Helicopter and butterfly

Citation

Igor, K. (2005), "Helicopter and butterfly", Aircraft Engineering and Aerospace Technology, Vol. 77 No. 4. https://doi.org/10.1108/aeat.2005.12777daf.001

Helicopter and butterfly

Helicopter and butterfly

Keywords: Aircraft, Helicopters

In the course of the historical development of the species in the animate nature flying had been “invented” several times: winged insect (dragon-flies and cockroaches) populated the forest of the Carboniferous period 280-300 million years ago, butterfly flitted into the air about 200 million years ago, and birds, too, have been proficient in flying for about 200 million years. As to man, the air space has remained closed to him until the most recent past. Full of envy he had to look on cabbage butterflies and insects were sailing ostensibly without effort in their element, dragon-flies as arrows were chasing victims, and stormy petrels withstood hurricanes on the high seas child’s play. He himself continued to be fettered down to the earth’s surface. It is therefore only too understandable that at least in his legends the heroes were able to fly with the aid of sophisticated technical devices like Daedalus and his son Icarus in Greek mythology (Figure 1). According to the legend Daedalus made wings for his son and himself out of bird’s wings and wax in order to be able to escape from a prison in Crete.

Figure 1 Daedalus and his son Icarus

The helicopter is an aircraft that uses rotating wings to provide lift, propulsion, and control. The helicopter with its rotary wings can generate these forces even when the velocity of the vehicle itself is zero in contrast to fixed-wing aircraft (glider), which require a translational velocity to sustain flight. This aircraft therefore has the capability of vertical and translational flight.

A history of helicopter development is usually begun with mention of Chinese top and Leonardo da Vinci. The Chinese flying top (c. 400 B.C.) was a stick with a propeller on top, which was spun by the hands and released. Among da Vinci’s work (late 15th century) were sketches of a machine for vertical flight utilizing a strew-type propeller. Launoy and Bienvenu (France, 1784) demonstrated a model of a machine for vertical flight to the French academy of sciences. It had rotors constructed of feathers. These models had little impact on helicopter development. However, with the growing of scientific knowledge and the development of technology there have been-starting since first half of the 20th century – increasingly numerous attempts by bold and imaginative men to force open the gate to active flight by Man. Paul Corny (France, 1907) constructed a machine that made the first flight with a pilot (Corny). This helicopter achieved an altitude of about 0.3 m for 20 s. In the years 1910-1912, The Russian Yuryev devised the now generally common helicopter principle with a large horizontally mounted rotor and a small vertically placed after propeller serves torque compensation. In 1942 Igor Sicorsky (Sicorsky Aircraft Co. in the United States) built the R-4 (VS-316). Sicorsky’s aircraft is generally considered the first practical, truly operational helicopter, although a possible exception is the work of the German Professor Focke (Focke – Achgelis Fa- 223, 1941). Sicorsky’s success gave great impetus to the development of the helicopter in the United States. Many other designs began development and production in the next few years. The operation use of the helicopter has grown to a major factor in air transportation system. The invention of the helicopter may be considered complete by the early 1950s. Helicopter engineering is thus now involved more with research and with development than with invention.

The initial development of rotary- wing aircraft faced three major problems that had to be overcome to achieve a successful vehicle. The first problem was to develop a light and strong structure for rotor blades while maintaining good aerodynamic efficiency. The second problem was to design the quiet helicopter. Aircraft noise is an increasingly important factor in air transportation, as it is the primary form of interaction of the system with a large part of society. Moreover, the acoustic detectability of the helicopter is often determined by the rotor noise, unless some effort is made to quit it. The final problem was to minimize the vibration of the helicopter. The rotor is a source of vibration, hence increased maintenance cost, passenger discomfort, and pilot fatigue. All these factors can be overcome to design a highly successful aircraft.

A progressive orientation in engineer work – referred to as bionics. Its main objective has been to create while following the example of animate nature new kinds of highly effective machines for the benefit of man. For this bionics prepares the ground by systematically investigating the multiplicity of biological structures, form and processes and ways these are functionally interrelated. Graphic examples of bionical research are given in the following paragraph.

