Building a Turbulent

[dubbleyew eight]

it is very easy in life to sit back, contribute nothing, and just criticise.
it is far harder to put the gonads on the line and contribute as you are about to see. :E

What Tools are needed?

http://i1274.photobucket.com/albums/y421/dubbleyew8/toolsused_zpsea787490.jpg

in the photo are all the tools I used to build a Turbulent.
in addition I found a home made linisher very useful for some things.

http://i1274.photobucket.com/albums/y421/dubbleyew8/linisher_zpsd4c6a2c3.jpg

the only other tool I needed was a table saw with a tungsten carbide tipped blade for stripping planks into building sizes. the linisher was then used to sand the stripwood to a nice smooth finish.

[dubbleyew eight]

wing ribs are made from strips of wood about 5mm by 9mm.
you build a rib jig (I'll go into that later) and position the next part on top of the construction, mark the angle of the cut with straight edge and pencil.
transfer the piece over to my bench vice. hold the piece without crushing it.
use the coping saw to cut just beyond the line.
use the sanding blocks to sand back to the precise length and position.
glue in place and you're on to the next piece.

plywood in the thicknesses used is easiest cut with sharp scissors or the stanley knife and a straight edge.

honestly in the photo above are all the tools I ever needed.

the vernier caliper is used to check strip sizes and adjust the saw as needed.

[dubbleyew eight]

If you build a Turbulent properly, and this goes for any wooden aircraft, you can expect that it will be still serviceable and flying in a hundred years time.
It might have been refurbished a few times along the way but there is no reason why it will not be flying.
As proof of this, two hangars from where I store my Tailwind, sits Bernie Pietenpol’s original Sky Scout. It is the actual aeroplane described in the Flying and Glider Manuals originally published in the 1930’s and reprinted by the EAA. It has been refurbished seven times now but sits immaculate in the hangar and is totally airworthy and is still flown.
it is 80 years old now.

Building a Turbulent is not as intimidating as it looks you know.
If you study the plans you will come to realise that the structure of a turbulent is glued up from a lot of small pieces of wood. most are probably less than a foot long.
The only long pieces of timber needed are the 4 longerons that form the corners of the fuselage and the spars in the wings and the tailfeathers.
The longerons can be spliced and can be made from pieces shorter than the fuselage.
The only pieces that need to be immaculate wood are the spars.
So what this means is that the task of finding suitable wood for most of the aeroplane is no where near as intimidating as it may first seem.

Spruce came into widespread use during the first world war because it had the best strength to weight ratio of all the woods and it could be obtained in long straight grained lengths that were clear of imperfections like knots. It isnt necessary to use spruce though. there are a lot of other suitable woods in the world.
Spruce that is suitable for building aeroplanes has a density of around 28 pounds per cubic foot.
Woods that have densities up to 35 pounds per cubic foot are still quite suitable for building aeroplanes.

