Category Archives: radial engine

How Jim Savage Gets the 1939 Spartan Executive So Shiny

This blog is republished with the permission of Jim Savage. His website, which features his 1939 Spartan Executive, is a must visit.

What do you do to make your Spartan so shiny?  That’s a question I am often asked and the answer may be easier than you expect. Obviously, it takes quite a bit of polishing, using quality polishing supplies and good polishing techniques. Most experienced metal polishers with bare metal airplanes already know that.  The missing piece has to do with how light behaves when it reaches the airplane. Specifically, it is either reflected or it is absorbed.  The more light that is reflected, the shinier the airplane appears to be.  The trick is to eliminate anything that absorbs the light. In the case of Spartan NC17634, it has minimal paint trim, so there is more surface available to reflect light. Of course, that holds true for many bare metal airplanes.  The other source of light absorption is the tiny black rings around each of the rivet heads.  Although often unnoticed unless you are specifically looking for them, almost every bare metal has these light absorbing rings, including ones that have been judged as Grand Champions.  They originate during the normal polishing process and over time and many polishings, they slowly accumulate.  With the passage of time, these rings become extraordinarily difficult to eliminate.  

While a tiny black ring around a single rivet doesn’t seem like much, consider what the cumulative amount is if you have 9000+ polished rivets, as is the case with NC17634. To the best of my knowledge, there is no magic potion that easily removes the black residue.  It is simply a matter of finding a process that works best for you and then proceeding, one rivet at a time.  While the removal of all traces of black from every rivet of an entire airplane is a daunting task, the results are clearly noticeable.   

Here are some close-up pictures rivets on a 1939 Spartan Executive.  The first nine pictures show examples of what domed rivets with black rings look like on a highly polished airplane.

The next nine pictures show similar views of the same Spartan, after removal of the black rings.

For those of you who are really, really curious about how long it took to remove all traces of black from the 9000 rivets on the Spartan, it is probably far more than you can imagine and likely far more than you will believe. It took approximately 800 hours of effort but as the following picture shows, the final results can be stunning.

More Pictures of the Spartan 1939 Executive

Radial Engine Cylinder Head Checks: Following the Service Bulletins

This is the seventh and final topic in the series about the 100 hour / annual inspection. This series focuses on maintenance performed during an effective 100 hr. inspection on R1340 and R985 engines:

  1. Oil Change with filter/screen & sump checks.
  2. Valve adjustment – Positive or compression.
  3. Ignition timing check – Spark plug servicing.
  4. Compression check – differential.
  5. Air filter and carb – heat system check.
  6. Fuel System Screens.
  7. Cylinder Head Checks.

It has been amazing to see the engine log books for both the R-985 and R-1340 engine cores coming back to us for overhaul with not one entry reflecting the visual inspections called for in Airworthiness Directive 78-08-07 (R-985-SB 1785) and AD # 99-11-02 (R-1340-SB 1787)!

The service bulletins outline an Ultrasonic inspection of the 985 cylinder heads and Florescent Penetrant inspection of the 1340 heads that must be done at each overhaul. However, there are instructions for visual inspections to be done on the cylinder heads of both engines at specific intervals! The AD Note 78-08-07 (985) stipulates visual inspection of the heads on a 150 hour interval while AD 99-11-02 (1340) states inspections must be done on a 100 hour basis!

The AD notes state the inspections must be done in accordance with the SB’s. SB 1785 which reads as follows: REASON FOR BULLETIN: (2) Provide instruction for visual inspection, at each periodic maintenance interval. The 1340 SB reads: REASON FOR BULLETIN: 3. Provide instructions for inspection of cylinder heads at periodic maintenance.

You are looking for cracks in the aluminum head that are evidenced by jet-black combustion residue deposited at the root area between two fins in the designated areas. The coloration will not be visible in areas that aren’t cracked and leaking combustion residue. It is possible for oil leaks to burn onto the cylinder cooling fins but that is usually dark brown colored and typically involves a larger portion of the head. Combustion residue is dark black and may be oily and gritty feeling. I have included a couple of scanned illustrations showing the areas of the head identified in the bulletins:

radial engine cylinder head checks

The pictures seem to indicate that the 1340 head doesn’t experience cracking around the side of the head and that the 985 doesn’t crack across the top. However, cylinders of both the R-985 and R-1340 engine can develop cracks in either location on the heads!

Some careful reviews of the requirements are in order due to the confusing wording of the AD notes vs SB’s!

R-985: The AD affecting the 985 states: “To prevent cylinder head separation from the barrel, perform the following in accordance with Pratt & Whitney Aircraft Service Bulletin No. 1785 or later FAA-approved revision.” (Paragraph) 1. “Visually inspect cylinder heads in accordance with Part B of the bulletin as follows: (Sub-paragraph) B. “Cylinders Ultrasonically inspected, inspect within 150 hours time in service after effective date of the AD, and thereafter at intervals not to exceed 150 hours time in service.”

Service Bulletin 1785 references the R-985 Wasp Jr. Engine Maintenance Manual, Part No. 118611; Periodic Inspection. That inspection table places the check of the rear of the cylinder head for cracks or evidence of exhaust gas leakage in column “B”; 100 hours! To correctly comply with the AD the 985 cylinder heads must be visually inspected on a 100 hour basis!

R-1340: The 1340 AD and Service Bulletin are no less confusing! The AD instructs the mechanic to inspect the cylinders in accordance with SB 1787 dated September 07, 1983. However, the AD states that cowled and baffled installations should have an initial inspection at 125 hours and subsequent inspections at 250 hour time in service since last inspection. All other installations (translates “Cropdusters”) are to have an initial inspection at 50 hours and subsequent inspections at 100 hours! The SB allows for cowled and baffled engines to be inspected at 500 hours and un-baffled or “cropduster” type installations at 200 hour intervals. Sadly, the AD note is the law! You get to inspect using the technique given in the respective SB and accomplish the inspection at the intervals specified in the AD! Oh well, what do you want? Good looks and money too!

By the way; if it isn’t written in the log book, it didn’t get done!

We hope you have learned a few things from this series!

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.

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

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

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.

Continue reading Vultee BT-13 aircraft profile

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.

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

“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

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.