A variable geometry wing offers the prospect of being able to vary the wing’s area and geometry in flight to make the most of the soaring conditions encountered. A high aspect ratio could be selected for ascending in a thermal and a low aspect ratio (and hence a smaller area) for getting the maximum speed in cross-country flying.

The Stuttgart Akaflieg’s solution to this problem in the high-performance single-seater FS-29 is a telescopic wing in which the span can be varied in flight from 43 ft 7.5 in to 62 ft 4in (13,3m – 19m) by outer telescopic wing sections sliding over fixed inner sections.

These outer wing sections are of glassfibre/foam/carbon-fibre sandwich construction and are extended and retracted manually by means of push-rods. A stub spar protrudes from each inner wing to provide a mounting for the guide rails on which the outer panels move. The inner wing is built up from a box spar of glassfibre/Conticell foam sandwich and is fitted with Schempp-Hirth air brakes on the upper surface which are only effective when the outer panels are extended. The plain ailerons are of similar construction to the outer wings. The fuselage is very similar to that of the Schempp-Hirth Nimbus 2, using the same landing gear and cockpit canopy, while the tail unit is the same as the Nimbus 2’s.

Design work on the FS-29 began in 1972 and the problem of devising a wing which could be varied in span solely by manual power, and yet keep to the required aerodynamic and structural safety factors, meant that the first flight was not made until June 1975.

FS-29
Wing span: 43 ft 7.5 in to 62 ft 4 in / 13,3m – 19m
Length: 23 ft 5.75 in / 7.16 m
Height: 4ft 2in / 1.27m
Wing area: 92.2 to 136.2sq ft / 8.56-12.65 sq.m
Wing section: Wortmann FX-73-170 (inner); FX-73-170/22 (outer)
Aspect ratio: 20.67 to 28.54
Airfoil: FX 73-170
Empty weight: 787 lb / 365kg
Max weight: 992 lb / 450kg
Water ballast: None
Max speed: 155mph / 135 kt / 250 km/h
Stalling speed: 39 kt / 72 km/h
Max rough air speed: 135 kt / 250 km/h
Min sinking speed: 1.8ft/sec / 0.54 m/sec at 46mph / 40 kt / 74 km/h
MinSink: 0.56 m/s @ 81 kph
Best glide ratio: 44:1 at 62mph / 98 kph / 54 kt
No. of Seats: 1
No. Built: 1

Notable as being the first sailplane to be built of glassfibre, design of the Phonix was started at the Stuttgart Akademische Fliegergruppe under R.Eppler and H. Na’gele in 1951. Originally balsa wood construction with a strengthened outer skin of paper-and-glue layers was decided upon with the idea of reducing weight, but the Phonix project had to be abandoned for a time. Work was continued when the Stuttgart Fliegergruppe was given a grant for further research, and by now the new glassfibre strengthened polyester resins were available as a constructional material. The Phonix was redesigned to a basically balsa wood/glassfibre sandwich form of construction. At first a conventional tail unit with low-set tailplane was featured, but this was later replaced by a T-tail; the landing gear is retractable.

The monocoque fuselage is built in two pieces with a sandwich skin with balsa wood filling and an outer skin of glassfibre. The wing attachment, controls and fittings are installed before the two pieces are glued together with the glassfibre skin overlapping; weight carrying points and the edge of the cockpit canopy are strengthened with plywood. The wings and tail unit are of similar balsa wood/glassfibre sandwich construction, the ailerons, flaps and rudder being cut out after the glueing process is completed.

The first made its first flight on 27 November 1957. Altogether eight Phonixes were built and all of them were still flying 21 years after the prototype first took to the air.

FS-24 Phonix
Wing span: 16m / 52 ft 6 in
Length: 22 ft 5.25 in
Wing area: 14.36 sq.m / 154.6 sq.ft
Empty Weight: 180kg / 362 lb
Gross Weight: 330kg / 584 lb
Wing Load: 22.9kg/sq.m
Aspect ratio: 17.83
Airfoil: Eppler 91
Max speed: 107 mph
L/DMax: 40 @ 79kph / 48 mph
MinSink: 0.59 m/s @ 68 kph / 1.7 ft/sec at 43 mph
No. of Seats: 1

FS-24 Phonix-T
Wing span: 16.0 m / 52 ft 6 in
Length: 6.84 m / 22 ft 5 in
Wing area: 14.36 sq.m / 154.6 sq ft
Wing section: EC86(-3)-914
Aspect ratio: 17.83
Empty weight: 164 kg / 362 lb
Max weight: 265 kg / 584 lb
Water ballast: None
Max wing loading: 18.5 kg/sq.m / 3.79 lb/sq ft
Max speed: 97 kt / 180 km/h
Stalling speed: 31 kt / 58 km/h
Min sinking speed: 0.51 m/sec / 1.7 ft/sec at 37 kt / 69 km/h
Max rough air speed: 54 kt / 100 km/h
Best glide ratio: 40 at 42 kt / 78 km/h)

Some attention has been paid by designers to varying the wing geometry or area in flight to cater for different soaring conditions, and one approach to this problem is embodied in the Mu 27, which features slotted Fowler-type flaps enabling the wing area to be varied in flight. At high speeds a good gliding angle can be obtained by retracting the flaps while at low speeds or in a weakening thermal the flaps can be extened to give more lift, increasing the wing area by 36%.

The wings are a glassfibre/Conticell sandwich with aluminium spars and metal webs, and air brakes on the upper surface are at the 50% chord line. The ailerons are linked to the flaps. The semimonocoque fuselage is entirely of glassfibre with a large two-piece sideways-hingeing cockpit canopy, while the T-tail unit is of glassfibre/foam sandwich construction. The landing gear consists of a retractable monowheel and a fixed tailwheel.

