Making a propeller - Print Topic

Having nearly finished the structure of my plane, it's time to make a propeller. True, I could just buy one, but where's the fun in that?

First step is to design the prop. There's lots of data, curves, books etc on the web & in the shops, so no lack of information. But it is generally agreed there is a bit of 'black art' in prop making, given a few basic parameters are met.

Using various - often conflicting - sources, I established the diameter & pitch of the prop. Diameter was easy - as big as I could swing without cutting grass. I've gone for 65", but may have to shave a bit off if that's too big. The pitch I arrived at is 42", assuming a usable 60hp and a top level speed of 85mph (given the horsepower & drag reduction I intend - though this may be optimistic).

Incidentally, I'm using 'pitch' rather than blade angle. Pitch is the distance a propeller would advance through the air with no slip. It is a function of horsepower & airspeed. I have allowed 12% slip since air is not solid - a typical value. The blade angle will change depending on where it is measured along the prop, as the blades are twisted helically.

To establish this twist, I drew the diagram shown top right in the 1st pic. The x axis is the propeller circumference in inches, the y axis is the pitch drawn to scale. The x axis is divided into 10% stations, and these are joined to the top of the pitch line. Thus the blade angle at any given station can be measured.

I drew the blade profile, giving it a fairly generous blade area to absorb the 60+ hp from my MZ202 engine, as the diameter is smaller than I would like, and I didn't want to make a 3 or 4 bladed prop initially at least. This wide blade also allows the blade angles (& hence pitch) to be adjusted slightly in the light of experience.

I then transferred the blade angle at each station onto the profile drawing. The blade section can then be drawn for each station, with the appropriate thickness to chord ratio for that station.

The taper can then be drawn on the side view. This stops at roughly 50% diameter because the prop is not 'deep' enough to allow a truly helical blade all the way to the hub. But fortunately the inner portions aren't doing as much as the outer ones, and hey, life's a compromise.

Sorry my drawings don't photograph too well - they're done in hard pencil so only give poor contrast.

Next step is to cut the blade profile on each laminate. I made a plywood pattern, & drilled a 1/4" dia hole through the centre. I used a 1/4" rod to locate the pattern over the laminate (also drilled in the centre), & rotated the pattern 180 deg to give a symmetrical profile. Then I cut it out using a bandsaw.

This was repeated for all 4 laminates before gluing together.

Then the laminates were glued together, again using the 1/4" rod to keep them aligned. The taper was marked on the side, and sawn to shape.

The 10% stations were marked along the blades, & saw cuts made at the angles found from the drawings. That's the last pic shown in the post above. This was done on the flat 'underside' first, leaving the upper surface uncut.

The wood was now chiselled out between each sawcut, just roughly at first. The rough cut was refined, first with a finer chisel, then a hand plane, and finally coarse sandpaper, to give the flat underside shown.

This process was repeated for the other blade, then for the upper curved surface.

For the upper (forward) surface, the sawcut is to a line tangential to the curve of the airfoil section.

At each station, the angle of the blade is checked against the drawing & its opposite blade, using a sliding bevel square.

The upper airfoil shape can now be formed. I mainly used a spokeshave here. I only used a power disc sander for some initial rough work, as it is too easy to take too much material off, which ruins the prop & wastes all that work.

The leading edge radius was added, and the trailing edge fined down. Both these radii were left slightly larger than ideal, again to allow for 'post flight' adjustments.

Each blade was again checked against the drawing & the other blade, this time using a profile gauge.

The prop was then sanded all over, with 240 grade abrasive.

to balance the prop, I made a simple jig. This is just a wooden channel with two metal straightedges clamped to it, leveled in both planes.
The 1/4" rod is fitted to the centre hole, and the prop should balance level in 4 positions. (unlike the pic, taken before final balance achieved).

I did this at each stage of manufacture, so I would never be wildly out.

For final balancing, careful measurements with the profile gauge are needed, with very small amounts of wood removed. This needs a lot of patience & care!

When the prop was balanced in all 4 positions, and smoothed all over, I gave it a thick coat of WEST epoxy.

I will leave this as is until I drill the prop bolt holes, to guard against dings while I'm doing this. I will make a 1/4" thick steel jig for the bolt hole drilling, again locating it on the central 1/4" hole.

When the holes are drilled, I'll flat down the epoxy finish with fine grade wet & dry, then coat with varnish against u/v damage.

Pics show prop to this stage, which took 37 hours. Last shows it against my new engine, hanging from the hoist while I sort out its installation (another story. . .)

When I am finally happy with the pitch, I will remove the leading edge radius & rebuild it with WEST thickened with colloidal silica. I can then re-shape this to the original profile, & rebalance. That way, I have a nice hard leading edge that doesn't need prop tape.

