My primary goal when I started this project was to create a large scale quadcopters. An electric powered design would have been the most straightforward approach. Electric motors are more efficient than gas, but the power density of gasoline is much greater than today's batteries. So until battery technology improves, gas power seemed the way to go. Goliath uses a single 30HP engine and a belt system to transfer power to the four propellers. The setup was chosen because at this scale, four smaller engines have a smaller power to weight ratio than a single larger engine.
An electric quadcopter would traditionally maneuver by varying the speed of each propeller to control thrust. Since Goliath uses fixed pitch propellers and all the propellers turn at the same speed due to the belt drive, maneuvering will be done by control vanes similar to those used to steer hovercraft.
Flight control will be performed using the Pixhawk controller running a modified version of the Ardupilot flight software. Modifications to the Ardupilot software are needed to work with Goliath's unique control system. Both the Pixhawk and Ardupilot are open source. The modifications made will be open source as well. A USB radio receiver will be attached to the flight controller and setup to receive ADS-B signals. These signals will allow the operator to be aware of other aircraft operating in the area.
Additionally Goliath will have a WiFi interface allowing the public to interact and connect with Goliath. Data and Video Feeds will be available and observers can notify the operator of potential issues.
Goliath itself will be open source with the creative commons license, and whenever possible open source components were used. It's currently a work in progress, and even when completed it's intended as a starting point for future vehicles.
1×30 HP vertical shaft gas engineShould equipped with a starter and alternator
4×Center Beams ( 2 1/2" x 1 1/2" x 39 3/4")Galvanized Slotted Angle
4×Side Beams ( 2 1/2" x 1 1/2" x 29 1/4" )Galvanized Slotted Angle
4×Cross Beams ( 2 1/2" x 1 1/2" x 30" )Galvanized Slotted Angle
4×End Beams ( 2 1/2" x 1 1/2" x 18" )Galvanized Slotted Angle
8×Outer Prop Supports ( 1 1/2" x 1 1/2" x 30 3/4" )Galvanized Slotted Angle
4×Inner Prop Supports - Fore ( 1 1/2" x 1 1/2" x 28 1/4" )Galvanized Slotted Angle
4×Inner Prop Supports - Aft ( 1 1/2" x 1 1/2" x 32 1/4" )Galvanized Slotted Angle
8×Shaft Mounts - ( 2 1/2" x 1 1/2" x 6")Galvanized Slotted Angle
4×End Cross Bars ( 1 1/2" x 1/16" x 18" )Galvanized Slotted Bar
If you hadn't seen the previous post yet, the gas engine was started for the first time, but during the process the vehicle was damaged.
Afterwards, I took an assessment of the damage to Goliath and tried to figure out where things went wrong.
After watching the video a few times and looking at the damage to the vehicle, I think I have a good idea of what went wrong and what I can do to prevent it from happening. Things were running good until the engine was shutoff. At this point one of the belt started losing tension. You can see this in the video at the lower right hand belt starts to flap. This was likely do the the engine spinning down faster than the belt. At some point the belt gets so much slack, that the belt bounced up and the propeller went under it and the belt got wrapped around the prop. Once it was tangled the belt cinched up really tight and bent two of the propeller shafts and the belt tensioner support. The other propeller attached to the belt was sheared off when it's axle was bent and the propeller hit the angle iron support.
The changes I need to make to the vehicle to keep this from occurring again are:
Add a one-way (overrunning) clutch to the engine pulley
Add belt guards to prevent the belt from flying up into the path of the propellers
I may also need to add some auto tensioners, I need to do a bit more research into it.
Otherwise the test went well. We could really feel the wind coming off the vehicle. I'm really amazed that the belts are as strong as they are. I would have thought that the belt would have snapped in this situation. I've already started on making two new propellers and hopefully the process will go faster now that I've done it a few times.
UPDATE - Great news, got the engine started! Bad news, I broke lots of stuff! See bottom for details
I'm taking advantage of a four day weekend by doing a series of tests, eventually building up to a hover test. I'll be updating this post over the weekend as I make progress towards the hover test. I plan on sharing whatever happens good or bad, and since these things seem to never go according to plan, please be patient if it seems like it's taking too long or nothing happens at all.
As I laid out in the last post, Today (Friday) the plan is:
Friday - Attach the rotors and test everything out using only the starter
Saturday - Run the engine on gas for the first time, but only at low speed
Sunday - Run the engine at higher speeds, building up to a hover test
Monday - Not cleaning up debris from a failed test
So the weather this weekend does not look good. Today (Friday) is an 80% chance of rain, Saturday 90 %, Sunday 80% and Monday is 40%. Things might slip a bit. I was running out of room in the workshop to actually work on the vehicle with the rotors attached so I moved it outside under a pop-up canopy to get the last two rotors attached. I have them attached, but I'm still adjusting all of the pulleys to make sure they are all aligned. It started raining so I lowered the shade to cover Goliath better and waiting for a break in the rain. I did get one other important item installed (below).
I got my T-Shirt this week from the "Astronaut Or Not" Challenge, specifically the "Most Outrageous Component" round. They also sent included a few stickers, one of which is now placed on the engine.
So the rain let up for a while and I was able to finish adjusting the pulleys and belts. Goliath is finally starting to look like a quadcopter.
Next step is getting ready to remotely start the engine. Previously I had connected an riding lawn mower ignition switch to Goliath for testing out the starter and other hardware. Later the ignition will be controlled with the Pixhawk controller, but for doing some preliminary testing. I need to start the vehicle remotely for safety reasons, so I rigged up an extension for the ignition.
Last step is making sure Goliath isn't going to go anywhere while just running the starter. I'll get more serious with ties down for the hover test.
With all those things done, it was time to try it out.
Everything seemed to work as expected, so everything looks good for running the engine on gas next.
So after meeting all the goals on Friday, Saturday did not go as well. The first part of the day was doing a bit more research into the Pixhawk controller. I'd like to have the Pixhawk included as part of the hover test and I'm working on making sure I have everything I need. Turns out I misunderstood some of the documentation on the Pixhawk. While the Pixhawk has 8 main and 6 aux PWM outputs, the Pixhawk does NOT provide power for servos.
To power servos off the Pixhawk, a BEC needs to be connected to one of the servo inputs to provide power to the rest. So this morning I called around and got a Castle 10AMP BEC that will convert the voltage from the 12 V Battery currently on Goliath to what the servos need. This will be in addition to the Voltage Converter that powers the rest of the Pixhawk.
After making a trip to the Hobby Shop, I got ready for today's testing. First thing was getting the fuel solenoid tied in to the ignition switch. After that I connected a temporary gas setup consisting of nothing more than a 1 gallon gas can with 1/4" tubing shoved in it and taped off.
I had wanted to keep the gas separate from the rest of the vehicle, and it was setup underneath at first, knowing that the change in height might be too much for the fuel pump. However after turning the engine over several times it was obvious the fuel wasn't reaching the engine. The...