b. Supplemental Information
(1) Non-reported corrections: Some corrections, such as those for frequency response of various system components, do not have to be reported if they are normal characteristics of the equipment. These corrections should be minor if the equipment meets the requirements of the regulation. Possible errors due to equipment characteristics will usually be small.
c. Procedure
(1) Data Reporting: All of the data recorded during the test program are to be reported. Any problems with the measurement system and any adjustments that were required in addition to those specified in the regulations are to be reported.
(2) Reporting Potential Errors: In most cases, potential errors due to characteristics of the measurement system should not be significant, but an assessment of these possible errors is required.
395. Section G36.109(b)
Measured and corrected sound pressure levels obtained with equipment conforming to the specifications in section G36.105 of this appendix must be reported.
a. Explanation
This section identifies the noise level reporting requirements for Appendix G noise certification testing.
b. Supplemental Information
(1) None.
396. Section G36.109(c) & (d)
(c) The type of equipment used for measurement and analysis of all acoustical, airplane performance, and meteorological data must be reported.
(d) The following atmospheric data, measured immediately before, after, or during each test at the observation points prescribed in section G36.101 of this appendix must be reported:
(1) Ambient temperature and relative humidity.
(2) Maximum and average wind speeds and directions for each run.
a. Explanation
This section presents the Appendix G atmospheric reporting requirements.
a. Supplemental Information
(1) Regulatory Interpretation: The regulation identifies “each test” and “each run.” For clarification, this regulation refers to each test series (test) and each test overflight (run). The (meteorological) measurements should be made at the time of each test run, since each noise measurement will be corrected by use of the meteorological data.
(2) Wind Measurement: The requirements of section G36.101(b)(4) (paragraph 378, above) set the limits on testing, based on a 30-second average wind not to exceed 10 knots, with a 5-knot crosswind limitation.
There are no additional limitations based on the surface wind. It is sufficient to report the 30-second average wind and crosswind that are within the regulatory limitations for each test overflight measurement.
397. Section G36.109(e)
Comments on local topography, ground cover, and events that might interfere with sound recordings must be reported.
a. Explanation
Sufficient information regarding the local topography and test site characteristics should be recorded and reported to show that the requirements of section G36.101(a) have been satisfied.
b. Procedures
(1) Sufficient information should be provided to show that the requirements of section G36.101(a) have been met.
398. Section G36.109(f)
The aircraft position relative to the takeoff reference flight path must be determined by an approved method independent of normal flight instrumentation, such as radar tracking, theodolite triangulation, or photographic scaling techniques.
a. Explanation
Recorded noise level data adjustments include those based on the height of the airplane above the microphone station. The acceptability of each data run is also based on the lateral angular offset at the microphone station. An independent approved method is to be utilized to determine the airplane position relative to the approved flight track and height over the microphone.
b. Procedure
(1) Airplane Position: The height and the angular offset from the vertical at the microphone position are to be determined by an approved method, evaluated to be within the test limitations, and reported.
399. Section G36.109(g)
The following airplane information must be reported:
(1) Type, model, and serial numbers (if any) of airplanes, engines, and propellers;
(2) Any modifications or nonstandard equipment likely to affect the noise characteristics of the airplane;
(3) Maximum certificated takeoff weight;
(4) For each test flight, airspeed and ambient temperature at the flyover altitude over the measuring site determined by properly calibrated instruments;
(5) For each test flight, engine performance parameters, such as manifold pressure or power, propeller speed (rpm) and other relevant parameters. Each parameter must be determined by properly calibrated instruments. For instance, propeller RPM must be validated by an independent device accurate to within –1 percent, when the airplane is equipped with a mechanical tachometer.
