The Fastest Airliner You’ve Never Heard Of

The Convair 990

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A Convair 990 of Swissair – note the four anti-shock bodies on the trailing edge of the wings

Well here we are for the second part of this series of interesting aeroplanes you’ve probably never heard of. There’s quite a few good candidates for these articles and a few which stand out particularly well. One of them is the Convair 990. Why is this an interesting aeroplane? It was a direct contemporary of the much better known Boeing 707 and Douglas DC-8, but it was designed to fly faster. In fact to this day most sources will tell you the Convair 990 was the fastest subsonic airliner ever built, yet it remains a mere footnote in aviation history. In this article I hope to tell you a bit about its story.

You may not have heard of Convair before. Convair was formed in 1943 by the merger of Consolidated Aircraft and Vultee Aircraft to form the Consolidated Vultee Aircraft Corporation which in time was contracted to “Convair”. Whilst Vultee never left a big mark on aviation history, Consolidated designed and built two very well known aircraft; the B-24 Liberator and the PBY Catalina. Convair would go on to produce such aircraft as the F-102 Delta Dagger, the F-106 Delta Dart and the B-58 Hustler. Convair was a significant player in the civil airliner market in post-war America. In 1947 it introduced the CV-240 twin engine piston airliner which went on to spawn a family of derivative types of which 1,181 were built before production ended in 1954. In 1953 Convair was purchased by General Dynamics and subsequently operated as a division of GD for the rest of its existence.

In 1958 Pan Am started operating the new Boeing 707, America’s first jet airliner. The following year Delta and United introduced Douglas’ rival DC-8. In 1960 Delta also introduced Convair’s entry into the jet airliner market, the CV-880. Heavily influenced by Howard Hughes, owner of TWA, Convair took a different tack to both Boeing and Douglas. The 707 and DC-8 were both very similar in terms of size, passenger capacity and performance. With the CV-880 Convair had designed a smaller, faster aircraft which it hoped would capture the prestige segment of the market. With a top speed of Mach 0.9 (880 feet per second at typical cruising altitude, hence the name) the CV-880 was faster than both the 707 and DC-8, cruising around 20-25mph faster. However, it carried 60 fewer passengers and its range was nearly 400 miles shorter.

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A Convair 880 of launch customer Delta Air Lines

Some airlines indicated they wanted a larger and even faster version of the CV-880. American Airlines, keen to steal a march on its rivals requested an aircraft which could fly from New York City to Los Angeles 45 minutes faster than the 707 and DC-8. Boeing engineers balked at the idea and said it couldn’t be done. Convair took up the challenge and embarked upon developing the CV-880 into a larger and even faster aeroplane.

To fly from NYC to LA 45 minutes faster than the 707 or DC-8 would require an aircraft which could fly at Mach 0.96, or 630mph. The new aircraft was dubbed the CV-990. American Airlines ordered 25 with an option for another 25. Orders also came from Swissair and SAS (Scandinavian Airlines).

The first part of turning the CV-880 into the CV-990 was very straightforward. Convair stretched the fuselage a bit over 10ft to increase the maximum passenger capacity from 110 to 149 passengers. The larger, heavier CV-990 would need more powerful engines, especially if it was to fly faster than the CV-880. The latter was powered by four General Electric CJ-805-3B turbojets. The CJ-805 was a non-afterburning civilian version of the J79 engine which powered the Lockheed F-104 Starfighter, McDonnell F-4 Phantom and the Convair B-58 Hustler. The new CV-990 would need to be powered by a turbofan engine.

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The Rolls Royce Conway was the world’s first production turbofan engine but was soon followed by similar engines from the American firms Pratt & Whitney and General Electric

The first generation of jet airliners were all initially powered by turbojet engines. Rolls Royce pioneered the turbofan engine with the Conway which was first used to power the RAF’s Handley Page Victor B.2 in 1959. The Conway later went on to power the Vickers VC10 and selected variants of the 707 and DC-8. The American engine manufacturers weren’t far behind. Pratt & Whitney took the JT3C turbojet which powered the early 707 and DC-8 variants and developed it into the JT3D turbofan. Likewise General Electric had developed the CJ-805-23B which was a turbofan version of the CJ-805-3B used on the Convair 880. The new -23B powered the CV-990, thus making it the first turbofan powered airliner.

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The first turbofan powered aircraft was the B.2 variant of the Royal Air Force’s Handley Page Victor, one of the famous V-Bomber triad. Note the Blue Steel standoff nuclear missile carried by this Victor

Up until the advent of the turbofan, the turbojet was by far the most prolific type of jet engine in use. In a turbojet all the air induced into the engine passes through the engine core and the thrust is derived by expanding the hot exhaust gas through a nozzle at the rear of the engine. A turbofan however, adds a larger diameter ducted fan driven by the hot gas turbine in the engine core. All of the air passes through the fan, but only some of it passes through the engine core, the rest of it bypasses the engine core (hence the term bypass engine). The fan itself produces a significant amount of thrust, and the mixing of the cold bypass air and hot exhaust from the engine core reduces the noise from the engine. The net effect is an engine which produces more thrust, is more fuel efficient and makes less noise than a turbojet.

