Flight load factor is the component of the aerodynamic force acting normal (i.e. at right angles) to the longitudinal axis, divided by the aircraft weight. A positive load factor is one in which the force acts upward, with respect to the aircraft; a negative load factor acts downward
VA is the design maneuvering speed. It must be greater than: Vs1* sqrt(n), with Vs1
the flaps-up stall speed and n the maneuver load factor. It need not be greater than Vc
or the speed at which n is reached at CLmax
.VB is the design speed for maximum gust intensity. It is defined in one of two ways. Basically, it is theairspeed at which the required vertical gust, produces maximum CL on the aircraft. However, VB neednot be greater than
Vs1* sqrt(ng) where ng is the gust load factor at Vc and Vs is the stalling speed withflaps retracted.
These velocities are specified as equivalent airspeeds and are linearly interpolated between 20000 and50000 ft.So, to construct the V-n diagram at a particular aircraft weight and altitude, we start with the maximumachievable load factor curve from the maneuver diagram. We then vary the airspeed and compute thegust load factor associated with the VB gust intensity. The intersection of these two lines defines the velocity VB
3.2.35 VS—stalling speed or minimum steady flight speed at
which the airplane is controllable (flaps retracted)
normal-force
Sec. 23.1 Applicability.
(a) This part prescribes airworthiness standards for the issue of type
certificates, and changes to those certificates, for airplanes in the normal,
utility, acrobatic, and commuter categories.
(b) Each person who applies under Part 21 for such a certificate or change
must show compliance with the applicable requirements of this part.
Sec. 23.3 Airplane categories.
(a) The normal category is limited to airplanes that have a seating
configuration, excluding pilot seats, of nine or less, a maximum certificated
takeoff weight of 12,500 pounds or less, and intended for nonacrobatic
operation. Nonacrobatic operation includes:
(1) Any maneuver incident to normal flying;
(2) Stalls (except whip stalls); and
(3) Lazy eights, chandelles, and steep turns, in which the angle of bank is
not more than 60 degrees.
(b) The utility category is limited to airplanes that have a seating
configuration, excluding pilot seats, of nine or less, a maximum certificated
takeoff weight of 12,500 pounds or less, and intended for limited acrobatic
operation. Airplanes certificated in the utility category may be used in any
of the operations covered under paragraph (a) of this section and in limited
acrobatic operations. Limited acrobatic operation includes:
(1) Spins (if approved for the particular type of airplane); and
(2) Lazy eights, chandelles, and steep turns, or similar maneuvers, in
which the angle of bank is more than 60 degrees but not more than 90 degrees.
(c) The acrobatic category is limited to airplanes that have a seating
configuration, excluding pilot seats, of nine or less, a maximum certificated
takeoff weight of 12,500 pounds or less, and intended for use without
restrictions, other than those shown to be necessary as a result of required
flight tests.
(d) The commuter category is limited to propeller-driven, multiengine
airplanes that have a seating configuration, excluding pilot seats, of 19 or
less, and a maximum certificated takeoff weight of 19,000 pounds or less. The
commuter category operation is limited to any maneuver incident to normal
flying, stalls (except whip stalls), and steep turns, in which the angle of
bank is not more than 60 degrees.
(e) Except for commuter category, airplanes may be type certificated in
more than one category if the requirements of each requested category are
met.
normal force coefficient curve CNA
Sec. 23.333 Flight envelope.
(a) General. Compliance with the strength requirements of this subpart must
be shown at any combination of airspeed and load factor on and within the
boundaries of a flight envelope (similar to the one in paragraph (d) of this
section) that represents the envelope of the flight loading conditions
specified by the maneuvering and gust criteria of paragraphs (b) and (c) of
this section respectively.
(b) Maneuvering envelope. Except where limited by maximum (static) lift
coefficients, the airplane is assumed to be subjected to symmetrical
maneuvers resulting in the following limit load factors:
(1) The positive maneuvering load factor specified in Sec. 23.337 at speeds
up to VD;
(2) The negative maneuvering load factor specified in Sec. 23.337 at VC;
and
(3) Factors varying linearly with speed from the specified value at VC to
0.0 at VD for the normal and commuter category, and --1.0 at VD for the
acrobatic and utility categories.
