Frequently Asked Questions
Frequently Asked Questions
How does the Thrust Drive System work?
The thrust drive system converts thrust generated utilizing a jet engine, ducted fan, or propeller into useful rotor shaft torque needed to power helicopter rotor blades. In the thrust drive system, the required torque is delivered to the rotor shaft when thrust is applied tangentially at a distance from the rotor shaft. The thrust generated by the engines is introduced into thrust tubes near the rotor shaft and at an optimum distance from the rotor shaft, the thrust is expelled from a nozzle at the outer extremity of the thrust tubes. The resultant thrust causes the thrust tubes to rotate around the axis of the rotor shaft. Since each thrust tube is connected to the rotor shaft, the rotor shaft and its connected rotor blades rotate. For example, the torque delivered to the rotor shaft attach to the 10' rotating thrust tube assembly would be approximately equivalent to 10 times the amount of thrust delivered from the thrust tube outlet. In the event of a power loss, a one-way clutch disconnects the thrust support hub allowing the rotor blades to rotate uninhibited for autorotation. The thrust drive system usually has two or more thrust tube assemblies that are directly connected to the rotor shaft and cause its rotation. Additional thrust can be delivered by adding more thrust tubes and more powerful engines allowing this system to provide an incredible amount of shaft torque delivery while utilizing very powerful yet compact jet engines. In the thrust drive system, the rotor shaft is isolated from the fuselage eliminating the need for gearboxes, tail rotor, and related drivetrain.
Why is a tail rotor not required with the Thrust Drive System?
In a conventional helicopter, the engine is attached to a gearbox that converts engine power into mechanical torque to rotate the rotor shaft. When force is applied to spin the shaft an equal and opposite amount of force is created that "pushes" back against the gearbox and in turn wants to spin the helicopter in the opposite direction. To prevent the helicopter from spinning out of control an anti-torque device such as a tail rotor uses some of the engine power to counteract the force created from driving the rotor shaft. In the thrust drive system engine thrust is converted to the torque that is delivered directly to the rotor shaft which is isolated from the fuselage eliminating the need for a tail rotor. The resulting equal and opposite force of the thrust coming from the thrust tubes are what provides the "push" to spin the thrust tubes and connected rotor shaft in the opposite direction to thrust exiting the tubes. Without the need to continuously mitigate torque from the engines, only a small force applied directly to the fuselage is required to provide yaw control. The pilots' pedal input operates a fixed vertical stabilizer, a set of energized electromagnetic coils act against the outer edge of a magnetic disk attached to the rotor shaft to provide yaw control of the aircraft. Another simple option for yaw control is an electric brushless fan unit located behind the fuselage with an adjustable pitch, the fan to be only activated during takeoff and landing. Ideally, the fuselage profile should be as streamlined as possible not only for good aerodynamic performance but also to limit the surface area that wind can act against causing uncommanded movement.
How does the Thrust Drive System compare to past tip jet or pressure jet technologies?
Past attempts to employ a reaction drive system have either utilized separate engine powered compressors to produce thrust or engines attached directly to the end of the rotor blades. In the pressure jet configuration, a large centrifugal compressor provided air pressure from the fuselage up to the rotor blades to provide the reaction power needed for the rotor system. The engine or APU used to provide adequate pressure through the rotor blades was much larger and less fuel-efficient to comparable conventional turboshaft engines. The weight and fuel consumption of the larger engine created a disadvantage compared to conventional technology. Also, the delivery of thrust utilizing relatively small diameter tubes in the thin profile of the rotor blades at very high pressures contributed to low overall system efficiency. Tip jet helicopters where the engines are located at the tips of the rotor blades are another example of using a reaction drive power system and arguably the best of those examples is the Hiller Hornet. The Hornet along with other similar designs utilized a ramjet engine directly connected to the end of the rotor blades. The result was great performance for the 31 pounds of thrust delivered per ramjet engine but the ramjets also proved to have extremely high fuel consumption that greatly limited operational range. Further a lack of safe autorotation from altitude due to the engines being located at the ends of the rotor blade that moves at the highest velocity introduced tremendous drag in the event of power failure. These issues and others brought an end to the pursuit of these designs. For more information see https://www.fantasyofflight.com/collection/aircraft/currently-not-showing-in-museum/korean-war-post-wwii/1956-hiller-hornet/
What sets the thrust drive system apart from these prior examples is the greatly improved thrust delivery system that isolates power delivery from the rotor blades through a decoupling clutch for greater safety in the case of power loss. In prior pressure jet designs, thrust delivery was inefficient due to the delivery of airflow at very high pressure and low volume as opposed to the high volume and low-pressure propulsion of the thrust drive system, which also contributes to the thrust drive systems quieter operation. The much higher inertia of the thrust drive system allows more time to respond to the loss of power without the drag penalty of the power system due to the decoupling clutch. Also leveraging vastly improved engine technology that is now more compact, easily available, and far more affordable than in the past provides for an aircraft that is easier to manufacture and maintain.
