EPNLs should be calculated for the revised NPD plot. Guid elines on the elements of an acceptable projection procedure are provided in this section. The process is also illustrated in Figures 4-24 and 4-25.
It is not intended that the procedure illustrated in Figu res 4-24 and 4-25 should be exclusive. There are several options, depending upon the nature of the powerplant noise sources and the relevance of individual noise sources to the EPNL of the airplane. The method presented does however specify the main features that should be considered in the computational procedure.
It is also not necessary that the computations illustrated in Figures 4-24 and 4-25 should always be carried out in the order specified. There are interrelations between th e various steps in the procedure which depend on the particular form of the computation being followed. Hence the most efficient manner of structuring the computation cannot always be predetermined.
Combine to give total aeroplane noise and adjust for propagation effects Compute EPNL Extrapolate to aeroplane flight path Account for airframe noise Adjust for aeroplane configuration Adjust for noise source motion and engine flight conditions Adjust for noise source position Separate into contributing noise sources Separate into broadband and tone noise Correct for test environment Acquire static noise data Broadband Tones Normalized flight spectra Flight noise source breakdown Static noise source breakdown Normalized freefield static spectra Figure 32. Generalized projection of static engine data to airplane flight conditions (Refer to 402(a)(3)) Page 175
Acquire time-averaged static test data under stabilized power conditions over angular range embracing 10 dB-down times of flight time-history.
For each individual measured angle* perform one-third octave (and possibly narrow-band) analysis and adjust data for static propagation effects under reference atmosphere conditions.
Identify all relevant discrete tones and subtract from one-third octave spectrum to give broadband spectrum.
Consider tones and broadband separately.
Broadband Tones Using approved source identification techniques, conduct source breakdown.
Adjust for source motion effects, accounting for relative velocity, Doppler frequency shift, convective amplification, directivity effects, etc., and engine thermodynamic changes.
Adjust for number of engines, relevant installation effects and sources of interaction/airframe noise, and reconstitute one-third octave band sound pressure levels.
Extrapolate to aircraft height (slant distance) and allow for ground reflection, lateral propagation and other relevant effects to produce quasi in-flight one-third octave band SPL and PNLT.
Integrate projected PNLT over time corresponding to height, flight path and airspeed.
Compute EPNL Identify origin of tones where appropriate.
PNLT* Narrowband SPL SPL One-third octave Frequency Frequency Jet Other Fan Aeroplane Fan tones Airframe PNLT 10 dB Time Repeat for other angles EPNL Figure 33. Example procedure for projection of static engine data to airplane flight conditions There are several engine installation effects which can modify the generated noise levels but which cannot be derived from static tests. Additional noise sources such as jet/flap or jet/wind interaction effects may be introduced on a derived version of the airplane which are not present on the “flight datum” airplane Far-field noi se directivity patterns (i.e., field shapes) may be modified by wing/nacelle or jet-by-jet shielding, tail plane and fuselage scattering or airframe reflection effects. However general methods to adjust for thes e effects are not yet available. It is therefore important that before the following procedures are approved for the derived version of the airplane, the geometry of the airframe and engines in the vicinity of the engines be shown to be essentially identical to that of the “flight datum” airplanes so that the radiated noise is essentially unaffected.
402(a)(3)(D)(ii) Normalizat ion to reference conditions [ETM 4.2.1.3.4.2] Page 176
The analyzed one-third octave band sound pressure level static test data should be normalized to free-field reference atmospheric conditions specified in B36.7(a)(k)(5) of part 36.This adjustment can be applied only with knowledge of the total spectra being the summation of all the noise so urce spectra computed as described in the following paragraphs. The required adjustments include:
a) Sound attenuation due to atmospheric absorption . Adjustments to account for the acoustical reference-day sound attenuation are defined in Reference T16. In the event that minor differences in coefficient values are found in Reference T16 between equations, tables or graphs, the equations should be used. The sound attenuation coefficients should be computed over the actual distance from the effective center of each noise source to each microphone, as described in 402(a)(3)(D)(v) of this AC.
b) Ground reflection. Examples of methods for obtaining free-field sound pressure levels are described in References T5 and T15. Spatial distribution of noise sources do not have a first order influence on ground reflection effects and hence may be disregarded. It is also noted that measurements of far-field sound pressure levels with ground-plane microphones may be used to avoid the large spectral irregularities caused by interference effects at frequencies less than 1 kHz.
