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Structural Design Considerations and Airspeeds
Cargas em Aviões
ITA – Instituto Tecnológico de Aeronáutica
Airspeed Definitions
•True Airspeed (ktas)
is the true airspeed of the airplane, at the altitude it is flying.
•Equivalent Airspeed (keas)
is an airspeed at sea level that would result in the same dynamica pressure experienced by the airplane flying at altitude at its true airspeed.
•Indicated Airspeed (kias)
is the airspeed the pilot reads on the airspeed indicator in the cockpit.
•Calibrated Airspeed (kcas)
is the Indicated Airspeed plus airspeed correction due to instrument and static source error, which may be a function of flying altitude, Mach number and flap position.
Indicated Airspeed Correction
Standard atmosphere parameters
for H < 36.089 ft for H > 36.089 ft
keas , 287,661
75187,0
22336,0
ft , 22336,0ln089.36
5,0
3
es
x
c
e
kH
keas , sound of speed
ratiodensity
ratio pressure
ft altitude,
24,089.36x
ft 03,806.20
ft 57,445.145
3
3
1
esc
H
kH
k
k
keas , 287,661
1
1
ft , -1
5,0
2561,41
2561,51
0,190261
esc
kH
kH
kH
Speed relations
MMaV eE5,0287,661
12,01
12,015,3
2
5,32
0
M
aVC
1112,015
5,315,320 MaVC
Given altitude and Mach number, calibrated airspeed is calculated
Given altitude and calibrated airspeed, Mach number is calculated
1112,01 5
5,315,3 20 aVM C
Given calibrated airspeed and Mach number, pressure ratio is calculated
Given Mach number and altitude, equivalent airspeed is calculated
5,0ET VV
Given altitude and equivalent airspeed, true airspeed is calculated
Variation of Airspeeds with Altitude
Equivalent and true airspeeds for constant calibrated airspeed
05.000
10.00015.00020.000
25.00030.00035.000
40.00045.000
300 400 500 600 700
Airspeed (knots)
Alti
tude
(fee
t)
True Airspeed at 350 kcas350 kcas
Equ
ival
ent A
irspe
ed a
t 350
kca
s
Variation of Airspeeds with Altitude
Mach Number for VC = 350 kcas
0
10.000
20.000
30.000
40.000
50.000
0,5 0,6 0,7 0,8 0,9 1,0 1,1 1,2
Mach Number
Alt
itu
de
(fe
et)
Equivalent velocity fo VC = 350 kcas and M < 0,86
0
5.000
10.000
15.000
20.000
25.000
30.000
35.000
40.000
45.000
200 250 300 350 400
Airspeed (keas)
Alt
itu
de
(fee
t)
Part 25 § 25.335 – Design airspeeds.
The selected design airspeeds are equivalent airspeeds (EAS). Estimated values of VS0 and VS1 must be conservative.(a). Design cruising speed, VC. For VC, the following apply:(1). The minimum value of VC must be sufficiently greater than VB to provide for inadvertent speed increases likely to occur as a result of severe atmospheric turbulence.(2). Except as provided in § 25.335(d)(2), VC may not be less than VB + 1.32 UREF (with UREF as specified in § 25.341(a)(5)(i)). However VC need not exceed the maximum speed in level flight at maximum continuous power for the corresponding altitude.(3). At altitudes where VD is limited by Mach number, VC may be limited to a selected Mach number.(b). Design dive speed, VD. VD must be selected so that VC/MC is not greater than 0.8 VD/MD, or so that the minimum speed margin between VC/MC and VD/MD is the greater of the following values:(1). From an initial condition of stabilized flight at VC/MC, the airplane is upset, flown for 20 seconds along a flight path 7.50 degrees below the initial path, and then pulled up at a load factor of 1.5g (0.5g acceleration increment). The speed increase occurring in this maneuver may be calculated if reliable or conservative aerodynamic data is issued. Power as specified in § 25.175(b)(1)(iv) is assumed until the pullup is initiated, at which time power reduction and the use of pilot controlled drag devices may be assumed;(2). The minimum speed margin must be enough to provide for atmospheric variations (such as horizontal gusts, and penetration of jet streams and cold fronts) and for instrument errors and airframe production variations. These factors may be considered on a probability basis. The margin at altitude where MC is limited by compressibility effects must not be less than 0.07M unless a lower margin is determined using a rational analysis that includes the effects of anyautomatic systems. In any case, the margin may not be reduced to less than 0.05M.
