Low Airspeed Measuring Devices for Helicopter Usage Monitoring Systems

Low Airspeed Measuring Devices for Helicopter Usage

Monitoring Systems

C. G. Knight

Air Vehicles Division

Platforms Sciences Laboratory

DSTO-TN-0495

ABSTRACT

Low airspeed measurement is problematic on helicopters due to adverse flow fields from the
rotors impinging on the standard pitot static system. A variety of alternative methods exist to
measure airspeed.

This note presents a brief review of some of the low airspeed measuring devices available for
aircraft and, more particularly, helicopters. This note also introduces a novel technique that
was studied within Air Vehicles Division as a concept demonstrator.

RELEASE LIMITATION

Approved for public release

Low Airspeed Measuring Devices for

Helicopter Usage Monitoring Systems

Executive Summary

A helicopter manufacturer usually specifies the retirement lives of fatigue life-limited
structural components based on assumed usage spectra. This is dependent on the
aircraft’s mission profile and measured flight loads. A more accurate method measures
the actual, in-service, usage. This method, Usage Monitoring, aims to identify how the
helicopter is being used and then estimates how much fatigue life remains for specific
components before they need to be replaced.

One of the key parameters for Usage Monitoring is the helicopter’s airspeed.
Unfortunately traditional methods of measuring airspeed, such as the pitot-static
system, often do not work below 30-45 kt. As some low speed manoeuvres, such as
hover, sideward and rearward flight, do significant damage to components in terms of
fatigue life accrual, it is important that low airspeed be measured accurately.

No current Australian Defence Force (ADF) helicopters have low airspeed indicators.
However, low airspeed measurement is critical on attack helicopters, due to the low
speed targeting requirements. The ADF is about to start operating an attack helicopter,
the Eurocopter

Tiger

. Thus knowledge of low airspeed methodologies will also aid in

the introduction to service of this new platform.

This report outlines some potential methods for measuring low helicopter airspeeds,
where the normal airspeed sensing system does not work adequately.

In addition to a review of available methods, some additional work in this area was
completed as part of a proof-of-concept demonstrator. A simple variable incidence
anemometer was designed and tested in the low airspeed wind tunnel at the Platform
Sciences Laboratory.

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1. Introduction

An aircraft manufacturer usually specifies the retirement lives of fatigue life-limited
structural components based on assumed usage spectra that are dependent on the
aircraft’s mission profile and measured flight loads. Over their service life, Australian
Defence Force (ADF) helicopters are often not used in the same mission profile as
assumed when they were designed. In addition, the size of the ADF fleet for any given
platform often does not warrant recalculation by the original equipment manufacturer
(OEM).

These differences in usage spectra have implications for the retirement lives assigned
to components. The calculated lives could be too high, leading to safety concerns, or
too low, leading to higher than necessary operating costs. A means of determining the
actual damage incurred by components is therefore desirable. One possible solution is
to monitor the mission profile of the helicopters and recalculate the retirement times
based on the actual type of flying undertaken. This monitoring could be continuous or
done at intervals. A monitoring device could be installed that establishes the type and
duration of manoeuvres flown allowing for an adjustment of the retirement lives of the
components. This process is known as Usage Monitoring.

Usage Monitoring aims to identify how the helicopter is being used and then estimate
how much fatigue life remains for specific components before they need to be replaced.
There are two main ways of conducting usage monitoring; loads monitoring and flight
condition monitoring. Flight condition monitoring is the more common method, and
involves the recording of various aircraft parameters from which flight conditions can
be deduced. To convert information about flight conditions into fatigue damage for a
particular component, a relationship is required between each flight condition and the
resulting flight loads on that component. This relationship is generally obtained by
conducting a comprehensive flight test program on a fully instrumented aircraft. The
aircraft is flown through each flight condition and the required component loads are
recorded.

