曝光台 注意防骗
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f V, = 0.1733
50,000
t, = ~9 8 ) 36~49~ tan-1(0.1733)
- 23.9681s
155
156 PERFORMANCE, STABILITY, DYNAMICS, AND CONTROL
We have
. T-D
sin y : -- at V
W
- Vl - 0.2472
y - 14.3153 deg
The airborne distance S2 and time t2 are given by
15
S2 = tTrri y
- 58.7863 m
S2
t2 - VI osy
- 1.0617 s
Total takeoff distance -. Sl + S2 - 742.8928 m, and the total takeoff time -
tI + t2 = 25.0298 s.
2.10 Hazards During Takeoft and Landing:
Windshear and Microburst
Windsheat*,5 is the changein wind direction and/or speed in a very short distance
with respect to the ground. Whereas a change in the horizontal wind velocit3r
affects the airspeed, the vertical component of windshear affects the flight path
angle. Windshears are normally caused by thunderstorm activity, weather fronts,
and jet streams. While windshear can occur at any altitude, it is most hazardous if
encountered during landing and takeoff. The windshear occurring at low altitudes
close to the ground is called low-level windshear.
Another dangerous form of windshear, which has been recogru;red in recent
years, is the nucroburst. A rnicroburst is formed when a column of air at high
altitudes quickly cools because of evaporation of ice, snow, or rain and, becoming
denser than the surrounding air, falls rapidly to the ground. On hitting the ground,
this mass of air spreads radially outward in all d:irections. A core of such a down-
burst can be of about 2-3 km in width, and the winds generated can be as high as
150-200 km/h. The life of a microburst is typically 5-15 min.
A pilot flying through a microburst will experience within a minute or less,
a bewildering combination of events during which he will encounter headwind,
downburst, and tailwind as shown schematically in Fig. 2.38. The headwind will
cause the air speed to rise and make the aircraft start climbing. The pilot,ifunaware
of what may happen next,is likely to throttle back the engine and reduce power to
maintain his original flight path angle. However, the headwind quickly disappears
and is followed by the down draft, which willinduce a high rate of descent. At
this point, the pilot will attempt to restore the full thrust by opening the throttle.
While the engine takes some time to react, the pilot encounters strong tailwind that
AIRCRAFT PERFORMANCE
Fig.238 Schemahc diagr.am of microburst during takeoiWlanding.6
157
reduces air speed further with a potential risk of an imminent stall. Depending on
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