Dornier Do Y

Do Y
	A Dornier Do Y of the Royal Yugoslav Air Force
Role	Bomber
National origin	Germany
Manufacturer	Dornier Flugzeugwerke
First flight	17 October 1931
Primary user	Royal Yugoslav Air Force
Number built	4

The Dornier Do Y was a trimotor German monoplane bomber of the early 1930s designed by Dornier Flugzeugwerke (Dornier Aircraft Works) to an order by the Royal Yugoslav Air Force (RYAF). The Yugoslavs originally intended to use war reparations to pay for them, but ultimately had to use their own funds. Only a pair of aircraft were initially ordered to give the Yugoslavs experience with multi-engine bombers and Dornier failed to sell any more Do Y's. Two improved versions were subsequently built on speculation, but failed to find purchasers until the RYAF bought them in 1935.

All four Do Y's were declared obsolete in 1939 and relegated to training and transport missions. They played no role in the 1941 Invasion of Yugoslavia by the Axis Powers and were captured undamaged. The Germans decided to transfer all of them to the Air Force of the Independent State of Croatia later that year, but only one can be confirmed to have served with the Croatians.

Background

In early 1930 the RYAF decided to order some multi-engined bombers from builders with experience with multi-engine aircraft to gain experience with the type and to evaluate different models before making a decision on purchasing future aircraft. The RYAF had two main requirements: the first was that they use three license-built Gnome-Rhône Jupiter VI air-cooled radial engines and the second was that the bombers had to be delivered by 30 August 1931. Dornier had no suitable design in production that could meet such a tight deadline and had to perform design work in parallel with the Do 11 twin-engined bomber. For example both aircraft used a semi-elliptical wing design based on that of the earlier Do K3 prototype. The company was able to begin construction of two prototypes before the Yugoslavs signed a contract for both on 21 May 1931. These aircraft were unarmed as Germany was not allowed to build military aircraft by the terms of the Versailles Treaty.^[1]

The RYAF intended to pay for them using war reparation monies owed to Yugoslavia by Germany, but the effects of the Great Depression on the German economy in 1931 caused U.S. President Herbert Hoover to propose suspension of the reparations payments effective on 1 July and this was later accepted by the signatories to the Versailles Treaty. The Weimar government cancelled all deliveries funded by reparations as of 1 July and Yugoslavia was forced to pay for the Do Ys and other aircraft that it had ordered with its own money. The first prototype flew on 17 October.^[2]

Dornier began construction of two improved Do Ys on its initiative the following year as the Do U and unsuccessfully attempted to sell them as airliners to Deutsche Luft Hansa. While negotiations were ongoing, the aircraft was redesignated as the Do 15. Dornier then offered them to the Heereswaffenamt (Army Weapons Department) on 14 January 1933, one civilian aircraft for Lufthansa and the other for the Reichswehr (German Armed Forces). Work continued on the Do 15s until Hermann Göring, the acting Aviation Minister of the new Nazi government, ordered that work be suspended in March. Dornier stored the incomplete aircraft until 1935 when they were inspected by a Yugoslav delegation which placed an order for both in November. Designated the Do Ym by the RYAF, the first flight of these bombers was made on 1 July 1936.^[3]

Description

The Do Y was a shoulder-wing monoplane with an all-metal monocoque fuselage formed from duralumin. The structure of the empennage was also duralumin, but it was covered in fabric as were all of the control surfaces. The two-spar cantilever wing was also covered in fabric except for the area behind the octagonal engine nacelles which was duralumin and could be walked upon to facilitate maintenance. There were some issues with wing flutter under certain conditions, but these were mostly resolved during trials. The wing was built in three sections: a center section that extended just past the nacelles and two outer wing panels. The center section housed the bomb bay and the two 1,045-litre (230 imp gal; 276 US gal) fuel tanks. Each engine was provided with a 70-litre (15 imp gal; 18 US gal) lubricating oil tank. The Do Y had fixed conventional landing gear that included a tailwheel at the extreme end of the fuselage. The telescopic legs for the main wheels were attached to the bottom of the forward wing spar and to the fuselage with V-struts and bracing wires.^[4]