(c). Design maneuvering speed, VA. For VA, the following apply:(1). VA may not be less than where n is the limit positive maneuvering load factor at VC; and VS1 is the stalling speed with flaps retracted.(2). VA and VS1 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 CNmax curve intersects the positive maneuver load factor line, whichever is less.(d). Design speed for maximum gust intensity, VB.
(1). VB may not be less than where:
Kg = gust alleviation factor = g = airplane mass ratio =VS1 = the 1-g stalling speed based on CNmax with the flaps retracted at the particular weight under consideration;VC = design cruise speed (knots equivalent airspeed);Uref = the reference gust velocity (feet per second equivalent airspeed) from § 25.341(a)(5)(i);w = average wing loading (pounds per square foot) at the particular weight under consideration. = density of air (slug/ft3);c = mean geometric chord of the wing (feet);g = acceleration due to gravity (ft/sec2);a = slope of the airplane normal force coefficient curve, CNA per radian;(2). At altitudes where VC is limited by Mach number--(i). VB may be chosen to provide an optimum margin between low and high speed buffet boundaries; and,(ii). VB need not be greater than VC.
Part 25 § 25.335 – Design airspeeds.
nVS1
w
aVUKV Crefg
s 49811
gg 3.588.0 agcw 2
(e). Design flap speeds, VF. For VF, the following apply:(1). The design flap speed for each flap position (established in accordance with § 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.
§ 25.341 -- Gust and turbulence loads.(5). The following reference gust velocities apply:(i). At the airplane design speed VC: Positive and negative gusts with reference gust velocities of 56.0 ft/sec EAS must be considered at sea level. The reference gust velocity may be reduced linearly from 56.0 ft/sec EAS at sea level to 44.0 ft/sec EAS at 15000 feet. The reference gust velocity may be further reduced linearly from 44.0 ft/sec EAS at 15000 feet to 26.0 ft/sec EAS at 50000 feet.
Part 25 § 25.335 – Design airspeeds.
VC and VD profiles
Low altitudes:
VC = 340 kcas (chosen)
VD = VC/0,8 = 425 kcas (calculated)
High altitudes:
MC = 0,85 (chosen)
MD = MC + 0,07 = 0,92 (calculated)
0
10000
20000
30000
40000
200 250 300 350 400 450
Airspeed (keas)
Alt
itu
de
(ft)
VC speed
VD speed
340 kcas
425 kcas
M = 0,85
M = 0,92
The maneuvering speed VA is the maximum airspeed at which the pilot may apply a single application of any one control surface to its maximum angle, limited by 300 lb of pilot effort, stops or blowdown. This control surface motion is intended to be in one direction only and does not include oscillatory inputs.(1). VA may not be less than where n is the limit positive maneuvering load factor at VC; and VS1 is the stalling speed with flaps retracted.(2). VA and VS1 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 CNmax curve intersects the positive maneuver load factor line, whichever is less.
