Making a Wooden Prop

[Dave Hull]

The conversation here on Stunthanger about selecting a good wood type for propeller making reminded me of an article I wrote for our club newsletter back in 2022. After I posted a picture of it on Curare's thread I received a few questions about it. So here goes....

A while back, I needed a propeller that was between the commercially available sizes to test on a new engine/plane combination. I wasn’t getting enough drive from a 4 inch pitch prop, but the engine didn’t want to pull a 5 inch pitch without going down in diameter. I could start trimming and reworking props, but decided instead to just make one. I had a nice piece of straight-grained beechwood left over from someone’s kitchen cabinet job—so why not have some fun?

My objective was to make a 10.5x4.5 prop with conventional blade shapes, airfoils and a helical pitch distribution. The main tools used were layout and dimensioning tools, a bandsaw, a drill press to get the hole straight, flat and half-round wood rasps, various home-made sanding blocks of different grits and a peaceful afternoon to get started.

Photo 1:  An oversize piece of beechwood at the beginning of the project. Beechwood isn’t the lightest prop material, but it is hard and quite durable. It also holds a crisp edge which is important considering all the handling it will go thru during fabrication. I have also made nice props out of birch. Here, I’ve started to get the layout tools out of the toolbox.

[Dave Hull]

Photo 2: There’s a good bit of progress at this point. A section has been cut from the board, and then it was resawed to get to the correct blank thickness. In this case, 1/2 inch. The blank width at this point is 1”. The scrap piece that came off during resawing is laying above the prop blank. This new face was carefully sanded to be parallel to the uncut opposite face. It would be ideal to run it thru a thickness sander, but I don’t have one, so I did it the hard way. Digital calipers were used to ensure getting the surfaces parallel and on-dimension. Next, I began sketching out the blade shape I wanted after I drilled the shaft hole. All of the subsequent layout work references the hole.

[Dave Hull]

Photo 3: I must be making progress—the collection of tools on top of the table keeps growing! Once I had sketched out a blade shape I liked on one side, I cut and sanded it to final outline. Then, to transfer the exact same shape to the other blade I made a template from aluminum. I find that aluminum roof flashing material is excellent for things like this. You can keep everything registered using a ¼” dowel pin. While cutting the template it is handy to use double-sided tape to hold it to your wood blank. I used a minimum number of stations spaced at one inch. Marking these with a fine point ink pen is desirable, on both the template and the blank. A Sharpie is excellent for the aluminum, any fine point gel pen should work well on the wood. Having an accurate center line in each axis is very important. The use of an axial (along the blade) centerline is critical. This prevents having the blades come out 178 degrees apart—or worse.

[Howard Rush]

Groovy

[Motorman]

Looks easy so far.

[Istvan Travnik]

Uhh-uhh...
You forgot to install the most essential machine tool: the sandpaper-covered disc!
Simple, primitive looking tool: take a 3-phase, 1440 rpm asynchronous electric motor, with some 200-500 W power.
Make an 3/4" or 1" thick, wooden, or plywood disc, , 12" dia, with some 1/4" radiused edge, and some 2-3° conical side, and lacquer it very well by floor  lacquer, since the sandpaper will be bent on wet. After perfect drying you can use. A steel frame is needed to bend on the wet paper. (Similar to barrel's frames)
A narrow horizontal table in the middle point is practical...
That question, if the very first, or tenth, or hundredth prop will be perfect, depends on your talent, practise, and precisity...
Here is a disc (rem: the paper is not bent on! )
https://i.ytimg.com/vi/PavR5SxbZ0o/hqdefault.jpg

[Dave Hull]

Photo 4: Once the planform of both blades has been cut and station lines marked out, the blank can be thinned towards the tips. I like to leave the forward face of the prop orthogonal to the shaft and put all the taper in the back side. They simply look better this way. You can see the thin wedge cut from the first blade here. You might also notice the squiggly lines on the portion of the blank that is intended to be cut away. Kind of like building both a left and a right wing, you need to the get the taper on the correct side of both blades.