Most people who pontificate about wood throw you a theoretical text on the structure of wood as though it will all be obvious. It took me ages of working with the wood to work out what was actually important. (you can still go and read a theoretical text if you want)
Wood as it is in the tree has a soft cellular structure full of moisture. to turn the green wood into a material suitable for use in the structures we build it must be dried out. This is called seasoning the wood. Once dried out the cellular structures become harder and becomes the engineering material we know as timber. Timber has about a 10% moisture content.
The cells are held together by lignins. I write lignins in the plural because lignin is not one molecule but is a whole family of related molecules. Some of those molecules soften when heated and some dont and this is one property of timber important to us in building aircraft.
Some of the pieces we build need fairly tight curves and we achieve this by heat softening the wood and shaping it while in the softened state. the upper curves of the wing ribs are formed this way. obviously a wood that does not soften with heat will make forming some of these parts nigh on impossible.
all things being equal the strength of a seasoned timber will vary pretty well according to its density. The timber used in the design work and the original builds of the Turbulent was spruce. How will we know if the wood we are looking at will be as strong? well its density will be a pretty simple guide.
Wood grows out in the open countryside. it is subjected to a never ending cycle of seasons. in warm conditions the wood grows fast and in colder seasons the wood grows slowly. The fast wood is less dense the cold weather wood is denser. so this means that wood does not have a uniform strength across the growth rings. What we do is select wood that has lots of growth rings close together to achieve a material with more uniform strength characteristics.
much of the wing rib trusses are just 5mm x 9mm wood so lots of growth rings will give us a more trustworthy structural material.
We are gluing the wood to build the structures so we need to avoid woods that will not allow us to achieve a strong glue bond. Resins and waxes occur naturally in some woods and these prevent good glue joints so it is pointless trying to use them.
A point to bear in mind here is that not all trees of a species grow the same so you may find some woods quite resinous and other woods of the same species will be quite acceptable.
Some woods turn out to be brittle, they break easily.
You can tell a lot about a wood by taking a long piece about 5mm x 9mm and bending it in your hands until it breaks. look at the break. if it broke on the outside of the curve the tensile strength is lower than the compressive strength. if it broke on the inside of the curve the compressive strength is lower than the tensile strength. if it pretty well broke evenly across the bend then both the compressive strength and tensile strength are the same. if it didnt bend far before it broke then it is a brash timber and will not be useful. could you twist the timber without it breaking? if it was a supple timber that bent and twisted a long way before breaking then it will probably be suitable.
Selecting a wood for aircraft building is a case of knowing what you want to achieve and selecting suitable examples of timber.
Stains, moulds and other discoloration are indications of bacterial and fungal problems in the timber. avoid these timbers and dont store such timber near your building stocks to prevent contamination.

Ok we think we have a timber that is suitable. We now need to check that we can achieve structurally sound glue joints using the timber. we need to saw up some timber into 5mm x 10mm strips. sand these so that they are smooth. we then cut them into lengths about 2 inches long, these are glued as in the photo and the glue is allowed to set completely before testing. (so 2 inch long pieces of wood glued with an inch overlap)
http://i1274.photobucket.com/albums/y421/dubbleyew8/gluetestpiece_zps6dfc490e.jpg
You can see the little ball pein hammer in the photo. to test the test piece we give it a hit with the hammer in the position of the hammer in the photo. we need to hit the test piece hard enough that we break it. our glue joints should be stronger than the wood that they join, achieving that strength consistently means that we can take the joints for granted and only be concerned by the strength of the wood. the reason that we hit the test piece with the hammer is to subject the glue joint to a sudden shock load.
should we ever find that the test piece breaks through the glue joint then that test has failed. if we have glue joints done in a similar manner in our aircraft then those joints must be seen as incompetent and they must be removed and done again.
what we should always see is that the glue joints survive the test and that the wood beaks.

Gluing will be the subject of another post. There are aspects of gluing that require a meticulous approach but once understood it isnt hard.

For much of the wood needed in the turbulent it is possible to saw it from planks that have knots. I used a plank 5” x 1” and about 8ft long for most of my wing ribs. the plank had 57 knots along it but about half an inch from the knot the grain was nice and straight again.
at the end of cutting the rib pieces from the straight grained sections I had two small hand fulls of knotted reject timber and over half the ribs made.

[dubbleyew eight]

what does good wood look like?

http://i1274.photobucket.com/albums/y421/dubbleyew8/wood_zps819221b1.jpg

if you can find wood that looks like that on the edge grain and it meets all the other tests for lack of brashness, suppleness, lack of brittle failure and you can prove it glues well you just about have it made.

the timber in the sample above was sold as swimming pool decking would you believe.

do I need to tell you the species. not really because this is about selecting suitable timbers from what is around you.

if you flip the piece over...

http://i1274.photobucket.com/albums/y421/dubbleyew8/knot_zpse956a3d7.jpg

you'll see a bloody big knot in it, but notice the grain away from the knot. it is possible to cut from this timber near perfect pieces of strip wood just by cutting away from the knot.

right time for a break.
I should really have been making aluminium spacers.

anyone any questions other than what was the wood? alaskan yellow cedar btw.

