UAV Navigation in Depth: Basic Airspeed Concepts
In aerodynamics, the flight envelope defines operational limits for an aerial platform with respect to maximum speed and load factor given a particular atmospheric density. The flight envelope is the region within which an aircraft can operate safely.
This flight envelope is normally defined during the design phase. A chart of airspeed versus load factor (or V-n diagram) is a way of showing the limits of an aircraft's performance. It shows how much load factor can be safely achieved at different airspeeds.
In order to understand all the information that these diagrams may provide, it is essential to know the different airspeeds used in aeronautics for navigation and control are:
- IAS (Indicated Airspeed): the airspeed indicated by the measurement systems as the difference between total pressure and static pressure.
- CAS (Calibrated Airspeed): the IAS corrected for calibration errors due to the positioning of the instruments.
- EAS (Equivalent Airspeed): the CAS corrected for airflow compressibility.
- TAS (True Airspeed): the EAS corrected for air density variation with altitude. It represents the actual speed at which the aircraft is flying with respect to the surrounding air.
- GS (Ground Speed): the corresponding ground speed of the vehicle, taking into account the effect of wind (WS).
It is important to note that the speed used to navigate and control VECTOR APs is the so-called Indicated Airspeed (IAS). This is the airspeed indicated by the onboard sensors, typically a combination of the pitot tube and static ports. The total pressure measured through the pitot tube is the summation of the static and dynamic pressure. Dynamic pressure is proportional to air density and increases with the square of the airspeed.
But this airspeed is indicated in reference to the sea-level standard density of air. As we know, air density varies through the atmosphere with altitude; thus, IAS will differ significantly from TAS (True Airspeed) and GS (Ground Speed). To simplify, in the system, we talk about IAS and TAS.
TAS and IAS are related by the following formula:
The IAS is the TAS at sea level. If the UAS flies higher than sea level, the air density is less, and TAS and IAS will not be the same. For the same "airspeed", TAS will be increasing with the altitude.
Air density then plays an important role. A common way to estimate density is through a standard atmosphere model and an "OAT" probe.
With the previous equation, and knowing that the relation between TAS and GS is:
UAV Navigation’s VECTOR autopilots are commanded in IAS; this simplifies most operations as stall speed is kept constant with altitude. Systems are calibrated from delivery, so IAS, CAS, and EAS are all equivalent from the operator’s point of view.
Airspeed is one of the most critical concepts in aerospace engineering. Understanding it helps to interpret the flight envelope and ensure flight safety as it shows the behaviour of the aircraft and the autopilot system.
UAV Navigation are aeronautical systems with advanced algorithms refined thanks to our years of experience. A well-instructed team with deep technical knowledge makes us provide our clients with aerospace-grade systems.