CHECK SIX: The Area Ruling of the Cessna Citation X

If you look closely at the aft fuselage by the engine nacelles of the Cessna Citation X, it’s pinched in quite considerably to conform with supersonic area ruling, the same reason supersonic fighter jets have “coke bottle waist” mid-fuselages- If you were to plot the cross sectional area of an aircraft from nose to tail, the ideal curve on such a graph would be a smooth elliptical curve. But in most aircraft, the wings, tail, or engine nacelles make that curve “bumpy”- so you can take away some fuselage area by the wings. In the Citation X, the fuselage is pinched in to compensate for the engine nacelles increasing the cross-sectional surface area which is what transonic area ruling- if you didn’t there would be a big increase in drag. The pinching also creates a more constant width channel between the nacelle and fuselage- this keeps air from speeding up locally and then slowing down, which would cause it to become turbulent, also increasing drag (that kind of drag is called interference drag). If you’re going to cruise at Mach 0.92, you’ll need every trick in the aerodynamics book!

Further reading: 

The Cessna 310 as a U-2 Trainer

A Short History of the Ground-Breaking Williams FJ44 Engine

Photo: JP Santiago

A Short History of the Groundbreaking Williams FJ44 Engine

The Williams FJ44-3 engine. The fan is 23 inches in diameter

The Williams FJ44 engine of the CitationJet family has a fascinating history as it's based on the F107 turbofan used on American cruise missiles like the ALCM and the Tomahawk. Williams International started out with marine turboshafts and APUs before getting into small turbofan engines. Sam Williams and his namesake company would rise to prominence in the aviation industry with miniature turbofans but his real dream was a civilian turbofan for general aviation. In the 1980s Williams envisioned a new class of turbofans based on the F107 design that would allow for a new class of general aviation aircraft that were jets half the cost and size of current bizjets with the field performance and economy of turboprop twins. 

Sam Williams believed so much in his FJ44 engine design that development continued through the 1980s in the absence of any launch order or production application. Keep in mind that this class of engine had never been used in general aviation and Williams International had never made a civilian production jet engine. Through the 1980s, Williams lobbied numerous general aviation manufacturers about his concept for a light jet aircraft and how he had a superbly economical engine to make such an aircraft feasible. The first manufacturer to agree was Rutan's Scaled Composites who flew the Triumph in 1988. Rutan's design never entered production but it did mark the FJ44's first flight. The next manufacturer to agree to use twin FJ44s was Swearingen in its SJ30 design- even though the aircraft flew, Swearingen and the SJ30 have passed through many ownership changes serial production has yet to occur. Williams' big break came with the third manufacturer- Cessna was looking for an efficient and higher performing successor to its iconic Citation line and launched the Cessna 525 CitationJet family in 1992 with a very substantial order of Williams FJ44 engines, vindicating Sam Williams' dream.

Sam Williams

The FJ44 is a joint venture between Williams International and Rolls-Royce and was the company's first civilian engine and first manned aircraft engine. Simplicity and reliability were key to this new class of aircraft and as such, the FJ44 engine has only 700 parts total, 1/4 that of many other bizjet powerplants. The Cessna CitationJet series was the first production application for the FJ44 engine. It wasn't enough to have a simple engine, it also had to have performance and basing the FJ44 on the F107 cruise missile engine gave the FJ44 that foundation. In fact, the F107 that the FJ44 is derived from was such an impressive accomplishment that it won the Collier Trophy- it has 1/10th the weight of the Pratt & Whitney JT8D engine but has the same specific fuel consumption and thrust-to-weight ratio. That performance pedigree translated over well into the FJ44 and it made the engine successful. On the Cessna CitationJet series, the economy and performance of the new engine made the new jet a leap in performance over the Citation 500 series it replaced. While aerodynamic improvements are part of the equation, the FJ44 engine's performance and economics are a large contributor. Compared to a Citation 500 at an identical mid-cruise weight, the CitationJet goes 13% farther with 17% less fuel and it does this 40% faster than the original Citation with the FJ44s having a lower thrust output than the JT15Ds used on the first Citations.

