de Havilland Canada DHC-3 Otter experimental
STOL conversion
In 1956 de Havilland Canada (DHC) started a research program to
assess the aerodynamic performance, stability and control problems of
STOL aircraft. Using a DHC-3 Otter on loan from the RCAF (3682 c/n
40), the program was supported by the Canadian Defense Research Board
(DRB).The Otter was chosen because its existing configuration well
suited it for the intended research. The initial phase of the program
was devised to assess the potential of a deflected slipstream configuration.
Therefore the aircraft was fitted with large flaps on the wing
sections inboard of the wing fences and ailerons, this necessitated
enlarging the tail surfaces and incorporating dihedral on the
tailplane. The unusual undercarriage was based on the standard
seaplane chassis as the simplest way to simulate a tricycle landing
gear. First flight took place in 1959 and this configuration made it
possible to maintain stable flight at speeds 10 mph slower than the
standard Otter.
In the next phase the large flaps were removed and a 2,450 lb (1,111
kg) thrust General Electric J85-GE-7 turbojet was installed in the
rear fuselage. The engine was fitted with adjustable nozzles to give
the pilot positive control of the rate and angle of decent (a crude
form of vectored thrust) and to make accurate pinpoint landings. The
unusual undercarriage was modified to provide about twice the
standard energy absorption.
Flight testing during 1961-1962 proved that minimum speeds as low as
48 mph (77 kmh) were obtainable through the use of airscrew
slipstream deflection at the Otter’s normal all-up weight of
8,000 lb (3,628 kg). STOL landing techniques were investigated using
the unique control capability of in-flight reverse thrust. This
permitted a higher degree of STOL landing performance consistency,
even for very short landing distances. Take-off was improved by
deflecting the thrust rearwards.
In the third phase the aircraft was fitted with two 500 hp Pratt &
Whitney Canada PT6 turboprops attached to the Otter wing. The
original Pratt & Whitney R-1340 radial was removed from the
forward fuselage and a solid nose fitted. In this configuration with
the GE J85 still operable the gross weight was increased to 9,500 lb
(4,309 kg), 1,500 lb (680 kg) over the standard Otter. The landing
gear was modified again to absorb the increased landing weights and
rates of decent.
The program came to an end in 1965 and much of the data gathered
during the tests flights and roughly the configuration were used to
design the DHC-6 Twin Otter.