402(a)(3)(D)(iii) Separation into broadband and tone noise [ETM 4.2.1.3.4.3] The purpose of procedures described in this section is to id entify all significant tones in the spectra: first, to ensure that tones are not included in the subsequent estimation of broadband noise and, second, to enable the Doppler-shifted tones in-flight to be allocated to the correct one-third octave band at appropriate times during a simulated airplane flyover.
Broadband noise should be derived by extracting all significant tones from the measured spectra. One concept for the identification of discrete tones is the one used in Appendix A of part 36 for tone correction purposes (i.e., considering the slopes between adjacent on e-third octave band levels). Care must be taken to avoid regarding tones as “non - protrusive” when the surrounding broadband sound pressure level is likely to be lower when adjusted from static-toflight conditions, or when classifying a closely grouped pair or series of tones as broadband noise. One technique for resolving such problems is the use of narrow-band analysis with a bandwidth of less than 50 Hz.
Narrow-band analysis can also be used to check the validity of other tone identification procedures in establishing the spectral character at critical locations in the sound field (e.g., around the position of peak PNLT) or where predominant turbo-machinery tones exist.
402(a)(3)(D)(iv) Separation in to contributing noise sources [ETM 4.2.1.3.4.4] The number of noise sources which require identification will to some extent depend on the engine being tested and the nature of the change to the engine or nacelle. The separation of broadband noise into the combination of noise generated by external jet mixing and by internal noise sources is the minimum and sometimes adequate requirement. A more sophisticated analysis may be necessary depending upon th e significance of the contri bution from other individual sources, which could involve identifying broadband noise from fan, compressor, combustor and turbine. Furthermore, for fan and compressor noise, the split of both the broadband and the tone noise between that radiating from the engine intake and that from the engine exhaust nozzle(s) could be a further refinement.
To meet the minimum requirement, the separation of sources of broadband noise into those due to external jet mixing and those generated internally can be carried out by:
a) estimating the jet noise by one or more of the methods identified below; and
b) adjusting the level of the predicted spectrum at each angle to fit the measured low frequency part of the broadband spectrum at which jet noise can be expected to be dominant.
There are three methods which have been used to obtain predicted jet noise spectra shapes:
a) for single-stream engines with circular nozzles, th e procedure detailed in Reference T12 may be used; the engine geometry however may possess features which can render this method inapplicable; sample procedures for coaxial flow engines are provided in Reference T11;
b) analytical procedures based on correlating full-scale engine data with model nozzle characteristics may be used; model data have been used to supplement full-scale engine data, particularly at low power settings, because of the uncertainty in defining the level of jet noise at the higher frequencies where noise from other engine sources may make a significant contribution to the broadband noise; and
c) special noise source location techniques are available which, when used during full-scale engine tests, can identify the positions and levels of separate engine noise sources.
402(a)(3)(D)(v) Noise source position effects [ETM 4.2.1.3.4.5] Static engine noise measurements are often made at distances at which engine noise sources cannot be truly treated as radiating from a single acoustic center. This may not give rise to difficulties in the extrapolation to determine the noise increments from static data to flight conditions because noise increments in EPNL are not particularly sensitive to the assumption made regarding the spatial distribution of noise sources.
However, in some circumstances, for example where changes are made to exhaust structures and where the sources of external jet-mixing noise are of overriding significance, it may be appropriate to identify noise source positions more accurately. The jet noise can be considered as a noise source distributed downstream of the engine exhaust plane.
Internal sources of broadband engine noise may be considered as radiating from the intake and the exhaust.
There are three principal effects to be accounted for as a consequence of the position of the noise source differing from the “nominal” position assumed for the “source” of engine noise:
a) Spherical divergence. The distance of the source from the microphone differs from the nominal distance, in which case an inverse square law adjustment needs to be applied.
b) Directivity. The angle subtended by the line from the source to the microphone and the source to the engine centerline differs from the nominal angle, in which case a linear interpolation should be made to obtain data for the proper angle.
c) Sound attenuation due to atmospheric absorption. The difference between the true and the nominal distance between the source and the microphone alters the allowance made for sound attenuation.
Source position can be identified either from noise source location measurements (made either at full or model scale) or from a generalized database.
Note.— No published standard on coaxial jet noise source distribution is currently available. An approximate distribution for a single jet is given by the following equation (see References T6 and T17):
x/D = (0.0575 S + 0.0215 S–2 )–½ where:
— S is the Strouhal number fD/V i;
— x is the distance downstream from the nozzle exit;
— D is the nozzle diameter based on total nozzle exit area;
— Vj is the average jet velocity for co mplete isentropic expansion to ambient pressure from average nozzleexit pressure and temperature; and — f is the one-third octave band center frequency.
