How many ways are there to define dihedral? We can think of at least 8.
For a constant chord wing, dihedral is a lot easier to define, since the dihedral angle of the top surface, bottom surface and the centerline of the airfoils would be the same. However, when the wing panels are tapered, and even worse when different airfoils are used for the root and tip and these airfoils are not fully symmetrical, your dihedral angle specification can be interpreted many different ways.
So, the for worst case, a tapered wing panel with different airfoils, and lets say you need some tip washout as well, there are at least 8 ways our customers have defined dihedral for their foam cores.
1. Height distance under the wing tip. This is the classic method used in the instructions of a lot of model airplane kits and plans. A "tip block" of a defined height is placed under the wing tip while the root end of the wing panel is block sanded perpendicular to the building board.
2. Dihedral angle of the wing bottom surface. This is the angle formed by the bottom surface of the wing panel and the building board.
3. Dihedral angle of the centerline of the airfoils.
4. Dihedral angle of the wing top surface.
5. Wing bottom surface is flat. This is the same as #2 at zero degrees.
6. Wing top surface is flat, same as #4 at zero degrees, sometimes referred to as "Top Flat".
OK then, what does "no dihedral" mean?
7. Zero degrees of the centerlines of the airfoils. Pretty simple for a fully symmetrical airfoil, but a semi-symmetrical airfoil will have a slight amount of anhedral.
8. Tip centered. For this method, both the root and tip airfoils are centered in the block of foam making the dihedral angle of the wing bottom surface the same as the top surface. For a non symmetrical airfoil, this will result in a slightly different result than #7.
So, which method do we use to define dihedral? We really don't care as long as you tell us how you want your dihedral. If you are providing plans, drawing or a sketch of your project, the method will usually be clear.
By the way, our Quick Order Cores are cut with #8 "tip centered", just in case you were wondering.
Here are some other dihedral related subjects to consider.
We will normally set the dihedral angle by adjusting the height of the root and tip airfoils in the block of foam if possible. This method works well for methods #5, #6, #7 and #8 and will work for all other methods if the dihedral angles are slight.
However, if your dihedral angle definition would require the block of foam to be thicker than normal, we will cut as much angle in the foam block as possible, and make a dihedral angle cut on the root end of the panel to get the remainder of the dihedral. This keeps the foam block thickness at a minimum, which lowers both core costs, and shipping costs, particularly on large wing panels.
