2 months ago •
The design of the new rotors for Goliath is complete. There's also been a lot of effort spent making a separate repository for the rotors. The goal is to provide everything a person would need to manufacture their own rotors from scratch using the process used for Goliath. Manufacturing of the rotors has started, but there has been some issues machining the new rotor cores.
To avoid any confusion, the numbering system for the cores started with original cores built for Goliath (Cores 1 through 4). Two were destroyed in the second test and were replaced with rotors 5 and 6.
For the 7th core, in addition to use the new tool paths, there was an attempt to optimize the cutting path and reduce the cut time. The optimization worked for one side, but ruined the leading edge on the other. It was decided at that point to hold off the optimization until after enough cores were completed.
The 8th core came out acceptable with no issues. The 9th core had a bad trailing edge. It appears that the bottom cuts were performed with the z-axis slightly too low and too much material was cut off the rotor and the trailing edge broke. To help ensure the Z-axis was aligned properly for the following propellers the spoil board was re-machined to ensure it was level.
The next core, the 10th was completed without issue and was likely the cleanest of the cores that have been made. The 11th core, was the first of the new clockwise cores since cores 8 and 10 were counter-clockwise rotors. The tool path was loaded and the router was left to rough out the top of the rotor. This was the result after returning the the router:
No idea what happened.
Making additional rotors is on hold all the birch plywood is used up. Meanwhile the new tool paths will be tested out on a purely foam blanks to avoid wasting more birch. Work can also start on fiberglassing cores 8 and 10. There's still a lot of work left to do, so it'll be several weeks until all of the new rotors are completed.
3 months ago •
Thrust measurement and weight reduction has been the focus of the work on Goliath over the last few weeks. Prior to the last test, Goliath weighed 238 lbs, about 40 lbs more than the target weight when the project started. With the vehicle design still in flux it's not time to move away from the slotted angle yet, but there were a few areas that could provide some potential weight reduction.
Bolts and Washers
When Goliath being assembled for the first time, the final design was somewhat nebulous. To make things easier, the same sized bolts where used for the frame assembly. Most of the bolts don't need to be the full length, so wherever possible, the original 3/4" long bolts have been replaced with 1/2" long bolts. The size difference is shown below. About 75 of these bolts have been changed out for smaller bolts
While the bolts were being swapped out, the traditional lock washers where replace with external toothed lock washers. The primary reason for replacing the washers is that the traditional lock washers aren't very vibration resistant and every test would see at least one or two come loose. A bonus to replacing the washers is that the external tooth washers are lighter than the traditional lock washers. The picture above also shows the external toothed and traditional lock washers.
Cross Beams and Side Beams
The side and cross beams previously used the wide slotted angle. It was felt that they could be swapped out for the narrow slotted angle with a minimal reduction in stiffness. Below on the right is the wide cross beam and on the left is the narrow cross beam replacement installed.
The cross members were another area improved. Previously the two cross members on each side of Goliath spanned the whole length of the side and angle wasn't ideal for stiffness. Each of the cross members where removed and trimmed into two smaller pieces, with a little bit of excess. These pieces where then used to form two smaller crosses on each side. Below is a shot of the side beams and cross members being reconfigured on one side of the vehicle.
After completing the structure modifications, the end result is that Goliath looks a bit leaner, and the weight was reduced by 5 lbs to a total weight of 233 lbs. The slotted angle structure is probably as lean as it's going to get.
Another issue that needed to be addressed was the axles for the two propellers powered by the double sided belt. After inspecting the vehicle it was found that the axles were slightly deformed (see picture below). The double sided belt needs additional tension, probably because of the extra length and the difference in the angles. Eventually the all-thread axles will be replaced with thicker axles, but for now an easy fix was to replace the 5/8" zinc plated all thread rod with 5/8" stainless all thread rod. The stainless has a higher strength and has more resistance to being deformed.
The thrust measurement portion of the # Drone Test Stand has been completed. You can read the details there, but the test stand has been setup to measure the thrust while Goliath is running. Below is a picture of the remote display with Goliath in the background.
In the above picture the reading says 220.92 lbs. This was because some of the hardware had been removed from Goliath. Note that there is no data logging capabilities built it. The measurements are being recorded visually for now.
Three more tests have been conducted since the last posted test. There hasn't been time to compile the video yet, it'll probably get posted over the weekend.
Test 22 tested the latest changes in the frame. The test went smoothly and Goliath was brought up to full speed without the ignition switch coming loose. There was no flexing in the frame visually, so the new design appears to be working.
Test 23 attempted to get full thrust measurement data. However the double sided belt tensioner was not performing satisfactorily...
Read more »
3 months ago •
The last few tests have shown that under full power Goliath was unable to produce enough thrust to lift itself. Analysis shows that Goliath should have sufficient power to lift itself, but it's difficult to say why Goliath is not hovering without additional data on the actual thrust and engine power being produced. The electronics for measuring the thrust produced is almost complete. Measuring the engine power requires measuring the engine RPM and a hall effect sensor is being installed that will be triggered by the magnet on the flywheel.
Meanwhile the design analysis has been revisited again to make sure there wasn't something's that been missed. The power required calculations were shown to be sound, so a closer look was at the propeller design was next.
Originally when designing the shape of the propellers, analysis was performed using simplified blade element theory. This consisted of some code borrowed from a Matlab code that I got from a University of Cambridge website. An attempt to make a spreadsheet out of this was unsatisfactory and Prop Designer was tried instead. Aircraft propellers and helicopter rotors work on the same principle, but operate in different regimes. Since the hover condition (static thrust) was the design condition for Goliath, Prop Designer gave an answer, but the software warned it may not be a good answer. With the deadline for the 2014 Hackaday Prize looming, it was decided to push forward and design the propeller with an angle of attack at the maximum L/D of the airfoil to get an efficient design.
Taking a closer look at the propeller design required having a more accurate analysis. This time a python code was written to analyze the design and the issues encountered previously were resolved. While the new python code is giving the best results for the propeller design so far, it still needs to be validated with data. However if it is right, then the new analysis shows a reason why Goliath isn't flying and more importantly how I can change the design to get Goliath flying.
Below is a plot showing the predicted power curves for different propeller designs using the new code.
The current design has a angle varying from 20 degrees at the root to 12 degrees at the tip. The problem with the design could be that it produces too much thrust at lower RPMs. The predicted power is too much for the engine to get above 2400 RPM, limiting the power to 21 Hp. The predicted thrust is at this condition is 220 lbs. Goliath weighed 238 lbs when last tested, so this could be why Goliath isn't flying.
If this is indeed the case, then the fix is to lower the tip angle. The plot above shows 3 additional designs where the tip angle is decreased in increments of 2 degrees. The last design in the list has a tip angle of six degrees and looks to be the most promising. This design could allow the engine to run up to 3300 RPM generating 27 Hp and the predicted total thrust is 290 lbs which should provide more than sufficient thrust.
The next test will provide thrust data and possibly engine RPM data. This will help validate the software and provide a good path forward to get Goliath flying.