The bomber was initially powered by three 450-horsepower (340 kW) Gnome-Rhône Jupiter VI radials, driving two-bladed wooden propellers. Two engines were located at the front of the wing while the third was mounted on struts above the fuselage. The RYAF requested that they be replaced by license-built 480-horsepower (360 kW) Gnome-Rhône Jupiter 9Ak engines before delivery, although it is uncertain when this was actually done.^{[Note 1]} The wooden propellers were replaced by metal ones before delivery.^[6] Aviation historian Sime Ostric believes that the Jupiter radials were first replaced by 420-horsepower (310 kW) Gnome-Rhône 9Ad engines and then by 480 hp Gnome-Rhône 9Ae models in 1937.^[5]

The pilot and copilot sat side-by-side in an open cockpit at the top of the fuselage, forward of the wing, each with a minimal windscreen. The copilot also served as the bombardier and could control the Do Y's rudder from the bombardier's position in the lower nose of the aircraft. The rest of the crew consisted of a radio man, a mechanic and possibly an observer. The bomber was armed with five 7.7-millimetre (0.303 in) Darne machine guns, arranged in two twin-weapon installations, one in the upper nose above the bombardier's position and the other in the upper fuselage between the wing and the vertical stabiliser, and a single belly gun. The radio man also served as the forward machine gunner and the mechanic operated in the dorsal machine gun position. The observer, if one was carried, manned the belly gun. The bomb bay could accommodate up to 1,200 kilograms (2,600 lb) of bombs in two racks that were limited to bombs no larger than 200 kilograms (440 lb).^[7]

The RYAF required that the license-built 625-horsepower (466 kW) Gnome-Rhône 9Kers engines with three-bladed propellers be fitted to the Do Ym and Dornier replaced the engine nacelles with more efficient NACA cowlings to provide better cooling. In addition the wingspan of the new aircraft was reduced and the fabric covering of the underside of the wing was replaced by duralumin to eliminate the wing flutter experienced by the earlier models. A much more minor change was that more windows were added to the fuselage to improve visibility.^[8]^[9]

Operational history

The Do Y's gave demonstrations in Austria, Hungary and Romania in early January 1932 in an unsuccessful attempt to gain more orders for the aircraft. They arrived in Yugoslavia later that month and were assigned to the 6th Aviation Regiment (6.Vazduhoplovni Puk) based at Zemun. All of the multi-engine bombers were transferred to the newly formed 261st Aviation Group (261.Vazduhoplovna Grupa) in September 1933. Later that month they participated in a parade in honour of Prince Nicholas of Romania and ferried the prince to Kraljevo and Novi Sad. On 15 October 1934, the Do Y's escorted the train carrying the body of the assassinated King Alexander I from Zemun to Belgrade. The following year the unit was redesignated as the 261st Independent Trimotor Bomber Group (Serbo-Croatian Latin: 261.Nezavizna bombarderska grupa tromotoraca).^[10] The bombers finally received their bomb racks and were fitted for their defensive armament that same year,^[11] although there is no photographic evidence that the Darne machine guns were ever actually mounted.^[12]

The two Do Ym bombers arrived in Yugoslavia in early March 1937 and Prime Minister Milan Stojadinović flew in one of them on 13 March. The Do Y's began general overhauls at the Kraljevo Aviation Depot on 8 October; one aircraft was completed by the end of the year, but the other was not finished until August 1938. The 261st Group was redesignated as the 81st Aviation Regiment during 1938. All of the trimotors were scheduled to be replaced by the Savoia-Marchetti SM.79 in early 1939, but those aircraft were delayed until September. Most of the aircraft were then relegated to transport and liaison roles, but one remained with the 81st Regiment for training duties through mid-1940. Both of the Do Ym aircraft were fitted with large doors on the side of the fuselage, probably for use by paratroopers.^[13] All four Do Y's were transferred to Kraljevo at the beginning of 1941,^[14] with at least one of the older Do Y's already being overhauled.^[15]

