MOUNTAIN GOAT STOL™
High Performance Bush Plane and General Aviation Aircraft
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2.  MOUNTAIN GOAT STOL™ BACKGROUND
Montagne MOUNTAIN GOAT STOL™ and PIPER SUPERCUB™ at Friday Creek, Alaska. A heritage, and a new era of bush and general aviation flying.
Montagne MOUNTAIN GOAT STOL™ and PIPER SUPER CUB™ at Friday Creek, Alaska. A heritage, and a new era of bush and general aviation flying.
RESEARCH AND DEVELOPMENT

The MOUNTAIN GOAT STOL™ is the result of seven years of development, addressing the particular needs of Alaska bush pilots. Two years were spent researching the requirements and disposition of bush pilots, including perceived shortcomings of the existing bush plane fleet; useful load requirements; undesirable flight qualities of the existing bush plane fleet; undesirable structural qualities of the existing bush plane fleet; determination of the real world performance of the existing bush plane fleet; what happens in typical crashes; FAA regulations related to certification processes; design criteria needing to exceed the requirements of FAA regulations; and available, state of the art technologies capable of benefitting design goals predicated by the research.

Some things we didn't learn until after we built our first prototype. We found for instance that in some cases, field repair capability is considered preferable to advantages measured purely in terms of performance. Over a period of several more years, each issue raised by the bush pilot community was addressed. A composite wing was eliminated over concerns of "in field" serviceability. A new technology for an all metal wing was developed which provides the cost benefits of a metal wing while achieving the aerodynamic benefits of a composite wing. Engine power output was increased to 180 horsepower factory specs, with additional performance derived from exhaust and propeller refinements. We developed an exceedingly quiet, high-performance exhaust. Additional aerodynamic modifications were made to achieve better performance. One of the greater challenges was to design an empennage providing the performance required for FAA certification standards, while providing equally outstanding performance at 180 mph IAS and above.

THE FIELD AS A PROVING GROUND
Montagne MOUNTAIN GOAT STOL™ and PIPER SUPERCUB™ at Friday Creek, Alaska.
Montagne MOUNTAIN GOAT STOL™ and PIPER SUPER CUB™ at Friday Creek, Alaska.

The MOUNTAIN GOAT STOL™ uses as standard equipment, a McCauley 76-inch diameter, 66-inch pitch, fixed pitch prop. We regularly operate out of short, rough strips at any altitude, taking off and landing in the same distance as a good, lightly loaded Super Cub™ with an 82 inch prop with 40 inches of pitch — a rig which substantially benefits the Super Cub™ to achieve speed and climb.

As soon as we are off the ground, we are climbing at over 1,000 ft per minute — at a faster speed than the Super Cub™ will fly at full throttle and level. If the Super Cub™ is in stable, maximum rate of climb, full power, with the 82 / 40 prop, and the MOUNTAIN GOAT STOL™ has the same load and full throttle, the Super Cub™ will be climbing at about 1,200 ft/min, at about 63 mph at 3,000 ft. The MOUNTAIN GOAT STOL™ will be climbing at 2,200 ft/min, at 110 mph.

The MOUNTAIN GOAT STOL™ can take off in short distances because the wing has more than twice the lift of a Super Cub™, and less than half the drag. The wing uses a state of the art airfoil developed from aerospace research by Bill Montagne, implemented in a flush riveted wing, flaps, and flaperons. The accurate structure of this wing, and its state of the art mechanical and aerodynamic technology, distinguish MOUNTAIN GOAT STOL™ performance above what are really antiquated designs, far behind the curve of modern developments. We don’t use an 82 /40 prop, because we can achieve substantially superior performance without the diameter and pitch, and without the inherent landing clearance risks.

REAL WORLD PERFORMANCE OF THE EXISTING BUSH PLANE FLEET
Montagne MOUNTAIN GOAT STOL™ at Friday Creek, Alaska.
Montagne MOUNTAIN GOAT STOL™ at Friday Creek, Alaska.

The fact is that the Super Cub™, Husky, Maule, or Scout will not carry heavy loads legally, or safely. There isn't even room for a heavy load.

This is because these planes fail FAA flight tests at heavier loads than they were certified for. The FAA flight tests are there to ensure aircraft are safe within the weight and balance envelope, air speeds, and flight conditions encountered in an airplane's intended and practiced scope of use.

REAL WORLD RAMIFICATIONS OF THE PERFORMANCE OF THE EXISTING BUSH PLANE FLEET

Over-stressing of a structure is cumulative.

Any form of over-stressing manifests in a degraded structure, incapable any longer of withstanding cycles of stress it was formerly capable of withstanding. An over-stressed structure may not break immediately — but may indeed ultimately break at any time thereafter, as the result of some future stress event, "usually" far below the original design limit of the structure.

This is exactly and unequivocally why the FAA regulations are defined as they are, and why it is extremely unwise to load an airplane beyond its certified weight and balance envelope. The laws of physics are not malleable. The consequences of every event, acting on the given condition of given objects, are exacting.

