Improving system efficiency and enhancing performance


Prioritizing simple robust performance

Thrust Drive System test and application summary


General technical overview of the project:

The concept consists of a reaction drive propulsion system used on a helicopter that converts the kinetic energy of the thrust generated by means of a jet, ducted fan, or propeller into useful rotor shaft torque needed to power helicopter rotor blades. For example, a 1500 lb two-seat helicopter requires ~600 ft/lbs of rotor shaft torque to fly. In a conventional helicopter, this power is generated by either an internal combustion engine or a turboshaft engine connected to a transmission system that utilizes gearing to achieve the torque requirement for the main rotor shaft along with the power required by the tail rotor. In the thrust drive system, the required power is generated when thrust is applied tangentially at a distance from the rotor shaft. For example, if you deliver 30 lbs of thrust from a nozzle that is oriented at a 90-degree angle located 10 feet away from the rotor shaft you will generate 300 ft/lbs of shaft torque and with a pair of engines, a total of 600 ft/lbs is produced. Since the torque generated by the reaction drive is connected directly to the rotor shaft no transmission system or counter-torque device (tail rotor) is required.


Full-size helicopter performance comparison:


The Composite-FX XET equipped with a single T62-T2A turboshaft engine was chosen for performance comparisons. This affordable light helicopter was chosen due to its low cost and ease of modification for experimental use. The XET has a max gross weight of ~850lbs (including the pilot) and fuel consumption of 750 ml/min at full power. Shaft torque of 400ft/lbs required for max takeoff weight. The T62-T2A weighs 65lbs and produces 95 shaft horsepower. The transmission gearing allows for just over 450 ft/lbs of rotor shaft torque production. Typical transmission losses are on average 3-5% of rated shaft power due to internal friction of drive train gears and related components. Tail rotor power requirements are unknown however on average tail rotors require around 10% average and 20% transient engine power to provide adequate control and anti-torque function.


Turbojet thrust tube efficiency testing:

In order to determine the efficiency of the thrust drive system, a test assembly was constructed that consisted of a small turbojet engine that was affixed to a ball bearing supported linear rail system. The turbojet engine is of a single-stage compressor and turbine construction with a length of 10.2 inches and a diameter of 4.3 inches with a dry weight of 3.2 lbs. Rated fuel consumption is 380 ml/min. The engine produced an average thrust of 38.5 lbs measured by a load cell attached to the end of the engine cradle with conditions of 94°F ambient temperature and 62% humidity. An air intake velocity stack and an 8' section of 4” diameter stainless steel pipe with a ninety-degree loose radius elbow was attached directly behind the turbojet engine to entrain thrust exiting the engine.

Thrust exiting the end of the completed assembly was measured and recorded at just over 36lbs of thrust for a total efficiency loss of only 6.8%.



Testing has shown that a pair of turbojet engines similar to the unit tested with thrust exiting 10ft from a rotor shaft is capable of producing 720 ft/lbs of torque with combined fuel consumption of 760ml/min. In comparison to the single conventional turboshaft engine utilized in the XET helicopter, the thrust drive system produces far more torque for almost identical fuel consumption and likely far better fuel efficiency at the required power levels. In addition, the transmission, drivetrain, and tail rotor components are not required in the thrust drive system which saves considerable weight, complexity, manufacturing cost, and maintenance. Multiple engines also provide redundancy in the event of engine failure and in this example forward flight could be sustained on a single-engine. U.S.A. and foreign PCT patent applications have been filed and are pending.