A key parameter for flight condition monitoring is the helicopter’s airspeed.
Knowledge of the airspeed is therefore important. Unfortunately traditional methods
of measuring airspeed, such as the pitot-static system, often do not work below 30-
45 kt. As some manoeuvres such as hover, sideward and rearward flight do significant
fatigue damage to components it is important that low airspeed be able to be measured
accurately.

Low airspeed measurement is also important on attack helicopters, such as the
Eurocopter

Tiger

, Bell

Cobra

and Boeing

Apache,

as low speed targeting requires

accurate knowledge of helicopter speed.

This report outlines some potential methods for measuring low helicopter airspeeds,
where the normal airspeed sensing system does not work adequately.

DSTO-TN-0495

2. Helicopter Airspeed Measurement

Airspeed is normally obtained via pitot-static tubes mounted near the front of the
helicopter. If the helicopter is travelling faster than approximately 30 knots (56 km/h),
then the speed indication works accurately as the pitot-static tubes are not in the
downwash of the rotor system (Figure 1). However, if the helicopter is travelling at less
than 30 knots then the pitot-static tubes are within the downwash from the rotor blades
giving an error for the airspeed readings (Figure 2). For some helicopters, low-speed
regimes are significant in terms of the fatigue damage experienced by components.

Low-speed regimes consist of hover, low-speed forward, sideways, and rearwards
flight.

Since the readings from the pitot-static tubes are not reliable at speeds below 30 knots,
other methods of determining the helicopter’s speed are required.

Downwash
Stream Lines

Figure 1: Speed greater than 30 knots - downwash has no effect on pitot-static tube readings (Ref

. 1)

Downwash
Stream Lines

Figure 2: Speed less than 30 knots - downwash affects pitot-static tube readings (Ref. 1)

Some methods that have been researched include using the Global Positioning System
(GPS) to measure ground speed (Ref. 1). The methodology behind this is discussed
briefly in Section 4.

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Some other possible solution to the problems of low airspeed sensing lies in methods
such as mechanical anemometry or laser based anemometry methods. Both these
methods will be discussed in this report.

3. Mechanical Methods of Anemometry

Development of mechanical methods of accurately estimating low airspeed began in
the 1950s when preliminary concepts were developed and flight-tested. These systems
involved mounting sensors above the rotor hub as well as in the wake beneath the
rotor. The sensor has generally been a rotating anemometer, pitot tube, accelerometer
or pressure transducer.

In general these methods rely on gimballing the sensor to measure local wind speeds
and directions or taking averaged airspeed measurements from less turbulent areas of
airflow around the helicopter and calibrating this to true airspeed.

3.1 Gimballed Methods

The main source of error in a pitot-static system of airspeed measurement is that the
pitot tube is no longer aligned with the direction of airflow. A general solution to this
problem is to allow the pitot tube to align itself with the local airflow and then calibrate
the system to read corrected helicopter airspeed. Figures 3 and 4 show one such system
as mounted on the Boeing AH-64D

Apache

. The system is mounted in dual on arms that

extend outward from the top of each engine cowling, as indicated by the arrows.

Figure 3. An example of mechanical anemometry onboard the Boeing Apache AH-64D.
(Picture from www.boeing.com)

The basis of the LORAS (Low Range Airspeed Sensor) system is shown in Figure 4. It is
a standard pitot-static system mounted on a gimbal that allows the system to rotate in
line with the local airflow. A circular vane that acts like a weathercock aids the

DSTO-TN-0495

alignment. The angle of the sensor can then be measured and a calibrated airspeed
signal is produced.

Figure 4. The LORAS low airspeed measuring system (Ref. 2).

This style of low airspeed measurement is used on some attack helicopters such as the
Apache and the Bell AH-1Z Cobra. However, the Cobra appears to only use one
system mounted on an arm above the cockpit. Here the flow field is perhaps more
uniform, and thus a backup is not required. The system costs approximately
US$110000.

