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Race Engine Technology
The Premier Engine Magazine

The Mark-15 gearbox is a new, lightweight, high-performance design which draws heavily on the success of the Mark-9 (described by several users as "bulletproof), and includes refined vibration absorption technology, improved gearing design, and new housing technology to reduce costs and increase flexibility. It can be configured to handle in excess of 1600 lb-ft. of engine torque with an acceptable margin of safety.

The technology and implementations have been proven in thousands of hours of operational experience with the Mark-9. Inspections of Mark-9 gearboxes with more than 500 flight hours have revealed all the internal parts to be in like-new condition.

The Mark-15 incorporates all the proven features of the Mark-9 into a less expensive housing, which is easier to machine and assemble. The two-mesh version incorporates a new design feature which cures the only known Mark-9 problem (idler-shaft fretting).

The vibration technology embodied in the proprietary EPI engine-to-gearbox coupling system ELIMINATES over 99% of all the torsional pulsing generated by a V8 engine, and over 99.8% of the torsional pulsing generated by an even-fire V12 engine. For a clear insight into this vibration technology, follow these four links, in order:

Part 1,       Part 2,       Part 3,       Part 4.

The Mark-15 housing design can be machined to attach to several popular engines, including: (a) the ubiquitous Small-block and Big-block Chevy engines, (b) GM LS-1 / LS-2 / LS-7 engines, (c) Ford Windsor engines, and (d) the Jaguar V12 engine. Other interfaces can be designed and built by special request. (Current M-15 status is at the bottom of this page.)

The picture below shows the Mark-15 with the optional Lord-Mount "ears".

The housings and propshaft are designed to provide a comfortable margin of safety when exposed to the extreme gyroscopic, bending, thrust and torque loads generated by a 106-inch, 175-pound metal propeller spinning at 2000 RPM when subjected to the combined loads produced by the extreme conditions specified by 14-CFR-23.371(a)(2).   (Those simultaneous operating conditions are: max thrust and max torque with 2.5 vertical g's, and aircraft rotations of 2.5-rps yaw velocity and 1.0-rps pitch velocity.)

The gears have been extensively analyzed under both static and dynamic loading conditions. As a result of the Mark-15 gear-design parameters and our proprietary vibration-absorbing technology, the Mark-15 can be equipped with gears which can handle over 1600 lb-ft of input torque with a good safety margin.

Our gear and vibration technology is so effective that the width of the output gear for 800 lb-ft. of input torque is only 1.0 inch. At 4000 RPM and 800 lb-ft. of input torque, the highest-stressed gear has a dynamic-load safety factor in excess of 1.8, and a calculated wear life in excess of 4000 hours.

Some of the other purveyors of propeller reduction gearboxes preach about the "requirement" that the PSRU must use different lubrication than the engine. One expert proclaims: "Sharing engine oil with the PSRU is asking for engine and/or PSRU failure in advance."    Check out our Lubrication page to see just how wrong this expert is. (That same expert also says that planetary gears can not work in a propeller gearbox. He might be interested in examining the planetary reductions in the P&W 1830, 2000, 2800 and 4360, the Wright 1820, and the Garrett TPE-331.)

EPI gearboxes use hydrodynamic fluid-film bearings (just like the crankshaft bearings in your engine) for the propeller shaft and the drive-gear. These bearings have far greater capacity than rolling-element bearings of the same size, and are much lighter and physically smaller than rolling element bearings which could carry the large loads required. In addition, the thrust-handling capacity of ball bearings (which some PSRU's use for their propshaft) is EXTREMELY limited. The EPI hydrodynamic bearing system provides CONTINUOUS thrust capabiltiy in excess of 3,000 pounds in either direction.

Our hydrodynamic shaft bearings and high-capacity thrust bearings, and therefore, the gears, are completely lubricated and cooled by engine oil. That eliminates the complexity of a separate oil supply, a separate oil cooler and the associated ductwork and cooling drag.

Hydrodynamic bearings require a continuous supply of pressurized lube oil. The oil required to support the bearings and to cool the gears is in the neighborhood of 4 gallons per minute (depending on the power being transmitted).

In most cases it is very expensive and unduly complex to replace an engine's native oil pump with one having nearly twice the volume capacity. That is especially true of the newer crop of engines in which the engine oil pump is a gerotor design which runs on the nose of the crankshaft, tucked inside the torsional absorber. Doubling the pumping capacity (gpm) often requires the use of an expensive, multi-stage belt-driven external pump and a snake-nest of complex, expensive Aeroquip hoses.

To avoid that unneeded expense and complexity, the Mark-15 gearbox has an optional integrated, two-stage oil pump, which is visible in the picture below. The pressure stage draws fresh oil from the engine sump, pumps it through a dedicated gearbox oil filter and into the gearbox internal plumbing passages. The second stage draws the used oil from the gearbox sump, through a removable finger-screen, and pumps it back to the engine sump.

