Geared Low-Pressure Turbine Blade Tuning for Noise Shift
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Solution Overview
Problem
Gas turbine engines produce noise at frequencies sensitive to human hearing, particularly from the low pressure turbine, which can be mitigated by controlling the vane-to-blade ratio, but this approach increases weight and reduces aerodynamic efficiency.
Innovation Solution
Incorporating a gear reduction with an epicycle gear train having a ratio of greater than 2.5:1 between the fan and the first turbine, and designing the low pressure turbine blades to meet a formula where (number of blades × speed) / 60 ≥ 5500 Hz, ensuring noise frequencies are less sensitive to human hearing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the vane-to-blade ratio is controlled to be above a certain number (e.g., 1.5 or greater) to achieve acoustic cut-off and reduce noise sensitivity, then noise sensitivity to human hearing is reduced, but weight increases and aerodynamic efficiency decreases
Solution Approach 1:
The patent changes the operating parameters of the turbine by controlling the product of blade count and rotational speed to satisfy (number of blades × speed)/60 ≥ 5500 Hz. This parameter change shifts the noise frequency spectrum to higher frequencies where human hearing is less sensitive, achieving noise reduction without requiring increased vane-to-blade ratio that would add weight and reduce efficiency.
Solution Approach 2:
The patent introduces dynamic operation by allowing the turbine to operate at variable speeds while maintaining the noise frequency criterion. The low pressure turbine blades operate at least some of the time at a rotational speed that satisfies the formula, enabling flexible operation across different flight conditions while consistently meeting noise requirements without fixed geometric constraints.
2Object-affected harmful factors
If the vane-to-blade ratio is controlled to be above a certain number (e.g., 1.5 or greater) to achieve acoustic cut-off and reduce noise sensitivity, then noise sensitivity to human hearing is reduced, but aerodynamic efficiency decreases
Solution Approach 1:
The patent changes the operating parameters of the turbine by controlling the product of blade count and rotational speed to satisfy (number of blades × speed)/60 ≥ 5500 Hz. This parameter change shifts the noise frequency spectrum to higher frequencies where human hearing is less sensitive, achieving noise reduction without requiring increased vane-to-blade ratio that would add weight and reduce efficiency.
Solution Approach 2:
The patent introduces dynamic operation by allowing the turbine to operate at variable speeds while maintaining the noise frequency criterion. The low pressure turbine blades operate at least some of the time at a rotational speed that satisfies the formula, enabling flexible operation across different flight conditions while consistently meeting noise requirements without fixed geometric constraints.
3Object-affected harmful factors
If the gear reduction ratio is increased to greater than 2.5:1 to reduce fan speed and noise, then noise sensitivity is reduced, but device complexity increases
Solution Approach 1:
The patent introduces an epicyclic gear train as an intermediary mechanism between the low pressure turbine and the fan. This gear reduction system with a ratio greater than 2.5:1 acts as a mediator that reduces the fan rotational speed and consequently the noise generation, while the epicyclic configuration provides a relatively compact and efficient implementation of the speed reduction function.
Data Source
AI summary
An aircraft system includes, among other things, an aircraft and a gas turbine engine coupled to the aircraft. The gas turbine engine includes propulsor means for providing propulsion, compression means for compressing airflow from the propulsor means, reduction means for reducing a rotational speed of an output that drives the propulsor means, and expansion means for driving the input of the reduction means. The expansion means includes a number of turbine blades in each of a plurality of rows, with the turbine blades operating at least some of the time at a rotational speed, and the number of blades and the rotational speed being such that the following formula holds true for at least one of the blade rows of the second turbine: 5500 Hz≤(number of blades×speed)/60 sec≤10000 Hz. The gas turbine engine is rated to produce 15,000 pounds of thrust or more. A method of operating an aircraft system is also disclosed.