There are four engines in the FJ44 family. The lowest powered one was the first one, the FJ44-1 and it powers the Cessna CJ1 and CJ1+ of the CitationJet family and is also used on the Cessna Citation M2 as well as the Swedish jet trainer Saab Sk60. It has a thrust rating of 1900 to 2100 lbs. The FJ44-1 series first flew in 1988 and went into production in 1992. 

The second engine in the family is the FJ44-2 and is based on the core and LP turbine of the FJ44 paired up with a larger fan and new compressor section for increased thrust. Its applications include the Cessna CJ2, the Beech/Raytheon Premier I,  and the latest incarnation of the SJ30 design, Syberjet SJ30. Two of Rutan's designs fly with the FJ44, the Scaled Composites Proteus and the Virgin Atlantic Global Flyer that Steve Fossett flew around the world in solo nonstop in 2005. It's also used on two re-engining programs for the Citation 500 and the Learjet 25. The FJ44-2 has a thrust rating of 2300-2400 lbs thrust and went into production in 1997.

The next engine in the family is the FJ44-3 which has a new fan and compressor section for an increase in thrust rating to 3000 lbs. It powers the Cessna CJ2+ and CJ3+ and is used in Nextant's re-engining and remanufacture program of the Beech 400 as well as on a re-engining program for later Citation 500 variants. It went into production in 2004. 

The top end member of the FJ44 family is the FJ44-4 with an increase in thrust to 3600 lbs with a larger fan and enlarged compressor. It also features a dual-channel FADEC for more efficiency. It powers the Cessna CJ4 along with the Beech 400XPR remanufacture/upgrade program for the Beech 400. The engine will also power the upcoming Pilatus PC-24 jet. 

There is also a scale down of the FJ44 engine, the FJ33, which went into production in 2004 and has a thrust rating of 1000 to 1900 lbs. The fan is only 19 inches in diameter and it powers the Diamond D-Jet and the Cirrus Vision. 

Unlike other manufacturers of small jet engines, the FJ44 has no turboprop derivative and there are no turboprops in the Williams portfolio. This is a reflection of Sam Williams' philosophy that given time and technological progress, light turbofans like the FJ44 will displace turboprop engines for most civilian aircraft applications. The FJ44 is one of the landmark engines of aviation history and Sam Williams has earned honors for creating an engine that gave rise to a whole new class of general aviation aircraft.

Source: The History of North American Small Gas Turbine Engines by Richard A. Leyes and William A. Fleming. American Institute of  Aeronautics and Astronautics/Smithsonian Institution Press, 1999, pp383-429. Williams International http://www.williams-int.com/. Photos: Williams International

The Cessna 310 as a U-2 Trainer

The Cessna U-3A "Blue Canoe"

In 1957 the United States Air Force selected the Cessna 310 twin as its new utility transport for light cargo, liaison, and administrative missions to replace the Beech C-45 which was a military adaptation of the stalwart Beech 18 transport. Eighty were ordered and originally designated L-27 (in 1962 the rationalization of military aircraft designations led it to be redesignated the U-3). That first group of 80 aircraft were delivered within the year thanks to Cessna's already high production output to meet the civilian market. Another 80 were ordered later that same year and in 1960 came the final contract for another 35, this time of the swept fin version of the 310. The older version with the straight vertical fin became the L-27A, later U-3A, and that last group of aircraft were designated the L-27B, later the U-3B. The distinctive blue and white colors used by the USAF on the aircraft led to its nickname, "Blue Canoe" and even though the US Army and the US Navy ordered the aircraft and used their own service specific color schemes, personnel still referred to the aircraft as the "Blue Canoe". But one of the most unique uses of the U-3 was also its most little-known and in the smallest of numbers by the military- that of a trainer for the early U-2 program!

