Design work began on the Hi 21 at the start of 1942 in response to a RLM requirement for a multi-purpose training glider. It was to be capable of providing both basic and advanced flight training, including high performance flying, aerobatics and three-point landings. A retractable undercarriage and side-by-side seating to optimise the instructional process were specified. The majority of two seat gliders have had tandem seating, with the student's seat over the centre of gravity (c.g.) so that its occupation would not affect the trim. Since both occupants of the Hi 24 sat ahead of the wing leading edge for good visibility, this was not an option and it was decided to employ a variable sweep wing, moving its aerodynamic centre to the c.g., rather than shift the c.g. with movable weights.[1]
Apart from the variable sweep, the Hi 21 was a conventional cantilevermid-wing design with a straight tapered wooden wing ending in rounded tips. The inner third of the wing had a plywood covered torsion resisting box section around the leading edge from the single spar; the ply skinned nose extended to the tips and behind the spar the wing was fabric covered. Minimum sweep placed the leading edge normal to the flight line so that the quarter chord sweep was forward; with maximum sweep the trailing edge was normal and the sweep rearwards. This arrangement allowed a variation of the c.g. by about 300 mm (12 in) or 20% of the chord with only a small weight penalty. Adjustment was made with a pull-up, lockable lever placed between the seats. The outer thirds of the trailing edges carried two piece ailerons with their chords increasing outwards. Four segment Schempp-Hirth type airbrakes were mounted inboard of the ailerons and immediately aft of the spar.[1]
The fuselage of the Hi 21 was largely a wooden, ply skinned monocoque but the forward part around the cockpit was steel tube framed to protect its occupants. They sat side-by side ahead of the leading edge under a multi-part glazed canopy which ended, slightly raised above the upper fuselage line, over the wing. The empennage was conventional, with a straight edged tailplane mounted on top of the fuselage and carrying straight edged, rounded tipped and balancedelevators. A small, straight edged fin at the rear of the tailplane carried a balanced, rounded rudder which reached down to the keel and worked within an elevator cut-out.[1]
The Hi 21's retractable undercarriage was unusual for a glider. A pair of wheels, with a track a little wider than the fuselage, were swung up into it around longitudinal axes by a single lever above the pilots' heads. They retracted into a space immediately behind the seats. There was a single, fixed tailwheel.[1]
Several variants were considered: Wolfgang Hütter detailed a simplified, fixed sprung monowheel version, reducing the weight so a shorter span, lower wing area design could still meet the RLM's required glide angles. Karl Schewyer and Ferdinand Schmetz both designed tandem seated competition variants, respectively with a high wing and demountable undercarriage and a low wing, retractable undercarriage. There is no evidence any of these were completed.[1]
Operational historyedit
The Hi 21 was first flown on 4 May 1944. The variable sweep mechanism proved easy to use in flight and was used to test a wide range of c.g. positions. On 7 June 1944 it was test flown by an RLM engineer who, unused to gliders, stalled on landing from about 1 m (3 ft) and severely damaged it. It did not fly again until after the war had ended, when it was repaired in the Nabern factory. Because there were few surviving training gliders in post-war Germany, the Hi 21 was much used and made some 14,000 winch launches in the eighteen months to the end of 1946.[1]
Maximum glide ratio: 28:1 at 101 km/h (63 mph), measured
Rate of sink: 0.66 m/s (130 ft/min) minimum
Wing loading: 23.8 kg/m2 (4.9 lb/sq ft)
Referencesedit
^ abcdefgSelinger, Peter F. (1978). Segelflugzeuge - Von Wolf zum Mini-Nimbus [Sailplanes - from Wolf to Mini-Nimbus] (in German). Stuttgart: Motorbuch Verlag Ltd. pp. 88–92, 109. ISBN3-87943-448-4.
