The inventor of the slat, Gustav Lachmann, at the end of the thirties of the last century, proposed to equip the tailless with a freely floating winglet placed in front of the wing. This winglet was equipped with a servo rudder, with the help of which its lifting force was regulated. It served to compensate for the additional dive moment of the wing that occurs when the flap is released. Since Lachmann was an employee of the company Handley-Page, she was the owner of the patent for this technical solution and this idea is mentioned under this brand in the technical literature. But there is still no practical implementation of this idea! What is the reason?
Balancing loss
An aircraft wing that creates lift has a concomitant, one might say negative, by-product of a dive moment tending to bring the aircraft into a dive. To prevent the aircraft from diving, there is a small winglet on its tail - a stabilizer, which prevents this dive, creating a downward, that is, negative, lifting force. Such an aerodynamic scheme of the aircraft is called "normal". Because the stabilizer lift is negative, it adds up to the aircraft's gravity, and the wing must have lift greater than gravity.
The difference between these forces is called balancing losses, which can reach up to 20%.
But the first flying aircraft of the Wright Brothers did not have such losses, because a small wing - a destabilizer that prevented a dive, was located not behind the wing, but in front of it. Such an aerodynamic scheme of the aircraft is called a "duck". And in order to prevent the aircraft from diving, the destabilizer must create an upward, that is, positive, lifting force. It adds up to the lift force of the wing, and this sum is equal to the gravity of the aircraft. As a result, the wing must create a lift force that is less than the force of gravity. And no loss of balance!
Stabilizer and destabilizer are combined into one term - horizontal tail or GO.
However, with the massive development of take-off and landing mechanization of the wing in the early thirties of the last century, the "duck" lost this advantage. The main element of mechanization is the flap - the rear part of the wing deflected downward. It approximately doubles the lift of the wing, due to which it is possible to reduce the speed during landing and takeoff, thereby saving on the mass of the chassis. But the by-product of swooping moment when extending the flap increases to such an extent that the destabilizer cannot cope with it, but the stabilizer can handle it. To break is not to build, in this case a positive force.
In order for the wing to create lift, it must be oriented at an angle to the direction of the oncoming air flow. This angle is called the angle of attack, and with its growth, the lifting force also grows, but not infinitely, but up to a critical angle, which is in the range from 15 to 25 degrees. Therefore, the total aerodynamic force is not directed strictly upwards, but is inclined towards the tail of the aircraft. And it can be decomposed into a component directed strictly upwards - the lifting force, and directed backwards - the aerodynamic drag force. The ratio of the lift force to the drag force is used to judge the aerodynamic quality of the aircraft, which can range from 7 to 25.
In favor of the normal scheme, such a phenomenon as the bevel of the air flow behind the wing, which consists in the downward deviation of the direction of the flow, is greater, the greater the lifting force of the wing. Therefore, when the flap is deflected due to aerodynamics, the actual negative angle of attack of the stabilizer automatically increases and, consequently, its negative lift.
In addition, in favor of the "normal" scheme, in comparison with the "duck", such a circumstance as ensuring the longitudinal stability of the aircraft flight also works. The angle of attack of an aircraft may change as a result of vertical movements of air masses. Aircraft are designed with this phenomenon in mind and tend to resist disturbances. Each surface of the aircraft has an aerodynamic focus - the point of application of the lift increment when the angle of attack changes. If we consider the resultant increments of the wing and GO, then the aircraft also has a focus. If the focus of the aircraft is behind the center of mass, then with a random increase in the angle of attack, the increment in lift tends to tilt the aircraft so that the angle of attack decreases. And the plane returns to the previous flight mode. At the same time, in the “normal” scheme, the wing creates a destabilizing moment (to increase the angle of attack), and the stabilizer creates a stabilizing moment (to decrease the angle of attack), and the latter prevails by about 10%. In the “duck”, the destabilizing moment is created by the destabilizer, and the stabilizing moment, and it is about 10% larger, is created by the wing. Therefore, an increase in the area and shoulder of the horizontal tail leads to an increase in stability in the normal scheme and to its decrease in the "duck". All moments act and are calculated relative to the center of mass of the aircraft (see Fig. 1).
