Make your own wooden prop

[Markdun]

Bruce, casein is made from bovine milk protein a monomer which is polymerised to a polymer. The simplest technique is warming the milk with an acid which makes the milk curdle. The curds are dried and formed into shape. So, I would guess it would denature with heat or be consumed by microbes. But it would not remember and return to being milk.  Next time your offspring produces a baby and it up-chucks its milk feed, go collect it to make casein glue or milk plastic buttons.  And if that doesn’t get you some attention you could see about collecting the kids urine to make a urea based polymer, but that requires formaldehyde.  You could make some plastic jewellery?

 

 Yenn, I don’t think it works quite like that. From the ‘get go’ the carbon and wood will share the loads.  The issue is whether the greater stretch in the timber as load is applied will cause delamination. Sometimes carbon fibres are mixed with Kevlar. The carbon gives the laminate an exceedingly stiff but brittle characteristic. The Kevlar on the other hand yields more easily and combined with carbon reduces the negative of carbon in not being able to absorb sudden loads and the negative of Kevlar in not being very stiff. The wood carbon mix does much the same. The other thing is that carbon epoxy is much better at handling compression loads. So a 6mm square rib member with a 2mm square carbon insert on one side gives 32mm2 area of wood and 4mm2 of carbon. If the part is a beam with the lifting force away from carbon insert the tension side will be handled by the wood and the carbon would handle the compression load There’s also practical factors like a 2mm x 2mm carbon bar may be sufficient for a wing rib, but will it have sufficient surface area for a glue join or to attach fabric or plywood to?

 

Rot in timber structures is really only an issue if you don’t manage moisture content.  Spruce, balsa, radiata pine, & many of the eucalypts all have terrible resistance to rot if you let moisture in. Don’t. Encapsulate it all in epoxy and polyurethane. Nicks in props should be fixed. If you see water ingress as inevitable, try using western red cedar, Huon pine or cypress pine which are full of toxins and resist rot.

[Ian]

Apologies for joke in relation to forestry, I was trying to be funny but it may have missed the mark. I'm sure that structural analysis, material design analysis & testing etc is all part of the curriculum. However your formal training should make you appreciate the relevance of the research relating to epoxy-wood bonds.

 

In a related query I was recently pointed to Robert McGavin who works for DAF in Queensland recently wrote "Barriers to the Effective Adhesion of High-Density HardwoodTimbers for Glue-Laminated Beams in Australia" which is an interesting read if a bit sad from an adhesive point of view. If anyone ever has an adhesive related question he's probably the right person to chat with.

 

Resorcinol is a technically more difficult glue to use correctly which I alluded to, however there are also some PU and Isocyanate glue which also don't tend to exhibit the moisture related delamination issues which epoxy is prone to which are gap filling. From a propeller construction perspective it where the "joint" is simple from a topology perspective it makes sense to use the best adhesive. For a more complex joint where clamping pressure can't be applied epoxy might be the most suitable. However if the joint is simple, clamping pressure can be applied, and maximum durability and strength is required then in most cases resorcinol is a better choice. It also washes up in water which makes cleanup simple. Pretending that epoxy has obsoleted all other glues is somewhat misguided, not a golden hammer. What it comes down to is whether you'd feel safer buying a used wooden aircraft or boat built with epoxy or resorcinol especially if weather exposed. Resorcinol correctly applied has an outstanding record of maintaining a bond in difficult environment, epoxy not so much, but it is certainly easier for non-craftmen to apply and create something.

 

If you clicked on the links associated with the strength of the timbers involved you may have seen that Ironbark in this instance was grey ironbark or "Eucalyptus paniculata". Rather than write the full botanical name I thought that most people would find the term Ironbark simpler. In relation to making propellers out of ironbark I haven't heard of it being done either however there may be some aerodynamic advantages due to it's superior strength allowing the use of lower profiles.

 

In relation to your query about seasoned or unseasoned timber I suspect that this is a bit of a red herring. If you read the papers they all deal with seasoned timbers with specific moisture contents. The issue relates to moisture exposure post seasoning which impacts the adhesive performance across a variety of tests.

 

While internal framing can be fully shielded this protection can't be provided to propellers. Stone chips, rain etc and the exposed nature of propellers makes it likely that the timber will be exposed to moisture ingress as you can't control the weather when flying.

 

 

 

 

[Ian]

2 hours ago, Markdun said:

Rot in timber structures is really only an issue if you don’t manage moisture content.

A couple of the articles that I posted relate to acetalysed timber which greatly reduces the likelyhood of rot as it reduces the water absorption of timber.

https://www.accoya.com/au/acetylation-what-is-it-and-what-is-acetylated-wood/

Its available on the market but I'm not sure at what price. But I think most of us won't live long enough to care.

2 hours ago, Markdun said:

Spruce, balsa, radiata pine, & many of the eucalypts all have terrible resistance to rot if you let moisture in.

