The design I am analyzing has full-span control surfaces with a deflection range from -15° (upward) to 30° (downward).
Until recently, I had been assuming a bottom-surface piano hinge. However, other geometric constraints are pushing me towards a top-surface piano hinge.
For the bottom-surface piano hinge, I had been assuming a control surface geometry a bit like this:
Black is basic airfoil, red is max upward deflection, blue is neutral, green is max downward. With this geometry, the control surface is thickened a bit relative the basic airfoil, so that the point of intersection of the control surface and the basic airfoil is constant. That increase in thickness is on the top side, and is of less than boundary layer thickness, so it is unlikely to have a significant negative drag impact, and as discussed elsewhere may promote attached flow on the control surface to higher positive deflection that would otherwise be present. In addition, the primary seal (Blenderm over piano hinge) is on the positive pressure side, reducing its tendency to come off; and a secondary seal placed on the top surface junction is in pure sliding rather than sliding and flexion.
Unfortunately, the same "trick" to thicken the surface just enough to give a constant point of contact for the surface opposite the hinge breaks down for a top-hinged surface:
The higher maximum deflection in opposite the hinge increases the increased thickness of the control surface, taking it outside the boundary layer; and the bulge is now on the positive pressure side of the wing, not promoting attached flow, so it's a pure negative. The alternative is to go to what I assume is the more common control surface matching the basic surface outline:
This seems fine, but the maximum gap is sizeable (2.7 inches at the quite large root chord), which seems like a lot of width for a mylar seal. (The seal would obviously have to even bigger than that to allow a reasonable curvature.)
The top-hinged approach has other downsides (e.g. a 4% reduction in wing area at max deflection compared to bottom-hinged for the same basic planform, which leads to more wing area being needed), so I'm continuing to play with ways to return to bottom-hinged... but I did want to solicit advice as to how to deal with the top-hinge geometry in terms of sealing a giant variable gap vs avoiding having a giant gap to seal (assuming, for discussion, that I do want sealed control surface gaps).
Until recently, I had been assuming a bottom-surface piano hinge. However, other geometric constraints are pushing me towards a top-surface piano hinge.
For the bottom-surface piano hinge, I had been assuming a control surface geometry a bit like this:
Black is basic airfoil, red is max upward deflection, blue is neutral, green is max downward. With this geometry, the control surface is thickened a bit relative the basic airfoil, so that the point of intersection of the control surface and the basic airfoil is constant. That increase in thickness is on the top side, and is of less than boundary layer thickness, so it is unlikely to have a significant negative drag impact, and as discussed elsewhere may promote attached flow on the control surface to higher positive deflection that would otherwise be present. In addition, the primary seal (Blenderm over piano hinge) is on the positive pressure side, reducing its tendency to come off; and a secondary seal placed on the top surface junction is in pure sliding rather than sliding and flexion.
Unfortunately, the same "trick" to thicken the surface just enough to give a constant point of contact for the surface opposite the hinge breaks down for a top-hinged surface:
The higher maximum deflection in opposite the hinge increases the increased thickness of the control surface, taking it outside the boundary layer; and the bulge is now on the positive pressure side of the wing, not promoting attached flow, so it's a pure negative. The alternative is to go to what I assume is the more common control surface matching the basic surface outline:
This seems fine, but the maximum gap is sizeable (2.7 inches at the quite large root chord), which seems like a lot of width for a mylar seal. (The seal would obviously have to even bigger than that to allow a reasonable curvature.)
The top-hinged approach has other downsides (e.g. a 4% reduction in wing area at max deflection compared to bottom-hinged for the same basic planform, which leads to more wing area being needed), so I'm continuing to play with ways to return to bottom-hinged... but I did want to solicit advice as to how to deal with the top-hinge geometry in terms of sealing a giant variable gap vs avoiding having a giant gap to seal (assuming, for discussion, that I do want sealed control surface gaps).