Wooden Propellers

[gdw]

Hi all, I have a wooden propeller on a Lyc O-320 which can do with some refinishing and balancing. Can anyone let me know where I might go to have the work done, or at to least seek advice. I'm based in NSW.
Thanks

[dubbleyew eight]

I'm sure this will give those in industry yet another opportunity to jump on me...
I've been refinishing my wooden prop once or twice a year for the past decade.
previous owners removed the brass leading edge protection and sheathed the blades in fibreglass to render them resistant to light rain.
the sheathing worked but the absence of a metal leading edge has meant that any appreciable moisture in the air erodes the paint at the leading edge.
so...
when the leading edge paint gets eroded I get sick of the 5 knots knocked off the cruising speed that this causes and off comes the prop for a refinish and rebalance.

I'm assuming you have a prop that was competently manufactured.
this means that the blade geometry will be symmetrical.
if you rub back the finish confine the rubbing back to the varnishes used on the prop. do not erode the actual wood. this will keep the original blade geometry intact.

on my prop I have realised that the leading edge paint will always be subject to erosion. even using two pack polyurethanes will not prevent the erosion.
so I have taken another tack on my finishes. I use acrylic enamels that can be wiped off with a bit of elbow grease and either acetone or MEK.
when the prop needs refinishing I simply wipe off the damaged paint back to a level and rebuild the paint layers back up to the desired finish.

balancing the prop is quite easy.
I use a half inch diameter shaft with two machined aluminium washers that have tapers that can press into the central prop hole and hold the prop absolutely concentric to the shaft.
two six inch wide boards are placed on my absolutely level dining room table.
all air movement in the house is suppressed.
the prop on its balancer is placed on the boards and allowed to rotate.
I balance my prop by adjusting the paint.
typically I spray another coat of matt black on the back of a light blade.
I check the balance both ways on the jig and when the prop can be placed either way without any tendency to rotate it is balanced.
this sounds crude but a post it note stuck on a blade will rotate it easily.

the take home message is that you should never neglect your wooden prop.
if you can't find a commercial prop shop that will do the job for you
you can do it quite competently yourself.
W8

[jas24zzk]

I'm not going to jump on you, as I don't know enough.
Just raising questions based on limited experience, in the hope the more learned will offer up the truth.

What I do know, is that the Paint will begin eroding the moment you first start the engine, thus ruining your fine balance with continued use.

I would think that you would be better off balancing to that accuracy, and then painting and ignoring the imbalance the paint brings in.

A knowledge of the permitted imbalance error would be helpfull....

I base that on, what a coat of paint actually weighs, and seeing how much material I've seen engineers remove dressing aluminium props without a rebalance.

I'm probably wrong, but without further education, it sounds like you are going a little overboard

Cheers
Jas

[Squawk7700]

Jas, you balance the prop by using the weight of the paint to your advantage... W8's procedure is bog standard for a timber prop except for the acrylic part.

[dubbleyew eight]

What I do know, is that the Paint will begin eroding the moment you first start the engine, thus ruining your fine balance with continued use.

I would think that you would be better off balancing to that accuracy, and then painting and ignoring the imbalance the paint brings in.

A knowledge of the permitted imbalance error would be helpfull....

ok first correction. the paint doesn't erode at all in flight unless something impacts it. dry air does not erode paint at all.
water looks like a splashy soft liquid but at high speed it is as solid as concrete.
at propeller flying speeds a cloud of mist is as effective a paint remover as bead blasting.

second correction. there really is no allowable imbalance for a propeller.
the propeller must be in perfect balance for vibration free flying.
if you do fly with an out of balance prop every part of the aircraft will be subject to relentless vibration.

as an illustration I went flying with a friend in his aircraft. the prop balance was so bad that the vibration coming through the rudder pedals was painful at first and eventually sent your foot to sleep. we flew by alternating in sacrificing our feet to the pedals. they really were painful.
I made a rude comment about not wiring his toolbox to one blade of the prop in future since it threw the balance out.
I don't think he'd ever realised that his prop was out of balance, thinking that vintage aircraft were like this.
after he had the prop balanced all the vibration vanished and the aircraft was a pleasant long distance flier.

keep your prop in balance. you will increase the aircraft's fatigue life measurably.

[ELaw]

second correction. there really is no allowable imbalance for a propeller.
the propeller must be in perfect balance for vibration free flying.
if you do fly with an out of balance prop every part of the aircraft will be subject to relentless vibration.
Um... that's ridiculous. If after a flight, you notice a dead gnat stuck to the hub of your prop, are you going to ground the airplane and send it for prop balancing? Not likely. OTOH if you notice a dead raccoon impaled on one of the blade tips, you probably ought to get it checked out. ;)

From the webpage of a company that balances props (http://www.expaircraft.com/PropBalance.htm (http://www.expaircraft.com/PropBalance.htm)):
"We typically see beginning vibration levels from 0.25 - 0.60 IPS (Inches Per Second) or more, anything above 0.20 IPS is considered out of limits and requires balancing. The industry standard after balance limit is 0.15 IPS, this where your average balance shop will say it's "good enough". We guarantee a level of 0.07 IPS or less after balancing, less than half the industry accepted after balance limit!"

Note that they do *not* state the limit, even their "stricter than standard" limit, is zero.

