Darmstadt D 28 "Windspiel"
Type
Single seat glider
Dimensions
Length 5,98 m, span  12,0 m, wing area 11,4 m2
Weights
Empty a) 54 kg b) 72 kg , max. flying weight 152 kg, wing loading 13,3 kg/m2, wing 27 kg, fuselage 17 kg, empennage 6 kg, instrument 4 kg, pay load 75 kg
Performance
Max. speed 180 km/h, glide ratio 1:23 at 56 km/h, lowest descent rate 0,48 m/sec. at 48 km/h
Type
Werk.Nr
Registration
History
     
Designed by R. Kosin, R. Schomerus in 1933/36  Profile Göttingen 535 Worldrecord 1934 cross country flight 240
km, Target Darmstadt - Saarbrucken 140 km at 8/3 1935 by Schnee
In 1932 the Akaflieg started ihc development Of the D 28.. The aim was to get good performance even in poor weather conditions by building a rela-tively small plane to the lowest possible weight. In this respect the D 28 is unique still to this day; the empty weight was 54 kg  with a wing span of 12 m In 1935, while Fischer was in the cockpit of Windspiel on Airfield Griesheim, a plane crashed while landing the glider. Fischer escaped with a few bruises, but the glider was destroyed. The glider was rebuilt, renamed D-28B. , reconstructed Windspiel with ailerons and flaps separated. The empty weight was somewhat higher: 72 kg, but without much impact on performance
The main student designers were R. Schomerus and R. Kosin.  Kosin later played an important role as chief aerodynamicist for the first jet bomber ARADO 234.
The one-piece wing had one spar and a plywood torsion nose box. The wing was attached to the fuselage by three duralumin fittings. The full-span ailerons were operated in common mode for airfoil camber control and in opposite mode for roll control. Rudder and ailerons were coupled by a special gear, so with increasing rudder deflection the differential function of the aileron was increased. This gear is shown in the Figure 2. The fin and rudder arrangement of the D 28 was „semi-damped". The fin moved together with the rudder, but only with 1/3 of the rudder deflection. These special deiails were unique; ihey were never used on other airplanes except the Darmstadt D 30, which also had the semi-damped lin and rudder arrangement.
Personal comment of Hans Zacher: :The manoeuvrability of the D 28 at the low speed of 60 km/h was really exceptional due to the semi-damped rudder and the variable differential function of the aileron. The L/D of 23,5 was remarkable compared to the DFS Meise with 15 m span, a design which occured five years later. I liked to fly the D 28 and moved it lively around the three axes".
The front fuselage was a conventional wood structure; the rear fuselage was a plywood tube over frames without stringers.
The small weight was achieved by extremely careful design and construction and exceptional skill in the workshop.
The D 28 was completely designed to the pre-war German sailplane airworthiness specifications BPS and was even certified for simple aerobatics. The following measures contributed to the low weight Careful and individual selection of highest quality materials. Spruce and high quality plywood was used for the wooden structure. All metal fittings were constructed from Duralumin and the nuts and bolts wherever possible. For all structural parts the individual strength of the material was tested and the parts were individually designed for full usage of the material's strength. During glueing the overflowing glue was immediately removed to save weight! The airframe was covered with a special lightweight silk.

The maiden flight of the ā€˛Windspiel" took place on the 8th of July 1933. With the pilot Hans Fischer the D 28 immediately joined the Rhon competition in 1933. The success was only moderate; during the competition the plane was damaged by a stone during landing.
On 16th of June 1934 Hans Fischer achieved a world dis- tance record with 240 km from Darmstadt to Montmedy in France. In the 1934 Rhon competition Hans Fischer with the D-28 became winner in the distance class and third in the duration class.
1935 saw the end of the D 28 prototype in an unusual accident. In the vast grassfield of the Griesheim airport a small sportsplane found no other landing spot than the D 28! During 1936 ihe Akaflieg Darmstadt built a second example "D-28b".
Due to some changes and additional equipment the empty weight increased to 72 kg. The main design change was the division of the wings trailing edge flap into an inboard flap and an outboard aileron. Also with this configuration for camber control the ailerons could be moved together with the inboard flap. This plane became famous especially with a flight from north to south across the Alps with Hans Osann at the controls. He started at Salzburg and landed after 240 km near Udine.
During the 1937 Rhon-Competition the D 28b was flown by Hans Osann; he finished with sixth place. Bernhard Flinsch piloted the D 28b during the 1938 Rhon-Competition and reached 12th place, flying against a crowd of better and much faster ships like DFS Reiher, DFS Weihe. Minimoa and Condor II. His best achievement was a cross country flight of 365 km.
The D-28b survived the war. was captured by American troops and then shipped to USA. Sadly enough the plane wasnot packed professionally and due to very rough sea it was destroyed on the Atlantic.
Due lo happy circumstances the workshop drawings of the D 28 survived the war. the complete set of drawings exists still today in the archive of the Akaflicg.
VGC News
No. 100 Summer 2000