The Mark-15 is available in several different lengths (the distance from the engine block to the propeller flange face). The minimum length is 14.00 inches, but that can be varied within limits by means of an extended propshaft. We prefer to extend the propshaft rather than using a spacer between the propshaft and the propeller.

The current TARGET price for the basic, two-mesh, 800-lb-ft. capacity gearbox is $9,500. Prices will vary depending on options, ratios, engine type, etc. The prototype is expected to begin testing in Mid-2008. First production availability is expected to be in the late third quarter-2008 vicinity.

Note that some of the basic specs for the Mark-15 have recently (01/01/07) changed. The first customer of the Mark-15 was originally a cropduster operator who had the idea he could get the FAA to allow ag-planes with uncertified engines to operate in revenue-service. That project suffered a premature expiry.

The original vertical offset and the range of reduction ratios had been tailored toward the ag-plane requirements. In late 2006, we decided to reduce the weight of the package and increase the application flexibility, which resulted in the reduction of the vertical offset from the original 8.300 inches to the new 7.900 inches, and the elimination of the two highest ratios so that the housing could be made smaller. All other specs, including structural strength and torque capacities, remain unchanged.

FEATURES

Here is a summary of the Mark-15 features:

1. The vertical offset between the crankshaft centerline and the prop centerline is 7.90 inches;
2. The available ratios are: 1.724, 1.821, 1.926, 2.038, 2.160, 2.292, 2.435, 2.591, 2.762, 2.950, and 3.158;
3. It can provide either direction of prop rotation with either direction of engine rotation;
4. The input torque capacity exceeds 1600 lb-ft. (depending on the required safety-margin);
5. The propeller thrust capacity (all configurations) is 3000 lbs. in either direction (pusher or tractor applications);
6. The propeller flange conforms to SAE ARP-880, which is the 8-bolt, 2-dowel configuration used on the P&W PT-6 and Garrett TPE-331 turbine engines, and most Continental GTSIO-520 engines;
7. The minimum length from engine to prop flange is 14 inches. It is available with extended propshafts to move the propeller further away from the engine to assist with specific cowling issues;
8. The integrated controllable propeller system is the common single-acting hydraulic configuration (Hartzell, McCauley, AeroComposite, MT, Hoffmann, etc.) found in most propeller-driven aircraft. The hydraulic prop governor drive is integrated into the gearbox and provides an AND-20010 governor mounting pad, vibration-isolated gear-drive, and internal prop control plumbing;
9. The gearbox includes provisions for a reversing prop and the PT-6-style beta-feedback mechanism for the control of a reversing prop;
10. It uses engine oil for lubrication, cooling and propeller control, so it requires no additional heat exchangers, which helps to reduce cooling drag;
11. Because most engines do not have an oil pump with enough EXTRA capacity to supply the gearbox, the Mark-15 contains its own pressure pump and spin-on filter to lubricate and cool the gearbox, and a scavenge pump to return the oil to the engine;
12. The Mark-15 contains several gearbox "health-monitoring" features, including (i) a chip detector, (ii) two temperature sensors (for incoming and exiting oil) and (iii) an oil pressure sensor location.
13. The lightest version (single-mesh) weighs 86 pounds (NOTE: weight varies with ratio, torque capacity and direction of prop rotation).

APPLICATIONS

The 7.90 offset Mark-15 gearbox was designed initially for a cropduster application using a high-output (725 lb.-ft. torque) big-block Chevy-based engine. With the 7.90-inch vertical offset and very high torque and thrust capacities, the Mark-15 is also ideal for replica-fighter applications.

Here is a picture of the Mark-15 on the EPI Cropduster big-block engine, with a 20-inch spinner.

The Mark-15 is also being used with the EPI 471-HP LS-2 based engine in a new, pusher-configuration kit aircraft currently being designed. Here is a picture of the Mark-15 on an LS engine, with an 18 inch spinner.

The fact that the Mark-15 can be configured for either direction of prop rotation (same-as-engine or opposite-of-engine) makes it ideal for a lightweight 500-HP V-8 conversion for a high-performance cabin-class twin-engine aircraft (Cessna 421, Pressurized Navajo, Aero-Commander 685, for example) using counter-rotating propellers for the safest engine-out performance.

DRAWINGS

We have recently made available a 3D-solid-model file of the Mark-15 to help potential customers with aircraft design and lofting efforts.

To download this 3D model, or to view drawings which show the major exterior dimensions of the Mark-15 gearbox and side-views of the gearbox with various spinners attached, CLICK HERE.

STATUS

(April 08): The design and analysis of the Mark-15 gearbox, and all the engineering drawings and process sheets are complete. After receiving absurd quotes for the housing patterns ($50k and up!), the company which did the patterns for the Mark-9 made time in their schedule for us, and now we expect the first housing castings for the prototype Mark-15 by July 1. From that point, it should be roughly two months to complete the prototype and get it into ground testing. Based on out previous successes, we expect the ground test phase to be uneventful, in which case we will advance to flight testing ASAP.