An “Ugly Duckling” of Aviation: The one and only turbojet biplane.
… Time is running out inevitably. People are also leaving this world synchronously with the time. If the information from these people is not recorded properly – it will utterly disappear along with the people. The only rumors, speculations, fantasies and legends will stay with the next generations. This article is more about people and time than about technology; however, readers will have a chance to dive into 60-70s and aviation challenges of that time.
So, at first 1960-s the USSR had the one and only airplane used in agricultural aviation – famous Antonov An-2 (time had shown that it is a super-successful airplane which is being in use in many countries and for variety of missions even today), but it was a biplane with ASh-62IR 1000 hp 9-cylinders Piston propeller aircraft engine, which was obsolete technical solution even for that time (spoiler: time had shown that this is not true even for 2020-s, but this is the another story!).
Soviet Agricultural Bush plane An-2 is spraying chemicals
An-2 productivity was, by the evaluations of Soviet officials in the 1960-s, insufficient, especially in aviation-chemical works in Agriculture. In those days, agricultural airplanes sprayed huge amounts of dry and liquid chemicals for various purposes: fertilizers and pesticides to control weeds and pests at fields and forests, these airplanes defoliated cotton and solved many other tasks. Nevertheless, it was impossible to seriously increase the performance of the An-2: this was clearly demonstrated by the operation of An-2M. According to some reports, this machine, in comparison with An-2, gave an increase in productivity by 30-40% and a decrease in the cost of processing fields by 20-25%, however, this was considered insufficient. Production difficulties, as well as numerous comments in operation, led to the decision to stop An-2M production. Almost all the An-2Ms were gifted to the allied countries: Cuba, Hungary, Bulgaria and Yugoslavia, while Soviet Aeroflot remained with its An-2s without the letter “M”. So, generally speaking, a new aircraft with new design solutions was strongly needed.
In 1964, the aircraft plant in Arsenyev, in Russian Far East, started mass production of the record A-15 glider and Antonov An-14 “Bee” airplane. To support the plant production, the group of Antonov OKB engineering representatives from Kyiv, Ukraine headed by the lead engineer Riamir Izmailov was sent to Arsenyev for a “business trip”. Previous missions of that small team during their work in Kyiv included (but was not limited to) the searching of ways to increase the productivity of aircraft intended to perform the agricultural chemical works. Although the main task of the representative office was to support the serial production of An-14 and A-15, the designers took the initiative and began to work on the project of an agricultural aircraft. They continued their mission in Kyiv: to collect and study the information on existing agricultural aircraft produced all over the world. However, since nowhere else as in the USSR the aircraft were not used in such volume to spray chemicals to the fields, the statistics collected did not give the engineers the useful information to make their own decisions. The overwhelming majority of agricultural aircraft had a significantly lower carrying capacity than the An-2 had. Depending on the altitude and speed of flight, the design of the spray equipment and required amount of chemicals per acre of the field and covered strip width was different and varied between 10 and 25 meters. This parameter was called the "working width". To increase the aircraft's productivity, it was necessary to significantly increase the working width. The question was how to do it? It should be noted that, in the main, aviation-chemical works were carried out with use of dry substances. Most types of fertilizers, compositions for liming the soil, even some types of pesticides were made in powdery or crystalline form. The amount of the substance introduced into the soil was measured in tens and hundreds of kilograms per hectare/acre. Therefore, the designers paid their main attention to working with dry substances. The “capture width” strongly depends on the state of the scattered substance. In those days, the Soviet chemical industry was tasked with producing chemicals in a form-factor that would ensure their reliable storage and delivery to the destination. However, this requirement was not met, which was a weak point in the entire agroindustry. Not only the plane, but also the tractor could not evenly introduce a chemical into the soil if it was a sticky lump weighing tens of kilograms. The solution to the problem was to manufacture the chemicals in granular form and deliver them to the point of use in a sealed package. This has already been done in the world and was planned in the USSR.
The sole purpose of the flight is chemicals delivery
Finally, it was decided to focus on granular chemicals since in this case the problem of a radical increase in the swath width can be solved with the aid of pneumatic transport: granular chemicals can be "fired" away from the flight line through nozzles located spanwise in the air channel. It was only needed to have a source of compressed air. The higher is firing speed, the further the granules fly. In addition, it had to be necessary to increase the aircraft's useful load to at least 2000-2200 kg to avoid too often returns to the base for reloading. Obviously, empty flights reduce the aircraft efficiency. Thus the Izmailov’s team figured out the concept of a specialized agricultural aircraft focused on performing only chemical works, and therefore more efficient in this particular field than the multipurpose An-2.
Where to take the proper engine?
