Re-Drilling a Wooden Propeller

Why would you buy a prop that doesn't fit?

If there are no FAR's that cover this, I'd rotate things 30°. I wouldn't do it in the first place, actually.

Is there an FAR for plugging a wooden prop?

Something about this scares me, but, it's your neck.

P-51 crashed into the stands at Reno today. Many dead and wounded. Probably wasn't the prop.

Good luck.

Thats the annoying part - it was supposed to fit. However, I'm too impatient to send it back to Thailand where it was made

My primary prop is a two-piece carbon one, but I was curious how this one would perform, and figured I would keep it as a back-up

Being as paramotors fall under FAR 103 - they really aren't regulated much

Im really not concerned about it being a safety issue - as the prop is built way over-kill for the power of my little 185cc backpack, and after i fill the holes, the strength wont likely be altered much.

It was more of a curiosity as to what people would recommend than anything.

Thanks

The six holes that are already drilled are positioned to allow the maximum continuation of the grain of the wood through the hub and disturbs only three hole diameters of hub width, roughly 1/4 of the width of the hub. Plugging the holes will not restore the grain and rotating the pattern and drilling six more holes will further interrupt the grain. Less than half the width of the hubs continuous grain will be left intact. If you had a brand new prop with a hub that was half the width of the one you have, would it be strong enough for your 185cc engine and the centrifugal force, strain and vibration produced? Go with your answer.

If there is a difference in hardness between the plugs and the hub, the new holes may tend to wander off center while drilling. I think the 30° rotation is better.

I agree with Tornado. Something like a forstner bit might minimize any tendency to wander, but why take the risk.

Drilling at 30deg seems like the better option but there may be other consequences or considerations.

Don't one determine top dead center relative to the stance of the prop? or would like to have the prop at rest in a certain place?

Plugging the holes with oversize plugs will add some stress.

Drilling at 30 deg will increase the number of weaker lines across the grain.

By drilling the holes at 30deg you would not be able to use your machine gun and that may be a negative.

I think a bit of patience should be practiced. (and make certain of the quality)

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Better a sitting duck than a dead duck - Hendrik

Rotating 15° might be even better than 30°; that way no two holes would lie along the same grain lines in the wood.

BTW, that's a beautiful looking prop. Maybe a new hobby for Del?

I like the idea. Looking at most other wood props, it seems that they are almost all drilled with the holes like the original holes in picture, but I'm not certain why.

Flying a paramotor, or making props? I'd recommend the former :) much more enjoyable

i would never try to put a band-aide on this. what the hell where you thinking of when you sent it to a third world nation to have it repaired?

hmmm, well first off - I never sent it anywhere to be repaired. I bought it new from a guy in Thailand who machines them on a pattern router (the same way wooden props are made most anywhere else) and then sands and hand finishes them to an incredible finish. Its a well made prop, no matter who made it and where they were located. The fact that it came with the wrong hole pattern is a bit frustrating, but really not that big of a deal. So as much as I appreciate your anonymous concern.....I think that I will kindly disregard it

Why not talk to the pros ? Wings of History museum has the only FAA repair shop on the west coast for wooden props. Drop them a line. It may take a few days for a response. They are 20 miles south of San Jose, CA.

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nice information

i like it

I'd be concerned about doing this Band-Aid approach, for two primary reasons:

1. You may induce increased stresses into the prop wood, even if you plug the original holes.

2. You must be absolutely "dead-on" when you drill the new holes and fill the original holes, otherwise the prop will NOT be ABSOLUTELY balanced and therefore you may induce severe vibrations when it is turning under power. The torsional stresses may become too high for the prop material to withstand.

I'd be very wary about a DIY approach unless you have the proper equipment to statically and dynamically balance the propeller. My advice to you is to hire the services of a properly equipped (and FAA certified) machine shop or aviation repair shop (w/ FAA certified mechanic) can do a stellar job fixing this screwed-pooch of a prop!

Good luck......keep 'em flying!

I think you should be fine, however, once you finish, I would spend the extra bucks to have it tested and balanced................on the ground.

Rotate 30 degrees and use either a mill to drill or a Morse taper drill rather than a Jacobs chuck. Also do drill the other holes; if needed, so a proper fit of dowels can be allowed or fill resin instead.

What about using an adapter plart? Put the propeller bolts through then bolt it to the engine, set the prop and fasten with locking nuts & safety wire. Drew K

I agree with Drew.An adapter makes more sense than risking an ad-hoc engineering fix on a critical part such as a prop.