[Dave Hull]

Photo 5: After cutting the back of the prop to get the taper and dressing it flat, you can lay out the cut line along the leading edge. Note that the blank shape, planform, and desired pitch all need to be figured out beforehand. Note that these layout lines define the back surface of the blades. This surface will be measured with a pitch gauge, but this is not the true pitch of the blade! The true pitch would be measured at the neutral lift line of the airfoil, which this type of gauge cannot measure directly. It is easier just to work to a slightly lower measured value of the back face. The gizmo on the table above the prop blank is a prop holder for a Prather pitch gauge. An essential piece of layout equipment is the Faber Castel plastic eraser used to “improve” the accuracy of your layout lines. A 0.5mm mechanical pencil works well on close-grained hardwoods. You want crisp layout lines to work from. Or, keep your traditional pencil SHARP!

[M Spencer]

https://stunthanger.com/smf/index.php?action=dlattach;topic=65692.0;attach=350998;image

Is that your best hammer ? .  Thanks for posting this . 

[Dave Hull]

AirMiseries,

No, it's just my smallest hammer. I think it was handed down from my uncle. I used it to centerpunch the hole in the prop blank for the crankshaft. When I need a hole to be where it belongs (so as not to start over with another blank) I jab the blank with the yellow-handled pick first, then follow up with punch and hammer. The punch has a pretty flat grind so the drill bit follows easily. One key is picking wood that has good uniformity and is NOT ring-porous! Another good method is to fit a sharpened steel pin into the drill press and align your marks to the pinpoint, then maintain pressure on the quill while you clamp down the wood blank. Then swap out the sharpened pin for your 1/4" drill bit. I use a piece of scrap wood under the part to keep from blowing out the hole on the back side. Sharp drill bits are nicer to work with, too.

For metals, (like my aluminum template) I use a Starrett automatic centerpunch. As they say in the southren latitudinals--it works a treat!

The Divot

[Motorman]

When you say the back of the prop do you mean the face of the prop? because you're talking about the flat side right? It's not considered by how it relates to the airframe. Flat side is the face, hump side is the back in prop terminology.

[Dave Hull]

Not going to argue about front or back faces of props. The industry and users haven't been consistent as far as I'm concerned. We might as well be arguing that ft-lbs is not a torque since the vogue term today is lb-ft.

[Dave Hull]

Photo 6: This view shows both the planform and the edge views together, so hopefully it will make more sense. I typically work out the shape on one side first before doing the second blade. That way, if I don’t like how it is turning out, I have much less work in the job. The trick is, don’t get yourself into that situation in the first place.  Notice that I left the blank a little bit long so that I could make a decision on the final diameter at a later point. If you look carefully, you will see that I "curve-fit" a straight line to the measured tick marks at the individual stations. That is just using one technique to improve the accuracy of the first method.

[Dave Hull]

Photo 7: Now things really start to take some shape. This is one of the more difficult steps—cutting away a lot of material to create the back of the prop—without taking too much off and ruining it. One way to do this is by freehanding with the bandsaw as shown here. But, that is one of the more sketchy operations that you can do with a bandsaw. If you are not steady and precise, the blade can grab and ruin the workpiece or even break the blade. Needless to say, your hands need to be a good distance from the blade at all times. This is not the place to screw up! There are several viable alternatives to this: use a coping saw or else use a coarse wood rasp for roughing out. A good rasp will cut away a lot of material in short order with none of the drama. As with any power tool, know the dangers and set your personal limit based on skills. As a point of information, the old wooden boat builders had a much larger bandsaw that could cut a continuously changing bevel on frames while the work was laying up against the moving table. In contrast, one of the most famous rocking chair makers, Sam Maloof, could saw out long and large chair pieces freehand. He had the skills to match his tools.... In the case of this propeller, I started sawing at the tip and progressed inward, trying to put in the correct twist along the way. At about the 1-3/4" station, I had to stop and reset as I was cutting too deep and getting too close to the layout line. As you get closer to the hub you have to change the twist quickly. Using a fine-tooth blade with plenty of set helps. A dull blade--especially one with drift--is a good way to produce kindling....

