THE WORLD’S SAFEST AIRPLANE

The remarkable Helio Courier was the brainchild of Dr. Otto Koppen, professor of aeronautical engineering at M.I.T., and Prof. Lynn Bollinger of the Harvard Business School. Their goal, in the mid-to-late forties, was to design and manufacture an airplane that not only had STOL capabilities, but also had the combination of the highest coefficient of lift and the lowest possible drag. Prior STOL designs, such as the Curtiss Tanager that was entered in the Guggenheim Safe Flight Competition, were slow, aerodynamically dirty, and depended on low wing loading for short field performance. They also could not be safely maneuvered when operating near their stall speed. The plane they envisioned would not stall or spin at any airspeed, could make steep turns even when flying at 30 mph, and would have a good useful load, range, and a high enough cruise speed to make it a practical cross-country airplane. Such a plane, they reasoned, would have great appeal to the flying public.

As a starting point, the two professors and a third investor each put up $6,000, and purchased the fuselage, wing panels and horizontal tail surfaces of a Piper Vagabond. They engaged Wiggins Airways of Norwood, Massachusetts to modify the airframe to their specifications, the work being done at the old Norwood Airport in Canton. The fuselage was lengthened by four feet, the landing gear was made taller and was placed farther forward to insure prop clearance, and the vertical tail was increased in area and height. A salutary side-effect of the new gear position was that it allowed full braking at touchdown, without danger of the plane flipping tail over nose. The rudder was split into top and bottom halves, with the bottom serving its conventional function, while the top acted as a giant trim tab, interconnected with the full-span, slotted flaperons, to counteract adverse aileron yaw. A mechanical “brain” increased the deflection of the upper rudder in direct proportion to flap deflection. Leading-edge slats were fitted to smooth out airflow over the wings’ top surface at high angles of attack, almost doubling the amount of lift available. Amazingly, according to Dr. Koppen in an interview, these slats provided 64 percent of the total lift.

According to Dr. Koppen’s calculations derived from wind-tunnel tests, the Helioplane should have left the ground in 100 feet. Actual flight tests showed that 125 feet was required for the ground roll. This was the source of much speculation amongst the staff at M.I.T., until one of the engineers theorized that the coefficient of lift measured in the wind-tunnel derived from steady-state conditions, whereas in reality, the airplane would accelerate quickly through its liftoff speed, and the circulation around the wing lagged somewhat behind. By the time it “caught up” to the point where enough lift was being created to allow the craft to leave the ground, another 25 feet would have been covered. Understandably, the team found the extra few feet quite acceptable, and moved on. Power was supplied by a four-cylinder Continental engine of 85 horsepower, swinging a wide-chord, nine-foot Aeromatic propeller via a V-belt reduction drive. This combination adhered to Dr. Koppen’s formula for ideal STOL performance, which involved squaring the propeller diameter in inches (81) and dividing the result by the horsepower (85); the resulting number should be close to unity. At that time, the Aeromatic came the closest to constant-speed performance as was available for light planes in those days. By balancing aerodynamic forces against springs and counterweights, it provided low pitch at takeoff and high pitch at cruise.

In spite of the lengthened landing gear, the ideal ground clearance for the prop, at least nine inches, was not achieved. In fact, it was a negative 1.5 inches! The C.A.A. pounced on this, and sent no less than four test pilots to Canton, with the stated intent of making the prop strike the ground. Try as they would, they were not able to do so. With the application of full throttle, the plane would rise up on its twelve-inch-travel oleos, and would be off the ground before they could even think about raising the tail. In fact, the tailwheel was always the last to depart. Likewise, touchdowns always occurred in either the three-point attitude, or tailwheel-first. Finally, the C.A.A. threw in the towel, and accepted the design, as-is. Jack Phillipps, who is still very active after a career as head of sales for Wiggins, during which he sold over 1,000 Piper aircraft for them, was the first test pilot on the Helio project. The above-described “proof of concept” aircraft, as the C.A.A. called it, was actually named the Helioplane. It now hangs in the Smithsonian, a tribute to this pioneering effort.

The second design, purpose-built as a four-place and considered the prototype Helio Courier, utilized a Continental O-300 engine of 145 horsepower, swinging an eleven-foot prop. It had only spoilers for roll control, and as Mitsubishi MU-2 pilots will attest, and as Dr. Koppen described it, spoilers are a nasty means of causing an aircraft to bank. At first, the airflow remains somewhat laminar over the rising spoiler, and the plane doesn’t respond. With further input, the spoiler will “snatch”, and a greater-than-intended bank will result. This plane was destroyed in the vicinity of Concord, Massachusetts, when the pilot attempted to fly through a line of squalls and crashed. Realizing that such large props as the prototype used were impractical, the professors followed up in 1954 with an all-metal design using a Lycoming GO-435 engine with planetary reduction drive to a constant-speed metal propeller just over 100 inches in diameter. This provided more than adequate static thrust, essential for the quick acceleration needed for STOL operations. Slow tip speeds made for a very quiet and efficient propeller. Cooling drag, which is usually 2.5% of the total, was reduced by the pioneering use of ejectors, also known as augmentors. Exhaust was dumped into the engine cooling air exit plenum, and the high velocity of the gases would entrain the cooling air, accelerating it and creating the needed pressure differential ordinarily produced by drag-creating cowl flaps. Eventually, it was determined that ejectors were a maintenance headache, and were not cost-efficient, so they were eliminated in favor of conventional cowl flaps.

By David Keith Part 2 next month...