3.2 Other Mechanical Systems.

Other mechanical systems exist which measure airspeed using a variety of methods.
For example the Omni Directional Air Data System (OADS) uses two venturi tubes on
opposite ends of a rotating arm. This system is installed above the rotor hub to
measure the magnitude and direction of the local airspeed. The differential pressure
between the two sensors is used to calculate the airspeed and sideslip angle.

A similar Russian system, mounted on the Mi-28

Havoc

attack helicopter, has pitot

tubes on the end of the rotor blades rather than as a separate system (Ref. 2).

A brief search of the US Patent Office reveals numerous examples of mechanical
systems for measuring low airspeed. There are many similar systems, with varying
degrees of accuracy and capability. Appendix A contains the cover page from similar
patented systems. Patent 5874673 (February 1999) uses an accelerometer mounted on a
swinging arm in phase with the main rotor. The amplitude of the sensor peaks
corresponds to airspeed and the phase of the peaks is an indication of the direction of
the aircraft.

DSTO-TN-0495

Patent 4893261 (November 1987) is a system that uses the sinusoidal variations of
pressure on the rotor along with a Fourier Transform analysis to determine aircraft
velocity. Of note here is that the assignee is United Technologies Corporation. This
company is the owner of Sikorsky and manufactures the various Hawk helicopters, of
which the

Black Hawk

and

Seahawk

are in Australian military service.

Often these mechanical systems require slip ring assemblies or some other means of
transferring data from the rotating reference frame of the rotor to the fixed reference
frame of the fuselage.

Patent 5299455 (March 1992) attempts to account for this by measuring the air velocity
from the three major axis directions with standard hot-wire anemometry. If this system
was calibrated accurately then it should be as accurate as the gimballed pitot tubes
discussed above. However hot-wire anemometers are prone to fouling due to insects
and dust in the air. Thus it is not clear how effective this system would be in a real
helicopter.

4. Non-Mechanical Methods

4.1 GPS as a Low Airspeed sensor.

The Global Positioning System (GPS) can be used to obtain a helicopter’s ground
speed. However, as the required parameter for flight regime recognition is airspeed,
not ground speed, the use of the GPS as a system for measuring low airspeed will
require knowledge of the wind speed. The relationship between airspeed and ground
speed is a simple vector sum as shown in Figure 5.

A possible solution for determining wind speed, whilst the helicopter is flying at
speeds greater than 30 knots, is to obtain the airspeed from the pitot system and the
ground speed from the GPS, and then determine the wind speed vector. Assuming that
the wind speed stays relatively constant during short periods whilst the helicopter is
travelling below 30 knots, an estimation of the airspeed can be obtained via the GPS by
removing the last wind speed vector from the GPS ground velocity.

WindSpeed

AirSpeed

GroundSpee

Figure 5: Relationship between Airspeed and Ground Speed

Wind Speed

Airspeed

Ground Speed

DSTO-TN-0495

The accuracy of this system is dependent on the ability of the GPS to accurately
measure ground velocity and the validity of the assumption that the wind speed vector
is relatively constant during the times that the airspeed decreases below 30 knots.

A field trial with a GPS unit, mounted in an automobile, at the Army Test and
Evaluation Agency (ATEA) testing grounds, at Monegeetta, Victoria (Ref. 1), proved
that the GPS could be used. As noted below, this system did have some limitations.
The velocity results obtained via the GPS matched well with the measured velocity of
the vehicle even in a dynamic environment. Some observations from the trial were:

•

The velocity solution can momentarily become inaccurate whilst switching between
satellites. The problems of inaccuracy that arise whilst changing satellites may be
reduced in updated GPS receivers which now routinely track up to 12 satellites or
by receivers designed for use in a dynamic environment.

•

The loss of a satellite, from four to three, provides fewer problems than the
switching of satellites as described above.

•

The algorithms used must be able to handle the transient and sometimes erratic
results that a GPS outputs when it switches satellites. One possible solution is to
ignore the GPS output for a certain period of time when the sensor detects that the
GPS has switched satellites.

•

The time lag measured between the vehicle speed and the GPS speed is several
times greater than that claimed by the manufacturer of the GPS. The lag is large,
but still acceptable. Newer GPS receivers will certainly be better in this regard.

