Wood type for wood prop?

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Aviacs

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As a woodworker, but not a propellor maker-

I don't like urea resin glue. It is not gap filling, whatever someone told you, and it tends to degrade over time, intrinsically, especially with heat in service. As has been said, epoxy is the only gap filler. Resorcinal is the most permanent. All the glues mentioned have specific surface treatement and other prep requirements, and, for lack of a better work "learning curves". It is also not quite the same for all of them. For instance, the 2 tradional glues require smooth knife planed surfaces and lots (literally tons) of pressure. Epoxy requires less pressure, and the best bonds are with wood processed by abrasive planing, rather than knife planing. For some dense wood, epoxy has a 2 step process that includes an open rest, in application. Appropriate fillers need be used. (Sanding for prep and open rest would be bond killers for the other 2 glues)

If you are not a woodworker, contouring, & getting and keeping a smooth finish on oaks is not the easiest of the choices you mention. Hickory would seem to be wonderful based on strength, but it would be low on my list due to low stability compared with other wood, and also the open wide grain, not quite as coarse as oaks.
Walnut is not the hardest of the choices either. It is a fairly soft wood. Softer than birch, oaks, hard maple, and hickory. It is among the most stable and rot resistant. Mahogany is nice, if you can read grain. A lot of mahogany, especially chosen for appearance, has a lot of short and changeable grain. It can also have grown togther short fractures, which are solid in aspect, but visible, and weaker than surrounding grain. It sure is nice to work. Maybe just a little easier than walnut.

Hard maple is still just maple. It’s just some is denser than others
This is not true. Commercially, hard maple is Acer saccharum. There are a couple other hard maples, none as hard as Acer saccharum; & the 2 other hard maples are not common commercially. Soft maple essentially boils down to "everything else". Compare the Janka hardness: Acer s. is over twice what say, silver maple is, and that is big. (1,450 lbs vs 700 lbs) It is also 30% heavier (more dense). OTOH, while Hard maple is also generally superior in other attributes like strength (tensile and compressive), elastic modulus, etc these qualities may be adequate in "lesser maples. (I don't know).

I believe black cherry wood (forest/commercial cherry), would make a good propellor wood (based on technical attributes) but it is another that requires perhaps better ability to read grain; and commercially, birch is better/cheaper.

None of the lumber mentioned should include sapwood in the propeller - except maples and yellow birch, which, commercially, are the sapwood & should contain minimal heartwood for technical constructions. :)


The NFPL database can be searched for more info on specific adhesives, testing, and bonding methods.

smt
 
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Appowner

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82
For what it's worth I use to carve props for RC models. I used Maple almost exclusively. I'd laminate using a very slow epoxy and a fair amount of pressure, an old car battery usually. You can use a fabric dye to color the epoxy for seam lines or purchase a dye made for epoxy. My props ranged as long as 13 inches diameter and would spin up to 16,000 rpm without failure. And I did NOT fiberglass them but that isn't a bad idea.

I don't carve anymore but I do refinish any wood prop I get for a model. Usually to make it look like a metal one. But in so doing I check the pitch and fine tune the balance. You'd be surprised how far off some of them can be. My models today run 24" props upwards of 9,000 rpm.
 

Mike von S.

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Feb 4, 2021
Messages
117
As a woodworker, but not a propellor maker-

I don't like urea resin glue. It is not gap filling, whatever someone told you, and it tends to degrade over time, intrinsically, especially with heat in service. As has been said, epoxy is the only gap filler. Resorcinal is the most permanent. All the glues mentioned have specific surface treatement and other prep requirements, and, for lack of a better work "learning curves". It is also not quite the same for all of them. For instance, the 2 tradional glues require smooth knife planed surfaces and lots (literally tons) of pressure. Epoxy requires less pressure, and the best bonds are with wood processed by abrasive planing, rather than knife planing. For some dense wood, epoxy has a 2 step process that includes an open rest, in application. Appropriate fillers need be used. (Sanding for prep and open rest would be bond killers for the other 2 glues)

