Wood rotors revival? Nature's own composite.

[AdamFrisch]

When I last walked in to my garage (auto repair shop for those over the pond), I found to my dismay fake carbon fibre woven mat stickers and tape to buy. This presumably so you could stick them on your dashboard or spoiler and pretend you had a carbon fibre one.... That was the catalyst for me and I decided that in these carbon crazy times, it was time to fight the corner of nature's composite for awhile.

Venice still stands on Alder wood plinths (sure she's sinking now, but after 1000 years what metal would have lasted longer?) and wood is the most used structural material in the world. It used to be the material of choice for rotors, but got de-throned by aluminium, only to be shuffled aside for the new wonder material carbon fibre - a material that has yet to prove its long term capabilities. In fact, so uncertain are the FAA of it's long term structural abilities, that it requires it be built with a 2x safety load factor, whereas aluminim and wood only need 1.5x, negating many of the weight savings the material offers.

And wood in rotors is terribly out of fashion these days. It's just sooo uncool. But is it really? Let's take a look at the facts.

1. Wood does not fatigue. All metals known to man will fatigue. That's why they have run times and TBO's for aircraft - cause they're made out of a material that will break eventually (even if you never load it to it's design limit it will still fatigue).

2. Wood does not corrode. All structural metals corrode in contact with air and water. Once again, that's why they're on run times and TBO's.

3. Wood (Birch in this example, but Ash, Oak and many other woods can have even more) is 1.7 times stronger than 6061 Aluminium in Specific Strength (strength per density). In fact, Aluminium is 3.9 times heavier than Birch, but only 2.3 times as strong. Bet you didn't think that.

4. Wood is repairable and patchable on rotors - metal and composites need to be replaced. In fact, the Bell 47 wooden rotors that are still on many 47's have no time limit - they're on inspection. The metal ones are 1200 hrs only. But more on that later.

5. Wood flies smoother and flutters less. This is from many Bell 47 and Hiller pilots.

6. Wood is easy to shape and work with.

7. Wood is cheap.

8. Wood is environmentally friendly.

9. Wood is easy to inspect. Especially if you use S-glass glassfibre on top - then you can detect any buggery with your bare eye.

10. The Kaman K-Max - a true heavy lift champion - has wooden laminate main spars in their blades. Read here: Composites take off ... in <i>some</i> civil helicopters: COMPOSITESWORLD.COM (http://www.compositesworld.com/articles/composites-take-off--in-ltigtsomeltigt-civil-helicopters.aspx).
So did their now retired HH-43 Huskies and they had an excellent track record.

11. Extra's aerobatic 300 aircraft has wooden main spars, because in such high load situations metal would fatigue rapidly. Many other high stress aircraft share that. And so do many of the still flying vintage aircrafts - most of them on the same spar they had when they were made 50-70 years ago. The Lancaster bomber had a very modern wooden geodesic space frame that was almost unbreakable, for instance. Most homebuilts also have wooden main spars. Many Hiller's still fly with wood blades. The list goes on.

Did I hear you say dry rot? Dry rot needs two things - moisture and air. A sealed glassfibre blade will let neither in. You can also Cuprinol treat wood these days which will further eliminate the problem. And just to be sure - with a clear glass blade you could spot it with your eye if it against all odds would appear.

The traditional way to build wood rotors were as a laminate, at least in the Bell's. Staring from the front the leading edge was normally a Birch laminate. The main section after that was either Birch or spruce and after that came balsa as a filler shape (it's equiv to honeycomb, basically) on the non structural parts. The last trailing edge bit was normally a bit of spruce.
At the front of the leading edge was then attached a stainless steel to protect the blade from erosion. I include a cross section of the Bell blade.

I'm thinking maybe it's time for wooden blades again - and when I talk about wooden blades incidentally, I mean modern laminate wood core covered with glass fibre or even carbon fibre. Just to be clear. Not just varnished planks.