(6) Airspeed, position, and performance data necessary to make the corrections required in section G36.201 of this appendix must be recorded by an approved method when the airplane is directly over the measuring site.
a. Explanation
These airplane parameters must be determined and reported to determine the engine and propeller test conditions that are used for adjusting the noise level data to reference conditions.
b. Supplemental Information
(1) Required Measurements: These parameters are required in order to provide the information used in the adjustment process. Mechanical tachometers are subject to potential indicating errors as a result of the cable drive system. Separate validation of the in-flight reading is therefore required if a mechanical tachometer is used. All equipment utilized to determine the required parameters is to be calibrated, and the calibrations are to be applied before being reported to the FAA in the test report and before being used to make reference airplane corrections. The temperature at the airplane height is required for tip Mach number correction.
400. Section G36.111 Flight procedures
(a) The noise measurement point is on the extended centerline of the runway at a distance of 8200-ft (2500 m) from the start of takeoff roll. The aircraft must pass over the measurement point within –10 degrees from the vertical and within 20% of the reference altitude. The flight test program shall be initiated at the maximum approved takeoff weight and the weight shall be adjusted back to this maximum weight after each hour of flight time. Each flight test must be conducted at the speed for the best rate of climb (Vy) –5 knots (–9 km/hour) indicated airspeed. All test, measurement, and data correction procedures must be approved by the FAA.
a. Explanation
This regulation identifies the overhead flight position (height and lateral position) of the airplane during noise certification testing. The FAA-approved airplane weight and airspeed are controlled by this regulation.
b. Supplemental Information
(1) Airplane Position: The regulation requires determination of the noise level at a single location, specified relative to the start of take-off roll. Limits on the permissible deviation from the reference flight path are specified for the flight tests. These limits are based on the ability to obtain consistent, representative results, without placing excessive restrictions on the flight test. The initial take-off weight is required to be equal to the maximum approved take-off weight, and after an hour of flight time, the weight is to be increased back to this weight to account for fuel burn. This procedure ensures that the flight parameters, primarily angle of attack, do not vary significantly from the reference. The airplane position is to be FAA approved for each test overflight.
(2) Best Climb Speed: The choice of Vy for the climb speed is representative of actual airport operations. These speeds, and the resulting rate of climb, are usually available in the documentation (Pilot's Operating Handbook and/or Approved Flight Manual). The tolerance of ±5 knots from this speed allows for normal variations due to pilot technique. Variations in excess of this would be an indication of turbulence and/or anomalous winds, conditions that are unsuitable for testing.
(3) Lateral Deviation: The approved deviation tolerance of 10° from the vertical can result in a reduction in measured noise level of approximately 0.1 dB, relative to the noise level for flight directly over the microphone. There are no specified corrections for this. Deviations from the reference height can occur for many reasons, including wind, airplane weight, test site elevation, and temperatures, as well as the procedure used to calculate the reference height (in section G36.111(b)). The regulation permits test heights to be within ±20 percent of the reference and requires an adjustment to account for the difference.
c. Procedures
(1) Normal Take-off: Unless an equivalent procedure is used, the airplane takes off in a normal manner and is then set up in a climb. The noise level is measured at a point 8,200 feet (2,500 m) from the start of take-off roll.
(2) Equivalent Procedure: In practice, most applicants elect to use an equivalent procedure referred to as an intercept technique. In this technique, the airplane remains in flight and intercepts as closely as possible the reference climb path. The intercept height and location may be varied so that the correct height ±20 percent is achieved at the microphone. This procedure is acceptable because the noise level is measured only when it is at a maximum, and any maneuvering before and after that time does not affect the result, as long as the operational parameters are stabilized.
401. Section G36.111(b)
The takeoff reference flight path must be calculated for the following atmospheric conditions:
(1) Sea level atmospheric pressure of 1013.25 mb (013.25 hPa);
(2) Ambient air temperature of 59 deg. F (15 deg.C);
(3) Relative humidity of 70 percent; and
(4) Zero wind.
a. Explanation
This regulatory section identifies the reference atmospheric conditions for noise certification testing.
b. Procedures
(1) Calculation of the take-off reference flight path is a simplified representation of actual take-off and climb procedures. For the purpose of this part of the regulation, it is assumed that the atmosphere is homogeneous throughout the take-off and climb, with no change of air pressure, temperature, or relative humidity, and no wind. The selected conditions correspond to sea level and standard atmosphere, with the exception of the relative humidity. The combination of 59°F and a relative humidity of 70 percent results in low absorption of sound as it propagates from the airplane to the ground, representing a worst-case scenario for the noise certification evaluation.
