Evolution of any engineering system obeys objective laws. Parts of engineering systems develop in accordance with certain trends [1]. Let us try to identify the main evolution trend of an aircraft propulsive device by analyzing its evolution line starting from the first airplanes and finishing with spaceships.
There are a great many of aircraft propulsive devices based on different principles of operation. For instance, a propeller. In technical literature, one can find descriptions of propellers of various shapes and sizes, with different number of blades, manufactured of different materials, with adjustable and non-adjustable angle of installation of blades, etc. There are jet engines of airplanes, spaceship engines. To orientate in this diversity, it would be useful to construct the “Evolution Tree” Within the framework of this article, we are only going to describe the construction of one of this tree's lines.

A sailplane flies without an engine. In non-turbulent air, it is moved by the gravity force and its wings provide support while sliding from an invisible air mountain. If there are ascending streams, the sailplane wings receive a lift from the vertical air streams. In contrast with a sailplane, an airplane has an engine. The engine activates a propulsive device – a propeller or a reaction jet, which pushes the aircraft forward. When flying in the atmosphere, the wings change the motion speed of an aircraft into a lift that holds the aircraft in the air.

Thus, a propulsive device is what an aircraft uses to push off the surrounding space while flying.

The traditional propeller of the first airplanes was an airscrew, a propeller formed of one or several obliquely installed shaped blades. When rotating, the blades cut the air and threw it back moving the aircraft forward.

If we line up the versions of a propeller in a certain sequence, the starting position from the point of view of the evolution laws will be occupied by a single-blade version. It has a very simple structure – one shaped blade and a short massive counterbalance. It is easy to manufacture and statically balance such a propeller by modeling the blade and properly choosing the counterbalance weight. But it is impossible to dynamically balance it because a rotating single-blade propeller will always press on the air with one, unbalanced blade. This is quite an exotic construction only used in low-power target airplanes and airplane models having high-speed engines.
Much more popular is a double-blade propeller. It, on the contrary, is fully balanced dynamically but requires careful static balancing and a perfectly alike blades' shape.
Increasing the engine power requires a respective increase in the propeller efficiency. Such resources as the increase of the rotation speed and screw diameter are limited. Already at 2000 rpm the ends of the blades of a propeller having 3 m in diameter move at a supersonic speed. This causes airflow stall at the ends of the blades, the screw efficiency drops, whereas increasing its efficiency by other methods is attended with known difficulties. Increasing the propeller diameter increases the flying machine size, so it is necessary to lengthen the landing gear struts to provide a necessary clearance while taking off and landing. One of the methods to partially solve this contradiction is the inverted gull wing.
The most radical method for increasing the propeller efficiency is increasing the number of blades. The next step in our line is a multi-blade propeller with several blades (four, five, maximum eight). But the number of blades is also limited. If they are too many, they start interfering with each other, shield each other and the efficiency of the propeller decreases.

 

 

The following step naturally suggests itself: if it is impossible to increase the propeller diameter and the number of blades, why not to arrange them in two rows? It seems as if we can double the efficiency, but it is not that simple! If you install two multi-blade propellers on the engine shaft one after another, then the rear propeller will work under much worse conditions than the front one. The blades of the rear propeller will try to additionally accelerate and whirl the already turbulent air. But they will rather decelerate it.
To improve the efficiency of double-row propellers, they began installing them with spacing, at the front and rear ends of a powerful engine shaft. This spacing decreases the aerodynamic shielding of the rotating blades thereby increasing the efficiency of the aircraft power plant. But the desired effect was not produced.

The efficiency of double-row propellers could be considerably increased by making them rotate in opposite directions (Fig. 2). The air whirled by the blades of the first propeller gets to the blades of the second one not tangentially, but with a necessary stop thus gathering speed.

One blade, two… four, two rows of blades… The number of the propeller blades is growing. It seems that the TRIZ trend known as “Mono-bi-poly” , manifests itself. Don't hurry!

In the 40s of the last century, the speed of the propeller-driven aircraft reached its maximum. One of the fastest propeller airplanes was P-51 Mustang. It could travel at a speed of a little more than 700 km/hr. Further increase of speed was only possible by using a jet principle of thrust generation.
A transitional variant between an airscrew and jet propeller is the gas turbine principle of thrust production [3]. Thrust is produced by the joint action of the propeller blades and the reaction jet created by the turbine that rotates the propeller. In addition to the main propeller, this engine has a great number of small blades located in the turbine. Strictly speaking, the blades of the turboprop engine do not participate in thrust generation, but serve to rotate the propeller and supply air to the combustion chamber.

The next version of the aircraft propulsive device is a reaction jet. It can be produced by using a turbojet engine that is widely employed in modern aviation. The internal blades of this engine only accelerate the oncoming air and supply it into the combustion chamber. The engine thrust occurs due to the abrupt expansion of air while burning and the formation of a reaction jet. The jet engine itself does not have any blades. The propulsive device is a reaction jet formed while burning the fuel mixed with air and oxidizer.

The next step is transition to the level when the aircraft propulsive device consists of parts of atoms – an accelerated ion flow. Ion engine has the following structure (Fig. 3. The ion engine operating on “Deep Space 1” station).   The cathode bombards the fuel (for instance, xenon) with electrons. These knock electrons out of xenon atoms leaving positively charged ions (Xe+). The ions are accelerated by charged lattices, thereby forming a high-speed reaction jet, and accelerate the aircraft. The power sources for creating electrostatic fields on low-power engines are solar cells while on more powerful ones passive nuclear generators like those installed on orbital satellites are used [4].

