The history of aviation is one of constant innovation. When you consider that mankind went from powered flight to breaking the sound barrier in a mere forty-four years, finding this continuous improvement and reinvention is not surprising. Thus, the history of the aircraft themselves is often full of rapid gains in technology and just as rapid obsolescence of that technology, a paradigm that remained throughout the bulk of the 20th Century.
For example, World War II and the technology that spun from it led to the development of jet aircraft for every role imaginable, from fighters to bombers to transports. Because of near-constant improvements and subsequent leaps in capability, aircraft of the late 40s and 50s were arguably obsolete by the time they arrived in the field.
Many famed aircraft in those days had tours of duty we would consider minuscule at seven or eight years in service before being superseded. Two aircraft of that era (well, one of the 50s and one of the 60s) have defeated that paradigm. While very different aircraft, they share some commonality of mission and share a great deal of American military history.
The military aircraft with the longest continuous production is the venerable C-130 Hercules, whose first flight in 1954 began an unprecedented and still continuing six decades of service. The other, the oft-maligned but always desired CH-47 Chinook, celebrates 54 years since its first flight on September 21, 1961. And, spoiler alert, I’m kinda biased towards the Chinook!

A rare photo of a US Coast Guard Piasecki HRP-1G, predecessor of the Chinook.

The tandem rotor design was pioneered by Frank Piasecki and his company Piasecki Helicopter in the 1940s. While complex in function and design, it solved a simple problem of physics. Single rotor helicopters produce a large amount of torque by spinning the main rotor. Unchecked, the helicopter fuselage would spin rapidly in the opposite direction, creating the need for the tail rotor (more specifically known as the ‘anti-torque’ rotor) to counteract this torque reaction and hold the helicopter on the heading you desire.
This leads to an inherently unstable platform (requiring constant input from the pilot, as power is increased and decreased), as well as ‘stealing’ the power required to turn the tail rotor and make it function. Tandem rotor systems turn in opposite directions, each one countering the torque of the other and allowing all the available power to be used to produce lift.
Piasecki Helicopter went on to become Vertol Aircraft Company (after a parting of the ways with its founder, Frank), and was bought out by Boeing in 1960. Vertol took advantage of the rapid increase in turbine engine development and reliability for use in helicopters. Its first design, the YHC-1A (which would become the CH-46 Sea Knight), was put forth for Army trials in 1958.
HC-1B, what would become the CH-47A

Capable of hauling 20 troops, the Army liked it but felt it was too big for the assault role and too small for cargo hauling. The Army decided to upgrade the Huey for assault and develop a larger, heavier transport helicopter. This new design, known as the YHC-1B, would have its first flight on 21 September 1961. In 1962, the HC-1B was designated the CH-47A under the 1962 United States Tri-Service aircraft designation system, also adopting the name Chinook, using the Army’s Native American naming convention.
The Chinook was a key component of the ‘air mobile’ land war concept of the day. By 1966, 161 aircraft had been delivered to the Army. The 1st Cavalry Division brought their organic Chinook battalion with them to Vietnam in 1965. These aircraft, and the ones that followed, would be critical to the war in Vietnam. They routinely placed artillery batteries in high mountain positions that were inaccessible by road, and kept them resupplied. They moved troops and their equipment from widely-separated bases to the battles they fought, all across the country.
Additionally, they were in constant use recovering downed aircraft, enabling these damaged airplanes and helicopters to be repaired and returned to service. Over the course of the war, Chinooks recovered over 12,000 aircraft, valued at over $3.6 billion.
CH-47B Vietnam Huey Lift
CH-47B preparing to recover a UH-1 Huey in Vietnam.

The CH-47A model was capable, but limited in payload by the design of its rotor blades. There was also the constant temptation to fill the capacious cabin (well beyond the ability of the aircraft to lift such a load). Almost immediately, work began on the CH-47B, an interim solution that incorporated new asymmetrical rotor blades and minor airframe modifications to increase lift and improve flying characteristics. All of the A models were upgraded to the B standard.
In 1967, development of the CH-47C began, bringing greatly improved engines, rotor blades, and transmissions. Multiple variants with different combinations of engines and rotor blades were produced over the next ten years, regularly increasing payload and gross weight by thousands of pounds with each improvement.
Missions for the Chinook were as varied as the theaters they supported. In Vietnam, it was aircraft recovery, troop transport, and resupply. In Cold War Europe, they shuffled nuclear warheads from cache to cache, playing the ‘shell game’ with their Soviet counterparts. Rarely was there a commander who didn’t find a need for this versatile airframe.
By the 1970s, the airframe was over a decade old, on the third version, and working on a fourth. The CH-47D began fielding in 1979, and brought a massive generational shift forward with more efficient rotor blades, more powerful engines, a triple cargo hook system, and a completely redesigned hydraulic and flight control system. To give a metric for the changes that had come in that 18-year period, here’s the Chinook, by the numbers:
CH-47D lifting HMMWVs in a ‘shotgun’ configuration.