[GallopingGhostler]

Off to a good start, looking good. I've never tried it but with good wood selection, can't see why one can't pull it off.

I remember reading about one making their own props from older books. Keith Laumer, who was into free flight back in the 1950's - 1960's showed how to splice two broken wooden props at the hub, to make a good prop. But, he was a sport not competition flyer I gather, had many interesting powered free flight sport aircraft, mostly half-A with whimsical names.

One over on Cox Engine Forums, Robert has made his own laminated wood props, look sharp and seems to be decent performers.

[Lauri Malila]

Nice work, Dave. I’m looking forward to see the finished prop.
You manage with really simple tools to make one-offs, but for serious use, I mean to be competitive (or better perhaps) against composite propellers, CAD-CAM technology is the way to go. That way I can just finish the prop, correct the pitch and I more or less know how it’s going to perform. And with very good repeatability. L

[Dave Hull]

Creating laminated props would be fun. It just takes more work. On full scale props, they are pretty much compelled to do it because you can't find a balk of wood that big that is uniformly good and with proper grain throughout. With laminations, you can optimize the grain throughout. You can easily find good clear wood for a model prop, but if you want to make one that is really scale....

Keith Laumer is one of my modeling hero's. When I was just a little kid, I was given a copy of his book "How to Design and Build Flying Models" when I was in the hospital recovering. I absolutely devoured it. I've built some of his designs and plan to build more. If you are a SciFi person, know that you should read about his Bolo books. This short-story concept continues on with guest authors writing in his style. ("The Best of the Bolos: Their Finest Hour" is especially good.)

No CAD-CAM in my garage so I won't be doing things that way. It might also be too much like my former career in aerospace and maybe defeat the purpose of being retired? For those with the desire and the equipment--have at it!

Edit--Fixed a typo in the book's title!

[Dave Hull]

Photo 8: From this angle, you can see the twist cut into the back surface of the blank. The trick to sawing the larger waste area out is to make a smooth continuous cut while rotating the blank. Again, you can see where I got behind on the rotation as I got close to the root area. But I didn’t cut into the corner of the blank which is a reference edge. So as the cut progresses, you must twist the blank faster and faster to keep up with a constant feed rate. I'm probably using a 1/8" fine tooth (14TPI) .018" thick blade here. That's kind of my standard blade for everyday use.

[Dave Hull]

Photo 9: After working the rear face down to the reference lines at LE and TE using a progression of flat and half-round rasps, then files and coarse sandpaper I felt like I deserved to do a premature fitcheck. Here's the partially completed prop on my BigNose Vector just for fun. The plane has an oversized spinner on it with the top deck reshaped to fit.

[Dave Hull]

Photo 10: Here the prop is on the Prather pitch gauge and getting the rear face checked--and marked down on the stock. Each of the slots in the base of the gauge is numbered, and you must read off the measured pitch on the correct column on the angle plate. I'm going to finish the rear face on both blades before starting on the forward faces. I'm using a "flat-bottomed" airfoil simply because it is easiest both to make and to measure accurately. But it means that the numbers being measured and recorded are not actually true pitch. One of the last steps will be to gently radius the leading edge a specific amount which will increase the pitch slightly. Recall that we were shooting for a 4-1/2" pitch prop.

[Dave Hull]

Photo 11: Making good progress now on the back of the prop. On the left, the surface has been worked down to the reference edge (LE) and the layout line (TE). It is not final sanded yet, but very close. On the right, you can see the roughed out back face of the second blade. The numbers penciled onto the blank are the measured pitch numbers for the back face at each marked station. They are close to helical, but not quite equal yet. A bit more work to go on the left, and a good long distance to go on the right side.

[M Spencer]

"  I mean to be competitive (or better perhaps) against composite propellers, CAD-CAM technology is the way to go. "

Yes , BUT :

One CAN do em , BY EYE . ( If you have a good eye ) Mark One Side . Use a Virneer to transfer opposite . Repeat . Even a ruler would do . Or a Mark on a Stick . 

Two I did , one , is still beyond what a ' normal ' propellor , could ever do . Even De Tooned .

