Author Topic: Introduction and Question to Doug Fir versus Spruce and Epoxy  (Read 2154 times)

Offline Pilotarix

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Hi all,

I am relatively new to the world of Part 103 Ultralight Airplanes, but I am not so new to aviation and the idea of building my own plane. I have a couple of hundred flying hours on the usual suspects (Piper Cessna Diamond) and some hours on Schweitzer and R44.
The above experience is quite a while ago, and I am now trying to get back into aviation.
I am looking for low and slow. Just a couple of hours a month, no cross-country or the like, no going into bigger airports, no stress, not spending huge amounts of money. That was what brought me to Part 103.
First, I stumbled across the Affordaplane and was initially delighted by the simplicity of construction.
However, thereafter I came across the Legale Eagle and was immediately excited, even though the welding part was somewhat intimidating. Meanwhile, I have gotten the plans and waiting on the supplemental wing plans from Iron Design.
I brushed the dust off my TIG welder and started working on my welding skills, currently wasting tons of Argon and 5/8 4130 tubings. I know that down the road, I will need some Oxy/Acetylene as well.

I want to start the Projekt with the wings, mainly because I have to finish other pending projects before I can put up a worktable in the required size, and as mentioned above, I still have to improve my welding skills.

Making ribs does not need a lot of space, and will keep me busy for a while.

Now my questions.
I have a very nice piece of Doug Fir in my Garage, 2x6 20' very good quality, clear straight grain, a leftover from a boat project.

Can I use Doug Fir instead of Spruce?

For wooden boat projects, I use West System Epoxy. Can this be used on the wings?
West System is a high-quality Epoxy system, and in boat projects, I have experienced that the wood gives up but not the Epoxy joint.

Thanks,
& happy 4th of July
Christoph

Offline Dan_

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Re: Introduction and Question to Doug Fir versus Spruce and Epoxy
« Reply #1 on: July 04, 2021, 09:55:46 AM »
Welcome aboard!

Doug fir is heavier but stronger and can be substituted and even reduced in size to make up some of the weight disadvantage.  AC43-13 has the low down on it if you want to take up the engineering and test pilot challenges.

The weight budget on the LE and XL is extremely tight and this change will likely put you into experimental territory.

Also here is some other reading on it: 


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Offline Pilotarix

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Re: Introduction and Question to Doug Fir versus Spruce and Epoxy
« Reply #2 on: July 04, 2021, 02:14:51 PM »
Thanks for the welcome and the linked information.

I wasn't aware of the weight difference, but that is a good reason not to use Doug Fir. In Boats, the weight is not so critical but the strength. I made spars from Doug Fir with West Epoxy, and they can take quite a beating.
Anyhow, I read through the documents you recommended. It seems to be concerning to use what has not been tested and/or has proven that it will meet requirements. Understandable. Inflight separation of critical wing components will make, in most cases, a smooth landing impossible.
It looks like I have to source some spruce and T88. The last time I used T88 was in a model airplane project.

Offline Dan_

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Re: Introduction and Question to Doug Fir versus Spruce and Epoxy
« Reply #3 on: July 04, 2021, 05:30:13 PM »
It's a journey, if not an odyssey... So worth it however.
--Stay with it..!


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Offline Pilotarix

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Re: Introduction and Question to Doug Fir versus Spruce and Epoxy
« Reply #4 on: July 04, 2021, 07:01:27 PM »
Thanks, I will do my best. As we all know, those projects are very dependant on the overall life situation of the builder. Something changes significantly, and the project is the first thing to be tossed out. Years ago, I built a 1:16 scale piper PA18 fuselage and landing gear from stainless steel and 4130 tubings.  This followed pretty much the same principle, just on a smaller scale, and I used brazing, which is way easier than welding. https://www.rc-network.de/threads/baubericht-piper-pa18-rumpf-in-stahlrohr-bausweise-nach-plaenen-von-piper.300786/ sorry language is not English.
Moving to a new house interrupted the process. I never restarted; it's still sitting in my basement.
Hope I can do better this time.

Offline Aaron

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Re: Introduction and Question to Doug Fir versus Spruce and Epoxy
« Reply #5 on: July 05, 2021, 05:57:41 AM »
"For wooden boat projects, I use West System Epoxy. Can this be used on the wings?"

In regards to west system I did my entire LE wing with west system, worked great. In my opinion, I like west system more than any other wood glue. Don't get me wrong wood glue does a great job but heavily relies on clamping force. West System likes clamping force but it is not necessary to achieve an airworthy joint. I demonstrated this by mixing up a little bit and butt gluing 2  1/4" sticks of spruce together (the ones used in the wing ribs), No clamping pressure, just a suitable amount of epoxy, and the sticks broke before the joint did. Also note that fischer aircraft glue all their prebuilt fuselages together with West System (at least used to). Point being, if you are willing to spend $ on the high quality epoxy I would recommend. If you don't already know, caution when mixing, mix small amounts building the wing because you often don't need a lot, and spread it out or it will kick fast on you. Get the slowest hardener, you will need it, and the best application utensil on the wing I found was to be my index finger ;) Enjoy!

