User:Talha Syed1/sandbox

= Morphing Wings = Morphing wings refers to the ability of an aircraft to change it's wing geometry mid-flight. This concept has excited aircraft designers since the beginning of manned flight starting with the Wright Brothers and their Wing Warping systems to allow control of the lateral roll of an aircraft. There are numerous other aircraft that demonstrate some level of morphing such as the F-14 or the B1-B which demonstrate variable wing sweep to reduce drag at high speeds. These are just two examples of non-traditional thinking when it comes to aircraft design and engineering, however the majority of wing and aircraft designs only display one type of morphing. True morphing refers to a much larger scale of adaptability where the wing structure has the ability to make large shape changes to cause significant variations in aerodynamic performance. There are a significant amount of engineering challenges associated with the development of morphing wing aircraft but as the future of aviation heads towards increasing efficiency and sustainability, having an aircraft with the ability to make the optimal wing structure adjustments based on the changing aerodynamic conditions/loadings is important because it allows for a reduction in power requirements, cost and increased fuel efficiency among other benefits.

Background
Like all aircraft designs, inspiration for morphing wing structures start with birds because they accomplish this task the best with the ability to continuously alter their wing shape and tail to exploit any aerodynamic advantages. By studying the way birds change their wings and what effects those changes induce, we can take that information and apply it to the new technologies we're looking to develop.

The common swift (Apus apus) is a great bird to look at as an example for flight efficiency due to the fact that they can remain airborne for anywhere between 6-10 months. Being airborne is one of the most energetically demanding tasks so many birds will take time to rest on the ground or water, however the common swift has to accomplish a numerous amount of tasks in flight such as feeding, roosting and even sleeping. In a study conducted by the Experimental Zoology Group at Wageningen University they examined the flight characteristics of different orientations and placements of swift wings to determine how the wing morphology effects their glide performance. It was found that variable sweep enlarges the aerodynamic performance envelope of swift wings and swept wings contribute to low drag coefficients at low angles of attack; extended wings contribute high lift coefficients at high angles of attack. With increasing speed, the polar in figure 2f at first maintains its shape and shifts to lower drag values. Beyond 15 m/s, the enveloping polar breaks off at lower and lower speed-specific lift values because less swept wings break under the extreme loads; only the more swept wings are left to build up the enveloping polar. Extended wings provide the best glide performance. Five of the six charts reach an absolute maximum with extended wings. The cost-related maxima occur between 8 and 15 m/s. At 10 m/s, within this optimal speed range, choosing extended over swept wings triples all three turning indices. Nevertheless, swifts sometimes choose higher glide speeds. Structural Integrity of the wings at different sweep angles and speeds were also tested. They measured loads of up to six times the bird’s weight and observed two types of structural failure: One extended-wing specimen bent to the point of breaking at 15 m/s ; another started vibrating violently at 15 and 20 m/s, which ultimately led to failure at the bone. Swept wings avoid static failure by bending and twisting under lift-loads which reduces the effective angle of attack at the hand wing and thereby caps aerodynamic load. Overall, extended wings are superior for slow glides and turns; swept wings are superior for fast glides and turns. This superiority is due to better aerodynamic performance—except for fast turns. Swept wings are less effective at generating lift while turning at high speeds, but can bear the extreme loads.

With smaller birds like swifts energy usage is easier to extend since the requirements are much lower when compared to larger birds, but these larger birds also utilize similar tactics to improve their efficiency while migrating in particular. For example, during migration the flight mechanisms used by Canada Geese (Branta canadensis) are also optimized for efficiency. The typical flapping method used in migration utilizes a retraction of the hand wing which results in a more focused vorticity intensity close to the wing surface of the hand wing. The natural flapping method doesn’t utilize such a hand wing retraction so the vortex intensity is spread out over the wing instead of converging on one point and intensifies the velocity gradient, increasing drag. Small differences in the drag and lift coefficient end up making a large impact when it comes to drag reduction. It was also found that airflow on the typical flapping wing has a smaller velocity gradient and in turn a smaller drag force than an outstretched wing. When taking a look at the drag and lift coefficients more closely, it is found that the lift coefficient is a maximum at mid-downstroke and minimum at mid-upstroke and the drag coefficient is a minimum at mid-downstroke and maximum at mid-upstroke and even becomes negative in the downstroke, indicating an enhancement in the lift forces. The same happens for the lift coefficient, but the negative lift coefficient is larger than the negative drag coefficient. The large differences between the lift coefficient and small difference between the drag coefficients showed that the typical flapping method was the most effective during migration and high speed flapping flight. This changing morphology of the wing results in an overall energy savings of 15%.

Types of Wing Morphing
When it comes to morphing wings there isn't an explicit definition of what classifies a structure or aircraft as "morphing" or not apart from the general consensus that traditional hinged devices of flap controls like ailerons don't count. Wing morphing is broken down into two types: In-plane morphing and Out of plane morphing.

In Plane Morphing
In-plane morphing refers only to alterations regarding the wing planform changes, meaning configurations by resizing the span and chord lengths, changing the sweep angles as well as a combination of these changes. Wing span resizing is when the length of the wing is changed and can be accomplished by a telescopic design with segmented sections such that each segment can be accommodated in the adjacent inner section and can be controlled by pneumatic actuators, another approach is to use a scissor like mechanism. The benefit of changing wing spans is to allow for a much lower drag to be maintained throughout a range of lift coefficients.

