Sunday, March 21, 2021

Installed motor test run!

With a good friend, we did a few test runs of the installed motor & power system.  

Here is a very short run just to show you it spins.

Firstly, it is quite scary sitting that close to a spinning motor putting out 4.5kW.  Wearing a helmet helps, but it's just a LOT of power super close to your body.  Neither of us ran beyond 100A and we both kept our feet on the ground out of the plane of the propeller ... just in case.

At about 60-70A, there is a slight structural resonance in the nose tube assembly.  It's not present on either side of that RPM, so we just tried to avoid that.  Balancing the propeller would help.  Tightening things up more should help too.  Most of the play appears to be in the nose down tube connections, at either end, so I probably will start with getting the proper length bolts everywhere and snugging things up.  Replacing the nose tube with an aero strut would probably make a difference.

The throttle in the end of the control stick did not work for some reason.  It moves a servo just fine, but did not move the ESC.  I suspect the ESC calibration needs to be done using the same throttle driver.  For this test, I used a different servo driver that was the one used to set up the ESC in the first place.

The batteries and the ESC and the wires all were quite cool to the touch, even after a 60s run.  Rough math suggests about 10min run time at 4.5kW.

I did try balancing on the wheel to see if the thrust would roll Goat forward.  The ground is pretty soggy and the tire was partly flat, so there was no forward motion.  Howe er, it was a definite feeling of being pulled nose-down.  This is not surprising given the high thrust-line and low pitch rotation axis (here: the wheel axle), but it was nice to feel thrust as an external force, if that makes sense.

There is a fair amount of air movement along the structure, most visible in some vibration of the inboard edge of the flaps.  There was a little drumming of the lower wing skin covering.  The rudder tended to center up with the airflow, but the elevator had enough bungee that it mostly sat stationary.  Nothing was scary.

Also, a weight check with a bathroom scale came in at 158lb including motor & batteries.  The motor itself is 5.6lb, so I am clearly below the Part 103 glider 155lb weight limit without the power system.  Adding propulsion essentially ups the legal weight limit to 255lb, so I'm also clearly below that.  Super.


What improvements are on the short list?

  • Fix the throttle knob --- having throttle in the stick would be much nicer
  • Improve the throttle wiring --- need a second look at the throttle wiring & battery location
  • Tachometer --- having RPM would be really nice
  • Move the wattmeter display? --- it was awkward looking down to the right for power readings
  • Larger propeller --- the 30x10 just looks teensy.  May have to couple this with re-winding the motor for a lower KV, which is a much larger project

I'm also getting some hangar rash in the covering from strut or jury strut bolts poking into the covering.  Some of these looked avoidable.

Gap seal for the control surfaces is a necessary next step.  I really don't want to fly without gap seal.

Strut fairings is on the semi-short list.  It will fly fine without them, but I know the drag is unhelpful.

A trim color along the leading edge would be sweet too :-) 

Here's the battery tray installed.  It works.

Here is how the motor tube mounts.  It's sturdy in itself, but the nose tube seems like what is vibrating.

Saturday, March 20, 2021

More work toward a motor-Goat

A test assembly outdoors was necessary to get the thrust angle to match with what I picked for the starting visual.  I suspect that some thrust-line changes will be needed based on cruise attitude and the pitch coupling (noting the Romanian Goat said it took a lot of up elevator in this mount location), but for now I simply set it parallel with the bottom of the wing and match-drilled.

Mounting the battery is the next major task.  With 25ft of 8AWG wire tripled over for an ~8ft extension, I was able to reach from the motor, down the nose tube, back along the nose side rail, to beside the seat.  Like most others, I'll mount the battery beside the seat.  Instead of mounting to the struts, I decided to extend the seat tubes to make a little shelf of sorts. It did take removing the seat and destroying a couple torque seal marks to get it all, but that was a fun afternoon.

Seat off!  You can also see the final structural mounting for the motor and see the wiring runs.

New little shelf.  In retrospect, I should have put it on the left side so my right hand can stay on the stick, but maybe next version.

And here's the first mock-up of the battery tray.  The ESC is going here also.

