The effects of density altitude on sUAS operations
Jan 21, 2018
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The effects of density altitude on sUAS operations
Effects of Weather on Small Unmanned Aircraft Systems I’m going to go through the information that comes from the FAA’s, Remote Pilot - Small Unmanned Aircraft Systems Study Guide.
Basically, as the temperature goes up the performance of your aircraft decreases. As the humidity goes up, the performance of your aircraft decreases. As you go up in altitude, the performance of your aircraft decreases.
Density Altitude The more appropriate term for correlating aerodynamic performance in the non-standard atmosphere is density altitude – the altitude in the standard atmosphere corresponding to a particular value of air density.
As the density of the air increases (lower density altitude), aircraft performance increases. Conversely, as air density decreases (higher density altitude), aircraft performance decreases. A decrease in air density means a high-density altitude and an increase in air density means a lower density altitude.
Density altitude has a direct effect on aircraft performance. Air density is affected by changes in altitude, temperature, and humidity. High-density altitude refers to thin air, while low-density altitude refers to dense air.
The effects of density altitude on sUAS operations
The conditions that result in a high-density altitude are high elevations, low atmospheric pressures, high temperatures, high humidity, or some combination of these factors together. Lower elevations, high atmospheric pressure, low temperatures, and low humidity are more indicative or low-density altitude.
Effects of Pressure on Density Since air is a gas, it can be compressed or expanded. When air is compressed, a greater amount of air can occupy a given volume. Conversely, when pressure on a given volume of air is decreased, the air expands and occupies a greater space.
That is the original column of air at a lower pressure contains a smaller mass of air. In other words, the density is decreased. In fact, density is directly proportional to pressure. If the pressure is doubled, the density is doubled and if the pressure is lowered, so is the density.
This statement is true only at a constant temperature. Effects of Temperature on Density Increasing the temperature of a substance decreases its density. Conversely, decreasing the temperature increases the density.
Thus, the density of air varies inversely with temperature. This statement is true only at a constant pressure. Effects of Pressure on Density In the atmosphere, both temperature and pressure decrease with altitude and have conflicting effects upon density. However, the fairly rapid drop in pressure as altitude is increased usually has the dominant effect.
Hence, pilots can expect the density altitude to decrease with altitude. Effects of Humidity (Moisture) on Density The preceding paragraphs are based on the presupposition of perfectly dry air.
The effects of density altitude on sUAS operations
In reality, it is never completely dry. The small amount of water vapor suspended in the atmosphere may be negligible under certain conditions but in other conditions, humidity may become an important factor in the performance of an aircraft. Water vapor is lighter than air; consequently, moist air is lighter than dry air.
Therefore, as the water content of the air increases, the air becomes less dense, increasing density altitude and decreasing performance. It is the lightest or least dense when in a given set of conditions it contains the maximum amount of water vapor.
Humidity, also called relative humidity, refers to the amount of water vapor contained in the atmosphere and is expressed as a percentage of the maximum amount of water vapor the air can hold. This amount varies with the temperature – warm air can hold more water vapor, while colder air can hold less.
Perfectly dry air that contains no water vapor has a relative humidity of zero percent. While saturated air that cannot hold any more vapor has a relative humidity of 100 percent. Humidity alone is usually not considered an essential factor in calculating density altitude and aircraft performance, however, it does contribute.
Sounds like a lot when you get reading through it and it seems like some things are contradicting each other. Go through it again a time or two if you need to. But the bottom line is, in your small unmanned aircraft, as the temperature goes up, performance is not going to be as good and, as your altitude goes up, the performance is not going to be as good.
As the air becomes moister, your performance is going to go down. So, when you have those three things, performance is going to be bad. If you put them all together, it’s going to get even worse.
The effects of density altitude on sUAS operations
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