EXAMINATION OF THE RELATIONSHIP BETWEEN ROTARY WING UAV SYSTEMS AND FLIGHT PERFORMANCE
Relationship between Propulsion System and Flight Performance
Approximately 75% - 90% of the total power consumption of rotary wing Unmanned Aerial Vehicle (UAV) systems is generated by the propulsion system. For this reason, it is important to design the propulsion system, which is directly related to the flight performance parameters such as aircraft flight time, maneuverability, resistance level to operational conditions, and the payload capacity, suitable for the weight of the aircraft and highly efficient.
The fact that a UAV system with a high level of autonomy can navigate in the air for very short periods of time and has low wind resistance renders the autonomy capabilities of the aircraft meaningless. On the other hand, it is meaningless for an aircraft that can stay in the air for a long time to have a very low image transmission range and image quality. The important thing in this regard is to realize the desired capabilities and flight performance characteristics with an aircraft of minimum weight and dimensions, or to allow the aircraft to hover longer without compromising other flight performance characteristics compared to similar products with similar weight, size and capabilities.
Rotary-wing UAV propulsion system consists of engine, propeller, ESC (Electronic Speed Controller) and battery components. The propulsion system architecture is schematically shown in Figure 1.
Figure 1: Propulsion system architecture
The ESC adjusts the engine speed according to the commands given from the ground control station and the signal it receives from the flight controller. On the other hand, the ESC must be capable of transferring the maximum current drawn by the motor. Similarly, the battery must be able to supply the maximum current transferred by the ESC. As it is known, if the ESC cannot transfer the maximum current drawn by the motor, the ESC fails, and if the ESC cannot supply the current transferred by the battery, the battery swells.
While the ratio of Maximum RPM (Revolutions Per Minute) and total thrust created by brushless motors to aircraft weight is expected to be above a certain value, this value determines the relationship between flight time and wind resistance. In order to examine the relationship between the propulsion system and flight performance, the test data of different brushless motors and propellers of different geometries belonging to T Motor company, which are already in the market, were taken as basis and aircraft flight times were calculated under certain assumptions. Test data for the mentioned engines and propellers are given in Figures 2, 3, 4 and 5.
Figure 2: Test data of MN1804 model T engine with 5*3 propellers
Figure 3: Test data of MN1804 model T Engine with 6*2 propellers
Figure 4: Test data of MN2204 model T engine with 5*3 propellers
Figure 5: Test data of MN1806 model T engine with 7*2.4 propeller
Considering the test data of each propeller and engine pair in Figures 2, 3, 4 and 5, for 100% throttle, the efficiency of the propellers, the thrust produced and the current drawn, the weight of each engine and the total thrust generated by the 4 propellers are calculated. The ratio information to the total weight of the aircraft is given in Table 1.
When calculating the Total Thrust/Total Weight value, it is assumed that the total weight of all systems except the engine is 300 grams and that each propeller has the same weight.
Total Thrust/Total Weight ratio is an important parameter that directly affects flight performance and is taken into account when evaluating flight performance. The larger this value, the more resistant to wind and more maneuverable the aircraft propulsion system. In addition, this value, which is expected to be around 2 in terms of flight safety, confirms another general principle in propulsion system design, that the aircraft can hover with 40% - 60% throttle.
The flight time of rotary wing UAV systems can be calculated with the following equation.
E = 0.8. (C/I).60
In Equation 1, E (minutes) is the flight time, I (A) is the current drawn by the system in hover mode, and C (Ah) is the battery capacity.
Using the test data in Figures 2, 3, 4 and 5, the current drawn by each propeller and engine combination for the hover mode of the aircraft and the flight times information obtained as a result of the calculations made using Equation 1 are given in Table 2. In the calculation of each flight time, the battery capacity is taken as 3000 mAh and the current drawn by the subsystems other than the propulsion system is neglected.
Looking at the data in Table 1 and Table 2, the maximum Total Thrust/Total Weight value has emerged with the choice of the engine and propeller group no. 2 and the aircraft platform with the highest wind resistance has been obtained. It is seen that preference 2 is more advantageous than preference 1 and 4 in terms of both flight time and wind resistance.
However, it should be noted that the aircraft flight time will be less with option 2 compared to option 3. When the 1st and 4th options are compared, the 1st engine and propeller group can be preferred because the 1 minute flight time difference is at an acceptable level and the wind resistance is higher. On the other hand, the best flight time was obtained with the engine and propeller group no. 3, and as a result of the lowest Total Thrust/Total Weight value in the product preference, a structure that is less resistant to wind and rain has emerged.
As a result, the aircraft propulsion system should be optimized and designed in the best way, taking into account the environmental conditions in which the product will be used and the required operation time.