402(a)(3)(D)(vi) Engine flight conditions [ETM 4.2.1.3.4.6] Some thermodynamic conditions within an engine tested sta tically differ from those that exist in flight and this difference should be taken into account. Noise source strengths may be changed accordingly. Therefore, the values for key correlating parameters for component noise source generation should be based on the flight condition, and the static database should be entered at the appropriate correlating parameter value. Turbo-machinery noise levels should ேభbe based on the in-flight corrected rotor speeds √ఏమ Jet noise levels should be based on the relative jet velocities that exist at the flight condition.
The variation of source noise levels with key correlating parameters can be determined from the static database which includes a number of different thermodynamic operating conditions.
402(a)(3)(d)(vii) Noise source motion effects [ETM 4.2.1.3.4.7] The effects of motion on jet noise differ from speed effects on other noise sources and, hence, are considered separately during static-to-flight projection.
(a) For external jet noise Account should be taken of the frequency-dependent jet-relative velocity effects and the convective amplification effects. Broadly speaking, two sources of information may be used to develop an approved method for defining the effect of flight on external jet noise:
1) for single-stream engines with circular exhaust geometries, Reference T12 provides guidance. Additional supporting evidence however may be needed to show when jet noise is the major contributor to the noise from an engine with a more complex nozzle assembly; and
2) full-scale flight data on a similar exhaust geomet ry can provide additional evidence. In general however, because of the difficulty of defining high frequency effects in the presence of internally-generated engine noise, it may be necessary to provide additional sup porting information to determine the variation of EPNL with changes of jet noise spectra at high frequencies.
(b) For noise sources other than jet noise In addition to the Doppler frequency effect on the non-jet noise observed on the ground from an airplane flyover, the noise generated by the engine’s internal components and the airframe can be influenced by source amplitude modification and directivity changes:
1) Doppler effect. Frequency shifting that results from motion of the source (i.e., airplane) relative to a microphone is accounted for by the following equation:
݂௦௧௧݂௧ ൌ ሺ1 െ ߣݏܿܯ Page 179
w h e r e :
— ݂௧ is the flight frequency;
— ݂௦௧௧ is the static frequency;
— M is the Mach number of the airplane; and — is the the angle between the flight path in the direction of flight and a straight line connecting the airplane and the microphone at the time of sound emission.
It should be noted for those one-third octave band sound pressure levels dominated by a turbo-machinery tone, the Doppler shift may move the tone and its harmonics into an adjacent band.
2) Source amplitude modification and directivity changes. One-third octave band sound pressure level adjustments to airframe-generated noise that re sults from speed change s between the datum and derivative versions provided below.
For noise generated internally within the engine (e.g., fan noise), there is no consensus of opinion on the mechanisms involved or on a unique adjustment method that accounts for the detailed source modification and sound propagation effects. If an adjustment is used, the same technique must be applied to both the flight datum and derivative configuration when establishing noise changes. In such instances the adjustment for the one-third octave band sound pressure level changes that result from the motion of the source (i.e., airplane) relative to the microphone may be accounted for by using the following equation:
ܮܲܵ௧ ൌܮܲܵ ௦௧௧ െ݈݃ܭ ሺ 1െ ߣݏܿܯ ሻ where:
— SPL flight is the flight sound pressure level;
— SPL static is the static sound pressure level; and — M and are defined above and K is a constant.
Theoretically, K has a value of 40 for a point noise source, but a more appropriate value may be obtained by comparing static and flight data for the “flight datum” airplane.
402(a)(3)(D)(viii) Airplane configuration effects [ETM 4.2.1.3.4.8] The contribution from more than one engine on an airplane is normally taken into account by adding 10 log N, where N is the number of engines, to each component noise source. It might be necessary however to compute the noise from engines widely spaced on large airplanes, particularly in the approach case, if they include both underwing and fuselage mountings. The noise from the intakes of engines mounted above the fuselage is known to be shielded.
If engine installation effects change between the “flight datum” airplane and a derived version, account should be taken of the change on one-third octave band sound pressu re levels which should be es timated according to the best available evidence.
