Dr. Pepper Plane

My last “real” job left an hour in the middle of every day that was wasted. I couldn’t stand the thought of going out and getting lunch or just surfing the internet for an hour, so I began to build something unique.

 I figured the one thing I had in abundance was aluminum. I drink one can of Dr Pepper every day with lunch. Each can yields about 32 square inches of material and the bottom is a very sturdy round thing. The problem is what to do with all those cans I had saved. Recycling them in the typical way seemed like such a waste, so the most logical solution was…. Build An Airplane!

Flaperon Servos

Flaperon Servos in Wing Box

  Not an actual airplane, but a model airplane. But wait, why just build a model? If I’m going to all that trouble, shouldn’t it be able to fly? Of course it should! The main objective was keep the weight down.  

 I spent a whole lot of time on R&D, because how does one use cans to build an R/C airplane? R&D was really an ongoing process because it was a design-build project, so I can’t put an actual number on it. All I know is that I worked on this project every day at lunch for about fifteen months. I also stayed late and worked on the nights when my wife worked.

 Some time in June of 2010, I stopped work on it because I decided to prepare for the GMAT during lunch. I figured it was a better use of my time. Ever since, I only do things which have potential profitability. I will finish the plane some day because I always finish things, and I can see how it might make for a good commercial, thus satisfying the profit potential criteria.

The testing showed that it will absolutely fly. I need a suitable electric powerplant though.

This project took longer, and was better documented than any other. There are way too many photos to post, but I believe these are the most significant.


Rib Design Progression

The first thing I did was try to make the ribs. Without an airfoil, it would never fly. The problem with cans is that they are too thin to support any in-plane compressive load without buckling, so I stapled a small L-shaped stiffener on its chord line. (left) I made sure to leave 3 tabs so the rib could be attached to the spars and trailing edge cap. (Lower Right) I should also mention that I never used a stapler because it crushes the metal. I first poked holes with a thumbtack and then inserted a staple and closed it with pliers to ensure a tight connection.

(Right) After a lot of tracing and cutting ribs, I Set them in the lower spar at 3″ intervals. This spar was my first attempt that later proved to buckle too easily. The spars were made by bending 7″ strips into small C-channels. They were butted together and connected with smaller C-channels and epoxy on the inside of the spar.

I figured out that staples and half-staples would not work everywhere, so I had to figure out some way to occasionally glue stuff together. I ended up settling on epoxy after slightly roughing up the metal. It works well, as long as there is no tendency for the parts to “peel” apart. It works very well in planar shear. I ended up using the epoxy method to secure the ribs to the spars. (above right) Once I had ribs and spars glued together, I attached the trailing edge cap. There was sufficient space to use staples, so I did. You can see the top spar and trailing edge cap in the above right picture, waiting for installation.


Skinned wing tip before flaperon installation

Once I had an assembled skeleton, I had to figure out how to skin it. The initial thought was to use cans, but I wantedpeople to be able to see the internal structure, so I needed something clear. I settled on transparency film because it is rigid in 2 dimensions and can be “flat-wrapped”. I also fabricated small cap strips for every other rib to help support the skin. The cap strips were held in place by small bends that clipped over the spars and trailing. The L-bracket on the ribs kept the cap strips from falling off the front of the thin ribs. (Right) You can also see the trailing edge end cap that I installed. I did not want the flaperons to go all the way to the edge in case the wing tip clipped the ground on landing. So there is a trailing edge on the last 2″ of the wing.

 Now before I went much further with skinning, I had to install some sort of hinge for the flaperons. (Wait a minute, what is a flaperon?!) I’m glad you asked, it’s a combination flap/aileron to save weight and trailing edge space. Its function is controlled through the R/C transmitter software. Anyway, perhaps more on that later. The picture below right shows the tiny hinge pieces I used. The hinge pins were simple U-shaped pieces of staples. The hinges were then installed with staples.


