Rotors
are similar to eddies in the water. A good way to visualize
where a rotor is is to picture water flowing in the
direction the wind is going and where the flow would
carry the swirling water/representing air, created by
the rotor trigger. Wherever a trigger is, picture how
the terrain downwind of the trigger will affect where
the swirling air will go. If the rotor runs into a slope
behind the trigger, the airflow including the swirling
rotor will be deflected upward. Rotors will funnel into
low spots as well. Thermals can even lift rotors high
into the air.
On
a cliff that has a flat or shallow slope behind its
edge, the rotor will rise at an angle of about 20 degrees.
If there is a ridge or slope behind a front rotor trigger,
the rotor will rise at steeper angle. Learning how to
predict rotor triggers and where the rotor will be carried
is a constant process while flying. If you stay upwind
of any rotor triggers, you will not fly into rotors.
When you top land on any mountain or cliff, learning
to see where the rotors will be created and carried
is necessary to avoid flying into the turbulence created
by the triggers. Landing in stronger winds on top is
extremely hazardous if you get your wing into a rotor.
A hike up from a beach is a better option than top landing
in a violent rotor.
If
there is a steep cliff with a flat spot (like a road
or flat trail) across its face, the airflow will be
travelling up the cliff and the edge of the road will
create a rotor. If the cliff above the road is sloped,
the rotor will travel up the above cliff with the slope.
Flying or scratching near the cliff above such a trigger
is a common issue that catches new and sometimes even,
experienced pilots from time to time.
Click Image to view full size picture
below ...
Rotors
dissipate as they travel downwind. In lighter winds
below 12 mph, about 200 - 300+ yards downwind of a steep
cliff (this also depends on the sharpness of the edge
and the vertical height of cliff and the relationship
of height to the landing area), you might be far enough
downwind for the rotor to be managable. On a stronger
day, the rotor can travel much further, as far as 10
times the height of the rotor trigger. Most ridge soaring
sites have landing areas near the cliff, where the paraglider
airfoil can be be kept above the swirling air during
the approach and landing.
At
some sites in more moderate or light conditions, you
can land far enough downwind of the cliff edge that
the rotor will have dissipated enough to allow for a
safe landing. Local knowledge is a golden rule when
flying new sites. Always find out from the locals where
the best places to land are and how conditions change
the options. The important point to keep in mind is
to not be let the airfoil encounter the rotor at any
point close enough to the trigger point to cause any
collapses or loss of control for the landing approach.
Pilots can sometimes get into trouble with vertical
edges of a cliff. If the wind in cross and pilots will
try to stay very close to the cliff, this is called
scratching. But, if there are vertical or even horizontal
edges to the cliffs or anything directly upwind of where
you are flying, rotors can occur.
So,
on a cross day, you need to see such a rotor trigger
and give yourself padding where such a rotor trigger
might spit its mechanical turbulence.
I
have an entire article written about wind
management and avoiding getting blown back. Getting
blown back behind a ridge will put you into a rotor,
so please read this article to help you learn how to
avoid that as well.
