Lakhasly

Good morning, everyone.I also thank the School of Engineering faculty and technical staff for providing access to the necessary resources.(a) Cantilever boundary conditions do not consider the flexibility of wing-fuselage attachments (b) For flutter analysis of V-Tail and buffeting, complete aircraft might be analyzed (c) Following phenomena which render the flutter model non-linear, have not been considered, and hence, they might be covered in future projects a. Stall flutter b. LCOs (especially the subcritical regime) c. Control surfaces freeplay, etc

Slide 27.(b)The aircraft is free from flutter and divergence throughout its specified flight envelope o Min flutter speed (at sea level)299 kts o Min divergence speed (at sea level)531 kts (c)Comparison of flutter results from the three schemes show that o All the methods exhibit reasonably close agreement o k method offers conservative results for all altitudes o pk method predicts higher flutter speeds than those given by semi-analytical framework

Slide 25.In aviation it can lead to phenomenon like Control Reversals - where control inputs produce opposite effects, Divergence - where instability leads to unbounded deflection and Flutter- where aerodynamic forces interact unfavorably with a structure's natural vibration modes, potentially leading to rapid and uncontrolled oscillations.The main problems detected in the CAD model included: [Click] Inconsistent Geometry, Gaps Between Bodies, Component Intersections, Missing Elements These geometric inconsistencies were systematically addressed through careful CAD healing operations to ensure the model accurately represented the physical structure.The main aim of this work was to carry out the aeroelastic flutter analysis of the wing of Medium Altitude Long Endurance (MALE) Unmanned Aerial Vehicle (UAV) to determine the critical speeds corresponding to divergence and flutter.Because surface dimensions of all the components were significantly larger than the thickness, from the solid model, a reduced shell model was constructed by removing the thickness dimension and adding it as thickness to 2D shell elements.This analysis identifies the natural frequencies, mode shapes, and damping characteristics of a structure, which are essential for subsequent aeroelastic and flutter analyses.Therefore, stability characteristics were evaluated at different flight conditions where speed was fixed at 184 kts (340.74 kph) and altitudes were varied from sea level to 30,000 ft (9144 m) which is 5,000 ft higher than the maximum operating altitude with interval of 5,000 ft (1524 m).Naturally, it makes sense as with the increase in altitude, the value of density decreases, and the magnitude of dynamic pressure required to exert aerodynamic force sufficient for flutter onset implies a higher value of velocity.k method offers conservative results for all altitudes, whereas, pk method predicts higher flutter speeds than those given by extended eigenvalue framework.As the aircraft wing can be adequately modelled as a cantilever beam, we find that its stiffness is a function of Modulus of Elasticity (E), Moment of Inertia (I), Polar Moment of Inertia (J), its length (span) and support conditions.(d)Neutrally stable oscillatory behavior at the operational conditions indicates the possibility of LCOs (e)Variation in the thickness of the ribs do not imply any significant trend or effect on the flutter speed.(f)In comparison with the mass, aeroelastic characteristics are relatively more sensitive to the structural (bending and torsional) stiffnesses of the wing, which are directly proportional to the first and third powers of the thickness of the spars, respectively.The field of Aeroelasticity got prominent after the famous Tacoma Narrows Bridge collapse, which highlighted how aeroelastic phenomena can lead to catastrophic outcomes.In this analysis, the first ten natural modes of vibration have been extracted, however, in the flutter analysis, only the first six modes will be considered owing to the high significance of first modes and their markedly decreasing contribution as the number of modes goes towards the higher side.The analytical formulation of the flutter model starts with the derivation of the equations of motion from the Lagranges equations followed by the eigenvalue analysis which is commonly known as p method.Having determined the stability characteristics and critical speeds corresponding to flutter and divergence, it makes sense to analyze how the aeroelastic performance parameters of the wing can be enhanced.The maximum speed of the UAV was assumed to be 160 kts (303.5 kph) which implies that according to MIL STD 8870, the UAV should not encounter flutter till at least 184 kts (340.74 kph).In this project, I'll be presenting our research on flutter analysis, a critical phenomenon in structural and aerospace engineering.Subsequently a parametric study was carried out to study the effect of variation in rib and spar thickness on critical flutter speeds.Modal analysis is a critical step in understanding the dynamic behavior of structures, providing valuable insights into their natural vibration characteristics.[Click] As all the eigenvalues are pure imaginary, the system will undergo neutrally stable oscillations - thereby indicating flutter-free state

Slide 15.k method has established its supremacy in the industry practices as one of the fastest methods of flutter root extraction.Whereas p method is accepted to be the most accurate method of flutter analysis owing to its mathematical formulation.A parametric study was carried out in which the effect of variation of thickness of structural members such as ribs and spars on the critical flutter speed was studied.Today I'll be presenting my Aerospace project on the Flutter Analysis on the wing of a UAV.Slide 2.

