The rolling of an aircraft about its fuselage produces centrifugal forces which affect the stiffness of the wings. A number of previous studies explain the effect of centrifugal stiffening in rotating beams and consequently on the frequencies of the beam. Multiple cases of the rotating beam are explored in this thesis to investigate effects of mass distribution and boundary conditions on the frequencies of centrifugally stiffened beams. It is found that for a uniform beam with all degrees of freedom free on both ends, the rigid modes of the beam are affected and are no longer zero when it is stiffened from centrifugal forces. This thesis aims to set up a model to investigate the stiffening effects using the mAEWing2 aircraft. A preliminary analysis is done for the mAEWing2 aircraft and the roll rate, control surface deflection and angle of attack are identified as the parameters to be studied. For a given angle of attack and control surface deflection, the centrifugal forces in the aircraft in steady roll are determined using trim analysis. These are used to pre-stress the model for modal analysis. It is found that in mAEWing2 aircraft in steady roll maneuvers, the centrifugal stiffening effect on the natural frequencies is not significant. It emphasizes the need to conduct a sensitivity analysis to include centrifugal stiffening in the dynamic analysis while designing an aircraft. This, along with some de-stiffening due to gravity loads might be important for the future N+3 aircraft with their high aspect ratio large wingspans. / MS / Structural analysis is mainly concerned with determining the behavior of a structure when subjected to a disturbance. The natural response of a structure to some disturbance is termed as free vibration of the structure. The term vibration describes repetitive motion that can be observed in a structure and is influenced by its material and structural properties. These vibrations may cause fatigue in the structure and the performance of the structure may be adversely affected. Consequently it becomes necessary to study and eliminate these vibrations.
The vibration characteristics of a system are described by its natural frequencies and mode shapes. Natural frequencies of a structure are the frequencies at which the structure naturally tends to vibrate if it is subjected to a disturbance. The deformed shape of the structure vibrating at one of its specific natural frequencies of vibration is termed its normal mode shape.
In the case of a rotating beam, the centrifugal force acts axially along the length of the beam. When the rotating beam deflects upwards, the centrifugal force creates a downward bending moment, reducing its net deflection. The ratio of force to displacement increases, increasing the stiffness of the rotating beam. This effect is called the stiffening effect.
There is a large volume of literature that presents the effect of stiffening on the natural frequencies of a rotating beam model, for various boundary conditions. Such a stiffening analysis has also been done for the blades of a turbine and turbo fans. In addition, there are models available for analyzing the aerodynamic model of an aircraft in roll, considering stability derivatives of the aircraft. However, there are gaps in the available literature in analyzing an aircraft in roll from the perspective of structural analysis. The rolling of an aircraft about its fuselage produces centrifugal forces which affect the stiffness of the wings.
A number of previous studies explain the effect of centrifugal stiffening in rotating beams and consequently on the frequencies of the beam. Multiple cases of the rotating beam are explored in this thesis to investigate effects of mass distribution and boundary conditions on the frequencies of centrifugally stiffened beams. It is found that for a uniform beam with all degrees of freedom free on both ends, the rigid modes of the beam are affected and are no longer zero when it is stiffened from centrifugal forces. This further motivates the need for investigating the effect of centrifugal stiffening in spinning spacecraft and aircraft in rapid roll maneuvers.
This thesis further aims to set up a model to investigate the stiffening effects using the mAEWing2 aircraft. A preliminary analysis is done for the mAEWing2 aircraft and the roll rate, control surface deflection and angle of attack are identified as the parameters to be studied. For a given angle of attack and control surface deflection, the centrifugal forces in the aircraft in steady roll are determined using trim analysis. These are used to pre-stress the model for modal analysis. It is found that in the mAEWing2 aircraft in steady roll maneuvers the stiffening effect on the frequencies is not significant. It emphasizes the need to conduct a sensitivity analysis to include centrifugal stiffening in the dynamic analysis while designing an aircraft. This, along with some de-stiffening due to gravity loads might be important for the future N+3 aircraft with their high aspect ratio large wingspans.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/92698 |
| Date | 09 February 2018 |
| Creators | Deshpande, Revati Rajeev |
| Contributors | Aerospace and Ocean Engineering, Kapania, Rakesh K., Ragab, Saad A., Patil, Mayuresh J. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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