Aviat Pitts S 1: The Ultimate Aerobatic Biplane

Introduction

The Aviat Pitts S-1 is a single-seat, high-performance aerobatic biplane that has become an iconic aircraft in the world of aerobatics. Originally designed by Curtis Pitts in the 1940s, the Pitts Special series, including the S-1, has dominated aerobatic competitions for decades and continues to be a favorite among pilots for its agility, performance, and relatively low cost. The Pitts S-1 is renowned for its compact design, powerful engine, and remarkable maneuverability, making it an ideal aircraft for both competitive aerobatics and airshows. This article delves into the history, design features, performance specifications, and continued relevance of the Pitts S-1.

History of the Pitts S-1

The story of the Pitts Special begins with Curtis Pitts, a visionary aircraft designer who sought to create a small, lightweight biplane capable of performing complex aerobatic maneuvers. The first Pitts Special took flight in 1944, and from that point forward, it became a standard in aerobatic aviation. The Pitts S-1, specifically, was designed as a single-seat version of the aircraft to cater to individual pilots who wanted an agile and responsive plane for aerobatic competitions.

Curtis Pitts’ initial designs were hand-drawn and intended for homebuilders. Over time, as the aircraft gained popularity, more professional plans were made available in the 1960s. By 1973, Aviat Aircraft began producing factory-built versions of the Pitts S-1S, which had been certified by the FAA. The design became so successful that it dominated international aerobatic competitions throughout the 1960s and 1970s.

One notable event in the history of the Pitts S-1 occurred in 1972 when Bill Strohmeyer flew the aircraft upside down at 164 mph over Blackbushe Airport in England, setting world records for both speed and height while inverted. This feat exemplified the aircraft’s extraordinary capabilities and cemented its place as one of the most successful aerobatic planes ever built.

Design Features

The Aviat Pitts S-1 is distinguished by its compact size and lightweight construction. The fuselage is made from welded steel tubing covered with fabric, while the wings are constructed from wood with fabric covering. This combination of materials ensures that the aircraft remains light yet strong enough to withstand the extreme forces encountered during aerobatic maneuvers.

Key design features include:

• Single-seat configuration: The Pitts S-1 is designed for solo flights, making it ideal for pilots who want complete control over their aerobatic performances.
• Biplane structure: The dual-wing design provides greater lift at lower speeds compared to monoplane designs, which is crucial during tight turns and loops.
• Fixed tailwheel landing gear: This traditional landing gear setup is simple yet effective for small airstrips and grass fields.
• Open cockpit: The open cockpit design adds to the thrill of flying this aircraft but also requires pilots to wear protective gear such as helmets and goggles.

The aircraft’s wingspan measures approximately 6.10 meters (20 feet), and its overall length is about 5.41 meters (17.75 feet). Despite its small size, it packs a punch with its powerful engine options.

Engine and Powerplant

The heart of the Pitts S-1 lies in its engine. Most variants are powered by a Lycoming IO-360 engine that produces around 200 horsepower, though earlier models featured smaller engines like the Lycoming O-290 or O-320 with outputs ranging from 125 hp to 180 hp. The IO-360 engine is fuel-injected and air-cooled, providing reliable performance even during intense aerobatic routines.

This engine allows the Pitts S-1 to achieve remarkable performance metrics:

• Max cruise speed: 113 knots (209 km/h)
• Rate of climb: 2,600 feet per minute (13.21 meters per second)
• Service ceiling: 20,000 feet
• Travel range: Approximately 298 nautical miles (552 kilometers)

These specifications make it clear why the Pitts S-1 has remained a favorite among aerobatic pilots—its power-to-weight ratio allows it to perform complex maneuvers with ease while maintaining excellent control at both high and low speeds.

Performance Capabilities

The performance capabilities of the Pitts S-1 are what truly set it apart from other aircraft in its class. With a roll rate of up to 300 degrees per second and a high thrust-to-weight ratio, this biplane can execute rapid rolls, loops, hammerheads, spins, snap rolls, and other advanced maneuvers effortlessly.

Some key performance data include:

These figures highlight just how nimble and efficient this aircraft can be in various flight conditions. Whether performing at low altitudes during airshows or climbing rapidly to higher altitudes during competitions, the Pitts S-1 delivers consistent performance across all scenarios.

