The Akademische Fliegergruppe of the Technical University of Darmstadt (
Akaflieg Darmstadt) was first formed in 1921. It was, and is, a group of aeronautical students who design and construct aircraft as part of their studies and with the help and encouragement of their University. Design work on the variable wing geometry D-40 began in 1980 but the first flight did not take place until 15 August 1986. As understanding of thermal soaring grew in the 1930s, glider pilots and designers became aware of two conflicting requirements for cross country flights. The aircraft needed good climb characteristics and low
stalling speeds to enable tight turns within thermals but high speeds in the sinking air between them. These respectively called for low and high
wing loadings on wings with high and low camber. Several designs, e.g. the 1938
Akaflieg Hannover AFH-4 and the later
LET L-13 Blaník and
Beatty-Johl BJ-2, had added large area
slotted Fowler flaps on the inner part of the wing to increase camber and add area when extended. These satisfactorily reduced stall speed and with it the turn radius, but disappointed hopes of improving climb rates because of vortex generation (
induced drag) at the tips of the flaps, seriously decreasing the lift to drag ratio. A solution to this problem was to extend the whole trailing edge, including the
ailerons, and this route was taken by both the disappointing, heavy and complicated
Operation Sigma Sigma, the more successful but still heavy and complex
Akaflieg München Mü27 and the World Championship winning 15 m class
Akaflieg Braunschweig SB-11. These last three designs changed the wing geometry by extending the wing rearwards at right angles to the trailing edge. Akaflieg Darmstadt took a different approach, pivoting the single piece flap near the
tip and sliding it out from within the wing
trailing edge, gaining the mechanism the nickname "penknife wing". As it is extended, a track in the fuselage side guides the thin flap into its high camber position at the
wing root. The wing area is increased by 21% with the flaps extended. Although this arrangement avoids the vortexes at the flap tip, like any area increasing method used on a fixed span wing it results in a lower aspect ratio and hence a lower lift to drag ratio. The D-40 is constructed from a mixture of plastic-fibre
composites,
glass reinforced plastic (GRP),
carbon fibre reinforced plastic (CRP) and
aramid reinforced plastic (ARP) with some use of
balsa wood. The
shoulder mounted wing has a
spar built from CRP
rovings and GRP-balsa
webs. The
skin of both wing and flaps is an ARP/CRP/balsa sandwich; the flaps have CRP
stringers. In plan the wings are straight tapered, with an increase in taper close to the tips; these outboard sections carry the short span ARP
ailerons. The wing has 1° of
dihedral and is swept forward at 2.3° at 25%
chord. There are
Schempp-Hirth upper surface
airbrakes slightly forward of mid-chord near to mid-span. When the flaps, pivoted immediately inboard of the ailerons, are deployed the wing trailing edge becomes almost straight, making the plan almost triangular, and 12° of
washout is generated by the now strongly cambered inner sections. This produces a lift distribution close to that of the ideal
elliptical wing, with its minimum
induced drag. The D-40 uses the fuselage and empennage of the
Rolladen-Schneider LS3. This is a GRP shell, slender aft of the wings and with a
T-tail with straight tapered surfaces. The rather wide cockpit occupies most of the deeper forward fuselage and has a long, one piece, front hinged
canopy. A retractable monowheel undercarriage was assisted by a tail bumper. ==Operational history==