(2) A worked example of reference flyover height and reference conditions for source noise adjustments is provided in Appendix 5 of the Technical Manual (TM).
402. Section G36.111(c)
The takeoff reference flight path must be calculated assuming the following two segments:
a. Explanation
For analysis purposes, the flight path is separated into two segments, namely take-off roll and climb segments, as discussed in paragraphs 403 and 404, below.
The term “segment” is typically used in performance testing of airplanes and should not be confused with noise certification flight path segments. ICAO Annex 16 Chapter 10 uses the term “phases” to avoid confusion.
b. Supplemental Information
(1) Available Information: For the purpose of noise certification, the assumed take-off flight path is considerably less complex than an actual take-off, which consists of more than two segments. The assumed take-off flight paths are an artificial situation used only for noise certification purposes. Calculation of the reference flight path requires only information that is readily available to the applicant and published in the Pilot's Operating Handbook and/or Approved Flight Manual.
(2) Approval Requirements: Any information used in these data corrections or analysis calculations are to be FAA approved.
BRAKE
RELEASE
FIRST SEGMENT SECOND SEGMENT
REFERENCE HEIGHT 8200 FT 50 FT
REFERENCE FLIGHT PATH
TEST FLIGHT PATH FROM
DISTANCE BRAKE
RELEASE
HEIGHT (FT)
Figure 15: Typical Test and Reference Flight Paths
Figure 15, shown above, illustrates the difference between the test and the reference flight paths.
Calculations may be used to determine corrections to the measured noise levels to account for the difference between the flight paths. For a sample calculation of reference and test conditions, see Appendix 5 of the Technical Manual (TM).
c. Procedure
(1) Any information used in these calculations is to be FAA approved.
403. Section G36.111(c)(1)
First segment.
(i) Takeoff power must be used from the brake release point to the point at which the height of 50-ft (15m) above the runway is reached.
(ii) A constant takeoff configuration selected by the applicant must be maintained through this segment.
(iii) The maximum weight of the airplane at brake-release must be the maximum for which noise certification is requested.
(iv) The length of this first segment must correspond to the airworthiness-approved value for a takeoff on a level paved runway (or the corresponding value for seaplanes).
a. Explanation
The airplane weight used in the calculation is the maximum take-off weight. The first segment starts at the point of brake release and ends at the point where the airplane reaches a height of 50 feet above the runway.
b. Supplemental Information
(1) Airplane Configuration: Usually, the applicant will select a flap setting that minimizes the takeoff distance to 50 feet. In some cases, there may be an advantage in selecting the position of the center of gravity to further reduce the take-off roll. There is no requirement to use the selected center of gravity position during the actual testing, nor does it become an operating limitation. The take-off configuration may be any configuration that is approved from an airworthiness point of view, and the distance required to reach a height of 50 feet is to be that listed in approved documentation.
c. Procedure
(1) Airplane Performance: The required reference airplane performance data are to be determined from approved available information (e.g., the Airplane Flight Manual).
404. Section G36.111(c)(2)
Second segment.
(i) The beginning of the second segment corresponds to the end of the first segment.
(ii) The airplane must be in the climb configuration with landing gear up, if retractable, and flap setting corresponding to normal climb position throughout this second segment.
(iii) The airplane speed must be the speed for the best rates of climb (Vy).