402(a)(3)(D)(ix) Airframe noise [ETM 4.2.1.3.4.9] To account for the contribution of airframe noise, measured flight datum airframe noise on its own, or combined with an approved airframe noise analytical model, may be used to develop an airframe noi se database. The airframegenerated noise, which can be treated as a point source for adjustment purposes, is normalized to the same conditions as those of the other (i.e., engine) sources, with due account given for the effects of spherical divergence, atmospheric absorption and airspeed as described in Appendix A of part 36.
Airframe noise for a specific configuration varies with airspeed (see Reference T5) as follows:
Δܮܲܵ ൌ 50logሺܸோாி/்ܸாௌ்ሻ where:
— VREF is the approved reference airspeed for the “flight datum” airplane; and — V TEST is model or measured airspeed.
The above equation is also valid for adjustments to EPNL where an empirically derived coefficient replaces the coefficient 50 since that number may be somewhat configuration-dependent. However, the approval of the FAA is required for values other than 50.
402(a)(3)(D)(x) Airplane flight path considerations [ETM 4.2.1.3.4.10] When computing the one-third octave band sound pressure le vels corresponding to the slant distance of the airplane in flight from the noise measuring point, the principal effects are spherical divergence (inverse square law) adjustments from the nominal static distance and sound attenuation due to atmospheric absorption (as described in Appendix A of part 36). Furthermore, account should be taken of the difference between the static engine axis and that axis in flight relative to the reference noise measuring points. The adjustments should be applied to the component noise source levels that have been separately identified.
402(a)(3)(D)(xi) Total noise spectra [ETM 4.2.1.3.4.11] Both the engine tonal and broadband noise source components in flight, together with the airframe noise and any installation effects, are summed up on a mean-square pressure basis to construc t the spectra of total airplane noise levels.
During the merging of broadband and tonal components, consideration should be given to appropriate band-sharing of discrete frequency tones.
The effects of ground reflections must be included in the estimate of free-field sound pressure levels in order to simulate the sound pressure levels that would be measured by a microphone at a height of 4 ft (1.2 m) above a natural terrain. Information in Reference T4 or T15 may be used to apply adjustments to the free-field spectra to allow for flight measurements being made at 4 ft (1.2 m). Alternatively, the ground reflection adjustment can be derived from other approved analytical or empirically derived models. Note that the Doppler adjustment for a static source at frequency (fstatic) applies to a moving source (i.e., airplane) at a frequency (fflight) where ݂௧ ൌ݂ ௦௧௧/ሺ1 െ ߣݏܿܯሻ using the same terminology as described above for the Doppler effect. This process is repeated for each measurement angle and for each engine power setting. With regard to lateral attenuation, the information in Reference T14, applicable to the computation of lateral noise may be applied.
402(a)(3)(D)(xii) EPNL computations [ETM 4.2.1.3.4.12] For EPNL calculations, a time is associated with each extrapolated spectrum along the flight path. Note that the time is associated with each measurement location with respect to the engine/airplane reference point and the airplane’s true airspeed along the reference flight path, assumi ng zero wind. For each engine power setting and minimum distance, an EPNL is computed from the projected time history using the methods described in Appendix A of part
36.
402(a)(3)(D)(xiii) Chan ges to noise levels [ETM 4.2.1.3.4.13] An NPD plot can be constructed from the projected static data for both the original (i.e., flight datum) and the changed configurations of the engine or nacelle tested. Comparisons of the noise versus engine thrust (power) relationships for the two configurations at the same appropriate minimum distance will determine whether or not the changed configuration resulted in a change to the noise level from an engine noise source. If there is a change in the level of source noise, a new in-flight airplane NPD plot can be developed by adjusting the measured original NPD plot by the amount of change indicated by the comparison of the static-projected NPD plots for the original and changed versions within the limitations specified in 402(a)(3)(ii) for EPNL.
The certification noise levels for the derived version of an airplane may be determined from NPD plots at the relevant reference engine power and distance, with an additional adjustment of [10 log Vnom/Vr] for the velocity of the airplane at the certification reference condition relative to the nominal velocity (Vnom) used in developing the NPD plots.
402(b) Propeller-Driven Airplanes Over 19,000 lb [ETM 4.2.2] The procedures described in this chapter have been used as equivalent in stringency for propeller-driven airplanes with maximum certificated takeoff weight exceeding 19,000 lb, as provided in Appendix B.
402(b)(1) Flight test procedures [ETM 4.2.2.1] 402(b)(1)(A) Flight path intercept procedures [ETM 4.2.2.1.1] Flight path intercept procedures, as described in 402(a) (1)(i), have been used to meet the noise certification demonstration requirements in lieu of full takeoffs and/or landings.
