Iron Sonata of World War II

The author has something to say, the unit will work overtime tomorrow, and tomorrow's chapters will be published in advance today.

Chapter 2 Correcting Mistakes

We know that the reason why the wing can generate lift is that the air flow speed on the upper and lower surfaces of the wing is inconsistent due to the influence of the shape of the wing and the angle of attack.Among them, the lower surface of the wing is the windward side, the air flow speed is slower and the pressure is higher, and this positive pressure area will form an upward lifting force; the upper surface of the wing is the leeward side, the air flow speed is faster and the pressure is lower, this The negative pressure area will also form an upward adsorption force.

The lift force of the air below the wing and the suction force of the air below the wing together constitute the lift force of the wing. The lift force of the air below the wing accounts for about 30% of the lift force of the wing, while the air above the wing The upper suction force accounts for about 70% of the lift force of the wing. It can be seen that the suction wind generated above the wing is the most critical factor for the wing to generate lift normally.

However, the generation of suction wind above the wing is not unconditional. This condition is that the air above the wing must flow along the direction of the wing surface without serious separation.So why does the airflow over the wing sometimes separate?It turns out that if the airflow wants to flow along the surface of the wing, it must overcome the viscous force and the pressure difference.

There is a tendency in all fluids to prevent them from flowing freely. This is caused by the interaction between the molecules that make up the fluid. This is the so-called viscous force.Air as a fluid is no exception, and although air molecules are much less viscous than paste molecules, you can also think of air as an extremely thin paste.The air viscous force can also be regarded as the friction force between the air and the wing surface. This friction force makes the air flow velocity near the wing surface slower than the unaffected part above. In this part of the slower flow area we It is called "boundary layer" or "boundary layer".

Viscous force is not all the factors that hinder the flow of air on the surface of the wing. When the air flows over the raised upper surface of the wing, it is like a river flowing to the place where the river channel narrows, so the air velocity gradually increases, and the pressure decreases accordingly. .Since the airflow flows from a place of high pressure to a place of low pressure in this process, with the help of this co-pressure, the airflow can easily overcome the influence of viscous force and continue to accelerate.

When the air flow passes through the narrowest part of the flow channel, the flow channel begins to widen again, the air speed slows down, and the pressure also increases.At this time, the flow of the airflow not only does not have the help of the forward pressure, but also overcomes the viscosity and the reverse pressure at the same time.If the energy of the airflow is not enough to overcome this double obstacle, it will no longer flow honestly along the airfoil.At this time, the boundary layer on the surface of the wing will curl up, forming a closed separation vortex.

So what is the "stall" we often say?It turns out that when the angle of attack of the wing gradually increases, the lowest pressure point above the wing will move forward, and the forward movement of the lowest pressure point will drive the forward movement of the airflow separation point.If this continues, when the angle of attack of the wing reaches a certain value, we will see that the separation point of the airflow moves directly to the leading edge of the wing. At this time, the suction wind on the upper surface of the wing will completely disappear, and the lift of the wing will be greatly improved. Descending, the angle of attack of the wing at this time is the "stall critical angle of attack".

Therefore, in order to improve the high angle of attack performance of the aircraft, it is necessary to increase the stall critical angle of attack of the aircraft; and to increase the stall critical angle of attack of the aircraft, it is necessary to delay the separation point of the airflow on the upper surface of the wing to move to the leading edge of the wing.So how do you delay the airflow separation point moving towards the leading edge of the wing?

The dawn of solving this problem appeared in 1918, when the German aerodynamicist Gustav Lachmann discovered that if a gap is opened between the leading edge part of the wing and the main part, the wing can be greatly delayed. Airflow separation on the upper surface increases the stall angle of attack of the wing and improves the lift-to-drag ratio of the wing at high angles of attack.

This is the origin of the leading edge slats that are common on later generations of aircraft.However, although Rahmann invented the leading edge slat, he did not know why the leading edge slat would have such an effect. When he submitted the invention to the German Patent Office, the officials of the patent office also felt that the invention was too much. Absurd, so Rahman's patent application was rejected.

