Principles of Flight

References Used Pilots Handbook of Aeronautical Knowledge (PHAK) FAA-H-8083-25A, Chapter 3 Available Online for Free at: http://www.faa.gov/library/manuals/aviation/pilot_handbook/

Atmosphere.... Brain teaser: What percentage of oxygen is found at 0 ft? What percentage of oxygen is found at 20,000 ft?

Atmospheric Pressure Air is very light...right? But, it still has mass and is affected by gravity... Because of its effects with gravity, it has weight Because it has weight, it exerts a pressure So where are we going with all this???

Atmospheric Pressure Cont’d. On the ground air has an average pressure of 14.7 PSI or 1013.2mb Air has a limited thickness right? So.... The higher the altitude, the less air there is above Which means...that at 18,000 ft. the pressure is about half what it is at sea level. Cool huh?

Problems...and Solutions Atmospheric pressure varies with time and location... Well...we need a standard unit of measure so that all airplanes worldwide are “on the same page” The standard atmospheric pressure was developed This means... Temperature of 59 ° F or 15 ° C & A surface pressure of 29.92 inHg or 1013.2 mb

Standard Lapse Rate -3.5 ° F or -2° C /thousand feet (up to 36,000ft) From 36,000-80,000 the temperature is said to be constant at about -65 degrees F or -55 degrees C Standard lapse rate is also when pressure decreases about 1 inHg/ 1000 ft (up to 10,000 ft.)

Pressure Altitude = 29.92 Height above a Standard Datum Plane (SDP), which is a theoretical level where the weight of the atmosphere is 29.92 inHg or 1013.2 mb. as measured by a barometer (An instrument measuring atmospheric pressure) Altimeter in essentially a sensitive barometer If you dial in “29.92” on your altimeter Then.... Your altimeter will indicate your PRESSURE ALTITUDE Why is this important? It will allow you to determine how the airplane will perform And it will be used to use to assign altitudes above 18,000

How to find Pressure Altitude? Dial in 29.92 into your Kollsman Window on your altimeter -or- PA=(29.92-ambient pressure)x1000 Indicated altitude: What is indicated on your altimeter

Examples and Sample ?’s Find PA : Altimeter Field Elevation PA 30.16 1,000 760 29.91 1,850 1,860 28.57 963 2,313 31.10 256 -924

What is density? Density= Mass / Volume What is Mass? The amount of material in an object What is Volume? It is the amount of space enclosed by a shape or object, how much 3-dimensional space (length, width, and height) it occupies.

Density Altitude SDP (Standard Datum Plane) is a theoretical pressure altitude (oft 15 degrees C) But...aircraft operate in non-standard atmosphere So, the term Density Altitude is used for correlating aerodynamic performance in the non-standard atmosphere Density Altitude: Pressure Altitude corrected for Non-Standard Temp

Effect of Pressure on Density Since air is a gas...it can be compressed or expanded When air is compressed (increased pressure) you can have more air in a space-like inflating a tire At a lower pressure, there is less mass of air So we can say that density is directly related to pressure If pressure is doubled then the density is doubled If the pressure is lowered then the density is lowered *All this is assuming a constant temperature

Effect of Temp. on Density + Temperature = - Density - Temperature = + Density Now, as you climb the pressure and temperature both generally decrease, thus having a conflicting effect on density. BUT... The deceasing pressure usually wins the battle and as you go higher, density decreases *This is only true with constant pressure

Effect of Humidity (Moisture) on Density Everything before assumed air was dry...but it’s not! Water vapor is lighter than air.... So that means that air with high humidity is lighter (less dense) As moisture increases, the air becomes less dense As moisture decreases, the air becomes more dense

Brain Teaser: Can air have a RH of 100%? Relative Humidity is the amount of water vapor contained in the atmosphere Brain Teaser: Can air have a RH of 100%? Warmer air can hold more water , colder air holds less So that means... Warm air with high humidity = less dense Air that is cold with less humidity is more dense

Tying Everything Together Pressure, Temperature and Humidity have a huge effect on aircraft performance because of their effect on density!!! Why? Because the wing and engine all depend on density to operate... The less dense the air is then the less molecules of air there are for the wing and engines to “grab” onto

Why is it important to know density? The HIGHER you go, the less dense the air is (less packed molecules) this is HIGH DENSITY ALTITUDE The LOWER you go, the more dense the air is (tighter packed molecules) this is LOW DENSITY ALTITUDE The less the density, the greater reduction in: Power because engine takes in air Thrust, because a propeller is less effective in thin air Lift, because thin air exerts less force on airfoils

Finding Density Altitude Write down current altimeter setting and indicated altitude Find your Pressure Altitude (29.92 on altimeter) Find your Temp (Celsius) Using your E6B, line up your temp with your press alt. (in decimal form using inner-inner scale) and read density altitude off arrow -OR- DA=PA+/-(120Vt) (THIS EQUATION IS AN APPROXIMATION ONLY) DA=PA +(120 *Temp Deviation from ISA) Find the ISA standard temp for your altitude (Use the Graph on Slide 24 and interpolate the altitudes/temperatures to find ISA for your altitude) Find the temp at the altitude in question Find the difference between these two Adjust the altitude by 120 ft for every 1 degree C deviation from ISA Standard temp Add altitude if temp is more than ISA or subtract for a temp less than ISA Use the graph

The Real DA Equation

Now you try... Altimeter Field Elevation Temp 30.18 1069 25 28.56 10,003 -2 29.92 0 15 31.36 -279 12 30.00 551 36

Website to help.... http://www.srh.noaa.gov/epz/?n=wxcalc_densityaltitude

More about DA... Regardless of you’re actual altitude, the airplane will fly as if it were at an altitude equal to the density altitude Air density is affected by changes in temp, altitude and humidity High density altitude refers to thin air. High altitudes, temperatures, humidity Low pressures Low density altitude refers to dense air Low altitudes, temperatures, humidity High pressures

Just a note.... Pressure, temperature, and humidity have a great influence on aircraft performance because of their effect upon density. There are no rules of thumb that can be easily conveyed but the affect of humidity can be determined using online formulas.