Dornier Do-335 "Pfeil"
By Heinz J.Nowarra, Edward T. Maloney
During World War II the German aircraft industry developed many outstanding military aircraft. In this stable of modern and unique aircraft was the most unconventional piston engine design to come out of the war. The Dornier Do-335 "Arrow" (Pfeil) featured a unique push-pull engine combination.
In 1942 the Technical Office of the German Air Ministry issued a request for a high speed bomber of twin engine design. Such an aircraft was already at hand—the Heinkel He-219 — but Ernest Heinkel was in disfavor by the German Air Ministry (R.L.M.) and his design was rebuffed. The design request was sent only to Junkers, Arado, and Dornier. All three firms undertook the project, but only the initial Dornier P-231 design was accepted.
In 1937 Claude Dornier took out patent rights on a push-pull engine aircraft. In 1940 he gave an order to Ulrich Hütter, a well known glider designer, to build an aircraft with an engine in front and another engined mounted on the aft fuselage driving a pusher propeller. This tandem engine design was to be used as a prototype design aircraft for future push-pull aircraft. Hütter drew experience from the Dornier Do-17 "Flying Pencil" for fuselage design and from the Dornier "Wal" flying boats.
Hütter's final design became known as the Göppingen Gö-9. It had a Hirth HM-60R engine of 80 h.p. buried in the fuselage under the wings, with an extended drive shaft to the propeller mounted in the rear. Dornier had had considerable experience with shaft driven engines in the Do-18 and Do-26 Flying Boats. These aircraft had proven very reliable over the Atlantic Ocean and North Sea air routes prior to World War II. Weight and speed tests were very encouraging, and this design led to the experimental Dornier Do P-231 project.
In early 1942 the war was going well for Germany. The R.L.M. — Air Ministry felt no urgent or current interest in the Dornier project. The officials disliked new revolutionary designs. However, in 1943 following German defeats in North Africa and Russia, the Luftwaffe urgently requested an aircraft which would be superior to Allied types of aircraft. The order was finally given to build this unique design. But in spite of this fact, construction of the first prototype, the Do-335 V-l took only nine months. Two years of valuable flight testing experience was already lost. Everyone who knew the events in Germany at that time viewed the situation with great alarm for the sands of time were slipping away for this outstanding aircraft and, like so many other German aircraft developments, Dornier "Arrow" would come too late.
The outline of the prototype Do-335 V-1 was generally similar to all future Do-335 design models. However, the oil cooler inlet on the Do-335 V-l model was mounted just below and aft of the front engine cowling. It was a single seat fighter of tremendous size. It carried the code letters CP+UT on the sides of the fuselage and under the wing. It did not carry armament and only limited ground to air radio equipment. The leading characteristic in identifying the Do-335 V-l on the ground was by the large circular wheel cover plates on the main landing gear.
The first test flights of the Do-335 V-l were very successful, and after the phase I test flights were completed at the Dornier factory in Oberpfaffenhofen Bavaria, the aircraft was transferred to the Luftwaffe Test Center at Rechlin.
A short time later Do-335 V-2 and V-3 left the factory. These two prototype models incorporated a new oil cooler ring mounted in the front engine cowling. The new location of the oil cooler improved engine oil cooling efficiency.
The Luftwaffe pilots who test-flew the Do-335 at Rechlin test center all praised the handling qualities of this outstanding design.
The only accident during the entire Dornier Do-335 testing program was caused by an engine failure. No structural trouble was experienced whatsoever. One day the factory test pilot Mosbacher was flying the Do-335 V-2 prototype when the rear engine became overheated and caught fire. The resultant crash and loss of the aircraft required a modification to the Do-335 V-3.
The Do-335 V-4 was the prototype model of the first two-place design Do-335. This project, which was initiated in 1943, was a much stronger and simplified version of the original Do-335. The fuselage was almost straight from front to rear except for the streamlined canopy. This aircraft was to be equipped with two Jumo 222's of 2500 h.p. each. This engine was never built in quantity.
