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typically found in a steep approach,
where the column of air remains
underneath your helicopter. To get
out of it, reduce power, enter
forward flight or autorotation, but
bear in mind that you will lose a lot
of height anyway. Better still, keep
out of it with positive forward speed
or by descending more gently, so the
doughnut is below the machine.
Power
The types of power required are to
overcome the various sorts of drag,
namely parasite power, rotor profile power
and induced power, which accounts for
about 60% of the power needed to
hover (rotor profile power takes up
the other 40%).
Here is a graph that shows the
relationship of power required
against various forward speeds:
You can see that a little power is
needed at first to prevent the
machine sinking as the lift vector is
tilted and reduced, and you
transition into forward flight out of
the ground cushion, then reduces
drastically until the effects of parasite
drag come into force and require
much more power for forward speed
(it increases as the square of speed).
The lowest point of the curve is the
speed at which the least power is
needed, and is therefore the best for
endurance. The maximum range speed
occurs slightly later, when the curve
starts to rise more sharply (where the
projected line from the origin cuts
the curve a second time). The best
rate of climb speed gives the maximum
Principles of Flight 179
altitude in the shortest range, and
because the maximum power is also
available then, is the same as the
endurance speed. In other words,
the best ROC is obtained when there
is the greatest difference between the
power required for level flight and
that available from the engines.
Blade Sailing
High winds and gusts will cause the
main rotor blades of helicopters to
flap up and down and be both a
danger to people near them and the
helicopter itself, as the blade stops
could be damaged, or a particularly
flexible blade could hit the tail
boom. At certain critical speeds (50-
100 RPM), blades will pass in and
out of the stall. Holding the cyclic in
the direction of the wind will keep
the pitch of the advancing blade to a
minimum and stop it lifting in the
first place.
Other ways of minimising the effect
include parking the helicopter away
from the downwind side of
obstructions or the downwash or
slipstream of other machines,
keeping the collective down, or
accelerating and decelerating the
blades as quickly as possible. In
addition, point the nose out of wind,
so that the lowest deflection is away
from the tail boom (exam question):
Having the wind from the rear helps
you keep an eye on the low blade at
the front, but this means landing
downwind in the first place.
Do not use pitch on the collective to slow the
blades down –droop stops depend on
friction for proper operation, and all
you will be doing is lightening the
load where it ought not to be.
Some Questions
1. What factors affect the amount of
lift produced by an aerofoil?
2. What are the 3 axes that an
aircraft moves around, and
associated stability.
3. Why does an increase in all-upweight
lower speed in a helicopter?
4. What is the relationship between
aspect ratio and wing tip vortices?
5. What is the boundary layer?
6. Why does the cyclic control in a
helicopter move across the cabin
towards the retreating blade side
when the hydraulics fail?
7. If its propeller rotates
anticlockwise, which way does the
aircraft yaw on takeoff?
8. Which way does the nose pitch if
you turn right with a right hand
propeller?
9. Why does the RPM of a fixed
pitch propeller increase during
takeoff?
Some Answers
1. The angle of attack, air density,
velocity of airflow, surface area.
2. Longitudinal (roll) – lateral
stability, lateral (pitch), longitudinal
stability, yawing axis – directional
stability.
180 Canadian Professional Pilot Studies
3. More collective pitch is required
and the retreating blade will stall
earlier.
4. The higher the aspect ratio, the
smaller the vortices are.
5. The layer of retarded air
immediately in contact with the
aircraft skin.
6. Feedback from the pitch
operating arms to the control orbit.
7. To the right (the downgoing blade
has the higher angle of attack). Blade
rotation is viewed from the cockpit.
　
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