Here's a rough rendering of the instrumented prop hub. The 4 large shaft collars in the center are the interface for our carbon prop spars. The bronze colored object is the thrust bearing which presses on three radial load cells. The two paddles on the back transfer the torque through two load cells to the hub itself.

Tell me if I turn you (on)

As we ramp down on the prop portion of the project (we'll show a final series of pic soon), it's time to start moving forward again on the chassis components.

We've spent quite a bit of time considering the instrumentation of the vehicle -- what sort of data streams would be valuable and how we would use them. We decided a while ago to build an instrumented propeller hub but spent more time than we would have liked choosing, locating and matching the parts to make that happen.

Through the magic of Ebay we finally picked up 8 affordable load cells. These cells are not exactly what we needed for all locations, but we have the ability to mill them, modify them and make them work on our budget.

We'll radially place three of these cells near the front of the hub to measure prop thrust, two will be placed at the back of the hub to measure torque and down below we'll build two into the frame of the chassis to measure the resistive force being place on the wheels by the ground. From these three streams we'll be able to derive prop efficiency as well as overall power transmission efficiency.


We had to figure out how to get our data off our spinning prop hub. We preferred to have access to this data 'real time', rather than storing it on the hub for later download.

In our real business, we work with a Canadian RF design shop regularly- Pacific Design Engineering (http://www.pde.com/). They helped us locate an off the shelf RF solution that will do the trick and then they generously sent us a sample to use on the project.

THANKS PDE!!

We'll be programming and testing this telemetry system over the next few weeks.

In the end, these are the data streams we plan to be logging on the vehicle:

propeller thrust
torque applied to prop hub
prop shaft speed
retardant force on drive wheels
drive wheel shaft speed

Additionally, we'll be measuring and logging GPS plots and relative wind direction and wind speed at three levels on the vehicle -- low, center and top of prop. We'll likely use digitally instrumented pitot tubes and will place these well in front of the vehicle and out of the region of prop influence.

Repeat as necessary

Building up the layers of glass. 3 per side.

The Professor's evil creation

Young Propenstein at work late at night in his evil lair.

The cooked foam episode

It's cold here in the garage. Resin likes about a week to cure in the cold and we don't have a week for each layer. Between us we have three 500w work lights so we fired them up and kept them around near the foam. We carefully monitored them to make sure they weren't too hot for the foam and got away with it for a day or so.

Next time you're building a DDWFTTW vehicle propeller in your cold garage and using quartz work lights, here's a little tip -- turns out that the last 5 minutes of the bulbs life, right before it burns out, it produces MUCH more heat than normal. We had a light that had been in the same place for hours and hours with not a problem (all the way back to the day before) and all a sudden we noticed that there was a blister on the foam and then 'poof' the bulb went dark.

After this episode, we went to Lowes and bought a propane salamander heater to bring the garage up to temp without risking the foam. We were able to sand out the blister and fill the void, but it could have been a lot worse.

Getting rid of what's no longer needed

Each foam section had 'legs' in the back that were the reference steps for the twist fixtures. Once the top layer of glass was holding the prop in shape, we flipped the blades over and with Rick's custom foam 'scissors' we hot wired off these legs and then sanded them smooth.



When we glued the foam sections onto the spar, the resin leaked out and ran down each seam on the back of the prop. This gave us a hard 'line' at each seam that didn't want to sand easily.

After a few different attempts, we finally found that a small sanding drum on a dremel tool could be used to lower the level of each seam below the desired surface. We'll then fill these seams and sand them level.

Home from the Holidays

After a couple weeks of only sporadic effort, the new year has brought the team back together again. School break for the SJSU students still has a week or two left in it (we have a team meeting coming up after they return on the 14th), but Rick and I have resumed work on the propeller in Rick's garage.

After putting one layer of glass on the front side of each prop blade to create some structure, we removed the twist fixtures from the table and built a couple of clamps sets. These grab the spars and allow us to rotate both blades for sanding and the remainder of the glassing.

The first of many

We've now started the rather long and rather laborious process of adding layer after layer of glass to each side of the propeller blades. We're going one layer at 0degrees and then one at 30d and one at 60d.

Here's the first layer of the first side of the first blade. While there will be no more 'shaping' on this blade, there will be a fair bit of trimming and sanding between layers.

No, it won't make the Honda Insight go DDWFTTW

With both blades of the prop bonded to the spar, we had our first chance to see the general look of the entire prop assembly.

One half is filled, shaped and sanded while the other is rough.

Rick putting the finishing touches on one of the blade shapes before we start wrapping it with glass.

Rick got the tapered aluminum spar extension (formerly a ski pole) glued into the carbon spar bushing and the final sections bonded. A bit of filling and sanding and we're ready for glass.

We learned today that for some reason, the glass fabric that we've had on order for a week or so was not shipped. A bit of a delay that we weren't expecting.