Wood has a lot to offer still. I'd like to see the return of the laminated wooden rotor again - no TBO's, no time limits, smooth, cheap, un-fatigued, un-corroded and safe.

http://www.adamfrisch.com/images/B-47%20blade.jpg

[500e]

(The Lancaster bomber had a very modern wooden geodesic space frame)
Do not forget the Mosquito ( wooden wonder)

www.raf.mod.uk/rafbramptonwytonhenlow/rafcms/mediafiles/9818FCBF_1143_EC82_2ED93F5D8312A0C6.pdf (http://www.raf.mod.uk/rafbramptonwytonhenlow/rafcms/mediafiles/9818FCBF_1143_EC82_2ED93F5D8312A0C6.pdf) - S (http://www.google.co.uk/search?hl=en&client=firefox-a&channel=s&rls=org.mozilla:en-GB:official&hs=Y2B&q=related:www.raf.mod.uk/rafbramptonwytonhenlow/rafcms/mediafiles/9818FCBF_1143_EC82_2ED93F5D8312A0C6.pdf)

[SASless]

Save on the wood consumed and go back to wood and fabric blades.....just mind the heavy rain!

[Impress to inflate]

I'm with you. Nothing would look nicer than a large a/c, say a Chinook with varnished Oak blades.

Our medieval ancestors would build a church or large building out of stone and oak, once finished they would plant several oak trees. After 800 years, when the oak trees were mature, they would cut them down and repair the church/large building because they reckoned that how long oak would last.

[rotorboater]

woodnt want to be the 1st to try it on a 44 :)

[AdamFrisch]

This is the built in prejudice wood has to fight.

If they've worked on a Bell 47 for 50 years or on K-Max's, then I'd assume they'd be ideal for the R44? In fact, didn't the R44 have de-lamination problems with its rotor? That couldn't happen on a wood blade.

[brett s]

Wood blades, at least on Bell 47's, really didn't like being exposed to the elements all the time. They also aren't nearly as tough as modern ones if/when they hit something.

They were popular with schools running Bell 47's because of on condition vs a fixed hour lifespan, where they spent most of their life on a nice airfield - not so popular with operators doing anything else.

Quality wood suitable for aviation use isn't cheap at all!

Not saying you can't build them, or that they wouldn't be suitable for some stuff - a homebuilt that never stays outside or gets flown in the rain comes to mind - but there's definitely a reason they went away on production ships.

[Um... lifting...]

I to I-
This an anecdote like you're thinking of?
Anecdote: The Beams of New College, Oxford (http://www.anecdote.com.au/archives/2008/01/the_beams_of_ne.html)

Modulus of elasticity of white oak is 1/6 that of aluminum alloy, specific gravity of 0.67 is substantially less as well, with aluminum's specific gravity about 2.7. Ash and birch have similar strength properties and slightly lower specific gravities. Locust and hickory are other options. If you're American, woods you'd make a baseball bat or ax handle from are likely choices. One should imagine the same would hold true for 'clicky ba's across the pond or all those yew trees that won't be made into longbows. Thermal and electrical conductivity of all woods, much lower than any metal or carbon fibre composite.
Balsa of course is the ne plus ultra of lightweight woods, with a sg of about 0.15. Too soft for impact loads, but easily capable of being used in fairings or trailing rotor surfaces. A hard varnish finish or a veneer can be used for more protection.
With modern wood glues, long grain to long grain glue joints are stronger than the woods themselves.

Most tropical woods, while high in strength, are probably too dense for practical aviation use.

Fellow I know in Key West was restoring a Waco from the wheels up. One wing (minus cloth and dope) was hanging above the workbench for safekeeping. A thing of beauty... belongs in a museum or an art gallery if it can't be aloft.