(iv) Maximum continuous installed power and rpm for variable pitch propeller(s) shall be used. For fixed pitch propeller(s), the maximum power and rpm that can be delivered by the engine(s) must be maintained throughout the second segment.
a. Explanation
This section presents the regulatory requirements for the climb segment, which extends from the point where the airplane reaches a height of 50 feet (end of the take-off segment) to a point over the microphone at a distance of 8,200 feet from brake release. It is assumed that this segment is at a constant climb gradient, constant configuration, and constant airspeed.
The engine power requirement for the second segment is an unharmonized item between Part 36 and Annex 16. Part 36 Appendix G allows the use of maximum continuous installed power for variable-pitch propellers during the second segment of the flight path, where as Annex 16 Chapter 10 requires maximum power if the engine is not time limited.
b. Supplemental Information
(1) Normal Take-off and Climb: In an actual take-off and climb situation, as the airplane climbs, the climb gradient, configuration, and airspeed may continuously change. In practice, the airspeed, engine power, landing gear position, and aerodynamic configuration (flap setting) at the end of the first segment cannot instantaneously change to those used in the reference calculation of the second segment.
(2) Reference Performance: For regulatory purposes, it is assumed that the airplane configuration, airspeed, and climb gradient instantaneously change following the first segment and remain constant throughout the second segment.
(3) Airspeed: The airspeed for the best rate of climb (Vy) is typical of that used in actual airport operations and that which is determined during the airworthiness certification for the airplane.
(4) Flap Settings: The airplane must be flown in constant configuration during the second segment.
(5) Configuration: There are different requirements for fixed, and variable-pitch propellers because of the different operational characteristics. A discussion of the issues that are relevant to noise certification is provided in paragraph 339, below. In an actual take-off situation, as the airplane climbs, the climb gradient changes.
c. Procedure
(1) Applicant’s Responsibility: The applicant is to determine the appropriate climb flap setting and airspeed from the airplane certification documentation (e.g., Airplane Flight Manual).
405. PART C - DATA CORRECTION
406. [RESERVED]
407. Section G36.201 Correction of Data
(a) These corrections account for the effects of:
(1) Differences in atmospheric absorption of sound between meteorological test conditions and reference conditions.
(2) Differences in the noise path length between the actual airplane flight path and the reference flight path.
(3) The change in the helical tip Mach number between test and reference conditions.
(4) The change in the engine power between test and reference conditions.
a. Explanation
Corrections are required to account for differences between the noise certification test conditions and the reference conditions.
The required corrections falls into two groups present below:
Corrections to the noise levels for atmospheric absorption of sound to adjust them to acoustical reference temperature (59°F) and relative humidity (70 percent).
Corrections to the noise levels to account for differences in propeller tip helical Mach number and engine power (of the actual flights) relative to those intended under reference conditions. The term “reference conditions” is used repeatedly in 14 CFR Part 36 Appendix G and means conditions that would occur if the airplane were flown “textbook fashion” for a Vy take-off under standard atmospheric conditions (i.e., sea level, 59°F).
Most of these corrections require some calculations before the test program starts (i.e., to determine the reference conditions) and immediately after each test flight or series of test flights.
b. Supplemental Information
(1) Atmospheric Absorption: The temperature and relative humidity of the air affect the sound propagation. This correction accounts for the difference in atmospheric absorption along the sound propagation path that occurs between temperature and relative humidity under noise certification test conditions and temperature and relative humidity under reference conditions (59°F and 70 percent Relative Humidity). Refer to paragraphs 408 and 411, below, for additional atmospheric absorption correction information.
(2) Noise Path Length: As stated in paragraph 400, above, the airplane test limitations are as follows: (a) height over the microphone within ± 20 percent of the reference height; and (b) lateral position within ± 10° of the vertical. The noise path length correction adjusts the measured noise levels for the difference in noise path length between actual noise test conditions and reference conditions. Refer to paragraph 411, below, for additional path length correction information.
(3) Helical Tip Mach Number: The noise generated by a propeller-driven airplane depends on the rotational speed of the tip of the propeller, more specifically the helical tip Mach number. Data corrections are based on the relationship between the helical tip Mach numbers determined for test and reference conditions.