[dubbleyew eight]

in the photos preceding I used a block of wood to illustrate a few points.
what is the density of the wood?

take a ruler to it and you measure dimensions of 3 1/8" wide by 1 1/16" deep by 14 1/4" long.

3 1/8" is 3.25", this divided by 12 = 0.26041 ft

1 1/16" is 1.0625", this divided by 12 = 0.08854 ft

14 1/4" is 14.25", this divided by 12 = 1.1875 ft

0.26041 x 0.08854 x 1.1875 = 0.02738 cubic feet

351.4 grams is 0.7746964 pounds

0.7746964 divided by 0.02738 = 28.293 pounds per cubic foot.

so about 28 pounds per cubic foot. a perfect replacement for the original spruce.

[BroomstickPilot]

Hi Dubbleyew eight,

First of all, thank you for taking the time to write such an informative article.

Did you attend any of the LAA's courses on home construction, or have you learned your construction skills from previous work experience?

What about the opportunity costs of this project? Have you had to rent working space in which to carry out this work, or did you already own a garage or shed in which to work.

Regards,

BP.

[dubbleyew eight]

what strange questions broomstick.

LAA? you'd have to be in england for that.

opportunity costs??? what on earth are you on about?

wing ribs. there are 20 of them.

you build yourself a building template on a piece of MDF. you lay out an accurate NACA23012 aerofoil then draw the internals of the rib. then you varnish it all. you screw on supports to aid positioning. then you go over it all with bees wax.
you have a tool there that is a foot wide and one and a half metres long.
how big did you think it would be?

the rudder and fin aren't that big.
the stabiliser and elevator are a bit bigger, about 6 ft 6 inches long.

the thing is you build an enormous amount of the aircraft as flat parts.
the fuselage when it is made into a 3D item is still only about 3 ft wide and a bit over 14 ft long.

it is only after you have made all the wing components and start assembling the full wing that you need a lot of space.

your question really should be where would you hangar the aircraft?
you would probably find lots of building space in the hangar.

I was a pilot who ran out of money due to family expenses.
I spent about 18 years as a 'builder of the model' aeromodeller so I suppose I built up some woodworking skills doing that.
I read a lot of aeronautical theory, aeronautical engineering and a lot on old construction techniques.
I then set about building my turbulent. I already owned another aircraft by that time and was back flying.
I followed the old estonian proverb "the work will teach you how to do it".
all the way along the process I sought to verify that my construction techniques were sound.

you will find a lot of people who follow ritual construction approaches.
I dont.
I seek to understand the fundamental aeronautical engineering first then to execute a construction that meets the structural needs.
during my turbulent building I spent a lot of time investigating the design.
I have the benefit also of having a crackerjack aeronautical engineer as a friend and he is also interested in small aircraft as a hobby.
he guides a lot of my investigations into aeronautical engineering.

I don't claim infallibility or any god like status. I'm quite happy to remain non-famous and anonymous.
if you disagree with what I write then post and argue your objections.

[dubbleyew eight]

The subtle art of gluing wood

Before we start into the artistry let us look at what we need in a glue.

In Australia an aircraft parked on the Tarmac at Alice Springs in summer will heat up.
The outside temperature might be close to 40 degrees celcius, no wind and the sun just beating down. In those situations the aircraft might very end up being over 50 degrees celcius.
In Japan at the moment the place is covered in snow. Daily temperatures can be as low as minus 15 degrees celcius.
neither scenario is very conducive to flying enjoyment but if you own a turbulent there the aircraft sitting parked may well be subjected to those temperatures.
So what we need in a glue is that it will achieve a stable chemical bond between the components over a huge range of temperatures.

Mankind has always been able to create glues stronger than timber, like casein made from milk, but it was only in the last century that glues of the necessary stability were first developed.

Resorcinol Formaldehyde was the first really useful glue for aviation and was developed in the early 1930’s. This is a two part mix of a red syrup and a brown powder. mixed in the correct proportions this produces a glue of amazing stability. The glue requires very tightly conforming surfaces, pressure and heat to correctly set it off.

