How weather affects flight – Weather knowledge

Drones, like all flying equipment, are easily affected by the wind. The type, size and weight of the drone have a major influence on how wind affects the drone in the air. A small and light drone is affected by the wind more easily than a large drone. Taking the effect of wind into account is different with helicopters and multirotor drones than with fixed-wing drones. When the drone hovers in place in relation to the ground, the wind effect creates an airspeed for the drone that is equivalent to the force of the wind, and the drone requires steering commands to stay in place, which may affect the motor power required as well as the consumption of battery charge. In a strong wind, the drone may not be able to maintain its position. A fixed-wing drone flies in relation to the air, and the drone’s groundspeed varies depending on the flight direction and the wind force. In general, wind force increases with altitude.

For all commercially produced drones, the manufacturer has stated the highest recommended wind force in which the drone can fly. The remote pilot must study the manufacturer’s instructions and follow them to ensure a safe flight. If the wind is too strong, the drone may drift away with the wind, or the pilot may lose control.

If the drone has a payload

Payload refers to a sensor, cargo, communication device or similar attached to the drone that is not used for the operation or steering of the drone, and is not a part of its body, motor or propeller. In operations in the ‘open’ category, no material can be dropped off of the payload.

Attention must be paid to attaching the payload to the drone, and the load must be correctly attached, taking the drone and the task into account. If the load can shift or come loose during the flight, this may lead to the centre of gravity of the drone changing. The manufacturer has determined the drone’s centre of gravity, and if it moves outside the permitted zone due to the load shifting, controlling the drone may become impossible. Installing a payload outside the drone’s body also increases its air resistance. This results in a larger wind surface area when hovering in place in relation to the ground and a larger wind effect. The increased air resistance also affects the motor power required for horizontal flight.

The wind may also affect the drone’s flight time by either reducing or increasing it. When the drone stays in place relative to the ground in a strong wind, the motor power required varies compared to flying in calm conditions. The remote pilot must monitor the battery charge levels carefully.

A drone flying with a payload may not necessarily be able to fly in the highest wind speed recommended by the manufacturer.

You can also use the wind to your advantage when flying a drone. Motorless drones in particular can be flown along a slope perpendicular to the direction of the wind, taking advantage of the rising current in the direction of the slope created by the wind.

A strong wind may prevent the drone from returning to its starting point

In many drones, the activation of the automatic Return To Home function RTH is set to be activated if the battery charge level drops so low that a flight back to the set home location is no longer possible (based on the distance between the drone and the home location). However, drone's equipment/systems do not take into account the effect of wind. The ground speed of the drone decreases as the drone flies in headwind, and at the same time the possible flight distance is reduced, making it not possible to return home location. The drone might make unplanned landing in a potentially unsuitable location, and the remote pilot may not be able to change the landing location. The risk of injury to outsiders is high. During strong wind conditions, the remote pilot should consider flying above the wind to ensure a safe return.

Most drones do not have an IP rating (a rating that indicates how well the electrical device is protected against external threats, such as dust and moisture), and as a result, drones are vulnerable to rain, thick fog and snow. Moisture can enter the drone’s body and damage the components of the control system. The moisture damage may only be revealed later as malfunctions in the drone’s operation. The protection level stated by the drone manufacturer and the permitted conditions must be reviewed before the flight in case of high humidity or rain. The drone’s structures may also suffer from moisture. Unprotected wooden structures in particular are vulnerable to moisture.

Thunder in itself is a formidable weather phenomenon. Thunder poses several risks for aviation. Thunder may develop quickly as a local phenomenon, or it may arrive as a large weather front. Heavy rain, hail, strong gusts of wind and lightning strikes may occur in connection with thunder. All of these weather phenomena can be hazardous to drones as well as remote pilots. The pilot may lose control of the drone due to strong air currents, hail may cause mechanical damage to the drone, and a lightning strike may damage the drone severely. All this may result in loss of control and the drone falling.

Humidity may affect the drone’s sensors negatively

Many drones have IR sensors that are sensitive to water, snow, direct sunlight and shiny and reflective surfaces. These disturbances may result in the sensor reporting incorrect distance information.

In addition, drones may have optical camera sensors, and humidity and rain may also cause these sensors to malfunction. Conditions that limit visibility, such as fog, rain and smoke, also limit the ability of these sensors to function.

Radar sensors are less sensitive to the effects of humid weather than the more common IR and optical sensors.

Effects of the weather on the remote pilot

The weather conditions also affect the actions and perceptual ability of the remote pilot. Weather conditions that limit visibility reduce the size of the flight area in visual range, and rain also affects the ability to operate. Special care must be taken when flying in bad conditions; cancelling the flight should also be considered.

Cold air affects different drones in different ways. As mentioned in the previous sections, remote pilots should study the manufacturer’s instructions and familiarise themselves with the limitations of their drones with regard to temperature, too, and follow the instructions.

The following issues should be taken into account when flying in cold conditions:

• formation of ice on the propellers or wings
• the batteries’ cold resistance
• the effect of cold on the drone’s structures
• the effect of cold on the remote pilot
• the effect of cold on the radio controller

The formation of ice on propellers or wings has a significant impact on the functioning of the drone and its ability to fly. The weight of the drone increases as more ice accumulates, but the changes in the shape of the propeller or wing due to the layer of ice have an even greater impact. The ice changes the profile of the wing and propeller, and as a result, they generate less lift. In a helicopter or a multirotor device, the accumulation of ice may reduce the lift generated by the rotors so much that the drone can no longer remain in the air. In a fixed-wing drone, the flow of air around the wing may also be disrupted so much that the wing no longer produces enough lift for a controlled flight.

