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Author Topic: Six inch carved balsa prop  (Read 1347 times)
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outofbalance
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« on: August 29, 2008, 12:42:20 AM »

I saw a plan form of prop carving in a recent NFFS journal so reduced the plan to a six inch prop and carved the prop in a couple of days. I hooked it to my Sorta Korda SQ Bostonian and it turned the beast into a speed demon!

I do not have a jig saw so I made a floor for my scroll saw (Skill) out of 1/4 inch plywood. I bored a hole in the plywood so the blade could stick through and then bolted the metal guide of the saw base to the plywood. I used a couple of "C" clamps to mount the plywood to my bench. The saw is underneath and the blade sticks up. Way cool.

I drew the measurements of the prop onto a one inch square by six and a half inch block and then drilled the center hole with my vertical drill (Ace Hdw). I used the scroll saw to cut the edges neatly. Worked okay. Then, I drew the side view and cut it out too.

I carved one blade first, both sides. Then I used that example to match the carving of the other blade. I sanded it and inserted a small 1/16 OD Al tube. It weighed just under 1 gram.

The balsa was VERY soft, so I slit the hub through the side and inserted a piece of 1/64 plywood, gluing up with Ambroid.

I applied several coats of 50/50 clear dope sanding down to 320 grit.

Then, I attached a layer of Tyvek® to the front side of each blade. I first separate the Tyvek® into two layers and remove most of the fibre snarles. I put the blade on the Tyvek® and trace around with a pencil so there is enough margin to slightly wrap around the blade edges.

I glued (Ambroid) the Tyvek® to the FRONT of the blade using a lot of glue. I smeared it around the front of the blade. Tyvek® is thirsty. After it set up, I took my sharp craft knife and slit the edges of the margin (not yet affixed with glue) so they could be wrapped around the LE and TE without leaving any corners. I added glue to this area and used my fingers to make sure I got a good seal.

I put the prop on my static balance and added clear dope to get the static balance right. The I sanded the front of the hub a bit and CAed a small lock washer in place for the free wheel. Voila!!!!!--2.3 grams.

Some photos are below.

Outofbalance
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Six inch carved balsa prop
Six inch carved balsa prop
Six inch carved balsa prop
Six inch carved balsa prop
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BG
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« Reply #1 on: August 29, 2008, 05:35:39 AM »

looks really good....may have to try your approach in the future!
B
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« Reply #2 on: August 29, 2008, 08:46:43 AM »

Hi BG,

This little prop is so strong for its weight!

I'm working on a second try, a seven incher but with a long, narrow profile.

I lived in Wiesbaden a couple of years (teaching) and my wife really enjoyed the Rhinegau (2004-06). The model shop I frequented had everything I needed, a model shop out of the 1950s, in some respects. It was a father and son operation; they had a lot of Spanish small size machines too. Some of their balsa (stamped Graupler or Graupner I think) was very high quality. I was the only Free Flighter around- I used to fly in a volkspark a few hundred meters from our apartment- huge, slow-rising thermals at midday.

OOB
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« Reply #3 on: November 21, 2008, 10:43:44 AM »

I have only made four props and only two of them were carved. The first was pretty poor; the second much better. I have tried to deal with problem of the shaft hole in soft balsa by gluing a tiny rectangular peice of 1/32 ply on both the front and rear faces of the prop. and then drilling the rquired size hole for the prop hook. Your ply insert is quite a hansome peice of work and I would guess it strengthens the whole hub area quite nicely.
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« Reply #4 on: November 21, 2008, 10:13:22 PM »

Hi Blackarrow et al,

I have carved a number of props and it's just a level of experience that lets you improve. I look for certain indicators; I made a lot of mistakes but, in the end, I finally came to realize a few general rules:

Once I have the sizes and drawing complete, I always carve the back side first and I leave it flat until I have the front side done; then I go back to the rear surface of the blade and create a gentle under camber. I use an old hammer handle to wrap the sandpaper around; it's somewhat circular and it helps me get a nice curve.

Recently, I carved another balsa prop but this time I used two 1/2 inch pieces glues together to form the block (for a one inch square block). I let it dry over night. I drilled the prop hook hole perpendicular to the glue seam. As I carved, I noticed how much MORE strength the form had as I got near completion. That carpenter's glue seam must be adding a bit of rigidity. We'll have to see how it holds up under impact!

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