•

The possible loss of GPS signal in a dynamic and obscured environment, such as
nap-of-the-earth flying and hovering amongst tree cover.

•

The use of the GPS for a low-airspeed sensor requires an assumption that wind
speed is constant during slow speed flight regimes for short periods of time. This
assumption has not been tested and would need to be validated before a GPS
system could be used.

A fuller review of the investigation into the use of GPS for low airspeed sensing is
contained in Ref. 1.

4.2 Other Non-Mechanical Methods

The previous search of the US Patent Office revealed a number of non-mechanical
methods of low airspeed anemometry. Each of these methods can be classified as using
either an algorithm approach or a neural network. There is one other type of non-
mechanical method, which uses a laser beam. That method is covered in more detail in
the next section.

DSTO-TN-0495

The cover page from a selection of these patents is contained in Appendix B. A search
of other patent offices, such as the British or European Patent Offices is expected to
reveal similar airspeed anemometry methodologies.

4.2.1 Algorithm Method

The algorithm method consists mainly of measuring various aircraft state parameters
such as altitude, pitch and roll angle, accelerations, and various control positions.
These values are inputs to previously calculated equations, which reduce to indications
of airspeed and, often, sideslip angle.

Appendix B contains a number of patents for algorithm based anemometers found at
the US Patent Office website. These represent only a small portion revealed by the
search.

A number of very interesting approaches have been tried and patented. The measure of
seriousness of this problem is indicated as much as anything by the company names
that the patents are assigned to.

Patent 5750891, which was assigned to GKN Westland Helicopters in 1996, is a method
of deducing airspeed from a measurement of radial air flow on the blade and the
rotational velocity of the blade. This gives an indication of sideslip angle. This angle is
then compared with a database of values that correlates the sideslip angle with
airspeed.

Patent 4300200 was assigned to the precursor of GKN Westland Helicopters, Westland
Aircraft Limited in 1979. The basic methodology here is to measure the instantaneous
weight and power of the helicopter and compare these values with a database of
known airspeed, weight and power characteristics.

Many other patents consist of a similar system to that identified in Patent 4794793 from
1989. The basic methodology is to measure the various control (cyclic, collective and
pedal) positions and use these as inputs to an algorithm that outputs airspeed. Patent
5214596 from 1993 is a variation on this theme.

4.2.2 The Neural Network Approach

The neural network approach of airspeed estimation is very similar to the algorithm
approach. That is, various state parameters are measured and an indication of airspeed
is calculated. The main difference is in the method of calculation. Neural nets need to
be “taught” the methodology for calculation. As an initial input, a series of known
parameters and known correct airspeeds are used to train the neural net.

DSTO-TN-0495

Once this is achieved a series of tests are run which allow the trainer to estimate if the
neural network has sufficiently learnt the methodology or needs additional training in
order to be more accurate.

Once the neural network has been successfully trained, the values gathered in service
can also be used to give additional knowledge to the neural net.

The first of the neural network approaches to airspeed estimation in Appendix B is
Patent 5901272. This patent was assigned to The US Navy and is based on work
presented in Ref. 4. The remaining neural network based system for airspeed
estimation is Patent 5063777, assigned to Sextant Avionique. The patents shown in
Appendix B are very similar in methodology. The main differences are in the different
parameters used to calculate the airspeed and in how the information is presented to
the pilot. Some detail of the methodology is provided in References 4 and 5.

The testing outlined in Ref. 4 was performed using a tandem rotor CH-46

Sea Knight

The accuracy noted for the 1996 tests shows some promise. The in-ground-effect (IGE)
was within 10 kt and the out-of-ground-effect (OGE) was within 6 kt. Of note is that the
reference airspeed was only accurate to 5 kt. So both these results are well within the
accuracy for Helicopter Usage Monitoring System (HUMS) requirements.

