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