As for water and weather resistance... a company in Maine called Grain Surfboards is marketing kit (and fully built) surfboards consisting of plywood frames and locally harvested white cedar skin. They recently began marketing in Australia as well, with locally harvested red cedar. Core of the board is hollow, which necessitates installation of a plug so that the board doesn't burst when it's put on the roof of the car in the sun. Most surfboards one sees these days that look wooden are really veneer over foam with glass fibre and epoxy covering the lot. For what is something of an environmental sporting pastime, surfboard manufacture has long been pretty hard on the environment. Grain is experimenting with a woven bamboo fibre cloth (apparently very successful) in lieu of the "traditional" glass fibre, and some form of vegetable-based resin in lieu of epoxy (no reports on the resin as yet). The boards have no practical life limit if you don't hole one on a rock (and even then, they're repairable).

The technology for large wooden aviation structures is certainly there... witness the "Spruce Goose" (which is really made of composite birch and resin, mostly) with a bigger span than the A380.

Most of the little I know of wood comes from R. Bruce Hoadley's excellent book "Understanding Wood" (a craftsman's guide to wood technology).

[Speedbird48]

Since when did the Lancaster have any wooden structure, let alone "GEODESIC"??

The Wellington had a "GEODETIC" structure, not the Lancaster, and that was also metal although fabric covered.

The Kaman K-Max has wooden blades with control via tabs on the trailing edges of the blades. The one I saw a couple of weeks ago could lift huge loads off of mountainsides, but vibrated so much in cruise that you couldn't let go to fold a map/chart!!

The cost of making wooden blades is very high due to the man hours. At least one, the Sycamore, had three blades that all had to be changed any time any of them sustained any damage. That gets expensive!!

Speedbird 48.

[BedakSrewet]

Our -Franklin powered- Bell 47G had wooden blades which were perfectly in line once the moisture was 'pushed' out by the centrifugal force....

:ok: Greatest helicopter ever made.

[IFMU]

Aluminum is all one huge conspiracy. When we are all flying around in wooden-bladed, wooden bodied synchrocopters the world will be right.

-- IFMU

[MartinCh]

IFMU,

I'm sure your gliding in old wooden gliders didn't have any influence on your bias...

As for using wood where plausible, fine.
What I see in the picture, is basically using wood as PART (albeit bigger in this idea) of the whole design. Sure, stuff the blades with wood, but you still have to wrap it and attach it to mast. So..

Just nail or screw big wooden plan on the MR mast, varnish the blade and off you go. I wouldn't.

If I had a car (and driver) at my disposal or bothered too much, I'd have gone to see 'Spruce Goose' as I was nearby recently. You can't expect them to build such HUGE aircraft without limiting weight with the engine technology they had that time.

I watched documentary on Mosquitos in WWII and their use while ago. I forgot the name already. Using wooden frame or wooden parts is fine with me in "small" airplanes. Thinking of using ONLY WOOD for heli blades, ehmm...

If IFMU's heretic ideas went forth, there'd be slightly modified 'fire in flight' emergency procedures (ie with wooden airframe :ooh: :eek:) for sure.

[helix47]

I think you will find that the wooden blades have a steel spar. The wood simply provides the shape.

[Fareastdriver]

I started off on the Sycamore with wooden blades and as Speedbird 48 said the set had to be changed in toto because they were flown in a tower together. The Mosquito, Hornet and especially the Sycamore had severe problems in Malaya mainly due to delamination mainly because of the glues used at that time but also from warping when left in the sun. You can treat, age, do anything you like to wood but it will change shape over time especially if it is exposed to the sun and rain. You can protect it with paint, fibreglass, resin or whatever but you cannot prevent thermal transfer and that is the killer. Wood moves all the time which is why on a house the roof only rests on the walls, not fixed, otherwise it would crack the masonry. In the building trade there are some incredibly strong laminates available but there is no guarantee that they will keep their shape because they are manufactured fron softwoods. Going back to the old biplane days the rigger was the genius who knew how to adjust the flying and landing wires to trim the aircraft.
Lightweight helicopters? Feasible, but anything bigger than that you are in a minefield.

[bugdevheli]

So come on Adam, What are you building?. All these blade questions, and now what materials to use. Sounds like the beginning of another homebuilt to me. Bug.

