More Tooling – 3D Printed Bending Die

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Taking a page from Brian Carpenter’s article and video on using 3D printed parts as bending dies, we created a simple tool that would allow us to bend the flanges on rudder rib #7.  We started by importing the DXF file into Onshape, our CAD software of choice and then projecting the bend lines to our sketch.  A simple extrusion and fillet to match the bend radius gave us our die.  After printing we aligned and clamped the rib to the die.  Because the rib is relatively thin aluminum we simply bent the flanges around the die by hand.

CAD model of the tooling die. The screw holes allow for mounting if necessary.
Printing on our Makergear2 with PLA and 70% infill.
Rib clamped to die, ready for bending.
Tooling is removed after this step and flanges bent further to perpendicular.
The finished part installed in the rudder.

“Rudder You Doing Today?”

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We started the day at 1:00 pm and began to take the plastic off of all of the parts we needed. After the plastic was off we de-burred the metal and started assembling the rudder. Before we could do anything else we had to mark and drill holes into the spar that was used to attach the ribs to. The problem that we faced was making sure that the ribs would be aligned and straight. When drilling the holes we had people spot to make sure that they weren’t at an angle. Once the holes were drilled we clecoed the ribs to the spar. Then we attached the assembled rudder to the tail of the frame via the hinge (eye-bolt to fork-bolt). The next obstacle was finding out if we could still get the plane out of the building by carrying it down the stairs. We folded up the horizontal stabilizers and used surgical tubing to hold them in place. Luckily we were able to carry it down and back up but had to turn it at different areas of the stairwell because the clearance was tight.

– Greta J.

Converting the decimal inch dimensions to millimeters to make it easier to transfer the hole locations using tape measure.
Using a C-channel extrusion to mark 4 lines every 90 degrees on the rudder spar.
Marking the rudder spar for drilling. The nylock nut plates have already been drilled and clecoed.
Filing the top end of the rudder spar to eliminate the interference with the top rib.
Removing the protective plastic film and deburring the rudder ribs prior to assembly.
Enlarging the holes in the nut plate.
It's coming together.
More cleco's.
Looking down on the rudder. Shiny goodness.
All that remains is making it permanent with some rivets.
Photo-op presented itself after performing the egress test from the building.
The school's 'Io (hawk) approves of the work we have done so far.

Horizontal Stabilizer Installation

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Friday started out with us discussing the emails which we received from Brian Carpenter addressing the issues concerning part placement and assembly. Once the issues were adequately resolved amongst the team, we proceeded with the assembly of the horizontal stabilizers. We began this process by laying out the parts for the horizontal stab on two separate tables. Once the general shape of the horizontal stab was achieved, we gathered the screws, nuts, and washers that were needed to fasten the parts of the horizontal stabilizer.

When we returned to work on Saturday, January 20th, we discovered that we needed to drill the holes that were required for proper assembly of the horizontal stab, so we spent a solid two to three hours prepping and drilling the leading edge, main spar, and rib tubes. Afterwards, we were able to attach the horizontal stab to the tail boom.

– Dylan F.

Laying out the parts to make sure we have everything to assemble that horizontal stabilizer.
Checking the specifications on the AN bolts.
Grip length and overall length are as expected and within tolerance. Now we know how to decipher the part numbers for the fasteners.
The left and right horizontal stabilizers are starting to take form.
Starting to drill the holes in the inboard and mid rib tubes.
We designed and 3D printed another flanged collar to ensure that we hit top dead center and stay lined up all the way through. The set screws prevent the tube from rotating during the drilling process.
The left side mostly complete except for the sheet metal ribs.
Leading edge of horizontal stabilizer and inboard rib tube interface.
Main spar and inboard rib tube interface.
Both left and right sides installed. Note our reserved N-number (November-One-Eight-Six-Three-X-Ray). The number 1863 represents the year our school was founded. The X is for experimental (E for electric was not available). We have the number for a year and if we don't get certified by then we will have to renew. Maybe that's incentive to pick up the pace.
It's getting dark outside. Time to go home. Ribs and vertical stabilizer next week.

