Week Eight 5/25/2017

During this week in lab, a new layout for the shrink wrap was implemented. Individual layers of the plastic were attached to both the bottom and top of the glider with scotch tape. This made production of the kite take less time, as the layers were not needed to be bonded together using the solderer used previously. This was effective because the previous design methods were slow and required great skill to implement.

Fig 1: The edges of this model are sealed with scotch tape at all seams, creating a stronger, quicker bond between sheets of polyolefin

Figure 2: Sealing "ribs" created by the soldering iron in order to seal the kite's two layers together

As seen in Figure 2, the "ribs" on the polyolefin are lines from where the solderer used in this case was not effective at sealing the edges. The temperature of the solderer was 168॰F, which is greatly reduced from what it had been during previous attempts. The group expected to see a greater efficacy of this technique at a lower temperature, but even at its best, melting the top and bottom sheets of polyolefin together is not effective. The bonds rip open when the kite is heat shrunk because the melted bonds cannot handle the tension.

The current flight ability of the kite can be seen below: 

The kite has flight with too shallow of an angle of attack, causing it to dive downward at a rather sharp angle. Weight at either end of the kite (specifically the back) has not made this angle of attack any higher (relative to the horizontal). It has only served to make the flight more unstable. 

Because of this instability, a fin on the back of the kite was constructed to give pitch stability to the glider. It is currently being tested for its usefulness in the final design.

Week Seven 5/18/2017

Final Assembly Attempt #1
Following up on the final design established last week, we once again began class by cutting out an appropriately sized piece of shrink wrap and began to crimp along the borders of the kite with a soldering iron. We then cut the model out from the shrink wrap and used the heat gun to tighten the material as evenly as possible, as seen in Figure 1.

Figure 1: The kite after being sealed using a soldering iron, before heat-shrinking

Due to minor inconsistencies in the crimping and the lack of space left outside of the frame for the polyolefin to shrink, several edges of the kite frame ripped through the skin. Patches were attempted but were unsuccessful. That being said, the kite was still very much functional and proved to fly quite well in tests. The kite itself weighed 33.8g, but a substantial amount of weight will need to be added to the front to reduce the likelihood of stalling.

Week Six 5/11/2017

During the sixth week of the project, a new idea was constructed for how to best support the kite. Carbon fiber rods will now be used to connect the cross beams of the kite (Figure 1), as the line that was used in the week prior did not retain its shape after the wrap was shrunk. The rods can handle both compressive and tensile load, which will allow them to resist any deformation of the frame during heat shrinking.

Figure 1: The new kite frame, with all long structural members being carbon fiber

The rods were connected by inserting them into wire insulation on the ends of the crossbeam (Figure 2). In addition, in order to conserve material, the long end of the wing was shortened to 90 cm.
For the shrinking process it was noted that a temperature of 300 degrees Fahrenheit is the optimal temperature for shrink wrap, so the heat gun was calibrated to slightly above this setting, 320 degrees, in order to heat the wrap consistently.

Figure 2: A close up of the "wire hubs" used connect the frame

Week Five 5/4/2017

     This week, the group continued to expand upon our ability to heat shrink objects of varying dimensions. Whereas during week 4, only small, solid objects such as markers were wrapped, this week, a a roll of tape was wrapped. This is important because it has one critical difference: there is an air gap. In wrapping the roll of tape, it was found that slowly, constantly moving the heat gun from a distance of about half a meter worked the best to evenly heat the shrink wrap, resulting in what is seen in Fig. 1.

Figure 1: The tape roll
with a very smooth, tense
surface after heating

     The key for success in this materials test was both slow heating and the rigid frame of the tape roll.  

     Using knowledge gained from that testing, we then tried to apply those heating principles to the kite. Knowing that the small diameter of the frame's carbon fiber rods would poke through the heat shrink, cardboard ends were added to the rods to spread the force over a larger area and decrease the likelihood of a tear. Onto these cardboard stoppers, string was attached between each end of the kite using sliding knots, which created a very tense frame.

     The entire kite was put between two sheets of heat shrink, and then it was sealed inside using a wood engraving tool. Excess wrap was cut off, and then the heating process commenced. 

     The result found was twofold. The first result was that by heating the top layer of the kite more than the bottom layer, a curvature will form, the severity of which is dictated by the difference in heat applied to each layer. The second, and more important, result was that as the curvature described above and pictured in Fig. 2 occurred, it decreased the distance between the 4 tips of the kite. This means that the strings that previously created a very tense frame for the heat shrink to adhere to were now limp and useless. This made the kite bow and made it incredibly three dimensional, which created a lot of drag forces and made flight a laughable concept.

Figure 2: The balloon-like structure of the kite created severe curvature as a result of a less rigid frame

    The main conclusion to be drawn from this iteration of the kite is that an entirely rigid frame must be made, and the next iteration will replace string with carbon fiber rods. These will prevent the 3-D balloon-like structure pictured in figure 2, and will allow for a thin profile, more rigid frame, and a tense kite skin.

Week Four 4/27/2017

During the fourth week of construction, the shrink wrap, nylon cord, and carbon fiber rods arrived. The goals for this week were to practice using the shrink wrap to properly construct the skin for the glider and to build the outside frame using the carbon fiber rods. While practicing with the shrink wrap, it was noticed that the edges only retracted as it shrunk, therefore it was necessary to crimp the edges using a hot blade. Using a heat gun, the wrap was then shrunk around various items. Markers, pieces of wood, and old kite prototypes were wrapped. In addition, the carbon fiber frame was lashed together using the thin-nylon cord. Superglue was then added to solidify the frame.

Week Three 4/20/2017

Week Three Summary: The original prototype was flimsy, small, and stayed in the air for a very short period of time due to the weight of its construct. A revised, slightly larger model was built today, consisting of straws, string, and thread rather than wooden coffee stirrers. This decision reduced the overall weight by 4 grams, despite the glider having a greater surface area. Ultimately, the new model works better in every way; the frame does not bend and does not require constant maintenance like the original one did and flies straighter. Additionally, there was no guidance string attached. Thus the product resembles a glider more than a kite, and will be thrown between people rather than pulled around a room.

Performance characteristics

Glide ratio: 3
Bend radius: 0
Weight: 15.5g
Glide speed: Variable, more trials needed
Lift: ?
Drag: ?
Dihedral angle: 20 degrees
Wingspan: ?
Dimensions: ?
Surface area: ?
Current total expenditures: $0

Material Concerns: The string at the lab is glossy and frays easily, making it a poor choice of material for any serious design. Thread is sturdier and does not slip as easily, but is still susceptible to breaking under stress. Fishing line and nylon string are being considered as better options, as they provide high strength without adding much weight.

Week Two 4/13/2017

In addition, towards the end of the lab lines were added balance the gliding action of the kite. However, it was decided that the final design will not feature this line, as a glider design will be now constructed instead.

The first prototype of the kite was constructed during week two. It is a scaled down version of the final kite design featuring a wooden frame and plastic skin. The kite measures 45.0 cm wide and 29.5 cm long, which is a little less than half scale. A rounded front and back were constructed using metal wire and straw to keep the plastic skin in a curved shape to allow for smoother air flow. The weight of the kite at this point was 12.0 g.

During preliminary testing, the kite stalled (angled upwards), so weight was added using extra wire and paperclips. This added weight moved the center of mass of the kite towards the front, balancing the lift force so that the kite flies steadily. The final mass for the prototype was  19.1 g.