None of the Do Y's played any part in the invasion of Yugoslavia in April 1941 and all four were captured by German forces. Both Do Y's were being overhauled and the two Do Ym bombers were parked in front of the State Aircraft Factory. Dornier documents dated 11 September state that all of the Do Y's were to be given to the Croats free of charge, but they must pay for any repairs. One of them was subsequently handed over to the Air Force of the Independent State of Croatia and was assigned to 9th Squadron in January 1942. It only made a few flights before it became a decoy at Sarajevo in mid-1944. The other three aircraft were cannibalized for spare parts and subsequently scrapped.^[16]

Operators

Independent State of Croatia

The Air Force of the Independent State of Croatia

Kingdom of Yugoslavia

Royal Yugoslav Air Force

Specifications (Do Y - 9K engines)

Data from Jane's all the World's Aircraft 1937^[17]

General characteristics

Crew: 4
Length: 18.2 m (59 ft 9 in)
Wingspan: 26.82 m (88 ft 0 in)
Height: 6.3 m (20 ft 8 in)
Wing area: 108.8 m² (1,171 sq ft)
Empty weight: 5,700 kg (12,566 lb)
Gross weight: 8,500 kg (18,739 lb)
Fuel capacity: 2,090 L (550 US gal; 460 imp gal) fuel in two wing tanks; 210 L (55 US gal; 46 imp gal) oil in three tanks
Powerplant: 3 × Gnome-Rhône 9Kers 9-cylinder air-cooled radial piston engines, 450 kW (600 hp) each at 4,050 m (13,290 ft)
Propellers: 3-bladed metal propellers

Performance

Maximum speed: 300 km/h (190 mph, 160 kn) at 4,000 m (13,000 ft)
Cruise speed: 250 km/h (160 mph, 130 kn) at 4,000 m (13,000 ft)
Landing speed: 105 km/h (65 mph; 57 kn)
Range: 1,500 km (930 mi, 810 nmi)
Service ceiling: 8,300 m (27,200 ft) service ceiling
Absolute ceiling: 8,800 m (28,900 ft)
Rate of climb: 2.7 m/s (530 ft/min)
Time to altitude: 4,000 m (13,000 ft) in 12 minutes 30 seconds
Wing loading: 78 kg/m² (16 lb/sq ft)
Power/mass: 0.15838 kW/kg (0.09634 hp/lb)

Armament

Guns: 5x machine guns in nose, dorsal and ventral positions
Bombs: 12 × 100 kg (220 lb) bombs carried internally in the fuselage

Notes

^ Ostric believes that this was most likely performed in 1932 or 1933.^[5]

Footnotes

^ Nikolic & Ognjevic, pp. 115, 117; Ostric, first part, p. 53
^ Nikolic & Ognjevic, pp. 117; Ostric, first part, p. 53
^ Nikolic & Ognjevic, pp. 125, 128–129, 131
^ Nikolic & Ognjevic, pp. 136, 138, 140–141; Ostric, last part, pp. 57–58
^ ^a ^b Ostric, last part, p. 58
^ Nikolic & Ognjevic, pp. 140–141
^ Nikolic & Ognjevic, pp. 139, 141; Ostric, last part, p. 57
^ Nikolic & Ognjevic, pp. 129, 131
^ Dornier, p. 121
^ Nikolic & Ognjevic, pp. 120, 122–123, 125
^ Ostric, first part, p. 55
^ Nikolic & Ognjevic, p. 141
^ Nikolic & Ognjevic, pp. 132–133
^ Ostric, last part, p. 55
^ Nikolic & Ognjevic, pp. 133–134
^ Nikolic & Ognjevic, pp. 133–135; Ostric, last part, p. 56
^ Grey, C.G.; Bridgman, Leonard, eds. (1937). Jane's all the World's Aircraft 1937. London: Sampson Low, Marston & company, ltd. p. 268c.