[AdamFrisch]

Haha - caught out.

Nah, I'm just a dreamer and it will probably not amount to anything, but I might just be working on a new gearbox-less turbine rotor system (not tipjet)....

As for thermal moving - this is a fact to consider, of course. But unless your doing solid platinum blades, all other materials will also expand, especially metals.

[hoggsnortrupert]

QUOTE:Speedbird48
The cost of making wooden blades is very high due to the man hours. At least one, the Sycamore, had three blades that all had to be changed any time any of them sustained any damage. That gets expensive!!
Speedbird 48.

No its not true,an average tradesman could/can laminate up a pair of blanks/planks, in minimal time, dependent on raw material quality.

If one was then handed a pattern blade, say carbon fiber or alluminium,whatever.

Take all to a CAD driven CNC machine and in all likelyhood you could turn out blades to your hearts content, only limited by the bit in the mill head and the amount of planks/blanks.

As for rain, formed stainless wrap around leading edges have been used before.

As a note in the building trade, a multi storey building with laminated floor beams is both safer from Fire and Earthquake, as compared to steel and concrete.

Steel bends when heated, and concrete beams explode.

Thats the tradesman in me coming out.:)

Chr's
H/Snort:ok:

[Graviman]

The big problem here is that aero grade spruce can only come out of the trunk between ground and first branch. Modern composites are just so much more consistent, which suits modern FE validated designs. Also wood does not lend itself to complex 3D shapes, unless you introduce a lot of fiddly operations. Otherwise for small volume production it is well suited.

Damage tolerance is also an issue, since wood will snap rather than bend or delaminate. The fatigue characteristics are interesting, but again this is a now a reasonably well understood area in metals and composites.

The fact that aircraft design has moved from wood to aluminium to composite is probably the best indicator of material suitability. All of the reasons mentioned will have been considered but the modern design direction is clear.

That said, if you could come up with a way of growing composite structures then the story might be different. Perhaps materials using wood fibre and resin?

[Ptkay]

Robin.

Still in production and very popular wooden aircraft:

APEX - Aircraft (http://www.capaviation.com/en/index.htm)

These sport airplanes are manufactured of wood - the natural composite with high strength to weight ratio! The luxury details are added by artisans, lovingly blended with high-technology required in modern aircraft.

[topendtorque]

As for water and weather resistance... a company in Maine called Grain Surfboards is marketing kit (and fully built) surfboards consisting of plywood frames and locally harvested white cedar skin. They recently began marketing in Australia as well, with locally harvested red cedar


This in Oz, I find a bit hard to believe as it is, 1) super expensive, and 2) now quite (very) rare.

A previous mate of mine and about whom we have recently spoken, Dave Norris, who is now not with us had a large shed full of the stuff in North Queensland, (Daintree) bet he wishes he was around now to start selling it.

I notice that you don't refer much to willow there Adam, mind you it's pretty useless in Old blighty these days, heh heh.

As far as the wooden blade versions go, i'm a total disciple, they teach the basics of respect for balance and smoothness like nothing else.
tet

[Um... lifting...]

tet-
Somewhere, buried in the Grain surfboard blog archives, was the article I saw where they mentioned red cedar, though I haven't the time to find it now. I don't think I'm mistaken about that once being in print. But in any case, it appears they have since shifted to Australian farm-grown pawlonia.

Grain Surfboards Blog Archive Grain Surfboards partners with Australia’s Capeboatworks (http://www.grainsurfboards.com/2008/01/07/grain-surfboards-partners-with-australias-capeboatworks/)

Wood, in general, properly seasoned, doesn't expand nor contract much at all based upon temperature. Moisture content is the driver for most dimensional instability in wood.

Outside the Honolulu Power & Light building there's an old wind turbine blade. Wood laminate. Much bigger than any helicopter blade I've ever seen. Yes, I know the rpms are slower, but the stresses are sustained for longer periods of time than a load of fuel. That's one of the weaknesses of wood, deformation over time under load. They seemed to have managed to sort that one out, rotor blades could be sorted out as well.