For People who DONT have a computer equiped basement , but can stand a rasp & a bit of 40 or 60 wt sandpaper , Rather than watch television , they can make things , like people did , before they had television , even .





the problem with crocodiles , when your invited to dinner , is that you never know if their smile is that theyre pleased to see you , or if because youre the main course . W.S.C.

[M Spencer]

Besides , it keeps them off the streets . And gives them a sense of achievement , at the end off the day .



Wood can twist , wether its ' C N C'd or not , AND a bit of variation would do people a bit off good . Youd have increments of pitch , off the shelf .

Ones I did , Pitch Lines , you can see if its ' Too much ' or ' too little ' without getting the instruments out . And try a few abstract interpretations .
As Propellor  Theory has been a bit neglected , since theyve left them off . Or used multitudious tiny ones .



Auto wotsit might be o.k. for these : take a bit to long with a hacksaw & file .    ACTUALLY , theres some with highly undercambered rather ornithalogical - turbine blade airfoils . Props , after all .

[Dave Hull]

Photo 12: To get to the desired 4-1/2” helical pitch, all of the stations need to measure out at 4-1/2 on the gauge. In reality, over most of the blade you are filing or sanding in a diagonal root-to-tip way such that it precludes abrupt pitch changes—and errors. At this point, I’m on target except at the root, where you have to decide just how close to the hub you want to cut into your blank considering strength issues. You may have noticed that the prop holder used here is not the Prather item; this one has a wider base made of steel which makes it more stable to work with. These are available from Lee Machine Shop.

[Dave Hull]

Photo 13: Another view of the nearly final finish on the back side of the prop blank. All of the coarse rasp and file marks are out, and it is sanded to within 1/32” of the reference edge on the right side of the blade. You must preserve your layout work (reference edges) or you will lose accuracy of the blade.  Note how the final blending was done at the root where the back face transitions into the preserved hub diameter. With the finish work, you can see the nice uniform grain flecks coming into focus. I believe these are the medullary rays responsible for radial transport in the living tree.

[Motorman]

How we doin' on balance?

[M Spencer]

You must preserve your layout work (reference edges) or you will lose accuracy of the blade.

Yip ; A stanley knife blade ,held perpendicularish to the surface , used  as a scaper , can getim to a microfarad . Viewed end on , in good light . Bit harsh of the skin, but .
As a kid Id found a discarded 3 edged  ' bearing scraper ' . Used to ' hand finish ' poured or undersized bearings . Whitemetal , to fractions of a thou. So de krect mental proach  elps .
There still around. Off the Shelf . Some ships & things have handsome bearings still . SO , gettingem inside a thou is nosewet , if you know whata thou is .

Those EDGE LINES ( etc ) are the Datums , Base Lines , and things like that . So the accuracy off THEIR positioning / Placement , is where the cat gets the cream .
Some off us might start in light soft pencil , then Vineer or good clean ruler maybe with another set vertical to edge , and other creative DEFINITIVE marking placers .

S SHARP 6 H or 4 H pencil , in a steady hand , can actually ' catch 1/10 th. m.m. . which is 63.5 micrometres . whatever they are . call it 63 or 64 , should close it .     
( practise + S H A R P  6H , draw twa parrallel at 1 m.m. exact . Split . ( Two Parts ) Split Again ( Thats four parts ) then two lines twixt . staggered - overlappeds . Youve drawn ten , or is that eleven . IN a milimeter .
No Sweat . Practised at that , you can visualise ' the split ' and catch fractions of m.m. , when marking out . the Hard Lead Tip will ' just ' give a lead , for a fine line ballpoint , to lay over . A few differant colours
If your doing multitudeinous longitudeinal cuts ( back to lines ) and ' scrapes , to deedonem . Throwing the Virneer on it occasionally . Tecnique X strokes , rotate ditto that ( blade ) then BALANCE is Equal as MASS
is exactly equal .

Sa can do in 1 1/2 Hr. if ' set up ( right tools out & bench clear )  with a clear head , and a clear eye . Tools ' weight ' or balance a factor too . Sometimes you tape your thumb & forefinger , so as to not wear through them .