The Mu 27 is a tandem two seater of largely glassfibre construction, and first flew on 24 February 1979 piloted by Thomas Fischer.

Mu 27
Span: 72 ft 2.25in / 22m
Length: 33 ft 9.5 in / 10.3 m
Height: 1.8m / 5 ft 11 in
Wing area: 189.5 sq.ft / 23.9 sq.m (flaps retracted)
Wing area 257.3 sq ft / 17.6 sq.m (flaps extended)
Aspect ratio: 20.2-27.5
Airfoil: FX 67-VC-170/136
Empty weight: 1,058 lb / 480 kg
Max weight: 1,543 1b / 700 kg
Water ballast: None
Wing Load: 8.19 lb/sq ft / 39.8 kg/sq.m
Max speed: 174 mph / 151 kt / 280 km/h
Min sinking speed: 1.8 ft/sec / 0.56 m/sec at 37 mph / 32.5 kt / 60 km/h
Best glide ratio: 47:1 at 63 mph / 54.5 kt / 101 km/h
L/DMax: 39 @ 88 kph
No. of Seats: 2
No. Built: 1

One of the principal prewar German high performance designs, the single-seater Mü 13 was developed at the Munich Akaflieg from the two-seat Mü 10 Milan, which had been built there under the direction of Dipl-lng Egon Scheibe, who later formed, at the end of 1951, the sailplane firm bearing his name. The first single-seat Mü 13 flew in 1936. It has an all moving tailplane and DFS airbrakes for approach control.

There were a number of variants, some later having air brakes instead of flaps while the rear fuselage cross section was later changed from a rectangular one to triangular. The Mü 13D-3 of 1943 had an increased wing span, longer fuselage and a modified fin and rudder. In May 1939, Schmidt set a new German record for a goal flight of 482km (299 miles) in an Atalante. Unlike most sailplanes of its size the Mu 13 was of mixed construction, using steel and chrome-molybdenum-tin alloy as well as wood. The wooden cantilever wing had steel tube flaps and fabric-covered steel-framed ailerons while the fuselage was a fabric-covered steel framework of basically square cross-section and the tail unit of conventional wooden construction. Particular attention was paid to good visibility, there being large window panels inset into the sides of the cockpit while a clear panel inset into the wing centre section leading edge gave the pilot a better view above and behind him; the cockpit canopy was detachable, and several different shapes were used. A monowheel landing gear and tailskid were featured.

Scheibe built a development of the Mü 13 in Austria known as the Mü 13E Bergfalke (Mountain Falcon), which first flew in prototype form on 6 August 1951 and this two-seat general purpose glider went into production as the Scheibe Bergfalke. The original Mü 13 made its first flight in 1936 and the two prototypes were named the Merlin and the Atalante, differing from each other principally in the fuselage structure and control arrangements. The Atalante with a number of modifications was built by Kurt Schmidt at the Black Forest factory and about 150 Mü 13s were built in all.

Akaflieg Munchen Mü 13 Merlin
Span: 52 ft 6 in
Length: 19 ft 9 in
Wing area: 174 sq.ft
Aspect ratio: 15.85
Empty weight: 375 lb
Max weight: 595 lb
Max speed: 124 mph
Min sinking speed: 2 ft/sec at 34 mph
Best glide ratio: 28:1 at 41 mph

Mȕ 13 Atalante
Wing span: 16.0 m / 52 ft 6 in
Length: 6.02 m / 19 ft 9 in
Wing area: 16.16 sq.m / 174 sq ft
Wing section: Mil
Aspect ratio: 15.85
Empty weight: 170 kg / 375 lb
Max weight: 270 kg / 595 lb
Water ballast: None
Max wing loading: 16.71 kg/sq.m / 3.42 lb/sq ft
Max speed: 108 kt / 200 km/h
Stalling speed: 27 kt / 50 km/h
Min sinking speed: 0.6 m/sec / 2 ft/sec at 29.5 kt / 55 km/h
Best glide ratio: 28 at 35.5 kt / 66 km/h

Akaflieg Munchen Mü 13 D Atalante
Length: 19.783 ft / 6.03 m
Wingspan: 52.428 ft / 15.98 m
Crew: 1

Schwarzwald Mu-13 D
Wing span: 16 m
Wing area: 16.16 sq.m
Aspect ratio: 15.84
Airfoil: Mu-profil 15%
Empty Weight: 185 kg
Gross Weight: 275 kg
Wing Load: 17 kg/sq.m
L/DMax: 28 66 kph
MinSink: 0.60 m/s 55 kph
Seats: 1

Schwarzwald Mu 13D3
Wing span: 16 m / 52.5 ft
Wing area: 16.16 sq.m / 174 sq.ft
Aspect ratio: 15.85
Airfoil: Mu
Empty Weight: 170 kg / 375 lb
Payload: 100 kg / 220 lb
Gross Weight: 270 kg / 595 lb
Wing Load: 16.71 kg/sq.m / 3.42 lb/sq.ft
L/DMax: 25 66 kph / 35 kt / 40 mph
MinSink: 0.62 m/s / 2.0 fps / 1.18 kt
Seats: 1

Scheibe-Flugzeugbau Mü 13E Bergfalke 1
Length: 51.837 ft / 15.8 m
Wingspan: 56.43 ft / 17.2 m
Max take off weight: 551.3 lb / 250.0 kg
Crew: 2