5 days ago •
I've been working on making changes to Goliath. As I discussed previously I had suspected the need for a one-way bearing to prevent slack from occurring during a shut down.
Helicopters use one-way clutches to allow for auto-rotation after an engine failure and one-way bearings are common in RC applications for cars and helicopters. The problem is that I haven't found a one-way clutch or bearing that will work well with Goliath. The simplest solution would be to swap out the QD Bushing on the main pulley with an idler QD bushing where the bearing is replaced with a one-way bearing.
The drive shaft for the gas engine has a 1 1/8" diameter shaft with a keyway. After doing a lot of research, it appears that one-way bearings only come in Imperial sizes up to 1". They are made in metric sizes, in 5 mm increments. There are one-way clutches that could be attached the drive shaft, but they start at $650 and they're aren't any HTD pulleys that will attach directly to the clutches.
There are electric clutches that are made for these size engines that would work, the problem would be then determining when the engine is shutting down and disengaging the clutch. Could be an option.
However after taking a look at the video again and the belt diagram, the portion of the belt that developed slack, would have been in tension during shutdown. This likely means that the belt failure just happened to occur around the time I shut down and that the cause was failure to maintain tension during normal operation.
Smerfi, pointed me to a document that addresses belt tension. I'd read it before when researching HTD belts about a year ago, but had forgotten some of the information. One of the important parts is that the belts stretch during initial use. This could have been the initial issue.
With that in mind, I've been working on making some changes to Goliath. The first is to change the belt tensioners. The two belts both attach to one of the tensioners, due to the nature of the belts, this means that if one gets tighter the other gets looser. I'm changing it so that they don't share any pulleys other than the main drive pulley. I'll also add springs to the tensioners to automatically adjust the tension.
I'm also stiffening up the structure vertically. I've replace some of the bolts with all-thread rods that go from top to bottom. This will eliminate some of the flexing that occurred during the testing.
When the changes are completed, the first test will be with some test propellers. This was a suggestion by a coworker. They are simply 1/2" plywood cutouts that will give the engine some load and can easily be replaced. Once the belt system has been tested up to speed then I'll switch back to the propellers.
Meanwhile, the replacement propellers are both cured and I'm back to the tedious process of sanding them. I've also been updated the description section for Goliath including an overview diagram and some more details on the electrical system. Things have slowed down a bit though since I had to go out of town for work and now I've gotten sick. But once the belt system changes have been made things should hopefully progress smoothly from there.
17 days ago •
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.
21 days ago •
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...
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