The question would be whether it could be done economically or if there wouldn't be other considerations. We've gotten rather good at laying up carbon fibre in such a way that it behaves precisely as we wish. Of course, if it catches on fire and you breathe so much as a whiff of it, you'll never be right again. Nasty stuff, carbon fibre.

Anyway, the point is moot, anyway... our next generation of aircraft will be made from ragworm jaws. I hope the little b*stards aren't unionized.

Economist.com (http://www.economist.com/science/PrinterFriendly.cfm?story_id=11785227)

[Lt.Fubar]

Mosquito, Halifax, Spitfire, and other British aircrafts from World War II are very poor examples in this conversation. They were not build from wood because it was superior material. They were made from wood, because the British had it - in contrary to aluminium. British industry in the time of war couldn't produce enough steel and aluminium alloys for aviation, so the airplanes were made from wood, except those that were ordered in US - those, of course, were made purely from Al alloys.

The main source for aluminium industry is Bauxite ore, which is rarely accounted for in western Europe, or Northern America. US could relatively safely import it from south America and Australia, and Germany got it from Greece and Africa... Even the Japanese industry was dependent on it, and without invading China - rich in that ore - they couldn't build their military might at the time.

Now, back to wood:
Apart form bamboo fiber, witch is very promising, the wood is not good for machinery of any kind. Sure, you can build structures from it - houses, bridges. But not machines. It contains water which is impossible to remove entirely without destroying the material. It needs longer seasoning than metals, and is not homogeneous ! That means you can't predict with high enough accuracy, how it will respond to different loads, and many same parts made in one bunch will respond differently. Will have different CofG, weak, and hard spots will be in different places... you don't have to care about it making surfboards, snowboards, or hull of sailing boats. But aviation is a whole different story :=

Metals are lot more predictable, and with right technology you can safely assume that all parts will react the same - under load, will crack exactly in the same place, exactly at the same time. For many years that was the problem with composites, but today's technology allow them to be as predictable as 1960s metallurgy or better, which is pretty good achievement.

Fun fact - century before World War II , the most precious thing money could buy apart from rare stones (diamonds, rubies) was jewelery made from ... aluminium. It was wrongly assumed that it is the most rare metal on the planet, and it came to cost more than gold, silver or platinum. Only some time later people discovered that in fact 8% of Earth solids weight is Al, and it become one of the cheapest metals of all.

[Speedbird48]

Sorry Lt.

If the Halifax and Spitfire were made of wood then my C**K's a Kipper!!

Hence my previous regarding the Lancaster. Definatly all metal although the purist will say that the Elsan had a wooden seat!!

As for the comment regarding using a CAD machine to make helicopter blades there was no such thing when Sycamore blades were being made. And by the way they were 6000UK pounds a set in the early sixties!! That at RAF lowest (only) bidder price.

Speedbird 48.

[Lt.Fubar]

Oh yes, there's sleep deprivation in the work. Of course I meant the Wellington and Hurricane, not Halifax and Spitfire (quite huge difference isn't it ? :( ). Of course the two were not entirely made of wood either - got steel or aluminium alloy frame, that gave the aircraft strength, wood and linen only made it aerodynamically sound. The Mosquito was an unique design as almost all wood (minus ailerons).

[IFMU]

I'm not really a wood fan. My building media of choice is aluminum. It seems like the big boys are going away from aluminum blades to composite, though there are still a lot of aluminum airframes out there.

In a thread started by the other AdamFrisch, there was this Nick Lappos post about wood in rotor blades:

http://www.pprune.org/2938373-post4.html

MartinCh,

I've never actually flown a wooden glider. Only metal and glass ones.

