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.
Just off the water jet we have the two tail wheel arms and the upper and lower control arms.
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.


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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.

Motor Mount

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The motor for our aircraft has been selected and ordered.  As we wait for it to arrive we design the motor mount structure by creating a mock-up and then modeling our solution in CAD.  Once the design is finalized we create the templates used to cope the tubes.   Then we cut, notch, and weld the tubes up to the fuselage.

The process begins with 3D printing a model of the motor we will be using. We also created a mock propeller that will help us determine the largest size prop while maintaining ground clearance and clearance to the underside of the tailboom.
We can use another temporary 3D printed part to prototype the motor mount location with an offset from the existing horizontal tube on the fuselage frame.
This is a CAD rendering that shows a central tube surrounded by 4 perimeter tubes that will be used to hold the mounting plate in place. We opted for a design where the center tube mounts directly to the existing cross member on the fuselage frame.
Here's a view from the rear of the frame showing how we plan to attach the motor mount. The central tube is 1" in diameter and the outer supporting tubes are 1/2"
This is a drawing of the bottom tube flattened. This will be printed in 1:1 scale, cut out, and wrapped around the tube for notching.
The motor mounting tubes have been cut to length and notched per the flattened templates. The red circular part is a 3D printed model that was used in the prototyping process to ensure CAD accuracy.
We begin by welding the center tube to the motor mount plate so that we can do a fit check with the perimeter tubes.
The support tubes have been welded to the motor mount plate. This assembly is temporarily clamped and taped to the fuselage frame for a fit check prior to final welding.
The aft section of the motor mount is fully welded and the assembly is clamped to the fuselage frame to enable the forwad section to be welded.
Prior to welding the forward section of the motor mount we will have to strip away the primer from the fuselage frame around the attach points.


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The team completed work on the ailerons recently.  It was a relatively straightforward process once we had all of the material and parts.  The ribs were provided by Rainbow Aviation and the stock for the leading and trailing edges came from Aircraft Spruce.  Follow the process from start to finish via the illustrations provided below.

We started the build process by deburring all of the aileron ribs.
Some of the rib halves were missing from our order so instead of waiting for the replacements to ship, we decided to cut our own on the water jet. They were cut from .025" 2024 aluminum sheet.
The next step involved a 90 degree bend in the ribs.
Prior to executing any of the riveting we clecoed all of the rib halves together and layed them out as they would attach to the leading and trailing edges of the aileron.
After cutting the leading edge of the aileron to length it's time to deburr the edges.
The ends of the leading edge are now drilled for the eye bolts that will be used to hinge the aileron to the trailing edge of the wing.
The hole locations for the ribs are marked on the leading edge. The ribs are then aligned to the mark, drilled, and clecoed into place.
All hands on deck for more drilling and clecoing.
Up until now we have used a pneumatic rivet gun to do all the riveting. Our Robotics team recently acquired this cordless, electric riveter which offers a bit more portability (no compressor or hoses) than the pneumatic machines.
The ribs have been riveted to the leading edge and the trailing edge has been clecoed in place. The last 4 ribs on the outboard side of the aileron are angled up to provide about 1-1/2" of washout.
Left and right ailerons mostly complete. The control horns still need to be riveted on. We will wait to complete this step until we've assembled the wings.
We will most likely use Oratex to cover the ailerons.

Landing Gear

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The E-Hawk airframe employs a conventional landing gear configuration (tailwheel-type) which consists of two main wheels forward of the center of gravity and a small wheel to support the tail.  In this edition of the blog the E-Hawk team  fabricates the main gear structure and suspension struts.  We also install the wheels, rotors, and brake calipers.  The tail wheel assembly will be fabricated and installed at a later date as will the brake lever and hydraulic lines. 

The 4130 tubing for the gear leg is cut, coped, and placed into the welding jig.
The gear leg spreader is tack welded into place.
Both left and right gear legs partially welded.
Tabs that provide structural integrity to the landing leg attach point are tack welded into place. We will then heat the tab via oxy-acetylene torch and wrap it around the axle and finish welding the seams shut.
The wheel axle tube is temporarily slid into place to check the fit.
Aft gear attach tabs have been cut on the water jet are fit checked prior to welding.
Forward gear tabs will require some grinding to close the gap prior to welding.
The gear leg bottom mount will be the attach point for the strut. It's cut from 1/4" 4130 steel. It's clamped into place to ensure alignment with the upper gear strut mount.
Here is the upper gear strut mount clamped for alignment.
The strut mounts are now in position and will be permanently welded into place. Note the wheel axle runs all the way from the left leg to the right to ensure that the wheels will run parallel.
Right angle grinder is used to cut the wheel axle tube to the correct length.
Time to start working on the struts. These are spring holders.
Using the mill to cut slots into the strut spring tube.
One side is almost done. The tube will be flipped and a matching slot will be milled on the opposite side.
Checking to make sure that the slots were milled correctly and that the plunger slides freely.
We start to install the landing gear sub-assemblies onto the fuselage frame. The gear legs go on first.
The struts are temporarily compressed using clamps which makes it possible to attach both the top and bottom bolts through the strut mounts.
Left side gear is complete.
Front view of gear. Right side strut is up next.
Both sides are on with the gear supporting the fuselage frame for the first time.
Union break.