The training data for the airspeed sensor components of the neural network used in the
2002 report (Ref. 5) was obtained during test flights performed at very low prevailing
wind speeds. This allowed the use of groundspeed measuring devices to effectively
measure airspeed. A number of different flights were completed with 4.5 hours of total
flight time captured. This involved flight in all directions (forward, sideways and
rearward) and during both in-ground and out-of-ground flight.

The results from Ref. 5 are an improvement on those from Ref. 4. The main differences
seem to be the application of various filters. The final root mean squared error of the
data was 3.4 knots. The error increases markedly when the airspeed is below 5 knots or
above 35 kt. No differences between OGE and IGE are noted in Ref. 5.

Ref. 6 is based on work with a Westland

Mk9 Lynx

, which is a single rotor helicopter.

The University of Warwick, CSE International and GKN Westland Helicopters
performed the work. The results from this study were more accurate with an error of
3 kt and the paper also notes a degradation of the accuracy whilst at IGE altitudes.

The inputs for the Ref. 6 study consisted of the four major controls, all angular
positions and rates (pitch, roll and yaw) and pitch and roll angular accelerations, rate-
of-climb, rotor torque, aircraft mass and CG position. While most of these can be
obtained from aircraft instrumentation, it is not clear how accurately aircraft mass and
CG position are known throughout the test flight period.

DSTO-TN-0495

5. An Example of Mechanical Anemometry

The search of available techniques revealed a number of issues. Firstly the available
methods often got no further into production than a patent or a journal / conference
paper. Secondly, those that actually went into manufacture were very expensive. For
example, the LORAS system presented earlier in this document costs approximately
US$110000. The actual cost of the unit is difficult to determine as it is usually sold as
part of the helicopter. Unfortunately, this cost was well outside that allowing AVD to
purchase and test the system.

A decision was made to design and test a simple mechanical anemometer. The design
is very basic. Figure 6 shows a picture of the final design. When mounted in a moving
airstream, the turbine rotates. After calibration the RPM of the turbine represents a
known wind speed. Due to the weathercocking effect, the axis of the anemometer
points directly into the wind. Thus the anemometer provides an indication of local
airspeed.

Figure 6.Major components of the AVD designed mechanical anemometer.

The counter-weight is mounted on a screw that allows the Centre of Gravity (CG) of
the anemometer to be adjusted. For the purpose of the reported experiments, the CG
was adjusted to coincide with the centre pivot shaft. This eliminated any turning
moment that can be caused by the vehicle’s acceleration.

The bearing support block carries the centre pivot shaft down to a coupling connecting
the shaft with the 0-10 Ohm rotary potentiometer. The potentiometer allows the
direction of the anemometer to be determined relative to some initial axis.

The turbine is a computer power supply fan. The turbine is mounted on a shaft (inside
the anemometer), which is connected to a slotted disk. The disk has 2 diametrically
opposed slots providing a twice per revolution tacho signal.

Centre Pivot Shaft

Counter

Weight

0-10 Ohm

Rotary Pot

Bearing Support Block

Turbine

DSTO-TN-0495

For wind tunnel testing, and for the testing while mounted on a car, it was only
necessary to have the anemometer capable of full rotation about the vertical axis, as
both of these environments were essentially two-dimensional, since no vertical wind
was anticipated. This device was not designed to be tested while mounted on an
aircraft; to do so would require an additional mount to allow rotation about the
pitching axis of the device.

One early concern was that the device would be unstable about the vertical axis and
oscillate in yaw. Both wind tunnel testing and preliminary car testing did not indicate
this problem. In fact, the device proved to be very stable at all speeds up to 25 m/s (50
knots).

After completion of the wind tunnel tests, informal car trials were completed to
indicate the oscillatory stability of the system in gusty and turbulent conditions. The
AVD Anemometer was visually tracked during these trials and its performance was
considered satisfactory to continue further testing. As in the wind tunnel testing, no
unstable oscillations were observed up to the maximum test speed of 100 km/h.