The ability of batteries to release energy is reduced in the cold. Both the current available as well as the storage capacity decrease if the battery is cold. The reduce in the release of energy may result in the battery suddenly running out of charge and the flight being interrupted, even in an uncontrolled manner. Batteries should be warmed up to room temperature before flight. Cold temperatures affect the durability of plastic structures in particular. When the temperature is below zero degrees Celsius, plastics harden and become more brittle than in warmer temperatures. In freezing weather, drones must be handled with special care due to the higher risk of mechanical damage.

The remote pilot must be well equipped for the cold weather. The necessity of handling the radio controller makes the choice of clothing challenging. One good operating model is to use the controller inside a separate protective hood created for the purpose. However, a protective hood can hinder careful monitoring of the camera screen.

The cold also affects the radio transmitter. Like the drone, the radio transmitter also has a battery. The transmitter requires much less power, and the low consumption does not heat up the battery while it is used; therefore, the battery may stop working faster than in warm temperatures. The transmitter screen and the separate camera screens do not function as well in the cold as in warm weather.

In general, the temperature decreases by approximately one degree Celsius per one hundred metres as the altitude increases.

The characteristics of atmosphere vary based on the density of air. Air density is affected by the altitude, air pressure and temperature. The standard atmospheric conditions can be found at sea level, at the temperature of +15 °C and air pressure of 1,013.25 hPa. This is the basic level when calculating location-based density height according to the prevailing conditions. In hot conditions the density height increases, in cold it decreases, and as air pressure decreases, the density height increases. In high density heights, the drone’s rotors or wings generate less lift, which may limit the largest flying weight even lower than the weight stated by the manufacturer. The drone’s user manual states the maximum take-off altitude.

Examples of density heights:

• height from sea level: 1,500 m, +35 °C, 970 hPa -> the air density height is approx. 3,000 metres compared to the standard atmosphere at sea level

• height from sea level: 50 m, -20 °C, 1,035 hPa -> the air density height is approx. -1,450 metres compared to the standard atmosphere at sea level

Turbulence occurs when the steady movement of an air mass in motion is disturbed and becomes turbulent. Several different factors may cause disturbances in the airstream. One common reason for turbulence is the radiant heat from the sun – solar radiation heats the different areas of the ground surface in different ways, and dark areas in particular warm up more than the light ones. The air above the warmed-up area also becomes warmer, and the air mass rises up. This phenomenon is known as a thermal. In thermals, the rising air mass often also rotates. The impact of thermals on helicopter-type drones is minor, and flight stabilisation systems can control the rocking caused by the thermal. Fixed-wing drones also deal easily with thermals. It is also possible to take advantage of thermals by flying the drone in the rising air mass, which makes it possible to gain more altitude without using motor power. This is known as soaring, and thermals are also used in unmanned aviation when flying fixed-wing motorless drones, known as gliders.

Another form of turbulence is mechanical turbulence, in which the wind flow is affected by an obstacle, such as buildings, forests or the terrain. Turbulence is created beneath the wind, and its strength is affected by the wind speed and the size and shape of the obstacle. Usually the drone’s flight stabilisation system can manage the disturbances caused by turbulence, but in some cases with a strong turbulence layer, it is possible to lose control momentarily. For example, a large turbulent zone may occur when flying into a strong wind from a calm area behind a large building. Other places causing turbulence include the edge of the forest. When rising up from the ground below the wind, possibly in nearly completely calm conditions, the wind strength may increase considerably at treetop level. Remote pilots should avoid flying near obstacles in strong wind.

Mechanical turbulence decreases as altitude increases and the effect of the friction layer on the ground surface is reduced. In general, the turbulence caused by the ground surface is no longer in effect at the altitude of 50 metres. On the other hand, the wind speed increases with altitude.

• The third factor that causes turbulence is a rapid change in temperature when ascending higher. In certain conditions, a clear air temperature boundary layer may occur as the altitude increases. In such cases the conditions on the ground are typically calm, and the wind above the boundary layer is strong. When reaching the boundary layer, losing control of the drone is possible as its airspeed changes rapidly. The phenomenon is common in winter during inversion.

How different weather conditions affect flight

Wind: Wind affects the drone’s ability to fly as planned, and wind may affect the external payload staying in place. An external payload must always be attached carefully.

Humidity: Many drones are not IP rated, which makes them vulnerable to the humidity due to rain, fog and snow. Drones may also attract lightning strikes, and the drone’s sensors do not work normally in humid conditions.

Cold temperatures: Ice may accumulate on the drone’s wings or propellers, and the performance of batteries deteriorates. The drone’s structures may become brittle in the cold.

Air density: Air resistance is lower in thin air, but less thrust and lift are also generated.

Turbulence: Mechanical turbulence may occur if you fly near buildings, forest or variable terrain; they disturb the flow of the air mass.

Regardless of the operating category in which remote pilots fly their drones, the pilots must stay up to date with the weather. You can check the weather conditions and the forecast from many different sources. General purpose local forecasts are often sufficient. More detailed information about weather services related to aviation can be found on the website ilmailusaa.fi.