In the field, as a consequence of the low useful loads of the existing bush plane fleet, and as a consequence of the needs of pilots to get so many pounds into so many flights, overloading of the conventional bush plane fleet is a regular occurrence. Higher stall speeds mean landing faster with that overload. The heavier weights mean stall speeds substantially higher than specified in the aircraft's documentation. Excessive flight loads in turbulence can over-stress the airplane. Structures not meant to survive the force of stress cycles and impacts substantially beyond designed tolerance, are regularly broken down as a consequence of inadequate mission performance.

INHERENT SAFETY AND REAL WORLD RISKS OF THE EXISTING BUSH PLANE FLEET
MOUNTAIN GOAT STOL™ in California.
MOUNTAIN GOAT STOL™ in California.

Climbing into an aircraft which has been overloaded should raise concern for the safety of all onboard, from the time of the overloaded condition forward. Just because the airframe did not break YET, does NOT mean that the airframe will not break under conditions substantially less severe than the original structure could readily survive.

Anyone flying an airplane which has been overloaded has become a test pilot. Every passenger of such an airplane is now a test passenger. An overloaded airplane has virtually been rendered into an experimental aircraft, even though the airplane has a type certificate. That airplane technically has violated its type certificate, and therefore is experimental from that point on.

Most pilots believe stalling the horizontal tail is the limiting factor. So when they fly with an illegal load condition, they keep the airspeed up. Stalling the horizontal tail however may not be the limiting load. As excessive flight loads in turbulence can over-stress the airplane, structural degradation and structural failures can and will occur in the air.

Any time a pilot overloads their airplane, they become a test pilot on every flight thereafter, because they have potentially overloaded every flight surface to some degree beyond the aerodynamic and structural limits of that airplane. The whole plane can be damaged. The whole plane can be rendered a substantially less effective and resilient aircraft — all the more disposed to future damage and failure.

MOUNTAIN GOAT STOL™ DESIGN AND PERFORMANCE ENVELOPE

The MOUNTAIN GOAT STOL™ was designed and tested with these needs and limits in mind. We know what Alaska bush pilots are going to do regardless what the flight manual says, so we wanted the MOUNTAIN GOAT STOL™ to fly WELL when flown as pilots need to fly their bush planes.

The useful load of the MOUNTAIN GOAT STOL™ needed to be twice that of the Super Cub™. The MOUNTAIN GOAT STOL™ needed to handle a much more aft CG condition. The MOUNTAIN GOAT STOL™ needed to have a large baggage box, and to carry much more fuel. The structures of our plane are meant to serve this useful duty range with even greater margins of safety.

The fuel capacity is 65 gallons, and unlike all other bush planes, the MOUNTAIN GOAT STOL™ has no significant center of gravity shift from full to empty, or with changes in angle of attack. Exhaustive solution of every such issue manifests in accumulated advantages in the end product. Particularly as the performance of any aircraft is only so great as its limiting deficiency, tremendous advantages are gained by superior design throughout. Elimination of significant load changes during flight itself improves safety and utility tremendously.

The MOUNTAIN GOAT STOL™ can hold the largest of pilots and all the baggage you really need. The baggage area behind the rear seat is limited to 350 lbs with the 4130 chrome moly alloy airframe, and to a seemingly incredible 430 lbs with the titanium fuselage airframe. The MOUNTAIN GOAT STOL™ exceeds FAA test requirements at these weights. In fact we have tested these weights in extreme turbulence (up to 100 kt winds).

Fill the MOUNTAIN GOAT STOL™ up with 65 gallons of fuel, two people, camping and hunting gear for a week, and easily cruise at 160 mph with the throttle set to baby your engine. As a flying machine, the MOUNTAIN GOAT STOL™ is solid as a rock.

MOUNTAIN GOAT STOL™ SPEED — WHY IS THE MOUNTAIN GOAT STOL™ SO FAST?
MOUNTAIN GOAT STOL™ in California. Front view shows the profile of a slippery airplane. Note left door latched open (right side of image).
MOUNTAIN GOAT STOL™ in California. Front view shows the profile of a slippery airplane. Note left door latched open (right side of image).

Why is the MOUNTAIN GOAT STOL™ so fast?

The MOUNTAIN GOAT STOL™ is a complete rethinking of how we approach aerodynamics, structures, maintenance, assembly, and propulsion systems. Numerous proprietary technologies contribute to the exceptional performance of the MOUNTAIN GOAT STOL™. The wing airfoil engenders less than half the drag and more than twice the lift of its competition. The aerodynamic struts increase cruise speed by 8 kts, and contribute dramatic strength to a more aerodynamic empennage.

SAFETY, CRUISE SPEED, AND STALL BEHAVIOR

Upon examining our aircraft diligently, Peter DeRuyter, an FAA DER certification engineer said, “This is probably the safest plane in general aviation history.”