If you are not a woodworker, contouring, & getting and keeping a smooth finish on oaks is not the easiest of the choices you mention. Hickory would seem to be wonderful based on strength, but it would be low on my list due to low stability compared with other wood, and also the open wide grain, not quite as coarse as oaks.
Walnut is not the hardest of the choices either. It is a fairly soft wood. Softer than birch, oaks, hard maple, and hickory. It is among the most stable and rot resistant. Mahogany is nice, if you can read grain. A lot of mahogany, especially chosen for appearance, has a lot of short and changeable grain. It can also have grown togther short fractures, which are solid in aspect, but visible, and weaker than surrounding grain. It sure is nice to work. Maybe just a little easier than walnut.


This is not true. Commercially, hard maple is Acer saccharum. There are a couple other hard maples, none as hard as Acer saccharum; & the 2 other hard maples are not common commercially. Soft maple essentially boils down to "everything else". Compare the Janka hardness: Acer s. is over twice what say, silver maple is, and that is big. (1,450 lbs vs 700 lbs) It is also 30% heavier (more dense). OTOH, while Hard maple is also generally superior in other attributes like strength (tensile and compressive), elastic modulus, etc these qualities may be adequate in "lesser maples. (I don't know).

I believe black cherry wood (forest/commercial cherry), would make a good propellor wood (based on technical attributes) but it is another that requires perhaps better ability to read grain; and commercially, birch is better/cheaper.

None of the lumber mentioned should include sapwood in the propeller - except maples and yellow birch, which, commercially, are the sapwood & should contain minimal heartwood for technical constructions. :)


The NFPL database can be searched for more info on specific adhesives, testing, and bonding methods.

smt
Aviacs,
Thanks for sharing your thoughts on various wood types. It's odd that Weick didn't mention sugar maple in his list of acceptable woods, considering how many props have been made with it over the years. Anyway, yellow birch seems to have the right combination of attributes, which, I hope, includes availability.
Regarding glues, I didn't mean to suggest that UF resin is gap filing in the same way that expoxies are, just that it is said to be more tolerant of pressure in the layup (and temperature in the cure) than resorcinol. Either way, we are talking "tons" of pressure. My lamination will be 66" x 5" = 330 sq in. At 150 lbs/sq in. that's close to 50,000 lbs. I'll be using 32 x 1/4" threaded bolts through pressure plates about every 4" along the length of the laminate. I know translating torque on a threaded nut to pressure is a black art, but this should add up to a big number. On the other hand, the guys who use UF resin for vacuum bag laminations are only exerting 14 psi max (albeit perfectly evenly distributed).
As for UF resin degrading at high temperatures in use, my understanding is that epoxy breaks down at about the same temp = 120 deg F.
I have also heard that for prop making, where typically 5 or 6 laminates are laid up, the fact that epoxy is very slippery means keeping the planks perfectly in line, even using dowel pins, is tricky.
FWIW
 

Aviacs

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318
UF glues are among the most reactive (good) and hence form some of the strongest bonds initially. Brief cruising NFPL data some tests show stronger (initial) bonds with yellow birch, than phenol resorcinal attains. My predjudice/fear with UF glues is that they tend to have a "half-life" (my term). According to tests i read at an impressionable age when starting in the late 70's, UF glues in hardwoods in interior use (typical range/change of ambient temp and MC over many seasons) UF glued side grain samples lost about 1/2 their bond strength in 20 years. Heat, sunlight, and high MC accelerate this (shorten that period) but do not directly cause it. Some of the degradation is due to acid &/or formaldehyde residuals, or production over time in the glueline. With some modifications, these factors may be considerably lessened or removed. However, this is considerably different from epoxies, which many may soften around 140F but return practically to full strength when heat is removed.

