Old Howard Hughes was no idiot!

The Hughes Flying Boat represents one of man’s greatest attempts to conquer the skies as the largest airplane ever constructed. It flew only one time on November 2, 1947. Conceived as a personnel and materiel carrier, the single hull prototype was designed to fly Trans-Atlantic to avoid World War II German submarines that were sinking Allied ships in large numbers. Completed in 1947 after the end of the War, the wooden winged giant is nearly six times bigger than any aircraft of its time. The press insisted on calling the Hughes Flying Boat the “Spruce Goose,� a name that its billionaire builder Howard Hughes despised. Most of the huge plane is actually made of birch, with only small amounts of maple, poplar, balsa, and, yes, spruce. Birch was chosen because testing proved it light, strong, and resistant to splitting, dry rot and deterioration.

Yes, spruce is much better for spars, I would replace the basswood.

In continuation of the discussion re virtues of basswood, many years ago I used a lot of basswood with excellent results, buying it from a local lumber yard in huge planks, then sawing and sanding it to any size I desired. I found it easier to work than spruce.

My wood design reference book calls basswood a hardwood, while spruce is, of course, a softwood. Basswood density is listed as 26 pounds per cubic foot, while both Sitka and White spruce are slightly heavier, at 28 pounds per cubic foot, all at 12% moisture content by weight.

Modulus of rupture is a useful indicator of solid beam bending strength. It is slightly higher than ultimate tensile strength, due to reduction of elastic modulus near the ultimate strength.

Basswood has a modulus of rupture of 8700 psi (pounds per square inch), while Sitka and White spruce chime in at 10,200 and 9800 psi, respectively. Thus, basswood appears to have a strength to weight ratio about 92% as high as Sitka spruce's.

Basswood's tensile strength across the grain is nearly the same as spruce's, at 350 psi versus 370 psi, so it should have about the same resistance to splitting as spruce. Basswood's shear strength parallel to the grain is a bit lower than spruce's, at 990 psi versus 1150 psi. Basswood's modulus of elasticity is given as 1460 psi, versus Sitka spruce's 1570 psi, so it is the same as spruce in relation to its weight.

Sitka spruce has been very popular among full-size aircraft builders for many decades. The reason may be its truly outstanding attribute, impact strength, at 56% higher than basswood's. Basswood finishes in a distant second place in crash damage resistance.

Basswood also takes a back seat to spruce in crushing strength across the grain, at 4730 versus 5610 psi. Sitka spruce appears to have the highest strength to weight ratio of any wood, higher even than that of hickory, the strongest wood listed.

I have conducted quite a few tests for modulus of rupture of balsa, and have found that balsa weighing at least 12 pounds per cubic foot to have a strength to weight ratio nearly the same as spruce's. Six pound stock had only about 1/4 the strength of 12 pound stock, so it has about half the strength to weight ratio, and soft balsa's impact strength is vastly inferior to that of the harder stuff.

Wood is a wonderful material for airplanes. It has a strength to weight ratio virtually the same as the WW2 vintage aircraft aluminum alloy 2024; much higher fatigue resistance and impact strength in relation to its weight; and it is easier to join. The success of the all-wood DeHavilland Mosquito bomber testifies to this.