The Aviat Pitts S-1 remains one of the most iconic aerobatic biplanes ever produced. Its combination of lightweight construction, powerful engines, and unmatched agility has made it a staple in both competitive aerobatics and airshows worldwide. Even decades after its initial introduction, pilots continue to be drawn to this aircraft for its thrilling flight characteristics and proven reliability.

In subsequent sections of this article, we will explore more about its variants like the Pitts S-2 series as well as its place in modern aviation training programs.

Citations:
[1] https://en.wikipedia.org/wiki/Pitts_Special
[2] https://www.skytamer.com/Pitts_S-1S.html
[3] https://en.wikipedia.org/wiki/Aviat
[4] https://mapsairmuseum.org/pitts-s-1-pitts-special/
[5] https://www.aeroexpo.online/prod/aviat-aircraft-inc/product-175926-60847.html

Variants and Evolution of the Pitts S-1

The success of the Pitts S-1 spawned several variants and evolutions, each designed to cater to different pilot needs and performance requirements. While the original Pitts S-1 was a single-seat aircraft primarily aimed at individual aerobatic pilots, later models introduced enhancements in power, handling, and even seating configurations. These variants have allowed the Pitts Special to remain relevant in competitive aerobatics for decades.

Pitts S-1S

The Pitts S-1S is perhaps the most well-known variant of the Pitts S-1 series. It was developed as a factory-built version of the original design and became certified by the FAA in 1973. This model is powered by a Lycoming IO-360 engine producing 200 horsepower, which gives it a significant boost in performance compared to earlier models that used less powerful engines.

Key features of the Pitts S-1S include:

• Increased power: The 200-horsepower engine allows for more aggressive aerobatic maneuvers and faster climb rates.
• Improved roll rate: With a roll rate of up to 300 degrees per second, the S-1S is capable of executing rapid rolls that are essential for competitive aerobatics.
• Certified production: Unlike earlier homebuilt versions, the S-1S was produced in Aviat’s factory, ensuring consistent quality and adherence to safety standards.

The Pitts S-1S quickly became a favorite among competitive pilots due to its enhanced performance and reliability. It was used extensively in both national and international aerobatic competitions throughout the 1970s and 1980s.

Pitts S-1T

The Pitts S-1T is another popular variant that builds upon the success of the S-1S. Introduced in the late 1970s, the S-1T features several upgrades designed to improve its overall performance and handling characteristics. One of the most significant changes is the introduction of a constant-speed propeller, which allows for better power management during aerobatic routines.

Other improvements in the S-1T include:

• Constant-speed propeller: This feature allows pilots to adjust the pitch of the propeller blades during flight, optimizing performance for different phases of flight (e.g., takeoff, climb, cruise).
• Enhanced control surfaces: The ailerons on the S-1T are larger than those on previous models, providing even greater roll authority and responsiveness.
• Refined aerodynamics: Minor tweaks to the aircraft’s aerodynamic profile help reduce drag and improve overall efficiency.

The combination of these upgrades makes the Pitts S-1T one of the most capable aerobatic biplanes ever produced. Its precise handling and powerful engine make it ideal for advanced aerobatic maneuvers such as snaps, spins, and tumbles.

Homebuilt Variants

In addition to factory-built models like the S-1S and S-1T, many pilots have opted to build their own versions of the Pitts Special using plans provided by Curtis Pitts or other sources. These homebuilt variants often feature custom modifications tailored to individual preferences or specific competition requirements.

Some common modifications seen in homebuilt Pitts aircraft include:

• Larger engines: Some builders have opted to install more powerful engines (e.g., Lycoming IO-540) for increased performance.
• Modified wings: Builders may choose to alter wing designs by changing airfoil shapes or adding additional bracing for improved strength.
• Custom avionics: Many homebuilt Pitts aircraft feature modern avionics systems that provide enhanced navigation, communication, and flight control capabilities.

The flexibility offered by homebuilt variants has allowed pilots to experiment with different configurations while still benefiting from the proven design principles of the original Pitts Special.

Two-seat Variants: The Pitts S-2 Series

While the single-seat configuration of the Pitts S-1 is ideal for solo aerobatic performances, there was also demand for a two-seat version that could accommodate an instructor or passenger. This led to the development of the Pitts S-2 series, which includes several two-seat variants designed for training purposes as well as recreational flying.