Acid catalysed Phenolics were once used but have fallen out of favour for aircraft use. The most common example of an acid catalysed phenolic is Selleys 308 marine glue.
this is a brown powder that is mixed with water to produce a watery paste that is applied to one side of the joint. the other side of the joint is brushed with formic acid and the joint assembled and pressed together. It is a truely brilliant glue. the problem that saw it discontinued in aviation use is that if too much formic acid was brushed on there would be some uncatalysed acid left in the joint and this would gradually attack the integrity of the wood, unpredictably weakening the joint.

The wonder child in aircraft glues though would have to be Epoxy. This has proven to be such a useful glue that it is used everywhere. The Laminex and Formica used in kitchen benchtops are paper and epoxy laminates. Entire aircraft have been built from fibreglass and epoxy, kevlar and epoxy, carbon fiber and epoxy.
Epoxy is basically two liquid chemicals that when mixed form a liquid that internally bonds to form a stable inert solid plastic. There are actually a number of different chemical formulations that can create epoxy. To achieve a total transformation to an inert solid the components need to be mixed together in exact proportions. There is a weakness in that if other contaminating liquids are entrained in the mix the reaction that produces the final solid form will not achieve the full strength in the final material. Oil and water are two strength destroying contaminants.

There are basically two epoxy formulations in use in wooden aircraft.

Ciba Geigy make a certified for aviation epoxy. The components are mixed in a 2 to 1 ratio. One of the components has titanium dioxide mixed in it as a water contamination indicator. If the resulting epoxy sets with a shiny surface there is no contamination, if it sets with a matte surface there is moisture contamination and the joint integrity is suspect.
While it is certified for aviation use it has the disadvantage that one of the components is a carcinogenic compound.

The other epoxy in common use is basically the WEST systems epoxy. WEST is the wood epoxy saturation technique used in boatbuilding and pioneered by the Gougeon Brothers.
This epoxy is a 1 to 1 mix by volumes. From memory it is a 4 to 5 mix by weight but I never use the weight method.

The west systems epoxy is what I use.

How much glue do you think you need to mix up at a time when building your Turbulent?

an intriguing question isnt it. you will find that a glue mix is usually just a few grams at a time. each glue mix has an optimal working time before the chemical change to the solid starts to become noticeable. you must get all the work done while the glue is liquid. so you need to do small glue mixes so that you are always be working with the fully liquid glue.
the west system epoxy is very easy to use in small batches and this is why I prefer it.

a story about glue contamination.
the airforce had developed a boron fiber structural patch that could be used to repair difficult to access cracks. while the technique worked it was subject to unpredictable failures. so the airforce hired the local composites expert to watch the process and see if he could determine the cause of the failures. for an entire week he watched patching work. it got to be quite boring and he could see nothing wrong so wanted to call it quits.
no! said the airforce, boring it might be but keep watching. we need to crack this problem.
the next week was even more boring and my friend was not enjoying it at all. come the final hours of the last day and mercifully the weekend and the end of the job. in one of the final ever so boring patches he had to watch the airframe fitter was having a problem getting the patch to sit where he wanted it, so leaning over he reached into his top pocket and took out a little screwdriver to push the patch into final position.
“stop right there” said my bored friend. “what is that thing in your hand??”
“my little screw driver. I use it for instrument changes.”
“where does it live?”
“in my top pocket.”
“yes but your top pocket goes all over the workshop and all around engines.”
“yes I suppose so.”
after further investigation the problem was traced to the ever so slight oil contamination on the screwdriver. it didnt look oily but it was just enough contamination to ruin the joint.

I’m sure people have many different approaches to glue mixes but this is how I did it while making my Turbulent.

http://i1274.photobucket.com/albums/y421/dubbleyew8/gluepots_zpsb9944e76.jpg

The little pots have proven to be very useful for mixing and seem just the right size.