402(b)(1)(B) Generalized flight test procedures [ETM 4.2.2.1.2] Generalized flight test procedures, other than normal noise demonstration takeoffs and approaches, have been used to meet two equivalency objectives:
a) NPD plots Noise data are acquired over a range of engine power settings at one or more heights. This information permits the development of generalized noise characteristics necessary for the certification of a “family” of similar airplanes. The procedures used are similar to those described in 402(a)(1)(ii), with the exception that the NPD plots employ engine noise performance parameters (µ) of propeller helical tip Mach number (MH) and shaft horsepower (SHP/δamb) (see Figure 34), where δamb is defined in 402(a)(1)(ii)(A).
Engine noise performance parameter ( ) SHP / amb Helical tip Mach number MH EPNL at a given height Figure 34. Form of noise-power-distance (NPD) plot for heavy propeller-driven airplanes In order to ensure that propeller inflow angles are similar throughout the development of the noise-sensitivity data as the airplane weight changes, the airspeed of the airplane used in the flight tests for developing the lateral and flyover data must be V2 + 10 kt (V2 + 19 km/h) to within ±3 kt (±5.5 km/h), as appropriate for the weight of the airplane during the test.
For the development of the NPD data for the approach case, the speed and approach angle constraints imposed in B36.7(c) and B36.8 of part 36 cannot be satisfied over the typical range of power needed. For the approach case, a steady speed of V REF + 10 kt (V REF + 19 km/h) should be maintained to within ±3 kt (±5.5 km/h) and the flyover height over the microphone should be 400 ± 100 ft (122 ± 30 m). Within these constraints, the test approach angle at the test power should be that which results from the test airplane conditions (i.e., weight, configuration, speed and power).
b) Noise level changes Comparisons are made of flyover noise test data for different developments of an airplane type (e.g., a change in propeller type). Such changes are used to establish certification noise levels of a newly derived version as described in 402(a)(1)(ii).
402(b)(1)(C) Determination of th e lateral certification noise level [ETM 4.2.2.1.3] For propeller-driven airplanes, B36.3(b) of part 36 specifies a full-power measurement point under the flight path as a replacement for the lateral measurement point. This section describes appropriate equiva lent procedures for those airplanes for which the two-microphone lateral measurement method was applicable.
Determination of the lateral certification noise level employing an alternative procedure using two microphone stations located symmetrically on either side of the takeoff flight path similar to that described in 402(a)(1)(iii) has been approved. However, when this procedure is used, matching data from both lateral microphones for each fly-by must be used for the lateral noise determination. Cases where data from only one microphone is available for a given fly- Page 183
by must be omitted from the determination. The following paragraphs describe the procedures for propeller-driven heavy airplanes:
a) the lateral EPNL from propeller-driven airplanes, wh en plotted against height opposite the measuring sites, can exhibit distinct asymmetry; the maximum EPNL on one side of the airplane is often at a different height and noise level from that measured on the other side;
b) in order to determine the average maximum lateral EPNL (i.e., the certification sideline noise level) it is therefore necessary to undertake a number of flights over a range of heights to define the noise versus height characteristics for each side of the airplane; a typical height range would cover between 100 ft (30 m) and 1800 ft (550 m) above a track at right angles to, and midway along, the line joining the two microphone stations; the intersection of the track with this line is defined as the reference point;
c) since experience has shown the maximum lateral noise level may often be near the lower end of this range, a minimum of six good sets of data, measured simultaneously from both sides of the flight track, should be obtained for a range of airplane heights as low as possible; in this case takeoffs may be necessary; however care should be taken to ensure that th e airspeed is stabilized to at least V 2 + 10 kt (V 2 + 9 km/h) over the 10 dB-down period;
d) the airplane climbs over the reference point using takeoff power, speeds and configuration as described in B36.7(b)(3) and B.36.7(b)(4) of part 36;
e) the lateral certification noise level is obtained by find ing the peak of the curve of noise level (EPNL) corrected to reference-day atmospheric absorption values and plotted against airplane height above the reference point (see Figure 35); this curve is described as a least-sq uares curve fit through the data points defined by the median values of each pair of matche d data measured on each side of th e track (i.e., the average of the two microphone measurements for a given airplane height); and
f) to ensure that the requirements of A36.5.4.2 of part 36 are met, the 90 percent c onfidence limits should be determined in accordance with paragraph 305.