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If the focus of the aircraft is ahead of the center of mass, then with a random slight increase in the angle of attack, it increases even more and the aircraft will be statically unstable. This mutual arrangement of the focus and the center of mass is used in modern fighters in order to load the stabilizer and get on it not a negative, but a positive lift. And the flight of the aircraft is provided not by aerodynamics, but by a four-fold duplicated automatic system of artificial stability, which “taxis” when the aircraft leaves the required angle of attack. When the automation is turned off, the aircraft begins to turn tail forward, this is the basis of the Pugachev Cobra figure, in which the pilot deliberately turns off the automation and, when the required tail turn angle is reached, fires a rocket into the rear hemisphere, and then turns on the automation again.
In the following, we consider only statically stable aircraft, since only such aircraft can be used in civil aviation.
The mutual arrangement of the focus of the aircraft and the center of mass characterizes the concept of "centering".
Since the focus is behind the center of mass, regardless of the scheme, the distance between them, called the stability margin, increases the GO arm in the normal scheme and decreases in the "duck".
The ratio of the shoulders of the wing and GO in the "duck" is such that the lifting force of the destabilizer with the maximum deviation of the elevators is fully used when the aircraft is brought to high angles of attack. And it will be missed when the flaps are released. Therefore, all the "ducks" of the famous American designer Rutan do not have any mechanization. His Voyager plane flew around the world for the first time in 1986 without landing or refueling.
The exception is Beechcraft Starship, but there, in order to use flaps, a very complex design with a variable destabilizer geometry was used, which could not be brought to a serially reproducible state, as a result of which the project was closed.
The shoulder of the wing depends to a large extent on how much the lift force of the destabilizer increases with an increase in its angle of attack by one degree, this parameter is called the derivative of the angle of attack of the lift coefficient or simply the derivative of the destabilizer. And, the smaller this derivative, the closer to the wing you can place the center of mass of the aircraft, therefore, the smaller the wing shoulder will be. To reduce this derivative, the author in 1992 proposed to carry out the destabilizer according to the biplane scheme (2). This makes it possible to reduce the shoulder of the wing so much that it eliminates the obstacle in using the flap on it. However, there is a side effect in the form of an increase in GO resistance due to biplaneness. In addition, there is a complication in the design of the aircraft, since it is actually necessary to manufacture two GOs, and not one.
Colleagues pointed out that the “biplane destabilizer” feature is available on the Wright Brothers’ aircraft, but not only a new feature is patented in inventions, but also a new set of features. The Wrights lacked the "flap" sign. In addition, if the set of features of a new invention is known, then in order for this invention to be recognized, at least one feature must be used for new purposes. In the Wrights, biplaneness was used to reduce the weight of the structure, and in the described invention, to reduce the derivative.
"Weathervane Duck"
Almost two decades ago, they remembered the idea of \uXNUMXb\uXNUMXbthe “weather duck”, mentioned at the beginning of the article.
It uses a feathered horizontal tail as a destabilizer - FGO, which consists of the destabilizer itself, pivotally placed on an axis perpendicular to the fuselage, and connected to the destabilizer servo. A sort of airplane of a normal scheme, where the wing of the airplane is the destabilizer of the CSF, and the stabilizer of the airplane is the CSF servo. And this airplane does not fly, but is placed on an axis, and it itself orients itself relative to the oncoming flow. By changing the negative angle of attack of the servo, we change the angle of attack of the destabilizer relative to the flow and, consequently, the lift force of the CSF during pitch control.
With a fixed position of the servo steering relative to the destabilizer, the CSF does not respond to vertical wind gusts, i.e. to changes in the angle of attack of the aircraft. Therefore, its derivative is zero. Based on our previous reasoning - the ideal option.
When testing the first aircraft of the “weather duck” scheme designed by A. Yurkonenko (3) with an effectively loaded CSF, more than two dozen successful flights were performed. At the same time, clear signs of aircraft instability were found (4).