When nurses dispense tablets to patients in hospital they dont touch the tablets to avoid communicating any bugs they come into contact with. They put the pills into the little containers. After the patient has taken the tablets the little containers get discarded.
On the wife’s visits to hospital she collects quite a few so when they are washed and dried these become my glue pots.

since we are gluing wood I use a wooden stick for the mixing. after each batch I let the glue cure and check that the stick has hard glue on it, then I stick it on the linisher and sand off the old glue. if the stick ever falls on the floor it gets set aside for the glue to set.
I break out a new uncontaminated stick. I make sure that the stick never gets contaminated with oil or water. any contamination and the stick is in the bin.

If you get epoxy all over you enough times you will become so sensitive to the epoxy that you will never be able to work with it again. one of the reasons for the long stick is so that you can work without getting epoxy on your hands.

more after I have a snooze in the afternoon heat.

[dubbleyew eight]

the first Turbulent built in Australia was first flown in 1958.
it is still being flown.

Adelaide Biplanes: Druine Turbulent (http://www.adelaidebiplanes.com.au/go/our-aeroplanes/turbulent)

still going strong at 56 years.

[dubbleyew eight]

I was once asked what I thought of PVA wood glue.
the common brand in Australia is called Selleys Aquadhere.
a chap I know was trying it in the wood parts of his aeroplane.
now while it is a really good glue for furniture work it is not ever to be used in aircraft.
remember that hot summer day I mentioned in the previous post?
what I asked him to do was get his heat gun and warm up one of his joints to see what happened.
next time I saw him (later that afternoon) all the colour had drained from his face.

all the glues that we use in aircraft are thermo setting plastics when cured.
there are two types of plastics, thermo setting and thermo softening.
if you heat a thermo setting plastic it just sits there getting hotter.
If you heat a thermo softening plastic it gets softer and softer until it melts.
Aquadhere, or PVA glue, is a thermo softening plastic and as it is heated it softens. as it softens the glue strength rapidly diminishes.
What my friend found when he heated the joint is that it fell apart.
On a hot summers day his aeroplane could have, would have, had a structural failure in flight.

We only learn new things by experimentation but you have to understand what you are doing.

[cockney steve]

W8... Thank you for your trouble to post this fascinating and informative thread.
Ihave seen a couple of boats built/ sheathed with the WEST system
One was constructed from Cedarwood strips,glued with epoxy. very light, enormously strong, covered with a skin of glass tissue and epoxy.
The resin soaks right in the wood and looks just like varnish!
The other was an aged Mirror Dinghy,stripped back to bare wood, dried right out and then Epoxied.

Great stuff,please keep going!

[dubbleyew eight]

When wood is turned into timber suitable for engineering use all but 8 to 10% of the moisture in it is dried out.
Usually the wood becomes quite stable but it is still subject to the atmospheric moisture around it.
If the timber is stored in humid environments it will absorb additional moisture.

two considerations arise form this hydroscopic characteristic.

The first is that we coat all surfaces in a wooden aeroplane with a varnish. this pretty well waterproofs the timber and slows the absorption of moisture.
The hollow parts in the aeroplane like the nose of the rudder and elevators are all varnished internally.
We leave no unvarnished surfaces on an aeroplane.
when gluing on a ply surface that would seal a hollow we mark on the ply where it will be glued and varnish all the rest of the surface. we then make sure that the unvarnished glue areas are all fully coated with epoxy when we do the gluing.

The second consideration is that as the moisture content increases in the wood it swells up, as it dries out it shrinks.
What that means is that if we glue together two pieces of wood with different moisture contents, they will eventually stabilise on a common moisture content. while doing this there will be noticeable shear forces set up in the glue joint. since all the loads on a joint are additive we are preloading the glue joint and in effect making the joint weaker.
storing all the wood together and allowing it to stabilise on a common moisture content by not getting around to doing the building for a few months is actually a good thing.

Ok what does a good glue joint look like?

where the two timber members meet the surfaces should be of the same shape. usually this will be a pair of flat surfaces.
the members should be cut to exact length. if we put in the joint a member that is not at exact length we will set up the forces that cause the section of the structure to warp and twist. since the timber sections are quite small in most cases it is not an expensive problem to cut another piece if we find that a member is too small. if we find it too long we just sand it off a little more.

