On Sunday, February 7, 2016 at 6:11:14 PM UTC-6, Martin Gregorie wrote:

Doesn't this follow from the fact that the yaw string is a fair distance
in front of the wing?

Think about it: if the tipwise axis of the wing is exactly aligned with
the radius of the turn, which it should be for maximum climb efficiency,
the yaw string should also be at right angles to the turn radius but,
being a good 2m/6ft or thereabouts in front of the wing, the turn radius
it is on is angled forward of the turn radius the wing is on, which
therefore makes it point out slightly out. This looks like a slight slip
to the pilot.

Draw this situation on a piece of paper, but exaggerate the situation by
drawing the circle diameter and about twice the wingspan and you'll see
what I described.

If you redraw the diagram with correctly scaled turn radius and wingspan,
the angular difference between the wing radius line and the yaw string
radius line will be very small, but this needs to be adjusted because the
curved shape of the canopy will amplify the angle of yaw string in a
slip. Now Dick Johnson's advice makes perfect sense.

Practically, if the yaw string is central or pointing slightly out in a
thermal you're doing it right. If its pointing a long way out or to the
inside of the turn you're sideslipping while turning and this is hurting
your climb rate by generating excessive drag.

Martin, I believe this is only part of the answer. You are correct that a perfectly coordinated turn (slip ball in the center) should show some slip if the yaw string is forward of the CG (and conversely, a centered forward mounted yaw string indicates a skidding turn - beware in the pattern!) but there is also the overbanking effect of long wings seeing different local airflow velocity in a turn; this can be countered by holding aileron out of the turn (inefficient and draggy) or by adding a little bit of slip via top rudder. This slightly increases the angle of attack of the lower wing and decreases the angle of attack of the top wing and voila, no overbanking, ailerons are flush and efficient, and you gleefully outclimb the newb with the perfectly aligned yaw string. It's really noticeable in my LS6 (probably due to the dihedral) - it settles into a nice slipping climb with the yaw string off about 10 degrees to the outside, no aileron deflection, and just a touch of top rudder. Extra bonus - you get some lift off the fuselage! (See "String Theory").

Kirk
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