Pt. 36, App. G

(c) No corrections for helical tip Mach
number variation need to be made if the propeller helical tip Mach number is:
(1) At or below 0.70 and the test helical tip
Mach number is within 0.014 of the reference
helical tip Mach number.
(2) 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.
(3) 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.
(d) When the test conditions are outside
those specified, corrections must be applied
by an approved procedure or by the following
simplified procedure:
(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 Section 36.6.
(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)
(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

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