Characterization of the HIAPER Aircraft in Progressive Science (CHAPS)
1. Background
NSF and NCAR established the HIAPER Progressive Science program to provide
NCAR and the scientific community with the opportunity to fully familiarize themselves
with the capabilities of HIAPER and to perform initial basic research with the aircraft.
Specifically, the following objectives were established:
1. ATD staff will be provided with a significant time period to become familiar with
the operation and performance of the aircraft. This includes the operation of the
various infrastructure components (e.g. data acquisition system, data display and
access software, state parameter sensors, SATCOM, etc.) on the aircraft. In
addition, performance of the aircraft in a variety of conditions is planned (e.g.
performance of flight management system in different weather conditions, cockpit
weather radar performance, response of aircraft to high altitude turbulence, etc.).
2. NCAR staff will be able to continue the development of fundamental
instrumentation for the facility during this period, including issues of payload
certification.
3. The capabilities of the aircraft will be showcased to the scientific community by
performing flight profiles to the maximum certified ceiling of 51,000 feet and
fully demonstrating the capability of the data acquisition, display, and transfer
capabilities of HIAPER.
4. Members of the Scientific Community will be provided with the opportunity to
perform initial, well-focused scientific missions with the aircraft.
For the last item, requests for participation in the progressive science program were
solicited through letters of intent, where information about the likely scope of possible
projects was discussed between the community and ATD/RAF. Letters of intent are now
being followed by formal requests for flight hour and other support items through the
normal OFAP/FAC request process. In this proposal we describe items 1 through 3
above, so that OFAP and the FAC can assess the requests for progressive science support
in the context of the other progressive science items planned by ATD. In particular,
some of the ATD flight testing will require dedicated flight time and other parts can be
done simultaneously with other progressive science missions. In most cases, we
anticipate research flights with portions of the flight being used for ATD tests and other
portions being used to support the group of progressive science missions that are now
being requested through the solicitation process.
2. Familiarization of ATD staff with HIAPER
Some crew familiarization flights are planned prior to the progressive science period.
These flights will involve checking out the flight crew on the aircraft systems on the
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aircraft, but will provide RAF flight crews little to no experience in the types of weather
conditions where HIAPER will conduct weather research. RAF estimates that
approximately 14 hours of dedicated flight will be required to familiarize the flight crew
with the performance of the aircraft in various weather conditions, especially high
altitude convective clouds with turbulence. During this period the flight crews will
penetrate a series of increasingly strong convective turbulence, while familiarizing
themselves with the operation of the cockpit weather radar, Flight Management weather
information systems, and aircraft performance items (e.g. airspeed margin above the stall
barrier system). These “first encounters” with weather conditions are not suitable for
research, but once the flight crews have had experience with the aircraft in suitable
weather, the capabilities of the aircraft in sampling weather can be demonstrated to
onboard scientists. Other familiarization activities can be combined with Progressive
Science research flights.
3. Dropsonde Testing
The GPS dropsonde system being developed for HIAPER is expected to be
similar to the unit currently being used on the NOAA G-IV aircraft. This system is not
part of the initial complement of instrumentation being delivered before the Progressive
Science period. As a high priority supplemental development task, the RAF expects to be
ready to install and test this system on the aircraft during the Progressive Science
interval. To do so, it will be necessary to take the aircraft out of flight status for 5 to 7
days to install the delivery chute, receiver antenna, receiver and associated cabling.
Space allocations for one standard rack and an operator seat will need to be included in
the Progressive Science floor plan. Functional testing of the release mechanism can be
incorporated into almost any of the high altitude Progressive Science flight objectives,
with only about an additional 4 hours of flight time anticipated to be dedicated to
dropsonde testing. Up to four drops will be made with full signal tracking to impact on
the surface to ensure proper performance of the receiver antenna. At least one drop will
be made from 50,000 ft MSL to confirm the functionality of the release mechanism at the
maximum cabin pressure differential.
4. State Parameter Calibration & Characterization:
Virtually all of the standard meteorological measurements taken from aircraft
platforms require some form of correction related to airspeed and attitude. These
corrections are platform specific and are empirically derived through extensive flight
testing with comparisons against some form of reference standard. Data should be
collected over the full range of the aircraft’s speed and altitude capabilities to fully
characterize the targeted measurements. An initial allocation of 20 test flight hours will
be flown prior to the Progressive Science period for the basic shakedown of the new
platform. This will enable the RAF to begin this “Calibration & Characterization”
process to the point where initial calibration coefficients can be determined, but not
optimized. There are two options that should be used to improve the calibration process:
flying a trailing cone on HIAPER and aircraft-to-aircraft inter-comparisons with an
existing well characterized aircraft.
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The trailing cone is the standard static pressure reference used by the aviation
industry to establish the platform specific correction factors for both static and dynamic
pressure measurements. These source variables are then used, along with total
temperature, to calculate the aircraft’s true airspeed – which is essential for calculating
the 3-dimensional wind fields from the gust probe system. Equipping the G-V with a
trailing cone is not simple (approximately $100,000 for a one-time installation through a
contractor), but essential to completely characterize the performance of HIAPER.
Calibration flights would include at least five level speed runs conducted at multiple
altitudes spanning the normal operating range of the aircraft (200 ft to 50,000 ft AGL). A
ferry to a coastal region would be required to conduct the low altitude legs. These tests
should require about two weeks during the progressive science period and approximately
12 flight hours, half of which might be combined with other progressive science
missions.
The standard meteorological systems on the NCAR C-130 have been well
characterized over its 12 years of conducting airborne research. This includes two recent
sets of calibration flights with a trailing cone system. We propose to fly two
HIAPER/C130 inter-comparison missions during the Progressive Science interval.
