(1) Cross-type
According to the direction of flight in relation to the fuselage, it is subdivided into +-type and X-type.
The X-type structure is currently commonly used because:
? More maneuverability
? The field of view of the front view camera is less likely to be obscured.
(2) Ring type
? More rigid than traditional cross type racks
? It can avoid the vibration generated by the rack in flight to a greater extent and increase the structural strength of the rack.
? Increased rack weight, rotational inertia, and reduced flexibility.
(1) Conventional Layout and **** Shaft Dual Propeller
1) Advantages of **** Shaft Dual Propeller
? No increase in overall multi-rotor size
? Reduces propeller obstruction of the camera's field of view
2) Caution
? Will reduce the efficiency of the individual propellers. Probably*** shaft twin propellers are only equivalent to 1.6 propellers
(2) Propeller disk angle
1) propeller disk horizontal equipment
Boom rotor aircraft, frame radius R and rotor maximum radius There exists a relationship between the following ( denotes the angle of inter-axis pinch)
Reducing the size of the multi-rotor fuselage has a great impact on the inertia of the multi-rotor, the payload, and eventually affects the maximum attainable angular acceleration α and displacement acceleration.
During design, the center of gravity needs to be designed into the center axis of the multi-rotor. Another question is whether to design the center of gravity above or below the plane of the paddle disk formed by the multirotor propeller.
(1) Multi-rotor forward flight scenario
? In the figure on the right, the induced incoming flow creates drag parallel to the plane of the paddle disk
? As in figure (a), if the center of gravity of the multirotor is below the plane of the paddle disk, then the moment created by the drag force will push the multirotor pitch angle to the 0-degree direction
? As in figure (b), if the center of gravity of the multirotor is above the plane of the paddle disk, then the moment formed by the drag force will push the multirotor pitch angle towards the direction of divergence until it flips.
Therefore, when the multirotor is flying forward, the center of gravity is below the paddle plane to stabilize the forward motion.
(2) Multi-rotor wind interference situation
? When gusts of wind blow, the induced incoming flow creates drag parallel to the plane of the paddle disk
? As shown in figure (c), if the center of gravity of the multicopter is down, then the drag creates a moment that drives the multicopter pitch angle in the direction of divergence until it flips.
? As shown in figure (d), if the center of gravity of the multirotor is on top, then the moment formed by the drag force will push the pitch of the multirotor towards 0. Therefore, when the multirotor is disturbed by the external wind, the center of gravity is on top of the plane of the paddle disk to suppress the disturbance.
(3) Conclusion
? Whether the center of gravity is above or below the plane of the paddle disk does not stabilize the multirotor.
? The multi-rotor needs to be balanced by feedback control. However, if the center of gravity is very high up on the paddle plane, it will make the multirotor very unstable in one of the motion modes. Therefore, in practice, it is recommended to keep the center of gravity close to the center of the multi-rotor, or slightly lower if needed. This makes it easier for the controller to control.
The ideal position should be at the center of the multi-rotor. If the autopilot is far away from the center of the vehicle, the centrifugal and tangential acceleration will cause measurement errors in the accelerometer, known as the "lever arm effect".
(1) Standard Mounting Orientation
(2) Alternative Mounting Orientation
The Pixhawk/APM2 autopilot can be mounted in the rack at an angle of more than 30 degrees, which requires the appropriate software setup to regain the standard orientation of the autopilot mounted on the airframe.
The shape is designed primarily to reduce drag during flight. This can be categorized according to its causes
(1) frictional drag
(2) differential pressure drag
(3) induced drag
(4) interference drag. To reduce the resistance, it is necessary to properly consider and arrange the relative position relationship between the components, parts of the connection as much as possible smooth transition, to reduce the vortex generation.
Design Recommendations:
(1) Consider the inclination of the multi-rotor when flying forward to reduce the maximum windward area.
(2) And design a streamlined fuselage
(3) Considering and arranging the relative positional relationship between the components, the parts connecting as much as possible round and smooth transition, and the surface of the aircraft should be as smooth as possible
(4) Calculating the drag coefficient through CFD simulation, and optimizing it continuously
(1) Stiffness and strength to meet the requirements of the load, and the airframe will not be shaken and bent;
(2) The lighter the weight, the better;
(3) Suitable aspect ratio, spacing of axes, and structural layout;
(4) During the flight process, the lower the vibration of the fuselage, the better;
(5) Beautiful and durable.