Maneuvering speed VA
Variation of of CNmax with Mach number
0,4
0,5
0,6
0,7
0,8
0,9
1
1,1
1,2
1,3
0 0,2 0,4 0,6 0,8 1
Mach Number
CN
max
nVS1
maxmax
level sea
2
positive negative
7,0
2,5
ft 1,18
in 500.2
lb 000.240
:dataaircraft Example
NLN
L
aN
w
CkC
k
ddCa
c
S
W
Buffet
Limited
Flaps Up
Maximum lift capability, flaps up
0,0
0,5
1,0
1,5
2,0
2,5
0 0,2 0,4 0,6 0,8 1
Mach Number
nzW
/ (
106 l
bs)
Maneuvering speed VA
keas 287,661
35,1481
lb/ft 35,1481
4,1 ; lb/ft 216,2116
21
21
21210
max2
max
22
20
20
00
2
MMcV
CSMWn
Mq
P
MPq
Pc
McV
Vq
SqCWn
sE
Nwz
s
s
wNz
Flaps Up
Buffet Limited
Maneuvering speed VA
Flaps up stall speed
0
10000
20000
30000
40000
154,0 156,0 158,0 160,0 162,0 164,0
VS1 (keas)
Alt
itu
de
(ft
)
max1 zSA nVV
maxívelincompress1 zSA nVV
Maneuvering speed VA
0
10000
20000
30000
40000
242,0 244,0 246,0 248,0 250,0 252,0 254,0 256,0
VA (keas)
Alt
itu
de
(ft)
Design speed for maximum gust intensity VB
keas 244
52,2498
1
keas 340
keas 7,153
ft/sec 56
734,03.588.0
653,262
level) sea(at Aircraft Example
1
B
Crefggust
C
S
ref
ggg
g
Vw
aVUKn
V
V
U
K
agcw
(1). VB may not be less than where ...
(2). At altitudes where VC is limited by Mach number--(i). VB may be chosen to provide an optimum margin between low and high speed buffet boundaries; and,(ii). VB need not be greater than VC.
05
1015202530354045505560
0 5 10 15 20 25 30 35 40 45 50
Altitude (1.000 feet)
Ure
f (ft
/s e
as)
w
aVUKV Crefg
s 49811
Design cruising speed VC
0
10
20
30
40
50
0 5 10 15 20 25 30 35 40 45 50
Altitude (1.000 feet)D
V (
keas
)
keas 8,2878,43244
:level seaat
CV
(a). Design cruising speed, VC. For VC, the following apply:(1). The minimum value of VC must be sufficiently greater than VB to provide for inadvertent speed increases likely to occur as a result of severe atmospheric turbulence.(2). Except as provided in § 25.335(d)(2), VC may not be less than VB + 1.32 UREF (with UREF as specified in § 25.341(a)(5)(i)). However VC need not exceed the maximum speed in level flight at maximum continuous power for the corresponding altitude.(3). At altitudes where VD is limited by Mach number, VC may be limited to a selected Mach number.
05
1015202530354045505560
0 5 10 15 20 25 30 35 40 45 50
Altitude (1.000 feet)
Ure
f (ft
/s e
as)
Design dive speed VD
VD must be selected so that Vc/Mc is not greater than 0.8 VD/MD, or so that the minimum speed margin between VC/MC and VD/MD is the greater of the following values:
(1). From an initial condition of stabilized flight at VC/MC, the airplane is upset, flown for 20 seconds along a flight path 7.50 degrees below the initial path, and then pulled up at a load factor of 1.5g (0.5g acceleration increment). The speed increase occurring in this maneuver may be calculated if reliable or conservative aerodynamic data is issued. Power as specified in § 25.175(b)(1)(iv) is assumed until the pullup is initiated, at which time power reduction and the use of pilot controlled drag devices may be assumed;
(2). The minimum speed margin must be enough to provide for atmospheric variations (such as horizontal gusts, and penetration of jet streams and cold fronts) and for instrument errors and airframe production variations. These factors may be considered on a probability basis. The margin at altitude where MC is limited by compressibility effects must not be less than 0.07M unless a lower margin is determined using a rational analysis that includes the effects of any automatic systems. In any case, the margin may not be reduced to less than 0.05M.