A Riveting Experience

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Before beginning to rivet as we had planned, the boom was checked for the twist that we had corrected the previous day, and no correction was needed.  Once that it was confirmed that the twist had been corrected, riveting began on the top and bottom of the boom, being that any
place that shouldn’t be riveted at this time would be very obvious (not drilled yet or too large a gap for the 4-2 stainless steel rivets).  

For each rivet, the thickness of material being riveted was checked; more than three layers would signify that the part was either incorrectly placed, or would need a rivet with a longer grip range.  Aside from a single rivet, the the process for both top and bottom went smoothly.  The top was riveted first, with two teams on each side of the boom, beginning from the front and moving toward the back at the same speed.  The boom was then flipped, and work on the bottom began in the opposite direction, advancing toward the front.  

At periodic intervals, the twist of the boom was checked, and if noticeable, was corrected with the same 2-1/2 “ PVC pipes.  Before riveting the sides, the locations where a rivet should not be placed were marked, and the riveting began.  Aside from a few rivets which had been discovered to need bolts instead of rivets, the process went smoothly, and those few locations were marked to be removed at a later date.

-Braxton L.

Twist to Level

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The last time we worked on constructing the fuselage boom of the plane we realized that there was a slight twist in the structure from forward to aft. We had attempted to correct this twist by clamping the boom down to a table, but this approach failed. 

After returning from a three-week-long Christmas break we made a second attempt at correcting the twist. This time we removed all non-essential cleco’s from the boom and stuck a 2-1/2″ diameter PVC pipe through the holes in the boom frame at either end. We then manually lifted the plane up and turned the pipes by hand in the opposite direction of the twist. This method quickly removed any twist in the plane. We began reinstalling the cleco’s, stopping every once in a while to check if the twist had returned and then re-adjusting the twist as needed. With four people working the process took around two hours to complete.  The structure is now level with all cleco’s inserted.  The next step will be to install the rivets.

-Ethan P.

We started by removing most of the cleco's which would alow the boom to flex more easily.
We then insert the PVC pipe through the boom forward and aft and gently 'persuade' the boom back to level.
Here we flip the boom over and add more cleco's all the while monitoring for any twist that may re-emerge.
Some additional persuasion... Repeat the process until all cleco's are in place. Tail boom is now straight and level.

E-HAWK Avionics

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Part of the E-HAWK project will include developing custom instrumentation and avionics for the aircraft.  The typical internal cumbustion engine instruments will be replaced with sensors and displays to monitor battery voltage (state of charge) and current, motor power, energy consumed, motor and controller temperatures, etc.  In addition to propulsion system monitoring, we will also be looking into additional instrumentation such as altimeter, airspeed indicator, compass, artificial horizon, attitude indicator, etc.  The readily available and inexpensive MEMS sensors offer a number of options.  The RC multirotor hobby has brought much of this technology to the forefront.  A capable multirotor flight controller with gyro, accelerometer, pressure sensors and GPS receiver can be had for < $50.  Add in some custom code and displays and you have yourself a custom “6-pack” instrumentation package found in a traditional cockpit.

With some parts laying around our electronics lab, we were able to put together our first prototype of an Altimeter.  It uses the ubiquitous Arduino (Nano), BMP180 pressure sensor, rotary encoder, and 128×64 OLED display (we’ll be experimenting with other types of displays to find the one with the best readability).  Add in a bit of code and surround it with a custom 3D-printed enclosure.

Our understanding of the FAA rules pertaining to Experimental Amateur-Built Aircraft is that no instrumentation is required for VFR daytime operations therefore no certification will be required.  Of course we will fully test any custom equipment to stringent requirements.