The plasma engine, in which thrust is generated by plasma outflow, is almost at the same level of hierarchy. Plasma is formed by exposing a solid (fuel) to powerful pulses of laser radiation or to pulsed electron beams. Plasma is accelerated at the expense of Ampere force by joint action of crossed electric and magnetic fields.

Our propulsive device is becoming smaller and smaller, the particles that accelerate the aircraft have passed to the level of molecules, atoms and their fractions. The logic conclusion of this chain is as follows: thrust needed to accelerate the aircraft is produced by a directed outflow of photons emitted by a photon engine. At the present stage of technology, it is impossible to produce a photon flow of a required power, but designs of engines that use a reflected flow of photons for acceleration, the so-called “light pressure”, are known and are being successfully realized. They are, first of all, space sales moved by “solar wind” (Fig. 4). Recently, experiments on using a laser beam for accelerating spaceships have been conducted.

So what do we have?
In the second part of the propulsive device evolution line, evolution is obviously directed towards reduction, segmentation of elements used by aircraft for pushing off the space: “small particles and molecules => parts of atoms => photons”. At the same time, the set of transformations in the first part of this line reminds strongly of the trend " “mono-bi-poly”: “one blade => two blades => many blades => two rows of blades”.
Now then, don't the laws work? Have we missed something? Let us try to study out the situation.

How can we distinguish “Mono- bi- poly” from “Segmentation”?
And why do we need doing this?

First of all, let us answer the second question. When analyzing the evolution lines of the “Evolution Tree”, it often happens that only several variants of object or system transformation are known. It is very important to correctly determine the trend according to which they are lined up, so as to adequately fill the gaps and bring the line to the logic completion.

“Mono-bi-poly” is easy to distinguish from "Segmentation:

Mono-bi-poly	Segmentation
1. Quantitative parameters describing the element's functions (efficiency, reliability, etc.) grow in proportion with the number of introduced elements and systems.	1. Quantitative parameters describing the element's functions (efficiency, reliability, etc.) remain unchanged.
2. Evolution is accompanied by the growth in the number of elements with simultaneous preservation or increase of their size and mass.	2. Evolution is accompanied by the growth in the number elements with the simultaneous reduction of their size and mass.
3. At the end of the line, transfer to a convolved polysystem (monosystem of a new level) is observed).	3. At the end of the line, transfer to the microlevel is observed.

Let us analyze the situation using this table.
When the increase in the number of blades is caused by the growth of the airplane engine power, the following reasoning will be suitable (Fig.5). If we use a single blade propeller as we transfer from the model on Fig. 5a to a sporting aircraft with a more powerful engine, then the propeller must be very large in order to provide the maximal load on the engine (Fig. 5b). To ensure the required diameter, we must mentally divide, segment one blade into two parts thereby forming a two-blade propeller. To transfer to an airplane with an even more powerful engine, we must segment the hypothetical huge two-blade propeller in the same way and transform it into a four-blade propeller of an acceptable size (Fig. 5c). And so on…

Consequently
1. The propeller thrust remains within allowable scope with respect to the aircraft engine power and does not grow with the growing number of blades.

When the engine power does not change, the optimal engine load is provided by reducing the size of propeller blades with simultaneous growth of their number (Fig. 6).

Consequently
2. The size of the propeller elements (blades) decreases with the increase in their number.

At the end of the second part of the aircraft evolution line, an obvious transfer to the microlevel is observed. This is an additional prompt which allows admitting an assumption that
The trend “Segmentation” prevails in the described evolution line of the aircraft propulsive device.

Let us tabulate the data:

We can make a blitz-analysis of the obtained through example and try to fill the empty boxes by seeking for analogy in literature and thinking a little.
Take as an instance granules. How can we use them for creating a moving pulse? There is a joky example when Captain Vrungel [5] accelerated his yacht by shooting corks with champagne bottles set to the stern. A similar analogy under space conditions prompts the following idea: to use the throwing away of stages for additional acceleration of a spaceship.
The next empty box: liquid. Here we see the possibility to use it jointly with the airplane propeller. If some water is sprayed in the propeller rotation area, mist is formed and the propeller thrust increases abruptly. Of course, it is costly to carry water all the time, but this effect may be used for a short time, for instance for increasing the airplane acceleration while taking off.
Vacuum. In this case, we mean something pulling, located ahead of the aircraft. For instance, fuel is burnt ahead of the rocket moving in the atmosphere. A low-pressure area is formed. It allows an abrupt decrease in air resistance [6].

We have not managed to fill the empty boxes quite correctly. The fuel tanks of the Shuttle bears little resemblance to granules. But this does not mean that the number of spaceship stages will not grow in future and their size will not decrease considerably.

The author is grateful to Georgy Severinets for assistance in preparing this article.

 

List of reference.
1. Althsuller G.S., Zlotin B.L., Zusman A.V., Filatov V.I. SEARCH FOR NEW IDEAS: FROM INSIGHT TO TECHNOLOGY (THEORY AND PRACTISE OF INVENTIVE PROBLEM SOLVING), Kishinev: Kartya Moldovenyaska Publishing House, 1989
2. Paolo Balocchi. Study of single-blade propulsion system retractable engine sailplanes. Alisport Srl, Italy. http://www.alisport.com/pdf/ostiv_IN.pdf
3. Turbo-jet engine NK-12. http://ephf.ispu.ru/avio/nk-12.htm
4. Deep space. http://nmp.jpl.nasa.gov/ds1/
5. A. Nekrasov. The Captain Vrungel's adventures. "http://www.03www.ru/skazki/s401.htm
6. Leonard David, “On Wings Of Light”, New Scientist 1998, N 21165