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KR-20260064653-A - Coaxial Counter-Rotating Asymmetric Blade Set for Wake Interference Reduction and Vibration Dispersion, and Method for Improving Thrust Efficiency Using Same

KR20260064653AKR 20260064653 AKR20260064653 AKR 20260064653AKR-20260064653-A

Abstract

The present invention relates to an asymmetric blade set for reducing wake interference and dispersing vibration in a coaxial counter-rotating aircraft, wherein an upper rotor (D1, z1, θ1, t1/c) and a lower rotor (D2, z2, θ2, t2/c) simultaneously satisfy four conditions: (1) diameter ratio 0.70 ≤ D2/D1 ≤ 0.85, (2) pitch angle difference 4° ≤ θ2 - θ1 ≤ 8°, (3) z1 > z2 and no common divisor other than 1, (4) t2/c ≥ 14% and t2/c - t1/c ≥ 2%. Under condition 1, the lower rotor avoids the center of the wake natural velocity, under condition 2, the reduction in the effective angle of attack is compensated, under condition 3, the BPF is desynchronized and vibration is dispersed, and under condition 4, acceleration flow stall is prevented. In the example (15-inch/12-inch, 3/2 blades, 12°/18°, NACA 4412/4415), improvements in thrust +38%, efficiency +62%, output/thrust ratio -27%, and peak vibration -40% were confirmed compared to conventional symmetric blades. The present invention is applicable to small drones, industrial unmanned aerial vehicles, eVTOLs, military unmanned aerial vehicles, small helicopters, etc.

Inventors

  • 김영호

Assignees

  • 김영호

Dates

Publication Date
20260507
Application Date
20260120

Claims (11)

  1. A coaxial counter-asymmetric blade set for reducing wake interference and dispersing vibration, comprising: (a) an upper rotor rotating in a first rotational direction, comprising a first airfoil cross section having a first diameter (D1), a first number of blades (z1), a first pitch angle (θ1), and a first thickness ratio (t1/c); and (b) a lower rotor disposed below and coaxial with the upper rotor, comprising a second airfoil cross section having a second diameter (D2), a second number of blades (z2), a second pitch angle (θ2), and a second thickness ratio (t2/c), and rotating in a second rotational direction opposite to the first rotational direction; wherein the upper rotor and the lower rotor simultaneously satisfy the following four conditions: (Condition 1) 0.70 ≤ D2/D1 ≤ 0.85; (Condition 2) 4° ≤ θ2 - θ1 ≤ 8°; (Condition 3) z1 > z2, and z1 and z2 have no common divisors other than 1; (Condition 4) t2/c ≥ 14%, and t2/c - t1/c ≥ 2%; an asymmetric blade set characterized by the simultaneous satisfaction of the above four conditions, wherein, by Condition 1, the lower rotor avoids the center of the natural velocity of the upper rotor wake, by Condition 2, the reduced effective angle of attack in the wake is compensated, by Condition 3, the blade passing frequency (BPF) of the upper and lower rotors is desynchronized to disperse vibration, and by Condition 4, the lower rotor operates stably without stall in an accelerated airflow, and by these combined effects, thrust is improved by more than 30% and peak vibration is reduced by more than 30% compared to a symmetric blade of the same total disk area.
  2. An asymmetric blade set according to claim 1, characterized in that D1 is 14 to 16 inches and D2 is 10 to 13 inches.
  3. An asymmetric blade set according to claim 1, characterized in that z1 is 3 and z2 is 2.
  4. An asymmetric blade set according to claim 1, characterized in that z1 is 5 and z2 is 3.
  5. An asymmetric blade set according to claim 1, characterized in that the first airfoil cross-section is NACA 4412 (t1/c=12%) and the second airfoil cross-section is NACA 4415 (t2/c=15%).
  6. An asymmetric blade set according to claim 1, characterized in that the axial spacing between the upper rotor and the lower rotor is 10% or more and 15% or less of D1.
  7. An asymmetric blade set according to claim 1, characterized in that the amount of twist of the lower rotor is greater than the amount of twist of the upper rotor.
  8. An asymmetric blade set according to claim 1, wherein the upper rotor and the lower rotor are manufactured from carbon prepreg laminates.
  9. An asymmetric blade set according to claim 1, wherein the upper rotor and the lower rotor are injection molded from glass fiber reinforced nylon or polypropylene.
  10. A method for improving thrust efficiency using an asymmetric blade set according to any one of claims 1 to 9, comprising: (S1) a step of generating a first thrust by accelerating stationary air to a low induced speed and a large flow rate using an upper rotor having a large diameter, multiple leaflets, a low pitch, and a thin airfoil; and (S2) a step of generating a second thrust by operating a lower rotor having a small diameter, small leaflets, a high pitch, and a thick airfoil within the wake of the upper rotor, thereby reducing wake interference, compensating for a reduction in the effective angle of attack, and dispersing vibrations according to the four conditions; wherein the total thrust, which is the sum of the first thrust and the second thrust, is improved by at least 30% compared to a symmetric blade set in which the upper and lower parts have the same specifications within the same total disk area, and peak vibration is reduced by at least 30%.
  11. A method for improving thrust efficiency according to claim 10, characterized in that, by BPF asynchronous operation according to condition 3 above, the BPFs of the upper and lower rotors and their integer multiples do not match in the 100~500Hz frequency band, thereby dispersing vibration energy over a wide band.