Pitts S-2A

The first two-seat variant was the Pitts S-2A, introduced in 1967. It features a tandem seating arrangement with one seat in front of the other, allowing an instructor or passenger to accompany the pilot. The aircraft is powered by a Lycoming AEIO-360 engine producing 200 horsepower, which provides ample power for aerobatic maneuvers even with two occupants on board.

Key features of the Pitts S-2A include:

• Tandem seating: This configuration allows for effective communication between instructor and student during flight training.
• Dual controls: Both seats are equipped with flight controls, making it easy for instructors to take over if necessary.

The Pitts S-2A became widely used as a training aircraft for aspiring aerobatic pilots due to its forgiving handling characteristics and robust construction.

Pitts S-2B

Building on the success of the S-2A, Aviat introduced an upgraded version known as the Pitts S-2B. This model features several enhancements aimed at improving both performance and comfort:

• More powerful engine: The Lycoming AEIO-540 engine produces 260 horsepower, giving it significantly more power than its predecessor.

With its increased power output and refined handling characteristics, the Pitts S-2B became a favorite among advanced aerobatic pilots who wanted a two-seat aircraft capable of performing at high levels in competitions.

Pitts S2-C

The latest iteration in this series is known as the Pitts S2-C, which incorporates further refinements aimed at improving both performance capabilities while maintaining safety standards required for training purposes

The Pitts S-1 in Modern Aerobatics

Despite its origins in the 1940s, the Pitts S-1 continues to hold a significant place in modern aerobatics. Its combination of lightweight construction, powerful engines, and exceptional maneuverability makes it a popular choice for both amateur and professional pilots. The aircraft’s design has evolved over the years, with various modifications and improvements that have kept it competitive even as newer aerobatic planes have entered the market.

Competitive Aerobatics

The Pitts S-1 has been a dominant force in aerobatic competitions since its inception. In the 1960s and 1970s, it was the aircraft of choice for many top aerobatic pilots, and it helped secure numerous national and international titles. Even today, the Pitts S-1 remains a potent competitor in lower categories of aerobatic competition.

One reason for the Pitts S-1’s continued success is its ability to perform complex maneuvers with ease. The aircraft’s high roll rate, combined with its powerful engine and responsive controls, allows pilots to execute precise maneuvers such as snap rolls, spins, loops, and hammerheads. These maneuvers are essential in aerobatic competitions, where pilots are judged on their ability to perform a series of complex figures with precision and grace.

In recent years, however, monoplanes like the Extra 300 and Sukhoi Su-26 have become more popular in top-level aerobatic competitions due to their higher speeds and greater agility. Despite this shift, the Pitts S-1 remains highly competitive in intermediate and advanced categories of competition. Its lower cost compared to modern monoplanes also makes it an attractive option for pilots looking to enter the world of competitive aerobatics without breaking the bank.

Airshows and Recreational Flying

Beyond competitive aerobatics, the Pitts S-1 is a favorite among airshow performers and recreational pilots. Its compact size and agility make it ideal for performing thrilling stunts in front of large crowds at airshows. The Pitts’ ability to perform tight loops, rolls, and tumbles at low altitudes creates an exciting spectacle that never fails to impress audiences.

Many airshow performers choose the Pitts S-1 for its reliability and ease of maintenance. The aircraft’s simple design means that it can be easily repaired or modified by skilled mechanics or even homebuilders. This has made it a popular choice for pilots who want to customize their aircraft to suit their specific needs or preferences.

Recreational pilots also appreciate the Pitts S-1 for its exhilarating flight characteristics. Flying a Pitts is often described as a “pure” flying experience due to its open cockpit design and responsive controls. Pilots who fly the Pitts often speak of the joy they feel when performing aerobatic maneuvers or simply cruising through the sky on a clear day.

Training Aircraft

While the single-seat configuration of the Pitts S-1 limits its use as a training aircraft, many aspiring aerobatic pilots begin their training in two-seat variants like the Pitts S-2A or S-2B. These models retain many of the same flight characteristics as the S-1 but offer space for an instructor or passenger.

The tandem seating arrangement in these two-seat variants allows instructors to provide hands-on guidance during flight training sessions. Both seats are equipped with flight controls, making it easy for instructors to take over if necessary. This setup is ideal for teaching students how to perform basic aerobatic maneuvers before they transition to more advanced aircraft like the single-seat Pitts S-1.