Millions of people have seen a Discovery Channel special featuring the MOUNTAIN GOAT STOL™ flying over convoluted terrain, just over barbwire fences, a few feet off the ground, at 30 mph. A highlight of the special shows the MOUNTAIN GOAT STOL™ taking off from a California runway. From a dead stop, with no wind, it routinely jumps off the ground to a spectacular climb rate, in a measured 50 feet.

In 2000, Bill Montagne, developer of the MOUNTAIN GOAT STOL™, received an award from Design News Magazine for the second best, new engineered technology. The technology was the flight characteristics and systems of the MOUNTAIN GOAT STOL™. The editor was a flight test engineer who realized how difficult it is to make an airplane fly so well at such low speeds — and just as well at such high speeds.

Cockpit, suspension, and cowling detail.
Cockpit, suspension, and cowling detail.

While most airplanes only cruise at three times their stall speed, the MOUNTAIN GOAT STOL™ cruises at nearly six times stall.

Primary reasons for the safe flight characteristics are the stall speeds and behavior, and the plane's characteristic behavior in recovering from stall.

Stall is preceded by a slight buffet. The airfoil pitches the nose down gently. And finally, without attendance, the low drag of the airfoil allows the MOUNTAIN GOAT STOL™ to accelerate by itself, back to flying speed, usually dropping only a few feet of altitude. The high lift and low drag coefficients allow the MOUNTAIN GOAT STOL™ to obtain flying speed almost instantly.

The MOUNTAIN GOAT STOL™ usually recovers from a stall by the time the nose has dropped to horizontal. This highly desirable, characteristic stall behavior is maintained even in steep turns below 30 mph.

Obviously, as load is increased, stall speed increases. You can expect stalls below 30 mph with two people and 20 gallons of fuel, and the engine at 1,000 RPM. Stall at gross weight, carrying an incredible useful load of 1,250 lbs (1,330 pounds with Titanium airframe) is still below 40 mph.

AIRFOIL AND BEHAVIOR
MOUNTAIN GOAT STOL™, left side — in the altitudes of Alaska.
MOUNTAIN GOAT STOL™, left side — in the altitudes of Alaska.

The MOUNTAIN GOAT STOL™ achieves its remarkable performance using an airfoil which, with flaps and flaperons deployed, has a CL max of over 3.8, and a wing efficiency factor of 0.95.

Because of our airfoil design, the MOUNTAIN GOAT STOL™ has no tendency to spin, even in slow skidding turns. Full aileron deflection can be made, right down to stall.

This is because, in the design of our airfoil, only the inboard half of the wing actually stalls. The drooped ailerons are set so that at full aileron deflection and maximum drooped position, they are still at an angle below the stall point.

The great stall characteristics of the MOUNTAIN GOAT STOL™ are also maintained with flaps and flaperons reflexed up. The stall speed just increases to 40 mph.

What does all this mean?

It means bush operators CAN use the MOUNTAIN GOAT STOL™ for their typical work, with much more safety. It means they can do almost three times the work in a day as they used to do.

It means MOUNTAIN GOAT STOL™ technology allows the typical pilot to fly at excellent cruising speeds, with far greater loads. Exemplary low speed capability and character mean every pilot can fly with the piece of mind that if the prop stops turning, they will have many more options. Even over the most severe terrain, pilots can easily fly slow enough that any impact will be much more survivable. The MOUNTAIN GOAT STOL™ does not need level or smooth ground to operate from.

GENERAL AVIATION APPLICATIONS
MOUNTAIN GOAT STOL™ in the hills of California.
MOUNTAIN GOAT STOL™ in the hills of California.

For those not shopping for a bush plane, consider that the MOUNTAIN GOAT STOL™ has 1,250 lbs of useful load, 65 gallons of fuel capacity, GROSS WEIGHT stall speeds below 40 mph carrying incredible useful loads of 1,250 pounds and upward, solo stall speeds in the mid twenties; and, with conventional aircraft tires, a fixed pitch prop, 5/8 throttle, and 2,600 RPM, the MOUNTAIN GOAT STOL™ can cruise at 180 mias at 8,000 ft.

FAA engineers have analyzed the MOUNTAIN GOAT STOL™ for Vne, and have concluded that our plane is FAA certifiable at 213 mph. The FAA in fact sees no problems in testing up to 235 mph, as this is the speed they require flying the MOUNTAIN GOAT STOL™ at, to test and certify their projected Vne of 213 mph.

Imagine therefore what you can do, and where you can go — and the safety in which you can travel in the MOUNTAIN GOAT STOL™.

The MOUNTAIN GOAT STOL™ was first intended to be the state-of-the-art bush plane. But with its speed and useful load, it will be a great plane for all types of sub-200-mph flying, particularly with its structural resiliency, and ready capability for incredibly graceful, "unusual" landings at under 40, and even under 30 mph.