(c) Gust envelope. (1) The airplane is assumed to be subjected to
symmetrical vertical gusts in level flight. The resulting limit load factors
must correspond to the conditions determined as follows:
(i) Positive (up) and negative (down) gusts of 50 f.p.s. at VC must be
considered at altitudes between sea level and 20,000 feet. The gust velocity
may be reduced linearly from 50 f.p.s. at 20,000 feet to 25 f.p.s. at 50,000
feet.
(ii) Positive and negative gusts of 25 f.p.s. at VD must be considered at
altitudes between sea level and 20,000 feet. The gust velocity may be reduced
linearly from 25 f.p.s. at 20,000 feet to 12.5 f.p.s. at 50,000 feet.
(iii) In addition, for commuter category airplanes, positive (up) and
negative (down) rough air gusts of 66 f.p.s. at VB must be considered at
altitudes between sea level and 20,000 feet. The gust velocity may be reduced
linearly from 66 f.p.s. at 20,000 feet to 38 f.p.s. at 50,000 feet.
(2) The following assumptions must be made:
(i) The shape of the gust is--
Ude 2(Pi)s
U = ---- (1 - cos ---- )
2 25C
Where--
s =Distance penetrated into gust (ft.);
C =Mean geometric chord of wing (ft.); and
Ude =Derived gust velocity referred to in subparagraph (1) of this section.
(ii) Gust load factors vary linearly with speed between VC and VD .
(d) Flight envelope.
[ ...Illustration appears here... ]
Flight Envelope
Sec. 23.335 Design airspeeds.
Except as provided in paragraph (a) (4) of this section, the selected
design airspeeds are equivalent airspeeds (EAS).
(a) Design cruising speed, VC. For VC the following apply:
(1) Where W/S'=wing loading at the design maximum takeoff weight, Vc (in
knots) may not be less than--
(i) 33 n where--
(i) VS is a computed stalling speed with flaps retracted at the design
weight, normally based on the maximum airplane normal force coefficients,
CNA; and
(ii) n is the limit maneuvering load factor used in design
(2) The value of VA need not exceed the value of VC used in design.
(d) Design speed for maximum gust intensity, VB. For VB, the following apply:
(1) VB may not be less than the speed determined by the intersection of the
line representing the maximum positive lift, CN MAX, and the line
representing the rough air gust velocity on the gust V-n diagram, or VS1
n where--
(i) n is the limit positive maneuvering load factor at VC; and
(ii) VS1 is the stalling speed with flaps retracted.
(2) VA and VS must be evaluated at the design weight and altitude under
consideration.
(3) VA need not be more than VC or the speed at which the positive CN max
curve intersects the positive maneuver load factor line, whichever is less.
(d) Design speed for maximum gust intensity, VB. For VB, the following
apply:
(1) VB may not be less than the speed determined by the intersection of the
line representing the maximum position lift CN max and the line representing
the rough air gust velocity on the gust V-n diagram, or (ng) VS1,
whichever is less, where--
(i) ng is the positive airplane gust load factor due to gust, at speed VC
(in accordance with Sec. 25.341), and at the particular weight under
consideration; and
(ii) VS1 is the stalling speed with the flaps retracted at the particular
weight under consideration.
(2) VB need not be greater than VC.
(e) Design flap speeds, VF. For VF, the following apply:
(1) The design flap speed for each flap position (established in accordance
with Sec. 25.697(a)) must be sufficiently greater than the operating speed
recommended for the corresponding stage of flight (including balked landings)
to allow for probable variations in control of airspeed and for transition
from one flap position to another.
(2) If an automatic flap positioning or load limiting device is used, the
speeds and corresponding flap positions programmed or allowed by the device
may be used.
