Category Archives: radial engine

The History & Story of the R-985 Powered Boeing PT-17 Stearman

Climbing over the narrow, wing-root walkway and stepping on to the cushioned seat of the tandem, two-place, blue and yellow fabric-covered open-cockpit Boeing PT-17 Stearman registered N55171 in Stow, Massachusetts, I lowered myself into position with the aid of the two upper wing trailing edge hand grips and fastened the olive-green waist and shoulder harnesses.  Donning era-prerequisite goggles and helmet, I surveyed the fully duplicated instrumentation before me and prepared myself both for an aerial sightseeing fight of Massachusetts and a brief, although temporary, return to World War II primary flight training skies.

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How 2 R-985s Aided the PT6 in Achieving Legendary Aviation Status

pt6a king air

The question of how a radial engine can be compared to a turbine engine is a question that has been asked many times over. Individuals in the Agricultural world are still asking themselves this question every year on a purely economic basis. However, the question can also be asked from a historic basis as well. In looking at the Pratt & Whitney family of Radial Engines and the PT6A family of engines, it is clear that the two are closely related.

A Bit of Background on Pratt & Whitney’s Engine Marvels: The PT6A, R-1340, & The R-985

A legendary engine deserves a story as extraordinary as it is, and such is the case with the early history of Pratt & Whitney’s PT6. This story begins decades before the turbulent history of the PT6 when radial engines were still the dominant engine for airplane use. The gas turbine engine of the PT6 revolutionized the industry, but not before the static, air-cooled radial engines had a few decades in the limelight.

Of all the radial engines, Pratt & Whitney’s R-985 was always a favorite since its inception in 1932. Simply sit back and watch a smile cross an aviation enthusiast’s face upon observing the sputter of the round radial engine as it starts up, and it is clear that these engines were something special.

However, the transition into the era of the PT6 was not an easy one. In fact, it was something of a miracle.

The Rise of the PT6

While the advancements of gas turbine engines were known to the aviation industry in the early 1950s, the expenses of the manufacturing, maintenance and repairing processes were problematic. However, that did not deter Pratt & Whitney Canada (PWC) while they forged ahead with their plans of designing a powerful gas turbine engine. They hired a team of specialists and proceeded with attempts to develop a 450 hp engine that had growth potential up to 500 hp. Their goal was to keep operating costs at a similar level as the previous radial engines, and their first foray into gas turbine engines was designed to fit small and lightweight airplane models.

However, they still needed to decide on a gas turbine technology, but eventually settled on a free turbine configuration that was more expensive, but had crucial advantages such as less starting power requirements, simplified controls for fuel and the ability for fixed-wing aircrafts to purchase off the shelf propellers rather than custom ones. Once the team decided to move in this direction, they still were not ready to get to work since they had to travel to Pratt & Whitney’s headquarters to convince the chief engineer that their plan was the right one. Upon securing his approval, the jubilant team started working on the ambitious project.

Unfortunately, their work was a blight on company balance sheets. The new design attempts led to a sort of development nightmare, but the chief engineer that approved the project still had faith in the vision. As a result, he sent a team of six experts spearheaded by a highly skilled engineer named Bruce Torell. The goal was to get the project back on track, and history reveals that this historic engine would have likely failed without his aid.

Progress was quickly made thanks to Torell’s engine expertise, but then the team faced obstacles from PWC itself. Despite aggressive attempts to terminate the project, work continued and was finally ready for flight testing in 1961. A search began for a suitable twin engine airplane to test with the PT6, and the team chose Beechcraft C-45 “Expeditor”. This Beechcraft Model 18 was equipped with two R-985s, meaning that the traditional radial engines played a huge role in the development and rise of the PT6. While further tweaks to the engine were made, the future of airplane engines was clear. Gas turbine technology was here to stay, it was just a matter of whether the PT6 was the engine that would dominate the airplane industry. It did, thanks to Beechcraft, the same company that used P&W’s radial R-985 engines of decades past. With that agreement, the PT6 finally saw mainstream success that produced its dominant run as one of the great engines of history and in fact was the first engine ever put on a King Air.

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Turbine Vs. Radial, Why the Comparison?