Design dive speed VD
weightgross airplane thrust;engine
started isupset which from airspeed at theflight for angle stabilized ; 5,7
airspeed true
sen
analysisupset
0req
0req
WT
V
CCWTgdtdV
o
LD
deployed bemay brakes speed and reduced bemay 5,1
5,1 to1 from variessec 20
1sec 200
numberMach ,
Tn
nt
nt
CftC
StqWntC
qSCWn
z
z
z
LD
wzL
wLz
Design dive speed VD
0
10000
20000
30000
40000
200 250 300 350 400 450
Airspeed (keas)
Alt
itu
de
(ft)
VC speed
VD speed
340 kcas
425 kcas
M = 0,85
M = 0,92
Design dive speed VD
Maneuvering envelopes
Maneuvering envelope flaps up at sea level
-1,5-1
-0,50
0,51
1,52
2,53
0 100 200 300 400 500
VE (knots)
nz
incompressível compressível
VA
Maneuvering envelope flaps up at 35.000 ft
-1,5
-0,5
0,5
1,5
2,5
0 0,2 0,4 0,6 0,8 1
Mach Number
nz
MA
Design flap speeds VF
For VF, the following apply:
(1). The design flap speed for each flap position (established in accordance with § 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 VS, with the flaps in takeoff position at maximum takeoff weight;
(ii). 1.8 VS, with the flaps in approach position at maximum landing weight; and
(iii). 1.8 VS with the flaps in landing position at maximum landing weight.
VS = stall speed at the flap position, design weight and power/landing gear configurations under consideration.
Maneuvering envelopes with flaps down (FAR § 25.345)
(a). If wing flaps are to be used during takeoff, approach, or landing, at the design flap speeds established for these stages of flight under § 25.335(e) and with the wing flaps in the corresponding positions, the airplane is assumed to be subjected to symmetrical maneuvers and gusts. The resulting limit loads must correspond to the conditions determined as follows:(1). Maneuvering to a positive limit load factor of 2.0; and(2). Positive and negative gusts of 25 ft/sec EAS acting normal to the flight path in level flight. Gust loads resulting on each part of the structure must be determined by rational analysis. The analysis must take into account the unsteady aerodynamic characteristics and rigid body motions of the aircraft. The shape of the gust must be as described in § 25.341(a)(2) except that-Uds = 25 ft/sec EAS; H = 12.5 c; and c = mean geometric chord of the wing (feet).(b). The airplane must be designed for the conditions prescribed in paragraph (a) of this section, except that the airplane load factor need not exceed 1.0, taking into account, as separate conditions, the effects of-(1). Propeller slipstream corresponding to maximum continuous power at the design flap speeds VF, and with takeoff power at not less than 1.4 times the stalling speed for the particular flap position and associated maximum weight; and(2). A head-on gust of 25 feet per second velocity (EAS).(c). If flaps or other high lift devices are to be used in en route conditions, and with flaps in the appropriate position at speeds up to the flap design speed chosen for these conditions, the airplane is assumed to be subjected to symmetrical maneuvers and gusts within the range determined by-(1). Maneuvering to a positive limit load factor as prescribed in §25.337(b); and(2). The discrete vertical gust criteria in § 25.341(a).(d). The airplane must be designed for a maneuvering load factor of 1.5 g at the maximum takeoff weight with the wing-flaps and similar high lift devices in the landing configurations.
Maneuvering envelopes with flaps down (FAR § 25.345)
0,0
0,5
1,0
1,5
2,0
2,5
0 50 100 150 200 250
Airspeed (keas)
nz
0,0
0,5
1,0
1,5
2,0
0 50 100 150 200 250
Airspeed (keas)
nz
MTOW
F30
MTOW F1
MlW
F30
Landing flaps at takeoff weight
Flaps down at takeoff and landing weights
52,2
lb 000.300
2ion configurat landing F30
53,2
lb 000.250
1ion configurat landing F30
64,1
lb 000.300
ionconfigurat takeoff1F
ft 500.2
max
max
max
2
N
N
N
w
C
MTOW
C
MLW
C
MTOW
S