Commin along well , there . But thats just the flat part , the airfoils Next .     

[Dave Hull]

The Beech was really uniform, so there were no balance issues as the project was completed. I didn't worry about balance until after I had both blades fully shaped and fine sanded.

Scraping is an effective method of working wood as long as the wood is hard enough. Just hold the scraper at the same diagonal angle across the back that you would the rasp or file (or sanding block, for that matter.)

I can see from looking at the pictures again that my pencil work was comprised of pretty fat lines. I should have been using a 0.5mm mechanical pencil with hard lead. Next time....

[Dave Hull]

Photo 14: Once the faces on the back side of the prop are final, you can begin on the front. Finish the back side first!  Here you can see the first part of the roughing work done on one blade. Once you start, you lose most of your reference lines. One more reason why you need to preserve the reference edges. You can still see on the TE of the blade in work the tick marks of the stations. There is also still a tick mark on the very end of the blade (not visible in this view.)  The sanding block in the background has become one of my two standard styles.  This type is about 7” long with rounded ends. Most critically, it uses very high-quality sanding cloth such as that used on belt sanders. When I blow out a 6x48 belt on my sander and it's not worn out (for example if the splice adhesive gets too old or it overheats and lets go), I cut pieces out of it with a utility knife and then staple it to the block. Full credit for selling me a few of these years ago goes to the late Dale Long of Riverside, California. With this kind of paper, you can use a crepe rubber block to clean it as you sand and paper lasts a long, long time.

[Dave Hull]

Photo 15: A bit closer view so that you can see the thickness of the blade at the tip. At this point the shape of the blade is mostly established, but as the tip shows, it is still too thick from root to tip. Looking at the thickness of the trailing edge illustrates the reason for that. But remember, the back face is done, final, complete! Do not remove any more material from the back!

[Dave Hull]

Photo 16: At this point, I am working the blade thickness down towards my target numbers. Here, I have measured the thickness at the reference stations using digital calipers and penciled in the results on the prop. If you are making a prototype and not a duplicate, it is unlikely that you will have a final numerical target at this point. I usually have a certain percentage thickness in mind, but as the blade gets thinner, I can change my target if it still seems too thick or too heavy. For example, the chord at mid-blade is around .950”, so a .187” thickness is around 20% which is still pretty thick. A beechwood prop doesn’t need to be as thick as some of the lighter woods.

[Curare]

Dave, your pics jogged my memory of something I noticed when I scanned the BY&O prop, and that is the profile (airfoil) of the BY&O, it's not a true airfoil, it's acutally an arc.

As a starter I plan to do a carbon-copy, but no doubt I'll mess around with profiles when I get some time.

Are you making a proper airfoil, or an arc, or something in between?

G

[Dave Hull]

Greg,

This prop is getting a recognizable airfoil, with a radiused LE, a tapered thin TE and a high point very roughly at 30% or so. The thickness of the outer portion of the blade is under 20%.

I'm looking at a few BY&O props right now because I found your comment interesting. On the first I noted that a 10x6 Beech was airfoiled but had the TE "rolled off" a bit, possibly from handwork. An 11x6 Beech looked pretty nice--but then I realized from the finish on it that I had reworked it so all bets are off regarding the factory shaping. In particular, the TE was thinner and had much less roll-off. Same on an 11x4 Beech; reworked and refinished. On what appears to be an Ash 10x4 it looks much more like a simple arc with the high-point nearly centered. I seem to recall that there was also a period of time where the person doing the cleanup on Brodak's props went back for more training when the airfoils started getting noticeably odd. I never ran across one like that, so can't verify the story.

One thing to check is to see how close your scanner can curve-fit, and does it create a math model or just a field of points? And, are you allowed to pick the type of curve-fit if it is doing it that way?

I can check out the shop tomorrow to see if I have a set of machinist's radius gauges that go large enough to make a better measurement. I think I also have an original Y&O prop out there that I can check. That also might be interesting.