As an engineer friend once told me:"You packed that chute, you jump with it."

Re the adapter plate:

Off set the holes on the adapter plate 30 degrees from the prop holes, and countersink and use flat head screws with nylok inserts with locktite red to attach to output flange.

This will allow plenty of room between holes for strength.(I am thinking 1/2" thick aluminum stock for the blank.)

Have a machine shop make the adapter plate for you, taking both the old and new prop for them to measure.You don't want an imbalance in this situation.It is very hard to get 8 holes exactly right without a milling machine.

I would go to a 45 deg, and redrill the holes , to fill the holes you can sand down some wood and then take the saw dust and add some elmers glue, mix it up. To save on the glue, you then take some old news paper and fill the holes half way and then fill the rest with the Home made glue that you just made. Let that harden up and then RE mark to a 27 deg off set and drill another set of holes. this way you will have enough holes that any way you put it , it will / or should fit. Tighten the bolts, then have a few Beers [i like bud] and when you get a buzzzz on , start it up and take her for a ride.If you are worried, have a few more beers untill you feel brave enough to take off.

have fun. you will be Flyin High ..any way you look at it.

No adapter/spacer. It moves the prop away from the bearings and will put more load on everything that rotates, making proper balance even more critical. It's another chance for failure.

Better to plug and re-drill.

Just call me Mr. Negative.

i agree with lyn here as the thickness of an adapter plate will offset the rotating load and increase the balance error accordingly. centering is the most important issue and rotating 15 degrees won't cut thru as many grain lines if they are in line with the face picture as they go thru their depth. as usual this can be done at many levels of accuracy by you or a machinist to achieve the best balance possible. as far as plugging those holes i would use epoxy as it does not strain a glue joint like any water based glue will so you will have a relaxed bonding at those holes

Lyn, I have to disagree with you this time. I know thw offset will multiply the force, but the bearings should have a factor of safety capable of handling this load. Load on his prop is uneven anyway because it is a backpack design. His body will block the prop unevenly and cause uneven loading so it is designed to endure the loads. I do wonder about the weight comparison between this wooden prop and the original carbon one. Drew

Drew,

I'm sure you're right about the over design of the the bearings.

Let's look at as if it were an electric motor with a service factor of 1.25. Sure, you can use the service factor in your design, but the added load will shorten the service life of the motor.

The ultimate decision rests with the OP. Me, I'd err on the side of caution.

Cheers.

I suppose somebody should do an analysis to determine the new endurance of the bearings considering the added weight (if any) of the wooden prop, consideration could be given to the adapter plate. Given the information about the crush plate, I would use a heavy plate and adapt it instead of removing more wood to make it fit. The adapter plate may not be an option anyway if the hub cannot be set up for the plate thickness, I doubt the op will cut or grind the studs down. Drew

You're right about no homework. I'm glad that you have put us all straight.

All the airplanes I've ever flown have had the engine/prop in front of the pilot and attached securely to the airframe.

GA from me, Jaguar. Even though his craft may not be regulated by the FAA, I would recommend using the FAA's AC 43.13 criteria for doing any repairs on this prop. The practice's that are out lined in AC 43.13 comes from years of experience dealing with failed attempts and what actually works for repairing aircraft and their appliances. DJ

If band aid consider this:

Aluminum plates on both sides. Maybe 5mm thick? Any machine shop would be able to make them in no time at all. Maybe have spikes on the timber facing sides. Better than bought .

Just blame me if you come down and survive.

Have one on me, Ky.

He already has the original crush plate for the front of the prop and a flanged adapter in the back that supplies the power. He only needs to match their bolt pattern?

Why would he need two more plates?

Laughing Jaguar

"Why would he need two more plates?"

To stabilize the grain? What difference does it make if you have added "spikes in timber" to control any shear? It would add 10mm or less to the sandwich, that's all. At least now I know what a crush plate is and that if it does what my plates would do you are right, they would be like ash trays on a bike.

Me talking about propellers, ridiculous, I know, Ky.

I wouldn't do it. Drilling new holes will lessen the structural integrity of the hub. It may be strong enough or it may not. A thrown blade in never fun. Especially if you are sitting in front of it at the time!