Maximum Gross Weight
CH-47A: 33,000 lbs.
CH-47B: 40,000 lbs.
CH-47C: 46,000 lbs.
CH-47D: 50,000 lbs.
Maximum Hook Load
CH-47A: 16,000 lbs. (It rarely could lift more than 8,000!)
CH-47B: 16,000 lbs.
CH-47C: 20,000 lbs.
CH-47D: 27,000 lbs.
Yes, you read that right; the D model could lift about 75% of the A model’s maximum gross weight, just on one hook.
Rare in the helicopter world, the Chinook is one of the few airframes that can recover one of itself (albeit with the blades and often engines removed…but still, it’s a pretty badass capability!).
The longevity of the helicopter means that even though some new models were produced with each new designation, the bulk of the airframes were sent back to Boeing and rebuilt to the new specification. It is entirely possible (and probably likely) that three generations of helicopter pilots have flown the same airframe in different configurations (A, B, C, or D models).
The aircraft truly excelled in the mountains of Afghanistan.  The tandem rotor design meant it could operate at altitudes beyond the reach of many single-rotor helicopters, and could haul a good amount of cargo getting there.  It became the go-to aircraft throughout the entire theater of operations.
CH-47D in Afghanistan

With the advent of more modern production techniques, the arrival of the CH-47F model in 2006 brought new production in place of remanufacturing; with minor exception, all of the F models produced were ‘new build’, incorporating a greater number of single-piece sections of milled metal, reducing vibration and flexing points, as well as reducing inspections and increasing service life.
With the addition of the Rockwell Collins Common Avionics Architecture System (CAAS), the latest versions now include full RNAV (aRea NAVigation) capabilities tied to the Digital Advanced Flight Control System (DAFCS), which when put together provide a full-axis autopilot and flight management system. The new model has brought a flood of orders from around the world, and will grow the list of over a dozen foreign military customers.
As covered in some other postings here, there have also been several special operations variants produced.  While the 160th Special Operations Aviation Regiment did utilize C models in their earliest incarnation, they moved quickly to the D model when it became available, which led to the first ‘MH-47’ version.  The aircraft had a long laundry list of incremental improvements, focusing on improved navigation, NVG operations, and eventually, in-flight refueling.  In the 1990’s came the purpose-built MH-47E, with larger main fuel tanks, multi-mode terrain following radar, FLIR, and many more mission aids.  By 2005, the MH-47G brought the Rockwell Collins CAAS cockpit and new optical sensors to the fight.
CH-47F Cockpit. (Photo courtesy of U.S. Army)
CH-47F Cockpit. (Photo courtesy of U.S. Army)

Boeing plans to introduce the Block 2 upgrades to the F model in 2020, with new engines boasting a 20% increase in power, a new composite-based advanced rotor blade (developed out of testing for the RAH-66 Comanche), and raising the maximum gross weight to 54,000 lbs. Long-range plans already in the works visualize a Block 3 update starting after 2025, with retirement dates of future models anticipated in 2060, one hundred years after the first flight. We all know that such plans are written in tapioca, but the aircraft is looking pretty spry at 54. It’s definitely not your daddy’s Chinook, and I wouldn’t bet against it!
A big part of the reason for the longevity of this helicopter is simple; there is no replacement available. Similar to the C-130, no other airframe can perform the missions it performs, nor is there a reasonable replacement on the horizon. In fact, with so much growth available, why would anyone really try to build one? It is much cheaper to continue to develop and improve on the foundation established by Boeing Vertol in 1961 than it is to clean-sheet an entirely new design to provide exactly what the current version does. And that says an awful lot for the original design itself.
While the underpinnings and bolt-on components may have changed a lot over the years, the airframe has changed no more than a few inches over that whole time. If you compare some of the photos, you’ll see the pure lineage from A to G, with only minor differences. That’s a pretty solid design.
Oh, and kids…it was done with a pencil and a slide rule. Perhaps there’s something to be said about doing it Old School.