I’ve been privileged to know both the PT6A and the 9-cylinder Pratt engines. Both engines operate on a different technique for deriving horsepower from the combustion process, but at heart they are still both internal combustion engines that share the same engineering DNA.

radial blower section

One of the most complex parts of the R-1340/R-985 engine, which has remained relatively unchanged since December 24, 1925 when the very first R-1340 roared to life, is the supercharger or blower section. The blower section, which also serves as the anchor-point when installing the engine, is attached to the rear power case. The circular case receives the fuel/air mixture from the impeller assembly through diffuser channels then delivers the fuel/air mixture to the cylinders via the intake pipes. The blower is driven directly by the crankshaft through a spring loaded gear coupling located at the aft section of the crankshaft assembly. This ingenious design helps protect the blower gearing from sudden acceleration or deceleration. The spring loaded gear drives the floating gear. The impeller assembly, being indirectly driven by the crankshaft, turns ten or even twelve times crankshaft speed.

rise of the pt6a impeller

In like manner the PT6A Impeller is located in the gas generator housing which is the anchor point when installing the engine. The centrifugal impeller delivers air through diffuser tubes to the combustion chamber. The hot gases flow through a series of turbines which produce horsepower to the propeller shaft.

rise of the pt6a

The impeller is only one area of similar design and function.  The reduction gearing in both the PT6A & R-1340G engines are remarkably similar as well as many other features. It is not difficult to see a common engineering theory. Many pilots and mechanics love the history and engineering that goes along with engines and aircraft. Certainly looking and comparing two of the legacy engines from Pratt & Whitney is enjoyable information for many in the aviation community. I have always found it entertaining that as the PT6A engine took its first breath of life, there were R-985 engines on each side! The photo (left) is of the first flight of the PT6A, being test flown on a Beech 18 (May 1961).

In closing, I am a mechanic that holds to the history of aviation. Learning about the past can certainly give insight to the present while possibly holding a glimpse into the future. Drawing a comparison between these two engines certainly does that.

– Rob Seeman, Covington Aircraft Operations Manager

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Vultee BT-13 aircraft profile

The Vultee Aircraft Corp. BT-13 “Valiant” was a single-engine, tandem-seat trainer produced for the U.S. Army Air Corps, U.S. Navy and foreign allies prior to and during World War II. The aircraft was selected and produced as a primary and follow-on intermediary trainer due to its ruggedness, forgiving flight characteristics and stability. Most of the pilots produced in the early years of World War II conducted initial training, or Basic Training, hence the BT name, on the BT-13.

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Radial Engine Time Between Overhaul: What’s My TBO for the R-985 & R-1340?

 

Please allow me to offer some information in regard to Pratt & Whitney R-1340 & R-985 engine Time Before Overhaul intervals (TBO’s) for engines utilized on current agricultural aircraft. A letter from Pratt & Whitney (P&W) faxed to the Federal Aviation Administration (F.A.A.) dated February 13, 1990 is useful in understanding the organization’s corporate position on the radial engine.

AT-302
Ayres AT-301 Air Tractors VH-ODB and VH-ODM at Tintinara SA in May 1989
Designed by Leland P. Snow, the AT-302 designation indicates 320 gallon hopper and P&W R-1340 radial engine
From http://www.goodall.com.au/photographs/aerial-agriculture-80-1/80saerialag-1.html

“Pratt & Whitney have no company or F.A.A approved methods for providing any engineering substantiation or manual/publication revision relating to new methods or procedures which are being accomplished by operators and overhaul shops on Pratt & Whitney reciprocating engines.”

This letter establishes a, “hands off” attitude on P&W’s part concerning the Reciprocating Radial engines. Oil consumption is a major issue and is addressed in a cautionary statement constituting part of the P&W TBO considerations given in the R-1340 & R-985 overhaul manual (part number 123440).

“Oil consumption is usually one of the best indications as to whether or not the engine requires overhaul, provided the engine is performing normally and there is no indication of possible trouble or irregularities requiring more than normal line maintenance attention. A sudden increase of oil consumption or a gradual increase of oil consumption to double that which has previously been average, is usually case for overhaul.”