Let's talk a bit about the battery tray.  I watched several videos about the e-help and related DIY systems (this one was particularly useful) to decide a rough plan was to make a u-shaped tray with Velcro loops to secure the batteries.  The other major decision was to place the ESC directly on the battery tray.  The battery to ESC leads apparently need to be short due to inductance issues, and let the three phase AC lines be the long runs (EMI and start-up routine are potential issues).  

I ended up making a small aluminum bracket to hold the ESC.  This was the first time I bent up a complicated sheet metal bracket, and it went pretty well.


Here's how the ESC is held in its bracket.  It will be riveted (or bolted) to the bottom of the battery tray.  There is lots of metal surface area to help with heat transfer.

The battery tray itself was fabricated in a similar manner, bent from sheet metal.  The cylindrical bosses are handles.

And here are the batteries and a watt-meter (this model) mounted in the tray.  I still need to put slots for the Velcro straps to hold the batteries in place.  There will also be a couple switches in the panel for ESC arming and the throttle power.

The last major component to solve is/was the throttle actuator.  For this ESC, I need a PWM signal going from 1.0 to 2.0ms and bringing its own power (the ESC is opto-isolated).  The awkward part to me was that no small servo driver had a convenient remote-mount potentiometer.  I ended up buying a $7 servo driver, de-soldering the rotary potentiometer, and remote-mounting it in a custom 3D printed mount that will fit in the end of the control stick.  I stuffed the servo driver inside the same 3D printed mount, so I only need to run it power and get back the PWM signal.

Here's the final servo driver and remote potentiometer arrangement.

The bracket fits inside the control stick tube (ignore the holes --- same size tube from the scrap bin!).

That's all for now.  I'm still working on tidying up the wiring for the panel and then will need another beautiful spring day for a test re-assembly and run in the back yard.  Stand by for more...

Sunday, February 7, 2021

Motor mounting comparison to other eGoats

I've decided not to mount the motor in this traditional location behind the seat.  That location is closer to the CG, so it makes sense for very heavy engines to have a smaller impact on balance.  That location is also large enough to eat a reasonably sized propeller (~49in).  But, it is almost entirely shielded by the pilot's body, meaning there is a lot of turbulence that the propeller has to eat.

Here's the Lima Goat (Pelican) with the typical motor mount location.  From what I can find, this setup is ~20kg according to Alex Ferrer in a YouTube comment (6.4kg motor Freer 202/80 27, 1.9kg KBL ESC 72V-144V/400A, 6.68kg six CNHL 6S 8.0Ah =1kWh, ~2kg wood prop 49x30in == ~30kg thrust, ~6min WOT or 10min sustain for 170kg TOW)

Instead, I'm going out front on a post ahead of the forward nose tube.  This is a traditional location for human powered aircraft (e.g. MIT's Daedalus), and the Romanian Goat was the first Goat that I've seen to do it.  It puts the prop in the cleanest air possible, allowing getting the most out of the propeller efficiency (prop selection matters too, but so is getting the most from that installation by putting it in clean air).  A typical downside is the forward location doesn't match well with heavy internal combustion engines without a long driveshaft.  Even human-powered aircraft usually have a long driveshaft arrangement to make it work.  For me, I went for a minimally sized motor, so the balance works out just fine, and I really need to maximize the propeller efficiency.

Here's the Romanian Goat with the forward motor mount.  From what I can find, this setup is 28kg (4.44kWh battery, 20kW motor (2350rpm, 78kg thrust), 1.3m Helix 3-bladed prop, 45min runtime).

Here's the first mock-up of my selected mount location.  The motor is mounted on its tube (yes, the prop is on backwards) and clamped very roughly in the right place.  I'm planning to pin the mount to the nose tube and support it up from the nose tube roughly parallel to the prop.  If it does not feel sturdy enough, I can add guy wires from near the motor to the leading edge of the wing, which should be more than sturdy enough.

Key weights so far come to 9.3kg (2.5kg motor, 0.4kg ESC, 3.2kg for four 6S 5Ah = 0.4kWh).

Saturday, February 6, 2021

What about a motor?

I visited the two local (hour and forty-five minute) grass strips to the West that almost certainly will not allow a car to drive on the runway.  So, what's plan B?

I found two local paved runways (about an hour and a half away) to the East that are in Class G airspace and look to be very low traffic.  Assuming I can't get a car on those either, what's about some power, cap'n?