(3) VF may not be less than--
(i) 1.6 VS1 with the flaps in takeoff position at maximum takeoff weight;
(ii) 1.8 VS1 with the flaps in approach position at maximum landing weight,
and
(iii) 1.8 VS0 with the flaps in landing position at maximum landing weight.
(f) Design drag device speeds, VDD. The selected design speed for each drag
device must be sufficiently greater than the speed recommended for the
operation of the device to allow for probable variations in speed control.
For drag devices intended for use in high speed descents, VDD may not be less
than VD. When an automatic drag device positioning or load limiting means is
used, the speeds and corresponding drag device positions programmed or
allowed by the automatic means must be used for design.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as amended by Amdt. 25-23, 35 FR
5672, Apr. 8, 1970]
Sec. 25.337 Limit maneuvering load factors.
(a) Except where limited by maximum (static) lift coefficients, the
airplane is assumed to be subjected to symmetrical maneuvers resulting in the
limit maneuvering load factors prescribed in this section. Pitching
velocities appropriate to the corresponding pull-up and steady turn maneuvers
must be taken into account.
(b) The positive limit maneuvering load factor "n" for any speed up to Vn
may not be less than 2.1+24,000/ (W +10,000) except that "n" may not be less
than 2.5 and need not be greater than 3.8--where "W" is the design maximum
takeoff weight.
(c) The negative limit maneuvering load factor--
(1) May not be less than -1.0 at speeds up to VC; and
(2) Must vary linearly with speed from the value at VC to zero at VD.
(d) Maneuvering load factors lower than those specified in this section may
be used if the airplane has design features that make it impossible to exceed
these values in flight.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as amended by Amdt. 25-23, 35 FR
5672, Apr. 8, 1970]
Sec. 25.341 Gust loads.
(a) The airplane is assumed to be subjected to symmetrical vertical gusts
in level flight. The resulting limit load factors must correspond to the
conditions determined as follows:
(1) Positive (up) and negative (down) rough air gusts of 66 fps at VB must
be considered at altitudes between sea level and 20,000 feet. The gust
velocity may be reduced linearly from 66 fps at 20,000 feet to 38 fps at
50,000 feet.
(2) Positive and negative gusts of 50 fps at VC must be considered at
altitudes between sea level and 20,000 feet. The gust velocity may be reduced
linearly from 50 fps at 20,000 feet to 25 fps at 50,000 feet.
(3) Positive and negative gusts of 25 fps at VD must be considered at
altitudes between sea level and 20,000 feet. The gust velocity may be reduced
linearly from 25 fps at 20,000 feet to 12.5 fps at 50,000 feet.
(b) The following assumptions must be made:
(1) The shape of the gust is
Ude 2s
U = --- (1-cos ------ )
2 25C
where--
s=distance penetrated into gust (ft);
C=mean geometric chord of wing (ft); and
Ude=derived gust velocity referred to in paragraph (a) (fps).
(2) Gust load factors vary linearly between the specified conditions B'
through G', as shown on the gust envelope in Sec. 25.333(c).
(c) In the absence of a more rational analysis, the gust load factors must
be computed as follows:
KgUdeVa
n=1 + ---------
498 (W/S)
where--
0.88g
Kg = ----------- = gust alleviation factor;
5.3+g
2(W/S)
g = ------ = airplane mass ratio:
rCag
Ude=derived gust velocities referred to in paragraph (a) (fps);
r=density of air (slugs cu. ft.);
W/S=wing loading (psf);
C=mean geometric chord (ft);
g=acceleration due to gravity (ft/sec**2);
V=airplane equivalent speed (knots); and
a=slope of the airplane normal force coefficient curve CNA per radian if the
gust loads are applied to the wings and horizontal method. The wing lift
curve slope CAL per radian may be used when the gust load is applied to
the wings only and the horizontal tail gust loads are treated as a
separate condition.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as amended by Amdt. 25-72, 55
FR 29775, July 20, 1990; 55 FR 37607, Sept. 12, 1990]