Refer to paragraphs 409 and 413, below, for additional helical tip Mach number correction information.
(4) Engine Power: Corrections are required to account for non-reference engine power settings that are used during noise certification tests. The procedures for determining of the engine power to be used in the calculations depend on the design characteristics of the engine-propeller combination. In most cases, this
power is not published, and does not have to be determined for airworthiness purposes. It is therefore necessary to determine the power for noise certification purposes. Refer to paragraph 414, below, for further engine power adjustment discussion.
c. Procedures
(1) Test Conditions: The noise certification test conditions of ambient temperature, relative humidity, propeller rotational speed, airplane height and engine power are to be measured or determined by an approved method during each test overflight.
(2) Reference Conditions: The certification reference conditions of propeller rotational speed, airplane height, and engine power are to be determined at reference conditions corresponding to sea level, 59°F and 70 percent relative humidity by an approved method.
408. Section G36.201(b)
Atmospheric absorption correction is required for noise data obtained when the test conditions are outside those specified in Figure G1. Noise data outside the applicable range must be corrected to 59 F and 70 percent relative humidity by a FAA approved method.
0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Temperature, F RELATIVE HUMIDITY, % (95%/35.6F) (80%/35.6F) (40%/59F) (40%/80.6F) (95%/80.6F)
Figure 16: MEASUREMENT WINDOW FOR NO ABSORPTION CORRECTION (Figure G1 of regulation)
a. Explanation
Corrections are required to account for situations where the test atmospheric conditions are not the same as the reference conditions. There are two methods of making adjustments for absorption of sound in air,
depending on the conditions. If the temperature and relative humidity are within the "no-correction" window shown in Figure G1 of the regulation, the adjustment is incorporated with the distance adjustment described in section G36.201(d)(2), below. If the temperature and relative humidity are outside the specified no-correction window, then a separate adjustment is required for atmospheric absorption.
b. Supplemental Information
(1) Regulatory Inconsistencies: The information included in the regulation has an inconsistency.
Figure 16 shows that the low temperature limit of the no-correction window is at 35.6°F. The low temperature limit for testing is 36°F, only 0.4° higher.
(2) A second inconsistency was removed during harmonization. Before harmonization, (Section G36.201(b)) specified that the noise data were to be corrected to an atmospheric condition of 77°F, 25°C.
Reference airplane performance is obtained for an air temperature of 59°F. This inconsistency between the acoustic and airplane performance reference conditions was corrected by adapting 59°F for both reference conditions.
409. Section G36.201(c)
Helical tip Mach number and power corrections must be made as follows:
(1) Helical tip Mach number and power corrections must be made if --
(i) The propeller is a variable pitch type; or
(ii) The propeller is a fixed pitch type and the test power is not within 5 percent of the reference power.
(2) No corrections for helical tip Mach number variation need to be made if the propeller helical tip Mach number is:
(i) At or below 0.70 and the test helical tip Mach number is within 0.014 of the reference helical tip Mach number.
(ii) Above 0.70 and at or below 0.80 and the test helical tip Mach number is within 0.007 of the reference helical tip Mach number.
(iii) Above 0.80 and the test helical tip Mach number is within 0.005 of the reference helical tip Mach number. For mechanical tachometers, if the helical tip Mach number is above 0.8 and the test helical tip Mach number is within 0.008 of the reference helical tip Mach number.
a. Explanation
For a variable-pitch propeller, adjustments are required for both helical tip Mach number and engine power. For a fixed-pitch propeller, helical tip Mach number and engine power adjustments are not required if the test power is within 5 percent of the reference.
ICAO Annex 16 Chapter 10 requires that the tip Mach number and power adjustments for both variable-pitch and fixed-pitch propellers be applied to the measured data regardless of engine power. This is an unharmonized item.
b. Supplemental Information
(1) Engine Power: Engine power depends on the atmospheric conditions, specifically, pressure altitude and air temperature. The test day conditions are likely to be different from the reference conditions, so the test engine power may not be the same as the reference engine power. Manufacturer’s approved airworthiness information can be used to determine engine power for test conditions and reference conditions.
