I Wood if I Could
I readily admit to being a little technophobic and thus biased towards simpler older, technology. Accordingly, I tend to believe wood is good!
Wood certainly has equivalent failings to other avian construction materials but these can be minimized, allowing the wood to become stronger with age (unlike aluminium which weakens with every work cycle). As with all else, wood isn’t overly keen on stress, however it does give a far more visible indication of an over-stress than either composite or metal.
On the other hand, wood does, or can, suffer from insect damage, unlike other materials, but again this is usually visible and, provided you don’t build any weakened wood into your structure, normal preservatives will prevent it.
Working with wood to construct your aeroplane is a completely different experience: wood wins hands down! It smells good, it’s warm to the touch, soft, light and a good piece of wood is so very pretty. How often have you heard anyone say that an aluminium or composite aircraft skeleton was too good to cover up?
Weight is always the enemy which can be maximised by using a little common sense. Select your wood carefully, especially when using substitutes for the benchmark Spruce. There are several acceptable alternatives including Douglas Fir or ‘ladder grade’ Western Hemlock as both are stronger than spruce and not much heavier.
Machine accurately to the size on the plans, making a 1×1 inch longeron a smidgeon larger for safety will add weight at a mind-boggling rate. Better to use 25 x 25 mm as an alternative.
The best designers will taper longerons etc. towards the tail where the loads are lighter but, even if the same cross section is called for on your plan, you can save weight by balancing the sticks and installing them with the heavy end to the front. You will be surprised how little ballast you will need come weight and balance.
As with metal and composite structures avoid sudden section changes; bevel all the doublers and patches as far as possible. This not only eliminates stress risers but makes things lighter. Keep joints nice and close fitting – “engineers work to 1000ths of an inch but woodworkers work exact.”
Pre-glue prior to glue application and clamping. Read the instructions for the glue you have chosen. Glue is heavy and the correct preparation and clamping will reduce the amount required. Wipe all excess off for the same reason, plus you want someone to say, “oh, it’s too pretty to cover up.”
Assemble bolts through wood either wet (glue or varnish) or into pre-sealed holes for longevity and use the correct length bolts. Extra washers and over-length bolts add weight. One builder even went to the effort of calculating how much weight could be saved by using reduced hex nuts: apparently he thought it was worth doing. Don’t crush the wood any more than about a washer thickness and reread the plans. If a hole is optional, drill the hole – they don’t weigh much!
Treat spar blanks the same as longerons: balance them and put the heavy end inboard. Routing of spars appears to remove about a third of the volume but trials have shown that a weight saving of almost 50 percent is more normal.
Varnish and sealers can weigh a fair bit, but do use them, as modern fabrics and paints may mean you don’t get to look at the structure for 20 odd years.
Wear masks as wood dust can be carcinogenic even though it smells so much better than aluminium or composites. At least splinters will eventually remove themselves from your finger, unlike carbon slivers which could be yours for life.
Bed fittings and brackets onto epoxy or varnish to prevent discolouration of the timber. You can also use a bed of thickened epoxy to true up those metal brackets that you didn’t really want to make at all.
One again, remember that the selection of wood is important. I doubt many of us are able to get to the suppliers in person any more, but even plywood sheets can vary by a considerable weight from sheet to sheet. Use the heavier sheets forward and inboard unless you want a bigger engine, electrical system, instruments, radios, retracting tricycle undercarriage, hostesses etc. In which case, you wont be using wood anyway.
Enjoy building your wooden machine-like sculpture and know it will give you a softer quieter ride, a safer smoother arrival and several lifetimes of great fun. You will feel the ripples in the air as wood provides more feedback than any other material I know of, and when you have finished with your aeroplane all the advantages of wood will apply to your boat project.
Plant more trees!