As said earlier regarding the Doug Fir, it is a perfectly acceptable material for aircraft. I haven't used it on Ultralights, but I would definitely do some math and dimension it down appropriately to keep your Part 103 weight in mind, every gram counts and it will creep up on you, however don't cheat yourself on strength, that is more important in my mind :)

Offline Pilotarix

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Re: Introduction and Question to Doug Fir versus Spruce and Epoxy
« Reply #6 on: July 05, 2021, 08:52:06 AM »
Aron,
thank for your input.
As mentioned, I used West System in all kinds of boat projects. For e.g. I mad a 19 foot main mast for a Caledonia Yawl in birds mouth technique and many other parts like centerboard or rudder. This stuff is expensive but one of the best epoxy glues out there.
I checked the fisher website but cannot find information about the Epoxy they used. In their videos it seem to be T88, they may have changed it. If yes, would be interesting to know why.
Typically for structural applications if there are gaps it should be thickened with a filler. I did this for many applications, there is no way to break a glue joint, the wood always breaks first.  Did you use a filler ?
I will do a littel bit more reasearch on that topic.
I used the "finger tool" alot, gloves and silicone finger caps ensure that there is not too much contact to the epoxy.

Thanks
Christoph

Offline Vince Carucci

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Re: Introduction and Question to Doug Fir versus Spruce and Epoxy
« Reply #7 on: July 05, 2021, 10:40:49 AM »
I've used both the West System [wood and fiberglass laminate for tail wheel spring] and the System Three [T-88 for the wing build]. Both are great epoxies. 

I found a forum on Fine Woodworking where members discuss comparing the use of both. It could be another source of useful input. 

You can see the engineering properties of System three and of West System here. 

What ever you do... DO NOT MAKE YOUR WINGS OUR OF STRAW!

Good luck.

Vince

Offline Aaron

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Re: Introduction and Question to Doug Fir versus Spruce and Epoxy
« Reply #8 on: July 05, 2021, 10:58:10 AM »
Sounds like you have plenty of experience with it Christoph, You are right it is pricey, but I agree the stuff is fantastic.

Interestingly, their website (fischer) does show in the FAQ's that they use T-88. To back up my earlier statement, my brother built a fp-303 from a quick build kit, in which all of the factory glued joints on the fuselage, tail feathers, wing ribs, geodetics, etc were glued with West System.

 I did not use a filler, I used timing to my advantage, gluing the stuff I could clamp right away and if a piece would be easier with a higher viscosity I would save it for later on, when the batch would start to kick and glue it up while the viscosity would be higher. That worked quite well for me. Note I'm no expert, just sharing my experiences and the decisions I made which I thought was best, so take it as you will...

You are also correct that wood usually will always break over the glue joint. Butt joints are the weakest joint you can make which is why I consider that a good test of the glue for airworthiness, thats just me :)

Also at Vince, I saw on Barnstormers a while back a LE for sale for 400$ or something like that. The wings were unairworthy, built with gorilla glue...dont do that either  ::)






Offline Dan_

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Re: Introduction and Question to Doug Fir versus Spruce and Epoxy
« Reply #9 on: July 05, 2021, 01:41:40 PM »
One thing...  Be sure the epoxy will take the build up of heat when the wings are out in the sun.  I think you will find T-88 is superior in that regard.


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Offline Pilotarix

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Re: Introduction and Question to Doug Fir versus Spruce and Epoxy
« Reply #10 on: July 05, 2021, 03:55:46 PM »
@ Dan, I appreciate your concerns, and somehow, mine are the same. However, West System Epoxy is actually an outdoor product typically used on boats, and they can be in pretty harsh environments. The Teak parts (West System and Varnish) of our old boat could get smoking hot in the summer but did not disintegrate.

Anyhow, I have an email out to West System technical support with my concerns. I will share the answer if there is one. 

Also, they seem to have a new product, looks like T88 ... 1:1 ratio longer pot life somewhat elastic, interesting.

My "glue corner" ... yep, could need some cleanup.  ::)


Offline Pilotarix

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Re: Introduction and Question to Doug Fir versus Spruce and Epoxy
« Reply #11 on: July 07, 2021, 02:37:33 PM »
Hi all,

I got an answer from WestSystem.
Their Epoxy will get somewhat soft above 130F but does not fail and go back to normal strength as the temperature normalizes. Above 250, it will finally fail. They claim that they have lots of aircraft builders and repairs shops as their customers.

Whatever this is worth. I am going to do a test. I have both WestSystem and T88.

Thanks
Christoph

Offline Dan_

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Re: Introduction and Question to Doug Fir versus Spruce and Epoxy
« Reply #12 on: July 07, 2021, 04:36:45 PM »
I'm pretty sure the link Vince sent in below gives the transition for T-88 as 180F. 

A lot of airplanes have plywood that is covered in light fiberglass cloth and West System epoxy...  This is where West System shines if you ask me.

Haven't heard of many planes with the wings falling off lately, and painting your wings white and providing shade when parked should help to quell much concern.  Don't do any high G maneuvers until the wings have had time to cool off, and a temp sensor may help your nerves as well...


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Offline Pilotarix

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Re: Introduction and Question to Doug Fir versus Spruce and Epoxy
« Reply #13 on: July 08, 2021, 10:11:44 AM »
That is correct. WestSystem has a special hardener that is curing to a very clear finish and excellent for covering wooden surfaces as long as there is also some UV protection with varnish. 205 and 206 are for structural applications.
Here is a picture from our Caledonia Yawl.




This is a lapstrake construction, only glue (epoxy) no other fastners. The main mast is laminated out of clear straight grain Doug Fir in birds mouthe technique using Epoxy with Micorfiber Adhesive Filler. It's a free standing mast and has to withstand a lot of bending force while exposed to the elements, so far no problems.
Anyhow, I am not fixated on one system. In all the information I have seen they used T88. So I am going to use T88 as well. I decided not to monkey around and stick with the plans and general recommendations.

Offline Dan_

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Re: Introduction and Question to Doug Fir versus Spruce and Epoxy
« Reply #14 on: July 08, 2021, 10:37:51 AM »
That's a great looking ride..!  Had to look up the bird mouth method.  Miles of glue surface.  You will need the strength if you ever have to heave-to.   
:emoji_u1f61c:


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