1 Airfoil
An airfoil can be defined as 'any shape designed to obtain a useful reaction from
the air through which it moves'. In helicopters, the rotors are the airfoils which
provide lift and propulsion. A helicopter's rotor functions in exactly the same
way as an airplane's
wing, although the shape of the former differs from that of
the latter in that it is much thinner as compared
to its length. Another difference is that the airflow through which the rotor moves is mainly
generated by rotation. This means that the rotor (functioning as an airfoil) can
generate lift even when
the aircraft is not moving. It is this mechanism that gives
the helicopter its much appreciated V.T.O.L. (Vertical Take Off and Landing) capability.
The airflow in which the rotor blade functions travels towards the blade with speed
'v'. Note that with an airplane, the airflow exists because the plane
flies forwards,
whereas with the helicopter it is the rotor which travels at
a certain speed through the air. The rotor speed is much higher than the
airspeed of the helicopter and
this is why it can still operate
in strong winds. Although there is no principal difference between a rotorblade and an airplane's wing, within this text the context used is always that of a rotating
airfoil.
The way in which an airfoil generates lift can be explained by more than one principle
of physics. In this section, an explanation along the lines of Newton's Laws of Motion
is utilised.
As the blades encounter air mass with speed 'v', they accelerate this mass downwards
(deflecting), resulting in it (the mass) moving with velocity 'w'. As we know, to
accelerate mass requires a working force. This is provided by the (rotating) airfoil
moving through the air. But, if there is a force acting from the blade onto the air
mass, there must be a reaction force (equal in magnitude but opposite in direction)
acting from the air mass which is working at the blade. It is this reaction
force that lifts the blade.
When air is deflected around an airfoil, the direction of T remains orthogonal to
it (the airfoil) as long as the deflected airflow follows it (no turbulence
or stall). The magnitude of T depends on the amount of air mass involved and the
magnitude of acceleration, with the latter being responsible for the vertical velocity component in the airstream.
2 Definitions
When studying (rotating) airfoils, we need to know some relevant terms and definitions. These are:
- Chord line: the straight line between the blades' leading and trailing edges.
- Blade angle (or picth angle): angle between a blade's chord line and the plane of
rotation.
- Relative airflow (RAF): the Relative Air Flow (RAF) is the airflow with speed and
direction, relative to the airfoil.
- Angle of attack: the angle between the RAF (Relative Air Flow) vector and the blade's
chord line.
- Inflow angle:. the angle between the RAF (Relative Air Flow) vector and the plane
of rotation.
- Induced flow: the airflow which exists solely as a result of the airmass that is
forced down by rotoraction.
2.1 Explanation
The rotor (illustrated here with a symmetrical airfoil) moves at speed Vrotor (due to its angular velocity),
and with some pitch (or blade) angle. Now imagine that this angle is zero degrees;
there will then be no induced flow. Induced flow only exists when the rotorblade accelerates air downwards, which does not occur when
the blade is 100% horizontal. The Relative Airflow (RAF) in this case is also at an
angle of zero degrees with the plane of rotation. Seen from the perspective of a
rotorblade, the only
relevant airflow is the airflow relative to the blade (RAF).
Now imagine that the blade isn't horizontal but is instead at an angle with its
plane of rotation: that is, the pitch angle is > 0 zero (e.g. 15 degrees). The
blades will now accelerate air downwards, which leads to a vertical speed component in
the airstream. This is called the induced flow (induced by the rotorblades). As a
result of this, the airstream relative to the rotorblade (the RAF) is no longer
parallel
to the plane of rotation, but is instead directed slightly downwards (less than the pitch angle
and greater than zero degrees). The angle of the RAF changes
because of induced flow.
The angle between the chord line and the RAF is called the angle of attack. This
parameter has a direct relationship to the rotorblades� lift and drag forces. The greater
the angle of attack, the more lift the blade will produce. An angle of attack of
zero results in no lift (assuming a symmetrical airfoil). Increasing the
angle of attack has two effects: firstly, more air mass will be accelerated down because
of the larger effective area that 'cuts' through the air; secondly, the rate of acceleration also increases as the air is forced down in a steeper angle (greater
change of direction, hence more acceleration).
Both effects lead to a higher reaction force and, thus, more lift.
Next topic > Lift & Drag
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