xp-soaring > Glider Polars

Simply put, the polar curve for an aircraft shows you how fast the aircraft will sink in the air for each forward speed. This performance curve needs to be modelled accurately in simulated sailplanes.

E.g. for the ASH25 Open Class sailplane:

This curve will be different for each model of aircraft, with a high-efficiency aircraft sinking less than a typical aircraft at the same forward speed. By this measure sailplanes are much more efficient than powered aircraft. For example, a modern glider of 18m span flying at 50 knots in still air will be sinking at about 1 knot and will be said to have a glide ratio of 50:1.

The glide performance of a given sailplane will alter according to its all-up weight and also its flap setting. Greater weight and/or a negative flap setting pushes the entire curve down and to the right, i.e. the glider can fly faster for a given amount of sink. This advantage in high-speed flight is complimented with a significant disadvantage when flying slowly. Many gliders carry water ballast, e.g. the ASW28 can carry 200Kg of water even though the aircraft has an empty weight of only 240Kg. This ballast provides exceptional high-speed performance but can be dumped in weak conditions. A pilot flying a sailplane with flaps will routinely be pushing the aircraft into negative flap when flying fast between thermals, and will pull the flaps back into a high positive setting when circling in a thermal. Flap settings may be referred to as (in order) Landing, Thermalling, Zero, Negative One, Negative Two. For any given speed there is an optimal choice for flap setting.

The polar curve at the top of this page is for the Schleicher ASH 25 25-meter high-performance sailplane which has flaps and carries water ballast:


The polar curve for the 25-meter ASH-25 shows a sink rate of 2m/s at a flying speed of 200km/h (In imperial measures that's 3.9knots sink at 108knots) for a glide ratio of about 28:1.

For comparison the polar curve below for the 18-meter ASW-29 shows it sinking at 1.75m/s at the same 200km/h flying speed, for a glide ratio of about 32:1. Counter-intuitively the cheaper smaller 18m sailplane is better at 200km/h than the ASH-25 25m sailplane. However, if conditions turn weak then the ASH-25 will win.

Simulator sailplane polar design

A note about units

If you want your math to actually work, stick to SI units, i.e. "meters per second" for speeds (airspeed or sink), "meters" for distance (e.g. distance travelled or height gained or lost), "kilograms" for weight. The internal math has nothing to do with the number the needle on a cockpit gauge will point to, i.e. "meters" can trivially be converted to "feet" for the altimeter, or "meters per second" to "knots" or "kilometers per hour" for the ASI. Americans are have a track record of cluelessness in this regard so we end up with FSX speed simvars in a random mix of units (such as "feet per minute" and "knots", so be careful) and NASA has satellites off Mars crashing because force units of Slugs per Banjo or whatever were mixed up with Newtons.

Polar basics

The basic sailplane polar is illustrated above. This is basically as 'advertised' by the manufacturer but you will see below this performance is actually one curve (the best) from an overlapping set of curves given different flight parameters. In still air, the glider sink rate is affected by at least the following:

Some key speeds on the polar diagram