This was a really painful problem for the Soviet aviation - where to get a suitable engine? This is no explanation why did the Soviet engine industry almost always lag behind not only the world level, but also from the urgent needs of own aviation! Moreover, it was a truly Hamlet question for all Soviet aircraft designers - to be or not to be for the new airplane. So, let’s consider which engines were proposed by the Soviet industry of 1960-s for the agricultural airplane:
1. Piston propeller engine ASh-62IR 1000 hp used on An-2s. Its power was obviously insufficient for the aircraft with a payload of 2000 kg.
2. TVD-10A Turboprop, 1000 hp, - clearly insufficient, moreover, this engine even had not been in full-scale production yet.
3. AI-24 Turbofan engine of 2400 hp. Powerful, but its dry weight was near 600 kg, and this is even without a propeller!
4. 2-cirquit Turbojet AI-25 with quite suitable takeoff thrust of 1500 kgf. Weight parameters were also suitable.
Well, what is there to choose from? No engine – no airplane! But the government demands an airplane! And also demands to get rid of the aviation petrol – so called “avgas”. This means that the engine have to be the gas turbine! So the Design group considered all pro et contra of usage of the turbojet engine and found more advantages than disadvantages:
- AI-25 had suitable dimensions;
- Was in full-scale production, had a resource about 6000 hours and was not heavy - about 300 kg dry weight;
- the absence of a propeller increases the safety of personnel when working at field aerodromes;
- allowed to take sufficient amounts of compressed air from both circuits to operate the pneumatic transport, hydraulic pump and cockpit ventilation.
Arguments against AI-25 were the following:
- high fuel consumption in comparison with turboprop, piston and turbofan engines;
- takeoff and landing parameters are worse than for the airplane with propeller engine: no airflow on the wing surface from propeller;
- nobody knew the turbojet engine behavior when it gets pesticides or dust at low altitudes.
So, having a rather poor choice, it was decided to install AI-25 turbojet on the agricultural airplane. Subsequently, this became the main source of criticism: "farmer, biplane type – and suddenly a jet!"
New Airplane aerodynamic configuration
The layout principles for the new turbojet agricultural airplane are the following:
1. Engine must be placed away from the ground to prevent it from the ingress of soil, dust and other objects and to minimize soil erosion from jet exhaust.
2. By placing the engine above the cockpit the designers got rid of the long inlet and exhaust pipe, reduced structural weight and drag.
3. In order to prevent the structural elements from jet exhaust a two-girder scheme and a U-shaped tail assembly was chosen. This layout was well known and previously used on jet airplanes around the world.
4. Spaced chemical tanks minimized the probability of ingress of chemicals to the cockpit. In addition, this increased the working width and saved tanks filling time: two tanks had the possibility to be filled by chemicals in parallel.
5. Air transport channels for dry chemicals should be hidden inside the wing, and the wing skin should be made of fiberglass to prevent corrosion. Since at that time it was impossible to build a monoplane with a composite skin, the biplane structure was chosen. This decision forced to abandon the lower wing mechanization.
6. The airplane should have the robust landing gear to be able to land on unpaved aerodromes and fields. So called “Bush plane” concept.
The new design bureau started working on a draft design and began to build a test model for the wind tunnel . The new aircraft was decided to be mass-produced in Poland, Mielec, under the brand name M-15, which meant "Mielec, model fifteenth". The intergovernmental protocol was signed between the USSR and Poland in 1971. The first flight was planned for 1972. In 1971, the Design Bureau relocated from Arsenyev to Poland. In the same year, a full-scale agricultural stand was built in Mielec for testing bulk chemicals pneumatic transport. Engineers from Warsaw Aviation institute built a flying laboratory based on An-2. At the same time, detailed design and preparation for serial production were on the way.
M-15 Manufacturing process, Mielec, Poland
M-15 First Flight. Poland, 1973
Totally 172 aircraft were built from 1973 till 1980, 155 delivered to the USSR. Soviet agroindustry was unable to operate M-15 normally. Chemicals were still stored in the open air, powders and crystalline substances clumped together. There was still no production of granular chemicals. There even were no adequate bootloaders to fill the airplane tanks. All this led to numerous negative reviews about the operation of this aircraft both from Aeroflot pilots and from authorities.
M-15 is spraying chemicals
In the 1970s a world fuel crisis began. Fuel prices began to rise in the USSR as well. AI-25 turbojet engines were not fuel-efficient: when they were designed, fuel-efficiency was not the first priority. So, in 1983 Aeroflot decided to terminate the M-15 program and dispose of all previously built airplanes of that type. As the ancient Romans said, "Sic transit gloria mundi".
Conclusions and epilogue
The main reason for this (and some other soviet airplanes) unsuccessful “destiny” was in the system itself: designers created, serial plants manufactured, financiers financed everything, consumers suffered - and all this was not connected into a single system! Decisions on the full-scale production of certain aircraft types were made before even flight tests completion. Then, the airplanes were improved during the operation, often with the stop of the entire fleet.
However, a positive result of this work was the creation of a qualified national Polish team of aircraft designers that later created a number of successful airplanes i.e, M-18 Dromader .