July 2021
Overview
I did this project while teaching a class on aircraft design for a weeklong summer camp for 7-10th graders. For this class we talked about the steps of the engineering design process and implemented them by designing and building a glider using foam and balsa wood. In addition to the design process, we also covered the main parts of an airplane and some different wing and tail designs and the tradeoffs involved with including them.
Objectives:
Create a plane that flies as far as possible
Gain a better understanding of aircraft design, especially for a glider
For my students to understand to understand why the engineering design process is useful and why planes look like they do so they can go and create a design on their own
Design
Specifications
Weight: 167 g
Planform Area: 150 in^2
AR: 7
Wingspan: 32 in
Airfoil: Clark Y
Root Chord: 6.7 in
Taper ratio: 0.4
Wing Incidence: ~4 deg
LE Sweep: ~ 4.5 deg
Total length: 30 in
Wing LE to Tail LE: 12 in
Wing Design
I chose the Clark Y airfoil because it is a solid well trusted efficient design. While there are many other more efficient shapes, I have read that the performance of the Clark Y is more forgiving of bad building. Since I was cutting the ribs out of foam by hand I knew I wouldn't be able to get a very precise cut and I was also covering it with wax paper which would make the shape even worse so this seemed like a good choice. I got my airfoil shape and Cl using airfoiltools.com.
To get my initial wing dimensions I used the following steps and equations. I estimated my weight to be 150g and throwing speed to be 15 mph which is about a light toss. While larger Aspect ratio (AR) is more efficient I only used an AR of 7 because I was afraid anything bigger would be too fragile.
Tail Design
To size my vertical and horizontal tail I used the equation show below. I think my horizontal tail was a good size because almost no incidence angle was needed for trimmed level flight. The vertical tail also was big enough since there was no "fish tailing" of the aircraft.
X is the distance from the LE of the wing to the LE of the tail. C is chord of the wing. S is the planform area of the wing. b is the span.
Construction
Wing
I decided to use a rib and spar method of building as it is popular, creates a strong wing, and resembles how real planes are made as well. I cut the airfoil shapes out of foam to make the ribs, then used 12" balsa wood as the spars. I used hot glue to connect everything together. For the spars since the wood was shorter than the span of the wing I had to glue pieces together, but I offset the space where the top was glued vs the bottom to increase strength. To give the wing strength between the halves I glued a piece of wood that connected them. This also had the added benefit of lifting up the front half of the wing and giving me the desired incidence angle of about 4 degrees.
I gave the wing some sweep by putting a foam wedge in the middle of the two halves of the wing. I glued wax paper over the skeleton as the wing surface. I also put a layer of packing tape over the leading edge to give it some extra strength.
Wing Attachment
At first I attached the wing using rubber-bands. This allowed me freedom to move thing wing if needed but also allowed the wing to pop off the fuselage upon landing. This proved to be especially useful when I hit a tree and the wing popped off and was completely damage free.
Once I felt comfortable with the design I glued the wing to the fuselage and was surprised by how durable the wing was. I took hard wing first crashes and hit trees but the wing was fine aside from a few cuts of the wax paper which I just taped over. It ended up being the hot glue connection that failed first.
Tail
I decided to use a T-Tail as it allowed me to easily adjust the tail deflection angle using a paper clip as show in the image above.
About 30g of nails was used as a ballast to balance the plane. I probably could've moved the wing backward to reduce this but maybe having higher wing loading helped with range.
Conclusion
We used a paper airplane as a baseline to measure the performance of our glider against. I was happy to see my glider outperformed my paper airplane by more than double the distance traveled.
While teaching this class I learned a lot of about teaching and how to get kids interested in involved. I also improved my understanding of aircraft design and things became a little more intuitive. Some examples are the effect of W/S on flight speed and range, and I also discovered catapult gliders and the strategy they use for getting long flight times (they add some rudder to yaw out of stall when the glider reaches max altitude then have aileron to straighten it out). I also learned new construction methods such as crimping the edges of the foam to make it more aerodynamic.