Technical Alternatives for Pitot Tubes?

[bob.arctor]

IAS Synthesizer

Goes to show there are a lot of complicated ways to measure airspeed.

Using a couple of vertical accelerometer sensors connected over WIFI to an iPod running dedicated low frequency audio software could be sufficient to give a decent reading (calculated with the static pressure). The reading should be more detailed by also using horizontal accelerometer data from the wing tips.

A good high school project perhaps.

specialist OSs are needed for safety.

And that would often times mean [hands on] by some version of Windows or Vista (referring to various EFIS devices here), right

[VnV2178B]

Bob.a,

by specialist I mean something like Integrity from Greenhills or LynxOs (OK the latter is sort of based on Unix/Linux) but certainly a certified to 178b system (hence my moniker)

VnV

[bob.arctor]

...alright, foremost I meant an IAS synthesizer well could be using Apple hardware

[VnV2178B]

Ok, but at the core of that hardware these days is still some Intel chip.

VnV

[MPGiles]

Master of the aUniverse

Very intersting stuff about Airspeed. Two questions How is speed calculted on the stealth aircraft which I presume don't have poles sticking into the breeze and what exactly are the failures in the Airspeed sytems in the AirBi?
The way it is phrased leads to me wondering if it was not actually icing but some other unknown failure

[Mad (Flt) Scientist]

Stealth AC in general have flush mounted sensors, AFAIK.

Those on the B-2, handled incorrectly with regard to the anti-ice system, caused the loss of the B-2 in Guam (?) IIRC so those systems are by no means foolproof either.,

[Towhee]

January 9, 2004
The End of the Tube?

UK engineers are developing a new generation of air-speed sensors that use the latest laser technology to improve on traditional methods. Jon Excell explains.

The pace of technological change - from passenger jets to helicopters - has been staggering over the past 100 years, yet there are still a few critical components that have remained unchanged.

One is the Pitot tube - a simple mechanical device at the heart of the speedometer on most modern aircraft - designed almost 300 years ago by French inventor Henri Pitot, to measure fluid velocity. The basic instrument consists of two coaxial tubes: an interior tube that is open to the flow, and an exterior tube open at 90 degrees to the flow. By measuring the difference between these two pressures the flow rate of the fluid can be calculated.


But the tube's lack of accuracy at low speeds and poor aerodynamic performance has led some in the aerospace industry to question its validity in the 21st century.

When mounted on an aircraft, the central passage in the tube points in the direction of travel. Meanwhile, a number of openings in the outside wall of the main tube lead to a second set of smaller passageways. These two sets of tubes are typically connected to either side of a pressure transducer attached to the base of the unit. The transducer measures the difference in atmospheric pressure in the two groups of tubes, and this pressure difference can be used to calculate the aircraft's speed.

While the tube is accurate at high speeds, its ability to resolve differences in pressure at low speeds is limited which compromises its performance.
An additional problem is that on fixed wing aircraft its profile significantly increases the amount of drag and therefore has an impact on fuel efficiency.

With these problems in mind, engineers at BAE systems Advanced Technology Centre have launched The Laser Air Speed Sensor Instrument programme (LASSI) a two and half year project aimed at developing air speed sensors that are both accurate at all speeds and won't contribute towards drag.

With the programme still in its infancy, LASSI project leader Leslie Laycock would not go into technical details. He did however reveal that the system will use a compact, short-pulse UV laser and a fibreoptic system. By firing the laser into the atmosphere, the nature of the light reflected from air molecules will change according to speed. This variation is measured by LASSI and used to calculate airspeed.

Laycock said that while engineers have been aware of the Pitot tube's drawbacks for some time, there hasn't really been a viable alternative. Engineers have looked at using lasers before, but they've been too bulky. Laycock claimed that it is largely due to cutting edge work in the UK on compact high-power lasers that laser-based systems are finally becoming viable.

Although LASSI has yet to yield any solid test data, Laycock said that initial estimations indicate that over the operating life of a long- haul plane, significant fuel savings could be achieved by replacing the Pitot tube with something that's flush to the airframe.

Clearly, the manufacture and installation of a system that requires compact lasers and fibreoptic cables will initially be more expensive than the Pitot tube. Laycock is confident however, that LASSI's ultimate benefits will far outweigh the initial cost. Laycock's team is also looking into the application of the technology in other areas. It could, for instance, be used to model airflow around buildings.

There's still a long way to go and at the end of the project Laycock hopes to be able to unveil a demonstrator that will be deployed either on an aircraft, or in a wind tunnel to both prove the principle and show just how small the system could be. If all goes smoothly, he said that the system could be appearing on planes in about five years.

from: The Engineer


also:

May 13, 2003
Can MEMS point the way ahead?