Bibliography

Dornier GmbH (1985). Dornier: Die Chronik des ältesten deutschen Flugzeugwerks [Dornier: The Story of the Oldest German Aircraft Company] (in German). Friedrichshafen, Germany: Walter Biering GmbH. ISBN 3-925505-01-6.
Nikolic, Djordie & Ognjevic, Akeksandar M. (2021). Dornier: The Yugoslav Saga 1926-2007. Lublin, Poland: Kagero Publishing. ISBN 978-83-66673-61-8.
Ostric, Sime (June 2003). "Laid et lourde: le Dornier Do-Y en Yugoslavie (première partie)" [Ugly and Heavy: The Dornier Do-Y in Yugoslavia (first part)]. Avions: Toute l'aéronautique et son histoire (in French) (123): 50–60. ISSN 1243-8650.
Ostric, Sime (August 2003). "Laid et lourde: le Dornier Do-Y en Yugoslavie (fin)" [Ugly and Heavy: The Dornier Do-Y in Yugoslavia (last part)]. Avions: Toute l'aéronautique et son histoire (in French) (125): 52–59. ISSN 1243-8650.

[6] Ostric believes that this was most likely performed in 1932 or 1933.^[5]

[1] Nikolic & Ognjevic, pp. 115, 117; Ostric, first part, p. 53

[2] Nikolic & Ognjevic, pp. 117; Ostric, first part, p. 53

[3] Nikolic & Ognjevic, pp. 125, 128–129, 131

[4] Nikolic & Ognjevic, pp. 136, 138, 140–141; Ostric, last part, pp. 57–58

[o8-5] Ostric, last part, p. 58

[7] Nikolic & Ognjevic, pp. 140–141

[8] Nikolic & Ognjevic, pp. 139, 141; Ostric, last part, p. 57

[9] Nikolic & Ognjevic, pp. 129, 131

[10] Dornier, p. 121

[11] Nikolic & Ognjevic, pp. 120, 122–123, 125

[12] Ostric, first part, p. 55

[13] Nikolic & Ognjevic, p. 141

[14] Nikolic & Ognjevic, pp. 132–133

[15] Ostric, last part, p. 55

[16] Nikolic & Ognjevic, pp. 133–134

[17] Nikolic & Ognjevic, pp. 133–135; Ostric, last part, p. 56

[JAWA1937-18] Grey, C.G.; Bridgman, Leonard, eds. (1937). Jane's all the World's Aircraft 1937. London: Sampson Low, Marston & company, ltd. p. 268c.

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[3]

[4]

[Note 1]

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[17]

v t e Dornier and Zeppelin-Lindau aircraft
Zeppelin-Lindau 1914-1919	Rs.I Rs.II Rs.III Rs.IV CL.I CL.II CS.I D.I Gs.I Gs.II V 1
Dornier designations 1919-1933	Do A Do B Do C Do D Do E Do F Do G (project) Do H Do J Do K Do L Do N Do O Do P Do Q Do R Do S Do T Do U Do V Do X Do Y Delphin Falke Komet Libelle Merkur Spatz Wal
RLM designations 1933-1945	Do 10 Do 11 Do 12 Do 13 Do 14 Do 15 Do 16 Do 17 (Operational history) Do 18 Do 19 Do 20 Do 22 Do 23 Do 24 Do 25 Do 26 Do 29 Do 212 Do 214 Do 215 Do 216 Do 217 Do 317 Do 318 Do 335 Do 417 Do 435 Do 635 P.59 P.85 P.174 P.184 P.192 P.231 P.232 P.238 P.247 P.252 P.254 P.256 P.273 P.1075
Dornier designations post-1945	Do 24 Do 25 Do 27 Do 28 Do 29 Do 31 Do 32 Do 32K Do 34 128 Do 131 Do 132 228 Do 231 328 328JET 428 528 728JET 928JET Aerodyne Alpha Jet Projekt 621 Seastar S-Ray 007
See also Claude Dornier and Dornier Museum Friedrichshafen

