Seat Modifications

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After sitting in the seat it was determined that our setup was not as comfortable as it could have been.  The seat back was too upright and some modifications had to be made to decrease the incline.  We lowered the seat back cross member which meant that the upright tubes had to be shortened.  After removing some rivets we cut the tube, redrilled some holes, and refitted the tube fittings and seat belt straps.

Jury Struts

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Our first post of the new school year has the team installing the wing jury struts.  The jury strut assembly is comprised of 4 sections of streamline aluminum extrusions that provide support between the main wing struts and the wing at the inboard compression strut.  The assembly is riveted together via strut connectors and attached to the wing and main struts via eye bolts.  The many pictures below document the process.

The first step of the process involves recreating the parts in our CAD system. We basically redraw the part based on the dimensions provided by the pdf file found in the EMG-6 database. From there we can export the dxf files to our water jet cutter.
Our internal drawing of one of the 6 unique jury strut connectors.
Freshly cut jury strut connectors. The smaller parts require tabs so that they don't fall into the abyss that is the water jet tank.
Jury strut connectors cleaned, deburred, and ready for installation.
We decided to print a full scale drawing of the jury strut assembly. We use it as a template to derive the length and angles for the 4 segments of streamline aluminum extrusions.
Lining-up the first cut in the chop saw.
The 4000 rmp chop saw effortlessly glides through the aluminum extrusion.
For the more drastic angles we turn to the band saw.
Some light sanding will remove any sharp edges prior to assembly.
Strut segments are all cut and the connectors are aligned with the scale drawing prior to drilling.
Drilling the 3/16" holes to accept the AN3 bolts that will connect the eye bolt.
More drilling.
The assembly is held and clamped in place while the connector rivet holes are drilled.
Cleco's are used to temporarily hold the parts together.
Cordless riveting tool makes quick work in sinking CCP4-3 stainless steel rivets.
More riveting.
Measuring from inboard the hole location for the main spar eye bolt.
Drilling main spar for eye bolt attach point.
Attaching the jury strut assembly to the main spar and the trailing spar.
Close up of the jury strut attached to the trailing spar.
Once the jury strut is attached to the wing we position where it will interface with the main struts and then mark and drill for the eye bolt.
Cold bending the lower eye bolt.
Here's where the main wing strut interfaces with the jury strut assembly.
Left wing jury strut complete.
A job well done. Can't wait to do the right side.

Wings

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We ended the 2019 school year by fabricating and installing the wings.  The tubes for the jury strut assembly still need to be cut and installed as do the wing tips.  Then it’s on to bending the wing ribs.  Once that takes place we will cover the wings with Dacron sailcloth.  The many pictures that follow illustrate the wing build process.

The first step was to cut all the tubing to length. This included the leading and trailing edge spars and all of the inserts used to reinforce the areas where the compression struts and aileron hinges attach. The aileron is temporarily placed in position to ensure that the hinges (fork and eye bolts) are aligned.
Cutting the tubing that will make up the drag struts.
Double checking the length of one of the compression struts.
The crew works on tightening all of the fasteners that will attach the compression and drag struts to the leading and trailing wing spars.
The lift strut attach fitting has been cut, heated and bent, and placed in position for welding to the fuselage frame.
Lift strut attach fitting welded to fuselage frame.
Lift strut fittings fabricated and painted. These will be inserted into the lower ends of the forward lift struts to make the connection to the fuselage frame.
Lift strut attach fitting bolted to fuselage frame.
The aircraft was raised off of the floor to make it easier to weld the lift strut attach fittings into place.
The lift strut ends have to be machined from round stock aluminum. They will be used to attach the outboard ends of both the forward and aft lift struts to the wing leading and trailing spars respectively.
Round stock inserted into lathe for manual milling.
Finished lift strut ends tapped to 3/8"-24 and drilled for AN4 through bolt.
Lift strut ends are tested for proper fit with fork rod ends and eye bolts prior to installation.
The ends of the lift struts are drilled to accept the through bolts that will hold the ends in place.
The hardware that secures the ends of the lift struts to the aircraft is installed. The large clamp temporarily supports the load of the wing during the install process.
The wing strut joiners were cut on the waterjet. They function to tie the lift struts together at the base of the fuselage frame.
Bottom view illustrating the strut joiner.
The remaining AN fasteners are installed and tightened.
Same thing for the right wing.
One side completed.
The bolt that secures the lift strut end to the trailing edge eye bolt is installed.
The wing fold fitting mates the inboard leading edge to the fuselage frame. We will most likely not use the wing fold feature as we will have to completely remove the wings in order to move the aircraft out of the building.
The aileron push-pull tube is shown here connected to the aileron control horn. Next step will be to adjust the linkages to achieve the correct aileron deflection. The rear spar also needs to be drilled to accept a bolt that will secure it to the tail boom.
Time to dial-in the dihedral for the wings. First we ensure that the center of the fuselage frame is level.
Using an inclinometer app on a smart phone we are able to measure the dihedral angle. Right wing indicates 3 degrees.
Same on left wing. Perfect!
E-Hawk stretching its wings.