Altimeter prototype comprised of an Arduino Uno, BMP180 Pressure Sensor, 128x64 OLED Display, housed in a 3D-printed enclosure.
Main screen shows altitude, vertical speed, and altimeter setting (QNH) that is adjustable using the rotary dial. This prototype is designed to fit in a standard round 2-1/4" cutout.
CAD rendering of altimeter.
More CAD. We use Onshape for most of our designs. It's been an invaluable tool for our students. It's easy to use, runs in your browser, allows for seamless collaboration, and is free for education.
Parts ready for assembly.
Microcontroller and sensor mounted to perf board. Connectors interfacing with the display and rotary encoder allow for easy assembly/disassembly.
Components installed in 3D-printed housing.
Rear of unit with USB port for power and programming.
We will also begin experimenting with an airspeed indicator. Here is a potential pressure sensor that may work. It's originally designed to work with the Pixhawk series of drone flight controllers.

Thinking About Batteries

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As we work on assembling the airframe, we are also thinking about our battery strategy.  In our robotics courses we have developed a number of different drones and up until just recently we have powered them using the traditional RC-type lithium pouch cell (also referred to as lithium polymer) battery packs.  These batteries offer high discharge rates which work well for the high power demands required to fly multi-rotor aircraft.  These high discharge rates however come at the cost of cycle life, specific energy (Wh/kg), and safety. 

As a result of these shortcomings, we have begun to experiment with the 18650 cylindrical lithium cell in our drone projects.  These cells have been around for a long time and are responsible for powering everything from laptop computers to cordless power tools to electric vehicles.  When compared to the RC lithium polymer type cells the 18650 cylindrical cells offer greater specific energy, energy density (Wh/L),  and greater cycle life.  Our aircraft will not be taking off vertically (at least not the first iteration) so our power demand should fall within the range of the lower discharge rates offered by these cells.

These cells also lend themselves well for custom configuring and packaging to achieve the desired voltage, amp-hour rating, and space constraints.  We have already  fabricated a couple of quadcopter battery packs using 18650’s.  We design battery holders using CAD software and then print the parts on our 3D printers.  After the cells have been packaged to achieve the desired series/parallel configuration, we use our custom-built spot welder to fuse the nickel strip electrical connections.  Early tests have led to a 45 minute hover on one of our medium size quadcopters.

With electric vehicles gaining more and more traction we can anticipate battery technology to improve significantly. The increase in EV’s will lead to additional battery options in the form of re-purposing existing electric vehicle batteries for our mission.  Obtaining a battery from an electric motorcycle or part of a battery from an electric car and modifying it to meet our requirements may hold promise.

Regardless of what route we take in battery development we will have to make sure that we incorporate the necessary battery management system that will help us monitor the health of the system as well as spot any anomalies that could potentially lead to catastrophic failures.  We will also have to ensure that our cell interconnection strategy and packaging will prevent single point failures from propagating.

Expect much more in the way of battery discussion as we move forward…

CAD rendering of an 18650 lithium battery pack used to power on of our drones.
Cell holders being printed using ABS plastic.
This is our custom built spot welder. We use a 12V deep-cycle lead acid battery to provide the short 7 millisecond pulses of high current to fuse the nickel strip to the battery terminals.
The spot welding process in action. Each terminal receives 2 to 3 welds.

Time lapse video of the spot welding process.

A partially welded pack. The series connections are made after the parallel connections. The nickel strip is 8mm wide by 0.2mm thick.
A larger 18650 pack comprised of 160 cells. The cells are arranged in a 20S8P configuration for 72V at 27Ah (~2kWh of energy).
Comparison between pouch cells and cylindrical cells.
This drone was used to test our first 18650 battery pack. It hovered for over 45 minutes on this 4S3P 10Ah battery with about 20% reserve energy.
This 650mm sized Hexacopter is scheduled to fly with a custom built 24 cell 18650 pack in a 6S4P configuration.

Click here to see some of the drones our robotics students have designed, fabricated, and flown.

Will It Fit?

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We are currently building the airplane on the 3rd Floor Project Space of the Sullivan Center.  It’s a perfect space for such an endeavor with only one real potential drawback.  Not having access to a set of hangar doors we will periodically have to check if the air frame sub-assemblies will fit through the stairwell.  As the plane grows in size we will perform ‘extraction’ tests.  The video below depicts our first test with the fuselage boom.  So far so good.