At the time, Lockheed U-2 pilot training was the responsibility of the 100th Strategic Reconnaissance Wing at Davis-Monthan AFB in Arizona. The first-generation U-2 spyplanes didn't have a two seat trainer version in use at the time, which made every pilot's first flight in the U-2 a solo flight- and even back then, the U-2 had the unenviable reputation as one of the most difficult aircraft to land properly. The 100th SRW had already been using a souped vehicle, the "Mobile", which back then was a Ford El Camino modified with a big-block engine which would sprint down the runway alongside the landing U-2 to assist the pilot with altitude information as the U-2's long span wings meant that the only way to land was to land in a full-stall. The Mobile called out altitude down in single foot increments until the aircraft was only a foot above the runway at which point speed was bled off to stall the aircraft into a touchdown. Needless to say, it was a completely counter-intuitive way to land an aircraft and was just one of many idiosyncrasies of the "Dragon Lady". But an aircraft was needed that could help train novice U-2 pilots to the point before the Mobile took over assisting with the landing. The 100th SRW evaluated different types of aircraft and quite surprisingly, found the Cessna U-3 ideal for the role. It was inexpensive, available, dual control, and it had a yoke which the U-2 also had as well. With most U-2 pilots coming from the fighter community, many hadn't used a yoke since basic flight training. The instructors at the 100th SRW specifically selected the straight-fin U-3A as the later swept-fin version was more unstable at the low approach speeds needed to train novice Dragon Lady pilots. 

An early generation U-2 from the U-3 chase plane

The U-3 had a turn rate, descent profile and pattern airspeed very close to that of the U-2; given that airspeed management was critical in the U-2 given its very narrow handling envelope, the "Blue Canoe" could be used as both a training aid and chase aircraft for a U-2 on approach. On training missions out of Davis-Monthan AFB, the pair in formation was known as the U-2 and "Me-Too". The U-3 would intercept the approaching U-2 at 15,000 to 18,000 feet at 160-180 KIAS and stay on the spyplane's right wing through the descent and traffic pattern until the landing was handed over to the Mobile. In addition, the sightlines from the student's seat in the U-3 were similar to that of the Dragon Lady and it was used as a pre-solo trainer. In preparing U-2 pilots for their first flight in the spyplane, the U-3 was known as the "Dragon Lady Pattern". The instructor would manage the flaps and throttles on the simulated approach to mimic the flying characteristics of the U-2 while the student in the left seat concentrated on the approach. Because the U-2 was stalled one foot above the runway in order to land, the student was required to level off and float the Cessna U-3 one foot starting over the numbers and to maintain that one foot altitude the entire length of the 12,500 foot runway at Davis-Monthan. If the student completed this exercise several times to the satisfaction of the instructor, the U-3 would then land and taxi to a waiting U-2 where the student would climb in for their first solo flight with the U-3 flying chase. For the first solo flight, the wing pogo gears were locked into place. 

Note the pogo landing gear still in place as seen from the U-3A.

The instructor in the U-3 then took off and waited at 15,000 feet for the student in the U-2. Once they had rendezvoused, the instructor by radio then walked the student through various approach-to-stall exercises as well as manually trimming the aircraft for landing by pumping fuel amongst the wing tanks to even the aircraft out. With a 100+ foot wingspan and a wet wing, it was critical that the fuel be evenly distributed through the wing tanks before landing. If the student had done this satisfactorily, it was time to shoot some approaches in what was called the "Dragon Lady Checkout". With the U-3 flying as "Me-Too", the student completed three traffic patterns with touch and gos with the Mobile participating in the final touch down. Completion of this exercise meant the newly minted U-2 pilot could proceed with advanced training. Only two Cessna U-3As were assigned to the 100th SRW at the time and the pace of training meant that no more aircraft than that were needed and only three instructors were checked out to train and fly chase in the U-3. The U-3As also flew chase in emergency situations to assist pilots in getting the U-2 back to the base and often flew chase for pilots returning from long-duration missions who were at their most vulnerable due to exhaustion. 

Source/Photos: Cessna Warbirds- A Detailed & Personal History of Cessna's Involvement in the Armed Forces by Walt Shiel. Jones Publishing, 1995, p127-132. Additional photos from the USAF Museum.

