Wood Constructed SSDR's

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Wood Constructed SSDR's

Postby tom@covehouse.demon.co.uk » Wed Oct 13, 2010 6:18 pm

I am newcomer to this area and to flying. I am keen to build myself an SSDR from wood. I have seen a lot of discussion and cross discussion about the use of uncertified wood.

There seem to be many candidates for the job other than sitka spruce, but the crux of the issue to me seems to be inspection and stress testing. There's a fair bit of literature on inspection; notable from the US, but very little on stress testing.

Does anyone out there have any info on constructing a test apparatus that you could use in the homebuilder's workshop?
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Postby ColinC » Thu Oct 14, 2010 8:45 am

Hi,

welcome to a minefield! You will find a number of discussions about it on the LAA Forum. You are not the first with the problem.

Here's something to read on testing which I thought was good. I have replicated the process described on some wood that has been independently tested and the result correlate:

http://www.bowersflybaby.com/tech/testing_wood.pdf

Also, I built an impact test rig to check brittleness to a design that I have seen in some old De-Havilland documentation and in ANC-19, there's a picture here:

http://www.laaeastmidlands.org/strut_ga ... 02&pos=-42

My suggestion is that if you are going to select your own wood then look first at ANC-19, BS2V-37 and BS2V-38. There is also a good EAA publication called 'Wood Aircraft Building Techniques'.

There are suppliers of a couple of suitable wood types in the UK but probably it still comes down to Sitka Spruce or Douglas Fir as the most efficient struturally, the latter being probably easier to obtain. It is possible to go your own way on wood supply, the LAA mag has published articles (if I remember rightly) by Richard Mole (Jodel D18) and Roger Partington (Menestrele).

However, and am not wishing to put you off building, but going your own route on wood selection with an SSDR may not be wise if as you say, you are new to the area. The LAA Inspector list will identify someone in your area who does have relevant experience and will be able to help you with the choice of materials and advise on suppliers. Also, given that by its nature it won't have too much wood in it you could probably buy it in for a reasonable cost.

My suggestions for 'aircraft' wood suppliers:

DudleyPattison - advertises in the LAA mag, also for ply.
Peter Johnson - Eastern Sailplanes
David Hodgkinson - Hodge Air Services

Hope that's useful for now.

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Postby merlin » Thu Oct 14, 2010 9:38 am

Colins info is good and you might do well to also have a look at the Flitzer site to seek info on wood.
There are a couple of US sources that I have used such as A/C Spruce and Spec. , dependant on exchange rate and shipping though.

The Minimax is probably a good example of lightweight construction, albeit not to an SSDR level.
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Un-certified wood

Postby tom@covehouse.demon.co.uk » Thu Oct 14, 2010 11:04 pm

Thanks for the info.

I have the EAA book and copies of ANC-19 + ANC-18, NACA Report 344 "Aircraft Woods: Their Properties, Selection and Characteristics, etc.

Coming from the Scottish Borders the number of LAA members (let alone inspectors) seem to be thin on the ground. I have made contact with the local Strut but have yet to attend a meeting. I believe from first contact that it is not very active.

Having an engineering background, I tend to be somewhat empirical in my approach to things - judging by results rather than theoretical input. In building an SSDR my intention is to keep cost down by use of un-certified wood, build a prototype for load testing to destruction and then (assuming successful results) duplicate the construction in a working version. I wanted to be able to test the wood to ensure repeatability in the construction so that the final product has wood in its structure that is at least as strong as the sacrificial version.

Perhaps a bit OTT but I would certainly feel much safer in the air in something that I am pretty sure is up to the job through actual loading tests rather than in someting that theoretically is strong enough.

Thank again for your input

Tom
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Postby ColinC » Thu Oct 14, 2010 11:55 pm

Tom,

forgive me for asking, by why the SSDR route? Is there not an existing LAA approved design that would satisfy your requirements and cut through the amount of work that you are setting yourself up for?

The Minimax mentioned earlier is a well thought out and supported design that ticks a lot of boxes.

For me, completing an aircraft project is hard enough, without trying to design it too.

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Postby Bill McCarthy » Mon Oct 18, 2010 10:19 am

Tom,
I would take a good look at some of the good designs out there and use the best points on simplicity, strength and cost. You will see in many past issues of LAA monthly mags that the method of strength testing is to load the flying surfaces with sand bags. If you "copy" an approved design, this would have been done in the acceptance process.
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Postby Brian Hope » Mon Oct 18, 2010 9:54 pm

Hi Bill, loading the structure is not a universal route to approval. If a design can be shown by calculation to be strong enough then it will not necessarily have to be load tested. Likewise designs accepted on a proven good service history overseas.
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Postby Ian Melville » Tue Oct 19, 2010 7:33 am

Oh how I wish it were that simple :roll:
Firstly 'borrowing' ideas from other design is standard practice in aviation since 1909. More evolution than revolution. However any stress/load testing cannot be assumed unless you copy almost the entire design, and that just wouldn't be cricket.