Structural Methods Applied to Improve Flight Performance
In his study titled "Rotor Sound Pressure Level Reduction Through Leading Edge Modification", Callender (2017) made modifications on the propeller, inspired by the wing structures of owls, which are among the quietest flyers in nature, in order to reduce the noise level. The modified propeller visuals used in the aforementioned study are in Figure 6, and the sound levels and thrust force data based on the number of revolutions obtained as a result of the study are given in Figure 7.
Figure 7: Modified propellers
Figure 8: RPM dependent sound levels and thrust data
Looking at the data in Figure 8, it is seen that there is a very small difference in the thrust value between the standard propeller and the modified propeller at low rpm, and there is a significant improvement in the sound level compared to the standard propeller at low rpm. At high speeds, the difference in thrust between the two propellers increases, while the difference between sound levels decreases.
Butt and Talha (2019), in their study titled "Numerical Investigation of the Effect of Leading-Edge Tubercles on Propeller Performance", made modifications on the propeller at different amplitudes and wavelengths, inspired by the wing structures of whales, in order to improve flight performance. The modified propeller images used in the study and called C1, C2, C3 and C4 are shown in Figure 9 and the propeller efficiencies obtained for 10000 revolutions are shown in Figure 10.
Figure 9: Modified propeller images
Figure 10: Modified yields
Looking at the data in Figure 10, it is seen that the efficiency of the modified propellers is higher.
As a result of the study, it was observed that the propeller efficiency was improved by 14.99%, 24.39%, 28.89% and 53.59%, respectively, compared to the standard propeller in C1, C2, C3 and C4 configurations for a coefficient of advance of 0.7 and 10000 revolutions. However, it has been observed that larger amplitude propellers are more efficient than smaller amplitude propellers. On the other hand, when the wavelengths are reduced from C1 to C2 and from C3 to C4 without changing their amplitudes, it is concluded that the propeller efficiency increases by 50.1% and 35.34%, respectively.
One of the methods used to improve flight performance is the ducted propeller application. In addition to improving flight performance, it can also be applied for passive sound reduction method by integrating the propeller with the acoustic damping frame.
In their study titled “Quadcopter Thrust Optimization With Ducted-propeller”, Kuantama and Tarca (2017) conducted thrust analysis of 3 different ducted propeller structures using CFD (Computational Fluid Dynamics) method.
Figure 11: Different ducted impeller structures
The results of the analyzes made with the propellers, each of which is 4 cm in diameter, are given in Figure 12.
Figure 12: Thrust data of different ducted propeller structures
As can be seen in Figure 12, a remarkable difference was observed in the thrust force value in ducted and standard propeller structures at high speeds (over 4000 RPM values). According to the standard propeller preference for 9000 RPM, an improvement in thrust force was observed at 1.38 N in the type α propeller structure and approximately 2.1 N in the type β and γ propeller structures.
Hrishikeshavan et al., (2012), in their study titled ''Development of a Quad Shrouded Rotor Micro Air Vehicle and Performance Evaluation in Edgewise Flow'', found that ducted and non-ducted standard propeller structures, in hover mode and at 1, 2, 3 and 4 m/ They presented the buoyancy force data under summer wind in the order of s as in Figure 13.
Figure 13: Thrust data for Ducted and standard propeller structures under different crosswinds
Looking at the data in Figure 13, it is seen that the deviation in the lift force value for a given RPM up to the crosswind of the order of 2 m/s for both cases is negligible. However, there was a 15% increase in thrust for ducted propellers and 12% for standard propellers compared to hover mode for 4 m/s crosswind and 7900 RPM. In addition, the thrust produced by the ducted propeller for each crosswind range was significantly improved compared to the standard propeller. The improvement in thrust for 7900 RPM turned out to be about 20 grams under hover mode and other conditions.
It should be taken into account that there will be some increase in aircraft weight due to their ducted structure for both studies. In this case, the aircraft Total Thrust/Total Weight value and flight time calculations should be done again for each propeller structure and the final results should be evaluated.
Conclusion
Since most of the total power consumption in rotary wing UAV systems is caused by the propulsion system, it is necessary to optimize the propulsion system in the best way by considering the required maneuverability, flight time and wind resistance, and structural applications should be applied to improve flight performance.
In addition, it should not be forgotten that each capability desired to be acquired by the aircraft has a restrictive effect on the flight performance characteristics of the aircraft. For all these reasons, in order to be competitive in the global market and to respond to operational requirements, especially in small-sized rotary wing UAV systems, where even one minute of flight time is significant, instead of targeting high performance for each of the flight performance parameters, it is possible to use the air according to a specific purpose and operational concept. It is concluded that the development of the intermediary system is more applicable.
DÖNER KANATLI İHA SİSTEMLERİNDE İTKİ SİSTEMİ İLE UÇUŞ PERFORMANSI İLİŞKİSİNİN İRDELENMESİ
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