What is the purpose of the piece of string taped to a glider canopy? Is it effectively a turn coordinator, but then what advantages would it have over a gauge such as the ones fitted in powered aircraft?
Yes, the yaw string on the canopy of a glider is there to show you whether or not you are coordinated, especially during turns. It indicates the relative wind, so will be straight unless the glider is slipping or skidding, in which case it will swing to the left or right.
For those not familiar with them, here is an image that shows one:
According to Wikipedia, they are also used on other aircraft (even jet fighters and the U-2!) although they are used in almost every glider.
Now, the advantages that it has over an actual instrument installed in the panel are numerous:
- It is directly in your line of sight while looking outside, so you don't have to look at the instrument panel to see if you are yawing.
- It is more sensitive
- It reacts faster to changes
- It is cheaper
- It is lighter
- It doesn't require electricity (or even an electrical system on the glider)
- It costs far less to maintain
- It is easy to recognize when it fails (it's either there or not)
On wings they are called tufts, and are a means of 2D-flow visualization. The yaw string just shows the yaw angle at a single location.
Wikipedia has a picture with tufts attached to a winglet Winglet with attached tufts of an KC-135A