Just offhand, I would note that for sport/stunt planes we generally configure them nowadays with a ton of extra horsepower. And so switching props of nominally the same diameter and pitch is often just managing engine loading where one prop is less efficient than another. Back in the day with much more limited power and the trend to larger models for advantages in "presentation," I'd bet that tuning props to get the best efficiency really mattered. Nowadays, the game might be much more oriented toward low rotating mass for a given dia/pitch for best maneuvering performance. Just a few things to wonder about coming from a mostly non-competitive guy....

[Dave Hull]

Istvan,

Sorry I missed your comment earlier about the disk sander. Yes, I have a combination upright belt/disk sander. That and the bandsaw are the most-used machine tools in my little shop--followed by perhaps the drill press and then the shop-vac? Of the two stations on the combination sander, I prefer the 6" belt over the 12" disk for nearly everything; so much so that I generally relegate the disk to just working on metal parts.

For making one-off props, i would use the stationary belt sander to help prep the squared-up blank. I would also use it wherever I can on the blade planview outline. After that, I pretty much use handwork. it is much more controllable, and I'm less apt to make a mistake. A lot of material can be quickly and safely removed with a selection of sharp rasps followed by 80 grit, cloth-backed sandpaper on an appropriate sanding block. Even on beech (the case here), eastern maple or yellow birch.

Dave

[Dave Hull]

Photo 17: The front face of both blades now worked down pretty close to final, but the root needs more work to blend it to the penciled hub circle. The thickness numbers (given in thousandths of an inch, but no decimal to ding the surface of the prop) are matching left to right within .004” or so, which is pretty good for a stunt prop. Final sanding will bring that in closer. You have to maintain your station marks as you work otherwise your thickness and chord measurements won't be consistent. I made the measurements with a digital caliper.

[Motorman]

it's not a true airfoil, it's acutally an arc.

There are always compromises in airfoil shape with a wooden prop. Stiffness must be maintained. Tip flutter and splitting are the problems.

MM

[Curare]

Greg,

This prop is getting a recognizable airfoil, with a radiused LE, a tapered thin TE and a high point very roughly at 30% or so. The thickness of the outer portion of the blade is under 20%.

I'm looking at a few BY&O props right now because I found your comment interesting. On the first I noted that a 10x6 Beech was airfoiled but had the TE "rolled off" a bit, possibly from handwork. An 11x6 Beech looked pretty nice--but then I realized from the finish on it that I had reworked it so all bets are off regarding the factory shaping. In particular, the TE was thinner and had much less roll-off. Same on an 11x4 Beech; reworked and refinished. On what appears to be an Ash 10x4 it looks much more like a simple arc with the high-point nearly centered. I seem to recall that there was also a period of time where the person doing the cleanup on Brodak's props went back for more training when the airfoils started getting noticeably odd. I never ran across one like that, so can't verify the story.

One thing to check is to see how close your scanner can curve-fit, and does it create a math model or just a field of points? And, are you allowed to pick the type of curve-fit if it is doing it that way?




I can check out the shop tomorrow to see if I have a set of machinist's radius gauges that go large enough to make a better measurement. I think I also have an original Y&O prop out there that I can check. That also might be interesting.

Just offhand, I would note that for sport/stunt planes we generally configure them nowadays with a ton of extra horsepower. And so switching props of nominally the same diameter and pitch is often just managing engine loading where one prop is less efficient than another. Back in the day with much more limited power and the trend to larger models for advantages in "presentation," I'd bet that tuning props to get the best efficiency really mattered. Nowadays, the game might be much more oriented toward low rotating mass for a given dia/pitch for best maneuvering performance. Just a few things to wonder about coming from a mostly non-competitive guy....

Dave, the scanner does not do any curve fitting or interpolation, it's merely a point cloud - the interpolation happens when i drag it into my CAD program and begin to take sections through the point cloud.. I assumed it was that way more for ease of manufacture, rather than any specific engineering requirements but I guess stiffness and reduction of splitting may have something to do with it as well.

THe prop that I scanned (for reference) was a 10x6, so your mileage may vary when measuring other props, or possibly even other 10x6's.