[gerry111]

dubbleyew eight wrote:


".. are placed on my absolutely level dining room table."


And: "typically I spray another coat of matt black on the back of a light blade."


I reckon that W8 lives alone. :)

[yr right]

Mmm perfect balance I think not. So what ips makes a perfect balance. What position of the prop to the firing of the cly to crank angle. Etc etc

[gdw]

Thanks for the advice W8, I'll give it a go.

[djpil]

Good stuff, w8.
I have seen my prop mechanic use a squirt of matt black to finish off the static balance of a metal Sensenich prop - then we did a dynamic balance with weights added to the spinner.
On my homebuilt I had a wooden Sensenich with no metal on the leading edge and later replaced it with a Hoffman. Both responded well to regular TLC.

[Dora-9]

W8:

ok first correction. the paint doesn't erode at all in flight unless something impacts it. dry air does not erode paint at all.Technically correct I guess. I had (still have, but now it's discarded on my hangar shelf) a Hoffmann HO21-198 wooden composite prop which would shed paint whenever I went flying, let alone flying in rain. Yes, I operate off a grass-over-gravel strip, but the erosion was far, far worse than it is with my Fairy-Reed (metal) prop.

[Squawk7700]

Partially off topic: I had a dynamic balance done on my timber MT prop once.... The LAME forgot to fully screw in a spinner screw which departed on the 10 minute flight home. Unbelievably the screw took out a huge section of the prop trailing edge, around 20 cm (along the prop) x 4 cm (into the prop from the trailing edge). To say I was mildly peeved was a bit of an understatement.

[dubbleyew eight]

Um... that's ridiculous. If after a flight, you notice a dead gnat stuck to the hub of your prop, are you going to ground the airplane and send it for prop balancing? Not likely. OTOH if you notice a dead raccoon impaled on one of the blade tips, you probably ought to get it checked out.

live in the real world will you!

if the prop in action isn't causing vibration then its balance is close enough.

yr right mentions a number of factors. like any engineering the answer is as good as the assumptions.
this was a question about maintaining a WOODEN propeller that has been in service on an aircraft.
the assumptions are that it is competently manufactured, i.e. both blades have the same pitch distribution, aerofoil distribution and blade area distribution.
that it has been properly set up on the aircraft and has now deteriorated.

I suppose that I should state the obvious. that the spinner is balanced.
that the spinner attach screws are symmetrically disposed about the backplate and are the same weight. the lockwire is close enough in balance that it isn't throwing the balance out. the propeller attach bolts are the same weight, i.e. the same length and that the number of belleville washers under each bolt is the same and that the crush plate is perfectly symmetrical and balanced.

if after all that your prop is still causing vibration then a dynamic propeller balance is the next option. Jimmy Williams in Albury does a good job of that.

mine, maintained as I have described, flies vibration free.

btw you guys in the SAAA should ask to see Shirley the redheads balancer.
I machined it for her.

resume own navigation.
your mileage may vary. :-)

[jas24zzk]

Very informative so far...except for the OP...no one has answered his valid question.


-------------------------------
I disagree with you DW8. You don't need moisture to cause erosion of a props surface. Even in a dry atmosphere, there are always airborne impurities to erode the surfaces...I'm pretty sure you would agree with that. The point you make tho, that water is really destructive, is true....we all know that water is the only element that cannot be compressed.

I also disagree with the remark that "water looks like a splashy soft liquid but at high speed it is as solid as concrete." As a keen water skier, I will tell you that the concrete is softer!!!! :}
_____________________
Elaws post is interesting because it puts up some numbers to what is an acceptable balance.....from reading that (and not exploring the link at this time), there IS an acceptable out of balance parameter.
It might be that these parameters are merely for that maintenance company, but surely the manufacturers also have published limits.

It doesn't matter what the prop is made of, you spin it, it will erode. Balance a prop and only fly it in ideal conditions...no nicks for dressing etc..i.e perfect world.......I bet it doesn't balance the same after 500 hours.

Learning lots here :ok:

[Squawk7700]

I'm interested in hearing more about airborne impurities.

[dubbleyew eight]

Very informative so far...except for the OP...no one has answered his valid question.


make a phone call to East Coast Propellers at Bankstown Airport.
if they don't touch wooden props I'm sure they will know who does.

I googled "propeller shops bankstown"

[Squawk7700]

+1 for Eric at Aust Air Props Bankstown as above. He did an exceptional job on the reco of my MT as a result of the incident I posted above.

[Jetjr]

Whats wrong with dynamically balancing?
Your balancing whole rotating mass too
Gear is out there and will get far more accurate results

[Andy_RR]

How about balancing the lift and drag coefficient for each blade too...?

[yr right]

Full electronic balancing of a prop is one of the best things you can do for your aircraft. Not only dose it smoth out the engine but also helps to stop vibration damage to your airframe. As I've said in earlier post hymonics destroy airframes.
We do this on turbines as part of normal maintenance and we suggest it to all our clients. Cheap and effective practice.

[Jenna Talia]

How often should this be done?

[yr right]

Generally any time you change or remove a prop or engine change or as the manufacturer recommend time. Also when changing some components like a starter generator

[yr right]

Generally any time you change or remove a prop or engine change or as the manufacturer recommend time. Also when changing some components like a starter generator