Selection And Use Of Flooded Gum (Eucalyptus Grandis/Saligna)
These notes are drawn from my experience and findings during the building of the Flying Flea HM290/3FB. They are not to be taken as definitive and should be added to/challenged by prospective users of Saligna.
1 – Timber Selection
From discussions with others, and in particular with Pieter Bosch and Anton Maneschijne, I deduced the following:
- Choose light wood. Usually but not always the white wood will be lighter than the red wood. Avoid wood with red/white interfaces. There may be a weakness at the interface. Avoid wood with grey dead areas.
- You can ignore the end grain. Use flat grain, vertical grain, etc.
- Choose straight grained wood.
- The wood should break with a fibrous appearance. Avoid carrot breaks.
- Obviously avoid knots and imperfections/intrusions.
The general view has seemed to be that Saligna is twice as strong as Spruce. The SA Forestry Research has done tests on a number of different sources of Saligna in SA and Zimbabwe but these results indicate a correlation between density and strength and density can vary considerably. Kiln drying can also introduce variables. Therefore you cannot assume that Saligna is twice as strong as Spruce.
2 – My Experience
I bought my wood from Silverton Timbers in Pretoria. They let me scratch about and dismantle the stacks of timber which was helpful in that respect. The wood is available in lengths of about 1.5 to 2.7m, rising in 300mm increments. The longer the plank the higher is the price per running metre. Cross section is about150/180mm X 25/32mm. Now this is rough timber and you need to use your eyes to follow the grain run etc. Hefting the timber can give an indication of density but the varying thicknesses are a problem. They will plane the timber for you but by then it is already yours, warts and all!
Another way is to go to Nelspruit and buy a crate of timber. When you get home you can sort it out and sell the planks you don’t want to a friendly furniture manufacturer. I believe that is what Pieter Bosch did.
I did my own sawing and planing. I think it would be difficult to get this done by a timber yard as you need to keep your eye on the timber quality and make choices as it gets cut up. Then you don’t want to cut too much wood at one time as it tends to walk about if given half a chance during storage. After cutting and planing wood I neatly packed the lengths together and applied clamps to stop warping. This worked well.
What I observed here is that you get some lovely straight grained pieces but I also found some where the grain became wavy. This wavy grain makes very pretty furniture as the grain direction changes back and forth on the surface, but don’t use it on aircraft!
Finally it is my opinion that you must test each and every plank for breaking strength when you saw them up. Breaking strength is extremely difficult to eyeball! As you will see in the test results that follow breaking strength varies considerably and the “twice as strong as Spruce” view is not to be counted on. Besides the test procedure is very simple and takes no time at all and the results certainly give one a comfortable feeling of confidence.
I have not reduced dimensions on my aircraft in accordance with the strength of the wood as I don’t have the knowledge to do so. I have calculated that I have added about 10kg to the weight of my aircraft and am not too bothered by this.
3 – Breaking Tests
The test method I used was given to me by the designer of my aircraft, Fred Byron of Australia. I understand it is used by glider builders. The test method and the results are attached.
The graph shows the general increase in strength with increasing density.
All samples tested, including the “carrot” break sample, are well above the minimum Spruce Factor of 1.
Last updated: 02-Sep-07
I only bought timber on two occasions. The first lot was both denser and stronger on the whole than the second lot. It would be great if others could pool test results to build up our knowledge on the strength of the run-of-the-mill Saligna available in South Africa.
Roy de Stadler
Tel: +27- 11 – 9570136
Email: destadler@worldonline.co.za