(1) The significance of vibration reduction
1) The acceleration sensor on the flight control board is very sensitive to vibration and the acceleration signal is directly related to the estimation of attitude angle and position, so it is very important. Specifically:
? The acceleration signal is directly related to the estimation of attitude angle and attitude angle rate.
? The flight control program fuses accelerometer and barometer, GPS data to estimate the position of the vehicle. And the position estimation is critical when the vehicle is in altitude fix, hover, return, navigation, fix and autonomous flight modes
2) Another important role of vibration damping is to improve the quality of the imaging so that it can be independent of the gimbal. This is critical for miniaturization of multi-rotors.
(2) Main sources of vibration
The main sources of vibration in the airframe are deformation of the frame, and asymmetry of the motors and propellers.
1) Frame
? The deformation of the frame, especially the arm deformation, will lead to the generation of asynchronous vibration, so the greater the stiffness of the arm, the better;
? A typical carbon fiber multi-rotor frame has adequate torsional and flexural characteristics;
? Aluminum frames are more rigid, but heavier;
? To ensure that the motor and arm mounting connection, as well as the arm and the control head mounting connection is safe and reliable, and has a certain shock absorption buffer effect.
2) Motor
? The motor can run smoothly and stably;
? The paddle clamps need to be and motor bearings, propeller center **** axis, to avoid eccentric force when the motor rotates;
? Motor balancing.
3) Propeller
? Propeller balance adjuster;
? The propeller should match the frame model and body weight and have the same toughness when rotating clockwise and counterclockwise;
? Carbon fiber is stiff but a safety hazard when rotating;
? Large propellers at low speeds are more efficient compared to small propellers at high speeds, but the vibration amplitude is larger.
(3) Vibration strength constraints
1) Generally in the multi-rotor transverse vibration strength is less than 0.3g, in the longitudinal vibration requirements of less than 0.5 g.
2) The actual engineering requirements of all axes vibration strength of ± 0.1g.
If the above issues have been considered, then only need to consider other means of vibration damping
(4) Self-driving instrument and frame vibration isolation
1) The traditional practice of double-sided foam tape and nylon buckle has been used to fix the self-driving instrument on the frame.
2) In many cases, because of the small mass of the autopilot, the foam tape or nylon buckles do not provide sufficient vibration damping. As shown in the image to the right, viable vibration isolation solutions that have been tested include Dubro foam, gel pads, O-ring suspension mounts, and earbud mounts.
3) There is also a flight control damper on the market, currently. It consists of 2 pieces of fiberglass mounts, 4 shock absorbing balls and 2 pieces of foam gel pads.
(1) The main hazards of propeller noise
1) The multi-rotor fuselage will be in the sound field directly radiated by the propeller, and the various sensitive sensors may be distorted by the noise.
2) Noise affects the surrounding flight environment and generates noise pollution. Especially the noise generated when the multi-rotor is flying in the residential area.
3) If not well considered, the fuselage structure vibration and acoustic fatigue induced by the noise radiated by the propeller may seriously affect the safety of the airplane.
4) Small multi-rotors need to be kept quiet enough for covert surveillance.
(1) Propeller sound principle
1) Rotational noise
? A propeller blade with a certain thickness periodically sweeps through the surrounding air medium and causes periodic non-stationary motion of air microclusters, so the thickness noise is generated;
? Load noise is a combination of tension noise and resistance noise, which is caused by the change of pressure field on the blade surface of the paddle.
2) Broadband noise
The broadband noise of a propeller is caused by random variations in blade loading resulting from the interaction between the blade and turbulence.
For aircraft propellers and various types of impeller machinery, the *** same characteristic is the sound emitted by the rotating blades. High-speed rotation of the propeller will lead to the generation of airflow pulsation, and the strong pulsation of the airflow will be released to the outside in the form of noise. In general, propeller noise can be further differentiated into rotational noise and broadband noise.
(2) Noise reduction measures
(1) Design a distinctive aircraft configuration that is easily recognizable.
(2) Vibration damping, because the vibration of the body mainly comes from the deformation of the frame, motor and propeller asymmetry, so in the case of the same weight and size of the frame, try to ensure that the frame has a stronger rigidity, and choose the motor and propeller with excellent workmanship. In order to prevent the impact on the flight control or camera equipment, it is necessary to further consider adding a vibration-damping gimbal.
(3) In terms of noise reduction, it is mainly achieved by designing new propellers. In this talk, we only give some design principles, but no specific design method. Then, under the same performance requirements, how to design a multi-rotor with minimum resistance, minimum vibration and minimum noise? In the multi-rotor appearance is similar today, these may be the future can be improved in one of the directions.