Factors Affecting Glider Performance (Part One)

Glider performance during launch depends on the power output of the launch mechanism and on the aerodynamic efficiency of the glider itself. The four major factors that affect performance are density altitude, weight, design, and wind.

High and Low Density Altitude Conditions

Every pilot must understand the terms “high density altitude” and “low density altitude.” In general, high density altitude refers to thin air, while low density altitude refers to dense air. Those conditions that result in a high density altitude (thin air) are high elevations, low atmospheric pressure, high temperatures, high humidity, or some combination thereof. Lower elevations, high atmospheric pressure, low temperatures, and low humidity are more indicative of low density altitude (dense air). However, high density altitudes may be present at lower elevations on hot days, so it is important to calculate the density altitude and determine performance before a flight.

One way to determine density altitude is to use charts designed for that purpose. [Figure 5-1] For example, you plan to depart an airport where the field elevation is 1,600 feet MSL. If the altimeter setting is 29.80, and the temperature is 85 °F, what is the density altitude? First, correct for nonstandard pressure (29.8 “Hg) by referring to the right side of the chart and adding 112 feet to the field elevation. The result is a pressure altitude of 1,712 feet. Then, enter the chart at the bottom, just above the temperature of 85 °F (29.4 °C). Proceed up the chart vertically until intercepting the diagonal 1,712-foot pressure altitude line, then move horizontally to the left and read the density altitude of approximately 3,500 feet. This means a self-launching glider or towplane will perform as if it were at 3,500 feet MSL on a standard day.

Figure 5-1. Density altitude chart.

Figure 5-1. Density altitude chart.

Most performance charts do not require a pilot to compute density altitude. Instead, the computation is built into the performance chart itself. A pilot needs only the correct pressure altitude and the temperature. Some charts, however, may require computing density altitude before entering them. Density altitude may be computed using a density altitude chart or by using a flight computer.

Atmospheric Pressure

Due to changing weather conditions, atmospheric pressure at a given location changes from day to day. The following is the METAR report for Love Field observed on the 23rd at 21:53Z (GMT) which indicates a local pressure of A2953, or altimeter setting of 29.53 “Hg. When barometric pressure drops, air density decreases. The reduced density of the air results in an increase in density altitude and decreased glider performance. This reduces takeoff and climb performance and increases the length of runway needed for landing.

KDAL 232153Z 21006KT 7SM -RA BKN025 BKN060 OVC110 12/11 A2953 RMK AO2 PRESFR SLP995 P0005 T01220106

The 12/11 notation of this report indicates the reported temperature and dewpoint for Love Field.

When barometric pressure rises, air density increases. The greater density of the air results in lower density altitude. Thus, takeoff and climb performance improves, and the length of runway needed for landing decreases.

Altitude

As altitude increases, air density decreases. At altitude, the atmospheric pressure that acts on a given volume of air is less, allowing the air molecules to space themselves further apart. The result is that a given volume of air at high altitude contains fewer air molecules than the same volume of air at lower altitude. As altitude increases, density altitude increases, and glider takeoff and climb performance is reduced.

Temperature

Temperature changes have a large effect on density altitude. When air is heated, it expands—the molecules move farther apart, making the air less dense. Takeoff and climb performance is reduced, while the length of runway required for landing is increased.

Consider the following METAR for two airports with same altimeter setting, temperature, and dewpoint. Love Field (KDAL) airport elevation of 487 feet versus Denver International (KDEN) at 5,431 feet.

KDAL 240453Z 21007KT 10SM CLR 25/15 A3010 RMK AO2…
KDEN 240453Z 24006KT 10SM FEW120 SCT200 25/15 A3010 RMK AO2…

The computed density altitude for Love Field is 1,774 feet; for Denver, 7,837 feet—almost twice the density altitude increase compared to the increase for Love Field. The effects of attitude and temperature are significant, a fact pilots must consider when computing aircraft performance.