I have to choose where to place the hinge line of the aileron of a large remote control aircraft which is to be optimised for endurance. Which is the most optimal placement of the hinge line of ailerons? Consequently, What will be effects of having the linkage shroud on top surface vs having it on the bottom surface?
Here's one thing large endurance aircraft can do: they use spoilerons instead of ailerons and rudders.
You simply leave off the downward pointing off-side aileron, which creates adverse yaw.
The inside one will create "proverse yaw". If you design it right, coordinating rudder will not be needed to turn.
Generally, bumps and irregularities are best placed underneath the wing, in the region of slower air and higher pressure. Any disruptions of upper wing airflow can be very detrimental to lift.
As a designer, another quandary would be trying to avoid holding ailerons in the turn. Planes with strong dihedral effect have this tendency. Avoiding swept wings would help here.
Of course, the best way to maximize range is not to turn for long periods of time in the first place.
The upper part of the wing is the one "doing the job" of keeping the airplane airborne and the airflow there should be left as much as possible undisturbed, especially toward the trailing edge. Anything there could lead to the detachment of the boundary layer and the subsequent stall of the wing. If you can choose where to locate the shroud, then on the belly of the wing is better.
The top picture shows an aileron with a center-line hinge line. You can tell because the circular leading edge of the control surface will sweep out an arc maintaining the upper surface seal -- the hinge line will be at the center of that arc. The bump on top is the pushrod of some sort.
The bottom picture shows a flap with an approximated Fowler action. It can deflect down only, with the hinge line far below the bottom surface of the wing.
Usually ailerons deflect symmetrically and usually have their hinge lines on the center line. I am sure there are exceptions -- most likely for construction simplicity.
One property of ailerons' hinges pivot point located at or close to the upper surface is differential increase or reduction of local wing chord.
Deflecting aileron up roughly won't change the local chord, while deflecting it down reduces the local chord noticeably. This geometrical property helps fighting adverse yaw.
Also that gap below the hinges creates more or less drag depending on deflection direction : deflected down, gap's drag is reduced, deflected up gap's drag is higher.
This also helps fighting adverse yaw.
In this upper located hinge position, regarding shroud and linkage location, and assuming equivalent moment arm therfore aileron's horn length,
shroud will be smaller if placed below the wing, and larger (more drag) if placed above.
Edit:
As a side note, since OP mentions unmanned rc plane whose goal is endurance flight, why not consider having a clean wing with no ailerons, and rely solely on two control axis, yaw and pitch.
2 axis controlled airplanes, if designed with adequate dihedral or swept back wings, can handle any kind of flight conditions, and provide quite high induced roll rates.