How is the Trust Drive System safer than a conventional helicopter?
The vastly simplified Thrust Drive System greatly reduces drive system components and their associated points of failure. There is no gearbox, tail rotor, and associated drivetrain, and maintenance is greatly simplified. The engines can be located far enough from the fuselage structure to not only greatly reduce cabin noise and vibration but also reduce the proximity of the largest potential ignition source in the event of an accident. The absence of a gearbox and associated drivetrain provides additional fuselage space. The potential to operate the aircraft with a lower risk of vortex ring state and much greater inertia to help prevent rapid decay of rotor rpm in the event of an emergency or complete power loss significantly improves autorotation capability. The ability to take off and land almost completely vertical with the high inertia rotor system allows approach and departure directly from high altitudes offsetting the need for long glide path zones and reduces the danger caused by wires and other obstructions contained in and around these zones. Tail rotor fan danger and accidents are eliminated - there is no tail rotor.
Does this new technology allow improved load capacity compared to a conventional helicopter?
The elimination of the gearbox, tail rotor, tail boom, and related drivetrain weight provides for more useful payload compared to a conventional helicopter.
Can cost of producing and owning a helicopter be reduced by the Thrust Drive System?
The elimination of the gearbox, tail rotor, and related drive train and tail components vastly simplifies and reduces the cost of manufacturing and owning a helicopter. Engine maintenance or replacement is greatly simplified. It is projected that insurance costs will be lower after a track record is established for the technology.
How is autorotation improved?
Due to the high rotational kinetic energy of the thrust drive system, more time is available for the pilot to react in the event of complete loss of power before autorotation is initiated. Also, the thrust drive system is automatically decoupled from the rotor shaft by a sprag clutch at the hub eliminating any drag or power loss from failed drive components during autorotation. This is in contrast to a conventional helicopter that encounters transmission friction even when engine power is lost and may also require negative anti-torque thrust to overcome transmission friction losses during autorotation. Additional friction preventing safe autorotation can also occur if there is a failure of any critical conventional drive component.
What size helicopter does this technology apply to?
Any size helicopter can take advantage of this propulsion technology from small drones up to heavy lift operations. The thrust drive system can be scaled and configured to create a vastly simplified and more efficient helicopter that will have greatly improved serviceability and safety due to its reduced parts count along with the lower costs associated with its power system components. Imagine the ability to carry a spare engine onboard the aircraft that is easily replaceable by removing a set of fasteners, an electrical coupler, and a fuel connector. The goal is to not only to improve the safety, reliability, and cost of ownership of helicopters but also to make the thrust drive system easy to inspect and service while greatly reducing the number of components that have in the past contributed to critical failures of the conventional drivetrain.
Can the Thrust Drive System reduce noise?
