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General control line discussion => Open Forum => Topic started by: Claudio Chacon on January 17, 2022, 08:45:46 AM
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Hello guys,
Have you ever repitched a wood prop using nothing but heat succesfully? (as with the carbon props, I mean...)
If yes, how did you do it?
Any advice?
Thanks!
Claudio
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Ooh. No, never done it, but I'm interested in the answer. It should be possible -- people bend wood with heat all the time. Should be just like re-pitching a CF prop, only different. It's the "how is it different" part I can't help you with.
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Yes, Claudio. I do it all the time.
A couple of random notes:
-I most often tweak near the blade root where airfoil is thick and allmost symmetrical. I always start with a prop with pure geometric 5,9 or 6" pitch, and reduce pitch. That makes some wash-out in the blade which I think is a good thing. You can also do it closer to tips but it's very easy to screw up the blade airfoil. Just hold something flat against the blade back when tweaking, and during cooling-down.
-For a permanent result you must heat the whole cross section to about 100 degrees C. Adjust your heat gun to 100..120C and heat very slowly, otherwise you'll burn the surface.
-I've found the best way is to lock the pitch gauge arm to the correct position with a small clamp, heat the blade root and quickly put the prop in the gauge. Then tweak the blade a little "over" the desired value, and when it's still hot, gently tweak it back to the correct pitch. Only that way, and only when the blade cross section is hot all the way through, can you expect permanent results. If it's not well heated, you'll leave interlaminar tensions in the wood and result is less stable.
-Wood becomes surprisingly soft at 100C.
-I make sets of big-ish props (about 13" dia) this way with 1/8" pitch increments and they stay exactly the same for years.
-Sometimes I know what I want, and do the tweaking before finishing the prop. That way the end result is a little prettier, as the clear coat always wrinkles or discolours a little during heating and deforming. But most of the time I don't care. L
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I do it all the time. I just heat the blade near the hub, and twist the blade (with a glove or rag in hand) in the direction I want to go, and hold while the blade cools down a bit. You can feel how hard you have to twist it. Then I check it on a pitch gauge and repeat as necessary. They seem to hold the adjustment just fine.I use a Monokote heat gin to heat the prop with. I though this procedure was more or less common knowledge.
Type at you later,
Dan McEntee
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Yes, Claudio. I do it all the time.
Wow! I have to say, it is not often than I hear of something completely unexpected, but I think this thread has crossed that threshold. I would never have even thought of it.
Brett
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Claudio,
Yes, I do it just like you would heat repitch an APC IC prop. It takes a bit of heat soaking as you apply the heat from the heat gun but it works and stays even after sitting in a hot car. I heard of this several years back, I had done carbon fiber props but always thought that for wood you needed to sand in the new pitch. Well, after I tried it and tested it by leaving in the car for several days then checking the pitch....it worked fine. I am lazy and only heat the hub area and set the pitch at the 70% mark on each blade.
I use a Prather pitch gauge and lock the pitch bar with a "C" clap so both blade are pitched to the same point. Just remember to heat and rotate the prop it takes a minute or so to get enough heat down into the wood fiber to get it to take the warp. You need to give it a good twist and hold a bit then check to see where you are. Wear gloves to heat and hold the blades, you should not cause the finish to curl or change color, if you do you applied to much heat. It doesn't take too many shots to get a feel for how much heat and twist you need.
Best, DennisT
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Wow! I have to say, it is not often than I hear of something completely unexpected, but I think this thread has crossed that threshold. I would never have even thought of it.
Brett
Oh Brett, I have props everywhere😂. L
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Wow! I have to say, it is not often than I hear of something completely unexpected, but I think this thread has crossed that threshold. I would never have even thought of it.
Brett
Heat bending of wood has been a staple for thousands of years. Look around your house and you'll likely find several things that were formed that way. Chairs, baskets, guitars....
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Heat bending of wood has been a staple for thousands of years. Look around your house and you'll likely find several things that were formed that way. Chairs, baskets, guitars....
I haven't seen any guitars that we are planning to spin up to 10,000 rpm while hovering inches away.
I am not criticizing, mind you, just pointing out that it never even occurred to me.
