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Airplane propeller rework

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Project by MalcolmLaurel posted 03-11-2020 01:07 AM 1058 views 1 time favorited 14 comments Add to Favorites Watch

Been a while since I’ve been here, too many other projects to do much woodworking, but some might find this interesting.

I’ve owned my Hatz biplane for about a year and a half now. It originally came with a metal propeller, but it wasn’t quite matched to the plane and for various reasons I wanted a wood prop so I bought one, custom made to my specs. The new prop is birch with a polyurethane leading edge for abrasion protection. After testing, it still wasn’t quite right… it needed to have the pitch (the theoretical distance it moves forward in one revolution, 47” in this case) reduced, kind of like downshifting a car to a lower gear. It was better, but still not quite right, so I decided to make the final adjustment(s) myself, bringing the pitch down to 44”.

A metal prop can be adjusted simply by twisting it, but for a wood prop the pitch can be reduced (but not increased) by shaving a bit of material off the back face trailing edge of the blades. The leading edge and front face are untouched. It’s a tiny amount, which varies by where you are on the blade, from .014” at the tip to .075” 12” from the hub… and it needs to be accurate. Of course I practiced on a scrap pine board first!

The modification starts with precise saw cuts 3” apart to establish the depth. I used one of Dad’s old Zona saws, to which I clamped an aluminum strip, which I adjusted with a dial caliper to set how far the teeth protruded. I then used an orbital sander to sand the back face down until the cuts disappeared. An orbital sander really isn’t made for heavy material removal so it’s slow, but it makes it less likely to accidentally take off too much. Thinning the blade leaves the trailing edge too sharp, so I rounded it back with a scraper, in the process making the blade slightly narrower.

After final sanding, 5 coats of rattle can polyurethane, checking the balance and adjusting it with a bit more varnish on the light side, then I put it back on the plane for further testing. It’s considerably improved, it turns about 75 rpm faster and the plane performs better. It’s now “good enough” but there may be room for further improvement, I need to do more testing first. If I do rework it again I’ll likely trim a bit from the diameter to get the rpms up rather than reducing the pitch any more.

-- Malcolm Laurel - http://MalcolmLaurel.com





14 comments so far

View Chris Cook's profile

Chris Cook

336 posts in 3164 days


#1 posted 03-11-2020 01:38 AM

That’s impressive! Nice work!

-- Chris, "all we are is sawdust in the dust collector""

View anthm27's profile

anthm27

1766 posts in 1993 days


#2 posted 03-11-2020 02:08 AM

Your prop tuning seemed incredible to me, Truly awesome.
Great work
Kind Regards
Anthony
(sent you a PM)

-- There is no hope for any of us if we keep apologizing for telling the truth.

View Wintergreen78's profile

Wintergreen78

96 posts in 623 days


#3 posted 03-11-2020 02:11 AM

Nice! That looks like a high-consequence modification, I bet the prop wasn’t cheap.

View LeeRoyMan's profile (online now)

LeeRoyMan

1422 posts in 610 days


#4 posted 03-11-2020 02:14 AM

Good thinking, Nice and methodical, I like it.

View ralbuck's profile

ralbuck

6598 posts in 3150 days


#5 posted 03-11-2020 04:07 AM

Looks great!

I would never have touched it myself thouugh! I am not even close to being accurate enough!
Looks like a sweet little airplane too!

-- Wood rescue is good for the environment and me! just rjR

View JimYoung's profile

JimYoung

390 posts in 2470 days


#6 posted 03-11-2020 12:50 PM

I’m impressed with the skill that you approached this with. I’ve adjusted and modified R/C airplane props, but not with this precision. I’ve used the same “a little more finish on the light side” technique to balance them.

Are there any FAA considerations in modifying a prop, or is this covered under the experimental category?

-- -Jim, "Nothing says poor craftsmanship more than wrinkles in your duck tape"

View TDominy's profile

TDominy

179 posts in 3425 days


#7 posted 03-11-2020 12:51 PM

Having worked in the aerospace industry for over 40 years, starting as a tool and die maker, retiring as a tech specialist for our suppliers, I understand how difficult this can be. I have seen props and helicopter rotors being balanced this is no small feat for the DIYer.

Get yourself your favorite beverage and toast yourself. Nice job.

-- By hammer in hand, all things do stand.

View ChuckV's profile

ChuckV

3333 posts in 4411 days


#8 posted 03-11-2020 01:04 PM

That is impressive! I once repaired an antique chair and was concerned about my work suspending a person 18 inches above the floor.

Your skill and confidence really show here.

-- “Big man, pig man, ha ha, charade you are.” ― R. Waters

View GR8HUNTER's profile

GR8HUNTER

7923 posts in 1596 days


#9 posted 03-11-2020 01:48 PM

a very kewl project … nice it worked out for you …. GREAT JOB :<)) GRATZ TOP 3

-- Tony---- Reinholds,Pa.------ REMEMBER TO ALWAYS HAVE FUN :<))

View sras's profile

sras

5594 posts in 4013 days


#10 posted 03-11-2020 02:27 PM

Impressive work! Did you get the results you were hoping for?

-- Steve - Impatience is Expensive

View MalcolmLaurel's profile

MalcolmLaurel

303 posts in 2506 days


#11 posted 03-11-2020 04:58 PM

Thanks all… it was intimidating at first but the actual work wasn’t that difficult. Just a deep breath before making that first cut in a perfectly good prop…

Wintergreen, the prop was $615, relatively cheap because it’s experimental. For a certified (factory built) plane it would be more than 4X that much.

Jim, yes, because the plane is registered experimental I can do pretty much anything I want to it. Changing the prop is considered a “major change”, however, so I have to notify the FAA and make test flights in a restricted area (25 miles from my home field) for the first 5 hours.

ChuckV, not a real risk… the amount of material I took off isn’t enough to affect its strength. If there was a significant performance hit it would be obvious while rolling down the runway on takeoff with plenty of time to abort.

Steve, I didn’t get quite the results I was looking for, though it’s definitely a significant improvement. It’s still a bit shy of the metal prop performance, but it’s usually said a wood prop won’t be quite as efficient because it can’t be made as thin as a metal prop. But on the plus side, wood is much lighter (always important in a plane), it damps vibration better, it breaks and protects the engine in the event of a prop strike, and just looks better on a classic biplane.

-- Malcolm Laurel - http://MalcolmLaurel.com

View swirt's profile

swirt

5573 posts in 3855 days


#12 posted 03-12-2020 01:40 AM

So glad you included the diagram to show the actual change. Cool that you did it with a zona saw.

-- Galootish log blog, http://www.timberframe-tools.com

View richarddorr's profile

richarddorr

5 posts in 226 days


#13 posted 03-14-2020 05:17 PM

Nice job and a beautiful airplane.

View PPK's profile

PPK

1802 posts in 1693 days


#14 posted 04-16-2020 06:21 PM

That is super cool. I’ve always looked at old wooden props with curiosity and wondered how they were made. I’ve always thought that there would have to be a lot of accuracy and balancing and true knowledge of wood characteristics that went into building one. Thank you for sharing.

-- Pete

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