"Super Resilience"
As it is not paradoxical, but the instability of the "weather vane" is a consequence of its "superstability". The stabilizing moment of a classical canard with a fixed GO is formed from the stabilizing moment of the wing and the counteracting destabilizing moment of the GO. In weather vane ducks, CSF does not participate in the formation of the stabilizing moment, and it is formed only from the stabilizing moment of the wing. Thus, the stabilizing moment of the "weather vane" is about ten times greater than that of the classical one. With an accidental increase in the angle of attack, the aircraft, under the influence of an excessive stabilizing moment of the wing, does not return to the previous mode, but “overshoots” it. After the “overshoot”, the aircraft acquires a reduced angle of attack compared to the previous regime, therefore, a stabilizing moment of another sign arises, also excessive, and thus self-oscillations occur, which the pilot is not able to extinguish.
One of the conditions for stability is the ability of an aircraft to level the effects of atmospheric disturbances. Therefore, in the absence of disturbances, a satisfactory flight of an unstable aircraft is possible. This explains the successful approaches of the YuAN-1 aircraft. In his distant youth, the author had a case when a new model of a glider flew in the evenings in calm weather for a total of at least 45 minutes, demonstrating quite satisfactory flights and showing bright instability - a nose-up alternated with a dive in the first flight in windy weather. As long as the weather was calm and there were no disturbances, the glider demonstrated satisfactory flight, but its adjustment was unstable. There was simply no reason to show this instability.
The described CSF can, in principle, be used in a "pseudo-duck". Such an aircraft is essentially a "tailless" scheme and has an appropriate centering. And his CSF is used only to compensate for the additional diving moment of the wing that occurs during the release of mechanization. In the cruising configuration, there is no load on the CSF. Thus, the CSF does not actually work in the main operational flight mode, and therefore its use in this variant is unproductive.
"KRASNOV-DUCK"
"Super-stability" can be eliminated by increasing the CSF derivative from zero to an acceptable level. This goal is achieved due to the fact that the angle of rotation of the FGO is significantly less than the angle of rotation of the servo caused by a change in the angle of attack of the aircraft (5). This is done by a very simple mechanism, shown in Fig. 2. CSF 1 and servo 3 are pivotally placed on the axis OO1. Rods 4 and 6 through hinges 5,7, 9,10 connect CSF 1 and servo 3 with rocker 8. Clutch 12 serves to change the length of rod 6 by the pilot to control the pitch. The rotation of CSF 1 is carried out not by the entire angle of deviation of the servo 3 relative to the aircraft when changing the direction of the oncoming flow, but only by its proportional part. If the proportion is equal to half, then under the action of the upward flow, leading to an increase in the angle of attack of the aircraft by 2 degrees, the actual angle of attack of the CSF will increase by only 1 degree. Accordingly, the CSF derivative will be two times less compared to the fixed GO. Dashed lines mark the position of CSF 1 and servo 3 after changing the angle of attack of the aircraft. Changing the proportion and, thus, determining the value of the derivative, is easy to implement by choosing the appropriate distances of the hinges 5 and 7 to the axis OO1.
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The reduction of the GO derivative due to feathering makes it possible to place the focus within any limits, and behind it the center of mass of the aircraft. This is the concept of aerodynamic centering shift. Thus, all restrictions on the use of modern mechanization of the wing in the "duck" scheme are removed while maintaining static stability.
"KRASNOV-FLUGER"
Everything is fine! But, there is a drawback. In order for CSF 1 to have a positive lifting force, a negative lifting force must act on servo 3. Analogy - the normal scheme of the aircraft. That is, there are losses for balancing, in this case, balancing the CSF. Hence the way to eliminate this shortcoming is the “duck” scheme. We place the servo in front of the CSF, as shown in Fig. 3.
CSF works as follows (6). As a result of the action of aerodynamic forces on CSF 1 and servo 4, CSF 1 is spontaneously set at a certain angle of attack to the direction of the oncoming flow. The angles of attack of CSF 1 and servo 4 have the same sign, therefore, the lift forces of these surfaces will have the same direction. That is, the aerodynamic force of the servo 4 does not reduce, but increases the lift force of the CSF 1. To increase the angle of attack of the aircraft, the pilot shifts the thrust 6 forward, as a result of which the servo 4 on the hinge 5 turns clockwise and the angle of attack of the servo 4 increases. This leads to an increase in the angle of attack of CSF 1, i.e., to an increase in its lifting force.
In addition to pitch control, the link provided by the thrust 7 provides an increase from zero to the required value of the CSF derivative.