The CJ-805-3B used on the CV-880 provided 11,650lbf thrust with a specific fuel consumption of 0.784 lb/(lb·h). The CJ-805-23B used on the CV-990 provided 16,100lbf thrust for 0.56 lb/(lb·h). That’s 38% more thrust with a greater than 28% improvement in specific fuel consumption. You can see why everyone jumped on the turbofan engine when it arrived. Another excellent British invention.

What you may find curious about the CJ-805-23B is General Electric mounted the fan at the rear of the engine. One can probably conclude this isn’t the optimal configuration as 60 years later nobody else has adopted this configuration and all turbofans have the fan located at the front of the engine.

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A front view of the CJ-805-23B – note the compressor turbine blades in the centre which provide the air to the engine core, and the annular duct for bypass air to the turbofan at the rear of the engine
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A cutaway of the same engine – the front of the engine is on the left. Note the compressor turbine stages for the air passing through the core. The yellow area is the combustion chamber and the red areas are where the hot exhaust gas passes through. The light blue area is for bypass air and you can see the turbofan mounted at the rear. Also note the clamshell type thrust reverser at the rear of the engine which are shown deployed in this exhibit

Now we need to talk about aerodynamics. Quite a complicated subject so pay attention at the back. When we talk about aeroplanes we often use the terms subsonic and supersonic. Subsonic of course is an aeroplane which flies below the speed of sound, and supersonic is an aeroplane which flies above the speed of sound. Right? Well no, not really. The speed at which air flows over the airframe isn’t uniform. The shape of the aircraft results in the air flowing at different speeds over different parts of it. Between Mach 0.8 and 1.2 it’s possible to have both subsonic and supersonic airflows simultaneously. This is called the transonic region. Consider an aeroplane moving through the air at subsonic speed. It’s going slow enough that the air can get out of the way fast enough to allow the aeroplane to travel through it. An aeroplane travelling at supersonic speeds is going so fast the air can’t get out of its own way fast enough and instead compresses in front of the aircraft, forming a shock wave which propagates behind the aircraft. The higher the supersonic speed, the more acute the angle of the shock wave. At transonic speeds localised airflows around the airframe become supersonic and these shock waves start to form. Where these shock waves intersect with parts of the airframe they create a lot of drag. This is called transonic, or wave drag. The speed at which these localised airflows begin to reach supersonic speeds is called the critical Mach number. It denotes the speed at which the onset of wave drag begins. Interestingly enough once an aircraft has accelerated through the transonic region the drag then decreases.

Why am I telling you this? The speeds the CV-990 was designed to cruise at put it more or less right in the middle of the transonic range where it would be subjected to very high wave drag. Not good. In 1952 an aerodynamicist called Richard Whitcomb working for the National Advisory Committee for Aeronautics (NACA, the forerunner of NASA) developed something called the Area Rule. This links wave drag to the aircraft’s longitudinal cross sectional area distribution. What the area rule shows is that an aircraft with a constant longitudinal cross sectional area distribution will be subjected to less wave drag than one with a large variance in its longitudinal cross sectional area. In other words, it doesn’t matter what the cross sectional area is, as long as it’s as close as possible to uniform along the length of the aircraft then the wave drag can be minimised.

Think about this for a second. On a conventional aircraft the longitudinal cross sectional area will increase where the wings are. This means the cross sectional area distribution will be weighted towards the aircraft’s midriff where the wings are and the variance in that cross sectional area distribution will cause wave drag. Whitcomb’s Area Rule led to a generation of aircraft designed in the 1950s and 60s with a so-called area ruled fuselage, sometimes referred to as a Coke bottle fuselage. The fuselage will pinch in around the wing to keep the cross sectional area as constant as possible and thus reduce wave drag. This was a hugely important concept in the era as it allowed the creation of the first supersonic combat aircraft.

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A Convair F-106 Delta Dart, a supersonic interceptor built for the US Air Force. Note how the fuselage pinches in around the aircraft’s midriff, the classic area ruled or “Coke bottle” fuselage

It was also discovered at this time that applying the same concept, one can add a shaped body to the leading or trailing edge of an aerodynamic surface to modify the overall cross sectional area distribution to a more desirable form and thus reduce wave drag. These are known as anti-shock bodies, sometimes referred to as a Küchemann carrot, named after Dietrich Küchemann of the Royal Aircraft Establishment at Farnborough. Küchemann carrots became quite a common feature of high speed aircraft in the 1950s and 60s.

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A period marketing pamphlet from American Airlines extolling the virtues of the new aerodynamic principles applied to the CV-990

Thus, the most significant aerodynamic change from the CV-880 to the CV-990 was the addition of four anti-shock bodies to the trailing edge of the upper wing. These would also be used to house additional fuel. The combination of the reduced wave drag and extra fuel would allow the CV-990 to achieve the necessary range to fly non-stop from NYC to LA at 630mph. In fact, the anti-shock bodies are the easiest way to tell a CV-880 from a CV-990.

