Wood and fabric airframes

[MichaelP]

I posted this on another thread, it would be unfair to take over that thread on another topic and so I suggest we continue the discussion here:

"moisture testing for wooden airframes"

This is very interesting.
I remember testing being done on WW1 airframes of considerable age and that the wood, though dry, retained more than 80% of its strength.

I am a firm believer in wooden airframes and their strength (with resin glues!). For aerobatics I trust wooden spars over metal ones.
But wood needs to be looked after.

I recovered two of my Condors which were built of wood and fabric in the same manner as the Jodel.
I recovered them in cotton fabric.
I will never willingly cover a wooden airframe with Ceconite or Stitts materials.
I have seen too much rot to have confidence in these materials.

If the cotton's rotten the airframe underneath needs looking at

I once changed the fabric on a tailplane because it had been on there 22 years. It was cotton but not rotten!
The woodwork underneath was in perfect condition, the Condor had been hangared all its life.
I recovered it in cotton.

I removed the Ceconite from above the walkway of the Slingsby T67A (wooden RF6B predecessor of the T67M Firefly). Nothing sticks to Ceconite it is a mechanical bond and so galleries form between the fabric and the wood and the fabric itself works like a greenhouse.
There was a lot of rot in there and I scarfed new wood in to repair it.

I have had the fabric off a Jodel wing and found rot in the undercarriage attachment area. In the D1050 Jodel Ambassadeur there is an inset in the top of the wing where the landing gear attaches and water can accumulate in the corners. A metal cover goes around the leading edge to cover this up and for the shape of the wing and so this area is not subject to preflight inspection.
The spar is a box with ply webs and spruce in the corners. It is very strong.
The single seat D9 Jodel has a 9 G wing and can be used for aerobatics.
The two seat D150 Mascaret was also aerobatic.

When I bought my second Condor I was told that two fabric panels in the top of the wing needed replacing... I thought about it, took out my knife and stripped the whole lot off.
The structure underneath was however perfect and so I bought some more DTD575A cotton fabric and recovered the wings, ailerons, rudder and elevators. I did the tailplane later as above.

There was a Cavalier at Delta Airpark and I had the job of checking its new owner out.
This aeroplane looked awful!
The water soaked ailerons had to be removed and totally rebuilt by the new owner.
There was some rot in the wings as well...
It was parked next to the Jodel, out on the damp grass.

Swordfish:
Excellent post, MIchaelP. It gives us an insight into wood & fabric structures that's an interesting wake-up for many of us unfamiliar with the subject.

I am wondering if the rot problems can be solved/alleviated by inside storage, as it is damp pretty well year round on the coast. Also, I didn't follow your discussion on Ceconite very well, and was wondering if you could amplify the difference between cotton and Ceconite. Thanks.

[MichaelP]

Inside storage alleviates the problems considerably.

The best storage I've seen was a steel Nissen Hut - Hangar where the bottom had rotted out. This allowed air to circulate while the aeroplanes were covered from the Sun and kept dry.

There are several wooden aeroplanes in the hangar besides where the Jodel was kept and they are in excellent condition.
There's a Jodel, a Turbi, a Super Emeraude etc in there. There is a Super Emeraude airframe in the rafters that's been there a long time (20 years) 'looks like beautiful workmanship but nothing can be done with it...

Ceconite is a man made material, it is very strong, and it is glued to the structure using super seam cement.
The bond is mechanical as nothing could stick to this material without disolving it and that would affect its strength.

Cotton on the other hand is a natural fibre and the dope becomes and integral part of the fabric. It is possible to bond cotton to the wood to the exclusion of anything else such as water and fully seal the structure.
I once tried to remove the Madapolin from the fuselage of one of my Condors, forget it! It was doped on to become totally integral with the wood.
Being a natural fabric Cotton rots... If the cotton's rotten the structure underneath needs inspection.

As Ceconite is a mechanical bond there are galleries formed between it and the wooden structure if care is not taken to exclude all the air. I used to soak the Ceconite in a mixture of super seam and dope and use a cotton rag to swab it down against the wood and exclude all the air between the fabric and wood.
But once hardened cracks can form in the superseam and create a passageway for moisture.

