Regarding previous posts about techniques other than pitot tubes to measure airspeed (such as doppler) I read about about system being trialled on helicopters right now, using l@sers.
It is said to be so accurate, it can function at speeds below 30 knots.
I'll try and find the reference...
Airbus Completes Flight Tests on Airspeed Sensor System for Helicopters - IHS Engineering360
Last edited by formulaben; 29th Dec 2014 at 21:27.
Reason: quote for reference