Let us assume that the aircraft entered the updraft and its angle of attack increased. In this case, the beam 2 rotates counterclockwise and the hinges 9 and 8 in the absence of thrust 7 would have to approach each other. Thrust 7 prevents convergence and turns the servo 4 clockwise and thereby increases its angle of attack.
Thus, when the direction of the oncoming flow changes, the angle of attack of the servo 4 changes, and CSF 1 spontaneously sets at a different angle with respect to the flow and creates a different lifting force. In this case, the value of this derivative depends on the distance between the hinges 8 and 3, as well as on the distance between the hinges 9 and 5.
The proposed CSF was tested on the electric cord model of the “duck” circuit, while its derivative was reduced by half compared to the fixed CSF. The loading of the CSF was 68% of that for the wing. The task of the check was not to obtain equal loadings, but to obtain precisely a lower load of the CSF compared to the wing, since if you get it, then it will not be difficult to get equal. In "ducks" with a fixed GO, the loading of the plumage is usually 20 - 30% higher than the loading of the wing.
"Perfect Plane"
If the sum of two numbers is a constant value, then the sum of their squares will be the smallest if these numbers are equal. Since the inductive resistance of the bearing surface is proportional to the square of its lift coefficient, then the smallest limit of aircraft resistance will be in the case when these coefficients of both bearing surfaces are equal to each other in the cruising flight mode. Such an aircraft should be considered "ideal". The inventions "Krasnov-duck" and "Krasnov-weather vane" make it possible to realize the concept of "ideal aircraft" in reality without resorting to artificial stability maintenance by automatic systems.
A comparison of the "ideal aircraft" with a modern conventional aircraft shows that it is possible to obtain a 33% gain in payload with a simultaneous fuel saving of 23%.
CSF creates maximum lift at angles of attack close to critical, and this mode is typical for the landing stage of flight. In this case, the flow around the bearing surface by air particles is close to the boundary between normal and stall. The flow separation from the surface of the GO is accompanied by a sharp loss of lift on it and, as a result, by an intensive lowering of the nose of the aircraft, the so-called "dive". An illustrative case of "dive" is the crash of the Tu-144 at Le Bourget, when it collapsed upon exiting the dive just after the dive. The use of the proposed CSF makes it easy to solve this problem. To do this, it is only necessary to limit the angle of rotation of the servo steering relative to the CSF. In this case, the actual CSF angle of attack will be limited and will never become equal to the critical one.
"Weathervane Stabilizer"
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Of interest is the question of using CSF in a normal scheme. If you do not reduce, but vice versa, increase the angle of rotation of the CSF in comparison with the servo steering, as shown in Fig. 4, then the CSF derivative will be much higher compared to the fixed stabilizer (7).
This allows you to significantly shift the focus and center of mass of the aircraft back. As a result, the cruising load of the CSF stabilizer becomes not negative, but positive. In addition, if the center of mass of the aircraft is shifted beyond the focus by the flap deflection angle (the point of application of the lift increment due to the flap deflection), then the vane stabilizer creates positive lift in the landing configuration as well.
But all this is probably true as long as we do not take into account the influence of braking and flow sloping from the front bearing surface to the rear. It is clear that in the case of the "duck" the role of this influence is much less. And on the other hand, if the stabilizer "carries" on military fighters, then why will it stop "carrying" in civilian life?
"Krasnov-plan" or "pseudo-vane duck"
The articulated destabilizer, although not drastically, still complicates the design of the aircraft. It turns out that a decrease in the derivative of the destabilizer can be achieved by much cheaper means.
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On fig. 4 shows a destabilizer 1 of the proposed aircraft rigidly connected to the fuselage (not shown in the drawing). It is equipped with a means of changing its lifting force in the form of an elevator 2, which, using a hinge 3, is mounted on a bracket 4 rigidly connected to the destabilizer 1. On the same bracket 4, using a hinge 5, a rod 6 is placed, at the rear end of which a servo steering wheel 7 is rigidly fixed. At the front end of the rod 6, next to the hinge 5, a lever 8 is rigidly fixed, the upper end of which is connected to the rod 9 by means of a hinge 10. At the rear end of the rod 10 there is a hinge 11 connecting it with the lever 12 of the trimmer 13 of the elevator 2. When this trimmer 13 with the help of the hinge 14 is mounted on the rear of the steering wheel 2 heights. The clutch 15 changes the length of the thrust 10 under the control of the pilot to control the pitch.