Damp days and sunny days can create the circumstances for the growth of all sorts of mold and green stuff
This is especially true of Ceconite which can create a greenhouse atmosphere perfect for rot.

If you keep a wooden aeroplane outside, then it is better if that aeroplane is covered in cotton rather than Ceconite or Stitts.
The Cotton will not hide the structural damage occuring.
It may last up to seven years before it will need to be removed and replaced, and I don't think a full airframe inspection is a bad thing.

One of my earliest recollections of Ceconite was of an Emeraude kept outside at Biggin Hill.
When the fabric was removed most of the airframe came apart
I resolved then to only cover my wooden airframes with Cotton, and I was as good as my word.
I covered other people's airframes or repaired them with Ceconite under a little protest.
I didn't mind covering Austers in Ceconite... The Steel Aeroplane!

[swordfish]

Many thanks. Cool stuff.

So I understand that these "galleries" to trap moisture are the problem. Why don't you shrink the Ceconite onto the ribs (& the rest of the plane, of course) and then redo the sealing between the wood and Ceconite where it contacts the wood, by soaking?

Also to alleviate this problem of the "greenhouse", why don't you install ventilation holes near the trailing edge to drain the moisture out?

[Old Dog Flying]

Swordfish: I was going to stay out of this but once again our resident Brit sticks feet in mouth...whether you are using cotton or dacron the finishing procedures give the same results if the underlying structure is properly prepared and finished. And that is the problem.

Wood structures need a high quality marine vanish finish before any fabric is attached just as steel structures require proper priming...preferably with epoxy. In either case if the preparation is not done properly there will be serious results.

Dacron material...by many names...is glued to the structure with the correct glue then shrunk in place by heating with an iron while cotton is glued in place with nitrate dope then wetted to shrink, which of course can be overdone damaging the structure. In either case the bonding is the same...if done correctly.

The final finish is then applied but not before "seaplane" drains are glued over 1/4" holes in each bay...at the lowest point in each bay.

Improperly prepared structures can rot or rust and I haved seen steel airframes that were held together by the fabric and paint just as I have seen wooden structures come apart for the same reason. Cotton will rot and that is a given but the dacron fabrics can last an extremely long time. In any case if the aircraft is allowed to sit outside in the elements, no amount of preparation will allow them to last.

I've worked on fabric covered aircraft for nearly 60 years, receiving my lessons from an old guy who very quickly adopted dacron when it became popular in the 1960s and it is still the fabric of choice.

[MichaelP]

Wood structures need a high quality marine vanish finish before any fabric

Once again Barney takes a pot shot at me... Oh well, you can't change that...

Like Barney I was taught by experienced old fitters who grew up with wood and fabric aeroplanes.
Rollasons who built the Condors and who were responsible for civilianising hundreds of Tiger Moths did not believe in coating the wood surfaces with anything but rather to let the wood breathe.
The Condors were unfinished under the fabric and they suffered little or no rot at all. I never found rot in a Condor, I did find a few broken ribs and failed glue joints, otherwise the naked structure of both my Condors was in excellent condition.

But the Slingsby was a much newer aeroplane, it was only three years old when I go it and yet the rot had taken hold under the Ceconite.

It all depends on the climate. A 'Dacron' covered aeroplane in a dry country is one thing, it's another in a damp climate.

Cotton and Linen shrink with taughtening dope, and that's the way the fabric stays.
Ceconite, Dacron, and Stitts, are elastic and after shrinkage they impose this elastic tension on the structure.
There was an AD to reinforce the trailing edge of the Jodel wing with bigger gussets because these new fabrics would warp the trailing edge and break the joints.

Yes there are opposing views like most things... But Rollason's Tiger Moths covered in linen and with bare structures are still flying in the damp UK climate.

Rollasons also used Ceconite from time to time, but then they used red dope on it as well... Red dope contained an anti fungal ingredient...