However, gold always shines. When more and more practical experience proves the role of leading edge slats, the academic community has to start to pay attention to the theoretical interpretation of the role of slats.For example, Ludwig Prandtl believed at the time that the slats worked because the airflow channels formed by them weakened the energy of the airflow blowing to the main wing, redistributed the energy in the boundary layer, and delayed the flight of the aircraft. wing stall.

From today's perspective, Plante's statement is naturally wrong, or at least inaccurate.Modern aerodynamics generally believe that the mechanism of leading-edge slats is that a part of the air on the lower surface of the wing flows to the upper surface of the wing through the slit, so that the air velocity on the upper surface of the wing increases and the pressure decreases.This increases the energy of the airflow over the wing's upper surface and relieves backpressure, thereby preventing the boundary layer separation point from moving towards the leading edge of the wing.

However, since Prandtl was the academic leader who created modern fluid mechanics and the founder of the "boundary layer theory", this wrong theory was regarded as the standard in the aerodynamics field at that time, and few people doubted that Prandtl would be in the In the "boundary layer theory" system he created, the overturning of Prandtl's theory has a negative impact on designers.

Due to the imperfect construction of aerodynamic theory at that time, under the guidance of this imperfect theory, whether the design of aircraft designers can work or not depends largely on intuition and luck.For example, in the previous plane, it was also a work designed by Messerschmitt. The high angle of attack maneuverability of the BF-109 fighter jet can be called the leader in the piston era, and the high angle of attack performance of the Me-210 fighter jet since then is simply a large The scene of the car accident!

The reason for this situation is not because Messerschmidt's design level suddenly dropped, but because the aerodynamic theory that the designer referred to when designing the aircraft at that time was full of loopholes, and the designers could only rely on These unreliable theories, coupled with my intuition developed in years of design experience, determine the aerodynamic layout of the aircraft.Therefore, even a designer who has just designed a good aircraft cannot guarantee that his next work will not be a fake. After all, the luck factor still occupies too much weight in the success or failure of the design.

In this era, there is no large-scale computer that can simulate the airflow on the wing at different speeds and angles of attack. Helena can only increase wind tunnel tests and flight bench tests in the field of aerodynamics research that she believes is the most valuable. investment scale.Germany's technical foundation in the field of wind tunnels is already good. Coupled with Helena's newly built series of equipment and grasp of the research direction, Prandtl finally discovered his own theoretical loopholes in a large amount of experimental data and began to correct them. His own theoretical model, Prandtl's most proud student, von Karman, was also invited by the teacher to join in this research work.

After the leading-edge slat technology was very successful on the BF-108 series aircraft, Helena specially converted a four-seater BF-108 tourist aircraft into a test aircraft as a professional aviation experiment platform for Prandtl and Feng Carmen led two masters to study the problem of airflow separation on the surface of the wing.The specific method is to paste multiple silk threads on the surface of the wing, then let the leading edge slats and trailing edge flaps of the aircraft be in different positions and gradually increase the angle of attack, and at the same time observe the state of the silk threads on the wing surface being blown by the airflow , until the aircraft exceeds the critical angle of attack and stalls, then open the drogue parachute to recover from the spin.

The chief test pilot in charge of this matter was Ernst Udet, the ace pilot of Germany during World War I. In the last plane, although this old man later became an admiral, he was finally defeated by his old boss Göring. He resigned due to a bitter power struggle with Milch and fears of a massive German invasion of the Soviet Union.After the resignation application was rejected by Goering, Udet finally shot himself on the night of November 1941, 11 because of unbearable heavy mental pressure.However, Udet is indeed a person who really loves flying. In the last plane, he personally flew almost every fighter plane developed by Germany. Udet in this plane also accepted Goering’s invitation without hesitation, and re- Started his own flying career.

As a practical person, Ude has a rebellious personality. Before that, he actually didn't think highly of academic authorities like Prandtl and von Karman who sat behind a desk full of books and materials and talked about the principles of flight.However, when Udet saw Prandtl and von Karman take off with him many times, just to observe the airflow changes above the wing for himself, not just sitting in the office and analyzing those video data, Udet In the end, he changed his attitude.

Helena also sat on the test plane with Prandtl and others more than once, but when the two masters were concentrating on observing the changes in the airflow on the wings, Helena spent most of her time observing the master.The more she watched these years, the more she felt that these great science gods were the reincarnation of cats, and they always had endless curiosity about new things.

