The 604 high performance single-seater by Hanle and Prosser is a 22m span version of the Kestrel 17, and in fact originated as a design study for a similar two-seater sailplane.
Originally intended as a development vehicle for a two-seat high performance sailplane, the 604 has a three-piece wing weighting more than 272 kg/ 600 lb.; just the center section weights 160 kg/ 353 lb. The wing has six flaps, the outer pair of which move at a 2:1 differential with the ailerons. Large spoilers deploy from the upper wing surface only, and there is a tail chute for added approach control.
The 604's wing consists of a centre section incorporating the fuselage top, and two outer panels joined to the centre section by the Hutter-Hanle method. The fuselage is 5ft 5in longer than the Kestrel 17's to give improved directional control with the longer span wing; the cockpit canopy, which is slightly shorter than the Kestrel's, is hinged to open upward and aft. There is a manuallly retractable monowheel with a brake, and a fixed tailwheel. Structurally the 604 is very similar to the Kestrel 17, and can carry up to 220lb of water ballast.
The prototype made its first flight in April 1970, only four months after construction began, and took part in the 1970 World Gliding Championships at Marfa, Texas, taking sixth place; it later took second place in the 1974 World Championships at Waikerie, Australia.
Only 10 604s were built but the type gained a number of competition successes and has set several world and national records, including one for speed over a 300km triangle, set by W. Neubert of West Germany in Kenya in March 1972 with a speed of 95.3mph, and the ladies' 100km triangular speed record of 79.1mph set by Adele Orsi of Italy in August 1975.
Wing span: 22m / 72 ft 2 in
Wing area: 16.26 sq.m / 174.7 sq.ft
Length: 24 ft 11.25 in
Height: 5 ft 5.75 in
Empty Weight: 420 kg / 926lb
Payload: 180 kg / 397 lb
Gross Weight: 600 kg / 1230 lb
Wing Load: 36.9 kg/sq.m / 7.57 lb/sq.ft
Water Ballast: 0 kg / 0 lb
Max speed: 155mph (smooth air)
L/DMax: 49 at 98 kph / 53 kt / 61 mph
MinSink: 0.50 m/s / 1.64 fps / 0.97 kt at 45mph
Aspect ratio: 29.8
Airfoil: Wortmann FX-67-K-1 70
No. Built 11