Honeywell says micro electromechanical systems could lead breakthrough in sensor and weapon guidance technology

by Graham Warwick

Honeywell is pursuing development of micro electromechanical systems (MEMS) as a "potentially disruptive" technology in aircraft avionics and weapon guidance. The company is already developing an attitude and heading reference system (AHARS) and standby display for commercial aviation applications using MEMS sensors.

MEMS combine micron-scale electrical and mechanical features on the surface of a silicon chip, and are batch-produced using integrated-circuit fabrication techniques. "MEMS is an enabling technology across Honeywell," says Eric Doremus, vice-president precision sensors and components. Applications range from biomedical sensors to aerospace devices including attitude and pressure sensors and inertial measurement units.


Doremus says MEMS offer significant reductions in cost, size, weight, volume and power over conventional sensors. The company is already producing air-data systems using MEMS precision pressure sensors. The next step is a flush-orifice air data system, now in development, which uses distributed pressure sensors to eliminate the pitot probe, he says. The first developmental MEMS-based inertial measurement units have been delivered to customers.

Honeywell is applying MEMS gyros to inertial systems small and robust enough to guide gun-launched projectiles. In the longer term, the technology promises to provide navigation-grade performance, allowing MEMS gyros and accelerometers to replace ring-laser and fibre-optic gyros in aircraft inertial systems. Doremus expects the Lockheed Martin F-35 Joint Strike Fighter's inertial navigator to be a MEMS-based device just 50cm3 (3in3) in size, compared with the F-16's 7,900cm3 laser-gyro unit.
Honeywell's MEMS-based AHARS is scheduled for introduction in 2004-5. GPS aiding will reduce errors, to provide an attitude accuracy of better than 0.1deg., says Doremus.

Back-up true-airspeed aiding will provide an attitude accuracy of 1-2deg.. The AHARS will be part of Honeywell's new flat-panel standby display, which will combine the unit with MEMS-based air data sensors and magnetometer. GPS integration and flight-control output will be optional features.

Honeywell has MEMS fabrication facilities in Redmond, Washington, and Plymouth, Minnesota. Both are capable of producing 150mm (6in)-diameter wafers, each containing 700 micro-scale gyros. The Plymouth site is being upgraded to handle 200mm wafers for the production of pressure sensors with 1.5 micron-sized features.

from: Flight International

[Obi Offiah]

Originally Posted by MPGiles

Two questions How is speed calculted on the stealth aircraft which I presume don't have poles sticking into the breeze

Michigan Aerospace:
"Pitot Probe/Static Ports/Alpha Sensors are used on the F-16, F-18, and the F-117. Flush Port Sensors are used on the X-15, Space Shuttle (SEADS Experiment), B-2, and the A-12. A hybrid system (Pitot Probe/Alpha Sensor/Flush Port) is used on the F-22. The B-2 has quad flush port sensors for the quad flight control systems. Flush Port Pressure Sensors are symmetrically placed on the airframe. The pressure measured at each port varies as the airframe maneuvers. Pressure differences between symmetrical ports correspond to changes in angle of attack and sideslip. Pressure data is derived from wind tunnel and flight tests. The B-2 air data system is made up of two gust load alleviation static ports, differential beta static ports, and left and right alpha/static ports. Each port contains its own 1553 terminal, which is tied to the Flight Control Computer (FCC) along with the total temperature (probe measured at engine inlet) and nose gear position. The air data computation routines in the FCC then determine angle of attack, angle of sideslip, static pressure, pressure altitude, mach number, dynamic pressure, and true air speed."

One of the advantages of laser based/optical air data systems, as well as the so called 'smart probes' is that they are very accurate (even down to very low speeds) and don't suffer from the pneumatic lag inherent in contemporary systems. This enables the FCS to have access to the most accurate current data, and aids flight stability.

Because OADS are capable of detecting air molecules in advance of the airframe, this gives the FCS the ability to act partially as a predictor 'crystal ball' system and issue corrective control surface commands before the airframe has encountered a particular body of air. I think this can be viewed as a flight safety as well as a flight comfort benefit. Turbulence detection and wind profiling

Obi

[Hyperveloce]

Hey
MEMS are related to the IRU part. They probably have a potential to cut prices. IRUs and GPS are very complementary to each other: IRUs have a drift tendancy (summing of errors on acceleration increments) while GPS will not drift but will have larger instantaneous errors. They can correct each others. With 3 GPS antennas, the GPS can also estimate the attitude angles, enabling a deeper hybridation with the IRUs. A complete hybridation (we call it sensors hybridation in French but in English, it might be termed as sensor data fusion ?) may be possible with all the air references, including new sensors like laser anemometry (which can remotly sense the 3D airspeed field, hence also mesure angles of incidence, slide, etc...) or a Lidar (new weather radar for improved detection of turbulences/CAT, with doppler/interferometry capabilities).