Chord length change is when the width of the wing is adjusted and is affected in conventional aircraft by means of the leading/trailing edge flaps. Most research into extensive chord length change involves the use of these flaps. Other potential chord length changing technologies are with the use of smart materials like DMF foam which is a shape memory polymer that is highly stretchable above the glass transition temperature and very stiff when below. However, very little research has been conducted regarding the use of smart materials in chord length change. Another application has been to use an interpenetrating rib mechanism that changes the chord length with smaller DC motors and lead screws.

Sweep is the angle that the wings are positioned adjacent to the body and the in-plane alteration in this case are the changes to increase or decrease that angle. The method of choice to change the sweep angle of aircraft has been to pivot the wings using hydraulic systems and has been implemented successfully in many operational aircraft. However the pivoting mechanisms used are generally very complex, difficult to install and maintain.

Out of Plane Morphing
Out of plane morphing refers to changes in the wing geometry that recast it outside of it's original plane. There are three types of this arrangement: Twisting, Chord-wise bending, and Span-wise bending.

In twisting, the airfoil profile itself isn’t changed, rather the airfoil camber is gradually changed along the span and is done either by a reconfiguration of the underlying structure or the morphing of the wing skin, methods can include internal mechanisms, piezoelectric actuation, and shape memory alloy actuation.

Chord-wise bending doesn’t involve a rotation of the wing, rather it changes the curvature of the airfoil and it’s mean camber line. This is often achieved by using SMA materials.

Span-wise bending is when the entire length of the wing is affected in shape by curving either upwards or downwards.

Morphing Wing Technologies
The difficulty in designing shape morphing aerostructures is that there is conflict or trade-off between being lightweight and compliant and being able to bear operational loads. In the past most wing morphing structures were either to flexible and weak or added too much weight and were overly complex and difficult to maintain so ailerons became the ideal choice. Recent developments in material science and engineering have brought about new ideas on how to create wing structures that are both compliant, rigid, and lightweight.

Lattice Wing Structures
A lattice wing is a structure composed of a multitude of individual cells all connected to each other at their nodes. These structures are ultralight and have a very high stiffness value, but the most significant benefit to using these structures is the combination of anisotropic programmabilty and the ability to have independent parameter control over the entire stiffness matrix. The project we are looking at in particular is a collaboration between NASA and MIT and it is called the Mission Adaptive Digital Composite Aerostructure Technologies (MADCAT).

The anisotropic programmabilty means that because the entire structure is made up of individual cells, you can replace different cells in different areas with a variety of materials. By having different stiffness ratios throughout the wing and arranging them accordingly you can very accurately achieve the specific deformation characteristics desired. For example, by using two different materials, aluminium and PTFE, (two materials with a Young's Modulus difference of 10^2) by making the inner half of the wing out of the less stiff material (PTFE) it would cause the tip to twist upwards under uniform loading and thereby increase the efficiency at lower angles of attack. The implementation of this "building block" system allows for a much higher degree of control in terms of how the wing behaves under different conditions. The most significant benefit of the lattice structures is their high stiffness values at very low densities, the structure used in this project had a stiffness of 2.6 MPa and a density of 5.6 mg/cm^3.

Control in Small Drones
Small winged drones face a wide variety of aerodynamic challenges during flight such as high maneuverability requirements, high wind resistance from strong headwinds and many more. However, these are all challenges that have been addressed by small birds and their solution is to change the profile of their wings to adapt to the different requirements. This research project takes inspiration from the morphing characteristics of small bird wings and applies it to a design for a micro-aerial vehicle. Bird wings mostly improve their flight capabilities by exploiting wing surface morphing by extending or retracting their outermost feathers which are called their primary flight feathers. The prototype vehicle designed by the researchers copies this overlapping feather design found in the primary flight feathers in order to modify the surface area and control the roll angle for turning.

The vehicle wings are designed with an innermost wing that remains fixed and a feathered outermost section that can be actively folded in and out. The feathered section is composed of eight artificial feathers connected to a leading edge and can be folded by rotating the leading edge with respect to the innermost wing. These individual wings are composed of a straight carbon fiber shaft that is bonded to a fiberglass frame and covered by a layer of polyester fabric called Icarex. The rotation is controlled by two artificial tendons, one for folding the wing and one for deploying. The folding tendon is controlled by a servo motor, while the deployment is pulled by a pre-stretched linear spring. The spring was chosen instead of another motor because it limits the backlash experienced and allows for an angular accuracy between 0.3 and 0.5 degrees, and it is pre-stretched to 3 Newtons which is enough to counterbalance up to 1.5 times the drag force generated when flying at 20 m/s. Overall this wing structure can experience a 41% reduction in total wing surface area.

Testing was conducted in wind tunnels and then compared to simulations that were run. It was found that the fully deployed wing configuration significantly enhances the maneuverability of the vehicle and the folded configuration of the wings is ideal for attaining low drag at high speeds and helps when dealing with strong headwinds. When fully extended, it can achieve a 32% higher lift coefficient, and when it is fully retracted the minimum drag coefficient reduced by more than 40%. Roll control is also improved by utilizing an asymmetric adjustment of the wings.