The Story of Aviation’s Largest Piston Engine

By Terry Dunn

Two prototypes of the enormous Lycoming's XR-7755 engine were built and tested before it was cancelled and doomed to relative obscurity.

During the early years of WWII, when the long-term survival of Britain was still in doubt, the US Army Air Corps began considering concepts for a bomber that could reach European targets from US bases. With the technology of the time, a bomber with sufficient range and payload had to be far larger than anything else in the sky. Engineers soon realized that their mammoth bomber would require engines of equally unprecedented size. The Air Corps' request would result in the largest piston-engine aircraft ever produced and the largest piston engine ever built for an aircraft.

Neither the airplane nor its intended engine were completed in time to be used in the war. The advent of the nuclear age kept the bomber project alive, but it would use smaller engines that were already available. Two prototypes of the enormous project engine, Lycoming's XR-7755, were built and tested before it was cancelled and doomed to relative obscurity.

The XR-7755 was originally developed for use with the enormous Convair B36. After the cancellation of the Lycoming engine, the B-36 used smaller radial engines and eventually jets as well. (UNT library photo)

The bomber was the Convair B-36. With a wingspan of 230 feet and weighing up to 410,000 pounds, the "Peacemaker" was a colossal airplane. Production versions of the B-36 were powered by six Pratt & Whitney R-4360 Wasp Major radial engines. Later models also added four jets on wing pylons, coining a popular phrase among Peacemaker crews, "six turning and four burning".

The 3500-horsepower R-4360 is the largest radial engine ever mass-produced. As such, the R-4360 provides the most relevant comparisons when discussing the XR-7755. Even so, parallels between the R-4360 and XR-7755 are difficult to find. For starters, the Wasp Major's 4362.5 cubic inch displacement, while huge by any standard, is only slightly more than half the displacement of the cancelled Lycoming engine!

Big and Different

The only remaining Lycoming XR-7755 is on display at the National Air and Space Museum Udvar-Hazy Center. The engine has a diameter of 5ft and stands 10ft tall.

As you might have guessed, the displacement of the XR-7755 is a whopping 7,755 cubic inches. Like the R-4360, the XR-7755 has its cylinders arranged in four rows. Pretty much all design similarity to the Wasp Major (or any other engine…ever) ends there. Lycoming's engine was not just an upsized reincarnation of an existing design. The engineers started with a clean slate. Their creation featured numerous innovations never before seen in radial engines.

The size of this powerplant is difficult to comprehend. On a recent trip to the National Air and Space Museum's Udvar-Hazy Center, I saw the only remaining XR-7755 among a large display of varied aircraft engines. It was the engine's enormous size that first caught my attention and spurred me to look into its history. The museum display has the engine positioned vertically, where it stands 10 feet tall, has a diameter of 5 feet and weighs more than 6,000 pounds.

The Air Corps' defined seemingly conflicting requirements for the engine by demanding high takeoff power and fuel economy. With a rated output of 5,000 horsepower, the XR-7755 definitely delivered on the former requirement. Yet, the engine consumed 580 gallons of AvGas per hour at full throttle. Let that number sink in for a moment. We know from other recent articles that the Packard Merlin in a P-51 uses about 60 gallons an hour in cruise, while the four Wright R-3350 engines in a B-29 burn a combined total of about 400 gallons every hour.

With such a prodigious thirst for fuel, it would seem that Lycoming totally missed the mark on the latter requirement. But that is not necessarily the case. The engine is so much larger than any of its contemporaries that comparisons of raw data provide a poor yardstick of efficiency. You have to look at figures that are independent of an engine's displacement. One particularly useful measurement is brake-specific fuel consumption (BSFC). This value is essentially a measure of how efficiently an engine converts fuel to horsepower.

Lycoming employees pose with the XR-7755, giving some sense of the engine's enormous size. (photo courtesy of Lycoming Engines)

According to an article by engine expert, Paul McBride, the XR-7755 attained BSFC values as low as .38-.41lb/hp-hour at low-cruise power settings. Put another way, the engine burned as little as .38 pounds of fuel each hour for every horsepower of output. McBride claims that this is an enviable BSFC value even by modern standards.

I could not find data regarding the XR-7755's power level at the lowest BSFC values, but we can spitball it. For the sake of argument, let's say that the engine was able to produce 2500 horsepower with a BSFC of .41 lb/hp-hr. Plugging in AvGas at 6 lb/gal, this theoretical setup would have burned a much more relatable 171 gal/hr.

Cooling the Beast

At full power, the XR-7755 generated a tremendous amount of heat. McBride states that the engine's cooling system pumped upwards of 750 gallons of coolant per minute to keep things in check.

Most radial engines are air-cooled. Therefore, multi-row radials have each successive cylinder row offset in order to expose all of the cylinders to oncoming air. Looking at an air-cooled radial from the front, you can see every cylinder, at least partially. As Pratt and Whitney found with the R-4360, it's tough to find fresh air for the rear cylinders by the time you stack up four rows. Perhaps that was a contributing factor to the engine's unsettling willingness to catch fire.