This prop must have been set up on a suitable mandrel when tested for dynamic balance by the Thai manufacturer. The mandrel would also have conformed to a recognized standard Boss/crushplate pattern. Would it not be safer to contact the supplier & acquire a crushplate for this prop compatible with your engine's output shaft. Good luck & happy landings!

Either the template holes or the original holes are not symmetrically placed, or so it appears with template overlaid, as viewed in the first photo. I guess the template was roughly made just for the demo.

have wooden dowels made at your local cabinet shop. they should be slightly oversized in diameter. i'ld recommend hickory or oak, but ask the cabinet shop what they'ld recommend. soak dowels in water for three days or until they're soft and swelled. apply "gorrilla" wood glue to the the dowels and drive them through the holes leaving a 1/4" extending on both sides. set prop in dry place for approx. a week or until they're completely dry. sand dowels flush with prop. cover them with a water proof sealer.

determine propeller postion in relationship to "TOP DEAD CENTER" on the compression stroke. use a dial indicater to get an exact measurement. take echings of the output shaft, and haul everything to your favorite machine shop and watch them laugh there asses off while you tell them what your planning on doing. wished i could be there sounds like a hoot.

Plan A is what happens if this arrangement does not fall apart in the air. What of plan B, in which the propeller falls apart and the paramotor is converted to a paraglider with a rather poor glideslope? Trying to find some figures, I came across
http://www.thepico.com.au/pico/images/b/bb/Drag_reduction_of_Paramotor.pdf

which suggests a lowest sink rate of about 1.2 m/s at a forward airspeed of about 38 km/h. Even allowing for flaring to bleed off forward speed and for a good headwind, this suggests that you are going to hit the ground running, literally.
My piloting days were limited to a few hours in gliders, so I am quite happy with the idea of no engine, but I am even happier with the idea of not having to put your feet down. What do you get taught for engine-out landings?

Of course, if one blade broke off, the wobble might addle you to the point where you would not guide well, even if you turned off quickly.

There are many people killed in ultrailghts of various sorts every year, is prop breakage a from modification a significant problem? A google search show these.

ultralight prop modification

prop failures in ultralights

I believe that use of a good grade bolt would allow you to attach the propeller with a single fastener. This would secure it as needed and have plenty of strength to prevent it becoming dislodged from your fireplace mantel.

Write the vendor and show him the mismatch, and request the shipment of another with the correct spacing on the holes, keeping the first one as a momento as the postage is not worth returning it. You will probably be able to keep the first and get a safe spare replacement prop. Woody No ask No get.

Rotating 30 degrees is self evidently the better option.
BUT... the only sensible option it to have it done by a qualifed specialist.
If it was blowing air around in a barn to dry hay, I would just go for it, but if your life depends on it then put your hand deep into your pocket, swallow hard and learn the lesson.
I'm a fully qualified cheapskate, but I just wouldn't risk it, even with your life.
Oh and if you do go ahead, then plug the holes with the same type of wood as the prop, and don't 'pound' them in, they should be a light press fit (IMO).
Del

"I was pleased to realize that I knew exactly what to say.So I said nothing."

Mark Twain.

"Sometimes I just have to ask the dumb question..."

It's not dumb at all and something I should have considered.

The limiting prop rpm is the tip speed. If the tips of the prop are moving into the supersonic speed, power is lost and the racket made is unbearable.

Because these are often small displacement engines, they do not put out enough power at crankshaft speeds suitable for the prop so some of those engines, like the popular Rotax, used small gear gear reduction box to allow the engine to spin faster while the prop rpm stays in the effective RPM range.

There are lots of other factors influencing this but it appears likely that a flanged hub with a different bolt pattern circle might be available.

Removing and replacing the hub could be labor intensive but that's another conversation.

Your intuitive nature serves you well

Laughing Jaguar

Thanks Laughing Jaguar - you pretty much hit everything right. A second reduction pulley for this motor would actually cost more than buying another prop (not to mention the time and annoyance in switching them back and fourth)

This is what the reduction pulley looks like FYI:

and with a carbon prop:

'

The prop and pulley lock co-axially via a ~1" dia central boss. This is what keeps the prop centered - the bolts really just provide the compression (not the alignment)

I epoxied in properly fitting dowels and gave it a couple days to fully cure - Tonight I turned an adapter to center the outer plate on the prop and used the plate as a drill guide.

Here is what it looks like before I re-cleared it:

Thanks for all the feedback - as always I am amazed by the number of people so willing to share their opinions. If I get out flying this weekend, I'll post some videos for anyone interested.