At that stage, a complete set of fully instrumented car mounted tests were being
planned. However, at about this time, the Optical Air Data Systems LANCE details
were found in Ref. 7. This system is detailed in Section 6 below. Its reported
performance, low weight and low projected cost would make this system ideal for a
low airspeed sensor for helicopter applications. Because of this, we have decided not to
invest further effort in pursuing the car trials of the AVD Anemometer, but to report
existing results to date.

6. The Optical Air Data Systems LANCE

The Optical Air Data Systems (OADS) Laser Night/day Combat and Training
Engagement (LANCE) system is a system originally developed to estimate range,
target wind velocity and muzzle velocity for the 30mm cannon carried by amphibious
assault vehicles (AAV).

The same basic principles evolved into a system to aid snipers in estimating wind
speed at the target. Extensive use of fibre optics and laser diodes has lead to a device
that weighs 500 grams and costs around US$2500. The system it replaces weighs 2.6 kg
and costs US$11000 (Ref. 7).

The next stage of development is as a down-range wind sensor for US Navy Landing
Craft Air Cushion (LCAC). This is a similar application to the initial fit out on the
Bushmaster amphibious assault vehicles (AAV). One of the major advantages of the
LANCE application to both the AAV and LCAC is that the system is mainly solid state
with very few mechanical components. This is particularly relevant to the salt laden

DSTO-TN-0495

atmosphere that the system operates in (Ref. 7). It also lends itself to the shipborne
helicopter operations.

The system has found applications with tilt rotor aircraft. The V-22 Osprey is to trial
the system in an effort to avoid the low speed phenomena of vortex ring state (VRS), or
“settling with power”.

VRS occurs when a helicopter is at low forward speed with a high rate of descent. In
this condition some portion of the airflow is upwards through the disk. To escape from
this state the pilot is required to reduce collective (and thus power) and gain forward
airspeed. Critical to recovery is the need for altitude. Unfortunately VRS occurs often
on approach to landing (Ref. 2).

The situation is even more critical on the V-22 as the two rotors may not enter VRS at
the same time. This will cause a rolling moment to develop. Although the conditions at
which VRS occurs are easily identified there is presently no way to provide
information on airspeed to the pilots because of the limitations of the pneumatic
system used. The LANCE system hopes to be able to provide the information on
airspeed and thus avoid VRS. This will allow the V-22 to remove its current limitation
of 800 feet per minute descent rate (Ref. 7).

Unfortunately, repeated requests for further information from Optical Air Data
Systems have not been answered. The referenced article (Ref. 7) and the company’s
website (www.oads.com) contain little in the way of technical information. What little
is available indicates that the system is based on an Erbium laser.

DSTO-TN-0495

7. Conclusion

This note has presented a review of some of the available technologies applied to low
airspeed indication on helicopters. For the purpose of this document low airspeed has
been defined as below 45 knots. However, in general it is that speed at which the
standard pneumatic system (pitot static tube) will cease to function reliably on a
helicopter.

A number of different approaches have been studied. These approaches fall broadly
into two categories. The first comprises the mechanical methods. These include
systems mounted dynamically, which are thus able to swivel into the local wind
direction and give a more accurate reading of local airspeed. Other mechanical
methods include systems based on accelerometers, or pressure transducers, which are
used to deduce local conditions. These local conditions are then calibrated back to a
global speed for the helicopter.

The second broad category is the non-mechanical system. These include GPS and laser
based systems, algorithm systems and neural networks. Both algorithm and neural
network systems are similar in that various state parameters are measured and
airspeed is calculated. The main difference between the two is in the method of
calculation. The algorithm method uses a set equation to convert the inputs into
airspeed, whilst the neural network needs to be taught the appropriate parameters but
is able to be more flexible in the application.

In the final part of this note two case studies were presented. The first was an example
of a mechanical system that was designed and tested within AVD. It was found to be of
reasonable accuracy, but was not tested in a real application on board a helicopter.