410. Section G36.201(d)
When the test conditions are outside those specified, corrections must be applied by an approved procedure or by the following simplified procedure:
a. Explanation
This adjustment is required when the ambient temperature and relative humidity combination is outside the no-correction window. It accounts for non-reference atmospheric absorption of sound when tests are conducted at conditions other than those within the allowed window.
411. Section G36.201(d)(1)
Measured sound levels must be corrected from test day meteorological conditions to reference conditions by adding an increment equal to
Delta (M) = (HT a - 0.7 HR) /1000
where HT is the height in feet under test conditions, HR is the height in feet under reference conditions when the aircraft is directly over the noise measurement point and a is the rate of absorption for the test day conditions at 500 Hz as specified in SAE ARP 866A, entitled “Standard Values of Atmospheric Absorption as a function of Temperature and Humidity for use in Evaluating Aircraft Flyover Noise” as incorporated by reference under § 36.6.
a. Explanation
Delta (M), in decibels, is a correction for the difference in atmospheric absorption of sound between the test meteorological conditions and the acoustical reference conditions of 59°F and 70 percent Relative Humidity (RH). The applicant may use the simplified procedure provided in the regulation or may propose another procedure, which will require FAA approval.
b. Procedure
(1) Atmospheric correction of the measured sound level data is necessary only if the meteorological conditions of temperature and relative humidity are outside specified in Figure G1 of 14 CFR Part 36, reproduced as Figure 16 in this AC.
Conditions that would be outside the measurement window include−
(i) Temperatures in excess of 80.6°F
(ii) Relative humidities below 40 percent or above 95 percent
(iii) Relative humidities between 80 percent and 95 percent for temperatures below 35.6°F
(iv) Combinations of temperature and Relative Humidity with values below the line in Figure 16 that connects 80 percent Relative Humidity, 35.6°F and 40 percent Relative Humidity, 59°F.
In general the correction, Delta (M), is to correct for differences in sound absorption in air at different combinations of temperature and relative humidity. The correction is given by−
Delta (M) = (HT α - 0.7 HR) /1000
where HT is the height (in feet) of the airplane over the noise measurement point during the actual test flight when the sound level was measured, and α is the rate of sound absorption per 1,000 feet at a frequency of 500 Hz, for the test-day temperature and humidity, as specified in the Society of Automotive Engineers (SAE) Aerospace Recommended Practice (ARP) 866A, entitled "Standard Values of Atmospheric Absorption as a Function of Temperature and Humidity." The value of α at various combinations of temperature and relative humidity can be obtained from Figure 10 of ARP 866A or from tabulations contained in Table 2 of ARP 866A,
which are accurate to the first decimal place, reproduced herein as Table 5 in the AC. For test humidities other than 70 percent, ARP 866A should be used.