My prayers go with you.

Huh? The metal plate shown most closely matches the filled-in holes, not the new ones.

Have a closer look at what is foggy plastic template and what is 'existing holes'.

But for what it's worth - I think most posters read it 'opposite'.

Hi

Why dont you buy another certified coupling that will fit your motor shaft and has the correct stud patern for the new prop.

I think this would be better than drilling new holes.

Cheers

Joe

Sure a lot of Holes. I know if it was me, and I take ALOT of chances , I would never use this prop.Dont for get, its made out of Wood. Spend a few bucks and you won't have to worry while you are up their, and worry about people on the ground when you crash.

Good luck, and you will need it.I think many of your friends will also tell you that you are foolish, or stupid.Sorry to be sooooo blunt.

I agree with you conntaxman, I sure hate to think that a departed propeller blade gets caught up in this guy's para-sail rigging and kills someone one the ground. At least I won't have it on my conscious if he crashes his powered para-sail because improper repairs done on this propeller. Just because this craft isn't govern by Fed's, doesn't mean that this propeller isn't govern by the same laws of physics as any other propeller. It's subjected to the same stress and aerodynamic loads forces as any other propeller. I just hope his liability insurance is paid up and current.

Having watched this discussion for some time; and considering the number of comments based in the perception the new holes would be a larger PCD, rather than smaller, no crush plate was to be employed and so on; I think much of this has distracted from the perhaps greater 'why not' of this substitution.

One reason for the paramotor props being extreme light weight, is the gyroscopic reactions. You may find the heavier prop converts directional and power change into body pitch motion, and vice versa.

Perhaps not such a great idea to go heavier when suspended on strings, without benefit of a tail plane. Do so with caution.

2 ¢

You too, can be nominated for "The Darwin's Award". I wish you the best of luck and safest of flights.

That's about what I expected. You didn't disappoint me a bit - Thanks :)

Not all of are against your innovation, we just advise caution because you are playing with your life...and I am a bit jealous at your avian abilities.

Drew K

Nice video

"(in certain regards)" please elaborate

The reasons aren't really material related

- the wood prop has a better pitch for my motor setup and produces greater thrust

- the wood prop is thicker and flexes less - making it easier to keep the shroud size down

- the wood prop isn't hollow and cant collect water - The carbon props are two piece designs and around the joint there are openings that expose the hollow cavity in the lamination. If water gets in the cavity and you spin it - it destroys the prop

- The wood props can be re-drilled if the bolt pattern is wrong ;p

I certainly have nothing against carbon, and if there were a higher demand for paramotors - I might design and build a better carbon prop for them, but for right now the fact that you can buy 3 wood props for the price of one carbon - makes the wood very appealing

Ha! ;p yourself. That wasn't my issue, :-)

No, seriously, any noticeable problem with direction change or power change (increased gyroscopic effects)?

And just out of interest, how does a carbon prop get water in it?

So far no noticeable change - Paramotors in general have a decent amount of torque steer - and you learn to compensate for it pretty quickly. I saw a guy who built a co-ax counter rotating prop setup to eliminate torque steer, but it adds a good amount of cost and complexity - not to mention the power loss.

When I go places with my setup, it sits on a hitch mount on the back of my car. I leave it uncovered to cut down on the wind resistance, and on one trip it was raining for part of the drive. Enough water got in the hollow cavity that when I ran it, the lamination split, and there was a puff of moisture out the tips of the blades. Needless to say, I now take the carbon prop off if there is any chance of it getting wet

Rather than contra rotation, which would still have the intake masking, has anyone tried two smaller counter rotating props each side of the pilot?

Possibly ducted, but what you save in cage and prop mass, would probably pay for a belt drive. Without the masking, bar the occasional elbow, it should give improved thrust and less torque reaction.

2¢

I have heard a lot of people talk about it - but haven't seen anyone build or test anything like it -

I saw this thread a couple years back on the topic:

http://avcom.co.za/phpBB3/viewtopic.php?f=16&t=49167

It reminds me a bit of the Martin Jetpack turned sideways - I read a while back that he began with counter rotating blades, but eventually found he could overcome the torque with proper ducting - I believe his most recent version has the two fans spinning in the same direction to save the weight of the gearbox

There is also this:

http://www.youtube.com/watch?v=EpfxULVP3tY

Its two Jet-cat engines under the seat of a paragliding harness - only downside is the cost of the jet engines and fuel usage / short flight duration

The 'ducted' thead looks like a photoshop exercise ...

