by Richard F. Honn

How about a 2,500+ ft/min. climb rate on your Skymaster? What if you had the additional single engine security of 310 hp from 520 cubic inches of turbo charged power? Is a 300 mph top speed appealing? Based on the overwhelming, positive response that SuperSkyrocket, LLC. received at the recent Sun & Fun show plus many daily telephone call-ins and Internet page "hits", there are a lot of interested Skymaster and potential Skymaster owners worldwide . But what does it really take to perform the FAA-approved conversion, which is covered by several STC’s? If your curiosity is aroused, the inside story of the Riley "Big Engine" conversion for a pressurized Skymaster follows.

Before getting to the "nuts & bolts", an explanation is in order. SuperSkyrocket, LLC., as the holder of all of the Riley 337 STC’s, is offering an engine-only Skymaster conversion as well as the complete Skyrocket & Super Skyrocket models. The six inch pile of CAD drawings that define the FAA-approved, engine-only conversion has been reviewed and updated. In addition, the entire parts list has been computerized, a vendor base has been established, and a complete quality control system has been implemented. Why are these actions so important?

As one prospective purchaser of an engine conversion stated, after thoroughly going over the re-engining portion of a Super Skyrocket that is nearly finished, "this is one gargantuan project". To be cost effective, and thus price competitive, this "gargantuan project" has to be run as the proverbial "well-oiled machine". Unfortunately, placing 520 cubic inch engines in engine compartments designed to tightly accept 360 cubic inch engines is much more involved than simply removing and replacing the existing engines and props!

It took Riley several years and multi-millions of dollars to satisfy the FAA requirements, during which time the conversion evolved into over five hundred in-house fabricated parts and over two thousand purchased parts, some of which have to be modified. Riley took on the challenge not only of fitting the "big engines" into the already cramped engine compartments but also successfully solved the historic engine cooling problems, especially in the back engine area. This required a totally different engine cooling baffle arrangement than the historic Cessna design. Recognize that the 337 aircraft presents an interesting challenge in this area, as there are two totally different engine installations to perfect. Thus, two totally new, largely different engine arrangements evolved with few common parts. It took many flight test hours to perfect these two new designs.. And oh, by the way, an all-new cabin pressurization system was developed which provides increased bleed flow for cabin pressurization, thus improving that system. In summary, you throw all engine-related items in both engine compartments away, relocate and alter the remaining firewall components such as control cables that must fit the 520 engines’ requirements, rewire the aircraft for 50 amp alternators and new engine instrumentation, and replace both the pilot and co-pilot instrument panels with a revised design. And then you add a customized three-bladed Hartzell propeller, new custom exhaust system, all new tubes and hoses, and on an on, and you have all the components that constitute the conversion. Now that you are impressed with the "gargantuan" magnitude of what is required by the FAA for the engine-only conversion, on to the "nuts and bolts".

Step 1 - Get the Parts

With the over 2,500 parts required for the conversion, it makes sense to buy/fabricate these before tearing into a customer’s airplane. Therefore, following receipt of the customer’s order, the computerized ordering effort is undertaken. The fabricated parts are ordered from the vendor base and consist of many diverse fabrication processes and materials, such as castings which are later machined, sheet metal parts that are bent and machined, thick steel parts that have to be bent, machined, heat-treated, and plated, fiberglass scoops and ducts, and all of the fuel, oil and vacuum hard and soft lines. Per FAA quality regulations, these vendors have to be supplied with certified material, vendor’s have to be audited for process approval, and all vendor supplied parts have to be thoroughly inspected to meet the drawing dimensions. This requires SuperSkyrocket, LLC management effort. As the vendors do not produce the same volume of parts for an airplane as they do, for example, an electronics industry, a narrow range of vendors is interested in making aircraft parts. These vendors are also concerned about the liability involved in producing aircraft parts. As a result, the vendors that will sign up to make these parts charge premium prices.

On the purchased parts side, all parts must be purchased from certified aircraft parts suppliers. They have the same thinking as the vendors in terms of quality and risk! Consequently, these parts are not cheap, especially from major companies such as Teledyne, Cessna, Hartzell, Woodward, BF Goodrich, etc. In addition, recent increases in component prices and, in the case of the engines, significant increases in price due to the cessation of OEM agreements and imposition of an engine core differential charge, have not helped the cost of the conversion. Once again, SuperSkyrocket, LLC has automated this process and negotiated the best possible agreements to hold the costs of the conversion in line.