[Dave Hull]

Photo 18: In the layout of this particular prop, the maximum blade width is matched to the hub diameter. In this picture you can see that when it is resting on the edge of the hub both blades are nearly touching the reference surface. You don't have to lay out a blade this way, but it is easiest and seems to work fine. For contrast, recall the shape of the old Grish props. They kind of look like each blade was shifted "forward" in the direction of the leading edge. For stunt, since most applications aren't flogging the prop within an inch of its life you could likely try a lot of different shapes and not have structural failures. But the faster you spin it, the more care you need to take regarding stiffness and flutter. I think there are stability benefits to keeping the zero-pitching moment line pretty straight down the blade length. This is the opposite of what you get with a pronounced scimitar planform. I have seen several of those (full-sized versions) promoted as having dynamic pitch or some such, intended to mean that they are aeroelastic. That's great as long as they are not structurally divergent. I recall one such prop, or at least the hub of the prop, on a Goodyear-type homebuilt. They left the blades out in the countryside somewhere....

[Istvan Travnik]

Dave,
Maybe my description was not clear, or precise enough. (Sorry for my poor English...)
That disc sander which I mentioned was not the usual, commercial one, since it is essential to bend on the sandpaper all around the radiused edge, and hold on by a steel rim.
The "Great Old Bisons" used this radiused edge, (not the plate of disc) surprising effectively and quickly. Naturally, to make some concavity for the bottom surface of prop, was not a big task...

[Dave Hull]

Photo 19: A quick fit-check with the intended spinner establishes the final shaping requirements for the blade root area. Here you can see that more of the prop root must be cut away for it to fit. Since I am already cutting on the prop, the decision was easy to cut the prop to fit the spinner, rather than cutting the spinner to fit a bulky prop. (All the commercial props I tried fit the spinner, so I didn’t want to put larger cutouts and then run a commercial prop. Ugly!)  You can see I have marked out the diameter and required relief at the root to make it fit. I chose to take all the material off the LE. Part of this hub work is to smoothly blend the blade planview into the hub diameter. Up to this point I have left it oversized, so now it's time to fix that.

[Dave Hull]

Istvan,

Ahhh, this is something I did not suspect. Thank you for clarifying. I looked at the picture in your link and said, "yes, I use this sometimes."  But it is not a "bent" disk by my new understanding. Thank you for clarifying. I'm still not sure how to roll the edge of a paper (or cloth) disk over the tool face....

Your English works well, my friend. But who are these "Great Old Bisons" you refer to? Master woodworkers from days gone by? Propeller makers?

Dave

[M Spencer]

"  I noticed when I scanned the BY&O prop, and that is the profile (airfoil) of the BY&O, it's not a true airfoil, it's acutally an arc. "

Actually a ARC is a ' proper ' airfoil . ( See in the B T/R Lutz plan ) Is CP & CL Stable , high subsonic . So apt ' race ' prop blade section .

Incidently , Square prop tips are noisyer . So , if you want to hear what your props up to , straight cut tips may let you hear it . A zzzz noise . Can always dress them , later .



Not a bad score , for $ 2 , at a Op. Shop .


Amougst other things . Some people believe a propellor makes a hole in the air , in front of it , that the aeroplane falls into . Apparently this is right , too .

[Istvan Travnik]

Dave,
Here is some photos on the mentioned sanding disk.
Now two of "Old Bisons" are living: Mr. András Meczner (he was multiple world champion with Hungarian team, in freeflight mechanic (F1C) in '60s and '70s,  and finally won in individual, at '81. He is now 84, but he cannot make fine woodwork with his invalide hands.
The other is Mr. Sándor Havrán, he is 78 now, formerly he competed in C/L Aerobatics, and organized many succesful contests. Now he is scoring judge, and sometimes makes 3-blade props, in best quality.
The sanding disk made with wet paper, and after fully drying, you can use it. Note the steel rim around!