It's a matter for each owner to decide, my decision is to cover a wooden aeroplane in Cotton or Linen.
The fabric itself will tell you if the structure is sound underneath.

[sportingrifle]

If you have ever seen a nitrate doped linen covered aircraft catch fire after an accident, you would be very appreciative of stits/ceconite's reduced flamability. The fabric doesn't just burn, it damn near explodes. We quickly went to the Stits process on our agricultural aircraft as a safety enhancement. It also seemed to resist harsh chemicals and cleaners better. If ceconite/stits had any major adhesion issues, they wouldn't have won so many STC's from the FAA and Gold Wrench awards at Oshkosh! Wood structures have to be kept dry, period. Doesn't matter what they are covered with.

[Hedley]

I know several people who tried Stitts and weren't happy
with the fact that it doesn't tighten up, so if you fly faster
than a Piper Cub (which it is well-suited for) it balloons.

There are much better STC'd fabric processes out there.

http://www.airtechcoatings.com/faq.html

[Old Dog Flying]

Hedley: Possibly the guy that couldn't get the Stits fabric to tighten up probably tried to apply it the same as cotton...believe m it happens to people who are very good with cotton but are so damned arrogant that they won't read the instructions. I had an AME at a flying club that tried to shrink Stits with water...which of course does not work.

My old Tiger Moth was cotton/nitrate and it went up in flames in about 30 seconds. But then again it was finished in the old ways before safer more efficient methods were developed.

[Hedley]

Two airshow pilot friends of mine independently
tried Stitts, and neither of them liked it.

One guy recovered just one wing with Stitts
and it ballooned at high speed, causing a yaw.

Another guy covered two Christen Eagles with
Stitts (at the same time) and they didn't like
it, either.

Like I said, it's ok for a Piper cub at 60 mph, but
if you want to go 200+ mph, look elsewhere
for fabric.

[MichaelP]

If you have ever seen a nitrate doped linen covered aircraft catch fire after an accident

Yes, I am aware of that, and most agricultural aeroplanes are metal so it really doesn't matter.
I used to cover Austers with Ceconite and I had no problem with it.

Regardless of the above, the subject is wooden aeroplanes and it doesn't matter much what you cover it with as the whole thing will burn anyway!
But wood will rot without evidence under the man made materials whereas with Cotton and Linen the evidence is likely to be there when you inspect it.

Having written that, a friend was topping his Taylor monoplane up from a petrol can... It caught fire and burnt all the fabric off leaving the structure ready for recovering

I flew a Stearman in a movie, they made a flare path of railway iron stakes topped with paraffin soaked sacks and set alight, it certainly concentrates your mind on keeping dead straight when flying an aeroplane covered in highly inflammable dope and fabric!

[airbusflyboy]

Super Emeraude CP-328 with STITTS fabric covering in 1999 ............................ aircraft has been parked outside under a covered type open air hangar.
Would anyone here suspect that there might be cause to do a "tap" on the wood wing to check for possible wood decay ? Has anyone here had any experience with this type of aircraft wood wing ? Interested in possible purchase but the "wood wing and body" is a little bothersome to me.
Thank you !!!

[robertsailor1]

I'm still having difficulty understanding how Stitts fabric would/could balloon on the wings. I've personally owned many different fabric covered airplanes some slow and some fast but I have never seen or heard of this. Assuming whoever did the recovering knew what they were doing it seems impossible but I'm always open to learning something new. After fabric is applied on the wings and then tightened by whatever method is applicable for the fabric each rib is covered with tape and is then rib stitched and the faster the aircraft the closer the rib stitching. No way this is going to move so that leaves the space between the ribs that would have to balloon and unless the fabric applied was way to light to begin with...well never heard of that either. By the way, Stitts or Poly Fiber as they now call it has a very large following among home builders. All these man made fabrics are really just Dacron in different weights except for Razorback which is a light weight fibre glass as I recall.

