Power system for Hot Wire foam cutter

What KidWind Challenge team would not want to be able to design and make aerofoil shaped blades for their competition turbine? Imagine being able to do it with minimal effort and cost. Imagine being able to repeat the process and make a set of blades in just a few hours! The blade in the picture shows an aerofoil blade with a twist made using this system. Read on and you too will be able to do this from low cost styrofoam insulation material and lamination film used in most schools. The design possibilities for your KW team are are endless.

Let me back up a bit and fill you in on the full story...

The idea started out simple, as most do. I would use my Schumacher 12 volt battery charger (remember that name) as a power source to heat up a piece of stovepipe wire. This wire would be curved to form the cross section shape of the aerofoil I wanted. It would be mounted in a board with the aerofoil shape sticking out. Then pieces of precut styrofoam would be fed through it. The extruded shape coming out the other side would be that aerofoil shape. Wallah! A groove for the blade mounting stick cut in similar fashion and glued in. Then the whole blade was covered with the plastic lamination film using a hot iron. Worked like a charm.

OK. So how could the process be modified to improve it by being able to make blades with a twist? The wheels started turning. To see this process you will have to go back to my 4/15/18 post titled "Making Air Foil Blades with a Twist". Lets just say sometimes your first idea may not be the best idea.

Moving on. I thought more about the hot wire and how I was using it. A good friend of mine that makes his own blades for hovercraft that he builds was consulted. He gave me all the help and information I needed to improve my early hot wire design. And he was using his Schumacher 12 volt battery charger. I was on fire! This was a great idea and I wanted to share it with any and all KW teams so they could use it also.


Working with teachers and knowing first hand the time and effort needed just to put together a KW team for a challenge was a lot. To help any team interested in using this hot wire process I made up 10 "kits" that included the Hotwire frame, grooving tool and foam holder that would be needed. What they would have to supply was the styrofoam and Schumacher 12 volt battery charger. A pretty common thing to find to borrow or they sell them at Farm and Fleet for about $75.



I took the system on the road to a couple of classroom visits to promote KidWind. The kids took right to it. They were making aerofoil shaped blades like hot cakes!


Here you can see the two girls are following the wooden cross section guides that are hot glued to the ends of the styrofoam blank being held in the clamping setup. You cannot see the Schumacher battery charger, just the two alligator battery clamps between the operators hands. Now that they had the hot wire and understood how to use it ALL they needed to do was get a Schumacher Model SE 1052       2 amp 10 amp 50 amp MANUAL battery charger and they would be in business. My work was done?

I though that I would have the Darlington team buy one of these Schumacher chargers and then bring my old model SE 1052 home. A funny thing happened on the way to a successful purchase. They have discontinued making the MANUAL SE 1052! The NEW models are AUTOMATIC with built in electronics that turn the charger down as the battery they are being used on charges up and will not heat up the wire in the foam cutter. What to do?

Got on line and found Jacobs Online at sales@jacobs-online.biz  to get the transformers I needed to match the stainless wire I had built the "Kits" with. These guys are great. They helped me get all the things I needed to stand-in for the Schumacher battery charger. Plus they were better matched to the resistance of the specific stainless steel wire I used and work better!


Here is what the parts look like. You will need to build one of these, as shown for the groove cutter and another using a different transformer for the main aerofoil shape cutter.


Here is top view of the power supply. I drew in the physical wiring connections so the KW teams can study this and maybe learn more about electricity. I will be working on a schematic for building this for another Blog Post in the future. Note: If you build one for MY "Kit" frame it will work. IF you make your own frame using nichrome wire you will need to do more work with Jacobs Online and pick a transformer that matches the specific resistance of the wire use. This will take more time but be a good learning experience!

If you or someone you know wants the details for building one of these just let me know and I will be happy to help out. The cost of materials for building one of these is around $75-$85. But they work and make the process possible.

Putting the Drag & Lift devise to the test

I have cut and prepared airfoil blades with six different degrees of twist from root to tip, ranging from 40 degrees to 23 degrees as shown above. And one airfoil blade with no twist (not shown).


The test. Each blade will be placed in the Drag & Lift devise in the wind tunnel at a wind speed of 4.5 m/s. Each blade with the exception of the one with no twist will be set to a pitch at it's tip of 0, 10, 20 and 30 degrees. A shown in the picture.


Four gauges as shown here, set to the individual pitches of 0, 10, 20 and 30 degrees will be used.


Each blade will be tested at the four different pitches and tested. The net drag and lift forces will be measured and calculated using the Vernier force sensors in the drag & lift devise and recorded. My hope was that the "best" blade would produce data that would be a predictor of how that specific blade design would preform when multiples were made and assembled into a rotor hub and generator.


To see if I could find a correlation I tested two previously made sets of airfoil blades. One set with no twist and one set of 6 with a 37 degree twist from root to tip. I would set these blades to 0, 10, 20 and 30 degrees at the tip as shown in the picture. Then record the energy produced in 30 seconds with 30 ohms of load running a KW generator on a 32:1 gear ratio.

Results:
Best energy output in 4.5 m/s wind speed -
6 blade airfoil with no twist for 30 seconds with 30 ohm load and 10 degree pitch at tip
322 rpm at rotor
10,310 rpm at KW generator
9.2 volt average
300 mA average
82.70 Joules produced

Conclusion: Although the airfoil blades with a 37 degree twist did well the airfoil blades with no twist out performed them with the following results in the wind tunnel. I can see no clear indicator in the drag and lift data that would have caused me to select the airfoil blade with no twist to choose as the "best" design and build a 6 blade rotor.

Next -
Using the hot wire and pattern process I will now make 6 airfoil blades with no twist but will thin them out toward the tip attempting to reduce drag at the tip and increase output performance above 82.70 Joules.

Results to follow in next post.