Brett
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Tweaking is not really a safety issue as the wood grain bundle, or blade, twists (is that a clear sentence..?) there are no forces that tear the wood structure apart. And unlike with softening chemicals, like ammonia, it’s more or less back to its original strenght when cool.
But as I said, it’s important to go well past the plastic deformation limit for good results.
Try it, it’s kind of funny what happens to wood at 100°. L
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I haven't seen any guitars that we are planning to spin up to 10,000 rpm while hovering inches away.
I am not criticizing, mind you, just pointing out that it never even occurred to me.
Brett
I gave this some thought when I first tried this. the grain in the wood goes span wise of the prop. This is where it's strength is. If you re-pitch a wood prop, you are just twisting the grain around each other, sort of repositioning it, and not stressing it. When I first tried this, I listened for any cracking or squeaking noises to indicate that I might be stressing the fibers and grain, and I have never heard anything. I don't pitch them that far, certainly not more than 1". That may be pushing things and have never tried it. In this I mean like taking a 6" pitch and trying to take it to a 4" pitch. It's easier to just try a 4" prop. I do it more to adjust a prop to be more like it's marked, such as making a 4" prop really 4", or a fraction of an inch in between. Like I said before, I have been doing thing a long time and just thought it was common knowledge. I'm sure I read about it somewhere, on SH or maybe Stuka, could even have been in Stunt News.
Type at you later,
Dan McEntee
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I haven't seen any guitars that we are planning to spin up to 10,000 rpm while hovering inches away.
I am not criticizing, mind you, just pointing out that it never even occurred to me.
Brett
I know Brett. But there's several luthier oriented folks here who understand the guitar reference. Bob Hunt, for one, does. He's doing some interesting form work using his foam methodologies. However haven't seen like at least a dozen guitars being smashed on stage and there's at least one song in the world written about it....
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I gave this some thought when I first tried this. the grain in the wood goes span wise of the prop. This is where it's strength is. If you re-pitch a wood prop, you are just twisting the grain around each other, sort of repositioning it, and not stressing it. When I first tried this, I listened for any cracking or squeaking noises to indicate that I might be stressing the fibers and grain, and I have never heard anything. I don't pitch them that far, certainly not more than 1". That may be pushing things and have never tried it. In this I mean like taking a 6" pitch and trying to take it to a 4" pitch. It's easier to just try a 4" prop. I do it more to adjust a prop to be more like it's marked, such as making a 4" prop really 4", or a fraction of an inch in between. Like I said before, I have been doing thing a long time and just thought it was common knowledge. I'm sure I read about it somewhere, on SH or maybe Stuka, could even have been in Stunt News.
Type at you later,
Dan McEntee
Speed fliers used to do it in days of old when wood props were common. Today most are composite which would lend it self to heat re-pitching. I've mostly just carved some under chamber in or moved the TE by thinning. Any more than that I figure like you do just move to the next pitch.
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Speed fliers used to do it in days of old when wood props were common. Today most are composite which would lend it self to heat re-pitching.
Wood is a composite material. It's cellulose fibers in a (mostly) lignen matrix. Some of it comes with a lot of entrained air -- we use that for building airframes.
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Wood is a composite material. It's cellulose fibers in a (mostly) lignen matrix. Some of it comes with a lot of entrained air -- we use that for building airframes.
Air is my hands down favorite thing to build airplanes with.
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Air is my hands down favorite thing to build airplanes with.
I thought that's why we call them airplanes. :o
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I thought that's why we call them airplanes. :o
Zackly y1
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Speed fliers used to do it in days of old when wood props were common. Today most are composite which would lend it self to heat re-pitching. I've mostly just carved some under chamber in or moved the TE by thinning. Any more than that I figure like you do just move to the next pitch.
That's ok for one-offs but kind of silly if you want to be systematical. You change pitch and another parameter (airfoil) changes at same time.
The best thing ever with wooden props is when they adopted the cnc-milling technology (I guess it was Yuriy who started it in F2B). Since then, repeatability has not been an issue. L
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Good morning guys!
Thank you, thank you very much for your replies!
I will start with a couple of broken props that I have, just to get the feel of the procedure...
In my case, I need to reduce the pitch by 1/2" to 3/4" on a 14" Top Flite Power Point... so we shall see...
Thanks again guys!
Later...
Claudio.