-- IFMU

[Senior Pilot]

Of course I meant the Wellington and Hurricane

Still not quite there: the Wellington was an aluminium construction :ok: There were wooden battens attached to the aluminium geodesic frames, but they were only there to attach the Irish linen fabric skin. There was no wood in the Wellington that formed any part of the stressed airframe :hmm:

The Hurricane was an all metal construction, also. The fuselage was a Warren girder-type fuselage of high-tensile steel tubes, and the wing was all metal, stressed skin, using Duraluminium. Again, no timber for any load bearing structures (the throttle cap in wood doesn't count, either!).

So, apart from the Mosquito, not too many all wood WW2 allied aircraft :p

[Freewheel]

The Miles M20 springs to mind as the only other WW2 aircraft made from wood by preference, and then only because the specification required it.

Wood's heyday was 20 or more years earlier.

In any case, the development of metal and now composite construction materials and methods has made wood an item of nostalgia for aviation.

[Lt.Fubar]

All wood Lavochkin LaGG-1, and later LaGG-3, somewhat 6300 build... all flew differently. Most of other early soviet designs, like Yaks, were similar to Hurricane - steel tube frame, and plywood over it - the plywood skin liked to fly off the aircraft at high speeds.

[AdamFrisch]

IFMU.

Nick Lappos is hard to argue with at the best of days due to his immense experience, but his wood figures are not correct. Aluminium is not 10 times stronger than selected hardwoods. Maybe 3 times at best and that's only in rupture. In specific strength (which is very important in aircraft design), many woods have higher strength.

Here's where I got that wood rotors are patchable and repairable - from Tulsa Rotors website:

Pictures (http://www.tulsarotorblades.com/pics/pics.html)

[AdamFrisch]

Fair enough.

I somehow would just feel a lot less secure riding in a helicopter with rotors full of filler patches and some epoxy touch-ups than on a wooden equivalent. To many bad memories of old S-glass patched boat hulls from the 60's, I suppose...:)

[Dave_Jackson]

AdamFrischAs for thermal moving - ....unless your doing solid platinum blades, all other materials will also expand, especially metals. Carbon composite actually shrinks ~ slightly. This has to be considered when using heat to cure composite items.

If you want to convert from wood to composite, here is an incomplete CNC machine for producing composite blades (http://www.unicopter.com/CNC_Workstation.html). ;)


Bugdevheli.
Could you use it for your new rotor blades?

Dave

[busdriver02]

something to consider when you talk weight, is that most modern composite structures are mostly air by volume. You have a honeycomb structure filling the void between the skins. The honeycomb turns into a pringles chip in bending. The result is that the skins can act in tension only (where carbon fiber excels). Which tends into the advantage of fiber based composites, you can design them to have very specific strengths, while maintaining flexibility in other directions. This is not to say wood is useless, the floor of a modern corvette is end grain balsa sandwiched between layers of fiberglass.

The achilles heal of modern composites is the material used to bond them together. If the layers disbond, the whole structure is done for.

[Graviman]

Adam,

The best solution is to compare material properties for wood, steel & composite.

YieldStress/Density for strength
E/Density for axial stiffness (never generally a problem)
EI/Density for bending stiffness
GJ/Density for torsional stiffness

This will put the thread on a more objective bearing. ;)

[bast0n]

Imagine if all the woodworm stopped holding hands at the same time.........:)

[bugdevheli]

Dave. Many thanks for pointing me towards the incomplete CNC machine! However for the moment I will have to stick to my method of forming one piece carbon skins over a plug (vac formed). Only eight to make for my new machine:) Regards Bug

[VfrpilotPB/2]

In the rafters of my garage( hell hole to my wife) but (Heaven to me) I have a pair of Wooden prop blades from a very early Spitfire, they are covered in a very thick black Plastic type substance and have a strip of coppper on the leading edge, with a couple of numbers handwritten in white paint on their face, I was (still intend to) make a fireplace centre piece from them, but cannot decide if I should strip them back to wood or clean them up as they are , trouble is once stripped theres no going back!


Peter R-B
Vfr