Practice makes ... incremental improvements

We put in a rather short day yesterday and did a bit of cutting practice on cheap construction foam. We had to glue two layers together for some because our pink foam is only 2" think.

We're hoping that we can get our cutting skills up to par before we throw down on the $250 chunks of blue foam.

Here's a root section and a section from about 2/3rds of the way out.

Checking alignment

Looking down the spar cavity.



Spar check.

First look

We finally got all the ribs on their stands and aligned in a proper row. It was our first chance to see the general shape of the prop in actual 3D.

All in the family

Here's a shot of the entire family of templates. Just behind them are components of the stands that will locate and align them on the build table.

The longest template chord is ~20" and the cord of the tip template is barely 2" long.

Smooth around the edges

The airfoil templates will guide the hot wire through the foam core section. The thin wire tends to catch on the fiberous edges of the plywood and if the wire pauses against the ply for even a moment, it will burn a groove in the wood.

You can make templates out of phenolic sheet or other dense and heat resistant materials, but we don't expect to use these particular templates more than once so we went the cheap route.

I sanded the edges with 600grit sandpaper and then trimmed them out with aluminum tape from the HVAC supply. The wire slides much better and a pause won't burn a groove nearly as fast. It's not perfect, but it's darn affordable.

Rick sent me a few profiles to post.

These are based on a 450lb vehicle/pilot weight. A Crr of .02. Frontal area of 20sf and a Cd of .3. Transmission efficiency of .8 and the prop thrust numbers from JavaProp.

We believe we can do better on some of those above numbers, but also we don't expect the prop to ever be quite as good as theoretical. In the end we think we can get quite close to the below.

Where's Waldo?

Somewhere in this block of blue Spyder foam is hidden a large propeller.

Picked up this slab yesterday and hope to have it in many curved pieces by the end of this next weekend.

Occasionally something turns out to be easy

We've been a bit concerned about how easy (or hard) it was going to be to get our foam accurately cut to fit our spars. As previously mentioned our spars are tapered carbon windsurfing masts and while these make almost perfect structural members for our prop, we were unsure as to how accurate we could mate the foam sections.

After laying out our build table with all the template positions, we transfered those marks to our spars and calipered the cones at those locations. We then made up a set of test templates designed to produce a hole in the foam to mate to the conical spar. If after the cut, the foam slid on too far or not far enough, we would then employ the method of cutting the blocks long and trimming them to length after the hole was cut. Rick had a far higher confidence than I that the foam would stop right on the mark when we slid it on.

Rick made a 'foam drill' from a piece of thin wall tubing and I modified the hot wire bow to allow the wire to quickly connect and disconnect. We threw the templates on a scrap of foam, drilled the pilot hole and pulled the wire from the cutter through the pilot.

After hot wire cutting out the plug we put it on the spar and it slid right to a stop on the mark. One less complication to worry about. We will cut our blocks to length and wire cut the airfoil and the spar hole in one operation.

Here's a shot of our test insertion.

Scott Nix donated two more masts

The team would like to thank local windsurfer Scott Nix for donating two carbon windsurfing masts to the cause. These two are particularly nicely matched, and will likely be used as spars for prop #2 in the reasonably likely event that our first effort comes out looking and working like a first effort.

Thanks Scott!

The plots thicken

Here are some 2D and 3D plots of our first production propeller design.

Though we have allowed room on the cart for a 20ft prop, this one is a 16ft. The windsurfing masts that we are using for spars allow for a very clean and simple 16 footer without needing any extensions at the root so we're going the easy route first round.

It's likely that we'll build more than just this first one. We'll put this one on the test stand and dyno, document it's actual performance curves, compare them to the theoretical and decide whether it deserves an actual test on the cart.

If we're careful enough during construction we might just pull it off with one, but as we've never built a prop of this size before, we're learning as we go. Both construction wise and performance wise we'll use what we learn on this one to improve round 2 if needed.

Religious symbols

Well, it's one thing to design a propeller -- it's another thing entirely to figure out how to get the design templates to print out properly and to scale.

We finally imported the points into my CAD program as survey data and faked it into drawing a line between all the points as if it were drawing contour lines on a topo map. Four hours (and countless sheets of 11 x 17 paper later) we finally figured out how to get it all scaling and printing.

We ran out a couple samples, glued them to our template material and whacked away on them to see what we thought. I think we'll be pretty happy once we get them sanded perfectly.

The tabs you see on the templates will sit on blocks located on our build table to get the alignment and twist correct.

The final product looks a bit like those Jesus fish with legs that you see on bumpers occasionally. Perhaps we'll be blessed.

A table for 2x

We've spent the last couple weeks working on our prop design. Rick's got a good handle on that now and we are close to starting construction on the template/ribs/etc. We'll post up some of the airfoil and performance plots in a bit.

Today we spent the day making up our build table. It has to be very straight and square so we can put our reference lines on it and also bolt guides to it etc.

Rick came up with a slick little set of adjustable braces that allow us to take any twist out of the table and still let us fold it up and put it against the wall when we aren't using it.

Here is Rick putting the last screws in the piano hinges before we put the legs on and turn the table over.

Table on it's feet for the first time.

A close up of the little adjustment bracket that I milled slots in. Loosen the wing nuts and slide the triangles relative to each other and the top twists/untwists.

Turns out we didn't need that.

As previously mentioned, we weren't exactly sure how the torsional stiffness of the chassis would be affected once we got the front steering assembly welded and installed. We suspected we could remove the front former altogether.

Once it was all bolted in we did a torsional load test, removed the front former and repeated the test. There was no difference between the two tests. The welded front assembly provides enough torsional rigidity that the front former is uneeded. Out with it.

Point us in the right direction and turn us loose

Rick and Steve spend Saturday running Java Prop sims on different prop sizes, planforms and airfoils. They're looking for the right balance of RPM and efficiencies that will give us the best transmission/prop combo and also have an airfoil sized right to accept our windsurfing masts as spars.

I spent the day welding and mounting up the steering fork for the front wheel.

The (almost) finished product. I haven't mounted the tabs for the wheel hub because I want to wait until we actually get the hub in our hands. I like to do things only once when possible.

Once we mount up the hub/wheel/tire, we'll decide whether we steer directly with our feet or with a linkage back to a stick.

Having a busy week at work so there's not much evening activity going on cart wise. We did spend a couple hours last night adding the composites to the center chassis former.

It was going to be a bit of a pain in the ass to use the carbon tow in this short space so we grabbed a few yard of unidirectional carbon fabric and wrapped with that. We again finished over that with one layer of 5.8oz glass to protect the brittle carbon.

Next up ... I'm going to weld up and install the front steering mechanism and wheel support. On it's own this will add some stiffness to the front portion of the chassis and after it's installation we will determine if the front former is just stiff enough, or needs composite layers added, or can be removed completely.

We build a set of crude headstocks so we could wrap the prop pylons by spinning them. It was a ton easier this way rather than looping the spool around and around like we were forced to do on the chassis itself.

Here's Rick starting the windings.

100 layers of carbon tow on each pylon and a layer of 5.8oz glass later:


Before the wraps we did a torsion test and recorded the results. Once this cures I'll publish the pics showing the before and after tests. Hopefully we added a good bit of stiffness.