Good morning, everyone. Today I'll be presenting my Aerospace project on the Flutter Analysis on the wing of a UAV. This project was conducted over 13 weeks under the supervision of Dr. Ehsaneh Essen Etemadi.

Slide 2. In this project, I'll be presenting our research on flutter analysis, a critical phenomenon in structural and aerospace engineering. Our presentation will walk you through our research journey, from theoretical foundations to practical applications and recommendations for future work in this field.

Slide 3. The field of Aeroelasticity got prominent after the famous Tacoma Narrows Bridge collapse, which highlighted how aeroelastic phenomena can lead to catastrophic outcomes. In aviation it can lead to phenomenon like Control Reversals - where control inputs produce opposite effects, Divergence – where instability leads to unbounded deflection and Flutter- where aerodynamic forces interact unfavorably with a structure's natural vibration modes, potentially leading to rapid and uncontrolled oscillations. In the field of UAVs, flutter is critically important as these systems fly for very long duration and at several flight conditions

Slide 4. The methodology followed during the project is as shown in the slide. First of all, CAD model was cleaned. Followed by a reduction in a shell model. Meshing was carried out and grid independence was achieved. Followed by modal analysis. Critical flutter speeds were identified at several altitudes. These values were validated using k and Pk method. Subsequently a parametric study was carried out to study the effect of variation in rib and spar thickness on critical flutter speeds.

Slide 5. Shown in the figure is the CAD model of the UAV wing. This wing has 2 spars and 21 ribs. The inner halves of the spars are metallic while the outer halves are made up of composites. The 21 ribs and skin of fuel tank located between first three ribs, are made up of Al 2024. The rest of the wing’s skin is made up of composites. Wingspan is 7.34m, with wing root and tip measuring to 1.3m and 0.47m respectively. The mean aerodynamic chord of the wing is 0.95m, which makes its aspect ratio nearly equal to 16.16, implying a high aspect ratio wing.

Slide 6. Upon analysis of the CAD model, several issues were identified that required correction before proceeding with further analyses. The main problems detected in the CAD model included: [Click]
Inconsistent Geometry, Gaps Between Bodies, Component Intersections, Missing Elements These geometric inconsistencies were systematically addressed through careful CAD healing operations to ensure the model accurately represented the physical structure.

Slide 7. Because surface dimensions of all the components were significantly larger than the thickness, from the solid model, a reduced shell model was constructed by removing the thickness dimension and adding it as thickness to 2D shell elements. This would result in significant saving of computational resources and time without compromising over the fidelity of the numerical solution.

Slide 8. As shown in slide, material properties were assigned to the reduced shell model. Ribs and fuel tanks were given the properties of AL-2024 and front and rear spars were given the properties of AL-7050 . The outer skin of the wing was given properties of composite materials.

Slide 9. After finalizing the model, the process of meshing was initiated. Quad-dominant structured meshing was carried keeping equal number of divisions on similar edges, as shown in the slides

Slide 10. Grid independence check with 6 different grid sizes was carried out for normalized natural frequency. As evident from the graph below, the grid with 343686 elements was chosen for further analyses.

Slide 11. Modal analysis is a critical step in understanding the dynamic behavior of structures, providing valuable insights into their natural vibration characteristics. This analysis identifies the natural frequencies, mode shapes, and damping characteristics of a structure, which are essential for subsequent aeroelastic and flutter analyses. In this analysis, the first ten natural modes of vibration have been extracted, however, in the flutter analysis, only the first six modes will be considered owing to the high significance of first modes and their markedly decreasing contribution as the number of modes goes towards the higher side.

Slide 12. The results of modal analysis, where we have determined the frequencies and mode shapes of 1st six modes

Slide 13. The analytical formulation of the flutter model starts with the derivation of the equations of motion from the Lagranges equations followed by the eigenvalue analysis which is commonly known as p method. Here I’d like to mention that p method only allows us to comment on the stability characteristics of a system at specified flying conditions. For example, it will tell us whether the MALE UAV is going into flutter at 25,000 ft 184 kts. Only yes or no. But if we wish to determine the speed at which it is going to encounter flutter, we will have to use PK method.

