The 21st Century learning experiences gained as the community, school and businesses design, plan and build a 32 foot long Town Lattice Truss covered bridge for the Village of South Wayne, Wisconsin.
Sunday, April 15, 2018
Making Air Foil Blades with a Twist
There is a lot to be learned here and I am just getting started. My hope is that others that learn will share their experiences to promote the craft. After many hours of designing, building and testing blades that can be used in Kid Wind Challenges I have learned a lot.
The goal here is to turn the piece of 1" styrofoam on the right into a wind generator blade on the left. An airfoil blade with a twist. Lets begin.
I chose to use a table saw to rip and cross cut a 4' by 8' sheet of 1" styrofoam into 6" by 24" blade blanks. Hand saws or utility knives would also do the job. Another goal in this blog post will be to make this process as user friendly as possible. No need to have a well equipped shop. Just the determination to "get'er done"!
Next you need to hot wire cut the air foil shape into the blade blank. OK so this step could be a deal breaker unless you are willing to do some experimenting. I used a 12 volt battery charger from Farm and Fleet. I also had a speed control potentiometer. I set the charger on 2 amp and the potentiometer on 60 %. I think a 6 volt charger would do the job without the potentiometer. For the wire I used 18 gauge Stove Pipe wire. Another. F & F product. Bend the wire (look close you can see the wire at the end of the styrofoam) to the air foil shape you want and run it through 2 holes in a piece of Oak hard wood (do not use Pine is will burn). Use sheet metal screws on the bottom side of the Oak piece to make the connections where you will hook up the battery charger leads. You will also need to make some sort of guide ( like the fence on a table saw) so that as you push the blade blank through it will stay straight. If you need help with this contact me and I will help talk you through it.
You will notice a line (groove for the blade spar a 1/4" dowel will be glued in) in the bottom picture of the blade blank. This is cut on the flat side of the blank and in the thickest part of the air foil edge. I made another hot wire cutter to do this job. A small loop of stove pipe wire through a small piece of Oak that I could hold in my hand and move along a guide to cut this groove. Again need help just email me andI will send you more details and pictures.
With the blank cut you will need to layout lines about 1" apart across the blank. You will cut the blade blank on these lines.
Of course I made a fixture that fit on my band saw and sliced them off like lunch meat in a deli. You could also cut them with a coping saw or utility knife. If you cut them by hand be sure to number them so that you can re-assemble them in the same order.
This is the part where we get creative and put the twist into the air foil. I have two 1/4" steel rods. One is held in the jaws of a small vise and the other is set so that the root of the blade will be at 45 degrees. Understand that the opposite ends of the two rods are held in a block of wood that has two holes drilled in it. The holes are slotted so that when you clamp them with bolts or a vise grips they lock on to the rods and hold them fast. This allows you to put the twist into the blade.
OK this should give you some more details of what you need to make to get the twist. Pieces are placed and hot glued one at a time until you reach the top. Now you have the twisted air foil shape.
Now you are going to get some 50 grit and 80 grit sand paper (abrasive paper to be technical) and glue it to a paint stir stick. This will be used to sand the edges of the rough twist down. The picture should give you the idea. This is just a short practice piece I used to get the hang of it. I also made a couple of holders to place the spar in to sport the blade while I sanded. Good idea to rid up something like this.
After about 30 minutes you should have something like this. The root end will be at 45 degrees of pitch and the tip of the blade will be at 0 degrees of pitch. Your next job will be to cover this blade with plastic lamination material like the model airplane people use. I just got some scrap end rolls of the lamination plastic from my local printer and ironed it on with a tacking iron. This will give a lot of strength to the blade and make it very smooth.
So the steps are - rip and cross cut the blade blank. Hot wire the air foil shape and spar groove. Slice into 1" rib pieces. Stack and hot glue the rib pieces on the rods with a 45 degree twist. Hot glue in spar ( note different colors). Sand twist in ribs smooth. Iron on lamination material to cover. CAUTION - Note the root end and the direction of twist. The thickest edge of the air foil is the leading edde of the blade. The incoming air will strike the flat side of the blade!
The orange colored spar is fiber glass. Much stronger than a wooden dowel and the end will last longer than wood. However you will need to cut three grooves into the end so that the Kid Wind Hub will lock on to them. I made a form tool for this and cut them on my lathe. You might have to put them in a drill and file them in by hand.
So where do you get 1/4" fiberglass rods? Well believe it or not, youth target arrows have shafts made of 1/4" fiberglass. They are hollow though so you will want to find a common nail that fits the hole and hot glue it in to fill the hole so that the hub does not crush it when tightened.
Oh and just when you thought you were done ( if it was easy everyone would do it ) one more thing. You will want to drill and tap (thread) every other hole on the Kid Wind hub. This will allow you to screw in a 6-32 thread cap screw. These six screws will allow you to really tighten the blade spars into the hub and keep the pitch you set. All you have to do it take your KW hub to your local hardware and tell them what you want to do and they will fix you up with the correct drill bits, 6-32 tap, cap screws and hex wrench. Easy peazie. Note: you need to drill clearance holes (holes that are bigger than the threads of the screw) through the top half piece of the hub assembly so that the cap screw can tighten the two halves together.
OR - I made a drill guide bushing and if you send your hub to me I will drill and thread it for you!
I had some time on my hands and thought I would recycle the scrap styrofoam and make a nice blade carrier with them.
So if you are having trouble with the pitch changing because of the wooden spar and want to increase your performance this should help. By the way you will want to space out the distance between the KW mount and hub with 3 or 4 1/4" nuts on the shaft. Let me know if you have questions. need or want more help. If we share what we learn everyone learns.