CHAPTER 4: PERFORMANCE Soaring is all about performance. When you are flying an aircraft without an engine, efficiency counts! In this chapter, you will learn about the factors that affect your glider s performance so that you will be better equipped to get the most out of each flight. 4.1 Glide Ratio The glide ratio is the distance the glider travels through the air divided by the altitude lost. It is a measure of the glider s performance. The glide ratio varies with airspeed. As the airspeed increases, the glide ratio decreases due to increasing parasitic drag. Determining Glide Distance If you know the glide ratio, you can determine how far the glider can glide from a given altitude. Simply multiply the altitude above the ground by the glide ratio. For example, consider a glider with a glide ratio of 23:1 flying 2,000 feet above the ground, as shown in Figure 4.1. Figure 4.1 From an altitude of 2,000 feet, the glider can cover a distance of 23 x 2,000 feet, which is 46,000 feet, or 7.6 nautical miles. To determine the distance the glider will cover over the ground, you multiply the altitude of 2,000 feet by the glide ratio of 23, to get a distance of 46,000 feet, or 7.6 nautical miles (there are 6,076 feet in a nautical mile). Determining Required Altitude Suppose that you are climbing in a thermal and want to know how high you must climb in order to glide 12 nautical miles. Also, assume that you want to reach your goal with 1,000 feet of altitude left for a pattern. This situation is shown in Figure 4.2. Performance Section 4.1 41
Figure 4.2 To fly 12 nautical miles at a glide ratio of 23:1, you will need 1/23 of 12 nautical miles, or 3,170 feet, plus 1,000 feet for a pattern, for a total required altitude of 4,170 feet. Since we want our answer in feet, the first thing we need to do is to convert the distance of 12 nautical miles into feet, by multiplying it by 6,076. This gives us a distance of 72,912 feet. Next, we divide the distance in feet by the glide ratio, 23, to get the required altitude of 3,170 feet. We then add the pattern altitude of 1,000 feet to get the altitude which we should climb to, 4,170 feet. 4.2 Glider Polars A polar is a plot of a glider s sink rate versus its airspeed. Most glider flight manuals include a polar. Each glider has a different polar, and various external factors, including dirt, bugs, and rain on the wings, or the weight carried by the glider, will affect the polar. Of course, the polar given in the glider manual is for brand new, polished wings. Your actual mileage may vary! Figure 4.3 Sample glider polar. When flying at an airspeed of 80 knots, the glider will be sinking at 3 knots. Section 4.2 Performance 42
Using the polar, you can determine what the sink rate will be (in straight and level flight through calm air) for any airspeed. For instance, the sink rate for the glider who s polar is shown in Figure 4.3, is about 3 knots when the airspeed is 80 knots. The glide ratio is determined by dividing the airspeed by the sink rate. In this case the glide ratio at 80 knots is 80 3, or approximately 27. Sometimes, the sink rate is expressed in feet per minute. 100 feet per minute is approximately equal to 1 knot. The airspeed can also be expressed in statute miles (5,280 feet per statute mile) instead of nautical miles (6,076 feet per nautical mile). Just make sure that when calculating the glide ratio, you convert both speeds to the same units. Maximum Glide Ratio The maximum glide ratio of a glider is probably the most important measure of its performance. A glider with a high glide ratio can travel further for a given altitude than a glider with a low glide ratio. The slope of a line from the origin to any point on the polar curve is the glide slope at that airspeed, since the ratio between horizontal speed and vertical speed is the same as the ratio between horizontal distance traveled to vertical distance traveled. This gives us a way to determine the maximum glide ratio directly from the glider polar. All you have to do is draw a line from the origin tangent to the polar curve (i.e. the line just touches the curve); this line will have the shallowest slope possible. The speed you must fly to achieve the maximum glide ratio is the speed indicated by the point of tangency. This is referred to as the best glide speed, and in the example below, occurs at about 55 knots. The sink rate at this speed is 1.6 knots, resulting in a glide ratio of 34:1 (55 1.6). Figure 4.4 To determine the maximum glide ratio from a polar, draw a line from the origin tangent to the curve. Divide the resulting airspeed by the sink rate to determine the glide ratio. In this case, the glide ratio is 55 1.6, or 34. Performance Section 4.2 43