Just like the two in front of me who can analyze the data recorded by the instrument on the ground, they have to get on the plane to experience the changes in the state of the plane during the formation and development of the separation vortex.In fact, even in the later generations with more developed sensor technology, there are still many technicians who like to go on the plane to observe the dynamics of the plane in person.

But hard work pays off. By summarizing a large amount of wind tunnel data and flight data, Prandtl and von Karman began to establish a new theory. They summarized the role of leading edge slats as including slat effect, circulation effect and dumping It is the result of a series of aerodynamic effects, including aerodynamic effects.Helena is sincerely happy with their results, because this is very close to the conclusion of later aerodynamics on the principle of slats.

However, in the last stall test flight before the end of this academic project, a small accident occurred.At that time, Udet carried Ludwig Plant and von Karmen into the sky as usual, and Helena also boarded the plane in the name of celebrating the completion of the new subject of the two great gods.But this time after flying the subject of wing stall, the spin recovery parachute failed to open normally.So the plane whirled from more than 3000 meters to more than 900 meters. Fortunately, Udet was experienced enough to correct the tail spin at the last moment and finally landed safely. Otherwise, Helena would have to explain it on the spot with the two great gods.

Although Prandtl and von Carmen both maintained a good attitude of "hearing the truth in the morning, death in the evening" after the incident, Helena at least on the surface looked as stable as an old dog.However, this incident frightened the German military and academic circles enough. The final conclusion of the incident investigation was that there was a quality problem in the aircraft's spin recovery parachute.

So Helena took the opportunity to propose that in the future when the military purchases spin recovery parachute, deceleration parachute, parachute and other types of aviation parachute, the boss of the factory and the person in charge should act as guinea pigs during the random inspection, so as to serve her own interests With a responsible attitude, they will try their best to improve the pass rate of umbrellas.It is said that this measure has reduced the defective rate of various aviation parachutes in Germany to almost zero.

With the accumulation of a large amount of experimental data and the guidance of relatively scientific theories, the leading edge slats of the BF-109 have naturally made a huge leap compared to the two-position slats of the previous BF-108 that can only slide horizontally.

Chapter 2 and five are ingenious

The automatic leading-edge slats used by the Bavarian Aircraft Factory on the BF-108 series aircraft have achieved great success, but this slat can only be extended or retracted along the horizontal slide rail, so there is considerable performance limited.With the in-depth understanding of the principle of leading-edge slats in the German aerodynamics community, Ruzel is obviously no longer satisfied with this simple slat structure.

So on the leading edge slats of the BF-109 fighter jet, the designer made a number of major innovations.One of the most important improvements is to change the original horizontal slide rail of the leading edge slat into a circular slide rail with a downward curvature.Don't underestimate the radian added by Ruzel to the slat rails. This seemingly simple design makes the leading edge slats of the BF-109 no longer simply stretch out horizontally when they are opened, but stretch forward. while deflecting downward.

When the aircraft's angle of attack just begins to increase, the slats will slide forward along the rails, but will not separate from the main wing immediately, while the wing area and leading edge camber will increase as the slats deflect downward. Increase.In this state, the function of the leading edge slat is somewhat similar to that of the leading edge flap, the increase of the camber of the leading edge of the wing will increase the lift coefficient, and the increase of the wing area will reduce the wing load, both of which are It is beneficial to improve the maneuverability of the fighter.

If the angle of attack of the aircraft continues to increase at this time, the slats will continue to expand until they are completely separated from the main wing and split out of the slit.At the same time, the downward deflection angle of the slats will continue to increase.In this way, at a high angle of attack, the airflow blowing in front will first bypass the leading edge of the slat smoothly and without separation, and then merge with the airflow flowing out of the slot, which further reduces the reverse pressure on the upper surface of the wing Gradient, which delays airflow separation, increases the stall angle of attack of the wing.

All in all, this new type of slat using curved slides has higher aerodynamic efficiency than traditional slats using horizontal linear slides, no matter at low angles of attack or at high angles of attack.But the change in the shape of the slide rail is not all the subtlety of the BF-109 leading edge slat. The real finishing touch of this slat system is actually its vortex generation system.