Lycoming took a very different approach with the XR-7755. It is a liquid-cooled radial. This negated the need to offset the cylinder rows. All 4 of the 9-cylinder rows are aligned fore to aft. Looking at the XR-7755 from the front, you see only the 9 cylinders of the forward-most row.

At full power, the XR-7755 generated a tremendous amount of heat. McBride states that the engine's cooling system pumped upwards of 750 gallons of coolant per minute to keep things in check. Again, I'll give you a minute to wrap your head around that number. That flow rate is equivalent to a class B fire hydrant, or enough to fill your average in-ground swimming pool in about 25 minutes.

In-Flight Adjustments

The Lycoming's unique cylinder arrangement permitted each bank of 4 aligned cylinders to share a common cam shaft. This resulted in an engine with a total of 9 cam shafts. What was truly unique about the cams was that there were two sets of lobes for each cylinder. One set of lobes optimized the operation of the engine valves at high power settings. The other lobes were better suited for cruise power. Moving the cam forward or aft along its axis determined which lobes were engaged. The pilot (or more likely, a flight engineer) would have been able to select the appropriate cam position for each phase of flight. Part of the XR-7755's fuel efficiency can be attributed to this variable cam arrangement.

The XR-7755 features numerous innovations such as variable cam shafts and a two-speed gearbox. (photo courtesy of Dr. Robert Ribando)

In the interest of avoiding a tangent into the complex relationships of engines and propellers, I think the purpose here is fulfilled by simply stating that most engines require a reduction gearbox in order to spin a propeller with acceptable efficiency. The XR-7755 went one step further by having a two-speed gearbox between the engine and propeller.

Think of the engine as having low gear and high gear. Low gear gives you additional torque to climb and accelerate, while high gear lets you drop the engine RPMs during cruise. When combined with variable pitch propellers (which most military aircraft had by the time WWII began) and the adjustable cam shafts, it seems that the XR-7755 would have had unprecedented freedom to operate efficiently under many varying conditions.

Would It Have Succeeded?

Here, the XR-7755 is set up in a test cell at the Lycoming factory. None of the prototype engines were ever fitted to an airplane for testing. (photo courtesy of Dr. Robert Ribando)

Before the XR-7755 could be fully tested, the Air Force's (the US Army Air Corps became the US Air Force in 1947) lingering doubts about the viability of jet power had faded. The military decided that any further development of piston-engine designs was wasted dollars. Neither of the prototype Lycoming engines was ever mounted to an airplane or even fitted with a propeller. According to Dr. Robert Ribando, whose father was a Lycoming engineer, Lycoming employees delivering their engine to US Air Force for testing, were told to "Dump it on the ground."

Because the engine was only ever operated in Lycoming's sterile test facility, the overall efficacy and robustness of the XR-7755 will never truly be known. Rigorous flight testing would have been necessary to flesh out any problems with the design. In that sense, the XR-7755 gets to enjoy something like a James Dean legacy among engines.

As an engineer, I can't help but be impressed by the ingenuity and original thinking that went into the design of the XR-7755. At the same time, it's disappointing to not know whether those innovations were fruitful. Many of the ideas withered on the project's cancelled vine. Perhaps the only lingering concept relates to the variable cam shafts. Some modern automobile engines feature a different form of variable cams, but it is not yet widely used.

Dr. Robert Ribando poses with the surviving XR-7755 prior to its restoration. (photo courtesy of Dr. Robert Ribando)

At the time when the XR-7755 was being developed, aircraft designers were faced with fundamental tradeoffs in selecting engines for their designs. Air-cooled radials offered simplicity and ruggedness at the expense of a fat, draggy profile. Liquid-cooled engines were typically much more svelte, but added coolant, plumbing, and radiators to the bill of materials. Interestingly, Lycoming's design captures both negatives: the cross-sectional area of a radial and the plumbing needs of a liquid cooled engine. Judging by the cooling requirements previously discussed, I'm sure this engine would have needed one heck of a radiator. Again, it would be satisfying to know how the concept might have materialized.

I also wonder what propeller(s) would have been utilized on the XR-7755. It takes quite an airscrew to absorb 5,000 horsepower. Even with the less powerful R-4360s, the B-36 had propellers with a diameter of 19 feet! Again, the motor gearing and overall efficiency come in to play. Some sources indicate that the XR-7755 could have used smaller, contra-rotating propellers (co-axial propellers spinning in opposite directions). The gearing required for contra-rotating propellers, however, is no trivial thing. In fact, it is believed that the Northrop B-35 flying wing project was cancelled due to troublesome contra-rotating systems on its R-4360s.

At a minimum, the Lycoming XR-7755 represents a marvelous feat of engineering. It's hard to imagine that it will ever be surpassed by a larger piston aircraft engine. Best of all, the engine's unanswered questions allow armchair engineers like me to ponder the ins and outs of what could have been.

Terry spent 15 years as an engineer at the Johnson Space Center. He is now a freelance writer living in Lubbock, Texas. Visit his website at TerryDunn.org and follow Terry on Twitter: @weirdflight