Full rotor shaft torque is available even at lower rotor speeds which allow for larger blade cord, diameter, and quantity which reduces noise by allowing slower rotation and increases load-carrying efficiency without the power robbing effects of counter torque needed during transitional loading of the rotor blades. The drawback of reducing the speed of the rotor blades too much is the reduction in the top speed of the aircraft in forward flight. In addition, the thrust tube structure's airfoil can be adjusted to a slight positive angle allowing lower disk loading on the main rotor blades further reducing the amplitude of disruptive low-frequency sound emitted from the rotor blades. Tail rotor noise is completely eliminated, there's no tail rotor. Flight and power system tests indicate that the flight path approach and landing area related noise would be greatly reduced due to the ability to ascend and descend almost completely vertical to and from higher altitudes. A greater margin of safety is obtained due to the higher energy of the rotor system that can offset some of the forward flight speed needed on approach and takeoff.
Can the Thrust Drive helicopter operate with zero-emissions?
The Thrust Drive System can operate using electric or fuel power.
In the case of zero-emissions operation, hydrogen fuel can offer an efficient way to produce electricity for the electric motors with only water vapor as exhaust or it can be burned directly by the engines resulting in ultra low exhaust emissions. Another advantage is hydrogen’s energy density. The diesel fuel we use in our test engine has an energy density of 45.5 megajoules per kilogram (MJ/kg), slightly lower than gasoline, which has an energy density of 45.8 MJ/kg. By contrast, hydrogen has an energy density of approximately 120 MJ/kg, almost three times more than diesel or gasoline. In electrical terms, the energy density of hydrogen is equal to 33.6 kWh of usable energy per kg, versus diesel which only holds about 12–14 kWh per kg. What this means is that 1 kg of hydrogen, used in a fuel cell to power an electric motor, contains approximately the same energy as a gallon of diesel. Hydrogen used in fuel cells has the energy to weight ratio ten times greater than lithium-ion batteries. Consequently, it offers much greater range while being lighter and occupying smaller volumes. In terms of safety it's important to point out that diesel and jet fuel have an autoignition temperature of only 300 degrees fahrenheit where the autoignition temperature of hydrogen is over 1000 degrees.
How can the Vortex Ring State be reduced or eliminated?
There are several factors that affect the way vortices are disrupted by the thrust drive system. The thrust tube's airfoil angle of attack can be pre-set or actively controlled to aid in reducing the disk loading of the main rotor blades. Jet thrust exits the Thrust Drive System just above the rotor blades which interferes with vortex formation and continuously supplies a large volume of air from the outlet nozzle at the ends of each thrust tube which then flows outward radially above the portion of the rotor disk that is producing the greatest lift for the aircraft. The observed effect of this during smoke testing with a prototype thrust drive system and rotor was that any vortex formations were pushed out radially increasingly as thrust output was increased with rotor blade pitch only increasing the amplitude of the vortex further out radially to the rotor disk. The 1/6 scale model also demonstrated the ability to quickly and abruptly take off and land completely vertically even while fully loaded without any noticeable VRS effects as opposed to a conventional model of the same scale that showed noticeable settling under the same conditions. Another area of research being explored is that in the power system configuration there is an additional airfoil co-rotating and stacked above the main rotor that is able to adjust its angle of attack by naturally "weathervaning" into the prevailing airflow. By essentially placing a wing (airfoil) inside the main rotor upwash just outside the rotor span, that effectively increases the rotor span -- in this case, more blade area could produce more lift, but the vortex would not get stronger. The vortices can't fully be eliminated but relocating them to boost efficiency is a worthy objective.
How is drag created by the thrust tubes managed?
The cylinder-shaped thrust tube has an airfoil-shaped outer skin that greatly improves its drag coefficient. The drag produced by the airfoil is roughly equivalent to the power normally consumed by a tail rotor in the conventional helicopter. Other options for reducing the drag of the thrust tubes range from a simple splitter place behind the cylinder to more exotic surface blowing or low power dielectric dbd plasma actuators to reduce drag. Further reduction in drag is possible by changing the shape of the thrust tube to a flatter profile allowing for a more streamlined outer skin or multiple smaller round tubes but the tradeoff is an increase in internal drag of the thrust tube. In the case of power failure, any drag losses are prevented from effecting autorotation by the clutch connected to the main hub. Another approach is to use the thrust tube as the rotor blade and balance the external aerodynamic performance with the internal flow performance of the thrust tube.
