Helicopter Aerodynamic(part-1)
INTRODUCTION
Helicopter aerodynamic is branch of physics that deal with forces
and pressure exerted by air in motion
In order to fully understand the relation between forces and
pressure we will take quick look at propriety of atmosphere and how it affects
helicopter engine and rotor performance
.
· The
atmosphere
The atmosphere is mass of air that envelope the envelopes
the earth .it is composed of different layer
Figure1, Atmospheric Structure.
For
the purpose of our study we will focus just on troposphere
· TROPOSPHERE
Layer adjacent to the earth’s surface.
Large amounts of moisture and condensation.
Nearly all weather occurs there. It
extends from surface to28,000 feet over poles and 55,000 feet over equator.
Temperature normally decreases with altitude (lapse rate).
This mass of air consists of two types of
constituents;
1. Non-varying
2. Varying
For a volume of air the non-varying
constituents includes
v Nitrogen
v Oxygen
Figure2; troposphere constitution.
The
remaining one percent consists of varying of varying constituents of carbon
dioxide, helium, hydrogen, and water vapor.
- · ATMOSPHERIC PRESSURE
The atmospheric pressure at any altitude is the result of the
downward pressure exerted by the mass of particles above that altitude.
Therefore, the air on the earth surface is exerted by the greatest pressure
than the air at certain altitude.
·Pressure altitude;
The pressure altitude is static air pressure at certain
altitude in standard atmosphere (29.92 IN HG/15 C°).
On standard day the temperature is 15C°(59F) and the pressure is
Figure3; atmospheric pressure on
standard day.
- THE STANDARD ADIABATIQUE LASPE RATE
It is the cooling rate which corresponds
to 2 degree Celsius decrease in each thousand feet of altitude increase.
· Atmosphere density
It is greatest at earth surface and
decrease as altitude increase
Air is affected by multiple factor like
temperature, pressure and humidity.
There is reciprocal relationship between
temperature and humidity
Starting with temperature, when a mass of
air is heated molecule distance increase thus density decrease, the opposite is
true.
Humidity increase cause decrease in air
density cause water molecules occupy
More volume when humidity decreases
density increase
Figure4;effect of temperature on air
density Figure5;relationship between temperature and
humidity
Figure6; effect of humidity on air
density
·
Air density and helicopter aerodynamic
Air density affects the aerodynamic forces on the rotor blade as
well as engine performance therefore affecting power required and power
available.
It is important for helicopter pilot to understand the difference
between air density and density altitude (DA) which is the altitude in standard
atmosphere corresponding to a particular air density; Making correction for
temperature and humidity to pressure altitude we can derive density altitude.
There is direct relationship between density altitude and air
density .as one increase the other automatically decrease
· Determine
density altitude
There are two way to determine density altitude
1. Rules of thump
2. Using density altitude diagram
From the first rule there is mathematic formula we can use to
determine the density altitude
DA=PA+Ct+Ch
PA: pressure altitude
Ct: Temperature correction
[indicated outside air temperature at altitude-standard temperature at
altitude]*120.
Ch: HUMIDITY
CORRECTION=relative humidity/10%*100.
Example:
Determine the density altitude:
Pa=4000ft
RH=70%
AOT (indicated outside temp) =17c°
Solution
Knowing that standard temperature is
15 c° and as we climb 1000 feet temperature decrease at rate of 2C° so the
standard temperate at 4000feet is
[15-(4000*2/1000)]=7C°
Ct=17c°-[15-(4000*2/1000)]*120=1200
Rc=RH/10%*100=70/10*100=700
DA=PA+Ct+Ch=4000+1200+700=5700FT
The second method is
way to easier
Exemple
PA=5700
ft
AOT=15 F°/-10C°
DA=4000
ft
Figure7;desity alt diagram temperature in Fahrenheit Figure8; density altitude diagram temperature
in Celsius
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