KRONFELD ON GLIDING AND SOARING.pdf - Lakes Gliding Club

MY SOARING FLIGHTS IN ENGLAND 363
and their behaviour on steep slopes understand the
conditions perfectly. Previously,, on the occasion of my
Alpine expedition of 1928, we encountered, over a precipice,
vortices which became more powerful as the wind increased
in force. Only when the wind is very weak are the currents
regular or when the sun's rays are strong enough, through
thermal currents, to disperse the vortices. Wherever there
is a vertical wall with a horizontal plateau above it a " weather
vortex " forms itself on the windward side, which at
Scarborough was the angle betweeen the cliff and sea,
and its axis runs horizontal. Immediately against the
perpendicular wall and directly below the top edge there is
a strong down-current which in the case of less steep slopes
is an up-current. The wind swirls up from below over
the sharp edge and again a vortex is created, a lateral vortex,
which again causes the wind to reverse. Only at a height
of from thirty to sixty feet above the plateau is the upcurrent
to be found. Owing to the limits of the ground it
was necessary to start within the sphere of the lateral
vortex but this is not constant at any one place, it forms,
becomes larger, breaks up and draws away with the current.
In strong winds which build up particularly strong vortices
there come moments when a " start " is possible, just as a
vortex has broken up and before the next one forms.
These phenomena can be noticed from grass thrown up
high. On this occasion the wind even blew at times
against the main direction and under such conditions successful
starts are dependent upon fractions of seconds which it
is very difficult to estimate exactly. Even a particularly
powerful launch failed to overcome these adverse conditions.
The short time at our disposal did not allow us quietly to
study the conditions and I had to go on as there was yet
another demonstration to give in England. Although,
therefore, I was unable to fly properly I should like to
express my thanks to the Scarborough Club for their work
and to apologize to the launching crew for the fright I gave
them when starting.
At high speed I had to drive to London through the night
and rain with my " Wien " towed behind the Mercedes
car so kindly placed at my disposal by the London Agency