact, because its propellers weren’t too large, it could take off and land conventionally with no tilt to the wing at all.
This was a very ambitious program. Remember, when the contracts were signed, the UH-1, CH-46, and CH-47 were just being accepted for service.
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Five aircraft were built, and put through their paces.
The aircraft actually flew quite well, and its performance met the required specifications. But several things conspired to keep the XC-142 as a historical curiosity, and not a long serving warhorse.
First, the cross-linked driveshaft was troublesome. It produced excessive vibration (like virtually all its predecessors) and was less than wholly reliable. Problems with the shafting would lead to hard landings and damaged aircraft. And excessive vibration in a testing environment could only be seen as a harbinger of frequent failure in any future service environment.
Secondly, the utility of what would inevitably be an expensive aircraft was questioned. An XC-142 might lift 30 troops 100 miles twice as fast as a helicopter, but if it cost more than twice as much to buy and operate, buying two helicopters suddenly looks a lot more appealing.
Third, just as the XC-142 began to fly, the US was becoming ever more deeply involved in Vietnam, and the bulk of defense spending was going to fund that war and the machines needed right then, not some time in the future.
One by one, the each of the services in the Tri-Service program dropped out. The remaining aircraft were transferred to NASA, who used them for testing until 1970, when the last survivor was transferred to the Air Force Museum.
It’s odd that I couldn’t find a single decent video of the TC-4C, a plane with almost 30 years of service, but was instantly able to find quite a bit of good footage of this also-ran.
Thanks to Jason Camlic, who inspired this post via a post of his on Facebook. I can’t figure a good way to link traffic to him, but he’s always a great source of ideas and interesting tidbits from the world of aviation.