Eaton Char-Lynn Motor – Why Is The Aero Motor: Rubbing?
With the gradual improvement of aviation motor fuel consumption, thrust-to-weight ratio and other requirements, it is becoming more and more important to improve the operational efficiency of aero-motors, especially to improve the economics of civil aviation motors. The overall operating efficiency of an aeromotor is determined by a combination of aerodynamic efficiency, combustion efficiency, cooling efficiency and mechanical efficiency. Among them, improving the aerodynamic efficiency of the aeromotor is the most direct and effective means to improve the overall operating efficiency of the motor. A large part of the loss of aerodynamic efficiency is directly or indirectly caused by the gap between the top of the blade and the casing.
Can we solve this problem if we design a small gap between the top of the blade and the casing? It can’t, because too small tip clearance will increase the possibility of the blade and the casing being rubbed, which will lead to some catastrophic consequences.
The consequences are so serious, will the entire motor be broken in the event of a collision? According to the Eaton Char-lynn Motor : In fact, all motors will wear more or less the blade and the casing during their entire life cycle, and more than 99% of the collisions are harmless. These harmless rubbing mainly include the following two characteristics: the blade vibration caused by the rubbing is not large; the blade vibration caused by the rubbing can gradually disappear with the passage of time. Those that cause the blade to sustain and violently vibrate may cause more serious problems such as blade breakage, shaft bending and severe vibration of the machine.
The motor is running well, why is there a collision phenomenon? There are many reasons for rubbing. For example, when the aircraft is taking off or maneuvering, the casing will be deformed by the inertial force. If the deformation of the casing exceeds the tip clearance, the blade will The machine crashed there. The above reason is due to the deformation of the casing, and another reason is related to the shafting of the motor. In a typical modern civil turbofan motor configuration, the low pressure shaft of the motor is connected to the wheel by flanges or other means, and a fan, compressor or turbine blade is mounted on the wheel. When the shaft of the motor bends or vibrates due to mass imbalance or other reasons, the wheel or the wheel connected to it may be deformed or vibrated, which may cause the blade on the wheel to collide with the casing. The above are the two most important reasons for rotor wear in aero-motors.
The same is rubbing, why some will slowly disappear, and some will continue to violently vibrate? According to Newton’s third law, the frictional force is applied to the blade and the casing at the same time when the rubbing occurs, causing deformation or vibration of the blade and the casing. If, unfortunately, the frequency of the vibration of the casing, the frequency of the blade rotation and the natural frequency of the blade itself satisfy certain specific relations, the blade and the casing will be subjected to more and more frictional forces, resulting in more and more The large amplitude, which in turn creates unstable vibrations, is like the occurrence of a “resonance” in the sense that eventually leads to danger.
If people can have a deep understanding of the mechanism of rubbing and a series of deformations, vibrations, etc., it can be controlled and adjusted during the design or motor operation to ensure that the motor can work in various conditions. In the case of long-term safe operation, the aerodynamic efficiency loss caused by the tip clearance should be minimized, and the “monster” that is rubbed against it will be firmly controlled, and it will not be allowed to do anything to ensure the safety of people’s lives and property.
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