The Seat

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We took some liberties and strayed from the original seat design and proceeded with our own original design.  The many pictures that follow outline our design and fabrication process for the E-Hawk seat plates and cushions.

The first step consisted of clamping in some scrap pieces of plywood to act as a quick mock-up for the seat.
The seat mock-up gave us the opportunity to perform some basic checks for fit and comfort.
Once the fit check was complete it was time to install the tubular structure that would support the seat back plate. The seat bottom plate would be supported by the existing structure that is part of the fuselage frame.
The seat backrest supports are installed. We used generic aluminum tube connectors to attach 1" aluminum tubing to the fuselage frame.
Since the tube diameter of the section of the fuselage frame we were connecting to was 3/4" we had to fabricate some sleeves to fill the gap. The sleeves are 3D printed from TPU flexible filament.
Taking measurements for what will become the seat plate.
The measurements are transferred to CAD as we begin to design the bottom seat plate.
Rendering of what the bottom and back seat plates will look like when placed in the fuselage frame. The plates will be cut on the waterjet from 1/16" aluminum.
The finished bottom seat plate is fitted into position prior to drilling the rivet holes that will fasten it to the seat tabs on the fuselage frame. When you design the parts yourself you get to inconspicuously slip your names into the part.
Riveting the seat plate into place.
Installing the supports that will hold the back plate in place. "Something doesn't look right".
Time to attach the back plate to the support tubes with rivets.
Taking measurements for the custom seat cushion that will be produced in-house.
We decided to go with 2" thick, high-density polyurethane foam for the cushion. Here we mark the foam for cutting.
We are using our in-house designed a built hot wire foam cutter. Using a 2x4 as a fence we get a perfect cut.
The seat cushion covers are sewn using a water repellent canvas type fabric. The short edge of the cover includes a zipper so that we can remove the foam for cleaning or replacement.
Foam being inserted into the cover.
The seat bottom and seat back plates are lined with velcro which will hold the seat cushions in place.
Installation of seat cushions.
Now that the seat cushions are complete, we can do a fit check and cut the seat belt to length.
The completed seat with seat belt.
Aft view of the seat. The only thing remaining is to attach the lower seat belt to the fuselage frame.

Tail Wheel

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We decided to implement the steerable tail wheel on our E-Hawk.  There are options for a fixed tail wheel and also a tail skid.  Our design deviates somewhat from the original design.  The original design relies on a couple of complex aluminum machined parts.  Instead we decided to go with 4130 steel plate and tubing that would be welded.   

This is the CAD rendering of our modified steeralble tail wheel.
This drawing highlights the details of our design. All of the custom parts are 4130 cromoly steel that are welded together. The balance of the parts include AN fasteners and a couple of flanged bronze bushings used as bearings.
This is the original tail wheel assembly design.
Tailwheel parts layout on the waterjet. Twelve minutes and they are done.
Just off the waterjet we have the two tail wheel arms and the upper and lower control arms cut from 0.09" 4130 steel.
The parts are temporarily held together using fasteners and clamps prior to welding.
The tail wheel arms are welded to the control arm aft tube.
The next step is to weld the upper and lower control arms to the control arm tube.
The partially completed assembly is test fit to the tail skid tube prior to welding the sleeve bearing standoff tube to the tail skid connector tube. The flanged bronze bearings will allow for smooth rotation during steering.
The completed parts with a coat of primer.
The completed tail wheel assembly installed on the aircraft.
The upper tail wheel control arm is linked to the rudder control arm via springs. It is through this linkage that the rudder pedals actuate the tail wheel.

Indoor Taxi Test

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Now that the landing gear and brakes have been installed, it only made sense to initiate a taxi test.  Since the plane will not fit out of the building in its current state, we had to complete the mission indoors.  It goes without saying that this was the first taxi test ever within the Sullivan Center’s 3rd floor project space.  Who knows, this could even be the first ever indoor taxi test of an electric aircraft, by a high school student.  

Motor Run-up With Prop

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We completed our first motor run-up with the prop installed.  With our small 24S, 6.6Ah test pack we were able to peak at about 7kW before our 60A circuit breaker cut-out.  The graph below depicts data that was logged on the motor controller for a run not shown in the video.  The max power attained in the video footage tops out at about 4kW.  We are not sure why the data captured for current (and power) is so noisy.  We will have to investigate.

Data logged from a test run. Investigation as to why the current values are so noisy are underway.

