Continuous Gust Design Criteria
Appendix G to Part 25--Continuous Gust Design Criteria
The continuous gust design criteria in this appendix must be used in
establishing the dynamic response of the airplane to vertical and lateral
continuous turbulence unless a more rational criteria is used. The following
gust load requirements apply to mission analysis and design envelope
analysis:
(a) The limit gust loads utilizing the continuous turbulence concept must
be determined in accordance with the provisions of either paragraph (b) or
paragraphs (c) and (d) of this appendix.
(b) Design envelope analysis. The limit loads must be determined in
accordance with the following:
(1) All critical altitudes, weights, and weight distributions, as specified
in Sec. 25.321(b), and all critical speeds within the ranges indicated in
paragraph (b)(3) of this appendix must be considered.
(2) Values of A (ratio of root-mean-square incremental load root-mean-
square gust velocity) must be determined by dynamic analysis. The power
spectral density of the atmospheric turbulence must be as given by the
equation--
1+8/3 (1.339 LV)2
f(V) = s2L/(Pi) --------------------
[1+(1.339 LV)2]11/6
where:
f=power-spectral density (ft./sec.) 2/rad./ft.
s=root-mean-square gust velocity, ft./sec.
V=reduced frequency, radians per foot.
L=2,500 ft.
(3) The limit loads must be obtained by multiplying the A values determined
by the dynamic analysis by the following values of the gust velocity
U:
(i) At speed Vc: U=85 fps true gust velocity in the interval 0 to
30,000 ft. altitude and is linearly decreased to 30 fps true gust velocity at
80,000 ft. altitude. Where the Administrator finds that a design is
comparable to a similar design with extensive satisfactory service
experience, it will be acceptable to select U at Vc less than 85 fps,
but not less than 75 fps, with linear decrease from that value at 20,000 feet
to 30 fps at 80,000 feet. The following factors will be taken into account
when assessing comparability to a similar design:
(1) The transfer function of the new design should exhibit no unusual
characteristics as compared to the similar design which will significantly
affect response to turbulence; e.g., coalescence of modal response in the
frequency regime which can result in a significant increase of loads.
(2) The typical mission of the new airplane is substantially equivalent to
that of the similar design.
(3) The similar design should demonstrate the adequacy of the U
selected.
(ii) At speed VB: U is equal to 1.32 times the values obtained under
paragraph (b)(3)(i) of this appendix.
(iii) At speed VD: U is equal to 1/2 the values obtained under
paragraph (b)(3)(i) of this appendix.
(iv) At speeds between VB and Vc and between Vc and VD: U is equal
to a value obtained by linear interpolation.
(4) When a stability augmentation system is included in the analysis, the
effect of system nonlinearities on loads at the limit load level must be
realistically or conservatively accounted for.
(c) Mission analysis. Limit loads must be determined in accordance with the
following:
(1) The expected utilization of the airplane must be represented by one or
more flight profiles in which the load distribution and the variation with
time of speed, altitude, gross weight, and center of gravity position are
defined. These profiles must be divided into mission segments or blocks, for
analysis, and average or effective values of the pertinent parameters defined
for each segment.
(2) For each of the mission segments defined under paragraph (c)(1) of this
appendix, values of A and No must be determined by analysis. A is defined as
the ratio of root-mean-square incremental load to root-mean-square gust
velocity and No is the radius of gyration of the load power spectral density
function about zero frequency. The power spectral density of the atmospheric
turbulence must be given by the equation set forth in paragraph (b)(2) of
this appendix.
(3) For each of the load and stress quantities selected, the frequency of
exceedance must be determined as a function of load level by means of the
equation--
|Y-Yone=g|
N(y) = SUM tNo [ P1 exp ( - ---------------------)
b1A
|Y-Yone=g|
+ P2 exp (- ------------)]
b2A
where--
t=selected time interval.
y=net value of the load or stress.
Yone=g=value of the load or stress in one-g level flight.
N(y)=average number of exceedances of the indicated value of the load or
stress in unit time.
SUM =symbol denoting summation over all mission segments.
No, A=parameters determined by dynamic analysis as defined in paragraph
(c)(2) of this appendix.
P1, P2, b1, b2=parameters defining the probability distributions of root-
mean-square gust velocity, to be read from Figures 1 and 2 of this
appendix.
The limit gust loads must be read from the frequency of exceedance curves at
a frequency of exceedance of 2x10-5 exceedances per hour. Both positive and
negative load directions must be considered in determining the limit loads.
(4) If a stability augmentation system is utilized to reduce the gust
loads, consideration must be given to the fraction of flight time that the
system may be inoperative. The flight profiles of paragraph (c)(1) of this
appendix must include flight with the system inoperative for this fraction of
the flight time. When a stability augmentation system is included in the
analysis, the effect of system nonlinearities on loads at the limit load
level must be conservatively accounted for.
(d) Supplementary design envelope analysis. In addition to the limit loads
defined by paragraph (c) of this appendix, limit loads must also be
determined in accordance with paragraph (b) of this appendix, except that--
(1) In paragraph (b)(3)(i) of this appendix, the value of U=85 fps
true gust velocity is replaced by U=60 fps true gust velocity on the
interval 0 to 30,000 ft. altitude, and is linearly decreased to 25 fps true
gust velocity at 80,000 ft. altitude; and
(2) In paragraph (b) of this appendix, the reference to paragraphs
(b)(3)(i) through (b)(3)(iii) of this appendix is to be understood as
referring to the paragraph as modified by paragraph (d)(1).
[ ...Illustration appears here... ]
Figure 1 (graph)
[ ...Illustration appears here... ]
Figure 2 (graph)