Many flight schools around the world offer training programs that use Pitts aircraft as part of their curriculum. These programs typically focus on teaching students how to safely perform aerobatic maneuvers while also emphasizing important skills such as energy management and situational awareness.

Homebuilding Community

One unique aspect of the Pitts Special series is its strong connection to the homebuilding community. Since plans for building a Pitts were first made available in 1960, hundreds of homebuilders have successfully constructed their own versions of this iconic aircraft.

Building a Pitts from scratch requires significant skill and dedication, but many homebuilders find the process rewarding. The flexibility offered by homebuilt variants allows builders to customize their aircraft with different engines, avionics systems, or even wing designs depending on their personal preferences.

Organizations like Steen Aero Lab continue to offer plans and components for various versions of the Pitts S-1, including modernized variants like the S-1SS, which incorporates advanced aileron technology originally developed for world-class competition aircraft like the Super Stinker (S-1-11B)[2][3]. These modern upgrades allow homebuilders to create high-performance aircraft that are capable of competing at high levels while still retaining much of what makes flying a Pitts so special.

Maintenance and Upkeep

Maintaining an aerobatic aircraft like the Pitts S-1 requires regular inspections and adherence to strict safety protocols due to the stresses placed on both airframe and engine during flight maneuvers[6]. Aviat Aircraft continues to provide parts support for older models no longer in production while also offering service bulletins that address common maintenance issues[6].

Some typical maintenance tasks include:

• Inspecting control surfaces: Due to frequent use during aerobatic routines (e.g., rolls), control surfaces like ailerons need regular inspection.
• Engine checks: High-performance engines used in aerobatic planes require more frequent servicing compared with standard general aviation engines.
• Airframe inspections: The stresses encountered during high-G maneuvers can cause wear on structural components over time; regular inspections help ensure that any issues are caught early before they become serious problems.

Proper maintenance ensures that these iconic biplanes remain safe and reliable throughout their operational lives—whether they’re being flown competitively or recreationally.

Citations:
[1] https://mapsairmuseum.org/pitts-s-1-pitts-special/
[2] https://www.airliners.net/aircraft-data/pitts-s-12-special/46
[3] https://www.avweb.com/ownership/pitts-special/
[4] https://en.wikipedia.org/wiki/Pitts_Special
[5] https://aviataircraft.com
[6] https://aviataircraft.com/pitts-service-bulletins/
[7] https://pacificcoastairmuseum.org/aircraft/pitts-special-s1/
[8] http://www.pilotfriend.com/aircraft%20performance/Pitts%20S2C.htm
[9] https://www.steenaero.com/pitts-s1/history/
[10] https://www.skytamer.com/Pitts_S-1S.html

The Legacy of Curtis Pitts and the Pitts Special

The Pitts Special series, particularly the Pitts S-1, has left an indelible mark on the history of aerobatic aviation. Curtis Pitts, a self-taught aircraft designer, first conceived the idea for a high-performance aerobatic biplane during World War II. His vision was to create a small, lightweight aircraft that could perform the most demanding aerobatic maneuvers with ease. The first prototype of the Pitts Special took flight in 1944, and it quickly became apparent that Curtis Pitts had created something truly special.

Curtis Pitts: A Visionary Designer

Curtis Pitts was born in 1915 and grew up with a fascination for aviation. Despite having no formal training in aeronautical engineering, he had an innate understanding of aircraft design and mechanics. In the early 1940s, he began working on a new type of biplane that would be smaller and more agile than anything else in the sky. His goal was to create an aircraft that could outperform the larger, heavier planes typically used for aerobatics at the time.

The first Pitts Special was powered by a modest 55-horsepower engine, but even with this small powerplant, it demonstrated remarkable agility and control. Over time, Pitts continued to refine the design, increasing engine power and making adjustments to the airframe to improve performance. By 1947, the Pitts Special had become one of the most capable aerobatic aircraft in existence.

Betty Skelton and “Little Stinker”

One of the key figures in popularizing the Pitts Special was Betty Skelton, a pioneering female aerobatic pilot who became known as the “First Lady of Aerobatics.” In 1947, Skelton purchased one of Curtis Pitts’ early designs—a Pitts S-1 known as “Little Stinker”—and used it to win multiple national aerobatic championships. Skelton’s performances in “Little Stinker” captivated audiences across the United States and helped cement the Pitts Special’s reputation as a world-class aerobatic aircraft.

