Versatile Bomber — Do-217

by William S Friedman

Horizontal and dive-bomber. minelayer and torpedo-plane. the Dornier is proving itself a dangerous air adversary.

The mark of a real tactician is his ability to examine the adversary's forces and equipment objectively, note well all virtues and seek out both apparent and hidden weaknesses. Having once determined these, he can proceed to the attack with available equipment, exposing his strongest points diametrically to the opposition's weaknesses. Brigadier General Claire Chennault held the air over a considerable strip of China by the religious application to an air modification of a primary military theorem laid down by von Clausewitz in Von Kreig, originally written for Friedrich the Great more than a century ago. The object of these descriptions is neither to praise nor condemn the enemy's equipment. It is to study them through the eyes of the practical technician, fitting whatever information we can glean from the enemy into our stock of idea ammunition.


One of the most dangerous pieces of equipment that the German Luftwaffe has brought into the field is the Dornier Do-217E, an all-round weapon with enough performance in several classifications to make itself a nasty adversary. By the same token, it has the weaknesses of all general-purpose equipment, and has, with surprising frequency, fallen victim to specialized equipment. Many specimens of the Dornier series are currently in the possession of the British Air Ministry, and a comprehensive analyses have been issued.

Built for horizontal and dive bombing, mine-laying, and torpedo carriage, the Do-217E is a high-wing monoplane with twin fins and rudders, usually powered by two 1,460-hp 14-cylinder radial air-cooled engines. It carries a normal crew of four: pilot, bombardier, top-gunner-radio operator, and belly gunner. The ship is a direct and palpable evolution from the Do-17 series, and like the Do-17 and 215, crowds the entire crew into the section ahead of the leading edge. This gives the Do-217 the characteristic Dornier "snake's head" profile. Behind this the fuselage is long and slim, running to flattish sides with curved top and bottom. The landing gear retracts into the engine nacelles, the tail wheel is also fully retractable.

The ship's wingspan is 62'5", the chord at the root is 12'9", while the general aspect ratio is 6.62:1. The ship's estimated take-off load is 33,500 lbs, which brings the wing loading up to 59.4 lbs/sq ft. This, plus the presence of nose hook fittings and similar devices bolster the belief that the ship is operated with an assisted takeoff system. Tests with even an ordinary load gave the ship a takeoff run of 1,550 yards and an initial rate of climb of 638 ft/min. The ship's top speed at its most efficient altitude, 19,100 ft, was 309 mph.

Main Wing Structure

The Do-217's wing is made in three parts: a 29' 5" center section, which includes a portion of the fuselage, and two 16' 6" outboard panels which join the main member on the outer side of the engine nacelles. Two spars are used throughout the wing; each of which has a T-shaped top and bottom flange. In the center section, the front spar has a double or box web, the front one being. latticed, the rear, solid. The whole rear spar and the entire outer spar system are lattice web with T-shaped bracing members. Most of the ribs are built-up Warren truss type, alternated by a few solid blank ribs. Former ribs are placed between the front spar and leading edge at close intervals. Metal skin is flush-riveted to the skeleton.

The ailerons are slotted and are fitted with a device which permits; them to droop a little when the flap is lowered for takeoff. They are fitted with balance tabs and run from the wing-tip to the flap. Electrically motivated split trailing edge flaps are used; these extend from the aileron to the fuselage, interrupted by the back end of the engine nacelle. The flap is operated by a rather complicated electrical setup. A screw-jack, driven by an electric motor, moves a spring-loaded cam. This is attached to a link arm which, in turn is flexibly attached to the flap itself. The spring takes care of the recovery. Maximum flap angle is 55°.

Fuselage

As mentioned before, all the crew activity in the Dornier 217 takes place in the rather cramped section of the fuselage ahead of the wing. While this gives the men little elbow room, it has the advantage of taking them out of the proximity of the bomb bay, fuel tanks and other hazardous sections of the ship. This, however, is a product not of safety concern, but of the German Kamaradschaft principle, that the Nazi morale is improved fighting shoulder-to-shoulder. The fuselage itself is of stressed-skin construction with Z- and T-section stringers running the length of the ship. Z-section formers are used. The section of the fuselage from the crew section back is divided longitudinally in half. The lower portion forms the bomb cell, while the upper section contains transverse bracing to support the bomb load. There are three pairs of bomb doors. Two open simultaneously in ordinary operation, while the third one can be cut into the opening circuit at will, if the ship is used as a torpedo carrier.

Tail Unit

The tail section of the 217 contains several interesting features. In many ways, it resembles closely the predecessor types, chiefly in general outward appearance, single, unbalanced stabilizer and elevator equipped with adjustable stabilizer and trim tabs. Alongside of the trim tabs are a set of similar units which are motivated by electric solenoids which can be set before a diving attack to pull the ship out of a dive automatically.

Directly behind the tail is a section of the fuselage, a sort of false fairing which is used when the ship operates as a horizontal or torpedo bomber. When the ship acts as a dive bomber, the harpoon-type diving brake is installed. This system has been described in other editions of Air Tech much more completely. It consists roughly of a set of four flaps, set radially on a screw jack. The jack is powered by an electric motor. When the brake is opened, the flap sections move out into the air stream like an unfolding umbrella. The brake is closed by reversing the process. A mechanical provision is made for jettisoning the entire brake setup if it is badly damaged in combat.

Another interesting feature of the tail section is the incorporation of a full Handley-Page slot into the leading edge of the stabilizer. These are supposed to prevent stalling on one side of the stabilizer when flying on one engine. Thus the rudder will not overbalance, and the ship will not have any "crabbing" tendencies in single-engined flight.