Now, having stated my admitted deep predjudice (research supported, but by no means "true in every case"); UF glues come in a very wide range of formulations and modifications. In recent years, things like starch are added to make them "greener" (reduce already low formaldehyde emmissions). In some cases some glues appear to be stronger, in some cases weaker, than without that mod. Many UF glues are modified by the addition of other stocks, such a melamine, or phenols, which enhance attributes & reduce acid & formaldehye effects. Rather than search every case and complication, i choose to simplify my approach and use either WEST epoxy, or Phenol-resorcinal. P-R & UF require essentially the same prep, temperature, and clamp conditions, so the only thing UF saves is cost. Which admittedly can be significant.

Of course, "plastic resin" glue has been a staple of the aircraft industry since it was first developed, so it certainly can be adequate in service. Use best practices and you should be OK. Many people before you have done so with great success.

Yes, gluing up laminations with epoxy is like trying to glue a stack of greased planks! :)
So do have some control features or guides, or pins. OTOH, the glue up time (once the glue is spread) is close to an hour, with WEST. It is minutes with UF & only slightly longer with PR.

Your clamping approach sounds good. I, personally, would probably use 3/8" minimum althread, and use the high strength stuff. 1/4" threaded rod is only really/practically 3/16" at the thread root. Without greased nuts (complicating glue up cleanliness) there could be a tendency for the studs to twist off at the wrong moment, before attaining what you intend for pressure. (IOW, without doing the math I usnderstand that 1/4" coarse thread rod probably is adequate. However, there can be some practical glitches in execution, especially if you will use power tools to run the nuts down :) )

Use at least one extra ply at least 3/4" thick (needs to be flat, but does not need to be "good" wood), faced with clear plastic packing tape to resist glue, on each side of the glued up stack to spread the force. Pressure emamates downward from clamp-face contact at a 45deg angle each side. So the height of an additional caul intrinsically extends the area of equal force under each clamp position, and the stiffness of the caul extends the "sideways" reach of effective force a bit more.

Per your note i find vacuum bagging inconvenient for many constructions except large, wide, flat and gentle contours. As you note, pressures are low (though excellent for my choice of epoxy :) ). In some cases i use air actuated clamps with an array of Firestone "Airstroke" Actuators - those big air springs sometimes seen under the trailers of tractor trailors. A cheaper, more flexible method is the old firehose press type. From what i have seen, this is sometimes used by some wooden prop makers. I also use collapsible discharge hose, which comes in a range of widths and pressure capacities. The advantage of pressure bladders is the increased pressure available. The disadvantage is reacting that pressure with a suitable frame.

It will be great to see you progress with this, and looking forward to your carving methods and notes!
smt
 
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poormansairforce

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N

Nut factor being a guesstimate of force actually delivered to the bolt after thread and face frictions are discounted?
In essence, yes.
.
 

Rljj

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Nov 7, 2021
Messages
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Tips or outer half should be glassed to allow for thinner tips than wood alone. The wood is just core so it is a wood core composite skin. I like plastic resin.
I have never built a propeller for a plane but have for wind turbines and one problem we ran into was water/moisture getting into a glassed blade, the moisture will end up at the tip though centrifugal force this caused the tips to burst when they froze and other times to become unbalanced. We ended up running wood right to the end and left the end exposed and haven't had any problems since. Not sure if this applies as a tubine spins most of its life while a prop only spins while running.
 

Rljj

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Aviacs,
Thanks for sharing your thoughts on various wood types. It's odd that Weick didn't mention sugar maple in his list of acceptable woods, considering how many props have been made with it over the years. Anyway, yellow birch seems to have the right combination of attributes, which, I hope, includes availability.
Regarding glues, I didn't mean to suggest that UF resin is gap filing in the same way that expoxies are, just that it is said to be more tolerant of pressure in the layup (and temperature in the cure) than resorcinol. Either way, we are talking "tons" of pressure. My lamination will be 66" x 5" = 330 sq in. At 150 lbs/sq in. that's close to 50,000 lbs. I'll be using 32 x 1/4" threaded bolts through pressure plates about every 4" along the length of the laminate. I know translating torque on a threaded nut to pressure is a black art, but this should add up to a big number. On the other hand, the guys who use UF resin for vacuum bag laminations are only exerting 14 psi max (albeit perfectly evenly distributed).
As for UF resin degrading at high temperatures in use, my understanding is that epoxy breaks down at about the same temp = 120 deg F.
I have also heard that for prop making, where typically 5 or 6 laminates are laid up, the fact that epoxy is very slippery means keeping the planks perfectly in line, even using dowel pins, is tricky.
FWIW
I have done a number of cold molded boat with wood/epoxy and always vacuum bagged them.
 

