On this page you will find details of the various materials I will be using in my KR and the reasons behind the selection of each material. Note that the opinions and decisions expressed here around the use of alternative materials than those listed on the KR plans are my own and in no way imply a recommendation of any kind. If you are building your own KR please make sure you complete your own research if you intend to use materials different than those stated on the KR plans.

Aircraft Grade Spruce

When I started looking for local suppliers of aircraft grade spruce, I initially could not find one in New Zealand with aircraft grade Spruce in stock. As such I ended up asking another KR builder where he obtained his Spruce from. He told me he had not used Spruce but had used New Zealand White Pine (Kahikitia) instead which he had sourced from a timber supplier in Whangarei, New Zealand. (Whangarei is in the Northern part of the North Island of New Zealand for those reading this unfamiliar with NZ geography).

I rang the supplier (Paul Wood) and he told me that NZ White Pine was no longer available as Kahikitia is now a protected species in NZ but he did recommend Yellow Cedar as an alternative. I spoke to a couple of other aircraft builders knowledgable about timber and was referred to the following NASA document: "Report No. 354. Aircraft Woods: Their Properties, Selection and Characteristics". I googled this title and found a PDF copy of the document which I downloaded and printed. This document lists two types of Cedar that can be used as alternatives to Sitka Spruce, these being Port Orford Cedar and Alaskan Cedar. Seeing no mention of Yellow Cedar in the document I googled Yellow Cedar and found this reference which showed me that another common name for Yellow Cedar is Alaskan Cedar.

Paul Wood also provided me with the following PDF links to compare the specifications for Yellow Cedar, Sitka Spruce and Douglas Fir (Oregon) for strength and weight. I have summarised this info in the table shown at right. Click on the image to expand it to full size in another page. As you will see from the table Yellow Cedar sits between Sitka Spruce and Douglas Fir as far as weight and strength is concerned. It is slightly heavier (11%) that Spruce but has a better MOR (Modulus of Rupture) (14%) meaning it can withstand a greater bending stress for a given density of timber before breaking than Spruce. The net result of this research is that I am satisfied that Yellow Cedar is a good replacement for Spruce. It is also a considerably cheaper option than Spruce.

Spruce Cutting List

Unfortunately the information provided by Rand Robinson for the Spruce Cutting List is protected by the RR Copyright clause so I can't show it here. I can however show you the Wicks Aircraft Spruce Kit which is a pretty good match to the RR cutting list. If you click on the image at right this will show you the Wicks Cutting list.

After including the changes I intend to make to my KR2S design and comparing the RR cutting list to the standard KR Spruce kit available from Wicks, I produced the spreadsheet shown at right to send to my timber supplier for my KR. The dimensions highlighted in yellow on the Wicks list show those that differ from the RR lengths. The dimensions shown in yellow on the cutting list for my KR are those that have been changed for the laminated and lengthened spars I will be using. The dimensions highlighted in green are those from the Wicks list which I could not reconcile with the RR list.

Aircraft Grade Plywood

There is a considerable amount of information available on the web about what ply to use for the KR2S. The KR2S plans themselves call for Aircraft Grade 3 ply 3/32" Birch ply to be used on the wing spars/ribs and 3 ply 3/32" Mahogany or Birch ply for the boat skin. The plans also come with a buying list for plywood which is as follows;

     4 sheets 8'x4' 3/32 (or 2.5mm) 3 ply
  or 8 sheets 4'x4' 3/32 (or 2.5mm) 3 ply
     1 sheet 4'x4' 6mm 7 ply

Wicks Aircraft Supplies also provide the following list for their KR2S ply kit.

     3 sheets of 3/32" x 4' x 8' Mahogany 3 ply 90 Degree
     3 sheets of 2.5mm x 4' x 4' Birch 5 ply
     1 sheet of 6.0mm x 4' x 4' Birch 7 ply

Mark Langford's site recommends the use of Mahogony ply for the boat skin because it is lighter than birch and strong enough for the requirements of the KR.

With all the above in mind I called round plywood suppliers in New Zealand and Australia looking for Aircraft Grade GL1 3/32 Birch and Mahogony 3 ply and was told that GL1 ply of any size is now almost impossible to source as the factory in Finland which manufactured this wood went out of business about 18 months ago (sometime in early 2006). Plytech in Auckland did have some GL3 Model Grade Birch ply in the sizes I needed but I was not sure this would be satisfactory. I read the Germanischer Lloyd (GL) rules for surveying plywood for aircraft and this seemed to indicate that GL3 was not really going to be a satisfactory option for building a wooden aircraft like the KR.

I then spoke with Mike Tunnicliffe, a local aircraft builder who has considerable experience working with Aircraft Grade Ply (he is currently working on a Mosquito Project in a workshop close to where I live) and he said GL3 is ok provided you inspect the inner laminations for faults by shining a very bright light behind the sheets in a darkened room, marking out any imperfections and cutting round these. This apparently works for 3ply sheets up to about 3mm thick.

