Chapter 3b: Effects of Weather on Small Unmanned Aircraft Performance
What can wind shears cause?
-Dangerous -It can cause violent updrafts and downdrafts, as well as abrupt changes to the horizontal movement of the aircraft
How is air density affected?
Air density is affected by changes in altitude, temperature, and humidity.
If weight is added to an aircraft, what must be done for it to fly correctly?
it must fly at a higher angle of attack (AOA) to maintain a given altitude and speed.
as the water content of the air increases, the air becomes ____ dense, ______ density altitude and ______ performance.
less, increasing, decreasing
Warm air can hold _____ water vapor, while colder air can hold _____.
more, less.
What is temperature inversion?
-A temperature inversion is when the air at a higher altitude is warmer than the air at a lower altitude -usually shallow and found close to the ground -the layer of air at the top acts as a lid, keeping pollutants below it
Unstable air masses include what 4 symptoms?
-Cumuliform clouds -Showery perspiration -Rough air (turbulence) -Good visibility (except in blowing obstructions)
What is air density?
-Density altitude is the altitude relative to the standard atmosphere conditions at which the air density would be equal to the indicated air density at the place of observation. -In other words, density altitude is air density given as a height above mean sea level.
The cumulus stage of a thunderstorm:
-First stage -Rain not falling, but present -Moist air rises -Growth rate of the cloud may exceed 3,000 feet per minute -Inadvisable to fly in these clouds since they grow rapidly
Standing lenticular altocumulus clouds
-Have little movement -Smooth polished edges -Wind can be quite strong around these clouds -Presence of these clouds means strong turbulence is present
What are the effects of temperature on density?
-Increasing the temperature of a substance decreases its density. Conversely, decreasing the temperature increases the density. -The density of air varies inversely with temperature.
The mature stage of a thunderstorm:
-Second stage -Precipitation beginning to fall is a sign the mature stage has begun -Downdraft has developed -Strong updrafts occur (from moist air rising) -Rapid rise in pressure occurs -Wind gusts occur -Temperature drops -This stage is where the greatest intensity of a storm is -Very turbulent
Stable air masses include what 4 symptoms?
-Stratiform (layers of) clouds and fog -Continuous perspiration -Smooth air -Fair to poor visibility in smoke or haze
The dissipating stage of a thunderstorm:
-Third and last stage -Storm dies rapidly -Once rain has ended and downdrafts have stopped, the dissipating stage is complete
What are the effects of humidity on density?
-Water vapor is lighter than air -Water vapor is always in the air -Therefore, the more moist air is, the lighter the air -As the water content of the air increases, the air becomes less dense, increasing density altitude and decreasing performance.
What are the effects of pressure on density?
-When air is compressed, a greater amount of air can occupy a given volume. Conversely, when pressure on a given volume of air is decreased, the air expands and occupies a greater space. -Density is directly proportional to pressure
What is wind shear?
-Wind shear is a sudden, drastic change in wind speed and/or direction over a very small area. -can affect flight at any altitude -silent danger to aviation
What is a microbust?
-a sudden, powerful, localized air current, especially a downdraft. -this is the most severe type of wind shear -About a 1-2 mile diameter with a depth of 1,000 ft -Life span: 5-15 minutes -can produce downdrafts of up to 6,000 feet per minute (fpm) and headwind losses of 30-90 knots, seriously degrading performance. -can also produce strong turbulence and hazardous wind direction changes.
The primary factors most affected by performance are...
-the takeoff and landing distance -rate of climb -ceiling -payload -range -speed -maneuverability -stability -fuel economy.
Heating of the air near the Earth's surface creates...
...an air mass thunderstorm
When planning a flight over mountainous terrain, gather as much preflight information as possible on...
...cloud reports, wind direction, wind speed, and stability of air.
Standing lenticular altocumulus clouds are formed on the...
...crests of waves created by barriers in the wind flow.
The most turbulent clouds are...
...cumulonimbus clouds
Increasing the temperature of a substance...
...decreases its density.
The conditions that result in a high density altitude are...
...high elevations, low atmospheric pressures, high temperatures, and high humidity.
Decreasing the temperature of a substance...
...increases its density.
The conditions that result in a low density altitude are...
...lower elevations, high atmospheric pressure, low temperatures, and low humidity.
When the temperature of the air is reduced to the dew point...
...the air is completely saturated and moisture begins to condense out of the air in the form of fog, dew, frost, clouds, rain, or snow.
Overcast (OVC) means...
...the entire sky is covered with clouds.
The dew point, given in degrees, is...
...the temperature at which the air can hold no more moisture.
Cool, dry air is... (in relation to vertical movement and stability)
...very stable and resists vertical movement, which leads to good and generally clear weather.
A typical microburst has a horizontal diameter of ___ miles and a nominal depth of _____ feet.
1-2, 1,000
What 2 factors must be present for structural icing to form?
1. Rain/snow must be able to be seen and present 2. The temperature at where the water strikes the aircraft must be below 0 degrees Celcius or colder
A thunderstorm cell during its life cycle progresses through three stages...