RATE OF ATMOSPHERIC ABSORPTION
Temp : AIR TEMPERATURE, DEGREES F α : ATMOSPHERIC ABSORPTION COEFFICIENT, ALPHA, dB/1000 FT
500 Hz GEOMETRIC MEAN FREQUENCY AND 70 PERCENT RELATIVE HUMIDITY
Temp α Temp α Temp α Temp α Temp α 1 1.8 21 0.9 41 0.6 61 0.7 81 0.9 2 1.8 22 0.9 42 0.6 62 0.7 82 0.9 3 1.7 23 0.8 43 0.6 63 0.8 83 0.9 4 1.7 24 0.8 44 0.6 64 0.8 84 0.9 5 1.6 25 0.8 45 0.6 65 0.8 85 1.0 6 1.6 26 0.8 46 0.6 66 0.8 86 1.0 7 1.5 27 0.7 47 0.6 67 0.8 87 1.0 8 1.5 28 0.7 48 0.6 68 0.8 88 1.0 9 1.4 29 0.7 49 0.7 69 0.8 89 1.0 10 1.4 30 0.7 50 0.7 70 0.8 90 1.0 11 1.3 31 0.7 51 0.7 71 0.8 91 1.0 12 1.3 32 0.6 52 0.7 72 0.8 92 1.0 13 1.2 33 0.6 53 0.7 73 0.8 93 1.0 14 1.2 34 0.6 54 0.7 74 0.9 94 1.1 15 1.1 35 0.6 55 0.7 75 0.9 95 1.1 16 1.1 36 0.6 56 0.7 76 0.9 96 1.1 17 1.1 37 0.6 57 0.7 77 0.9 97 1.1 18 1.0 38 0.6 58 0.7 78 0.9 98 1.1 19 1.0 39 0.6 59 0.7 79 0.9 99 1.1 20 0.9 40 0.6 60 0.7 80 0.9 100 1.1
Table 5: Rate of Atmospheric Absorption (a) at 500 Hz Frequency
412. Section G36.201(d)(2)
Measured sound levels in decibels must be corrected for height by algebraically adding an increment equal to Delta (1). When test day conditions are within those specified in figure G1:
Delta (1) = 22 log (HT/HR)
where HT is the height of the test aircraft when directly over the noise measurement point and HR is the reference height. When test day conditions are outside those specified in figure G1:
Delta (1) = 20 log (HT/HR)
a. Explanation
This adjustment is based on the test height relative to the reference height. Different procedures are specified, depending on whether the test was conducted in or out of the no-correction window identified in paragraph 408, above.
b. Supplemental Information
Inverse Square Correction: The adjustment accounts for the inverse square spreading of the sound during propagation. The adjustment is based on the noise certification test height and the reference height over the microphone. Different regulatory corrections are specified, depending on whether the test was conducted in or out of the no-correction atmospheric window.
c. Procedure
(1) Determine the height, temperature, and relative humidity at the surface for each data point and compute the appropriate adjustment.
413. Section G36.201(d)(3)
Measured sound levels in decibels must be corrected for helical tip Mach number by algebraically adding an increment equal to:
Delta (2) = k log (MR/MT)
where MT and MR are the test and reference helical tip Mach numbers, respectively. The constant "k" is equal to the slope of the line obtained for measured values of the sound level in dB(A) versus helical tip Mach number. The value of k may be determined from approved data. A nominal value of k = 150 may be used when MT is smaller than MR. No correction may be made using the nominal value of k when MT is larger than MR. The reference helical tip Mach number MR is the Mach number corresponding to the reference conditions (RPM, airspeed, and temperature) above the measurement point.
a. Explanation
This section specifies the adjustments to be made to account for non-reference propeller helical tip Mach number.
b. Supplemental Information
(1) Fixed-pitch Propeller Correction: This adjustment is not required for a fixed-pitch propeller if the engine power within 5 percent of the reference. If the test engine power is not within 5 percent the reference engine power, an approved correction is to be made.
(2) Correction Coefficient k: The format of the relationship between noise level and propeller helical tip Mach number has been developed from theoretical studies and tests. The rate at which the noise changes with Mach number is defined by the coefficient k. This has been found to vary for different installations and operating ranges.
(3) Correction Procedures: There are two alternative adjustment procedures for test Mach numbers less than reference. The regulation allows the use of an adjustment formula with k = 150 when the test helical tip Mach number (MT) is less than the reference helical tip Mach number (MR). When MT is greater than MR, k = 150 is not to be used. For most propellers, the value of k is less than 150, so this may overestimate the adjustment, resulting in a noise level greater than it would be at reference conditions. If this possible overestimation is not acceptable or desirable, approved data may be used to determine k. Possible sources of approved data are supplementary tests in which the Mach number is varied over a sufficient range to obtain a value of k.
Section 4.1 of the ICAO Technical Manual (reference 3.j) provides guidance on deriving the noiseversus-tip-Mach-number relationship.