When the leading edge slats of the BF-109 fighter jet are opened, you can see a row of fine serrations on the upper surface of the leading edge of the main wing covered by the leading edge slats.When the aircraft is in level flight, since the leading edge slats are closed, the row of serrations are hidden under the slats, so it will not bring additional waste resistance to the cruising of the aircraft.When the slats expand as the aircraft's angle of attack increases, these serrations are exposed to the airflow.

That's right!It's just such a gadget that doesn't seem to have much technical content, but it's actually a built-in vortex generator, which is also the technical fruit of Helena's years of research and development investment.We must know that in the aviation field of the last plane, the vortex generator was only proposed in the late 40s and gradually popularized and applied.

Since the direction of these serrations and the direction of the airflow are designed to have a certain included angle, when the airflow flowing out of the slots of the slat blows over these serrations, many small vortices will be generated, and these vortices can reduce the velocity of the external flow of the boundary layer. The high air flow is involved in the boundary layer, which increases the energy of the air flow in the boundary layer, and the effect is to make the thickness of the boundary layer thinner and the flow speed faster, and it is not easy to get out of contact with the wing because the energy is not enough to overcome the reverse pressure and viscous force. face separation.

The leading edge slats with curved rails and the built-in serrated vortex generators occupy the outer section of the leading edge of the BF-109 fighter wing, and their mutual cooperation makes the BF-109 wing have excellent high angle of attack characteristics .This not only means that the wing of the BF-109 fighter has a stall critical angle of attack far exceeding that of contemporary models, but also means that the ailerons on the trailing edge of the BF-109 wing can not be separated in a larger range of angle of attack Therefore, this design can greatly improve the roll ability of the BF-109 fighter at high angles of attack.

Although the BF-109 in this plane has changed the leading edge slats compared to the previous plane, it is only to give the slat rails a certain curvature and add a sawtooth vortex generator.But as the so-called real kung fu is seen in the details, behind this little technical improvement is the accumulation of a large amount of experimental data and the continuous revision of the theoretical model. This process took more than ten years and a lot of money. Na, this super cheat can be realized without making mistakes.

In addition to the leading-edge slat design ahead of the times, the most amazing thing about the BF-109 fighter jet is that it also makes full and reasonable use of the exhaust gas from the engine.

In order to optimize the aerodynamic shape of the aircraft and improve the pilot's vision, the BF-109 fighter uses an inverted V12 engine, which means that the crankcase of the engine is on the top and the cylinder head is on the bottom.According to the conventional design, it is enough to directly install the exhaust pipes of the engine on the two sides under the nose of the aircraft using this inverted V12 engine. Under the influence, the BF-109 fighter plane on this plane finally chose to reject mediocrity.

The exhaust gas produced by the two cylinders of the BF-109 fighter engine on this plane will enter the root of the wing through the exhaust ducts under the fairing on both sides of the fuselage, and finally flow into the two ejector radiators under the wing.At this time, the exhaust gas will be accelerated to supersonic speed in the converging nozzle in the ejector, and then enter the mixing tube of the ejector.This high-velocity, high-energy airflow creates a strong ejector effect that draws cool air into the engine radiator.The inhaled cold air will first pass through the heat exchanger to cool the coolant of the engine and the intercooler, then fully mix with the engine exhaust gas in the mixing tube of the ejector, and finally spray out at high speed to the rear.

The first benefit of this design is that it increases the flow of cooling air and improves the cooling efficiency of the engine.Most of the piston fighters in World War II on the previous plane depended on the speed of the aircraft itself to let the airflow pass through the cooling device of the engine, while the cooling system of the BF-109 on this plane can rely on the energy of the exhaust gas from the engine to suck in the cooling air, that is to say, even When the aircraft is at low speed or even in a static state, the cooler can still maintain the most basic operation.

The second advantage of this design is that it makes full use of the energy in the radiator and engine exhaust to generate thrust.When the BF-109 on this plane is flying at high speed, after the cooling air flows into the radiator, it will be heated for the first time in the heat exchanger, and then it will be heated for the second time by the exhaust gas in the mixing tube. When designing the radiator nozzle, the pressure energy obtained during thermal expansion will be converted into kinetic energy to generate thrust, which allows the BF-109 to fly faster.In contrast, the Mustang fighter jets of the United States on the last plane ejected the air that was heated and expanded by the radiator from the converging nozzle, and the thrust obtained through the "Meredith effect" was simply insignificant.

