Groundwork

Measurement Apparatus For Determining A Velocity Of A Fluid Gets a Better Next Step

Helicopters have never had a reliable airspeed reading in the hover, and Airbus's newest answer is a bent rod - twelve years after its laser read the air to 2 knots.

Clay-diorama editorial illustration related to Measurement Apparatus For Determining A Velocity Of A Fluid.

What changes in this design

A reader might expect fluid speed to be read directly by a measuring device. The needed result is how fast the surrounding fluid moves relative to the vehicle, rather than simply the vehicle’s own speed.

Here, sensors are associated with a flexible piece. The processing system interprets signals about the fluid’s push or turning effect on it, together with its known form and stiffness, to determine relative fluid velocity.

It explains why the flexible piece’s form and stiffness matter: they are inputs to the processing system’s velocity determination. This helps clarify what information the apparatus uses.

Zero knots over Nebraska

On 3 August 2018 a helicopter was circling a golf course near Mullen, Nebraska, looking over a landing site. The pilot slowed from 100 knots to about 70 and rolled into a tight decelerating downwind turn. The airspeed indicator displayed 0 knots. The nose began to swing left, the yaw rate was not arrested, and the passenger was seriously injured, the helicopter substantially damaged. The pilot had 213 hours.

Investigators put the probable cause on the pilot's anti-torque pedal application, not on the instruments, and I think that's right. Look at the reading anyway. At the moment the pilot most needed to know how fast the aircraft was moving through the air, the gauge said nothing at all.

Why the needle dies below forty knots

A pitot tube measures the pressure of air pushed into a forward-facing hole. Pressure rises with the square of speed, so at low speed there is barely any signal, and what there is gets stirred by the rotor's downwash and by the strange angles a helicopter flies at. A patent granted to Lockheed Martin in 2025 puts numbers on it: probes struggle below roughly 40 knots, and below about 25 the reading is a poor predictor of actual speed.

This is an old problem, worked hard. A 1997 filing from a US Navy laboratory describes an earlier fix - two venturi tubes on an arm spinning above the rotor hub, needing slip rings to get the data down - and explains why most helicopters carry no low-airspeed sensor at all: mechanical complexity, expense, and added drag. In 2000 an American instrument maker filed for a rigid arm to hold a probe clear of the downwash. None of it became standard equipment.

The laser Airbus flew in 2014

In December 2014 Airbus finished flight tests at Marignane of a laser system that read the free airstream 50 to 60 metres ahead of a Dauphin. It was accurate to better than 2 knots across the entire flight, and it could read very low and even negative airspeeds, meaning flight backwards. The conventional floor is around 30 knots. The stated next step was certification with the supplier, aimed at commercial rotary-wing use.

Twelve years on, I can find no public evidence of a certified installation. The supplier still lists the product. Its page publishes no accuracy figure, no weight, no power draw and no certification status.

A bendable rod, filed in February 2026

Which brings me to the filing. On 13 February 2026 the German arm of Airbus Helicopters, at Donauwörth, applied for a measurement apparatus for determining the velocity of a fluid, published on 27 August. It claims a bendable rod attached to the vehicle with a predetermined shape and stiffness, sensors producing a signal indicating the force or moment the fluid exerts on it, and a processing system turning that into velocity. It is a stick in the wind with strain gauges on it.

The plausible customer is next door. The same site launched a three-tonne light twin in March 2025, with a four-axis autopilot and first delivery planned for 2028, aimed first at air ambulance operators. An autopilot holding a hover needs a trustworthy low-speed reading even more than a pilot does.

The square that will not go away

Here's my problem with it. Aerodynamic force on a rod also rises with the square of velocity, so the signal collapses in exactly the sub-25-knot band that kills the pitot tube it would replace. Then there is ice. The rules for transport rotorcraft require a heated pitot or an equivalent means of preventing icing malfunction, and set a calibration bar of 3 percent or 5 knots, whichever is greater, in level flight. Ice on a rod changes its shape and its stiffness, the two numbers the whole calculation rests on.

Two credible competitors had patents granted in 2025 that bet the other way. Lockheed Martin blends pitot data with speed estimated from rotor control commands and swashplate positions. Joby Aviation computes airspeed from a rotor power model fed by signals already going to its motors, explicitly without dedicated sensors. Neither puts anything new into the airstream.

What would have to be true

For the rod to be the right answer, some things have to hold. The predetermined stiffness must stay predetermined, through temperature, fatigue, rain and ice, or be measured continuously rather than assumed. A late-2025 paper on flexible strain-sensing whiskers for underwater flow named calibration complexity as one of its open problems. Same dependency. The rod also has to beat inference from the rotor, which costs nothing to install and cannot ice up.

What I keep returning to is 2014. The laser worked, on an Airbus aircraft, to 2 knots. Something after that flight made a bent stick look like the better bet, and I'd like to know what it was.

Sources

Reporting for this piece drew on the published filing and the references below.

The record, briefly. US20260253613A1, “Measurement Apparatus For Determining A Velocity Of A Fluid” An apparatus mounted to a vehicle uses sensors associated with a bendable rod of predetermined shape and stiffness. The sensors generate signals indicative of fluid force or moment on the rod, and a processing system determines fluid velocity relative to the vehicle from those signals and the rod's properties.

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Read around this one:Public patent record USPTO published figure