The engine’s primary accessories (Carburetor, Fuel pump, Magnetos, Starter, Propeller Governor, and Generator) are designed to run to engine TBO. It is our recommendation that they be overhauled at the same TSO as the engine. Ref: AC65-12A Chapter 10 Page 411 Par. Major Overhaul Our basic TBO recommendations are 1000 to 1400 hours operating time since overhaul. In order to determine this “recommended” Time Before Overhaul we have taken into consideration all forms of Agricultural utilization of the R-1340 & R-985 engine and have averaged the operating time between overhauls of engines submitted to us for overhaul over the last 25 years.

Weatherly 620A VH-WEA
Manufactuered in 1989, Weatherly 620A VH-WEA is powered by a 9-cylinder, Pratt & Whitney R-985 radial engine however the aircraft itself has a relatively low spraying capacity of 1200 litres. From http://airqueensland.blogspot.com/2015/01/r-mach-aviation_7.html

It must be noted that there is an Airworthiness Directive 68-09-01 issued to the R-985 engine. It is concerning Crankshaft flyweights and flyweight liner replacement. This AD mandates that it be accomplished at 1200 or 1600 hrs depending on propeller installation. In order to accomplish this, the engine must be disassembled to the point it is more economically feasible to overhaul than to limit to repair and replacement only. This Time Before Overhaul recommendation is made with the assumption that all manufacturers’ recommended/required periodic inspections are complied with in a timely manner throughout the life of the engine. This recommendation is not to certify or guarantee that an operator will achieve a specific number of hours operation time before an overhaul is necessary. This TBO recommendation should in no way be considered a maximum TBO limit as it is possible to safely operate an R-1340 & R-985 past 1200 or 1400 hours TSO. It is merely a RECOMMENDATION that, hopefully, will better enable an operator to develop a safe, economic engine overhaul schedule.

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The R-985 Powered de Havilland Beaver: Arguably the Best Bush Plane Ever Built

With a big, nine-cylinder Pratt & Whitney strapped to the front, this 5,100-pound workhorse boasts a useful load of around 2,000 pounds, and it’s built to operate out of short and rough airstrips.

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The Radial Engine: There’s No Beating a Classic

When it comes to airplane engines, radial engines are the true classic

radial aircraft enginesEver since Louis Bleriot crossed the English Channel in 1909, the radial engine has been an integral part of modern aviation.   Their simple yet powerful design has been improved upon throughout the 1900’s, but the basic principles behind the airplane engine have remained constant. Pratt and Whitney R-985 and R-1340 radial engines were designed starting in the 1920’s, and have withstood the test of time.  Many of these radial engines are still in use today, thanks to companies like ours who are able to perform the overhaul and maintenance required for these masterpieces.

The R-1340 radial engine was Pratt and Whitney’s first foray into airplane engines, and approximately 35,000 engines were produced.  The next engine to be mass-produced was the R-985.  This engine was manufactured from the 1930’s to the 1950’s, and over 39,000 of them were made

Radial engines played a major part in the First World War, outlasting and outperforming rotary engines over time.  The R-985 and R-1340 engines became the standard aircraft engine for World War II, powering airplanes used in basic training as well as military versions of civil aircraft.  After the war, R-985 engines were used in various smaller civil and military airplanes, including utility aircraft, small transports and agricultural aircraft.  Their simple design and reliability made them popular among airplane enthusiasts around the country.

Ford Trimotor, powered by 3 Pratt & Whitney R-985 Radial Engines loaded up for the first flight of the day, following quite a downpour. At least with the rains comes the benefit of the reflections of Lake Lyon. Via @markRFoster

Today, you’ll find most R-985 and R-1340 engines in bush planes and agricultural aircraft, as well as on WarBirds.  Parts for these engines are still available on the market, but repair and overhaul of R-985 and R-1340 radial engines requires a skilled technician.  As a certified FAA Repair Station, our Radial Engine Division is the largest R-985 and R-1340 overhaul facility in the world, and we pride ourselves on being true artisans when it comes to overhaul and maintenance of radial engines.

So, if you’ve got a plane with one of these engineering marvels, be sure to entrust its maintenance to a facility that specializes in the radial engine.  They can keep your engine running for years to come.

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