1. the cumulus 2. the mature 3. the dissipating
What are the four methods that the air can reach the saturation point:
1. when warm air moves over a cold surface, the air temperature drops and reaches the saturation point. 2. the saturation point may be reached when cold air and warm air mix. 3. when air cools at night through contact with the cooler ground, air reaches its saturation point. 4. occurs when air is lifted or is forced upward in the atmosphere.
Each station converts its barometric pressure by adding approximately ___ inHg for every _____feet of elevation.
1;1,000
Wind at mountain top level in excess of ____ knots suggests some turbulence.
25
A station at 5,000 feet above sea level, with a reading of 24.92 inHg on a barometer, reports a sea level pressure reading of...
29.92 inHg
If a small UA enters a thunderstorm, the small UA could experience updrafts and downdrafts that exceed ______ fpm
3,000
Wind in excess of ____ knots across a mountain barrier dictates caution.
40
What is the life span of a microbust?
5-15 minutes
can produce downdrafts of up to ______ feet per minute (fpm) and headwind losses of ________knots, seriously degrading performance.
6,000; 30-90
AOA...
Angle of attack
What does it mean when the air reaches its maximum saturation?
It is completely full of humidity and can not absorb any more water. Precipitation is near.
In a thunderstorm, is it possible to detect the transition from one stage of its life to the next?
It is impossible since they are subtle and not abrupt.
When air is decompressed, can more or less of it be in the atmosphere? Explain
Less. When pressure on a given volume of air is decreased, the air expands and occupies a greater space.
Is moist air lighter or heavier compared to dry air?
Lighter. Water vapor is lighter than air, so combined with air it makes it lighter than completely dry air (which never actually happens).
What is the most severe type of wind shear?
Microbust
When air is compressed, can more or less of it be in the atmosphere? Explain
More. When air is compressed, a greater amount of air can occupy a given volume.
Is air every completely dry?
Never
Humidity alone is usually ____ considered an essential factor in calculating density altitude and aircraft performance; however...
Not, it does contribute.
How does the wind on one side of a mountain compare to the other side of the mountain?
On one side of the mountain the wind can be smooth and gently help the air craft up the mountain. However, on the other side the wind can be very turbulent and will affect the flight of the air craft.
What is performance in aviation?
Performance is a term used to describe the ability of an aircraft to accomplish certain things that make it useful for certain purposes.
Stratified clouds mean what type of air is present? (unstable/stable)
Stable
With a barometer, what is standard sea level pressure defined as?
Standard sea level pressure is defined as 29.92 inHg and a standard temperature of 59 °F.
When do surface based temperature inversions occur?
Surface-based temperature inversions occur on clear, cool nights when the air close to the ground is cooled by the lowering temperature of the ground.
Ceiling (aviation)...
The CEILING is the height above ground or water of the lowest layer of clouds and is reported as BKN (broken) or OVC (overcast)
What determines the stability of the air and resulting weather?
The combination of moisture and temperature determine the stability of the air and the resulting weather.
What are the conditions of the worst atmospheric stability? When are they usually found?
The greatest instability occurs when the air is moist and warm, as it is in the tropical regions in the summer.
What is a front? (weather)
The zone between 2 different air masses is a frontal zone or front. Across this zone, temperature, humidity, and wind often change rapidly over short distances.
How can mountains, hangers, towers, or large manmade structures affect wind flow?
They can break up the flow of the wind and create wind gusts that change rapidly in direction and speed.
Visibility (aviation)...
Visibility refers to the greatest horizontal distance at which prominent objects can be viewed with the naked eye.
______ has a very pronounced effect on aircraft performance.
Weight
When is air the lightest or least dense?
When the the maximum amount of water vapor is in the air
A decrease in air density means...
a high density altitude
An increase in air density means...
a lower density altitude
Density altitude has a direct effect on...
aircraft performance
As the density of the air decreases...
aircraft performance decreases
As the density of the air increases...
aircraft performance increases
To pilots, the ______________ cloud is perhaps the most dangerous cloud type.
cumulonimbus
Rising air currents cause...
cumulonimbus clouds
Pilots can expect the density to ________ with altitude.
decrease
In temperature inversion, as air rises and expands in the atmosphere, the temperature _________.
decreases
low density altitude refers to _____ air.
dense
Cumulonimbus cloud appear in...
groups or alone
An increase in altitude also ________ the power required and _________ the power available.
increases, decreases
The density of air varies __________ with temperature. When is the only time this is true?
inversely. This statement is true only at a constant pressure.
The characteristics of the atmosphere have a major effect on...
performance
Density is directly _____________ to pressure
proportional
On a barometer, tracking a pattern of falling pressure at a single weather station generally indicates...
the approach of bad weather
On a barometer, tracking a pattern of rising pressure at a single weather station generally indicates...
the approach of good weather
In relation to the other side of a windy mountain (where wind breaks up), the stronger the wind, the greater...
the downward pressure and turbulence become.
The intensity of the turbulence associated with ground obstructions depends on...
the size of the obstacle and the primary velocity of the wind.
High density altitude refers to _____ air.
thin
The stability of the atmosphere depends on its ability to resist ______ ______.
vertical motion
What effects climb performance?
weight, altitude and configuration changes
Always be alert to the possibility of _____ _____, especially when flying in and around thunderstorms and frontal systems.
wind shear