Some of the eucalypts also have exceptional in ground durability as well. Red and grey ironbark, spotted gum, tallowwood  and turpentine all have greater than 25 year in ground durability, above ground > 40 years. However they are heavy which except for niche applications such as propellers might preclude their use.

 

There's been some work in Europe combining hardwood and softwood in laminated beams using the stronger woods on the top and bottom and the lighter weaker wood in the core with really promising results enabling lighter and stronger "composite" beams. The ratio that they found most beneficial was about 15% on the top and bottom of the beam or spar.

A similar approach has been used with Carbon fibre or Fibreglass replacing the stronger timbers.

[Markdun]

Ian, most of what you say in that latest post I would agree with. My point is there are several factors and the best, strongest etc all depends. From a structure & weight point of view, including moment of inertia, I’d build props from western red cedar: it’s stiff, light & dimensionally stable, glues well and that’s why it’s common in lightweight boat construction.  However, it’s soft and dings easily, which can be partially fixed by coating/encapsulating, but in my view this would preclude it for props, just as soft radiata is crap for furniture (unless you like dents).  But I’ve gone to a composite ground adjustable prop for the Jabiru and have the two props for the Corby: one made from Eucalyptus delegatensis 60”x46”; & a second one from Auricaria cunninghamii (58x47.5”).  Both have similar weights profile and thickness; the hoop pine slightly lighter and the alpine ash slightly stiffer. The ash one pulled me along in cruise at 110kts @3100rpm burning 14.5lph, around 1500fpm climb at WOT. Really a ‘climb prop’.  Haven’t  got around to installing the hoop pine one yet as having too much fun with the Corby as it is, but it’s intended to give me 115-120kts cruise.

[Ian]

All, good robust debate and getting the facts on the table is always interesting. The key points which I was trying to make are as follows.

• Fixed pitch propeller design can be optimised to your cruise speed using freely available software. This should enable anyone with access to a large format CNC to develop wooden propellers optimised for their aircraft. Of course it is possible to do this by hand but your mileage may vary.
• When choosing timber for the props, there are a wide variety of timbers easily available in Australia which weren't available to manufacturers in Europe and the US.
• The thickness of the propeller can be reduced by leveraging stronger materials, increasing efficiency. However there may be a weight penalty. Similarly carbon fibre composite density is even greater. Also note that the bending moment increases as thickness decreases. Some Australian species have some interesting properties which while historically haven't been used in propeller construction might facilitate some design efficiencies.
• Generally resorcinol will produce a better, environmentally more resistant bond than epoxy. However joint quality, clamping pressure and minimum temperature are very important. It is far easier to produce a good bond with epoxy, hence why resorcinol has fallen from favour.

As an aside I grew up (a long time ago) helping to build and occasionally sail moths out of Australian Red cedar. While soft and light it didn't rot and bonded well. Supply was always interesting though.

 

[Bruce Tuncks]

There is a very interesting post ( elsewhere on this site) about a bamboo composite material, which may turn out to be great for props. In the meantime, I am embarrassed to admit to buying a new Jabiru scimitar prop...  $2,500 dollars !

I reckon I still would like to make my own. Maybe copy the bought one? this would possibly be illegal, but it would be fun.

[Bruce Tuncks]

It was onetrack who made the posting, about bamboo,  and it was on the other site ( off topic gets you there ).

[spacesailor]

SO

I,m interested in the " Bamboo" prop design . Any one going to have a go at making those " Bamboo " props will have to go back to school for finding the right glue for the job .

OR

shall I just make three planks from Bamboo, epoxy resin, then laminate them together like an all Wood prop, ending with shaping as per normal .

spacesailor

 

[Ian]

Bamboo is different again from wood, plus its a composite. I'd contact the manufacturer but you might scare them if you mention propeller.

This paper might give you insight into the challenges.  https://www.sciencedirect.com/science/article/pii/S1359836822001603

All woods (including bamboo even though its a hardened grass) use fundamentally the same building blocks and strength is somewhat proportional to density. However some timbers of similar density differ in strength as they use the underlying building blocks more or less efficiently.

I'll leave you with the pictures to mull over from one of the earlier articles I posted. All the adhesives bar one produce wet wood failure. They also noted that they had issues with the RF application leading to lower strength of some test samples. This demonstrates Markdun's right relating to the fact RF can be more difficult to apply compared to epoxy and that it can introduce technical risk.

A later test using acelylated timber shows the difference in treated timbers.

I have searched for any studies demonstrating newer epoxy formulations resistant to the standard de-lamination tests however I've only seen published successes when surface preparation treatments are used.

The manufacturers of the new improved epoxies might be able to point to successes in this area.

 

 

 

Edited August 24, 2022 by Ian

[Marty_d]

Wouldn't a carbon fibre or fibreglass coating over the wooden prop both strengthen and prevent moisture?

[Bruce Tuncks]

Not necessarily Marty. The materials, having different stiffnesses, carry loads differently.

An extreme argument ( Yenn or old K, ) was that of carbon and wooden structures. Basically, the carbon carries all the load till it fails, only then does the wood carry load. This is indeed fairly accurate.

