Hey all,
So, hear me out. This is a mental exercise. It may turn into a real thing, though.
This week, I reverse engineered the Bearhawk Patrol structure (shhhh.... don't tell Bob) and analyzed all the tubes for buckling (primarily) and other things of the sort. Then I constructed it all in wood, ensuring that it meets the same strength requirements. My main focus was buckling, because that's wood's weakest point when compared to steel. Everything else is the usual stuff-in-tension truss. Bob designed a great structure. The optimized weight of the wooden fuselage is (theoretically) within 0.8 lbs of the steel one when constructed entirely of Douglas Fir using the same configuration.
Obviously the wooden structure is "chunkier" than its steel tube counterpart because of the amount of material it takes to match the strength of steel. But it's built to the same dimensions, centerline-corrected for the dimensions of the lumber. The base thickness of all the primary "tubes" on the plane is 1", with widths varying between 1" and 1-5/8", and a couple over that in high-compression areas like landing gear attach points and the lower longerons under the cabin. For reference, if it sounds big, this is a 2000 lb airplane, not an LSA or an ultralight.
As is pretty typical of the type, the cabin will be wrapped in plywood both inside and out from the firewall to the B-pillar (didn't know what else to call it), and then on the outside only to the back part of the cabin. Let's say 1/8" thick stuff, since that's what LMA specifies in their planes. After that, mainly open to the tail. To save some weight, will thinning out the forward stock to 3/4" thick be suitable, since it's covered in birch plywood on both sides, maintaining the 1" thickness?
I chose the 1" thickness of material for buckling resistance. If I went any smaller, then they were prone to buckling off-axis due to their slenderness ratio. Most of the tubing in the Patrol in the area where people sit is 3/4" steel, so a 1" wood piece there isn't at all taking away from capacity, which is nice. And it's not visually distracting. So if I need to keep it, then the ply-covered sections will be 1-1/4" thick. Again, not a real problem in the grand scheme of things.
~Chris
So, hear me out. This is a mental exercise. It may turn into a real thing, though.
This week, I reverse engineered the Bearhawk Patrol structure (shhhh.... don't tell Bob) and analyzed all the tubes for buckling (primarily) and other things of the sort. Then I constructed it all in wood, ensuring that it meets the same strength requirements. My main focus was buckling, because that's wood's weakest point when compared to steel. Everything else is the usual stuff-in-tension truss. Bob designed a great structure. The optimized weight of the wooden fuselage is (theoretically) within 0.8 lbs of the steel one when constructed entirely of Douglas Fir using the same configuration.
Obviously the wooden structure is "chunkier" than its steel tube counterpart because of the amount of material it takes to match the strength of steel. But it's built to the same dimensions, centerline-corrected for the dimensions of the lumber. The base thickness of all the primary "tubes" on the plane is 1", with widths varying between 1" and 1-5/8", and a couple over that in high-compression areas like landing gear attach points and the lower longerons under the cabin. For reference, if it sounds big, this is a 2000 lb airplane, not an LSA or an ultralight.
As is pretty typical of the type, the cabin will be wrapped in plywood both inside and out from the firewall to the B-pillar (didn't know what else to call it), and then on the outside only to the back part of the cabin. Let's say 1/8" thick stuff, since that's what LMA specifies in their planes. After that, mainly open to the tail. To save some weight, will thinning out the forward stock to 3/4" thick be suitable, since it's covered in birch plywood on both sides, maintaining the 1" thickness?
I chose the 1" thickness of material for buckling resistance. If I went any smaller, then they were prone to buckling off-axis due to their slenderness ratio. Most of the tubing in the Patrol in the area where people sit is 3/4" steel, so a 1" wood piece there isn't at all taking away from capacity, which is nice. And it's not visually distracting. So if I need to keep it, then the ply-covered sections will be 1-1/4" thick. Again, not a real problem in the grand scheme of things.
~Chris