The above illustration from the book Wing Theory by R. T. Jones shows a large naca airfoil with the same parasitic drag as the small round wire. For airfoils that generate lift induced drag represents the majority of drag losses.
How are the flight characteristics compared to a conventional helicopter?
All controls operate the same as in a typical helicopter and have a similar response. The thrust drive system adds a greater gyroscopic effect to the helicopter which gives it added stability and a greater "locked-in" feeling and resistance to uncontrolled movement when encountering wind gusts or other turbulence. This also allows the aircraft to maintain its heading and controlled position better when maneuvering similar to what is evident in archive videos of past reaction drive helicopters maintaining a stable hover with the pilot's hands completely off of the controls. Control responsiveness remains the same with less dampening or flybar stabilization required. Another noticeable flight characteristic is that the reaction drive helicopter hovers completely flat and level compared to conventional tail rotor equipped helicopters that usually hover left side low due to the tail rotor thrust being counteracted by the main rotor tilt.
Larger rotor blades with lower disk loading have been tested with
the prototype which allowed a sizable increase in takeoff weight without any negative yaw effects during fast transitions in blade pitch due to lack of counter-torque requirements with only a slight increase in rpm recovery during abrupt positive pitch changes.
Offloading of main rotor blades during hover and low-speed operation can also be incorporating by adding a small amount of positive pitch into the thrust tube airfoils which provides a reduction in main rotor generated noise and safer operation while loaded with higher takeoff weight. Co-rotating thrust tube airfoil and rotor blade structure can also be positioned slightly offset to reduced wake induced noise levels based on recent research.
It has been far easier to isolate the rotor shaft and rotor system produced vibration from the fuselage since there are no power train connections to carry vibration throughout the fuselage. Rotor shaft angle can also be adjusted during flight to optimize efficiency without power train interference issues.
How do you get rid of the long tail boom?
Since there is no longer a need for an anti-torque system a simple vertical stabilizer and rudder can provide yaw control. A horizontal stabilizer with dual rudders can also be utilized in place of a single rudder behind the fuselage to provide yaw control. Also yaw control can be provided even without a rudder or tail boom by utilizing a magnetic rotor affixed to the rotor shaft and then by energizing copper coils that are attached to the fuselage structure which would allow the aircraft to be actively controlled by a simple change in electrical polarity and controlled current output. This system is essentially just a brushless electric motor with the rotor shaft acting as the motor axle and the fuselage acting as the motor case
▲Most accidents involving conventional helicopters take place on landing. At top, the pilot flares the craft and the tail boom strikes the ground. Above, chopper balloons forward to level off and the main rotor tilts back, cutting into the tail.
Is there a need for special flight training or heliport facilities for the Thrust Drive System?
It's a Helicopter. Existing regulations, pilot skills, and takeoff/landing facilities are applicable. Lack of a tail boom gives the thrust drive helicopter a slightly smaller footprint.
How efficient is the thrust delivery with a turbojet engine?
The current test configuration consisting of a 40-pound thrust rated turbojet engine. The average static thrust delivered by the test engine depending on ambient temperature is usually around 39 lbs of thrust in the warmer summer weather. The current 10-foot thrust tube assembly delivers an output of just over 37 lbs at the 90-degree outlet for an average efficiency of 95% for a 4" tube and over 34 lbs of thrust for a 90% average on the 3" tube. Since this system is direct drive there are no additional mechanical losses or added component weight that is normally associated with a conventional helicopter drivetrain. Profile drag from the outer airfoil skin of the thrust tubes represents roughly 5-8% of produced shaft torque. Additionally, a portion of engine power is not required to continuously provide thrust to a tail rotor or anti-torque system further improving the overall efficiency of the aircraft. See test videos.
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