Came across the E-Help Student system on YouTube, built for ground launches of a hang glider.  With the similar wing loading of the Goat to an old hang-glider, this seems like it'll be just about right to be a sustainer.

Cue some pictures...

Mounting the Rotomax 150cc motor to a 2in tube took some sheet metal brackets.  Not much sheet metal work on a typical Goat, so went out to buy a small sheet metal bender from Harbor Freight to bend these two.

The wiring harness was not the most glamorous task, and I'm still going back to look at the XT90 connectors once I take some measurements of the current during ground runs.

First test runs were literally on the bench in pusher mode.  The ESC needs some settings tweaks for softer start, and the brake needs to be on, but she does spin.

Next is figuring exactly how to mount the motor.  The two main options are behind the seat, like most powered Goats, or on a post ahead of the seat forward of the wing leading edge.  That'll be tomorrow's game before the big game and snow.

Other to-do's:

  • Gap seals for the control surfaces
  • Trailer tags (paperwork in the mail for the fourth time)
  • Carrier cradles for on the trailer (started)
  • Line and release system for auto-launch

Sunday, September 29, 2019

Quick assembly and disassembly

I did a quick assembly with an old friend in order to adjust the aileron cables.  The aileron surfaces are now level with each other and move freely as they should, with the stick centered at neutral.

The one point I wanted to note is that the strips of Velcro around the wing to hold the aileron and flaps up made a WORLD of difference during assembly.  It was much easier to maneuver the wings during setup.  I am slightly disappointed that I didn't modify the ailerons to let them fold flatter (it was my change that caused the interference), but it'll be okay.

Also, the right order of operations is to connect the wing panels while flat on the ground, then stand them up using the cabanes and struts.  Last time I tried to stand them up and then connect left/right panels, and it was awful.

Next on the list is a tip dolly.  I already have the wheels, so just need to create the little dolly itself.  Should be a fun small project.

Tuesday, September 17, 2019

This Prius meant for towing

It only took a few minutes to realize that putting the wing on top of the Prius looks like a bad idea.  I mean it could work, but boy it looks awkward.

Setting the wing down behind the car, it looks MUCH better.

A trailer bed needs to be 4ft wide and 18.5ft long at minimum.  This picture shows 2ft between the car and wing, but 3ft looks a bit better for making turns.  If I can keep the overall box height to below 4ft from the ground, air should flow cleanly from the back of the car.

Sunday, September 15, 2019

Musings about transportation

There are not a whole lot of physical things left to do on the aircraft, so I'm looking at transportation as the next big task.

A trailer is the obvious choice.  I already have a 4x4 trailer base that could be stretched and a new top made to haul Goat, and I could enclose it to store the aircraft in the trailer outdoors.  The major downside is that I don't have a good place to store the trailer when not at the airfield.  It can't stay on my street for more than 10 days.  I could push it into my back yard, but don't want to be "that" neighbor.  The trailer is even too long to store inside my garage.  And the vehicle I have to tow is a Prius, which I have seen towing a Sunfish sailboat on a trailer, but towing isn't approved.

Here is a flatbed trailer that I was looking at pretty hard:

Here is the Yando Goat trailer, which would be similar to my plan for a home-built enclosed trailer:

The other option is a rooftop system.  A Prius is also not the ideal rooftop vehicle.  The Thule roof rack only spaces about 27 inches between bars, and the Yakima roof rack spaces 34 inches.  For a wing panel that is 18 feet long, there should be more support.  And I have to drive about 60 miles at highway speeds to either of the two places targeted for a maiden flight, so it needs to be well secured.

This guy had a narrow roof rack for the long Goat wing, so maybe it would work for me.  It just looks sketchy for highway use.

This guy has a forward support mounted somehow under his hood, but which gives a lot more support to the length of the wing panel.

The Prius does have a pair of screw-in eyebolt locations in the front bumper, typically used for towing.  These might be okay bolt-points for mounting a support frame.  But how about something on the rear bumper?  Apparently you can get these kayak/canoe bed extenders that plug into a hitch:

The combination of a roof rack and support T-frame would let me strap the wings to the roof rack, and slide them reasonably far aft to leverage the support coming off the hitch.  Would it look weird?  Probably.  But that just might be the best option I have!

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