402(b)(1)(D) Measurements at non-reference points [ETM 4.2.2.1.4] In some instances, test measurement points may differ from the reference measurement points as specified in Appendix B. Under these circumstances an applicant may request a pproval of data that have been adjusted from actual measurements to the reference conditions for reasons described in 402(a)(1)(v).
Noise measurements collected closer to the test airplane than at the certification reference points are particularly useful for adjusting propeller noise data because they are dominated by low frequency noise. The spectra roll off rapidly at higher frequencies and are often lost in the background noise at frequencies above 5000 Hz. Section 306(c), describes a procedure for background noise adjustment.
x x x x x x x x x x x x x x x x x x x x x x x x x x xx x x Noise levels from right side of track Noise levels from left side of track Noise certification level EPNL Aeroplane height Figure 35. Typical lateral noise data plot for heavy propeller-driven airplanes Non-reference measurement points may be used provided that measured data are adjusted to reference conditions in accordance with the requirements of A36.9 of part 36 and that the magnitude of the adjustments does not exceed the limits cited in B36.8(f) of part 36.
402(b)(2) Analytical procedures [ETM 4.2.2.2] Equivalent analytical procedures rely upon the available noise and performance data of an airplane type. The generalized relationships between noise levels, propeller helical tip Mach number, and shaft horsepower, as well as the adjustment procedures for speed and height changes in accordance with the methods of Appendix A are combined with certificated airplane performance data in order to determine noise level changes resulting from type design changes. The noise level changes are then added to or subtracted from the certifica tion noise levels that are demonstrated by flight test measurements for the “flight datum” airplane.
Certifications using analytical procedures have been approved for type design changes that result in predictable noise level differences. The type design changes include the following:
a) an increase or decrease in maximum takeoff and/or landing weight of the airplane from the originally certificated weight;
b) power increase or decrease for engines that are acoustically similar and fitted with propellers of the same type;
c) airplane engine and nacelle configuration changes, usually minor in nature, including derivative airplane models with changes in fuselage length and flap configuration; care is however needed to ensure that the Page 185
existing noise sources are not modified by these changes (e.g., by changing the flow field into the propellers);
and
d) minor airframe design changes that could indirectly affect noise levels because of an impact on airplane performance (e.g., increased drag ); changes in airplane performa nce characteristics derived from aerodynamic analysis or testing have been used to demonstrate how these changes can affect the airplane flight path and consequently the demonstrated noise levels of the airplane.
402(b)(3) Ground static testing procedures [ETM 4.2.2.3] 402(b)(3)(A) General [ETM 4.2.2.3.1] Unlike the case of a turbojet or turbofan powerplant, static tests involving changes to the propeller are not applicable for determining noise level changes in the development of a propeller-driven airplane /powerplant family because of changes in the aero-acoustic operating conditions of the propeller when run statically compared with conditions existing during flight. The propeller noise levels measured during a static test can include significant contributions from noise source components that are not normally important in flight. However, limited static tests on engines with propellers, which are used as engine loading devices, can be utilized to determine small noise changes, as described below.
402(b)(3)(B) Guidance on the test-site characteristics [ETM 4.2.2.3.2] Guidance on the test-site characteristics, data acquisition and analysis systems, microphone locations, acoustical calibration and measurement procedures for static testing is provided in Reference T13 and is equally valid in these respects for propeller powerplants (see 402(a)(3)(iii)).
402(b)(3)(C) Static tests of the gas generator [ETM 4.2.2.3.3] Static tests of the gas generator can be used to identify noise changes resulting from changes to the design of the gas generators, or to the internal structure of the engine, in the frequency ranges where:
a) there is a contribution to the airplane EPNL;
b) that part of the spectrum is clearly dominated by the gas generator; or
c) ancillary equipment under circumstances where th e propeller and its aerodynamic performance remains unchanged.
Such circumstances where the propeller and its aerodynami c performance remain unchanged include, for example, changes to the compressor, turbine or combustor of the powerplant. The effect of such changes should be conducted under the same test, measurement, data reduction and extrapolation procedures as described in 402(a)(3) for turbojet and turbofan engines. The noise emanating from any propeller or other power extraction device used in static tests should be eliminated or removed analy tically. For the purposes of airplane EPNL calculation, the measured “flight datum” airplane propeller contributions should be included in the computation process.
402(c) Helicopters [ETM 4.2.3] The objective of a noise certification test is to acquire da ta for establishing an accurate and reliable definition of a helicopter’s noise characteristics (see Appendix H of part 36) . This Appendix establishes a range of test conditions and procedures for adjusting measured data to reference conditions.