But the jet: I want one!

Yes, photoshop no doubt- and I don't know of anyone who has taken the concept beyond that - Not sure of a simple way to drive the counter rotating props either, or if fins in the duct would be sufficient to counter the torque of two spinning in the same direction. Any thoughts?

The one other approach that is of some relevance is the flycocoon -attempting to streamline the pilot and improve the intake flow- its a bit big an hokey though.

The jets are awesome - I think they are the same units that Yves Rossi uses on his wing.

Seeing as you lack a rudder, I'd go one double sided toothed belt and the appropriate idlers to get a 'reversed wrap' on one prop. 'One fail - all fail', not 'spin me up Scotty'

Alternately blend the ducts to one central 'nozzle' on the center line - but that is more loss and weight.

Not impressed with the flycoon - thrust is too far back from the center of resistance - unstable - particularly on takeoff with legs down, max power and no airspeed.

Duct blend is likely to be the same - 'too far back'.

I feel like I'm designing a cassette tape:

It would sure be nice if you could just put a twist in it:

It would, and you could, if you had the distance

Bear in mind the small props turn faster

Also, you only need 10 teeth, or 120⁰ [whichever is greater] for 'full strength' on toothed belts

Awesome!!!!!!!

Man that looks fun. I don't know if I'd have the stones to do it though. My palms are sweating just from watching the video.

Nice job.

Brave.

It's a bird! It's a plane! It's Superman!... It's Jason!

Nice job, and great video.

WOW! What 'Nads, you had, Grandpa!

I hope this wasn't Jason S, " An investigator from the Federal Aviation Administration will be in Rogersville Tuesday to look over what's left of a powered parachute that crashed Christmas Day." As reported from KTTS News.

Wasn't me- but thanks for the concern.

I fly a powered paraglider - not a powered parachute (I know it sounds like a minor difference, but there are actually significant differences between the two)

I'm glad that wasn't you and I hope that your prop is still working out for you. DJ

I know this is an old post,

but failure right here is imminent

Might be why we haven't heard from him for a while? There's reg's for a reason. Unless you have a death with

Building Experimental aircraft, the lack of regulations can be a death wish.

Hey guys,

Here are a few more details on how this finished up. As you can see in the first attached image below, I plugged the holes and drilled a new set 30º rotated from the original set. It worked out fine and the prop performs well. I've only put double digit hours on it as its not my primary prop, but I am highly confident in its stability. I don't think that plugging the holes actually added any/much strength back and it likely would have been fine just adding the extra set and no plugs.

As Laughing Jaguar commented the torque of the motor isn't transmitted through radial contact of the OD of the bolts and the ID of the holes in the prop - A clamp plate is used to press the prop against the hub and the frictional face-on-face contact is the primary torque transmission interface. The second attached image below shows what the clamp plates look like. There are actually props that use a single bolt/nut through the center of the prop which makes it easier to visualize this.

My only adjustment to the accurate details Laughing Jaguar added is that the concentricity alignment of prop to drive hub (at least on the paramotors I have experience with) is created through the center hole of the prop press fit referencing a protruding boss on the drive hub, and not the bolt holes which are oversized for clearance. I actually just used a drill press to add the new clearance holes after laying out the locations with a template.

If any of you would like to check out some of the pictures that I have taken while flying - my flickr page is below:

https://www.flickr.com/photos/jcs7928/

Just out of curiosity - Phoenix911, how did you happen to come across this ~8 year old post?

Regarding dj95401's comment "There's reg's for a reason. Unless you have a death with "

I'm not sure what a "death with" is but I certainly don't have a death wish. I think that the overly skeptical, obscenely critical, and non value adding comments might possibly deter someone naive from doing something foolish, but at least for me, they are just a bit frustrating and seems like a waste of time for those who write them.

Thanks again to all those who shared constructive inputs! They make digging through the crap worth-while :)

We always appreciate a good follow-up with good pictures and detailed explanations.

Glad to hear you are still flying, I still have dreams of flying something someday myself. Recently, I did catch a really good video of a glider that was launched by the operator running down a hill or jumping off like a hang glider. I think I would really like flying a glider and I love the cost of fuel for them! They are like the sailboats of the sky...except you can't drop a hook and raft up with other gliders to share strong drink and good food.

Drew K