M18 receives engine conversion STC

by Mike Reightley
Ag-Air Systems, Inc.

CALDWELL, ID - Another new turbine conversion rolled out the hangar door at Hubler Field in Caldwell, Idaho this season, however this time not on the usual Thrush. Ag-Air Systems and Bill Hubler with his Valley Air Service crew combined once again to produce a new Garrett turbine installation for the M18 Dromader approved under STC SA00946SE.

Bill Hubler, like any astute operator, has always kept a close eye on the changing market for aerial application in his area. This approach has allowed him to stay competitive in the midst of urban encroachment, while working small fields to the west of Boise as well as the wide open spaces of his Burley operation in central Idaho. Having operated a fleet of five turbine-powered Thrushes for over 15 years, Bill and the base manager of the Burley operation, Tim Shamblin, determined that it was time to add an aircraft in the 600 to 700-gallon range primarily for the operation in Burley. Bill as many know, is one of the pioneers of turbine conversions, designing and building his own long before turbines even were fashionable. It was his original Thrush conversion that evolved into the Ag-Air Systems conversion that many operators around the world have in service today.

After considering all the options for a 700-gallon aircraft, Bill approached me about stretching and modifying our existing Thrush conversion for installation on the Dromader. Apart from the desire to retain the favorable design qualities of the Thrush conversion, by using as many existing parts and systems as possible, the project was significantly abbreviated.

The engine of choice was the TPE331-10 engine with a 940 shaft horsepower takeoff rating. Through efficient design of the exhaust system, the equivalent shaft horsepower (ESHP) produced by the engine is 975 HP exceeding the original radial installation of 967 HP. By analysis of wing loading, reduced empty weight, and other issues, the -10 engine was determined to be adequate to handle the propulsion requirements of the Dromader. Using the -10 engine, in comparison to the larger turbine engines typically installed on other Dromader conversions, the cost of the conversion, as well as the operating costs, are substantially less. The -10 engine has also demonstrated an excellent service history on the Thrush as well as many other non-agricultural applications.

The powerplant installation incorporates signature Ag-Air Systems/Bill Hubler design features such as the industrial battery starting system, clean and simple design, and new instrument panel configuration. In addition, the nacelle was designed to provide good aesthetics and symmetry to the aircraft. Careful attention was paid to the propeller disc position, center of gravity, and empty weight on the aircraft to obtain desirable handling qualities and a useful load of 6000 lb. The empty weight of the aircraft equipped for agricultural operations is more than 1000 lbs. lighter than the original radial engine installation.

The test aircraft, an M18B, was an excellent aircraft to begin with. It started its ag career under the auspices of Mr. Dusty Dowd of Syracuse, Kansas and then later to Craig Bair of Ag Flight, Inc. in York, Nebraska. I think that it is a universal sentiment that any aircraft that is subjected to the handy work of Dusty Dowd emerges as a bona-fide "cream puff". It had been well cared for when Valley Air purchased it. Bruce Hubler, Bill's son, operates Ag-Air Turbines, Inc. a Garrett Service Center (or I should say Honeywell...old habits are hard to break you know), at Hubler Field. Bruce built up the engine for this project. The aircraft, as expected, performed flawlessly during flight testing. It, with the TPE331-10 engine, continued its reliable operation for the duration of the ag season. Tim Shambin, who was instrumental in doing the design on the aerodynamic characteristics of the aircraft, also flies the Dromader. He has reported favorable results particularly in the hot and high conditions that exist in the local area. Burley is approximately 4300' MSL and the high temperatures typically average 90¡F. The strip he flies from is 3200' long. With 550 gallons, he can take off comfortably on an 87¡F day. He can carry 600 gallons from the lower elevation of Hubler field in Caldwell under the same conditions. Once in the air, the aircraft will climb out at a rate of 500 fpm and ferry at 145 mph burning 65 GPH. Empty, the ferry speed is 155 mph at a 65 GPH fuel flow.

Tim, having flown a Thrush for many years, says he's sold on the handling of the Dromader after a couple hundred hours flying it. With the reduced empty weight and subsequently less wing loading, the handling according to Tim is on par with a Thrush in the turns and by far more favorable climbing over the wires at the end of a field. The aircraft has met its forecasted productivity figures, and thus far it appears the investment in the bigger aircraft is paying dividends for Valley Air Service.

Our original intent in designing a Garrett conversion for the Dromader was to accommodate Valley Air Service's immediate requirements. Given the soft market that has existed for several years now on ag aircraft, we have been pleased if not somewhat surprised at the level of interest that has been generated by the completion of this project. It is my expectation that we will have an opportunity to build a few more conversions, since the Dromader airframe and Garrett engine fit a pretty good economic niche. Considering that even with a new airframe and factory CAM engine, this package costs substantially less than the next alternative. Additionally, with the current uncertainty surrounding the future of the 660 Thrush, that means for now there is one less 600+ gallon new aircraft for operators to choose from.

Where we go from here will of course be dependent on the market demands. Having all of the basic engineering in hand for the Dromader, additional configurations with the air inlet up, or larger horsepower engines, won't require a significant development or certification effort.

After working with and studying the Dromader last winter, Bill Hubler is already designing a few basic airframe enhancements that he believes will even further improve the aircraft's performance. I won't say any more about that for now, but I can assure you that anything that comes from the Ag-Air Systems/Hubler "skunk works" that will benefit the industry, we will do our best to make it available.