Transcript Principles of Flight - Centennial Aviation Academy

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
30.16
29.91
28.57
31.10
Field Elevation
1,000
1,850
963
256
PA
760
1,860
2,313
-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 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
 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
-ORUse the graph
The Real DA Equation
Now you try...
Altimeter
30.18
Field Elevation
1069
Temp
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_densityaltitu
de
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.