I may be dreaming, but this full hybridation of IRUs, Air references, Laser Velocimeter or Lidar, barometric/radioaltimetry altitude, and possibly other air reference sensors (cinetic pressure transducers on the airframe or in the compression stages of the engines as suggested by others), implemented via a large extended Kalman model (mainly describing the airframe & flight/fluid mechanics, the sensors and their measures... the underlying physics) would provide an optimal error detection/isolation scheme (nothing to do with constant thresholds in magnitude/duration to detect faults as it is done) and the capability to estimate some failed air parameters via all the others and through the real time monitoring the aerodynamical authority of the airframe (and its control surfaces, via microorders added to the normal orders). This real time aerodynamical authority estimation (through slight modification of the "posture" of the aircraft) coupled with the direct sensing of the air stream and the resulting linear/rotational accelerations provided by the IRUs (and possibly laser/lidar imagery of the air stream) would be close to the proprioceptive sense used in bird flight. This extended Kalman model would "know" that spikes of a few degrees magnitude and of a few milliseconds duration on the AoA or pitch are not a real thing in flight mechanics it has to react to (fault detection & exclusion), all the more if other paramaters remain unchanged (vertical acceleration). It would also know that a CAS drop approaching 200 kts in amplitude in a blink of an eye is not possible if the control surface authority and other parameters remain constant. It would exclude corrupted measures in an optimal manner, relying on measures consistant with each others from an aerodynamical point of view. Otto would be a lot smarter ? This sensor hybridation scheme is used by some military flying vehicles.
Nothing to do with the air reference improvement but in difficult conditions, the reading of the instruments may be a problem according to some pilots: the military have head up displays, why not for the civilians too ?
Jeff

[timpara]

Estimating the aerodynamical efficiency of the control surfaces

Re Hyperveloce's idea of using control surfaces as part of a backup aurspeed system: I'd been thinking along similar lines. But a simpler way to calculate airspeed might be to measure the pressure required (in say a hydraulic system) to deflect a control surface (for example, rudder) by a known amount. This backup calculation could be done continuously.

[Mad (Flt) Scientist]

Requires a very good understanding of the hinge moments at small deflections. Not a trivial thing to do, since it also varies with other conditions. Doing something similar (but with known AS in order to estimate control effectiveness for FBW purposes) has been done before.

[rubik101]

Doesn't anyone like my idea of a bendy piece of plastic for airspeed and a lead ball on a piece of string instead a gyro? (or a blob of depleted uranium on a carbon fibre thread if you want expensive)

[Hyperveloce]

the real time estimation of the control surfaces authority would at least require a complete aerodynamical model of the airframe for all the flight enveloppe and control surfaces configurations, and a structural description of its deformations under load factors. This complete aerodynamical/structural model is of course available to the aircraft manufacturer.
Jeff

[Mr Optimistic]

complex hybrid systems

problem with complex hybrid systems is that it would be a bit of a task to elucidate teh myriad failure modes and fault combinations. Sometimes the burden of providing such 'truth' makes the enterprise doubtful

[thomas8]

BUSS ( Back Up Speed System )

Airbus designs a new function for A380 & A330, it is called "BUSS", does anyone know this function ?

[Mad (Flt) Scientist]

It's an Angle-of-Attack based display; it basically has a green (target) range of AOA that you are supposed to fly in, with the low and high ends being red "no go" areas representing overspeed (low AOA) or stall (high AOA).

Fairly simple in concept - though of course it requires the AOA sensors be operational, and depending on the nature of the airspeed problem that might be in doubt (ADC hardware failure would be a good case for using BUSS, but something affecting the outside environment, such as extreme icing (SLD) or hail (physical gamage to probes and sensors) could disable BUSS as well as conventional airspeed)

Still, it's a fairly good solution in that it uses existing installed equipment, and has some significant dissimilarities to enhance the chance of it being available.

[rgbrock1]

re: the somewhat snide comments about Mac OS X and Linux.
Yes, both operating systems may, on occasion, crash or even hang. (I'm a systems engineer by profession) But they certainly do so far, far less than
Windoze. And let's not even discuss the security implications. (Windoze being akin to the proverbial swiss cheese as far as security goes.) Nor should we discuss the inherent bugginess of any Windows release.

[chase888]

I will have to take my chances on being shot down, but I simply have not had the time to go back over so many posts.
Just wondering if any consideration has been made on how icicles form?
Water from the pitot drain would I suspect be only just above freezing, so meeting 800k/hour sub zero air on the outside would probably induce a further blockage in the form of a lump of ice externally.
Maybe the pitot needs an external heater also?

[Graybeard]

It might be instructive to compare pitot tubes and heaters certified on Airbus to pitot tubes and heaters on Boeings, Douglas, Lockheed, et al. Multiple pitot icing appears to be unique to Airbus.

GB

[Will Fraser]

Graybeard I know it's generally accepted that the pitots and heaters on Airbus produce Unreliable Airspeed data, can you tell me where that is demonstrated?? With the Ua/s incidents, (NOT accidents, AF is far from known), what is the data derived that implicates this equipment??

Will