v t e Reich Air Ministry (RLM) aircraft designations (list)
1 to 100	8-1¹ Fi 2 8-3¹ 8-4¹ Fi 5 DFS 6 8-7¹ Gö 8 Gö 9 Do 10 Do 11/Wn 11 8-12⁴/Do 12 Do 13 Do 14 Do 15 Do 16/Wn 16 Do 17 Do 18 Do 19 Do 20 8-21¹ Do 22 Do 23 Do 24 Do 25³/Kl 25 Do 26/Kl 26 8-27/Do 27³/Kl 27⁵ 8-28¹ 8-29/Do 29 Fw 30 Kl 31 Kl 32 Ju 33 Ju 34 Kl 35 Kl 36 He 37³ He 38/Ju 38 DFS 39 BV 40/DFS 40 He 41³ Fw 42/He 42 Fw 43 Fw 44 He 45 He 46/Ju 46 Fw 47/He 47 Ju 48 He 49/Ju 49 He 50/Ju 50 He 51 He 52/Ju 52 Ju 53 DFS 54/NR 54 Fw 55/NR 55 Fw 56 8-57⁴/Fw 57 Fw 58/He 58 He 59 He 60/Ju 60 Fa 61/Fw 61/He 61 Fw 62/He 62 8-63⁴/He 63 Ar 64/He 64 Ar 65/He 65 Ar 66/He 66 Ar 67 Ar 68 Ar 69 He 70 8-71⁴/He 71 He 72 He 73³ He 74 Al 75 Ar 76/FZG 76⁶ Ar 77 Ar 78³ Ar 79 Ar 80 Ar 81 8-82⁴ 8-83¹ Al 84 Ju 85 Ju 86 Ju 87 Ju 88 Ju 89 Ju 90 Ju 91³ Ju 92 Ju 93³ Ju 94³ Ar 95 Ar 96 Fi 97 Fi 98 Fi 99 Fi 100³/He 100
101 to 200	Al 101 Al 102 Al 103/Fi 103 R Fh 104 Kl 105 Kl 106 Kl 107 Bf 108 Bf 109/Bf 109R⁶ Bf 110 He 111/He 111U⁶ He 112 He 113⁶ He 114 He 115 He 116 He 117³/Hs 117 He 118 He 119 He 120 Hs 121 Hs 122 Hs 123 Hs 124 Hs 125 Hs 126 Hs 127 Hs 128 Hs 129 Hs 130 Bü 131 Bü 132³/Hs 132 Bü 133 Bü 134 Ha 135 Ha 136/Hü 136 Ha 137 BV 138 Ha 139 Ha 140 BV 141 BV 142 BV 143 BV 144 Go 145 Go 146 Go 147/Ju 147 Go 148³ Go 149 Go 150 Kl 151 Kl 152/Ta 152 Kl 153³/Ta 153 Kl 154³/Ta 154 BV 155/Kl 155³/Me 155 Fi 156 Fi 157 Fi 158 Fw 159 Ju 160 Bf 161 Bf 162/He 162 Bf 163/Li 163/Me 163 Me 164/MeC 164 Bf 165 Fi 166/FK 166 Fi 167 Fi 168 Fi 169³ He 170 He 171³ He 172 He 173³ He 174³ 8-175⁴ He 176 He 177 He 178 He 179⁵ Bü 180/He 180 Bü 181 Bü 182 Bü 183³/Ta 183 Fl 184 Fl 185 Fw 186/Ju 186 Fw 187/Ju 187² Fw 188³/Ju 188 Fw 189 Fw 190 Fw 191 Ao 192 Ao 193³/DFS 193 DFS 194/Me 194 Ar 195 Ar 196 Ar 197 Ar 198 Ar 199/FiSk 199 Do 200^4,6/Fw 200