[MichaelP]

Super Emeraude CP-328 with STITTS fabric covering in 1999

A wooden aircraft is best kept in a well ventilated hangar. A roof and a dry floor.
It sounds like this Emeraude is being kept properly.

Check inside the fuselage around the stern post and above the tailwheel attachments.
Check where the undercarriage mounts for hard landing damage.
Check the floor under the seats and around the whole centre section.
Ensure all drain holes are clear.
If she's built well, and looked after, the Super Emeraude is a delightful aeroplane.
In Europe these aircraft have lasted for fifty years or more... It all depends on how you look after them.

[airbusflyboy]

Regarding the SUPER EMERAUDE, I do appreciate your information on how to check out the aircraft ........................ problem is to FIND a mechanic who is willing and familiar with WOOD airplanes !!! This aircraft was built in 1978 as I understand it .................... current power plant is a Lycoming 0-290G with fresh rebuilt engine. Aircraft is located in OKLAHOMA

[MichaelP]

Skills and competence are difficult to find in most things in Canada and so if you find a genuinely good mechanic you should treasure this person.
Another way to do anything is by the "If you want a job done properly, do it yourself" philosophy.
Buying a homebuilt aeroplane complies with the do it yourself philosophy and will provide you with years of learning... We exist to learn so that's good, but it's also frustrating at times.

There's a lot of knowledge easily available on the web from all over the world so it's not difficult.
Always step back from a problem and think about it before acting. You save a lot of money doing this.

The Emeraude should live well in the Oklahoma climate, it's a much better proposition than a coastal based aeroplane.
Emeraudes built in the 50's are still flying although you should check the glue used.
If you need me to look at this aeroplane I will but I'm not certified as an inspector.
Where are you based?

[RatherBeFlying]

Where will you be keeping this plane?

Hangared above 0C in the Prairies should be OK, but think about leaving it inside on those -20C days.

Outside in BC, Ontario, PQ, Maritimes, NF would not be smart.

Varnish has to be renewed. Water trapped against varnish will lift it. Scratches and fasteners will allow water to get under the varnish and lift it. Any moisture trapped under varnish will do bad things to the wood.

Bare wood can breathe.

I'm a fan of penetrating oil finishes, but I suspect there's no certification data.

[airbusflyboy]

Aircraft is in central OKLAHOMA ...................... near Tulsa. Not sure of status of the sale, it might be sold .................................

[robertsailor1]

I'm not sure why people are afraid of wood. There are wood structures that are many hundreds of years old that are in excellent condition. Wood rots, steel rusts and aluminum corrodes, all materials have their strengths and weaknesses. Wood needs to be kept dry or at least be able to dry out. I wouldn't be too worried about modern glues, if your buying something glued in the 30' or 40's that's a different subject. Often local homebuilders are better informed on wooden aircraft inspection that some A&P's. The Emeraude was a classy aircraft in my books, easy on the eyes.

[airbusflyboy]

Down here in the USA, folks are trying to sell their Mooney M20 airplanes .................... wood wing, wood tail airplanes from the factory. The wood tail had to go away, due to A.D. directives, the wood wing Mooneys are supposed to be amongst the fastest of the airplanes produced during those times, I have heard this over and over from others on various websites. I like the idea of wood, it is very light and strong. Just that when you go to sell an airplane like that, it is harder to sell it compared to others out there.
Thanks to all of you on this site for your inputs and ideas !!!

[robertsailor1]

Your right about the Mooneys, great plane, I've owned 4 of them over the years. No question about the wood tail, it was not up to the job and I personally would not fly one. I don't even think they are allowed anymore. The wood winged Mooney's were another story. They were fast and they rode nicer in rough air. I had a Super with 200hp and my friend with his wood wing Mooney could damn near stay right with me and he was faster than a 180 hp metal wing Mooney by a good 7 or 8 knots. They are still a real good airplane but you need someone who knows them well doing the annuals because water and fuel could get to them around the wing centre section. No problem if you know where to look but like you say they suffer on the price and time to sell and its been that way as long as I've known (long time) I sure liked that fool proof armstrong gear and the rubber donuts in the suspension.