Otto Lilienthal (Germany, 1893) together with his brother Gustav were the first to investigate scientifically the flight of birds and the airflow past a wing. He also came to appreciate the importance of the arched profile. This first example of bionical research has a substantial impact on the development of aircraft. Otto Lilienthal wrote the book “Bird Flight As a Basis for the Art of Flight”, which appeared in 1889 and was translated into different languages. Vasiliy Slesarev (Russia, 1914) experimented with insects in a diminutive wing-tunnel. It was discovered that the high aspect ratio wings of dragonflies required for a good aerodynamic efficiency of the light and strong planes of insects. He was the first to use the large propeller disk area of aircraft. However, the latter effort reached a dead end in the early 1921s because of the time and place of its development. After the Second World War, W.O. Kramer carried extensive researches in the USA with dolphin skins, and designed an artificial two-layer damping skin similar to that of the dolphins, using rubber and silicon preparation. By means of this skin the drag of test profiles could be reduced to about 40 per cent.

Skin friction drag for a large commercial aircraft is of the order of 40 per cent of the total. Consequently, Boeing Aircraft, NASA (United States) and Airbus (Germany) tested specially airplane’s skin for this purpose. Experimental studies in the 1970s showed that small grooves (riblets) aligned with the flow had the property of modifying the near-wall structure of the boundary layer. This surface had less drag of a flat plate. In flight test, the film riblets, developed by 3 M company (United States), demonstrated a drag reduction capability of about 8 percent, when it is attached to surface of an A 340-300 airliner. The fuel consumption reduction data published in the technical literature is variable, but converging to figure of 2 per cent. The riblets are barely perceptible to the touch, and they appear like a matte finish on the aircraft skin. Early in the game of riblets research same investigators found confirmation of grooving’s effectiveness in a clue from nature: it was learned that fast swimming shark have riblet-like projection on their skins, called dermal denticles, they are made of the same material as shark’s teeth and typically have four or five grooves on what appears to the naked eye to be such a smooth surface. Airbus engineers have nicknamed riblets “shark skin” because of its similarity to the predator’s drag- cutting outer envelope.

Butterflies and moths both belong to the insect order Lepidoptera. The word “Lepidoptera” is derived from a Greek word meaning “scale wing”. The surface of the wings of these adult insects is covered with thousand of tiny scales. The scales are arranged in highly ordered rows in the same fashion as slate tiles on a roof. When we handle butterflies and moths, the “dust” that comes off is composed of these very small scales.

Investigation of the structures, forms and functions of scales is begun ever since medieval times. Theodore de Mayerne, physician to Charles 1 (England, 1634) described colors and patterns on the wings of butterflies. The development of microscope and the growing of scientific knowledge had a substantial impact on the research the cuticular appendages of insect planes. It was showed that the scales create the wonderful colors and patterns observed in butterfly wings. The wing scales of the Pyramies family of butterflies were among the first to be examined in the early 1930s in Austria.

Each scale is a long and flattened extension of a material called cuticle that originates from a single epidermal cell. Close examination of the cross- section of single Pyramies atalanta (L) butterfly wing scale shows clear evidence on a two-layered structure (Plate 1). The scale is comprised of two layers of cuticle and separated by a hollow region filled with air. These layers are held apart by tiny vertical rods of cuticle. The upper layer has grooves and discrete openings. The bottom layer is a thin cuticle film.

Plate 1 A vertical cross-section of Pyramies atalanta (L) butterfly scale

Nachtigall W. (Germany, 1970) examined the butterflies in a wing- tunnel. The studies showed that the presence of the scales acted as an aid to the aerodynamics of the fixed-wing. Wasserthal L. (Germany, 1975) examined a influence of the butterfly wing scale on regulation of body temperature. These experiments showed that the scales were involved in the process of temperature control of the body. Kovalev Igor (Russia and Israel, 1990) examined the moths in wing-tunnel. Three major effects of the scale coverage were discovered. The first effect is to minimize the vibration. The energy of the wing loads goes into the scale coverage rather than into body motion. The second effect is to decrease the noise produced by the flying insect. A low noise due to the absorption of significant part of the sound energy as well as the turbulent reduction of the wing surface by the scales. The final effect is to absorb the ultrasonic squeaks produced by bats. In this way the moths protect themselves against the echo- navigation system bats use to locate prey.

Moreover, nature studies showed, that insect scale coverage increased the aerodynamic forces of wing in flapping flight. However, scales removal limited the air maneuver capability of the insect. It follows that the scales are an important factor in insect maneuverability. This property of the scales allows the butterflies to overcome the predator attacks in the sky. It was suggesting the use of a scale-like skin on the rotor blade of helicopter.