Lead times, for most of the parts, are four weeks with the propellers and a few other major pieces at no more than eight weeks, based on the existing agreements. Since the vast majority of the parts are available for installation at the end of four weeks from date of order, the airplane is ready for conversion.

Upon aircraft arrival, the airframe is thoroughly inspected and the avionics/aircraft systems are exercised to determine deficiencies. The engine only conversion includes corrective actions in the engine and associated structural areas. At that point the forward and aft engine compartments are gutted, much of the forward engine mounting structure is de-riveted and cut away, and the aft engine cage and mounting arrangement are thrown in the trash. The aircraft interior and instrument panels are removed to provide access for the 50 amp service and new engine gauge wiring. The old engine cluster gauge setup is incompatible with the updated engine instrumentation. So now we have a big pile of junk in the corner and a gutted airplane!

Step 3 - Airframe Modifications

With complete access to the forward and aft firewalls, as well as all the engine cowling pieces being disassembled and ready for rework, a number of modifications are made. On the forward firewall, due to reduced clearance between the engine and the firewall coupled with 520 engine- required control cable relocations, substantially all component locations are changed. As this is a pressurized airplane, previous holes must be sealed from the cabin side of the firewall, which has restricted access. On the aft firewall, the areas where the increased strength engine cage and mounts attach must be strengthened with thick steel doubler plates that require drilling, riveting and sealing. Once again, control cables and other components must be relocated. The upper scoop sheet metal is modified to accept a flow divider that is part of the all new baffle system for engine cooling.

The forward engine mount structure is rebuilt using an all together different design. This design underwent considerable stress analysis and testing. It consists of an improved "box beam" structure which locates the engine mounts. An interesting factoid is that, while Cessna had the freedom to alter the cowls to accommodate the engine/propeller alignment, the new structure must be aligned to make sure the prop comes out the existing hole! A sophisticated alignment tool is used to make sure the prop sticks out that front hole!!

The back engine cage and mounting structure is an all new design with an associated structural analysis. It is much stronger than the original cage and utilizes ½ inch bolts at the attachment to the aft bulkhead. A special boring/reaming tool is used to enlarge the mounting holes in the rear bulkhead. Doubler plates strengthen the aft bulkhead in the area of the mounting holes.

Cowl modifications, consisting of creating new oil access doors, air inlet and exhaust openings, "bubbles" made of fiberglass for engine clearance, louvers for air cooling on the cowls and aircraft underbody, and the use of improved hinges and actuators are required to accommodate the new 520 engines. Because no two Cessna airplanes are created dimensionally alike, the cowl rework is an artisan task. Pre-formed fiberglass ducts are carefully hand fit to provide an aesthetically pleasing, as well as functional, part. All new stainless steel fasteners are used, as well as annodized hinges. Of course existing holes that are not utilized in the conversion must be structurally plugged. Even the static pressure port has to be relocated. Airframe modifications require the skills of a craftsman.

Step 4 - Aircraft Wiring

New wires must be run through the aircraft floor and forward and aft engine bulkheads for the 50 amp alternators, engine instrumentation, and engine electrical devices such as starters, voltage regulators, etc. With the cabin gutted and the floor panels accessible, the wiring work still requires several hundred manhours. All non-functional wiring is removed to facilitate future electrical work. Wiring through the bulkheads must be sealed to for cabin pressurization integrity. A complete set of schematics are included in the all new Service Manual.

Step 5 - Engine Buildup and Installation

To the fullest extent possible, all engine accessories, baffles, hoses, and tubes are installed on the engines prior to their installation in the aircraft. Once installed in the aircraft, the propellers, exhaust systems, intercoolers and turbochargers are attached. All hoses, tubes and wiring connections are finalized. The improved cabin pressurization/environmental system is installed.

Step 6 - Ground and Flight Testing

After all this work, its finally time to fire the engines! This is always an exciting occasion Ground checks are performed, resulting in the final rigging of the engine control cables. The aircraft landing and cowl doors are adjusted after jacking up the aircraft, and the aircraft is buttoned up for flight. Following initial flight testing, the aircraft is ferried to the paint shop. Following paint, the aircraft is checked for pressurized cabin integrity, the interior is reinstalled, and all aircraft and avionics deficiencies are corrected.

Another Skymaster owner is ready to enjoy a level of performance that others only dream of!

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