Doggzilla
Well-Known Member
So I was viewing some of the more professional projects on here, and I noticed that spars almost always are constant cross section, even though the force on the spar is greatly reduced at length. This means that a lot of spar failures are at the root, as you can see from a lot of airshow crashes.
If the spar is kept proportional to the load, reducing the cross section along the length, a 3 foot wing with 12.5% mean chord would have 54 inches at the root and 5.4 inches at 10 feet, and the proportional loading would still be higher at the root, because the skin is proportionally stronger as the wingspan increases, because its the same skin, but less load.
Because flutter is controlled primarily by stiffness and length, the weakest part of the wing determines when the entire wing will flutter. This is the root, and so flutter can tear a wing completely off, and leave nothing on the first side to break. To avoid this, the reduction in spar strength towards the tip may be constructed to flutter at that point, and at a lower speed than the rest of the wing.
I believe the way to do this, would be to have many small spars, which are reduced and eliminated along the wingspan. If there is a pair of long thin spars, and every foot you have another spar added, the shorter spars will take the proportionally heavier load at the inner areas of the wingspan. If the longest spars are the outside spars, the same airfoil sections can be used along the entire length, just cut the foam to work around the other spars.
And since flutter is controlled both by stiffness and frequency, the longest spar and the less stiff wing tip will flutter before the wing root. This is opposite of a constant cross section spar, and is easily constructed. No large molds, which can be hard to handle, and less chance of ruining one large piece.
Thoughts?
If the spar is kept proportional to the load, reducing the cross section along the length, a 3 foot wing with 12.5% mean chord would have 54 inches at the root and 5.4 inches at 10 feet, and the proportional loading would still be higher at the root, because the skin is proportionally stronger as the wingspan increases, because its the same skin, but less load.
Because flutter is controlled primarily by stiffness and length, the weakest part of the wing determines when the entire wing will flutter. This is the root, and so flutter can tear a wing completely off, and leave nothing on the first side to break. To avoid this, the reduction in spar strength towards the tip may be constructed to flutter at that point, and at a lower speed than the rest of the wing.
I believe the way to do this, would be to have many small spars, which are reduced and eliminated along the wingspan. If there is a pair of long thin spars, and every foot you have another spar added, the shorter spars will take the proportionally heavier load at the inner areas of the wingspan. If the longest spars are the outside spars, the same airfoil sections can be used along the entire length, just cut the foam to work around the other spars.
And since flutter is controlled both by stiffness and frequency, the longest spar and the less stiff wing tip will flutter before the wing root. This is opposite of a constant cross section spar, and is easily constructed. No large molds, which can be hard to handle, and less chance of ruining one large piece.
Thoughts?
