How Air Data Computers Can Deal With Downwash
In a world where most people can find out where they are and how fast they’re going by looking at their mobile phone, it may come as a surprise to many that most aircraft generally rely on pressure measurement devices to do this. One reason for this is that aircraft need to know how fast they are flying relative to the air – something that changes with wind direction and speed. Pressure probes are connected to an ‘air data computer’ that converts sensed data into a format suitable for various aircraft systems such as cockpit displays.
Pressure based systems have been proven effective for decades but there are limitations to their effectiveness, in particular for rotorcraft. Rotorcraft experience some unique pressure phenomena, such as downwash, because of how they operate and these can affect the accuracy of pressure measurements, particularly in take-off, landing and in the hover. At low speeds, air is pushed past the sensors by the rotor. This creates pulses of pressure, also referred to as pneumatic noise. Similarly, when a rotorcraft climbs vertically, a sudden increase in air pressure from the rotors blinds the sensors to the rate of assent.
If these effects are not corrected, then the information sent from an air data computer will be inaccurate. Current approaches to resolving these challenges are mixed. One tactic is to use a separate low speed probe, but they can have a poor reputation for being useful in practice currently. Future solutions may bring an extra level of accuracy to low airspeed that will be very useful.