In 1967 Operation Combat Dragon commenced in Vietnam which would be the baptism of fire for the Cessna A-37 Dragonfly. An attack version of the T-37 trainer, the intent of the Dragonfly was to provide a low-cost yet capable close air support aircraft, a role that the faster jet fighter in the theater weren't exactly tailored to perform.

Conceived by Lt. Colonel Lou Weber, a veteran of the World War Two Flying Tigers, Operation Combat Dragon would introduce the Dragonfly into combat in a unique manner- whereas most combat aircraft had their weapons certification, weapons loading and maintenance procedures hammered out in operational testing in the United States before deploying to operational units, the Dragonfly would gain its combat certification by testing in actual combat.

Thirty pilots were selected for Operation Combat Dragon, all of whom had no more than 25 hours in the T-37. All came from all types of aircraft, from fighters to transports- Weber wanted to make sure that a pilot with any experience from any level could fly the A-37 in combat. Most deploying units to Vietnam had nine month training programs stateside before heading overseas- but with the A-37, the training was going to be done "on the job". Combat missions were to be flown in the III and IV Corps area of South Vietnam and forward air controllers favored the A-37- its slower speed allowed for pinpoint accuracy in delivering weapons on target.

The unit had the provisional designation 604th Air Commando Squadron and was based at Bien Hoa AB. In its first 3,000 sorties, not a single A-37 Dragonfly was lost in combat. Operation Combat Dragon ran from August 1967 to December 1967 and in that time frame, Lt. Col. Weber's strategy was soundly vindicated with 19,000 weapons drops and the combat experience led Cessna to develop an improved version, the A-37B. After nine months in combat, the unit had flown an astounding 10,000 sorties and reflected that it only took two men to maintain and turnaround the Dragonfly where as most jet fighters required 10 or more men per plane. On many missions a Dragonfly could be turned around for the next sortie in as little as 90 minutes. The unit would average twice the number of sorties as the more advanced and faster jets in Vietnam.

Source: Air & Space Smithsonian, January 2010. "Super Tweet- The A-37 Dragonfly carried 6,000 pounds of weapons. Bonus: They hit the target" by Stephen Joiner, p42-49.

It was in the late 1970s that analysts at Cessna realized that the vast majority of utility aircraft used in remote areas were either unsuitable for the role or were, in the case of the Douglas DC-3 and the De Havilland Beaver/Otter series, aging with no ideal replacement in the works. So on 20 November 1981 the Cessna Model 208 Caravan was launched, John Berwick, the chief engineer at Cessna's Pawnee Division in Wichita, Kansas, brought on board two men who were instrumental in design and development of the De Havilland Canada Beaver- the first individual was Russ Bannock, who first flew the Beaver on its maiden flight in August 1947 and later rose to De Havilland Canada's sales director after having demonstrated the Beaver worldwide.

The other addition was Dick Hiscocks who was the head aerodynamicist on the Beaver and later became De Havilland Canada's VP for engineering. Both Bannock and Hiscocks had recently retired from DHC and were brought in regularly to advise Cessna and review the program on a monthly basis. Two years after launch, the Caravan made its first flight on 9 December 1982.

At about the same time, Federal Express was looking at a small cargo aircraft as a feeder to replace trucks that were used to fly packages to/from small towns to larger feeder cities that transferred them to the larger jets in the cargo fleet. The founder and president, Fred Smith, initially wanted only twin-engined aircraft for the feeder role but was convinced by Cessna to a flying demonstration- Smith was sold and in 1985 ordered 30 Caravans featuring an underfuselage cargo pannier developed for Federal Express (but now offered to all Caravan customers). In their first six months of operation with the company, the Caravans had a 99 percent dispatch rate in over 2,000 hours flying time. FedEx also drove the development of a stretched Caravan, the Grand Caravan Model 208B which had four additional feet of fuselage for more cargo capacity. The first 208B went into service for the company in October 1986 and by 1996, the Caravan fleet had flown over 1 million hours for the company, by which point they had 300 Caravans in service. The 500th Caravan built would go to FedEx.