Wingtip-to-wingtip inter-comparison flights will provide very useful information on the
full set of sensors on both platforms. Calibration flights would include three level runs
conducted at various air speeds and at multiple altitudes spanning a range safe for
formation flying (1000ft to 25,000ft AGL). A ferry to a coastal region would be
required to conduct the low altitude legs. Over water legs would also provide an
opportunity to crosscheck the static pressure correction against the radar altimeter
mounted on the C-130. This mission would be combined with the trailing cone mission.
10 hours of C130 flight time are requested to complete these tests in addition to the time
requested for HIAPER.
5.
Air Sample Inlets and Exhaust Systems – Performance Testing
HIAPER will eventually carry a wide assortment of gas and aerosol sampling
instruments. The overall quality of gas and particle measurements will depend
significantly on the performance of air sampling systems (inlets and piping). Ultimately,
there will be a variety of inlets for the aircraft, but for the Progressive Science missions,
only inlets for basic trace gas and particle measurements (CN and particles size through
differential mobility) will be provided by the RAF. These inlets include one forwardfacing diffuser inlet and one aft-facing inlet. These inlets will also support many of the
STANDARD and ROUTINE category measurements, as defined by the “HIAPER
Instrumentation Priorities” report.
We propose to test the performance of these basic inlets at all locations that are
available and suitable for air sampling and approved for use on the G-V. Inlets will be
installed and connected to gas and particle instruments rack-mounted inside the aircraft
cabin and the instruments will be connected to the exhaust system. Tests will include
monitoring the air flow rate, relative humidity, temperature, inlet pressure, and exhaust
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pressure over the full range of altitudes, airspeeds, and attitudes that are expected during
research flights. The community exhaust system will be tested over a range of total
instrument flow rates to assure that appropriate pressure drops and flow rates are
maintained at potential user ports. The measurements will be compared with inlet flow
modeling results for selected cases.
A range of flight maneuvers will be utilized to evaluate inlet performance,
including side-slip, pitch oscillation (porpoising), steep bank rolls, speed runs, plus rapid
climb and descent rates. We expect that about 8 hours of flight testing is required, but at
least half of these could be obtained during ferry flights associated with other progressive
science missions.
6. Performance of RAF Aerosol & Trace Gas Instruments
We propose to conduct basic tests of performance for aerosol and trace gas
instruments that are likely to be flown on the G-V as part of the standard or routine
instrument package. Because RAF currently has a number of these instruments available
now for the C130, we propose to use existing instruments, including CO, CO2, CN, and
the Radial Differential Mobility Analyzer (RDMA). A number of performance issues
will be examined. For example, sample pressures less than ~250 mb can lead to
decreased sensitivity of CN (threshold particle size is larger), high voltage breakdown
can occur in the RDMA chamber, and significant changes in the particle size range of the
RDMA are anticipated. The CO and CO2 instrumentation utilize pressure control and
will have been lab tested to verify performance over the full HIAPER performance
envelope; however, vertical profiles are necessary to confirm proper pressure control at
all aircraft altitudes and attitudes. Some tests of pipe joint integrity (leaks) will be also
done in flight. Flight maneuvers can change the pressure at inlets and outlets and can
affect flow rates through instruments. These tests will be done concurrently with the
other missions described here and the other progressive science mission.
7. Cabin Leak Testing and Boundary Layer Characterization
We propose to sequentially sample pitot pressures and air through a rake system
supporting several pressure ports and inlets, extending through varying depths of the
boundary layer, in order to quantify the outward extent of boundary layer air at each
potential inlet mounting location. This approach allows the velocity profile to be
continuously monitored while trace gas measurements are made from each of the inlet
locations at varying depths of the boundary layer. The rake device will be designed and
built by ATD and interfaced to the HIAPER data system for recording pitot pressures.
The cabin exhaust port is located forward of all inlet apertures on the HIAPER
aircraft. Flow modeling results calculated for a range of aircraft angles of attack were
used to select locations for aperture placement to avoid areas likely to intercept this cabin
exhaust. However, it is important to verify through testing that cabin air is not
contaminating the free stream air sampled through the inlets. In addition, fuselage leak
rates, though small, are of concern as sources of possible contamination for inlets on
HIAPER.
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Pulsed carbon dioxide will be released within the cabin at a controlled and
precisely monitored flow rate that is high enough to spike cabin air significantly above
ambient background, while the in situ CO2 analyzer continuously samples the ‘free air
stream’ from a particular inlet mounting location. The rake sampling method suggested
above will also be used as inlet to the gas sensor quantifying the extent of cabin air
contamination, acquiring a sample from each port during a set of controlled flight
conditions. With a cabin volume of 47.25 m3, and a typical cabin air exchange rate of 3
minutes, the dilution of pure CO2 at a flow rate of 1-2 L/min will provide a cabin CO2
equilibrium concentration of approximately 438 – 502 ppmV exiting the aircraft from the
cabin exhaust port. Given the G-V cruise air speed and relatively small separation
distance between this port and aperture locations, this test scheme should yield detectably
elevated CO2 levels if boundary layer air is entrained into the inlet to a significant extent.
We feel that the cabin leak testing can be done with approximately 10 hours of flight.
About half of these might be obtained during ferry flights associated with other
progressive science missions.
8. Summary
Given the above flight hour estimates, about 33 dedicated flight hours are needed
for the RAF testing during the progressive science period and about 15 hours might be
shared (e.g. during ferry flights) with other progressive science missions.
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