The CV-990 first took to the air in January 1961. Subsequent flight testing revealed that despite the use of anti-shock bodies transonic drag was too high and the CV-990 was going to be unable to meet the speed and range promises made by Convair to the launch customers. One of the pre-production aircraft actually achieved a speed of Mach 0.97 at 22,500ft which corresponds to a true airspeed of 675mph. However, at this speed the fuel burn was just too high for the aircraft to meet its range targets. What’s more it was found when fuel was carried in the outboard anti-shock bodies it would cause the outboard engines to vibrate on their pylons at potentially dangerous levels. Thus Convair concluded the outboard anti-shock bodies couldn’t be used to carry fuel and this reduced the aircraft’s range even further. The launch customers weren’t happy. In March 1961 Convair embarked on a “Speed Recovery Program” (sic it’s an American company!). American reduced their order to just 20 aircraft, the first 15 would be delivered “as is” whilst the final 5 aircraft had to have a downward revised cruise speed of 620mph whilst flying non-stop from NYC to LA. The unit price American paid for the aircraft was also significantly reduced. Swissair negotiated a similar deal to American, whilst Scandinavian cancelled their order entirely, although they would later operate two factory fresh aircraft leased from Swissair.

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A CV-990 of launch customer American Airlines

Convair knew they were in quite a pickle. They hired outside help and over a five week period introduced a raft of refinements and improvements to the CV-990 . They identified multiple areas where drag could be reduced including at the wing roots, engine pylons and nacelles. Very significantly though, the solution to the engine vibration was to shorten the anti-shock bodies by 28 inches, eliminating the engine vibration when the anti-shock bodies carried fuel. This actually decreases the critical Mach number and thus increases the transonic wave drag. Not an ideal solution, but the CV-990 needed to be able to carry fuel in those anti-shock bodies in order to meet its range requirement so the compromise was tolerated.

The result of the Speed Recovery Program was the CV-990A. The net result of the improvements and refinements was an aircraft which still couldn’t meet the original 630mph cruise from NYC to LA requirement, but could do it at the reduced target of 620mph set by American. Delays to the aircraft certification process meant the CV-990A wasn’t cleared for airline use until December 1961. Convair immediately started delivering the CV-990A from its factory and made upgrade kits available for the original CV-990s delivered to American and Swissair.

Unfortunately for Convair the CV-990A was too little too late.

In July 1960 United Airlines had introduced the Boeing 720 to service. The 720 could carry the same number of passengers as the CV-990 over the same distance and at a cruise speed only 9mph slower. Crucially, the 720 was a minimum change derivative of the 707-120 and as such Boeing could sell the 720 at a lower price than the CV-990.

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Spanish charter airline Spantax were the final airline operator of the CV-990 and flew their last flight with the type in 1985

The 1960s are renowned as being an era when oil was cheap and the skies were full of very thirsty jet powered aircraft. Yet even in this era potential customers balked at the fuel consumption of the CV-990. The CV-990 could carry 149 passengers over a distance of 3,500 miles at a cruise speed of 620mph. A Boeing 737 MAX8 as rolling off the production line today can carry 210 passengers over the same distance at a cruise speed of 520mph, but it only needs a little bit over one third the fuel the CV-990 required.

Following the lack of any new orders Convair shut down CV-990 production in 1963 after only 37 aircraft had been built. When considered alongside the CV-880 from which it was derived, Convair built only 102 of its CV-880/990 jet airliners. By contrast, Boeing ended up building 1,019 of its 707/720 family and Douglas built 556 of its DC-8 family.

The launch customer American Airlines removed the CV-990 from its fleet by the end of 1967, a remarkably short career with the airline. The last passenger flight by a CV-990 was with Spanish charter airline Spantax in 1985. NASA was the last operator of the CV-990 and they made their last flight with the aircraft in 1994. Interestingly enough in the 1970s NASA used the CV-990 to develop techniques for flying steep, high speed descents to landing for the Space Shuttle.

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NASA were the final operator of the CV-990, using the aircraft for research and development purposes up until 1994

Today the best place to see a CV-990 is at the Swiss Museum of Transport in Lucerne where an ex-Swissair example is on display.

All in all Convair lost $425 million on the CV-880 and CV-990. In 2018 US Dollars that’s over $3.5bn. At the time it was the largest loss ever sustained by a US corporation and still remain in business. Convair never built another jet airliner again, although they were subcontracted to build fuselages for the Douglas DC-10 in the decades that followed.

And that is how the fastest subsonic airliner ever built ended up as a footnote in aviation history. The cutting edge can be a very expensive and unforgiving place to be, especially so in aviation.

Oh, and Elvis Presley fans may be interested to know in 1975 the King purchased a second hand CV-880 from Delta Air Lines and had it refitted with a luxury interior crammed with 1970s bling. It was named Lisa Marie and is still on display at Graceland.

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The Boeing 707, 1,019 built from 1957-79
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The Douglas DC-8, 556 built from 1958-72

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The Convair 990, 37 built 1961-63

 

© Æthelberht 2018
 

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