Tiltrotor Proprotor Blade Frequency Decoupling
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Solution Overview
Problem
Tiltrotor aircraft face limitations in airplane mode due to forward airspeed-induced proprotor aeroelastic instability, which restricts maximum airspeed and is exacerbated by the conventional three-blade design, limiting the ability to decouple in-plane and out-of-plane frequencies effectively.
Innovation Solution
The proprotor system incorporates four blades with a first-in-plane frequency range of 2.2/rev to 2.8/rev, utilizing carbon-based materials and a positive delta 3 pitch-flap configuration, allowing for decoupling of lead-lag and flapping frequencies, enabling a shift from negative to positive delta 3, and accommodating a four-blade design without interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional three-blade proprotor design is used, then the structure is simpler, but the in-plane and out-of-plane frequencies cannot be effectively decoupled, leading to aeroelastic instability at high forward speeds
Solution Approach 1:
The proprotor system divides the blade configuration into four separate blades instead of three, allowing the frequencies to be decoupled. This segmentation enables independent tuning of lead-lag and flapping frequencies to avoid coupling instability.
Solution Approach 2:
The patent changes the fundamental parameter of blade count from three to four, and adjusts the in-plane frequency to a specific range (2.0-3.0 times the rotation frequency). This parameter change fundamentally alters the frequency coupling characteristics, eliminating the aeroelastic instability problem.
2Speed
If the proprotor operates at high forward speeds, then airplane mode performance is improved, but forward airspeed induces proprotor aeroelastic instability that limits maximum airspeed
Solution Approach 1:
The patent sets the in-plane frequency to 2.0-3.0 times the rotation frequency, which changes the dynamic characteristics of the proprotor. This frequency parameter adjustment prevents resonance and aeroelastic instability at high forward speeds, enabling safe operation at higher airspeeds.
Solution Approach 2:
The frequency decoupling design proactively prevents aeroelastic instability before it occurs. By designing the blade frequency characteristics in advance to avoid coupling, the system prevents the development of unstable oscillations that would limit maximum speed.
3Ease of manufacture
If the first-in-plane frequency is set below 2.0/rev, then the proprotor design is conventional and easier to implement, but frequency coupling occurs that limits maximum airspeed
Solution Approach 1:
The patent specifies the in-plane frequency should be 2.0-3.0 times the rotation frequency, which is a parameter change from conventional designs. This frequency range selection achieves frequency decoupling that enables higher maximum airspeeds while remaining implementable with standard manufacturing techniques.
Data Source
Figure 1A~1B
Figure 2A~2B
Figure 2C~2D
AI summary
A proprotor system (100; 200; 300; 400) for tiltrotor aircraft (10) having a helicopter mode and an airplane mode. The proprotor system includes a hub (102; 202; 302; 404) and a plurality of proprotor blades (112; 212; 312; 402) coupled to the hub (102; 202; 302; 404) such that each proprotor blade (112; 212; 312; 402) is operable to independently flap relative to the hub and independently change pitch. In the airplane mode, the proprotor blades have a first in-plane frequency greater than 2.0/rev.