Bush plane has the performance edge

DN Staff

February 26, 2001

6 Min Read
Bush plane has the performance edge

At a weight of 1,670 lbs., the Mountain Goat lands with a ground roll of only 120 ft. Landing at maximum gross weight of 2,475 lbs. stretches this to 275 ft.

Livermore, CA-For Bill Montagne, president of Kinetic Aviation, his love of flying and hunting come together in a unique airplane he designed for bush flying. He felt it could do better than existing aircraft in getting more payload into and out of the small clearings pilots must negotiate, and do it safer as well. Not only will hunters and sport fishermen benefit, but those engaged in supplying remote, inaccessible areas with cargo, medical care, and missionary work have all expressed interest in Montagne's Mountain Goat aircraft.

Wing's the thing. Key to the Mountain Goat's ability to haul more-longer, both faster and slower, into tighter spots (see table)-than its main existing bush plane rival, the venerable Piper Super Cub, is Montagne's patent-applied-for wing design and its fabrication. "Basically," notes Montagne, "Its trailing edge 'cusp' (reflex curvature under the trailing edge) helps to produce a large nose down moment from lift, which allows more to be carried in the aft cargo area, balancing out the moment" without much trim drag from the tail. The wing has relatively high curvature on its top and bottom to produce twice the lift with half the drag of the Super Cub's wing, according to Montagne. "The high pitch down moment helps raise the tail for less drag when taking off," he adds. If needed, the flaps and ailerons can be "reflexed" (set trailing edge up) by 5 degrees to cut drag and nose down pitch at light loads.

The wing skin is a single sheet of aluminum alloy wrapped around the leading edge. The skin is 0.040-inch thick back to about 30% of the way to the trailing edge, then the thickness tapers to 0.032 inch. Few joints and flush rivets produce a smooth contour for greater lift and lower drag, compared with fabric covered or conventionally riveted lightplanes. These thicknesses are much more robust, says Montagne, than the 0.015-inch skins of other aircraft, "which bush pilots appreciate to cut damage from bird strikes or landing among saplings." Simplicity is key for the wing structure, cutting parts count and fabrication costs. For example, all the wing ribs are identical. The outboard ailerons and inboard flaps are identical and interchangeable.

Thrust from teh Mountain Goats' proprietary propeller design is nearly uniform across the outer 40% of its disk area. Designer Bill Montagne says a thin supersonic blade-profile cross section (tailored to the local Mach number along the blade length) makes this possible, cutting noise from teh supersonic tip speeds and producing a highly axial flow that does not kick up many stones.

Montagne originally planned on producing the wings for retrofit onto Super Cub fuselages but scrapped that idea when he figured the lift from the new wing could damage the Cub's fuselage structure. Thus the shape of the Mountain Goat's fuselage resembles that of the Cub, however it is two inches wider to hold more cargo. But because there are fewer of the larger diameter, thicker-walled, aluminum tubes in the Goat's fuselage frame, "It is ten times stronger than the Super Cub," he says, which also adds to crashworthiness. The external struts holding the wing to the fuselage are his low-drag design (looking like a teardrop, in section, with a concave tail), and are made by extruding aluminum to the net section shape. "They give an 8 knot improvement in speed over conventional streamlined struts," Montagne adds.

On the propulsion side, the efficient propeller (see figure) is also aided by a patent-applied-for muffler design that extracts less of a horsepower penalty on the engine (which, by the way, is the same as that used in the Super Cub, rated at 180 hp). Montagne says that exhaust and cooling losses on the Cub reduce the power available to 147 hp. The Mountain Goat's tuned exhaust headers, velocity-stack (bell-shaped) intakes, and muffler yield 205 hp. The muffler looks like a rectangular box with the exhaust slot running its length. All Montagne can reveal right now is that the internal plates have a simple "two bends" contour and "the linear muffler dissipates sonic waves and progressively reduces the shock waves." Once a patent is in place, the device should be applicable to other vehicles.

Asked his greatest design challenge, Montagne says, "Getting the stick forces correct with the oversized ailerons considering the speed extremes of the airplane plus the correct leverage and feel in the flap handle [positioned above the pilot's head] to go slow before landing." Balancing the tendency for a stiff control at high speed with the lack of feel at the low end was done with attention to the bell crank geometries, he notes. "The system that brings the flaperons down (drooped ailerons) not only had attention to (bell crank) angles and ratios of moving parts but also used adjoining (tandem) bell cranks" to droop both ailerons to act as flaps while allowing them to move differentially to control roll.

Montagne has produced parts and assemblies for five aircraft. Kinetic Aviation is looking to set up full-scale production in Montana-a great state for mountain goats.

Additional details

Contact Bill Montagne, Kinetic Aviation, 7481 Northland Ave., San Ramon, CA 94583; Tel: (925)373-9396; E-mail: [email protected] .

Other applications:

  • Search and rescue

  • Power line/right of way patrol

  • Eco-friendly air touring

Goat vs. Cub

Performance attribute

Useful load (lbs)

Max baggage (lbs)

Cruise speed (mph)

Cruise range (miles)

Takeoff distance (ft)

Stalling speed (mph)

Data for aircraft with large, bush-type tires

Data from Kinetic Aviation

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