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Heinkel "Training Aircraft Type H.D 21",
This type was designed under the premise of creating a very special machine for the training of land and sea pilots. With this
in mind, special emphasis was placed on the design of the machine to achieve pleasant flight characteristics in connection with the most modern experience in aircraft construction, as well as to be able to use the machine for aerobatics.
The demands that modern school operations place on such types were largely taken into account. - The "overall structure of the aircraft is kept clear and simple in all
its parts, furthermore great importance is attached to operational safety, which is particularly evident in the engine system.
The aircraft is designed as a single-arm biplane with staggered wing decks and has only one support and mating cable in each half of the wing, which are arranged in one plane
In order to reduce the mutual influence of the two decks, their distance is particularly large. This wide distance between the wings, as well as the strong staggering, also achieves a particularly favourable angle of view for the two occupants.
The hull is a construction of four longitudinal wooden spars with wooden frames and is completely planked with plywood plates on the outside.
The two seats are arranged one behind the other and lie behind the tensioning tower struts, so that in the event of a possible rollover, the two occupants cannot be trapped by the tensioning tower ; furthermore, this seating arrangement allows for extremely comfortable entry. In the front fuselage part under the tension tower there is a space for luggage, in which there is an emergency seat that can be used for a third person.
If the aircraft is not to be used for schooling, the front pupil control system must be easily removed. - To enable better communication between teacher and
student, the fuselage frame between the two seats is provided with an opening. Both seats are adjustable.
The fuselage is closed off from the engine by a continuous metal fire bulkhead. The four fuselage spars reach up to this fire bulkhead and carry fittings at their
endpoints, to which the engine foundation is connected.
The engine is mounted on a tubular steel foundation, which is connected to the corresponding fittings of the fuselage spars with 4 bolts, a principle that has repeatedly proven itself in
our designs and is used almost exclusively. The engine foundation also supports the radiator system and the other units belonging to the
engine. As a result, the entire engine system can be removed in the shortest possible time and replaced by a new one. In addition, the complete
engine system can be put to the test and thus the possibility of subjecting the complete engine system to a detailed test before installation.
The bonnet is hinged for viewing the engine system.
The upper and lower support decks are divided. The spars are double-T-beams made of American Spruce.
All 4 halves of the wing deck have a normal profile throughout. This results in a significant saving in the factoryas well as easy and cheap repair options.
The staggering of the two wing decks is chosen in such a way that the rear beam of the upper support deck lies over the front rail of the lower support deck. The resulting
level is crossed out by 2 cables. This arrangement has the advantage of only two cables and in combination with the N-stem this arrangement gives a completely statically
determined cell.
The wings are constructed in such a way that instead of an internal bracing, the front and rear spars are connected on the underside by Fournier plates to absorb the
shear loads, thus achieving a lighter design and simplified repair possibilities.
The fuel is housed in two easily removable drop containers in the upper support deck. The fuel gauges are clearly visible on the underside of the upper deck.
The chassis is designed as a split axle with elastic bands. The fact that each half of the axle can swing independently of the other around the central pivot point
causes it to adapt to the uneven ground, which enables safe climbing and landing. Replacement of the land frame with a float frame is
planned.
The dismantling of the supporting deck is only done by loosening a turnbuckle on both sides of the opposite cable, after which the N-stem can be folded under the upper deck, and both halves of the supporting deck can be unhooked by turning downwards.
The take-off length of the machine is normal and is about 60-80 m when there is no wind. The climb speed is very good. The manoeuvrability of the machine is also good; the
aircraft lies firmly in the curve and has no tendency to spin. With the machine, all aerobatics can be carried out easily.
The 120 or 100 hp engine is usually installed; however, a correspondingly different engine between 100 and 150 hp can also be installed.
When the 120 hp Mercedes is installed, the aircraft is equipped with fuel for about 4 full throttle hours.
The main data of the aircraft are as follows when installing the 120 hp Mercedes: wingspan 10.00 m, total length 7.25 m, total height 2.95 m, empty weight 710 kg,
payload 270 kg, full load 980 kg, speed in level flight 145 km/h, climbing ability 1000 m in 5 min., 2000 m in 1.13 min., 3000 m in 1.28 min., range approx. 570 km.