So with this information in hand I was about to place an order with Plytech when I spotted an advert from The Vintage Aviator Company in Wellington indicating they had Aircraft Grade Ply for sale. I contacted Rob Fisher from The Vintage Aviator and he emailed me to say they had Birch ply in the thicknesses I needed. The prices he quoted were $5.40 per square foot for 2.5mm 5ply in 5' x 5' sheets and $11.50 per square foot for 3/32 3ply in 8' x 4' sheets. The price for the 3 ply was a bit steep for me but I wasn't sure if 5 ply was an acceptable alternative to 3 ply. I did a bit more reading and noted that Wicks Aircraft Supplies only provide 2.5mm Birch ply in 5 ply for their KR kits which seems to indicate the Birch 5 ply can be used in place of Birch 3 ply. With this information in hand I then placed an order with The Vintage Aviator for 8 sheets of the 2.5mm 5 ply and this arrived a few days later.

Epoxy Glue

Most of the KR websites I visited indicated that T88 Epoxy seemed to be the glue of choice for the majority of KR builders. I contacted a few local KR builders here in New Zealand and found that two had used T88 and one had used Epiglue. I then reviewed the Aircraft Glues section in the SAA Handbook sent to me by the New Zealand Sport Aviation Association when I joined that organisation. The handbook had a list of acceptable glues which included both T88 and Epiglue but also indicated that West Systems Epoxy is also a good glue to use.

Shortly afterward I went to visit Mike Tunnicliffe at the Mosquito Project hangar and I noted they were using West Systems Epoxy. Mike highly recommended using the West Systems Epoxy because of its excellent handling qualities and given his considerable experience in its use I have decided to follow his advice. As a consequence I visited Adhesive Technologies out in West Auckland and spoke with Nick Aldrich who gave me a quick run down on the West System products. It didn't take him long to show me that the West System products would definately suit my needs but what convinced me most however was that the West System Epoxy is not only an epoxy glue but the same epoxy resin and hardener can be used for fibreglassing also. I had thought I would have to buy one product for glueing and one for fibreglassing. This one product performs both tasks. As such I asked Steve to put together a kit of their products to get me underway.

An update on the above. A month or so after I purchased my West Systems Epoxy, Mike Tunnicliffe came over to visit my workshop and noted that I had purchased the 403 Adhesive Filler. Mike recommended I use the 404 filler as this is better to use in situations where high-cyclic loads are anticipated. This sounds very much like an aircraft airframe to me so I placed an order in to Adhesive Technologies for the 404 filler. Mike also mentioned that I need to be aware that the 206 Slow Hardener I have is only good down to about 16 degrees C (60 F). This should be fine given we are coming in to the Southern Hemisphere summer but I will probably need to change to the 205 Fast Hardener when the temperature dips down below 16 degrees. The 205 Hardener is good down to 4 degrees C (40 F).

Foam for Wing and Tail Structure

The KR plans indicate that urethane foam should be used when building the wings and tail members of the KR. Reading through a number of sites and watching the KRNet mailing list I noted that there are some quite serious health hazards associated with using this product. Firstly you must not hot wire cut the product as it gives off poisonous cyanide based fumes which are lethal in even small doses. Secondly the residue left over when sanding urethane foam is extremely irritating to skin, eyes and lungs. Descriptions of the rashes caused by the dust residue created during sanding indicate this product requires very careful handling when used.

I also spoke with Mike Tunnicliffe and he indicated that the mechanical properties of urethane foam mean that wing skins made with urethane can be susceptable to delamination due to the way vibration from the aircraft is passed through the wing skins into the foam core material. He indicated that urethane foam is quite rigid so where the foam meets the fibreglass there can be a tendency for a shearing effect to occur which can cause the foam and the fibreglass skin to separate. He recommended that instead of urethane foam that I use styrofoam instead. This is not the multi cellular type of polystyrene you get as packaging for electronic goods but the type which has a continuous even structure throughout the material. Mike said styrofoam is not as rigid as urethane foam and tends to transmit vibration and movement evenly throughout the material. This means it does not cause a significant shearing moment to occur at the junction between the aircraft skin and the foam core.

The downside to using this material (there always seems to be a downside to everything) is that it is more difficult to sand because it tends to cause pilling in the sanding residue. Also the particles which are sanded off are much lighter than urethane dust and will blow around the workshop unless vacuumed up. They also tend to contain a static charge which means they cling to your clothes and the tube of the vacuum cleaner.

I still need to research this material further before I make my decision over which to use. It may come down to buying some of each material and trying it out.

A bit of an update on this. There has been a recent discussion on the KRNET about the advantages of using Divinycell H-Series Urethane. I did some research and note that this material is way stronger than standard urethane. It resists crushing and shearing to a much higher degree and is impervious to fuels as well. I think I will probably use this material on my wings and tailplane. It is a bit more expensive but the extra strength is well worth it I think.