The next experimental Do-335 V-5 prototype served as a test bed for guns and armament. It was equipped with one engine-mounted 30 m.m. MK-103 cannon and two MG-151/15 fitted above the engine. These two MG-151/15 were of 15 m.m. caliber. They had a special long barrel which made for a higher muzzle velocity. The 30 m.m. MK-103 cannon also had a barrel of increased length. This armament system proved very successful.
The final Dornier test programs were completed with the Do-335 V-6, V-7, and V-9. The Do-335 V-7 was sent to Junkers in Dessau to serve as a test bed for the Jumo 213A and 213E engine, which eventually was also to have been fitted to the Do-335. It was destroyed at the Junkers factory outside Dessau during an air raid. The Do-335 V-9 version was delivered to Rechlin during the summer of 1944. It was the basic model for pre-production plans of the Do-335 A-O.
The Do-335 V-8 was sent to Daimler-Benz for testing the Daimler-Benz DB-603 E and L engines.
After the Do-335 V-9 was completed, ten Do-335 A-0 were manufactured using the prototype V-9 as the basic model. It was one of these Do-335 A-0 aircraft that the French Ace Pierre Closterman mentions in his book as having seen. All ten Do-335 A-0's were completed and delivered between August-October 1944.
The Do-335 was officially designated "Kampfflugzeug" (literally meaning combat plane, but actually meaning — in official use — bomber). In November 1944 an order was issued by the R.L.M. cancelling all bomber construction work. However, the officials recognized the real value of the Do-335 design and no cancelling order was issued for it. Goering ordered production at the Heinkel plant in Oranienburg. Finally, orders were issued to speed up Do-335 production and development, but up until 1943 no priority could be obtained for this aircraft.
When the Do-335 V-10 two seat night fighter was completed more production orders were issued. The Do-335 V-10 would serve as the prototype model of the projected Do-335 A-6 night fighter series. It proved out very well in all tests in spite of a second cockpit for the radar operator, and the large radar antennae, which were installed in the wing leading edge. The R.L.M. had placed great confidence in the Do-335 A-6 Night Fighter as the aircraft to stop the nightly Mosquito attacks. The R.L.M. decided that the Heinkel plant at Vienna-Schwechat was now to produce the Do-335 under license. The other Heinkel plant that had been ordered to produce Do-335 aircraft at Oranienburg was at the time already in danger from mass bomb raids. But the Heinkel plants were overloaded with work from the He-162 and He-219 programs so that no large scale production could begin. Heinkel was also involved in developing a new large wing for the high altitude version of the Do-335.
Two experimental Do-335 training aircraft were built to train pilots on this unconventional aircraft. In external appearance they resembled the Do-335 V-10. Two experimental models were built, the Do-335 V-11 and V-12. These were to serve as prototypes for the Do-335 A10 series. The Do-335 A-10 trainer was derived from the Do-335 A-0, while the second trainer version, the Do-335 V-12, was derived from the Do-335 A-1.
The Do-335 A-4 was an unarmed, high speed, photo-recon aircraft similar to the Do-335 A-0. No more than two or three of these aircraft were completed.
Two further prototypes were built at the Dornier Works in Oberpfaffenhofen. The Do-335 V-13 served as the prototype model for the Do-335 B-1 series. The Do-335 V-14 served as prototype model for the Do-335 B-2 series. The Do-335 V-14 had the heaviest armament of all Do-335 aircraft—one engine mounted 30 m.m. MK-103 cannon and two MG-151/20 in the cowl and an additional two MK-103 cannons in the wing leading edges.
All Do-335, except the A-4 and B-1, could also be used a fighter bombers. They each had a bomb bay and they could carry one 500 Kg. bomb or two 250 Kg. bombs. A Revi C-12D gunsight was installed.
In the event the pilot was forced to make an emergency exit by bailing out, the vertical tail surfaces and rear propeller were jettisonable.
Due to critical delays in materials, Allied bombings and initial project delays, only ten pre-production Do-335 A-0 aircraft and eleven Do-335 A-1's were completed in addition to the fourteen prototype models Do-335 V-1 — V-14 and a small number of trainers. Altogether only thirty-eight Do-335's were completed for the Luftwaffe.
With a maximum speed of 417 m.p.h. at 26,000 feet with full military equipment, the Do-335 was one of the fastest propeller-driven fighters of the Second World War.