Skelton’s accomplishments were groundbreaking not only because of her skill as a pilot but also because she helped break down barriers for women in aviation. Her success with the Pitts S-1 inspired countless other pilots—both men and women—to take up aerobatics and pursue their dreams in aviation.

The Evolution of the Pitts Special

After Betty Skelton’s successes with “Little Stinker,” demand for Curtis Pitts’ designs began to grow. In response, he started offering plans for homebuilders who wanted to construct their own versions of the Pitts Special. The first set of plans was made available in 1962 for what would become known as the Pitts S-1C, or “Flatwing” model. This version featured a two-aileron configuration (ailerons on only one set of wings) and was typically powered by engines producing around 100 horsepower.

As more pilots began building their own Pitts Specials, they started experimenting with different modifications to improve performance. One common upgrade was to replace the original engine with a more powerful Lycoming engine capable of producing up to 180 horsepower. This significantly increased the aircraft’s performance, allowing it to perform even more demanding maneuvers.

In 1973, Curtis Pitts introduced a factory-built version of his design known as the Pitts S-1S. This model featured four ailerons (one on each wing), symmetrical airfoils for improved handling during inverted flight, and a more powerful Lycoming IO-360 engine producing 200 horsepower[1]. The S-1S quickly became popular among competitive aerobatic pilots due to its enhanced roll rate and overall performance.

The Super Stinker: Pushing Boundaries

In addition to refining his original designs, Curtis Pitts also worked on developing new models that could push the boundaries of what was possible in aerobatic flight. One such model was the Pitts S-1-11B, also known as the “Super Stinker.” This aircraft was designed specifically for world-class aerobatic competition and featured advanced aileron technology that allowed for incredibly high roll rates while maintaining precise control[2].

The Super Stinker represented a significant leap forward in terms of both performance and technology. It was powered by an even more powerful engine than previous models—typically producing around 300 horsepower—and featured state-of-the-art materials and construction techniques designed to reduce weight while increasing strength[3]. Although only a limited number of Super Stinkers were built, they remain highly sought after by serious aerobatic pilots today.

Aviat Aircraft: Continuing the Legacy

In 1977, Curtis Pitts sold his interest in both the S-1 and S-2 series to Doyle Child, who later transferred production rights to Frank Christensen under Christen Industries[6]. Eventually, Aviat Aircraft acquired production rights for both factory-built versions and homebuilt kits based on Curtis’ designs.

Today, Aviat Aircraft continues to produce certified versions like the S-1S alongside two-seat variants such as the S-2C[4]. These modern iterations retain much of what made Curtis’ original designs so successful while incorporating modern materials and technologies that improve safety, reliability, and performance.

Aviat also offers support services including spare parts manufacturing for older models no longer in production—ensuring that these iconic biplanes remain operational well into future generations[8].

Citations:
[1] https://www.airliners.net/aircraft-data/pitts-s-12-special/46
[2] https://www.steenaero.com/pitts-s1/history/
[3] https://www.iac.org/pitts-70th-anniversary-at-airventure
[4] https://pacificcoastairmuseum.org/aircraft/pitts-special-s1/
[5] https://www.aopa.org/news-and-media/all-news/2017/september/pilot/budget-buy-pitts-special
[6] https://en.wikipedia.org/wiki/Pitts_Special
[7] https://www.steenaero.com/pitts-s1/
[8] https://www.aeroexpo.online/prod/aviat-aircraft-inc/product-175926-60847.html
[9] https://mapsairmuseum.org/pitts-s-1-pitts-special/

Cultural and Linguistic Impact of the Pitts S-1

The Aviat Pitts S-1 has not only made an indelible mark in the world of aviation but also left a significant cultural and linguistic legacy. As one of the most iconic aerobatic aircraft ever built, the Pitts S-1 has influenced generations of pilots, airshow enthusiasts, and even the broader public’s perception of aviation. This final section will explore how the Pitts S-1 has impacted both aviation culture and language, as well as its role in inspiring future innovations in aerobatics.

Aviation Culture and the Pitts S-1

The Pitts S-1 is more than just an aircraft; it represents a lifestyle and a passion for precision flying. Pilots who fly the Pitts often describe their experience as a blend of art and science, where every maneuver requires both technical skill and creativity. This unique blend has helped foster a subculture within aviation that is deeply dedicated to aerobatics.