Slide 14. As per MIL STD 8870, the aircraft must be flutter-free till at least 15% above its maximum flight speed at the specified altitude. The maximum speed of the UAV was assumed to be 160 kts (303.5 kph) which implies that according to MIL STD 8870, the UAV should not encounter flutter till at least 184 kts (340.74 kph). Therefore, stability characteristics were evaluated at different flight conditions where speed was fixed at 184 kts (340.74 kph) and altitudes were varied from sea level to 30,000 ft (9144 m) which is 5,000 ft higher than the maximum operating altitude with interval of 5,000 ft (1524 m). [Click] As all the eigenvalues are pure imaginary, the system will undergo neutrally stable oscillations – thereby indicating flutter-free state

Slide 15. Critical flutter speed increases with the increase in altitude. Naturally, it makes sense as with the increase in altitude, the value of density decreases, and the magnitude of dynamic pressure required to exert aerodynamic force sufficient for flutter onset implies a higher value of velocity.
Slide 16. k method has established its supremacy in the industry practices as one of the fastest methods of flutter root extraction. Whereas p method is accepted to be the most accurate method of flutter analysis owing to its mathematical formulation. pk method combines the benefits of both k- as well as p- methods. In this slide values for critical flutter speeds and divergence velocities are depicted showing slightly higher values.

Slide 17. It is noteworthy that all the schemes exhibit reasonably close agreement in their respective results. k method offers conservative results for all altitudes, whereas, pk method predicts higher flutter speeds than those given by extended eigenvalue framework.

Slide 18. Having determined the stability characteristics and critical speeds corresponding to flutter and divergence, it makes sense to analyze how the aeroelastic performance parameters of the wing can be enhanced. A parametric study was carried out in which the effect of variation of thickness of structural members such as ribs and spars on the critical flutter speed was studied. The variation in thickness would have a direct effect on the weight of the aircraft. So, in a nutshell, we would have a measure of variation in the flutter speed with respect to the thickness or weight of the structural members.

Slide 19. To conduct the parametric study, it has been kept in mind that any enhancement should not take much toll on other design concerns such as weight or major structural modifications.

This parametric study was carried out in two sets. In the first set, the thickness of ribs varied while keeping the thickness of spars constant and its effect on the critical flutter speed was analyzed. In the second step, the opposite was carried out by varying the thickness of the spars and keeping the thickness of the rib’s constant.

Slide 20. does not vary significantly with change in thickness of ribs, as they do not contribute much towards bending or torsional stiffnesses. They do have a direct contribution against wing warping. Direct relation between and change in spars’ thickness is observed.

Slide 21. Again, mass of ribs has very little contribution towards Direct relation between and change in spars’ mass is observed

Slide 22. As the aircraft wing can be adequately modelled as a cantilever beam, we find that its stiffness is a function of Modulus of Elasticity (E), Moment of Inertia (I), Polar Moment of Inertia (J), its length (span) and support conditions. Therefore, it can be concluded that bending and torsional stiffnesses of the wing are directly proportional to the first and third powers of thickness of the spars, and so is the flutter speed.

Slide 23. The main aim of this work was to carry out the aeroelastic flutter analysis of the wing of Medium Altitude Long Endurance (MALE) Unmanned Aerial Vehicle (UAV) to determine the critical speeds corresponding to divergence and flutter. Some of the important conclusions drawn during this exercise are as follows:

Slide 24. (a)​The original CAD model is found to be unfeasible for aeroelastic analysis. Reduced model is developed and utilized. The mass distribution and geometric profile matching of both models has been carried out.
(b)​The aircraft is free from flutter and divergence throughout its specified flight envelope
• Min flutter speed (at sea level)​​299 kts
• Min divergence speed (at sea level)​531 kts
(c)​Comparison of flutter results from the three schemes show that
• All the methods exhibit reasonably close agreement
• k method offers conservative results for all altitudes
• pk method predicts higher flutter speeds than those given by semi-analytical framework

Slide 25. (d)​Neutrally stable oscillatory behavior at the operational conditions indicates the possibility of LCOs
(e)​Variation in the thickness of the ribs do not imply any significant trend or effect on the flutter speed. However, the flutter speed is found to be directly varying with the change in thickness of the spars.
(f)​In comparison with the mass, aeroelastic characteristics are relatively more sensitive to the structural (bending and torsional) stiffnesses of the wing, which are directly proportional to the first and third powers of the thickness of the spars, respectively.

Slide 26. While this project provided valuable insights, several aspects could be explored in future work.
(a) Cantilever boundary conditions do not consider the flexibility of wing-fuselage attachments
(b) For flutter analysis of V-Tail and buffeting, complete aircraft might be analyzed
(c) Following phenomena which render the flutter model non-linear, have not been considered, and hence, they might be covered in future projects
a. Stall flutter
b. LCOs (especially the subcritical regime)
c. Control surfaces freeplay, etc

Slide 27. Before i conclude, I'd like to express our sincere gratitude to my project supervisor, Dr. Ehsaneh, for the guidance throughout this project. I also thank the School of Engineering faculty and technical staff for providing access to the necessary resources.

Thank you all for your attention. I would be happy to answer any questions you might have about this project. Thank you.