Description

Coaxial Counter-Rotating Asymmetric Blade Set for Wake Interference Reduction and Vibration Dispersion, and Method for Improving Thrust Efficiency Using Same The present invention relates to a blade set applied to a coaxial counter-rotating rotary-wing aircraft, and more specifically, to an asymmetric blade set and a method for improving thrust efficiency using the same, wherein the upper rotor is designed as a wake generation optimization structure and the lower rotor as a wake environment compensation structure, respectively, and four design conditions—diameter ratio, pitch angle difference, number of blades (combination without common divisors), and airfoil thickness ratio—are simultaneously satisfied to reduce wake interference losses inherent to the coaxial counter-rotating structure and achieve a vibration dispersion effect through blade pass frequency (BPF) asynchronous. The present invention can be applied to various rotary-wing aircraft using coaxial counter-rotating rotors, such as small drones, industrial unmanned aerial vehicles, eVTOL (electric vertical take-off and landing) aircraft, military unmanned aerial vehicles, and small helicopters. The coaxial counter-rotor system is a method of canceling out torque by placing two rotors on the same axis and rotating them in opposite directions, and it can generate about 1.4 to 1.8 times more thrust in the same area compared to a conventional single rotor, so it is widely applied in small drones, industrial drones, eVTOL aircraft, etc. However, conventional coaxial counter-rotating systems typically use blades of the same specifications (diameter, number of blades, pitch angle, airfoil) on both the upper and lower rotors. While this symmetrical design is advantageous in terms of manufacturing and inventory management, it entails the following two fundamental problems. First is the efficiency loss due to wake interference. The downwash generated by the rotation of the upper rotor alters the operating environment of the lower rotor. According to the Blade Element Momentum Theory (BEM), the axial induced velocity v_i induced by the upper rotor is distributed radially, and the effective angle of attack α_eff of the lower rotor blade element decreases as follows: α_eff = θ - arctan((V_∞ + v_i) / (Ω·r)) [Equation 1] Here, θ is the blade pitch angle, V_∞ is the free-flow velocity (0 during hovering), v_i is the induced velocity by the upper rotor, Ω is the angular velocity, and r is the radial position. If the lower rotor has the same pitch angle as the upper rotor, the effective angle of attack decreases by v_i, thereby reducing lift generation efficiency. According to the literature, when using symmetric blades of the same specifications, the actual thrust is known to be only about 1.6 to 1.8 times (wake interference loss of about 30 to 40%) compared to the theoretical thrust of twice. Second is vibration concentration caused by BPF synchronization. When the number of blades on the upper and lower rotors is the same, the blade pass frequencies (BPF = z × RPM/60) match, causing vibration energy to concentrate at specific frequencies. This leads to problems such as structural fatigue, increased noise, and malfunction of onboard equipment. The configuration and problems of the conventional technology are summarized in Table 1 below. [Table 1] General configuration and problems of conventional coaxial counter-rotating blades Meanwhile, some prior art discloses configurations in which the diameters or pitch angles of the upper and lower rotors differ; however, these are primarily merely simple variations of individual parameters and fail to provide quantitative design criteria for complex conditions to simultaneously achieve wake interference reduction and BPF asynchronous operation. FIG. 1 is a side conceptual diagram of an asymmetric blade set according to the present invention. Figure 2 is a conceptual diagram showing the relationship between the induced velocity distribution of the upper rotor and the diameter of the lower rotor. FIG. 3 is a conceptual diagram showing a comparison of the configuration of a conventional symmetrical blade and the present invention. FIG. 4 is a plan view of an upper rotor blade according to one embodiment of the present invention. FIG. 5 is a plan view of a lower rotor blade according to one embodiment of the present invention. Figure 6 is a graph showing the vibrational spectrum dispersion effect due to BPF asynchronous operation. Figure 7 is a graph comparing the performance of a conventional symmetrical blade and the present invention. Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. 1. Core Principle of the Present Invention: The Combined Effect of Four Conditions Referring to FIGS. 1 to 3, the present invention assigns different roles to the upper rotor (110) and the lower rotor (120), and achieves a composite effect through a combination of f