2. Research Aims
2.1. Overlapping Rotor System d/D = 0
2.1.1. Thrust Sharing
2.1.2. Vertical or Axial Rotor Separation (z/D)
2.1.3. Co-Rotating Rotor Performance
2.2. Overlapping Rotor System d/D > 0
Various Overlapping Rotor d/D Ratios
3. Rotor Geometry
4. Experimental Setup
- Isolated single rotor tests;
- Overlapping rotor tests d/D = 0 (co-axial); and
- Overlapping rotor tests d/D ≠ 0.
5.1. Isolated Single Rotor Tests
5.2. Overlapping Rotor Tests d/D = 0 (Co-Axial)
5.3. Overlapping Rotor Tests d/D ≠ 0
- A high quality small UAV off-the-shelf rotor has been measured to have a zero lift drag coefficient, = 0.019.
- A thrust sharing change of 21% was measured over the tested axial separation distances 0.05 < z/D < 0.85, when fixed pitch rotors were operated in the torque-balanced setup. The upper rotor produced more thrust than the lower at any tested z/D ratio.
- Highly-twisted small UAV rotors in a conventional torque-balanced co-axial configuration are 22% less efficient when compared to two isolated rotors whose combined solidity is equal to that of a co-axial system. Momentum theory predicts a 26% increase in the induced power for torque-balanced co-axial rotor setups. The rotor axial separation from z/D > 0.25 had no effect on the overall system performance. The lowest performance figure of 26% loss over two isolated rotors was observed at z/D = 0.05.
- When comparing upper and lower rotors in a co-axial torque-balanced setup, it has been observed that the upper rotor operates at a higher efficiency than the lower rotor throughout the tested 0.05 < z/D < 0.85 range. Interference of the upper to the lower rotor was apparent throughout 0.05 < z/D < 0.85. Lower to upper rotor interference was observed only at 0.05 < z/D < 0.6 and the peak decrease the upper rotor efficiency was observed to be 21%. On the other hand, the lower rotor efficiency increased by 10% over the same axial separation range (0.05 < z/D < 0.6). These upper and lower rotor performance deviations affected the overall performance of the system at z/D < 0.25.
- The upper and lower rotors were compared to a single isolated rotor’s performance when operating in a co-axial torque balanced setup. At the range of axial separation of z/D > 0.6 the upper rotor operates at a similar performance to that observed on a single isolated rotor. In contrast, the lower rotor at the same axial separation range experiences an efficiency loss of 42%, which decreased to 33% at z/D = 0.05.
- Swirl recovery effects were observed in the overall co-axial rotor performance by running two co-rotating rotors in a co-axial setup, when the upper and lower rotor torque was equal. The results were compared to the conventional co-axial setup running rotors in the torque-balanced mode. Overall, the conventional co-axial rotor system was measured to be approximately 4% more efficient throughout all tested axial ratios 0.05 < z/D < 0.85, revealing the magnitude of the swirl recovery contribution to the overall system performance.
- Swirl recovery effects were observed on the individual upper and lower co-axial rotor performance by running two co-rotating rotors in a co-axial setup when the upper and lower rotor torque was equal. This was then compared to the results of a conventional co-axial setup running rotors in the torque-balanced mode. The lower rotor in a co-axial co-rotating system runs approximately 4% less efficiently throughout all tested axial separations 0.05 < z/D < 0.85. No differences in upper rotor performance were detected. These measurements have revealed that swirl recovery has little to no effect on the upper rotor and all of the efficiency gains are linked to the lower rotor.
- The performance of a partially-overlapped (tandem) multi-rotor system, in a torque-balanced setup, was tested at an axial separation ratio z/D = 0.05 and the results were compared to two isolated rotors. The overall efficiency of the system increased through the overlap range of 0 < d/D < 1. The efficiency of the system started to decrease again at d/D > 1. The peak efficiency of the system was observed at d/D = 0.97, but was approximately 3% less efficient when compared to the performance of two isolated rotors.
- Performance of the upper (aft) rotor of a partially-overlapped torque-balanced system tested at z/D = 0.05 was better than the lower (front) rotor, up to d/D > 0.95. The lower rotor was observed to operate at a slightly higher efficiency in the region 0.95 < d/D < 1 at an overall thrust of 8 N. At a higher overall thrust of 15 N, such a performance change was not present. Dependencies on the overall thrust were recorded at d/D = 1 also, and with the increase of thrust the lower rotor performance decreased.
- Dependencies on the axial separation ratio z/D when rotors are operating in partial overlap of d/D = 0.85 at a torque-balanced state were recorded. Through the tested axial separation range of 0.05 < z/D < 0.85, approximately a 2% increase in overall performance was recorded. Most of the gains occurred at z/D < 0.35.
Conflicts of Interest
|Rotor disk area, , m2|
|Rotor blade chord, m|
|Zero-lift drag coefficient|
|Rotor diameter, m|
|Rotor separation, m|
|Rotor overlap ratio|
|Non-dimensional chord length|
|Induced power factor|
|Induced power factor from interference|
|Induced power factor from overlap rotor interference|
|Induced rotor power, W|
|Zero-lift induced rotor power, W|
|Rotor torque, Nm|
|Rotor radius, m|
|Distance from the blade root to the blade section, m|
|Non-dimensional radial position|
|Rotor thrust, N|
|Rotor axial separation distance, m|
|Rotor axial separation ratio|
|Blade section angle of attack, deg|
|Blade section pitch angle, deg|
|Density of air (sea-level), 1.225 kg/m3|
|Blade section inflow angle, deg|
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