There is doubt that nature is not a constructor in the sense that the engineer is. In the last resort she is inimitable. Even so the engineer should venture a glance at biological structures of the butterfly scale, he will hardly find ready-made solution of his own technical problems but he may expect a variety of interesting hints.

In order to eliminate the problems of rotor blades of helicopter, Kovalev Igor devised a metallic version of the butterfly scale coverage, called butterfly skin, or moth skin. This skin is composed of two layers (Figure 2). The recess separates the upper wall and the lower wall.

Figure 2 A model rotor with the butterfly skin

The surface facing the flow of the external wall is covered with a large number of grooves aligned with the flow. The ridges are formed between grooves. The grooves are provided with lines of perforations. The lower wall similar the thin sheet. Butterfly skin is attached to the smooth outer surface of model rotor, which was then tested alongside a non- coated model in wing-tunnel. The performance of the two models was markedly different. The butterfly skin was found to increase the trust force of wing of about 20 per cent, reduce flap resistance, decrease the noise, and reduce frequency waves by 15 per cent.

Reduce the drag reduction

When the helicopter is flying the radial flow present in the turbulent boundary layer align with and are fixed to ridges producing a 5 percent reduction in skin drag. There is same pressure difference in a turbulent flow. If a region of high pressure is involved on the upper wall, fluid is aspirated through the perforations in the recess from the flow. On the other hand, in a region of low pressure a discharge of low pressure a discharge of fluid in the direction of flow takes place. Consequently, the recess serves for pressure compensation. The discharge of aspiration through the perforations removes the low-speed areas of the boundary layers before they can participate in the growth and eruption of the eddies. This results in a total drag reduction of 10 per cent or more.

Decrease the vibration. In steady-state forward flight, the periodic forces at the blade are transmitted to the helicopter airframe, producing a periodic vibratory response. Riblets of upper wall are employed in combination with wall perforations minimize the helicopter vibrations by drawing air in the recess and returning in via the spaces between their sides. The energy of the transmitted vibration thus is highly dissipated into the butterfly skin.

Decrease the noise

The contributions to helicopter rotor noise can be classified as blade slap and rotational noise. The order of importance of the sources of main rotor noise is blade slap. Blade slap tends to occur most often in such maneuvers as flare to landing shallow descents, and decelerating steep turns, and at high forward speeds. Blade slap is produced by the periodic lift acting on the blade, which results in impulsive sound wave W radiation (Figure 3). The wave W produces change, with higher pressure downstream d.s. of the slap and the recess R forms a plenum through which fluid is transferred in a direction counter to the fluid flow f from the higher pressure region downstream d.s. of the slap to the lower pressure upstream u.s. of the slap. The permitting recirculation of boundary layer fluid in this manner is known for the reduction of slap effect, in particular the pressure drag. The principal source of rotational noise is boundary layer turbulence. It is known that any improvement in aerodynamic efficiency the rotor can be reduced both the helicopter rotor noise and vibration, which are transmitted to the fuselage. The butterfly skin improves the boundary layer flow (as described above), hence reduces both the rotational noise and vibration.

Figure 3 An axial section of butterfly skin in a fluid flow in which there is a sound wave W. P- perforations; f – fluid flow; u.s. – upstream; d.s. – downstream; r – riblets; R – recess; W – sound wave

Increase the trust force

Blade pitch motion allows control of the trust and other forces of the rotor on the aircraft that control its position, attitude and velocity. The relative motion of a wing surface with respect to the air generates aerodynamic forces. These forces are proportional to the surface area of wing. So the larger wetted area, the larger forces of the rotor. The blade with butterfly scale has both upper wall and lower wall. The blade without butterfly scale only has a smooth outer wall. Consequently, the rotor with butterfly skin has a larger wetted area. At high angles of attack this blade therefore increases the thrust force. In that case the butterfly scale becomes a very effective mean of improving performances of the wing. It is evidently that higher performance of main blades with butterfly scale will ensure outstanding flying quality, safety, comfort and maneuverability of aircraft.

In summary, the butterfly skin also could be used in transmission lines, on submarines, cares, sails, parachutes and in jet engine.

Kovalev Igor