Technology Notebook ContentsEffective Date: 15 June 98
In-Ground Effect Calibration
In-ground effect calibrations are performed using a ground based tracker as a reference. A runway survey may also be necessary. Tracker calibrations, before and after flight, indicate the difference between surveyed targets and laser elevations to be in the range of hundreths to thousandths of a foot. Therefore, laser elevation, which provides tapeline height, is considered to have negligible error. Calibration data is on file for future reference as required.
DATA ANALYSIS
The in-ground effect position error is presented in the form:
DVpc versus Vic
and
Vc versus Vic
Where:
Vic = Instrument corrected indicated airspeed
Vc = Calibrated airspeed
DVpc = Vc - Vic = Airspeed position error correction
Required data parameters were:
Space Positioning Data
HTBR = Tapeline height of airspeed system static pressure transducer above a zero reference at brake release (ft)
HTn = Tapeline height of airspeed system static pressure transducer above a zero reference at test point (n), (ft)
Ground Weather Station Data
TGR = Temperature at ground weather station (oC) converted to (oK)
HPGR = Pressure altitude at ground weather station
Airspeed System Data
PSIBR = Indicated static pressure corrected for instrument error at brake release (PSF)
PSin = Indicated static pressure corrected for instrument error at test point (n), (PSF)
PTn = Indicated total pressure corrected for instrument error at test point (n), (PSF)
Data processing then involved the following steps:
HPIBR = [1-(PSIBR/2116.216)1/5.2561]/(6.87525 X 10-6) = Pressure altitude at brake release (ft)
DHTn = HTn - HTBR = Tapeline altitude increment from brake release to testpoint (n), (ft)
TASTD = 288.16(1.-6.87535*10-6 * HPGR) (OK)
TA = TGR (OK)
DHPn = DHTn * TASTD/TA = Pressure altitude increment between brake release and test point (n), (ft)
HPIBn = HPIBR + DHPn = Pressure altitude at test point (n), (ft)
Pan = 2116.216(1-6.87535*10-6 * HPIn)5.2561 = Ambient p ressure at test point (n), (PSF)
DPSn = Pan - PSin = Static pressure error at test point (n), (PSF)
qin = PTn - PSin (PSF)
qcn = qin - DPSn (PSF)
Vin = 661.48 5[((qin/2116.216) +1)1/3.5 -1] (kts)
Vcn = 661.48 5[((qcn/2116.216)+1)1/3.5 -1)] (kts)
DVpc = Vcn - Vin = Airspeed position error at test point (n), (ft)
Result Presentations
Takeoff Configurations
No military specifications exist for in-ground effect calibrations. However, Federal Aviation Regulation 25.1323 requires a position error calibration from .8 of the minimum value of V1 to the maximum value of V2.
The in-ground effect calibration for takeoff accelerations with flaps up and takeoff flaps are sketched in Figures 1 through 4. Pilot's and copilot's system calibrations are presented as DVpc versus Vic data and Vc versus Vic curves.
Landing Configurations
No military specifications or Federal Aviation Regulations exist for elevations. However, in-ground effect curves for landing are incorporated in flight manual charts.
The in-ground effect calibration for landing decelerations with flaps up, takeoff, and landing flaps are sketched in Figures 5 through 8. Pilot's and copilot's system calibrations are presented as DVpc versus Vic data and Vc versus Vic curves. These data and curves for landing deceleration are applicable with engines set without reverse thrust.
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