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National Advisory Committee for Aeronautics, Technical Notes - Measurement of the Forces Acting on Gliders in Towed Flight

The magnitude, the direction, and the fluctuations of
tow forces exerted upon gliders by towing them aloft be—
hind an automobile were measured under a variety of condi-
tions covering a range from gentle to severe types of oper-
ation. Eor these tests,the glider towing force did not
exceed 1.6 of the gross weight of the glider. V-G records
obtained during the towed—flight period as well as during
the subsequent return glide to earth showed accelerations
in the range from 8 g to ~l g. The results of prelimi—
nary airplane tow tests are also presented.

While gliders and sailplanes in free flight are ex-
posed to air—load conditions similar to those of powered
airplanes, a different category of load is introduced in
towed flight. The towing of engineless aircraft has devel—
oped into an established technique for the purpose of (l)
launching them into the air to;a point from which they can
glide down in training hope, (2) launching them into the—air
so that they can continue to soar in favorable-air currents,
or (Z) ferrying them to a destination. Towing is effected
usually by automobile, motorboat, winch, or powered air—
craft. The tow force being an extraneous force, it can
assume magnitudes and directions independent of the flight
attitude of the towed craft. If excessive. it may surpass
the design allowance for such a force for either craft.

In the earlier days of towing, some accidents were actu-
ally attributed to excessive tow forces. It is therefore
necessary for design use to establish reasonable limits
of the tow forces that engineless aircraft-may encounter
in service. '

It has been recognized that the towing forces and the
loads which they can impose on.the glider structure depend
greatly on the towing_method and technique. Statistical
data on these forces under a variety of conditions are
therefore a requisite for establishing practical design
requirements.

Reference 1 describes a tow—force recorder built in—
to a towing pole mounted on a high—performance sailplane
("Austria," weight 1,070 lb.) with a maximum accelerating
force of 560 pounds, a maximum tow—force peak in flight
of 200 pounds, and an average of 110 pounds. The use of
a camera obscure for investigations into the climb of
sailplanes in auto tow is also described.