[Dave Hull]

Photo 20: Getting real close to final shape with more of the hub roughed in to clear the spinner. Note that the surface finish there is from the wood rasp. On this prop i used a "half round" file and a larger rat tail file both with coarse-cut teeth. On hard beechwood and using a light touch you can control the amount of material removed but it doesn't leave a raggedy surface like you might expect. The status of the shaping is described by two sets of measurements on both blades. The top row shows the blade thickness while the bottom row gives the chord width at the reference stations. You can see I have a balancing armature installed and was at the point of tweaking the blades for balance. I had also rechecked tracking, or how much the path of one blade tip varied from the other one. While you are fabricating a prop if you have a nice reference flat like a saw table, you can simply press the rear hub face down on the table and measure up to the TE at the tip. A feeler gauge stack, gauge pins or something equivalent works for this. You should get the tracking correct while you are creating the back side of the prop but if the wood is not stable, then when you remove a lot from the front it can move on you. This beechwood was KD 12% and sat around (inside) for years after that. By looking at the grain when you select your material you are trying to avoid anything that looks like reaction wood. The grain in this blank was really uniform and in the end it didn't move at all on me.

[Motorman]

the zero-pitching moment line

I call that the axis of the helix. On a standard type prop (rev up, tornado, zinger) it goes straight through the center of the hub but, there are vaiations to give you different shapes.  Al Kelly made a gull wing prop where the axis of the two blades meet above the hub and some of the APC props are true semitar where the axis goes front and back of center. I find that within certain parameters all the different shapes perform about the same, at least in speed events.

MM

[Dave Hull]

Photo 21: When the final shaping, balancing and inspection are completed, it is ready to finish. This prop is a bit thick, and that shows in the weight. I selected raked tips, so I left it a bit bigger in diameter than the originally planned 10-1/2”. That’s always an option if you are making your own parts. It’s a lot more interesting piece of wood with all those shapes than the piece of a board that you start with.  The final step will be to seal the wood. After trying a bunch of different methods, I have decided that I prefer to wipe on two coats of thin CyA. Very little weight gain. It adds to the durability, especially when using softer woods, for example if reworking a Master Airscrew woody. And it is quick. You can reshape a blade, balance the prop, and seal it in very little time. My next favorite method is using KlassKote clear epoxy paint. That takes a lot more time, but the results are both pretty and durable. None of the miracle spray cans have satisfied me, even though there are proponents who swear by them. I mostly swear at them, after the finish gets gummy and I see fuel and oil migrating thru the prop, starting where the finish gave out. Whatever you do, be sure to seal the hole well and don’t take off that finish by running your prop reamer thru it at the field!

Keep your hands and important body parts out of your new propeller and enjoy how it looks and performs.

Happy flying!

[Motorman]

Looks good, better than mass produced props.

[Lauri Malila]

I have tried many kinds of blade tips in order to reduce noise. I don't know why, but the nicest looking swept-back tips had always the biggest tendency to speed up in strong wind. It's weird because usually the lighter load does the opposite. But it probably depends on the engine, too. So, went back to ellipse blades or square tips. L

[M Spencer]

Yea , its come out very nice .

"  but the nicest looking swept-back tips had always the biggest tendency to speed up in strong wind. "

Had a 10 inch prop ( nylon ) on a FSR .25 , when on concrette the wing glued it to the deck for a bit , it flew much better , as it was a 9 1/2 square tip , after it unglued ( same flight ) .
It ended up with rounded L E , as trike gear & grass field wore them round .     

Ive a 13 x 6 I did like this . But its a big heavy clunky wood thick comercial one . I must THIN IT , I guess , the bulk is a bit excessive for a .60 .



You can see the L E reinforcing . emember a aeromodelor comment that Sirotkin must have trouble finding props , as He had repaired the leading edge / tips , on his prop at the Euro or World Champs .

Mayve been He had used harder wood there , or removed dings , from grass & rubble . So I had to try that to , on one . You can see the typical ' hard surface ' on this Aussie Spitfire Prop Tip . The TRAILING edge is rounded , the reinforced L E straight . Guess its so it grabs more air .



Just tripped over this , will throw F & R on , seeing as its pertinant perhaps .



A orstrawlyin 'd say its gotta the Wash'n .  onna the line .