[Bruce Tuncks]

Space, I reckon that the bamboo as described ( stronger that steel in tension and much lighter) would be ideal for a prop, alas its too early to try one. The material is just in a report from Zurich university, so there are years to wait.

[Bruce Tuncks]

Referring to a tension-member with carbon fiber "reinforcement", if the carbon fiber is ten times as stiff as the wood, then it will carry ten times the load that the wood does. ( assuming equal cross-sections of each material) The ten times as stiff does indeed refer to carbon fiber and bamboo composite.

[facthunter]

I've read in technical references that Carbon  fibre isn't that good in compression. Nev

[Ian]

It is best to look at the Modulus of elasticity  and the Ultimate Tensile Strength (UTS) or Modulus of Rupture (MoR). You may not get the desired outcome if one material loads to failure before the other approaches 20%. (Also below I know the ratios are a thousand or so out but you get the gist)

 

MoE of Ironbark is 23GP, MoR 185 ( Ratio ~8)

MoE of Hoop Pine is 13GP, MoR 90  (Ratio ~6.9)

MoE of Balsa is 3.7, MoR 19.6 (Ratio ~5.2)

From

http://www.performance-composites.com/carbonfibre/mechanicalproperties_2.asp

MoE Std CF 70 GPa and UTS 600 (Ratio ~8.6)

MoE Eglass 30 GPa and UTS 440 (Ratio ~14.7)

 

This might imply that a match up of Carbon fibre and Ironbark or Hoop pine would, from a structural point of view be better than E-Glass in a propeller.

 

However you could also argue from a protective point of view the E-Glass would allow a safer failure mode by being having the ability to absorb failures of the internal material to an extent allowing a visible inspection to easily identify failures. It would also be under less load when surface damage occurs managing the associated stresses around the point of damage.

 

 

[spacesailor]

MOST of my wooden props were covered with glass to protect them from the elements. 

As well as keeping moisture out !, it keeps the moisture IN.

It also Looks rearly good if finished well.

Our friend " pylon500 " tried to make ' composite props .His verdict .

" Never again ".

spacesailor

[Blueadventures]

On 25/08/2022 at 3:21 PM, Marty_d said:

Wouldn't a carbon fibre or fibreglass coating over the wooden prop both strengthen and prevent moisture?

Maybe if bonding is epoxy resin; coating with everdure (epi dure) would be worth considering as it is a thin epoxy resin type mixture used in timber boat building.  First coat is thinned and next a as mixed (two equal part mixture)  Prevent rot and seals and strengthens the timber.  Product info - International Everdure is a clear two pack epoxy-based material that can be used as a wood saturation system to help harden, densify and seal out moisture. Compatibility.  I use it all the time great stuff for boats.

Edited August 26, 2022 by Blueadventures

[Markdun]

Many poo poo ‘Evidure’ because it has solvents which theoretically reduces its waterproofing properties.  However I’ve used it to coat entire wing structures with good results, but then these aeroplanes haven’t been sunk in floods. For a prop, I’d guess it’s not so good because you want to build a

 reasonable thickness of hard epoxy on the outside surface to resist nicks and chipping.  Indeed some like the WEST guys suggest the main benefit of a light glass cloth and epoxy on the outside is that the cloth guarantees a reasonable minimum thickness of epoxy to prevent moisture absorption.  But then you have the risk of delamination of the cloth.

[Ian]

Does anyone have a figure on how many planes received a dunking in the floods? Are they written off or are they likely to appear on the market at some point?

 

[meglin]

On 8/24/2022 at 6:57 AM, Bruce Tuncks said:

There is a very interesting post ( elsewhere on this site) about a bamboo composite material, which may turn out to be great for props. In the meantime, I am embarrassed to admit to buying a new Jabiru scimitar prop...  $2,500 dollars !

I reckon I still would like to make my own. Maybe copy the bought one? this would possibly be illegal, but it would be fun.

What fabulous features should a propeller have for that kind of money?

[Bruce Tuncks]

Gosh Meglin, I was ripped off huh? What should a Jabiru prop cost? 

[Bruce Tuncks]

I would love to buy one from Ukraine, you guys are doing a wonderful job and I do financial support to Ukraine as befits a self-funded retiree.

 

[meglin]

I don't know. Maybe it's gold plated on the inside? Or has incredible performance? The price is more in line with a variable pitch propeller. We have our own prices. We've worked very hard on the Jabiru engine. Unfortunately, despite careful maintenance, none of the engines have lasted as long as they should. This may have changed now. We have tried a lot of different propellers on it. We found a suitable carbon propeller. We found the best wooden propellers for our flight speeds.  

[facthunter]

Who makes the Kiev props?  Nev

[Old Koreelah]

On 4/9/2023 at 5:37 PM, meglin said:

What fabulous features should a propeller have for that kind of money?

Pricey, but perhaps Meglin is thinking US dollars. Converting those 2,500 Aussie dollars is about $US1,650.