The ratio between the airspeed and sink rate is the same as the ratio between the lift and the drag. For this reason, the best glide speed in calm air is also referred to as the best L/D (spoken as best L over D ) speed. Minimum Sink Speed The minimum sink speed is the airspeed speed at which the glider is descending as slowly as possible through the air. In still air, flying at minimum sink speed is how you achieve maximum flight time. In Figure 4.5, a horizontal line has been drawn which is tangent to the highest part of the curve. The point of tangency marks the minimum sink rate, which is about 1.5 knots. By extending a vertical line to the horizontal axis, you can see that the airspeed needed to achieve this sink rate is about 45 knots. If you fly at this speed, you will be coming down through the air as slowly as possible. If you fly faster or slower, you will come down more quickly. Figure 4.5 To determine the minimum sink speed from a polar, draw a horizontal line tangent to the highest part of the curve. This is the minimum sink rate. At the point of tangency, extend a line vertically to the horizontal axis. For this glider, the minimum sink rate of 1.5 knots is achieved at an airspeed of 45 knots. 4.3 Effects of Wind Notice the disclaimer in calm air that keeps popping up in this discussion of the glide ratio. As you may have guessed, the performance of the glider with respect to the ground will change if the air is moving, whether horizontally (wind) or vertically (lift/sink). While wind does not affect the glider s performance through the air, it does affect the distance the glider can travel over the ground. Section 4.3 Performance 44
Figure 4.6 No matter what the wind speed or direction, the glider will arrive underneath the balloon at the same altitude. However, both the balloon and the glider will drift with the wind, so that the distance the glider travels over the ground is dependent on the wind speed and direction. Imagine a glider and a hot air balloon flying in the same air mass, at the same altitude. If the glider flies directly towards the balloon, it will arrive at the balloon a certain distance below it. This distance will be dependent only on the speed that the glider flies through the air, not on the wind speed. However, the wind will have an affect on where the balloon is when the glider arrives, and thus on how far the glider has traveled with respect to the ground. Headwind A headwind (or headwind component) will decrease the distance the glider can travel. As an example, imagine that you are flying a glider at its best L/D speed of 55 knots into a 55 knot headwind. The glider would come straight down. Clearly, the best L/D speed will not give you the best glide ratio. You can calculate the effect of a headwind on the glider by again using the polar. A headwind decreases the groundspeed of the glider by the amount of the headwind. For example, if a glider is flying at an airspeed of 55 knots into a 20 knot headwind, its groundspeed will be 35 knots. To determine the glide ratio over the ground, you divide the groundspeed of 35 knots by the sink rate, which is about 1.6 knots (at 55 knots airspeed), to get a ratio of 22. This is significantly worse than the still-air glide ratio of 34:1 that we calculated earlier. You can see this graphically in Figure 4.7. The 20 knot wind has the effect of shifting the origin of the graph 20 knots to the right. If you draw a line from the new origin through the point on the curve corresponding to 55 knots, the line will be steeper than in the case of still air. Performance Section 4.3 45
Figure 4.7 A headwind decreases the performance of the glider. A 20 knot headwind effectively shifts the origin of the polar 20 knots to the right. Note that the line is no longer tangent to the polar curve. This means that a better glide ratio can be obtained by flying at a different speed. Instead of flying the best L/D speed, you would want to fly a new best glide speed that you can determine by drawing a line from the new origin tangent to the curve. The point of tangency indicates the best glide speed for a 20 knot headwind. As Figure 4.8 shows, for this configuration the best glide speed for a 20 knot headwind is about 63 knots, or 8 knots faster than when in still air. Figure 4.8 To determine the best glide speed into a 20 knot headwind, displace the origin 20 knots to the right and draw a line from this point tangent to the polar curve. Extend a vertical line from the tangent point to the horizontal axis to determine the best glide speed. Section 4.3 Performance 46