Polar, Performance, and Water Ballast

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Have you ever wondered what differentiates the 'high-performance' gliders from the normal two-seaters, apart from the price tag and the rather fragile appearance? What are the ridiculously long wings of a Duo-Discus good for? Or more importantly, after you have paid the launch fee, what can you do to stay in the air for longer? The answers to these questions require an understanding of the performance metrics of the glider.

You might have heard of more experienced pilots talking about 'polars', or you might have seen the convex curve which is confusing to get started with. You might also have seen people adding or dumping water into and out of their gliders. Building on the knowledge of glider performance, we can have a closer look at how these tools help cross-country pilots to fly faster and further.

There is a wealth of text, published or online, discussing the topics mentioned above. However, some of these are rather scattered pieces of discussions on the forums, or they can be written in another system of conventions than what is adopted in Cambridge. Some are sloppy about their assumptions and approximations, and some dive straight into the calculus making it impossible to follow. This work aims to present the derivations of the governing equations and the polar functions in a clear and detailed manner, and summarises the implications for those who would rather not follow the mathematics.

The road map of this article is as follows:

  1. Consider the forces acting on the glider in unaccelerated flight: lift, drag, and glide ratio.
  2. Aerodynamic coefficients: definitions and meanings.
  3. Relationship between lift and drag.
  4. General method of solution, and assumptions necessary to simplify it.
  5. Analytical form of the glide polar.
  6. Implications of the polar: minimum sink speed, and best glide.
  7. Adjustments to the polar: headwind and sinking air.
  8. Adjustments to the polar: change of glider weight, first purpose of water ballast.
  9. More effects of water ballast and recommended readings.

This article is a major project which will take me at least a month to complete. I cannot save a draft on WiKi, so if you accidentally come here and see this page in its very much incomplete form, please bear with me and come back after some time.