201 to 300	Si 201 Si 202 DFS 203 Si 204 8-205¹ Fw 206 8-207¹ Me 208 Me 209 (I)/Me 209 (II) Me 210 Hü 211 8-212⁴/Do 212 8-213¹ Do 214 Do 215 Do 216 Do 217/Hs 217 Do 218³ He 219 He 220 Do 221³ BV 222 Fa 223 Fa 224 Ao 225/Fa 225 BV 226/Ho 226 FGP 227 DFS 228 Go 229/Ho 229 8-230/DFS 230 Ar 231 Ar 232 Ar 233 Ar 234 Do 235⁵ Fa 236³ BV 237 BV 238 8-239¹ Ar 240 Go 241 Go 242 Me 243³ Go 244 Go 245³ BV 246 8-247¹ Ju 248 BV 250/Ho 250 Ho 251 Ho 252 Ju 252 Fi 253/Ho 253 Ho 254/Ta 254 8-255¹ Fi 256 SK 257 8-258¹ Fw 259 8-260¹ Me 261/Me 261w Me 262 Me 263 (1941)/Me 263 (1942)/Me 263 (1945) Me 264 Fl 265/Me 265 Fa 266/Go 266³ Ho 267 Ju 268 Fa 269 He 270 We 271 He 272³ He 273³ He 274 He 275⁵ He 276³ He 277 He 278 He 279³ He 280 He 281³ Fl 282 Fa 283 Fa 284 Fl 285 Ju 286 Ju 287 Do 288^4,6/Ju 288 Ju 289³ Ju 290 Hs 291 As 292/Hs 292³ Hs 293 Hs 294 Hs 295 Ar 296/Hs 296 Hs 297 Hs 298 Ju 299³ Fw 300
301 to 349	DFS 301 8-302¹ 8-303¹ 8-304¹ 8-305¹ 8-306¹ 8-307¹ 8-308¹ Me 309 Zwilling Me 310 8-311¹ 8-312¹ 8-313⁴ 8-314¹ Hs 315 8-316¹ Do 317 Do 318 He 319 Me 320³ Me 321 Ju 322 Me 323/ZMe 323 8-324¹ Fa 325/Fw 325³ 8-326¹ Me 327 Me 328 Me 329 Fa 330 DFS 331 DFS 332 Fi 333 Ar 334³/Me 334 Do 335 Fa 336 Ju 337³ 8-338¹ Fl 339 Ar 340 8-341 WNF 342 He 343 Rk 344/So 344⁵ Go 345 DFS 346 Rk 347 8-348¹ Ba 349
Post-349 (non-sequential)	Ju 352 Fi 356/Ju 356 Me 362 Me 364² Me 368³ Ju 388 Ju 390 Fw 391 Ar 393³ Ar 396 Ta 400 Me 409 Me 410 Do 417 He 419 ZMe 423 Ar 430/Ka 430 Ar 432 Do 435 Ar 440 8-445⁴ DFS 446 Ju 452 Me 462² DFS 468 Ju 488 Fw 491 He 500 Me 509 Me 510 He 519 8-520⁴ ZSO 523 Ar 532 8-534⁴ Do 535/He 535 Me 600² Me 609 Ar 632 Do 635/He 635/Ju 635
¹ Not assigned ² Unofficial/proposed ³ Assigned, but not used before RLM was dissolved ⁴ Assigned to captured aircraft ⁵ Unconfirmed ⁶ Propaganda/cover designation Note: Official RLM designations had the prefix "8-", but this was usually dropped and replaced with the manufacturer's prefix.