Airshows and Public Perception

The Pitts S-1 has been a staple at airshows around the world for decades. Its compact size, open cockpit, and ability to perform extreme maneuvers at low altitudes make it a crowd favorite. Pilots flying the Pitts can execute complex routines that include loops, rolls, spins, hammerheads, and tumbles—often within a very confined space—creating an exhilarating spectacle for audiences.

Airshows have played a crucial role in popularizing aerobatics and bringing aviation closer to the general public. For many spectators, seeing a Pitts S-1 in action is their first exposure to the world of aerobatics. The aircraft’s distinctive appearance and thrilling performances have made it synonymous with high-energy flying displays.

Inspiring Future Generations

One of the most significant cultural impacts of the Pitts S-1 is its role in inspiring future generations of pilots. Many of today’s top aerobatic pilots credit their early fascination with aviation to watching Pitts Specials perform at airshows or reading about legendary pilots like Betty Skelton or Bill Strohmeyer. The accessibility of homebuilt versions of the Pitts has also allowed countless aspiring pilots to realize their dreams by building and flying their own aircraft.

Organizations like the International Aerobatic Club (IAC) continue to promote aerobatics as both a sport and an art form, with many members flying Pitts Specials in competitions around the world. The IAC’s mission to promote safety, education, and excellence in aerobatics aligns perfectly with the values embodied by the Pitts S-1.

Linguistic Influence: The “Pitts Special” as a Term

In addition to its cultural impact, the Pitts Special has also left its mark on language within aviation circles. The term “Pitts Special” has become shorthand for describing any small, agile biplane designed for aerobatics—even if it’s not technically a Pitts model. This linguistic influence reflects how deeply ingrained the aircraft is in the world of aerobatics.

Within pilot communities, phrases like “flying a Pitts” or “doing Pitts maneuvers” are commonly used to describe specific types of high-performance aerobatic flying. These terms have become part of the lexicon among pilots who share a passion for precision flying.

Technological Influence on Future Aircraft Designs

The success of the Pitts S-1 has also had a lasting impact on aircraft design, particularly in the field of aerobatics. Many modern aerobatic planes borrow design elements from Curtis Pitts’ original biplane concept, including lightweight construction, powerful engines, and high roll rates.

For example:

• Monoplanes like the Extra 300: While monoplanes have largely replaced biplanes like the Pitts at higher levels of competition due to their superior speed and agility, they still incorporate many design principles pioneered by Curtis Pitts—such as lightweight materials and powerful engines.
• Composite materials: The success of wooden wings covered with fabric on earlier models like the Pitts S-1 inspired later designers to experiment with composite materials that offer similar strength-to-weight ratios while being more durable.
• Advanced avionics: Modern aerobatic planes often feature state-of-the-art avionics systems that allow pilots to monitor engine performance more closely during extreme maneuvers—a concept that was first explored by homebuilders modifying their own versions of the Pitts Special.

Conclusion: The Enduring Legacy of the Aviat Pitts S-1

The enduring legacy of the Aviat Pitts S-1 lies not only in its remarkable performance capabilities but also in its ability to inspire pilots and captivate audiences worldwide. From its humble beginnings as a homebuilt biplane designed by Curtis Pitts during World War II, it has grown into one of aviation’s most iconic aircraft—dominating aerobatic competitions for decades while remaining a favorite among airshow performers.

Even today—more than 70 years after its initial design—the Pitts S-1 continues to be flown by pilots who appreciate its unique blend of power, agility, and simplicity. Whether competing at international events or performing breathtaking stunts at local airshows, these pilots carry forward Curtis Pitts’ vision for what an aerobatic plane should be: light, fast, responsive—and above all else—fun to fly.

As long as there are pilots who dream of pushing themselves—and their planes—to new heights (literally), there will always be a place for aircraft like the Pitts S-1 in both competitive arenas and recreational skies alike.

Citations:
[1] https://learningenglish.voanews.com/a/everyday-grammar-ok/3808829.html
[2] https://strategically.co/blog/grammar-tips/ok-vs-okay.-what-they-mean-and-when-to-use-them/
[3] https://www.smithsonianmag.com/smart-news/how-one-man-discovered-the-obscure-origins-of-the-word-ok-180953258/
[4] https://www.livescience.com/33252-why-do-people-say-ok.html
[5] https://en.wikipedia.org/wiki/OK