Appowner

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Mar 30, 2021
Messages
82
I have never built a propeller for a plane but have for wind turbines and one problem we ran into was water/moisture getting into a glassed blade, the moisture will end up at the tip though centrifugal force this caused the tips to burst when they froze and other times to become unbalanced. We ended up running wood right to the end and left the end exposed and haven't had any problems since. Not sure if this applies as a tubine spins most of its life while a prop only spins while running.
An often overlooked place for the introduction of moisture are the holes in the prop hub. Raw wood is often exposed inside those holes with only a bolt or shaft to close them off. Has to be a better way...
 

blane.c

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Wasn't (isn't) it common to have a phenolic bushing in the bolt holes of propellers and other structural wood members for example spars? I thought it was in there to protect the bolts from corrosion but it could work both ways reducing the chances of rotting the wood?

Scroll down on link and read the overview.

GRADE L LINEN BASE PHENOLIC ROD | Aircraft Spruce Canada
 
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TFF

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MT props have this happen. It’s one thing to fly through rain, it’s another to store one outside. The flight school at my old airport sent props off their DA40s enough to buy Hartzell replacements that could have stayed outside. He got rid of that set of planes.
 

Aviacs

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I use a prop cover when tied outside.
No doubt better than not; particularly considering UV.
But wonder if it is effective against, or contributing to condensation?
"Microclimates" can be funny.

Raw wood is often exposed inside those holes with only a bolt or shaft to close them off. Has to be a better way...
Maybe saturate with epoxy, and ream? (Or press in smooth UHMW dowels, while epoxy is wet, to be pressed out when set)?

smt
 

Rljj

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I use a prop cover when tied outside.
An often overlooked place for the introduction of moisture are the holes in the prop hub. Raw wood is often exposed inside those holes with only a bolt or shaft to close them off. Has to be a better way...
We even tried to put larger holes, fill with epoxies then drill out with proper size. Our conclusion was that the moisture at 6% was slowly migrating to the tips as after a period of time the tip would be as high as 20% and the root of the blade would be low enough to error on the tester.
 

TFF

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Most wood airplane props are two bladed. They are supposed to be stored lateral, not vertical for the moisture reason. That’s been known since the beginning. Multiple blade props is an issue. You would need to turn blades every few days to keep them even, or just fly a bunch. If it has to take a winter nap, off and stored on its back.
 

Dan Thomas

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My Jodel's prop was hard maple, built by Colin Walker of Surrey, BC. I don't know if he's around anymore. Maple has the right combination of hardness, split-resistance, weight and so on. The prop is the flywheel, so I wouldn't use a light wood like spruce. Too soft, anyway. Fir splits too easily. Considering the amount of time one will spend building hos own prop, I don't know why he would use a cheap wood. Or cheap glues. The urea-formaldhyde glues you find in the hardware store have wood dust as fillers in them, and some homebuilders have had poor luck with them. I can't find Aerolite glue listed anywhere anymore; that was the stuff used in certified wooden airplanes. I have used a bunch of it.

Colin Walker had a unique leading edge treatment: he milled off about 1/2" of the leading edge for some distance in from the tip, made a mold of some sort, maybe just tape, and filled it with a hard urethane from Devcon. Then shaped that to the blade's airfoil. A lot tidier than a brass sheath, and stuck on there real good.
 

Dana

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Colin Walker had a unique leading edge treatment: he milled off about 1/2" of the leading edge for some distance in from the tip, made a mold of some sort, maybe just tape, and filled it with a hard urethane from Devcon. Then shaped that to the blade's airfoil. A lot tidier than a brass sheath, and stuck on there real good.
Ed Sterba does the same thing.
 
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