Hinges for Flaperons

 


Assembled Flaperons, ready for skins

Once the hinge tabs were installed on the trailing edge, it was time to make the flaperons. I made leading and trailing edge caps the same way I did with the main spars, of course the cross section of the C-channel was different. The flaperon ribs are made out of small V-shaped pieces (bent at the thick end) that make a zig-zag pattern when glued in place. Each arm of the V is bent into an opposing L-shape to stiffen the ribs. This all resulted in a very sturdy structure. The hinge tabs were then stapled onto the leading edge cap and it was all skinned with film. (Left)

  It was now time to put the wing together. I beefed up the middle of the wing with a strip of can because that is where the most bending stress is. I also put a can skin on the innermost 3″ of flaperon to handle the twisting stress at that point. (Below  Left)

 

(Right) The can skin in the middle of the wing also provided a solid surface to mount the servos. A small servo carrier platform was fabricated that held the servos in place. Scotch tape was used a few times in this area. The servo horns (white crosses) protruded from the underside of the wing, but it didn’t matter because this area was to be covered with a fairing. Pushrods and flareron horns were made of paperclips. Two thin wing mounting tabs were also wrapped around the ribs and trailing edge cap. These tabs would plug in to opposing ones on the fuselage, then this area would be wrapped with a fairing, setting the wing 1″ above the fuselage.

 


Unskinned wing mounted to fuselage skeleton

The fuselage came next.  (Left) The spine of the aircraft was a triangular tube. This was to enhance vertical and lateral rigidity while saving weight over a square tube. Round would have probably been best because it can resist torque, but I couldn’t make a round one perfect enough and it was hard to attach things to. A flat top was made to attach the wing mount tabs, and two trapezoidal bulkheads were made to transfer all loads to the spine. A rear bulkhead fairing was made to look nice and stabilize everything. Also shown is my first attempt at landing gear. This proved to be unable to handle the stress at the bend, so I made a different set. The electronics were to be mounted between the bulkheads and the motor would have been hanging from the spine in front of the first bulkhead. This would all have been covered with skin. 


Initial Tail Assembly

I needed a tail next, so I calculated the proper size for a V-tail and drew an outline in AutoCAD. I then bent a bunch of L brackets and laid them out on the diagram before gluing everything in place. (right) Below is the tail in the assembly jig.


Tail Assembly Jig

 

Once the hinges were attached, it was time to put it all together and skin it. A can skin was was wrapped around the center section to handle the stress and make it easy to mount to the spine. I also had to trim the ruddervators inboard edges because they hit in the  full-up position. The tail was mounted to the spine by bending two paperclips into deep C-channels then driving them down through holes in the tail skin. The protruding clips were then wrappedaround the bottom edge of the spine (one at the front of the tail, and one in back), effectively clamping the tail in place while allowing it to the adjusted fore and aft if needed by loosening the clamps and sliding the tail. 


Tail Installed

Now I did initial strength and balance tests. It performed perfectly. The picture below shows it balancing perfectly on two cans. It also shows the initial configuration of the tail. I decided to change it because I decided I did not want the tail to also serve as a landing skid. The picture (below center) shows it hanging by the wing tips. The wing did not seem to sag at all.


Balance Test

 

Next I wanted to test the wing and make sure it was actually functional. I built a small wooden platform on the end of a stick that would allow the wing to roll when the flaperons were deflected. (Below Right)  I took it outside and it worked quite well, however on my way back inside, a gust took hold of it and buckled the spars. So I took my wing apart and built a new set of stronger spars with cross bracing. I also made it 6″ longer to decrease the roll rate and increase lift. You can see the new wing and some of the cross bracing in front of the old wing. (Below Center) I also braced the upper and lower spars with V-shaped straps to help hold them together. You can see this on the lower left portion of the picture (Below Right)

 

(Below Far Right) is the redesigned gear and mount. I am really proud of my mount design because of its complex fold geometry. You can see one completed strut with the mount and the pattern used to cut it out. I still used paper clips for landing skids, but this time I hinged the landing gear strut with a paperclip and used rubber bands for shock cords. The picture below on the left shows the landing gear in place and the rubber band during its initial fitting. The gear struts were kept very long to ensure propeller clearance upon landing. Another skid was installed on the tail spine to keep the ruddervators off the ground.

As I stated on the main projects page, I stopped working on this project in June ’10 because I began studying for the GMAT. As much as I loved working on this project, it was not really a productive use of my time. There were other things that I needed to do. I will come back to this project eventually though. I also had another one in the works at the time I stopped this one. It has a 9′ wingspan and high aspect ratio with solid spars for extreme strength. I’m hoping will be much like a U-2!

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