Coaxial Rotor Drive System Autorotation High Speed Flight
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Solution Overview
Problem
Conventional rotary-wing aircraft are limited by the tendency of retreating blades to stall at high forward airspeeds, leading to dissymmetry of lift and instability, which restricts forward airspeed to approximately 180 knots due to inadequate blade flapping and feathering, and increased rotor RPM causing supersonic issues.
Innovation Solution
A dual, contra-rotating, coaxial rotor system with a translational thrust system that offloads torque to the translational thrust system during high-speed flight, allowing the main rotor to operate in autorotation and reducing rotor RPM, thereby minimizing vibration and performance degradation by maintaining the retreating blade in flat pitch and reducing advancing blade lift.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If rotor RPM is increased to maintain lift at higher forward airspeeds, then lift is improved, but supersonic issues at the advancing blade tips occur
Solution Approach 1:
The patent implements dynamic control of rotor RPM during high-speed flight, allowing the rotor system to transition from powered rotation to autorotation. The rotor speed is dynamically adjusted to remain below supersonic thresholds while maintaining adequate lift through variable pitch control and exploitation of reverse airflow conditions on the retreating blade.
Solution Approach 2:
The system changes the operational parameters of the rotor by operating in reverse airflow conditions where the retreating blade experiences negative angle of attack. This parameter change allows the rotor to generate sufficient lift at reduced RPM by optimizing the aerodynamic conditions across the rotor disc, preventing supersonic flow at the advancing blade tips.
2Speed
If forward airspeed is increased beyond the point where flapping and feathering become inadequate, then higher speed is achieved, but retreating blade stall and negative lift conditions occur
Solution Approach 1:
The patent exploits asymmetric aerodynamic conditions by operating the retreating blade in reverse airflow where the angle of attack becomes negative. This asymmetric condition is deliberately used to balance the lift distribution across the rotor disc, with the retreating side generating negative lift to counterbalance the increased positive lift on the advancing side at high forward speeds.
Solution Approach 2:
The system converts the harmful reverse airflow condition that causes retreating blade stall into a beneficial mechanism for lift balance. By operating in the reverse airflow regime and allowing negative angle of attack on the retreating blade, the system transforms what was previously a destabilizing factor into a means of achieving lift equality across the rotor disc at high speeds.
3Speed
If collective pitch is increased to prevent rotor RPM from increasing during high speed flight, then rotor RPM is controlled, but negative lift on the retreating side increases
Solution Approach 1:
The system dynamically adjusts collective pitch in conjunction with translational thrust to control rotor RPM while managing the negative lift on the retreating side. The dynamic balance between powered thrust input and autorotative conditions allows optimization of pitch settings to minimize negative lift while maintaining RPM control during high-speed flight.
4Speed
If a translational thrust system is added to enable high speed flight, then airspeed is improved, but device complexity increases
Solution Approach 1:
The translational thrust system is designed to serve multiple functions: providing forward thrust during high-speed flight, controlling rotor RPM during autorotation, and managing the balance between powered and unpowered rotor operation. This multi-functionality reduces the need for separate dedicated systems for each function, thereby limiting the increase in overall device complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables higher airspeeds with reduced vibration and performance degradation by controlling rotor RPM and lift balance, allowing operation beyond conventional limits while minimizing vibration and maintaining stability.
Implementation Method 1
In high speed flight, the main rotor system is unloaded from the main rotor drive engines (or turboshafts), and means for controlling rotor RPM is limited to adjusting collective pitch. For a rotary wing aircraft in a high speed flight profile, however, rotor RPM is preferably decreased to prevent the rotor blade tips on the advancing sides of the rotor discs from entering a supersonic region
Implementation Method 2
As the forward airspeed is increased beyond a given point for a given rotor rpm, the flapping and feathering action eventually becomes inadequate to maintain substantial equality of lift over the rotor disc. At this point, reverse airflow across the retreating blade creates negative lift
Implementation Method 3
In contrast, the airflow velocity across the advancing blade increases with increasing forward speed. Dissymmetry of lift is thereby generated by forward movement of the helicopter
Data Source
Figure 1A
Figure 1B
Figure 2
AI summary
A drive system for a high speed rotary-wing aircraft includes a combiner gearbox in meshing engagement with a main gearbox. The combiner gearbox is driven by one or more engines such that a main rotor system and a translational thrust system are driven thereby. The engine drives the combiner gearbox and thus the main gearbox through an overrunning clutch. The drive system permits the main rotor system RPM to be controlled by offloading power to the translational thrust system during a high speed flight profile.