The presented destabilizer works as follows. In case of an accidental increase in the angle of attack of the aircraft, for example, when it enters an updraft, the servo 7 deviates upward, which entails a displacement of the thrust 10 to the left, i.e. forward and causes the trimmer 13 to deviate downward, as a result of which the elevator 2 deviates upward. The position of the rudder 2 height, servo 7 and trimmer 13 in the described situation is shown in the drawing with dashed lines.
As a result, the increase in the lifting force of the destabilizer 1 due to the increase in the angle of attack will be to some extent leveled by the upward deflection of the elevator 2. The degree of this leveling depends on the ratio of the angles of deflection of the servo 7 and the rudder 2 height. And this ratio is set by the length of the levers 8 and 12. When the angle of attack decreases, the elevator 2 deflects down, and the lifting force of the destabilizer 1 increases, leveling the decrease in the angle of attack.
Thus, a decrease in the derivative of the destabilizer is achieved in comparison with the classical "duck".
Due to the fact that the servo 7 and the trimmer 13 are kinematically interconnected, they balance each other. If this balancing is not enough, then it is necessary to include a balancing weight in the design, which must be placed either inside the servo steering 7, or on the extension of the rod 6 in front of the hinge 5. The elevator 2 must also be balanced.
Since the derivative with respect to the angle of attack of the bearing surface is approximately twice the derivative with respect to the angle of deflection of the flap, then with a twofold excess of the angle of deflection of the rudder 2 compared to the deflection angle of the servo 7, it is possible to achieve a value of the destabilizer derivative close to zero.
The servo 7 is equal in area to the trimmer 13 of the rudder 2 heights. That is, the additions to the aircraft design are very small in size and complicate it negligibly.
Thus, it is quite possible to obtain the same results as the "weather vane" using only traditional aircraft manufacturing technologies. Therefore, an aircraft with such a destabilizer can be called a "pseudo-vane duck." This invention received a patent with the name "Krasnov-plan" (8).
"Turbulence-ignoring aircraft"
It is highly expedient to make an aircraft in which the front and rear bearing surfaces in total have a derivative equal to zero.
Such an aircraft will almost completely ignore the vertical flows of air masses, and its passengers will not feel "chatter" even with intense atmospheric turbulence. And, since the vertical flows of air masses do not lead to an overload of the aircraft, it can be counted on a significantly lower operational overload, which will positively affect the mass of its structure. Due to the fact that the aircraft does not experience overloads in flight, its airframe is not subject to fatigue wear.
The decrease in the derivative of the wing of such an aircraft is achieved in the same way as for the destabilizer in the "pseudo-vane duck". But the servo does not act on the elevators, but on the wing flaperons. The flaperon is the part of the wing that functions as the aileron and flap. In this case, as a result of a random change in the angle of attack of the wing, the increase in its lift occurs at the focus in terms of the angle of attack. And the negative increment of the wing lift as a result of deflection of the flaperon by the servo steering occurs at the focus along the deflection angle of the flaperon. And the distance between these foci is almost equal to a quarter of the average aerodynamic chord of the wing. As a result of the action of the specified pair of differently directed forces, a destabilizing moment is formed, which must be compensated by the moment of the destabilizer. In this case, the destabilizer should have a small negative derivative, and the value of the wing derivative should be slightly greater than zero. RF patent No. 2710955 has been obtained for such an aircraft.
The totality of the above inventions is probably the last unused informational aerodynamic resource for increasing the economic efficiency of subsonic aviation by a third or more.
Yuri Krasnov
REFERENCES
- D. Sobolev. Centenary history of the “flying wing”, Moscow, Rusavia, 1988, p. 100.
- Y. Krasnov. RF patent No. 2000251.
- A. Yurkonenko. Alternative duck. Technique - youth 2009-08. Page 6-11
- V. Lapin. When will the "weather vane duck" fly? General Aviation. 2011. No. 8. Page 38-41.
- Y. Krasnov. RF patent No. 2609644.
- Y. Krasnov. RF patent No. 2651959.
- Y. Krasnov. RF patent No. 2609620.
- Y. Krasnov. RF patent No. 2666094.
Source: habr.com