The Prop

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The propeller is in!  We have decided on a NG-D 3-blade pusher, ground adjustable pitch propeller from E-Props.  The blades are full carbon as is the hub.  The diameter is 125 cm.  It’s rated for a max rotational speed of 3200 RPM when driven by electric motors.  In this blog we describe the process of making an adapter plate and installing the prop.

Rear view of the fully assembled and mounted propeller.
The prop kit included the blades, 2-part hub with flange plate, bolts and washers, digital protractor for adjusting pitch, and an instruction manual.
The full up weight of the prop assembly, including the adapter plate. The only parts missing from this weigh-in are the are 6 M6x24mm socket head cap screws.
This is the CAD model of the adapter plate. It allows us to attach the hub which has a 6M8d75 bolt pattern (common on Rotax 2-strokes) to our electric motor which has a 6M6d50 pattern.
Screenshot of the CNC path required for machining the prop adapter.
We used our X-Carve CNC router to machine the adapter plate from 1/2" 6061 aluminum stock.
Aft view of the adapter after some polishing.
Forward view of the adapter. The M6 holes are counterbored so that the hub can lie flush to the plate.
All of the components, less the blades, are assembled prior to fastening to motor.
The first step is to insert the M8 bolts from the front and then mount the adapter to the motor using the M6 socket head cap screws.
The pitch of the blades is set and the propeller is mounted with all fasteners torqued per spec.
Carbon fiber goodness.
A view looking aft. Next step is a prop run-up.

Brakes

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Time to get the brakes functioning.  The rotors and calipers were installed a couple of weeks ago when we attached the landing gear.  We are using hydraulic brakes actuated simultaneously from a single lever on the control stick.  So for now no differential steering.  In the future we might come up with some clever way to regulate pressure  individually to each caliper for differential steering if necessary. 

Carefully adding Automatic Transmission Fluid (ATF) to the master cylinder reservoir.
The CAD models for some mounting parts we designed. The splitter will be sandwiched between the green and blue parts. The remaining parts are standoffs that will hold the brake line in place for routing.
The splitter is mounted to the frame using the custom 3D printed mount and the brake lines are routed left and right using the standoffs.
More sauce (ATF).
Bleeding the brakes. The deep red color of the ATF and clear brake lines makes any air in the lines readily visible.
Brake line routing is complete.
Brake line routing along the landing gear leg.
From the master cylinder the brake line enters the control stick at the top end and exits near the bottom. Rubber grommets are installed at the entry and exit points and will prevent chaffing.
Brake lever mounted to the stick.

The Motor

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The motor and controller for the E-Hawk have arrived.  We decided on the Rotex Electric REX30 Motor and the HBC series controller from MGM Compro.  The plan is to run the motor at either 86V (24s) or 100V (28s) depending on the battery we end up with.  The motor is rated at 20kW max, and 8kW continuous.  The motor is a 4-turn wind which yields a kV of 40 rpm/V.  The controller is designed to work up to 120V and 280A continuous.  The weight of motor and controller are 5.2 kg and 1.5 kg respectively.

In this installment of the blog we also design and build  a minimal electrical system consisting of a small battery pack, circuit breaker, some enable switches, and a potentiometer.  This allows us to power-up the system, learn how to configure the controller, and perform some preliminary tests.

The REX 30 motor has a diameter of 205 mm and a height of 46 mm.
The motor has 6 phase leads exiting the rear which are combined into 3 phase pairs at the controller. The cable bundle exiting in the top of the photo contains the hall sensor signal wires and temperature sensor signal wires.
The motor controller is relatively compact and according to the manufacturer is capable of pushing out 33kW continuously.
On the back side of the motor controller is visible the heat sink with cooling fans. The quality Rubycon input capacitors are encouraging.
The motor is finally mounted to the fuselage frame.
Aft view of the motor. The red disk is a 3D printed mock-up of the motor-to-propeller adapter plate. The prop we will be using requires the standard Rotax 6M8d75 hub bolt pattern. The plate will later be machined from aluminum.
The motor is fitted with a prototype prop for a fit check.
Side view of the installation.
This is the CAD model for a battery power combiner box. This will allow us to connect 4 6s Lipo batteries with XT90 connectors in series.
The combiner box is printed, XT90 connectors are inserted and wired in series.
The completed combiner box parts ready for assembly.
Voltage check of the 24s test battery pack.
The motor controller is temporarily attached to the fuselage frame. A wooden platform is also installed to support the balance of our test setup.
A 60A circuit breaker is wired in. It will allow us to quickly disconnect the traction pack if anything decides to go wrong.
The control panel from left to right: circuit breaker, precharge switch, auxiliary power (12V control electonics), and controller enable switch. The potentiometer knob is below the aux power switch. The Arduino will be used to read performance data from the controller and later display as flight instrumentation.