The second case study was on a laser based system, designed by Optical Air Data
Systems. Although this system has not yet been fully applied to the problem of low
airspeed indication on board helicopters, the manufacturer’s claim shows great
promise in providing a low cost, low weight and accurate alternative to current
pneumatic based systems, at all speeds.

DSTO-TN-0495

8. Acknowledgements

The author would like to thank Mr Scott Dutton for his electronic design skills and Mr
Soon-Aik Gan for his extensive programming expertise.

9. References

Application of GPS to Low Airspeed Measurement for Helicopter Usage

Monitoring Systems

, SA Gan, SA Dutton, CG Knight, DSTO TN-0428, 2002.

The Art and Science of Flying Helicopters,

S. Coyle, Iowa State University

Press, 1996.

A Method Of Helicopter Low Airspeed Estimation Based On Measurement

Of Control Parameters,

AJ Faulkner and S Attlefellner, Paper Presented at the

European Rotorcraft and Powered Lift Aircraft Forum, Amsterdam,

September 1979.

A Neural Network Based Approach To Helicopter Low Airspeed And

Sideslip Angle Estimation,

KM McCool, DJ Haas and CG Schaefer Jr. Paper

number 96-3481 in Proceedings of the American Institute of Aeronautics and
Astronautics Flight Simulation Technologies Conference, July 1996.

A Virtual Sensor System For Helicopter Usage Monitoring,

OH Ben-Zeev, DJ

Haas and MA Morales, Paper Presented At The American Helicopter Society
58

Annual Forum, Canada 2002.

6) P

redicting Airspeed And Sideslip Angle Using An Artificial Neural

Network

, DA Goff, SM Thomas, RP Jones and CP Massey, Paper Presented At

The American Helicopter Society 55

Annual Forum, Canada 1999.

Catch The Wind

, JG Roos, Armed Forces Journal, February 2002.

The Measurement of Airflow

, 5

Edition, E. Ower and RC Pankhurst,

Pergamon Press, 1977

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2. TITLE

Low Airspeed Measuring Devices for Helicopter Usage Monitoring
Systems

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THAT ARE LIMITED RELEASE USE (L) NEXT TO DOCUMENT
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Document

(U)

Title

(U)

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(U)

4. AUTHOR

C. G. Knight

5. CORPORATE AUTHOR

Platforms Sciences Laboratory
506 Lorimer St
Fishermans Bend Victoria 3207 Australia

6a. DSTO NUMBER

DSTO-TN-0495

6b. AR NUMBER

AR-012-765

6c. TYPE OF REPORT

Technical Note

7. DOCUMENT DATE

May 2003

8. FILE NUMBER

M1/9/1087

9. TASK NUMBER

LRR 02/184

10. TASK SPONSOR

DSTO

11. NO. OF PAGES

12. NO. OF REFERENCES

13. URL on the World Wide Web

http://www.dsto.defence.gov.au/corporate/reports/DSTO-TN-0495.pdf

14. RELEASE AUTHORITY

Chief, Air Vehicles Division

15. SECONDARY RELEASE STATEMENT OF THIS DOCUMENT

Approved for public release

OVERSEAS ENQUIRIES OUTSIDE STATED LIMITATIONS SHOULD BE REFERRED THROUGH DOCUMENT EXCHANGE, PO BOX 1500, EDINBURGH, SA 5111

16. DELIBERATE ANNOUNCEMENT

No Limitations

17. CITATION IN OTHER DOCUMENTS

Yes

18. DEFTEST DESCRIPTORS

Anemometers, Air Speed Measurement, Measurement, Helicopters

19. ABSTRACT

Low airspeed measurement is problematic on helicopters due to adverse flow fields from the rotors impinging
on the standard pitot static system. A variety of alternative methods exist to measure airspeed.

This note presents a brief review of some of the low airspeed measuring devices available for aircraft and, more
particularly, helicopters. This note also introduces a novel technique that was studied within Air Vehicles
Division as a concept demonstrator.

Page classification:

UNCLASSIFIED

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