You will notice on the plan there are gussets on the joints. if you look at a joint area it isn't large. what the gusset does is increase the glued area of the joint many fold and substantially reduce the stresses in the glue bonds. This makes the glue joints much more reliable.

When we glue the actual joint we apply the epoxy to both surfaces. then we position the surfaces together and clamp them so that they have no relative movement for the entire time it takes for the glue to set and cure.
Once we have it clamped in place we then we scrape off all the excess glue. the glue bond is just in the area between the members being glued. glue fillets don't add any strength to a glue joint and just add needless weight to the aircraft.

Where Resorcinol Formaldehyde glue is used, dribbles and glue fillets are actually a liability since they crack over time and at the bottom of the crack we get bare timber which lets moulds and moisture get past the varnish protection.

When plywood is made the timbers are pressed in heated presses. we substantially improve the bonding to plywood if we give the ply a light sanding all over the glue area. this improves the glue bond. this is where our sanding blocks make the job easy.

I hope needless to tell is that we remove sanding dust from surfaces being glued.

A small anecdote on improving glue strengths.
a company here in Perth experienced poor performance from some of their laminated timber beams.
After lots of experimentation it was found that gluing old surfaces of wood compromised the glue joint.
What they found is that gluing a freshly machined wood surface made for a stronger bond. in Gluelam manufacture the section comes out of a thicknesser and is immediately coated with glue and placed in position.
In practical terms it seems that gluing a surface that was prepared no more than a quarter of an hour before consistently achieves sound glue joints.

[dubbleyew eight]

When you are mixing the glue you need to be very accurate in the proportions.
In plastics terms epoxy is formed when two components (each called a Mer) are joined together and form a polymer.

if we have inaccurate mix proportions then we end up with too many of one of the plastics components and the overall cured polymer is weaker.

also when we make a mix, in my situation in a little pill cup, we leave some of the epoxy in the cup.
first confidence check for our bonding is whether the epoxy in the cup set hard.

if we pull the hardened lump of epoxy out of the cup and find that the outside of it is sticky then we haven't stirred the mix adequately. stiring places the two mer parts in close proximity to each other so that the bonding that leads to a solid polymer can take place.
not stirring the mix completely can be as catastrophic as getting the proportions wrong.

if in a few years time you had a doubt about the soundness of glue joints in your turbulent, how would you convince yourself that the aeroplane was safe to fly.
simple really.
you glue up some test pieces with each batch of glue mixed, four or five is probably a good number of test pieces.

you break one after the glue has cured as primary confirmation that the glue batch was sound. the others you keep in a plastic storage box. in 20 years time if you have a doubt you get one or two out and break them with the hammer.

your hands can get an oily coating so when I spend a night gluing I wash my hands periodically to keep them oil free.

here is a rogue little question to exercise the mind and the research skills.
why is it that you shouldn't carry a bag of oranges or lemons in your wooden aircraft?

[Tinstaafl]

Citrus oils are solvents. That smell of citrus? That's from minute citrus oil droplets in the air released by the fruit. At least, that's my understanding.

[ExSp33db1rd]

Thanks for that tip ! I'll remember that when I next fly my Turbulent.

My Turb. was built in the early 60's, hope the builder read all the above !

[A and C]

What a refreshing thread ! Expert opinion well presented.

[dubbleyew eight]

tinstafl is on the money.

citric acid is a particular threat to epoxy. it almost unique in that it attacks the strength of the glue.

this doesn't apply to resorcinol formaldehyde glue. that has different characteristics.

What I hope I have done is give people a route to the confidence needed for building a turbulent.
there are really only two hard things about building an aeroplane such as a turbulent, or any aeroplane for that matter.
Deciding to start the project.
Completing the project.
the rest of it is just technique.

I am still looking for some spar strength calculations I did some time ago so I have another post or two to make. be patient.

[djpil]

Looking forward to seeing the spar calcs. (I did some many years ago.)