Flying in different conditions
Flying in moving air
The speed of your drone through the air is its airspeed, which you as the pilot control. If your drone is flying with an airspeed of 20 mph on a day when there is not a breath of wind then its speed over the ground (groundspeed) will also be 20mph. More often there will be some wind, and this can significantly affect the drone’s speed over the ground.
Imagine you are again flying your drone at 20 mph, but directly into wind, and the windspeed is 20mph. So basically you are flying at 20 mph within an enormous mass of air moving in the opposite direction at 20 mph. In this case your drone’s groundspeed will be 0 mph. If you now turn to fly downwind, maintaining the drone’s airspeed of 20 mph, its speed over the ground will be a rapid 40 mph – it is flying at 20 mph within an enormous block of air, at the same time as the block of air is being moved across the ground in the same direction at 20 mph.
Let’s now imagine that your drone’s top speed is 30mph, and the windspeed is 20mph, and you fly it to a position a mile downwind of you. (This takes less than a minute.) You now turn back into wind and the drone, at full speed is moving over the ground at only 10mph. That is 0.16 miles per minute. It will take over six minutes at full power to cover that distance. If the battery starts to run out you will never get the drone back to base. And if the terrain is hostile (eg a lake, the sea, a forest) you will never recover the drone. The message is that you should always be conscious of the windspeed and direction, and be very careful of venturing downwind in breezy conditions.
Measuring wind strength
The severity of mechanical turbulence is directly proportional to the wind strength – if the wind strength doubles the turbulence quadruples – so it is vital that you accurately check the wind strength before flying.
To aid you in making a reliable assessment, you can buy a hand-held wind-strength meter (anemometer). These are reasonably accurate, and as you become familiar with a certain meter you will soon begin to relate its readings to the actual flying conditions experienced, and so will very quickly be able to use it to help you judge whether the wind conditions are within your model’s limits.
The problem with anemometers is that they can only measure the wind at your position – so they are therefore very susceptible to localised effects. It is worth thinking through the possible airflow pattern around your site, and measuring the wind strength at other positions if there is any doubt.
Besides the wind strength you must check for gustiness, and you should monitor the general weather.
In the absence of an anemometer you can fall back on the Beaufort Scale – which was devised over 200 years ago and still holds good. Basically this relates the observed actual conditions to a wind speed.
|0||Calm||<1||<1||Smoke rises vertically|
|1||Light Air||1-5||1-3||Direction shown by smoke drift but not by wind vanes|
|2||Light Breeze||6-11||4-7||Wind felt on face; leaves rustle; wind vane moved by wind|
|3||Gentle Breeze||12-19||8-12||Leaves and small twigs in constant motion; light flags extended|
|4||Moderate Breeze||20-28||13-18||Raises dust and loose paper; small branches moved.|
|5||Fresh Breeze||29-38||19-24||Small trees in leaf begin to sway; crested wavelets form on inland waters.|
|6||Strong Breeze||38-49||25-31||Large branches in motion; whistling heard in telegraph wires; umbrellas used with difficulty.|
|7||Near Gale||50-61||32-38||Whole trees in motion; inconvenience felt when walking against the wind.|
|8||Gale||62-74||39-46||Twigs break off trees; generally impedes progress.|
|9||Strong Gale||75-88||47-54||Slight structural damage (chimney pots and slates removed).|
|10||Storm||89-102||55-63||Seldom experienced inland; trees uprooted; considerable structural damage|
|11||Violent Storm||103-117||64-72||Very rarely experienced; accompanied by widespread damage.|
Whilst learning to fly your drone you should not venture out in anything more than a Force 3 gentle breeze. Once experienced you might also fly in a Force 4 moderate breeze. Winds that are stronger than this would be beyond the capabilities of most drones and their pilots. (Small lightweight quads are less able to fight against turbulence and wind effects – with the very tiny ones best reserved for indoor flying and only ventured outside when it is calm.)
Turbulence is a swirling motion imparted to the air by some external disturbing force – and it can have very unwelcome effects upon low-mass aircraft.
When air flows around a smooth streamlined shape it will follow the shape. Sharp edges and sudden changes in shape will tend to produce turbulence.
When moving air (wind) encounters an object such as a building, eddies are produced, because the shape is too angular for the air to flow smoothly around it. Standing eddies can form: they do not change position, and because they spin around on the spot they are often referred to as rotors. You should also avoid flying in the lee of trees and buildings.
Turbulence will also be found around thermals and other forms of wind shear, such as in a wind gradient.
Air flowing across the ground is slowed down by contact with the surface and with crops, hedges, trees, buildings and so on. Because air is slightly ‘sticky’ (viscous), the layer of air just above the slowed layer is also slowed to a lesser extent. This effect carries on until eventually the true wind speed is found at some distance from the ground. It is not unusual to find that a very gentle surface breeze is a strong blow at 300 m (1000 ft). This situation is most common on relatively stable days when thermal currents are not vertically mixing the atmosphere.
The temperature has various effects on drone flying. Firstly low temperatures can result in clumsy pilots – but also it reduces the power output of your batteries quite considerably.
High temperatures result in thinner air, so your drone’s ability to lift its normal payload will reduce in significantly higher temperatures. Also, as the motors will have to spin the propellers faster to get the required amount of lift, your batteries will be drained more quickly.
Just as higher temperatures mean less dense air, higher altitudes also mean that your drone is operating in less dense air. So the worst combination is ‘hot and high’. If you attempted to operate your drone from even UK Lakeland fell tops on a hot summer’s afternoon you would notice a significant reduction in performance.
See the next section here: Laws of the Air