Power Plant And Landing Gear

The ship is usually powered by two 1,460-hp 14-cylinder two-row BMW radials. These engines use a system of fan assistance for their cooling. This is the same engine as is used in the Focke-Wulf FW-190. In order to reduce the frontal area of the engine, the NACA-type cowling is drawn closer to the engine. While it leaves adequate room for air circulation required for cooling in flight, when the airspeed falls below a certain point, the air supply is not sufficient for adequate cooling. To overcome this, the front of the cowl is fitted with a magnesium fan, rotated by a spur from the reduction gear of the engine. When the fan reaches normal cooling speed, it automatically spins free of the gear train, robbing no more power from the engine. Schwarz three-bladed full-feathering propellers, fitted with compressed wood blades are used. Their diameter is 13'.

The ship is equipped with an unique heat de-icing system which collects warm air by means of a muff on the exhaust manifold. From there pipes conduct the air to vents along the leading edge, where the warm air is exhausted against the inner surface of the leading edge skin to prevent ice formation. Part of this same air is ducted into the cabin to keep the crew warm. A flapper valve is incorporated into the system so that the warm air can be spilled out through the back of the engine nacelle if it is not needed for de-icing.

Like the flaps and other moving auxiliaries, the landing gear is also electrically motivated. The struts draw directly back into the engine nacelles. Each landing leg consists of two straight plunger arms, attached to oleo-pneumatic shock struts. The tail wheel is also fully retractable.

Cockpit And Armament Arrangements

The pilot on the 217 sits in the conventional post in the forward, left side of the airplane. He is seated on a platform arrangement, protected by a curved deflector plate in the back. His seat is made of welded armor plate and more plate is built into the top of the fuselage above his head.

The bombardier sits on a lightweight folding seat next to him. The right side of the ship's nose is shatter-proof glass for several feet back, and, in action, the bombardier folds his seat away and assumes the prone position required for bombing.

The radio operator sits in a balcony seat with his head in the plastic upper turret. He serves as upper gunner and rearward lookout while operating the communications equipment. This consists of standard equipment for receiving a wave band from 30.0-35.5 MHz. The system also has equipment for the Emfanger Blindl blind approach system, geared to the Siemens visual indicator that has been standard for over four years. The transmitter is the FuG-18 type.

Armament in the Dornier varies with the theater in which it is used. Maximum armament, thus far has been 8 guns. The heaviest is a 20-mm Mauser shell-firing cannon. This is mounted on a fixed-flexible mount, with a special arrangement whereby it can be operated by the bombardier when he is not occupied at his sight. If necessary, the pilot can take over the 20-mm cannon and use it as a fixed gun firing straight forward. The pilot himself has a 15 mm cannon fixed below him that he can fire by sighting the ship. This is usually the Oerlikon gun, although the Mauser short-barreled 15 has been used alternately at this post. Vent holes for four 7.9-mm light machine guns exist in all the Do series, although four guns of this caliber are seldom found in the airplane. These guns are carried in racks inside, and slid through the vents when needed. These guns are used by whichever of the crew happens to be handy.

The top turret, operated by the radio man had a 13-mm machine gun, believed to be the Rheinmetal type, built in frank imitation of our .50-cal Colts. It is mounted free on a 360° mount with about 80° of astral travel. Directly below him is the rear gunner's position. Lying prone and facing the rear, he fires a flexibly mounted 13-mm gun out of the bottom. Some armor plate is provided at both of these posts. When not at his gun post, a folding seat is provided the gunner directly behind the bombardier.

The existence of fittings for a nose hook and other fittings at the tail, plus the ship's poor takeoff under normal conditions indicate that the Do-217 is operated by assisted takeoff. The extraordinarily high wing loading enables this relatively small airplane to get off with a bomb load of over 5,000 pounds and full tanks.

While several launching systems have been reported, one simple one comes back most consistently. (See sketch) This one calls for the establishment of a semi-portable high-speed winch from which a light steel cable is run through a swivel block to a hook on the forward section of the airplane. The tail is lifted into flying position by a high dolly, which is rigged to drop off as soon as the weight is lifted from it. Power is applied to the winch, stretching the cable taut as the pilot applies the wheel brakes and opens up both engines. At a given signal, full power is applied to the winch and the pilot releases his brakes and is shot along the runway by the combined power of his engines and the ground winch. Long before the ship reaches the block, it has gained sufficient flying speed to be able to drop both the dolly and the tow-line.

Other guesses as to the assisting system include the use of high-speed electric locomotives, rocket propulsion and catapults, similar to those used by the navies of the world.

By its military record, the Dornier series has indicated itself to be a formidable aircraft. The 217Es are fast, well-armed, easy-handling, middle-altitude ships. It is evidently an easy ship to maintain. Their entire engine units unbolt from the spar, permitting easy standardized handling. However, all accessories are electric and the ship is supplied by engine-driven generators, and power-plant failure results in loss of accessory use. This means that flaps and landing gear handle badly on a dead-engine landing. The trend in German design, however, is apparently toward the total elimination of hydraulic devices.

This article was originally published in the April, 1943, issue of Air Tech magazine, vol 2, no 4, pp 39-41, 66.
The original article includes 2 photos, one of the nose, one in flight, seen from 9 o'clock low, 2 photos of the dive brake, deployed and closed, the figure above and the cutaway drawing from Flight magazine.
Photos credited to European, Lindgren.