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General control line discussion => Open Forum => Topic started by: Robertc on October 05, 2018, 09:18:49 AM
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I'm cutting down a carbon 12" prop to 11 1/2". Do I leave the tips square, round a bit or what? It's a carbon copy of a Rev-Up
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I think that I'd try to match the existing tip as close as I could..
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Don't use any power sanders that can generate heat and don't leave any sharp edges that could lead to splitting. Put a dab of CA glue on the tip when you're done.
What, you don't just get the engine going strongly and hold the prop up against the asphalt?
Dangit, that must be what I'm doing wrong.
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I'm cutting down a carbon 12" prop to 11 1/2". Do I leave the tips square, round a bit or what? It's a carbon copy of a Rev-Up
Hi Robert,
When we were building the Dreadnought Hawker Sea Fury to race at Reno a retired Douglas Aircraft aeronautical engineer named Bob Smith helped with the aeronautical modifications to make it as fast as possible. Bob was an engineer before and through WW2 and was very familiar with the NACA wind tunnel test data developed in the 1930s including propeller test data. The question came up when modifying the 13' 4" diameter 4-blade propeller to be installed on the Pratt & Whitney R-4360 radial engine. Bob said that NACA data showed virtually no difference in performance due to differences in the shape of the blade tip. His words: "Use whatever is your favorite shape." Just FYI.
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The first APC 12x6E I cut down to 10.5 I left the tips squared off.
For some reason, the next one I cut down I put some shape on the tips, making them swept back and rounded somewhat.
While performance was unchanged, the second prop was much quieter on the same airplane, same rpm.
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I built a fixture to get both tip equal. It also retains the original tip profile. Once marked I rough cut the tips on my jig saw. I then sand, on a disk sander to the line. I am sanding off very little so the heating of the prop does not seem to be a problem.
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Re-emphasizing what MM was getting at regarding delamination and splitting when trimming a carbon fiber prop: don't use a scrollsaw, jigsaw, bandsaw or the like. I suppose if I had to shorten the blades a lot, I would go get the finest tooth blade and cut chordwise, not thickness-wise.
CyA on the finished tip helps guard against future delamination and splitting.
For a molded, glass-filled prop like a standard APC (gray) practically any method works, so chop away to your heart's content.
Dave
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The way I've looked at it, if you make a raked tip (like an APC), and you do a slight prop-strike on take-off, it'll likely split a bunch of the TE off at least one blade, if not both. If you make a rounded tip, you might get away with it and the prop won't be trashed.
But I can't say what the in-flight performance difference is, if any. I do think the engine will reach a higher rpm with a raked tip, or if the tip is thinned to a pretty sharp edge like a new TT Cyclone prop. I have related how I buzzed a TT Cyclone, refinished the tips to a 10.75" diameter, with square, blunt blade tips. This prop would no reach the same rpm as the same prop at a full 11". My conclusion is that a sharp edge on the tip matters. y1 Steve
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while sanding Carbon, I hope you all wearing a dust mask.
One of my CL Buddies passed away because lung Cancer,
sanding Carbon and inhaling dust.
He never smoked his entire live.
Peter
H^^
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while sanding Carbon, I hope you all wearing a dust mask.
One of my CL Buddies passed away because lung Cancer,
sanding Carbon and inhaling dust.
He never smoked his entire live.
Peter
H^^
True, carbon is extremely harmful, and the irritating particles may never be cleared out of your sinus and lungs, and bronchial tract. Eyes too. Balsa dust is also a serious irritant, which I believe John Brodak mentioned once as suspect in his throat and respiratory ailments. I've heard that about balsa from other builders as well. Sanding and shaping balsa blocks or rib stacks, etc. gives me symptoms of a bad head cold for a day or two.
Rusty