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Studies of HIAPER's Flow Boundary Layer in Progressive Science

As part of the Progressive Science project, a pressure rake was fabricated and used to measure the profile of dynamic pressure within 30 cm of the aircraft skin at several aperture pad locations. There are six aperture pads on the NSF/NCAR GV (HIAPER), where air sample inlets and other equipment can be mounted. The pressure rake measurements provide estimates of air speed at the location of air sample inlets. This information can be used to adjust suction rates to optimize air sampling. The data also provides an estimate of thickness of the flow boundary layer (BL).  Inlets must sample air outside the BL in order to avoid contamination or interactions with the skin of the airplane. This page describes the hardware and shows some examples of data. Analysis is continuing and results will be posted when available.
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summary of flights two aperture pads on belly

 six aperture pads on top of fuselage

Other air flow measurements at the nose/radome location can be compared with pressure rake data.
static pressure

 radome gust system

 fuselage pitot


Pressure Rake

The rake has twelve small pitot tubes along its strut and a large pitot-static tube at the end. All pitot tubes share the same reference pressure, i.e., the total pressure of the large pitot-static tube. The large pitot-static tube has static pressure ports on its side. The twelve pitot tubes connect by small tubing to a NetScanner (Pressure Systems, Inc.) that is located at the foot of the rake, inside the aircraft. The NetScanner measures the twelve differential pressures at 50 Hz, and sends data to the aircraft data system.  Dynamic and static pressures of the large pitot-static tube are measured with other sensors and recorded separately.
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  1. larger_image,
  2. annotated,
  3. zoom-out


DYNAMIC PRESSURE PROFILES - speed runs

Air velocity can be calculated from the dynamic pressure, V ~ sqrt(dp). Details are in RAF Technical Bulletin 23. During Progressive Science, speed run maneuvers were conducted to characterize the response of aircraft attitude (angle of attack), the effects on pressure and temperature measurements, and the adjustsments of the flow boundary layer. During these tests, aircraft speeds covered the normal operating range and through the span of G5 altitudes. This figure shows vertical profiles of dynamic pressures in a speed run test at 30,000 ft during RF07.

 

The boundary layer depth can be defined as the distance from the skin where the speed reaches 99% of the local asymptotic velocity, or 98% of the asymptotic dynamic pressure. This is not necessarily the freestream value, further from the aircraft. In this figure, the BL depth was ~4.0" for both the fastest and slowest speeds. Note on this semi-log plot that the profile of dynamic pressures is approximately a straight line within the BL.

Also shown is the centerline of the HIMIL (HIAPER Modular Inlet). At 7.50", it was well beyond the top of the BL at the forward-most belly location (250-R).



DYNAMIC PRESSURE PROFILES  at different aperture pad locations

The pressure rake was moved between flights. This figure shows pressure rake measurements at different locations during level flight. Note that the BL depth is greater at locations further from nose, as expected. At the furthest aft position (498-R), the HIMIL centerline lies within the BL.



Dynamic pressures QCR (radome) and QCF (fuselage) were also measured. At forward locations (250 belly and 255 top), QCR and QCF are larger than the distal rake pressure. However, when  the rake was mounted further aft (498 top), QCR and QCF are smaller.

BL DEPTH - changes during maneuvers

Pitch maneuvers and side-slip maneuvers were done on several flights. The following figures show data from a ten-minute interval during RF07, when the rake was mounted on the belly at 250-R. Rake pressures follow those at the nose radome sensor (QCR). Small scale variability is evident at all points within the boundary layer (p00 to p05) but not outside it. Side-slip causes large variations in rake pressures.
Next figure zooms in on one-minute of the pitch maneuver. It shows that in the BL, there is slight asymmetry in the rake pressure traces. This suggests that the BL profile during pitch UP is different from pitch DOWN. The times of maximum and minimum pitch are marked by vertical lines for reference in the next plot.
 
Profiles of rake pressure were plotted during the greatest pitch excursions -4° and +7°. They suggest that the BL was slightly thicker during pitch up.

BL DEPTH - measured vs GAC study

Gulfstream Aerospace Corporation (GAC) estimated BL thickness from flow modeling studies of the GV and found that the thickness generally followed the 1:100 rule (thickness ~1% of distance from the nose) for distances as far as ~gvfs 800. Thicker BL excursions occurred where other structures joined the fuselage, for example near the wing root.

Pressure rake measurements during Progressive Scienceindicated that the BL is thicker than predicted in the GAC simulations. At gvfs 498, BL depth was about 9.0" whereas GAC flow modeling predicts ~5". This discrepancy may arise from the non-perfect surface of the NSF/NCAR GV (due to antennas and other hardware) or because the simulations and measurements are at different speeds and angles of attack. The following plot shows the rake measurements of BL depth at four locations on the GV fuselage, along with the centerline heights of standard (short) and tall HIMILs.


BL Profiles - modeling

Flow modeling studies at RAF can be compared with pressure rake measurements.

TURBULENCE

Future analysis can examine high frequency signals (50 Hz) in the pressure rake and other data.