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drag is composed of form drag and skin friction.
Form drag results from the turbulent wake caused by
the separation of airflow from the surface of a structure.
The amount of drag is related to both the size and
shape of the structure that protrudes into the relative
wind. [Figure 2-12]
Skin friction is caused by surface roughness. Even
though the surface appears smooth, it may be quite
rough when viewed under a microscope. Athin layer of
air clings to the rough surface and creates small eddies
that contribute to drag.
INDUCED DRAG
Induced drag is generated by the airflow circulation
around the rotor blade as it creates lift. The high-pressure
area beneath the blade joins the low-pressure air
above the blade at the trailing edge and at the rotor tips.
This causes a spiral, or vortex, which trails behind each
blade whenever lift is being produced. These vortices
deflect the airstream downward in the vicinity of the
blade, creating an increase in downwash. Therefore,
the blade operates in an average relative wind that is
inclined downward and rearward near the blade.
Because the lift produced by the blade is perpendicular
Aircraft Yaw—The movement of
the helicopter about its vertical
axis.
THRUST
Thrust, like lift, is generated by the rotation of the
main rotor system. In a helicopter, thrust can be forward,
rearward, sideward, or vertical. The resultant of
lift and thrust determines the direction of movement of
the helicopter.
The solidity ratio is the ratio of the total rotor blade
area, which is the combined area of all the main rotor
blades, to the total rotor disc area. This ratio provides a
means to measure the potential for a rotor system to
provide thrust.
The tail rotor also produces thrust. The amount of
thrust is variable through the use of the antitorque pedals
and is used to control the helicopter’s yaw.
Figure 2-11. The load factor diagram allows you to calculate
the amount of “G” loading exerted with various angle of
bank.
Load Factor - "G's"
Bank Angle (in Degrees)
0 10 20 30 40 50 60 70 80 90
9
8
7
6
5
4
3
2
1
0
Figure 2-12. It is easy to visualize the creation of form drag by examining the airflow around a flat plate. Streamlining decreases
form drag by reducing the airflow separation.
2-6
to the relative wind, the lift is inclined aft by the same
amount. The component of lift that is acting in a rearward
direction is induced drag. [Figure 2-13]
As the air pressure differential increases with an
increase in angle of attack, stronger vortices form, and
induced drag increases. Since the blade’s angle of
attack is usually lower at higher airspeeds, and higher
at low speeds, induced drag decreases as airspeed
increases and increases as airspeed decreases. Induced
drag is the major cause of drag at lower airspeeds.
PARASITE DRAG
Parasite drag is present any time the helicopter is moving
through the air. This type of drag increases with airspeed.
Nonlifting components of the helicopter, such as the
cabin, rotor mast, tail, and landing gear, contribute to parasite
drag. Any loss of momentum by the airstream, due
to such things as openings for engine cooling, creates
additional parasite drag. Because of its rapid increase
with increasing airspeed, parasite drag is the major cause
of drag at higher airspeeds. Parasite drag varies with the
square of the velocity. Doubling the airspeed increases
the parasite drag four times.
TOTAL DRAG
Total drag for a helicopter is the sum of all three drag
forces. [Figure 2-14] As airspeed increases, parasite
drag increases, while induced drag decreases. Profile
drag remains relatively constant throughout the speed
range with some increase at higher airspeeds.
Combining all drag forces results in a total drag curve.
The low point on the total drag curve shows the airspeed
at which drag is minimized. This is the point
where the lift-to-drag ratio is greatest and is referred to
as L/Dmax. At this speed, the total lift capacity of the
helicopter, when compared to the total drag of the helicopter,
is most favorable. This is important in helicopter
performance.
Figure 2-14. The total drag curve represents the combined
forces of parasite, profile, and induced drag; and is plotted
against airspeed.
0 25  50  75 100 125 150
Speed Drag
Parasite

Drag

 

Profile

Drag
Induced

Drag
Total Drag
Minimum
　
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