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 PressureAir is very light...right?

But, it still has mass and is affected by gravity...

Because of its effects with gravity, it has weightBecause 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 SolutionsAtmospheric 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 developedThis 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 = 29.92Height 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 barometerIf 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)x1000Indicated altitude: What is indicated on your

altimeter

Examples and Sample ?’sFind PA :

Altimeter30.16

29.91

28.57

31.10

Field Elevation1,0001,850963256

PA7601,8602,313-924

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 AltitudeSDP (Standard Datum Plane) is a theoretical

pressure altitude (oft 15 degrees C)But...aircraft operate in non-standard

atmosphereSo, 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 DensitySince 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 pressureIf pressure is doubled then the density is doubledIf 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

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 denseAir that is cold with less humidity is more

dense

Tying Everything TogetherPressure, 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 AltitudeWrite down current altimeter setting and indicated altitudeFind 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 questionFind the difference between these twoAdjust the altitude by 120 ft for every 1 degree C deviation from ISA

Standard tempAdd altitude if temp is more than ISA or subtract for a temp less than

ISA -OR-Use the graph

The Real DA Equation

Now you try...Altimeter Field Elevation Temp30.18 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, humidityLow pressures

Low density altitude refers to dense airLow altitudes, temperatures, humidityHigh 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.