Factors Affecting Helicopter Performance

1. HFH Ch. 7, PHAK Ch. 10, POH Sec. 5

2. Factors Affecting Performance
 There are three major factors that affect
performance
 Density Altitude
 Weight
 Wind

3. Density Altitude (DA)
 The altitude in the standard atmosphere
corresponding to a particular value of air
density
 High DA=low air density
 DA is the pressure altitude (PA) corrected
for non-standard temperature
 In order for us to understand DA we have
to understand PA

4. Factors Affecting DA
 Atmospheric pressure
 Elevation
 Temperature
 Moisture

5. Pressure Altitude
 Is the altitude, in feet,
above the standard datum
plane (29.92” hg)
 Datum (just like in W&B) is
an imaginary plane used for
measurement
 It makes allowances for
sea level pressure being
different than standard**

6. Atmospheric Pressure
 What is atmospheric pressure?
 Though air is very light, it has
mass and is affected by Earth’s
gravitational pull
 Therefore it has weight; because it
has weight, it has force
 Since it is a fluid substance, this
force is exerted equally in all
directions, and its effect on bodies
within the air is called pressure

7. Measuring AP
 To measure
atmospheric
pressure we use a
barometer
 Similar to a
thermometer is to
temperature
 Measures the weight
of the overlying air
column

8. Barometer

9. Atmospheric Pressure
 The pressure of the
atmosphere varies with time
and altitude
 Primarily caused by changes
in weather and uneven
heating of the Earth’s surface
 Due to the constantly
changing atmospheric
pressure
 A standard reference was
developed (ISA)
 Helped with consistent
performance planning

10. International Standard Atmosphere (ISA)
 Due to these changing atmospheric
pressures, a standard reference
was developed
 It was developed by International
Civil Aviation Organization (ICAO)
 It is often referred to as ISA or ICAO
standard atmosphere
 Any temperature or pressure that
differs from the standard lapse rate
is considered “non-standard”

11. ISA
 The standard atmosphere at sea level is a surface
temperature of 15 C and a surface pressure of 29.92
inches of mercury (hg)
 A standard temperature lapse rate is one in which
the temp decreases at the rate of approx. 2 C per
thousand feet up to 36,000 feet
 A standard pressure lapse rate is one in which the
pressure decreases at the rate of approx. 1” hg per
1,000’

12. Pressure Altitude Overview
 Is the altitude, in feet, above the
standard datum plane of 29.92” hg
 ISA
 It makes allowances for sea level
pressure being different than standard
 Let’s look at an example of this…

13. 1,000’
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Sea Level Pressure: 29.92” hg
28.92” hg
Example #1
Standard day
SLP: 29.92” hg
Altimeter setting: 29.92
Indicated altitude: 0’
Pressure altitude: 0’
Altimeter setting: 29.92
Indicated altitude: 1,000’
Pressure altitude: 1,000’

14. 1,000’
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Sea Level Pressure: 30.12” hg
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
29.92
Example #2
Non-standard day
SLP: 30.12
Altimeter setting: 30.12
Indicated altitude: 0’
Altimeter setting: 30.12
Indicated altitude: 1,000’
Pressure altitude: ?
Pressure altitude- the
altitude, in feet
above the standard
datum plane (29.92)
29.12” hg
1” hg=1,000’
29.92”-30.12”=-.20”
-.20”x1000=-200’
1000’-200’=800’ PA
800’
It makes allowances
for sea level pressure
being different than
standard

15. Four Factors Affecting DA
 Atmospheric pressure
 Elevation
 Temperature
 Moisture

16. Elevation/Altitude
 Air becomes thinner or
less dense with
increases of altitude or
elevation
 Less of the overlying air
column weight is above
you
○ Loaf of bread

17. Four Factors Affecting DA
 Atmospheric pressure
 Elevation
 Temperature
 Moisture

18. Temperature
 As warm air expands, air molecules move
farther apart creating less dense air

19. Four Factors Affecting DA
 Atmospheric pressure
 Elevation
 Temperature
 Moisture

20. Moisture Content
 Water vapor weighs less
than dry air
 Takes the place of air
molecules making the air less
dense
 Earth is composed of
Nitrogen 78%, oxygen 21%
and other 1%
 Nitrogen (1 gal) = 11.1
pounds
 Oxygen (1 gal) = 9.5 pounds
 H2O (1 gal) = 8 pounds

21. Calculating DA
 Tools:
 CX2
 E6B
 DA chart (Section 5
POH)
 Math
○ DA=PA+120(OAT-ISA)
 PA=Elev.+1,000(29.92-
current altimeter)

22. Weight
 Lift is the force that
opposes weight
 As weight is
increased, the
power required to
produce lift needed
to compensate for
the added weight
must also increase

23. Wind
 Translational lift occurs whenever there is
airflow across the rotor disc, wind or
forward movement
 Headwinds are best for performance
○ Takeoff
○ Landing
○ Hovering
 Crosswind
○ At low airspeeds requires more torque from the
engine
 Tail winds
○ Bad for takeoff landing and hovering
○ Takes away power from main rotor
○ Beneficial when we are flying cross country
 Faster ground speed