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Atmosphere. Gases and liquids flow freely. Both are fluids Just like liquids, gases have pressure as well and is measured in PSI. Atmosphere. Atmospheric pressure: Live in it, don’t feel it Divided into zones Exosphere – 600 miles & up. Temps to –4040 F on sun-side of object.

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Atmosphere

  • Gases and liquids flow freely. Both are fluids

  • Just like liquids, gases have pressure as well and is measured in PSI


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Atmosphere

  • Atmospheric pressure: Live in it, don’t feel it

  • Divided into zones

  • Exosphere – 600 miles & up. Temps to –4040 F on sun-side of object.

  • Ionosphere – 50-600 miles. Ions and free electrons present. Aurora Borealis


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Atmosphere

  • Stratosphere – 10 to 50 or 60 miles. Ozone layer 12 to 30 miles up. At 63000 ft blood boils at 98.6 F due to lower surrounding pressure. 50 miles: fry on one side and freeze on the other without protective suit.

  • Troposphere – surface to 4 miles at poles, to 11 miles at equator.


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Troposphere

  • Temp from 7 to 20 miles constant –70 F (or –55 C).

  • 50% of air by weight below 1800 ft (3.5 mi)

  • 78% Nitrogen

  • 21% Oxygen

  • 0.9% other gases (neon, argon, krypton, etc)

  • 0.1% water vapor and carbon dioxide


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Troposphere

  • Of primary interest because most of weather occurs here.

  • Clouds, wind, vertical air currents, storms, fog, rain, snow, temp changes.

  • Most pilots fly in this region


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Atmospheric Pressure

  • Column of air 1 sq in at base from the surface to top of ionosphere weighs 14.7 lbs

  • As altitude increases, pressure increases

  • Increase of 1 PSI per 2343 ft or 1 in of mercury per 1000 ft


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Absolute pressure vs Gage pressure

  • Gage pressure registers 0 PSI at surface.

  • Gage pressure is a relative scale.

  • PSIA = PSIG + 14.7

  • PSIG = PSIA –14.7

  • 29.92” Hg = 14.7 PSIA

  • Convert to Hg: PSIA x 2.03

  • Convert to PSIA: Hg/2.03


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Instruments that use pressure

  • Gage Pressure: Engine instruments: oil pressure, fuel pressure, hydraulic pressure, manifold pressure

  • Difference pressure: Airspeed indicator and some stall warning systems

  • Aneroid Barometer – Altimeter

    • Aneroid = “without liquid”

    • Sealed, corrugated box with most of air removed.

    • Variation in air densisty causes box to move through system of levers and pointers


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Instruments that use pressure

  • Aircraft use altimeters.

  • Only problem: Barometric pressure may be different at landing area. Pilot needs current barometric pressure to adjust

  • Cabin Pressure expressed in terms of equivalent altitude above sea level

    • Cabin pressure of 6000ft means pressure inside same as atmospheric pressure at altitude of 6000ft

    • 8000ft passengers +crew can ride in relative comfort without any special oxygen supply.


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Advantage to flying at high altitude

  • At 8000ft, air pressure is 10.92 PSI

  • Suppose we fly at 40000 ft where pressure is 2.73 psi

  • Difference in pressure = 8.19 psi

  • Lear 24D, pressurized area is 45000 sq in

  • Bursting force = A x pressure=368550#

  • Safety factor 1.33 - 368550 x 1.33=490172# = 245 tons

  • Aircraft must be constructed with ultimate strength of 245 tons


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Standard Atmosphere

  • If performance of aircraft is completed through a flight test or wind tunnel test, a standard reference condition must be set first.

  • Standard Atmosphere = 40 deg. Lat and sea level

  • P = 29.92” hg (76 cm Hg), T=59 F (15 C) and g=32.174 ft/sec/sec (gravity)


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Standard Atmosphere

  • Temp and Pressure decrease with altitude

  • Would appear density of atmosphere would remain same or fairly constant with increase of altitude – NOT TRUE

  • Pressure drops more rapidly than temp

  • Results in density decrease with increase altitude

  • Moisture in air also affects density

  • This moisture is called HUMIDITY.


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Humidity

  • Two forms

  • Absolute Humidity is the actual amount of water vapor in a mixture of air and water.

  • Relative Humidity is the ratio of the amount of water present in atmosphere to the amount that would be present if the air were saturated

  • Temp drop and absolute humidity remains constant, relative humidity increases. Less water vapor is required to saturate the air at lower temp..


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Humidity

  • Dew Point is the temp to which humid air is cooled to become saturated. If temp drops below dew point, condensation occurs.

  • Humid air is less dense than dry air. Take off performance is reduced since engine output is reduced.

  • Less air in fuel/air mixture results in an excessively rich mixture.


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Bernoulli’s Principle

  • Originally stated to explain the action of a liquid flowing through the varying cross-sectional areas of tubes.

  • Works with air since air is a liquid.

  • “When the speed of a fluid increases, pressure in the fluid decreases.”

  • Hold a sheet of paper in front of mouth and blow across the top surface, the paper rises.


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Bernoulli’s Principle

  • Used to make a wing provide lift.

  • Relative wind: direction of wind with respect to wing and is opposite to path of flight.

  • Angle of attack: angle between relative wind and chord

  • Critical angle of attack: air flow from the wing separates


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Air Flow

  • Formula for lift and drag:

  • s= wing area of both wings- one surface only

  • C is the coefficient of lift. Depends on wing shape and angle of attack.


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Air Flow

  • Drag Equation:

  • Airfoils have a NACA xxxxx Number (National Advisory Committee for Aeronautics)

  • Example: Aircraft with 600 sq ft of wing surface, flying at altitude of 10000 ft with an angle of attack of 6 degrees and airspeed of 286 mph.

  • What is the lift?


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Aircraft in a Banked Turn

  • What happens to lift in a turn?

  • To overcome the the loss of straight level lift in a turn, a pilot must increase airspeed or increase elevator back press. Otherwise, the aircraft will lose altitude.