And now for something really unusual...the 1955 de Lackner DH-4 heli-vector prototype

Flight Journal, Oct 2001 by GUdaitis, Frank

This very strange-looking rotary-wing aircraft might not actually be the world's most unusual, but it could certainly qualify as the mechanically most simple.

Although the U.S. Heli-Vector first successfully flew in 1955, the "inverted" helicopter concept originated 38 years earlier in the minds of three Austro-Hungarians: Stephen Petroczy, Theodor von Karman and Wilhelm Zurovec. Their PKZ-2 machine was a coaxial type powered by three, 120hp, air-cooled Le Rhone rotary engines. It was tethered to the ground by an ingenious system of steel cables fed from winches. It had only one control-the throttle. It was designed to be used as a substitute for military observation balloons, and it was never really intended to fly. In June 1918, this PKZ-2 helicopter was granted an Austria patent, number 137333, and Theodor von Karman went on to become one of the world's leading aeronautical scientists. The Donald de Lackner Engineering Co. of Mount Vernon, New York, designed and built various projects that included the Heli-Vector. The most astonishing difference between the Austrian and U.S. creations was that the former required three engines with a total of 360hp to lift itself and one man off the ground. The Heli-Vector did it with only 20hp!

The Heli-Vector was the creation of Lewis McCarty, a talented Canadian design engineer. It all began when he learned of the vector flight principle that had been introduced by the brilliant inventor, Charles Zimmerman. Zimmerman also invented the very low aspect ratio aircraft that were commonly called "Flying Flapjacks." His 1946 Chance Vought V-173 prototype and the U.S. Navy XFS-U were fixedwing aircraft. The latter could potentially fly safely at speeds that ranged from 25mph to well over 400mph. Unfortunately, the Navy destroyed the two XFS-U fighters and never gave this promising airplane a chance to prove its capabilities.

Zimmerman correctly theorized that a machine like the de Lackner helicopter could achieve 360-degree directional flight control simply by tilting its axis of rotation. To this control feature, McCarty added the capability to vary torque distribution between the two sets of rotors; he thus achieved azimuth control as well.

McCarty was a "hands-on" engineer; he participated in the construction of the prototype, and he built the first rotor blades in a woodworking shop. These rotor blades were made of many strips of mahogany, birch and balsa laminated together lengthwise and covered with fiberglass cloth. The leading edges contained many small lead inserts that served as a mass balance. The airfoil was a NACA .0018 at the root, and it tapered down to .0009 at the tip; the chord was correspondingly eight inches at the root to four at the tip. Unlike helicopter rotors, these rotors were fixed at a constant five-degree pitch. This eliminated the need for complicated and costly cyclic and collective pitch-control mechanisms, and the rotors didn't need the usual flapping hinges.

McCarty designed flexible blade tips that canceled the usual rolling forces in helicopter movement. The rotor diameter was 15 feet, and 550rpm was normal for flight operation. The first prototype was powered by a two-cylinder, liquid-cooled Kiekhaefer-Mercury, two-cycle outboard racing engine. Although it was rated at 20hp, more power could be obtained by revving the engine substantially higher. Subsequent production versions for the U.S. Army were powered by fourcylinder Mercury MK-55 engines that delivered 43hp at 6,000rpm.

The Heli-Vector's transmission consisted of large reduction gears that drove two V-belts coupled to planetary gears that would reverse the rotation direction of one set of rotors. The transmission also had a stable, automatic, torquebalancing unit and an autorotation cam clutch, so in case of engine failure, the entire unit could be disengaged to allow the Heli-Vector to autorotate to the ground.

One of the 13 claims allowed in the U.S. patent granted to McCarty was the provision for attaching a cargo lift line through the hollow rotor shaft. This provided automatic balance and trim and did not couple the inertia of the cargo in pitch and roll (his patent was no. 2,847,173, issued on August 12, 1958).

In the "cockpit," a hand-operated twist grip on handlebars controlled engine speed, and a dutch lever engaged the rotors' drive system. The only instruments on these handlebars were a tachometer that indicated up to 650rpm and a fuel-level gauge. The 12 machines that were eventually supplied to the U.S. Army were appropriately named "Aerocycles."

Contributing to the Heli-Vector's inherent stability was the location of its center of gravity: above the rotor-hub assembly; it roughly coincided with the level of the pilot's waist. Its inherent stability couldn't be obtained with very highly loaded rotors of the ducted type. When tilted, a ducted-rotor mach-ine tends to keep tilting, regardless of where its center of gravity is.

Following McCarty's and test pilot Chris Kopp's first flights, more than 200 free flights were made by a variety of qualified, fixed-wing and helicopter pilots and by people without flight training. They all found the craft very stable, and they were able to hover and make directional flights almost instantly without any problems.