Source: The Legend of Cessna by Jeffrey L. Rodengen. Write Stuff Enterprises, 1998, p205-212.

The last Avro Canada CF-100 Canuck flight took place on 28 June 1982, but it wasn't with an operational role. Since 1967, Pratt & Whitney Canada had operated a CF-100 on loan from the Canadian Armed Forces to serve as an engine testbed for the JT15D small turbofan. PWC needed a two seat aircraft (the rear seat of the CF-100 being occupied by a flight test engineer) and one with sufficient ground clearance for an underslung nacelle for the JT15D engine.

The CF-100's ability to cruise up to Mach 0.8 as high as 48,000 feet made it ideal for testing an engine destined for business jets. The aircraft arrived at PWC's St-Hubert test center in November 1967 for conversion. The first flight took place on 22 July 1968 with the first flight with a JT15D engine on 14 August 1968. The first air start of the JT15D took place less than two weeks later on 22 August 1968.

The loan of the CF-100 got extended several times as PWC tested different versions of the JT15D engine, ultimately making over 400 flights totalling 1,017.6 flying hours with the test engines.

The JT15D engine is unique in that it has a centrifugal high pressure compressor which was common on early generation jet engines. However, in the JT15D it made the engine more compact and simpler in terms of complexity and parts as opposed to what it would have been had it used a traditional axial-flow compressor. The JT15D first went into use on the Cessna Citation 500 (later rebranded Citation I) and has been used on the Hawker Beechjet/T-1A Jayhawk, Aerospatiale Corvette, and the Citation II, Ultra, and Citation V as well as the military T-47 and UC-35 versions of the Citation.

Source: Wings of Fame, Volume 18. AIRtime Publishing, 2000, "Avro Canada CF-100 Variant and Operator Briefing" by Jeff Rankin-Lowe, p114, 133.

The successor in the skies over Vietnam to the AC-47 Spooky gunship was the AC-119 Shadow which often carried the nickname "Big Dumb Brother" which was derived from a little known USAF evaluation program called Project Little Brother for a light, heavily armed ground-attack aircraft suitable for counter-insurgency roles. The hope was the South Vietnamese Air Force (VNAF) would be able to field this smaller gunship as well.

Based at Eglin AFB, USAF evaluators gathered a Cessna 337 Skymaster, a Cessna 206, a Beechcraft S35, a Piper Cherokee Six, all of which were leased from their respective manufacturers and retained in their civilian schemes. Also, a Cessna 0-1 Bird Dog and a Cessna 0-2A Skymaster were pulled from USAF training units at Eglin. Joining the mix was the American Electric Pirhana, a single prototype high performance attack aircraft weighing only 2000 lbs.

The Little Brother candidate aircraft flew missions over Eglin's extensive ranges with minigun pods, napalm, free-fall bombs, rocket launchers and various gun installations. The favored platform in the program was the Cessna O-2 and the proposed O-2C would have higher powered engines, armor/self-sealing tanks, a side firing 7.62mm Minigun as well as the sensors and electronics for a pilot and gunner crew to fly and fight the gunship mission.

By the early 1970s Project Little Brother wound down to a close, but the sensors and avionics developed would form the basis of the more capable night attack sensors that would end up on the AC-119 and AC-130 aircraft during the War.

Source: Gunships- The Story of Spooky, Shadow, Stinger, and Spectre by Wayne Mutza. Specialty Press, 2009, p61-65.

The airport for La Paz, Bolivia, sits at 13,313 feet elevation and is the highest elevation airport in common use and is the reason many jets are certified for airports up to an elevation of 14,000 feet above sea level. Recently, however, the Cessna Citation XLS received approval to operate at airports up to 15,000 feet elevation. The flight inspection division of China's Civil Aviation Administration operates seven Citations and recently it made its first landing at the airport of Qamdo Bangda in Tibet which sits at 14,219 feet above sea level. The Citation XLS' certification is to 15,000 feet as there are no other runways in the world higher than that of Qamdo Bangda.

Source: Flying, July 2009. "Airways: Industry News & Notes", p17.