March 16, 2024
hirth, multi, purpose, seat, side, side, configuration, training, glider, designed, built, germany, during, world, retractable, undercarriage, wings, variable, sweep, accommodate, centre, gravity, changes, only, completed, heavily, used, immediately, after, 21. The Hirth Hi 21 was a multi purpose two seat side by side configuration training glider designed and built in Germany during World War II It had a retractable undercarriage and wings of variable sweep to accommodate centre of gravity changes Only one was completed but was heavily used immediately after the end of the war Hi 21Role Two seat side by side training gliderNational origin GermanyManufacturer Wolf Hirth GmbH Nabern Designer Wolf HirthFirst flight 4 May 1944Number built 1 Contents 1 Design and development 2 Operational history 3 Specifications 4 ReferencesDesign and development editDesign work began on the Hi 21 at the start of 1942 in response to a RLM requirement for a multi purpose training glider It was to be capable of providing both basic and advanced flight training including high performance flying aerobatics and three point landings A retractable undercarriage and side by side seating to optimise the instructional process were specified The majority of two seat gliders have had tandem seating with the student s seat over the centre of gravity c g so that its occupation would not affect the trim Since both occupants of the Hi 24 sat ahead of the wing leading edge for good visibility this was not an option and it was decided to employ a variable sweep wing moving its aerodynamic centre to the c g rather than shift the c g with movable weights 1 Apart from the variable sweep the Hi 21 was a conventional cantilever mid wing design with a straight tapered wooden wing ending in rounded tips The inner third of the wing had a plywood covered torsion resisting box section around the leading edge from the single spar the ply skinned nose extended to the tips and behind the spar the wing was fabric covered Minimum sweep placed the leading edge normal to the flight line so that the quarter chord sweep was forward with maximum sweep the trailing edge was normal and the sweep rearwards This arrangement allowed a variation of the c g by about 300 mm 12 in or 20 of the chord with only a small weight penalty Adjustment was made with a pull up lockable lever placed between the seats The outer thirds of the trailing edges carried two piece ailerons with their chords increasing outwards Four segment Schempp Hirth type airbrakes were mounted inboard of the ailerons and immediately aft of the spar 1 The fuselage of the Hi 21 was largely a wooden ply skinned monocoque but the forward part around the cockpit was steel tube framed to protect its occupants They sat side by side ahead of the leading edge under a multi part glazed canopy which ended slightly raised above the upper fuselage line over the wing The empennage was conventional with a straight edged tailplane mounted on top of the fuselage and carrying straight edged rounded tipped and balanced elevators A small straight edged fin at the rear of the tailplane carried a balanced rounded rudder which reached down to the keel and worked within an elevator cut out 1 The Hi 21 s retractable undercarriage was unusual for a glider A pair of wheels with a track a little wider than the fuselage were swung up into it around longitudinal axes by a single lever above the pilots heads They retracted into a space immediately behind the seats There was a single fixed tailwheel 1 Several variants were considered Wolfgang Hutter detailed a simplified fixed sprung monowheel version reducing the weight so a shorter span lower wing area design could still meet the RLM s required glide angles Karl Schewyer and Ferdinand Schmetz both designed tandem seated competition variants respectively with a high wing and demountable undercarriage and a low wing retractable undercarriage There is no evidence any of these were completed 1 Operational history editThe Hi 21 was first flown on 4 May 1944 The variable sweep mechanism proved easy to use in flight and was used to test a wide range of c g positions On 7 June 1944 it was test flown by an RLM engineer who unused to gliders stalled on landing from about 1 m 3 ft and severely damaged it It did not fly again until after the war had ended when it was repaired in the Nabern factory Because there were few surviving training gliders in post war Germany the Hi 21 was much used and made some 14 000 winch launches in the eighteen months to the end of 1946 1 Specifications editData from Selinger 1978 1 General characteristicsCrew Two Length 8 22 m 27 ft 0 in Wingspan 19 60 m 64 ft 4 in Wing area 24 0 m2 258 sq ft Aspect ratio 16 Empty weight 323 kg 712 lb Gross weight 573 kg 1 263 lb Performance Maximum glide ratio 28 1 at 101 km h 63 mph measured Rate of sink 0 66 m s 130 ft min minimum Wing loading 23 8 kg m2 4 9 lb sq ft References edit a b c d e f g Selinger Peter F 1978 Segelflugzeuge Von Wolf zum Mini Nimbus Sailplanes from Wolf to Mini Nimbus in German Stuttgart Motorbuch Verlag Ltd pp 88 92 109 ISBN 3 87943 448 4 Retrieved from https en wikipedia org w index php title Hirth Hi 21 amp oldid 1050843041, wikipedia, wiki, book, 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