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New Rotor Shaft Mounts

A project log for Goliath - A Gas Powered Quadcopter

A BIG Gas Powered Quadcopter

Peter McCloud 07/04/2016 at 03:106 Comments

The upper set of new engine mounts are now complete. This log documents both the design and the process of building the new mounts.

The new hardware is considerably different from the previous design. The prior mounts were originally a 6" piece of angle iron bolted to the slotted angle frame and had a slot cut out for the shaft. The shaft was held captive between two shaft collars that also served to transfer the thrust between the shaft and the shaft mounts. One of the previous shaft mounts can be seen below, just above the closest rotor.

The mounts for the Mk. II vehicle had to go from the shaft and mount to the three aluminum tubes that comprise the rotor arms. This required a custom part that could be bolted to the tubes and also act directly as the shaft mount, instead of using multiple shaft collars. The design process started by making a CAD object using Cubify Design.

After that, the next step was making some prototypes out of wood on the CNC router.

These were also used in the Jig to align the tubes for the rotor arms.

Next, it was time to make the actual hardware. The parts were machined out of 6061-T6 bar stock. The parts were cut on the CNC router. A CNC mill would be better suited than the CNC router, but I don't have one. That being the case, it took a couple tests to find the right settings on the CNC router. The first test run had some issues with galling. The galling causes the poor surface finish and the feathering on the edges.

The fix was simply to slow down the feed rates and lower the spindle speed. After that the roughing passes came out a lot cleaner.

Once the roughing pass was done, a waterline pass was made.

Next, a parallel pass was made in the Y-axis to clean up the upper surface.

The last pass was with a 3/4" diameter ball end to clean up the grooves.

With the machining on the CNC Router done, the next step was to drill and countersink the holes for the bolts to mount to the frame. The last step was to drill the holes for the mounting.

When everything is together the shafts will be mounted using half of a shaft collar.

With the parts complete, it was time to attach them to the upper frame.

The next part of the project is building the engine supports. To make sure everything is right before it's riveted all together, which means that the parts need to be scavenged from the first vehicle. More on that next time.

Discussions

Smerfj wrote 07/12/2016 at 14:07 point

By the way, I love the new design. It has the potential to be much stiffer if you need it to be (fully boxing in the structure with very thin aluminum riveted on), and you have some room to lighten it up. You can add some swiss-cheese holes to the sheet metal without compromising the shear strength (see Bruhn C10.18).

If you don't already have it, a copy of Bruhn's "Analysis and Design of Flight Vehicle Structures" is indispensable. Be wary of FEA that mixes thin bodies with solid bodies, since solid analysis rarely indicates the normal failure modes of thin structures (buckling, crippling). 

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Peter McCloud wrote 07/12/2016 at 17:22 point

Thanks for the compliment!

I've been checking out books, but haven't decided on one yet. I'll be sure to get a copy.

I haven't delved into FEA yet, just the truss methods from Frame3DD. For the gussets, I've just been building oversized to get a design quickly.  I figured once I can show the Mk. II design can fly and is controllable, then I can spend some time with FEA on some of the individual parts to pull out some of the remaining weight. 

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Smerfj wrote 07/12/2016 at 19:50 point

To be honest, you've spent enough time on this that you can help yourself quite a bit by reading (or flipping through) a few industry standard design books and utilizing some of the practical section and joint designs. There are quite a few easy things you can do now that will significantly improve your design with little weight impact. Increasing the stiffness of your structure now will help your flight controller and is cheap insurance against structural failure. Don't take a structural shortcut now just to try to make it fly... 

For instance, I would go ahead and get some .020-.030" aluminum sheet and box in all of your struts. Fill in the sides between the corner brackets (the trapezoidal opening) on the center section and rivet it all 4 sides- rivet it to top and bottom tubes and the edges to the corner brackets with a small jog to overlap. Box in the outriggers on all 3 sides. Ensure that every edge of the thin aluminum sheet is riveted to something, or double it over on itself to make a crimped unsupported edge. This will increase the torsional rigidity of your airframe by at least an order of magnitude for very little work. One reason your previous design was so flexible was that you had no shear plane between top and bottom shaft supports, which made each shaft support a giant leaf spring... Right now your drive belts are not occupying the space between the top and bottom tubes of the arms, so box those puppies up and give yourself the maximum sectional area you can get! My guess is that you won't even add another pound if you use very thin sheet. 

The NASA structures manual I refer to below has a section on rivets (B1.1.1) that provides guidance on rivet spacing and size and things like how close the rivet hole can be to the edge of the sheet. These are fairly important things, and you can implement them without having calculated rivet loads to your specific form since you will be material limited, not material optimized. 

Check Section B10.1 for design of weight reducing holes.  Take a door hole saw and punch some swiss cheese in those sheets once you have a good guess on how big the holes can be. Then make sure to deburr and sand/polish the edges to prevent stress cracks. 

The following are industry standard resources for aerospace structural design. Most have similar information, all of them give you real-world calculations that cover factors that the college text books don't even mention. All provide acceptable calculations that would pass FAA certification scrutiny... which you may end up needing should this thing really take off ;-) There are a few thousand pages here, so use the TOC to pick what you want to read on each...

"Analysis and Design of Flight Vehicle Structures" by Bruhn (This is considered by many to be the "bible" of aerospace design) <search for PDF or hardcover from amazon>

"Airframe Structural Design" by Michael Chun-Yung Niu (consider this the "new testament" of the aero bible)  <search for PDF or hardcover from amazon>

NASA Manual, Vol 1:  http://ntrs.nasa.gov/search.jsp?R=19760071125

Stress Analysis Manual, Air Force Flight Dynamics Laborotory. Google search: AFFDL-TR-69-42 and here's the first link (warning pdf): http://www.dtic.mil/dtic/tr/fulltext/u2/759199.pdf

I know that was a long reply, but I think you can have a much better design in Mk. II with not much extra work. 

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Smerfj wrote 07/12/2016 at 13:43 point

A couple of safety concerns:

You may want to add a second shaft collar above the first. You have some very high forces on the pulley and the way that collar is attached, the bolts are going to see a good deal of bending. With vibration it is also putting them in a really bad fatigue situation. By adding a second collar, the top set of fasteners will be in tension to counteract the bending moment, and reduce the possibility of fatigue failure. 

Also, it appears that the half-shaft collar is attached with fasteners that are tapped into the bracket. Most aircraft manufacturers that I design for do not allow this and require a through-hole with nut and double locking features since this would be considered a safety-critical joint. I would encourage you to drill this through and use a lock washer and loc-tite. The reason being that the way it is, the threads can see some fatigue load or vibrate loose and could eventually fail without warning. Ideally you would also ensure that the shoulder on the fastener passes through the shear plane, but by inspection your fasteners are quite oversized there so having threads in the shear plane is probably fine (as long as you aren't threading directly into the bracket).

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Peter McCloud wrote 07/12/2016 at 17:30 point

I like the idea of having a second shaft collar.

As for the brackets, I should have taken some better pictures. They are though-hole with lock washers. I had been concerned about how well tapped holes would hold up. It's good to know that the through-hole method is a best practice from aircraft design. 

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Smerfj wrote 07/12/2016 at 17:56 point

It definitely is! You're welcome to shoot me a message if you have aero-structures questions. I've been designing military airframe structure for the last 8 years and I can give you some tricks and safety information when needed. I keep eyeing your updates too for any safety concerns...

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