For anyone wanting to have a go at making their prop, I can highly recommend a little book called 'Propeller Making for the Amateur' by Eric Clutton. (Yes, THE Eric Clutton of 'Fred" fame, for those who know this super little parasol plane). It's a great start point.

Bruce

And to think I was rather proud of my accomplishment by building something that at least looked like an airplane. :-/ Bruce, you never cease to amaze me.

Lynn

I admire you again Bruce. I would not dare to think of doing props on an unknown engine. Two degree here and there and you are way off where you wanted to be. I cant follow any other way today than try ground adjustable composite props.
Excellent job, I wish that the prop you made is the right one for this engine without re-work.
Peter

Thanks guys.

Yes, I'll be lucky to get it right first time. But I have tried to match the prop design carefully to all the variables: horsepower/torque/rpm; airspeed; blade area and diameter. Theoretically it should be close, & I can vary my existing prop by the equivalent of +- about 1.5 degrees.

And if all else fails, I can make another one for about $80 in a week - plus I have a nice ornament with a clock or altimeter in it for my workshop!

Bruce I don't know why this latest effort of your suprised me! I should have expected this from you! As always your work is outstanding. Your skill humbles me!

Quoted from beragoobruce

Thanks guys.

And if all else fails, I can make another one for about $80 in a week - plus I have a nice ornament with a clock or altimeter in it for my workshop!

Bruce for this prop you need to buy one of the clock mechanism with anti -clockwise turning. I did one for a birthday I still have self sticky 3 dials left . 10 cm diameter. Let me know if you need them :) :) :)

Thanks Brian. How's your build - or are you too busy flying?

Peter, those dials are great. If I use one in my prop clock, does that mean I get younger all the time I'm in my shop?

Bruce

Very beautiful work.

Tom

Wow what a great pictorial story on how you have built your propeller. You are a very talented person, thanks very much for the post. :)

What a work Bruce! Your talent and craftsmanship is just amazing.
Before I started to build my plane I downloaded a book on how to build wood props. I just read the thing to have an idea of the process, otherwise I still would working on that and my plane on the ground ;D
I wish you luck with the engine prop-combo. Just make sure your prop-ground clearance is enough for a high tail up landing.

Bruce
Again I am humbled by your craftsmanship.
The only people who are able to create beautiful objects on that skill level are Japanese ZEN masters.
Deep concentration paired with meditation must be the source of all this...
Great & inspiring work.
Cheers
Reto

Quoted from beragoobruce

Thanks Brian. How's your build - or are you too busy flying?

Peter, those dials are great. If I use one in my prop clock, does that mean I get younger all the time I'm in my shop?

Bruce

No, but it reminds you that you should have carved that prop differently way before .... :)

Thank you to all for your compliments. But it really isn't that hard - come on guys, you've built a plane!! I hope my efforts will encourage a few to have a go.

I'm off to visit my son & his family near Perth, Western Australia, for a couple of weeks. So prolly won't be online. I hope you all get some good building & flying in, before your weather gets bad.

Just coming up to summer here!

Bruce

Thanks for your posts Bruce.

Great thread, very educational.

Profile gauge? Now I know more.

Don't it just make you sick?
It just makes me sick.  :B
Not with Bruce, but with myself.  :'(

Thanks for the post, Bruce. The prop looks great and the explanation of how you have gotten this far is great. I'm anxious to  see how it works out.

And when everything is right you plan to remove the leading edge radius and remake it. I would think I was God's gift to wood if I could get the leading edge right one time, never mind redoing it.

I love to see what a real craftsman can do with any material.

Thanks again for taking the time to make your posts.

Quoted from beragoobruce

Thanks Brian. How's your build - or are you too busy flying?

Peter

Bruce

Have been busy at both! I have over 15 hours flying and LOVE IT! Moving forward very well with the Vmax! Also weight loss continues! Will be updating my build log in a day or so! I do not want to hijack your thread!
Back to the prop.. Do you feel static balancing is sufficient? Reason I ask is I have a prop for my Vmax and would like to refinish it. I know how to static balance. And could easily make a static style balancer. I would rather do it than send it out to someone else.

Hey Brian

Great that your build, weight loss, AND flying programs are all on track. Good on yer!

As regards static balancing of your prop, that's all I've ever done. I've never had the opportunity to use a dynamic balancer.

When I do a static balance, I check for balance in 4 positions. After the first position, I rotate the prop about its long axis, & check again. Then rotate it about the 'engine' axis, check balance, & finally rotate about the long axis for a final check.

This method ensures you have checked for 'sideways' balance (across the width of the blades) as well as blade-to-blade balance. and of course the 2 straight edges have to be exactly level in both axes - sideways and fore & aft.

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