Well, I mounted a yaw string on a post near (just aft of) the CG of my 2-meter Spider, and mounted a video camera on the fin, got a beautiful 40-minute sunset flight but when I landed the camera was off- have no idea how much footage I got
But I don't have any doubt about what the results were-- the yaw string surely streamed a little toward the outside of a constant-banked turn (showing some sideslip). I think this is going to be true of all aileron-controlled RC sailplanes with any normal rudder mix and any normal flying style (rudder usage). After all most RC pilots feel-- I believe-- that a wing geometry with lots of dihedral contributes a rolling-out torque in a constant-bank turn, causing the glider to need less outside aileron input (or in extreme cases, more inside aileron) input than would otherwise be needed. For example if the model requires zero aileron input in a constant-banked turn, and you then remove some dihedral from the wing, it will then tend to roll into the turn and will require some outside aileron input. Is this not the experience of most RC glider pilots?
This can only happen if the model is being allowed to slip a bit, so that the dihedral "feels" a sideways flow and thus creates a "downwind" roll torque-- toward the outside wing or high wing. With no slip, the dihedral creates no roll torque, and has no effect on the overall balance of roll torques acting on the model.
Allowing a bit of slip makes sense from a performance standpoint as well as a handling standpoint. Assuming that the model has some dihedral, isn't it more efficient to create some sideways flow over the wing and allow the dihedral to create some rolling-out torque, than to keep the model completely coordinated in yaw (slip) and rely solely upon deflecting the ailerons to create the rolling-out torque? (After all, some rolling-out torque is always needed to offset the rolling-in tendency that we would always see in a flat-winged model, or a model with dihedral that was not being allowed to sideslip.) (This rolling-in torque arises from the fact that the outboard wingtip is travelling faster, and covering more distance, than the inboard wingtip.)
Likewise consider the balance of yaw torques-- the outboard wingtip moves faster, and covers more distance, than the inboard wingtip, and so the outboard wingtip experiences more drag than the inboard wingtip. This creates a yawing-out torque, which tends to make the nose point some degrees toward the outside or high side of the turn, relative to the direction of the flight path at any given moment. Does it really make sense to try to eliminate this sideslip completely? Regardless of what the slip angle ends up to be, something must be creating the yawing-in torque that counterbalances the yawing-out torque from the tip drag of the outboard wingtip, bringing the net yaw torque to zero,so that the glider ends up with some constant yaw rotation velocity (zero yaw rotational acceleration.) What is the best way to create this yaw torque? If the fuselage is slender and streamlined (doesn't create lots of drag in a sideslip), then isn't it more efficient to allow the vertical fin to meet the air at a slight sideslip angle, so that its "airfoil" flies at a non-zero angle-of-attack and generates some yawing-in torque, rather than to rely solely on deflecting rudder to create the yawing-in torque, as would be the case if the turn were completely "coordinated" (zero slip as measured at some particular point--say for the sake of this example, as measured at the vertical fin)?
I guess this fin-based argument for allowing sideslip vanishes if the vertical fin is all-moving (no separate rudder)-- then you can have your cake and eat it too-- use the whole fin efficiently not just the rudder, but also keep the fuse streamlined with the airflow-- but it still might be more efficient to allow a bit of slip so that the wing's dihedral creates some rolling-out torque, so that less (or zero) outside aileron deflection is needed to maintain a constant bank angle.
Of course if the rudder is simply left centered in a constant-bank turn, then the vertical fin MUST be meeting the air at non-zero slip angle and creating a yawing-in torque. This is the only way that the glider can counterbalance the yawing-out torque from the increased tip drag of the outboard wingtip, bringing the net yaw torque to zero.
I suppose an exception might be a glider that needed lots of outside aileron input in a constant-banked turn, and was rigged with zero differential aileron travel-- then maybe the adverse yaw from the aileron deflection could create enough yawing-in torque to make the nose point toward the low side or inside of the turn, so that the whole fuselage (including the tail) was feeling a skidding flow (flow toward the low wingtip) rather than a slipping flow (flow toward the high wingtip)? But that would be a non-typical situation I think. Maybe we would see this when flying inverted-- we typically need lots of outside aileron to hold the bank angle when inverted, and if the ailerons had differential throw when upright, they'll have differential throw in the wrong direction when inverted! The only problem with this idea is, the skidding flow would interact with the inverted dihedral (anhedral) to create a rolling-out torque, removing the need for the outside aileron input. The fact that we do typically have to hold lots of outside aileron when inverted suggests that the wing is feeling a neutral (non-slipping) flow, or a flow toward the high wingtip (sideslip), presumably due mainly to the fact that the outside wingtip is travelling further, and creating more drag, than the inside wingtip, yawing the nose to point toward the outside or high side of the turn.
Finally consider that since the flight path is curved, the relative wind is also curved, but the fuselage is not curved. Just as the span of the glider is non-trivial compared to the turn radius (creating the difference in airspeed between the two tips), so too is the length of the fuselage non-trivial compared to the turn circumference. Or to put it another way, there is some significant curvature in the flight path and relative wind even along the length of the fuselage. Therefore the fuselage cannot be streamlined to the curving relative along its entire length. Let's say for the sake of argument that we decided that we did in fact want the vertical fin to be completely streamlined to the flow. Then every point forward of the fin must feel some slip (airflow toward the inside of the turn), unless we somehow make the fuselage curve like a banana to remain parallel to the curving flow along its whole length. Again this argument makes the most sense in the context of a glider with a slender streamlined fuselage-- in something shaped more like "Le Fish" or a Schweizer 2-33, we might want a point near the CG, where the fuselage has a broad flat side, to be streamlined to the flow.
Another way to look at the curving relative wind is to recognize that the glider is rotating as well as translating linearly. The rotation creates the difference in relative wind direction along the fuselage length. If the glider were only rotating, the difference in relative wind direction between the nose and tail would be obvious. We still have that same rotation even in a normal turn, where the rotation rate equals once per 360 degrees of turn.
Well I'll have another go in a few days. I predict that the yaw string mounted just behind the CG will deflect toward the outside of a constant-banked turn. If I add a second yaw string at the nose, it will be interesting to see if it deflects noticeably more than a yaw string mounted further back. It will also be interesting to see if the yaw strings deflect toward the outside or high side of the turn even during an inverted constant-banked turn....