For the home designer testing with sandbags is the only practical way unless you are, or are willing to pay for a stress engineer. Even if it passes, I doubt I would want to fly in that airframe. I see little point in building the test airframe in anything other than the correct materials, unless you are just checking fit of parts.

In industry, by calculation is the way to go as you are more likley to get it right first time. However load testing may still be needed to 'prove' the calculations. Even if just to 'limit' loads which should not damage an airframe.

Proven track record is the final method, but you need a fleet of identical aircarft, or a high hour single example. IIRC it's something like 2500 hours. I say identical because the FAR103 and experimental aircraft in the US are constantly 'fiddled' with. Sometimes to cringeworthy levels.

With SSDR, unless you want to sell the design as a plan or kit, there is no requirement to do any of this. Even then that would only be to protect your business.

Just get designing, but while you do, ask yourself if you would let your daughter or son fly this contraption? Have I done enough to protect them?
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Existing Designs

Postby tom@covehouse.demon.co.uk » Tue Oct 26, 2010 11:10 pm

I have been looking at existing designs including the Minimax (which I think I am correct in saying doesn't meet SSDR requirements because of its wing loading ie too smallwing area) - I don't like it anyway.

There are some designs from the US which seem to pass the test of (flight) time - I am particularly looking at the Ragwing designs which are based on classic aircraft designs and some of which have been around since the early 90's. Although Roger Mann (Ragwing's founder) seems very difficult to contact.

One of his designs is based on the Corben Baby Ace using wood rather than a welded tube fuselage and such things as thin webbed spars to keep weight to a minimum. He also has a Pitts Special look-a-like which there seem to have been a fair number constructed around the world and which is clained to fall under the SSDR weight limit if a light enough engine is used.

I am currently looking at the original Baby Ace plans substituting a spruce frame instead of the steel tubing fuselage and going through different loading cases to check how the structure would stand up in theory. It does seem that the original was massively over designed as evidenced by their ability to do aerobatics.

If I get to the stage where I have produced a completely original design or had even take an existing design and perhaps modified some of the construction methods, I would value my life (or indeed anyone elses) sufficiently to pay a qualified aero-engineer to check my calculations. As a retired civil engineer, this is standard practice for any structural design anyway. You can't "sign off" your own design.
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Postby Ian Melville » Wed Oct 27, 2010 6:30 pm

Tom,
Have you considered going to the Designing light Aircraft - More Methods and Tools Conference on 23/11/2010?

I am

BTW it would not take much to make some of the FAR103 designs meet SSDR regulations
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Conference

Postby tom@covehouse.demon.co.uk » Thu Oct 28, 2010 3:58 pm

Ian

Conference would be great, but I'm working that day and the travelling from Scottish Borders, where I live, would male it a three day trip.

No wings to fly in yet.
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Postby Ian Melville » Thu Oct 28, 2010 11:08 pm

I've taken a days leave, which is OK for me in the home counties.

BTW It may take a while to get there in a SSDR :D
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Conference

Postby tom@covehouse.demon.co.uk » Fri Oct 29, 2010 7:46 am

Probably twice as long as driving if the wind's as strong as it is today. Quicker getting home but might overshoot!!!
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Postby Ian Melville » Tue Apr 26, 2011 6:25 pm

I see that the Cloudster Ultralight is now available as a plan pack and soon to be available as a kit. Mostly wood and seems to fit within the SSDR specs even with a 1/2VW engine.

http://simplexaero.com/pops-props-cloud ... craft-kit/
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Postby G.Dawes » Thu Jun 30, 2011 7:49 pm

When this idea was first proposed I thought to myself that this is going to be dangerous, any person with a modicum of engineering can design and build an aircraft and probably quite a good one BUT the difficult one is to build a really light one where there is a limit set down. The reduction is sizes and strength is the difficulty as it must be near the borderline for structural safety and the person who put the limit on is therefore causing the danger. It is the extreme light end of the spectrum that requires to exact application of stressing and construction and there is no way that a little extra margin can be put on for safety, because of the LIMIT. If you consider the terrible Reliant three wheeler, apart for the stability???? the structural strength was Zilch. have an accident with any impact then you might be left holding a steering wheel and sod all else. It was because of the weight limit. The extreme low end of aircraft is dodgy as the weight carried is variable and therefore the loadings especially gust loading with a large wing area. Larger aircraft might lose out on carrying capacity if made too strong but at least it is the right way around.
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