Polar curve (aviation)
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A polar curve is a graph of the rate of sink of an aircraft, often a glider, versus its horizontal speed.
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[edit] Measuring a glider's performance
Knowing the best speed to fly is important in exploiting the performance of a glider. Two of the key measures of a glider’s performance are its minimum sink rate and its best glide ratio, also known as the best 'glide angle'. These occur at different speeds. Knowing these speeds is important for efficient cross-country flying. In still air the polar curve shows that flying at the minimum sink speed enables the pilot to stay airborne for as long as possible and to climb as quickly as possible, but at this speed the glider will not travel as far as if it flew at the speed for the best glide. When in sinking air, the polar curve shows that best speed to fly depends on the rate that the air is descending. Using Paul MacCready's theory, the optimal speed to fly may often be considerably in excess of the speed for the best glide angle to get out of the sinking air as quickly as possible.
[edit] Glide ratio
The glide ratio is expressed as the ratio of the distance travelled to height lost in the same time. The ratio of the horizontal speed versus the vertical speed gives the same answer. (If the glider flies at 40 knots for an hour and experiences a 2-knot (4 km/h) sink rate, it will travel 40 nautical miles and descend 2 nautical miles (4 km). The glide ratio is 20 using both methods.
[edit] Plotting the curve
By measuring the rate of sink at various air-speeds a set of data can be accumulated and plotted on a graph. The points can be connected by a line known as the ‘polar curve’. Each type of glider has a unique polar curve.
The origin for a polar curve is where the air-speed is zero and the sink rate is zero. In the first diagram a line has been drawn from the origin to the point with minimum sink. The slope of the line from the origin gives the glide angle, because it is the ratio of the distance along the airspeed axis to the distance along the sink rate axis.
A whole series of lines could be drawn from the origin to each of the data points, each line showing the glide angle for that speed. However the best glide angle is the line with the least slope. In the second diagram, the line has been drawn from the origin to the point representing the best glide ratio. The air-speed and sink rate at the best glide ratio can be read off the graph. Note that the best glide ratio is shallower than the glide ratio for minimum sink. All the other lines from the origin to the various data points would be steeper than the line of the best glide angle. Consequently, the line for the best glide angle will only just graze the polar curve, ie it is a tangent.
