Just fly FAAAAAASTER! That’s what people might will yell at your pilot once deadlines come closer… but aerial survey projects are not that simple. Yes, by increasing the aircraft speed your pilots will definitely finish the flight plan sooner but at what cost?
Aircraft Speed ≠ Aircraft Speed
Let’s first look at the definition of speed as there is various definitions in aviation. The most important one’s though are the following:
- Ground Speed
- Air Speed
- Cruising Speed
There is plenty more definitions of speed (i.e. all the “V speeds“) and even the above can be broken down into sub-definitions (i.e. Air Speed), but I leave that to the professionals to explain you.
Ground Speed
Simply explained, ground speed is the horizontal speed of an aircraft relative to the ground. The ground surface serves as a reference and therefore does not move. The speed measured describes how fast the aircraft moves past a point on the ground
The definition of ground speed is important as it is this definition that is assumed when selecting your parameters and preparing your flight plans.
Air Speed
Analogue to the definition of ground speed, air speed is the speed of an aircraft relative to the surrounding air. Here the aircraft is considered the static reference. The speed measured describes how fast the air moves past the aircraft.
In the case where the air is not moving, the speed of the aircraft passing the ground will be the same as the speed of the air flowing past the aircraft.
As soon as wind forces come into play, the situation can look very differently. To visualize this I will represent the speeds using vectors in the graph below:
With a head wind, the speed of the aircraft will be reduced by the wind speed therefore reducing the resulting ground speed. The air speed though will be increased as wind speed is added to the aircraft’s speed.
In a scenario with tail wind the aircraft gets an extra push by the wind and as a result the ground speed will be both wind speed and aircraft speed combined. A tail wind will though reduce the airflow over the wing as the aircraft speed gets reduced by the wind speed.
Air flow over the wings creates uplift for the aircraft. As a result a high enough air speed is of utterly importance for aircraft operations. Too low airspeed (by flying too slow or with a heavy tail wind) will lower the uplift of the wings and the aircraft will struggle to maintain altitude.
Cruising Speed
The cruising speed of an aircraft depends to a great extend on the engines used. Every combustion engine has an optimum work level at which it will consume the least fuel compared to the greatest mechanical output it produces. For a piston engine this is somewhere between idle speed and 25% away from full throttle. Flying at this cruising speed will therefore allow you to get the largest endurance time out of a single tank of fuel.
How fast can you go?
The aircraft’s speed is definitely an important factor to keep in mind when planning an aerial survey. After all, flying hours are a major contributor to the overall costs of a project.
In order to be able to maximize the efficiency of your flight plan, it is important to understand which parameter and factors are limiting the maximum speed for your survey.
Sensor parameter
Scan rate / Along-track point spacing: Changing the aircraft’s speed over ground will directly affect the resulting distance between pulses in direction of the flight. To counter this effect you could adjust the scan rate which regulates how quickly the next line is scanned. In fact some of the existing LiDAR scanner on the market would do this adjustment in real-time. Once the limits of the sensor are reached, you will have to decide if you can live with a larger forward gap between points or if you want the along-track spacing to limit your acquisition speed.
Pulse repetition rate / Across-track point spacing: The setting for the pulse repetition rate regulates the time between firing pulses while the scanner swoops from one side to the other. By increasing the rotation speed of the scanner the angle between two pulses and consequently the distance on the ground between two points will increase. This effect can be compensated by increasing the pulse repetition rate as this will reduce the angular difference between two pulses so that the same across-track point spacing can be maintained. You can therefore increase your aircraft’s speed by increasing the scan rate to control your forward point spacing and by adjusting your pulse repetition rate to correct the distance between points across the flying direction.
Terrain variation / Flight plan: As discussed in another blog post, your settings for pulse repetition frequency will directly affect the unambiguous measurement range of your sensor. Depending on the elevation changes in your project area and the type of sensor you are using, you might have to lower the pulse repetition rate (therefore increasing the measurement window) to allow the sensor to capture both the highest and lowest points in your strip. Careful consideration of the terrain while placing your flight lines can help to have less terrain variation within a single strip thus minimizing this issue.
Aircraft parameter
Maximum speed: Obviously the maximum speed of your aircraft will be the highest speed that can be used during aerial survey. Given the high fuel consumption at this speed though it can be be smart to consider flying at cruising speed as this could well allow you to cover more flight lines within a single flight.
Stall speed: Your aircraft’s stall speed will similarly dictate the lowest speed you can use during your survey flights. Keep in mind that this is normally defined in air speed and not ground speed. Depending on the wind conditions in the area your pilots will most likely ask you to add a safety margin to this number to allow for some amount of tail wind.
Summary
The aircraft you choose for your survey will determine the absolute maximum and minimum speed you will be able to fly. The terrain within the project area as well as the project specifications will give you more restrictions within which you are then able to tweak the settings for your sensor of choice in order to maximize the speed for your project.