The third benefit of this design is that the fairings of the exhaust ducts on both sides of the fuselage can exert a good aerodynamic effect. In order to accommodate the exhaust ducts, there are long strips of fairings on both sides of the front fuselage of the BF-109 Attached to the leading edge of the wing.This directly stamped fairing not only makes the transition between the front fuselage and the wing more smooth, but also generates a vortex flowing over the wing at a high angle of attack, thereby increasing the lift of the wing. This effect is similar to that of later generations. The strip wings are somewhat similar, but the effect is not as obvious as the side strip wings.

The fourth advantage of this design is that when the flaps at the rear of the wing are deflected, high-speed airflow can scour the surface of the flaps, thereby improving the lift efficiency of the flaps.The rear edge of the wing of BF-109 on this plane is from the inside to the outside: the inner receding slotted flap, the middle ordinary flap, and the outer aileron, and the ejector radiator is just located on the inner receding slotted flap the front of the wing.In this way, when the flaps are fully retracted, the radiator jet can flow smoothly under the flaps; but when the flaps are extended, the high-speed jet will directly wash over the flaps, which is equivalent to increasing the The area of ​​the flap improves the efficiency of the flap and the hovering ability of the fighter.

Chapter 2 Control System

Here it is necessary to focus on the receding slotted flap installed on the BF-109 fighter jet, which is a kind of flap lift system installed at the rear of the wing. The trailing edge blends into one.While working, the airfoil will at first retreat along the slide rail, and when the leading edge of the flap retreats to a position close to the trailing edge of the main wing, the flap will deflect downward to form a gap with the main wing.When retracting the slotted flaps, there are three effects on the wing:

The first effect is to increase the wing area.When the receding slotted flaps are retracted, most of the flaps overlap with the main wing, and the smaller wing area ensures that the aircraft has lower resistance in level flight; but when When the backward slotted flap is working, as the flap moves backward, the part that overlaps with the main wing is exposed, which increases the wing area and reduces the wing load, which will increase the flight resistance and improve The lift of the wing.

The second effect is increased wing camber.When the receding slotted flaps are retracted, the wing shape is relatively flat, with low lift and drag; but when the flaps are retracted and deflected downward, the shape of the wing becomes more curved , the effect of this change is similar to the effect of increasing the wing area before, and it will also increase the lift while increasing the drag of the wing.

The third effect is to delay the separation of the surface airflow over the flap.This point is relative to ordinary flaps and unslotted receding flaps. What plays a decisive role is the seam formed when the flaps are fully opened.The principle is similar to that of the leading-edge slat, which guides a part of the high-pressure airflow under the airfoil to the upper surface of the wing through the slit, increasing the energy of the airflow above the airfoil, thereby blowing away the separated vortex and delaying the airflow. The wing stalled.

This set of receding slotted flap system is installed on the inner side of the BF-109 wing trailing edge, just behind the nozzle of the jet exhaust ejection cooling system, so when the receding slotted flap is fully opened, the wing surface is exactly on the In the high-speed jet flow generated by the mixture of engine exhaust gas and radiator cooling air, this method of increasing lift called flap blowing allows the BF-109's receding slotted flaps to play a role far exceeding its own area. Control effectiveness.

In addition to improving the efficiency of the retreating slotted flaps, flap blowing also brings an unexpected benefit, that is, even at very low speeds, the fighter can also adjust the position of the flaps on both sides to make the sides The wings generate lift of varying magnitudes, thereby generating counter-torque to counteract the torsional force applied to the body by the rotation of the propeller.

And because the jet flow intensity of the exhaust jet cooler is positively correlated with the energy contained in the engine exhaust, so when the engine power increases and the torsional force increases, the anti-torque force that the blowing flap can provide will also increase synchronously, which means It is tantamount to eliminating in advance the problem that the torsional force exceeds the design margin of the airframe due to power upgrades in the future.

In the last period of World War II in the last plane, the BF-109K series fighters that were changed from Germany to the end of the world had this problem. The DB1850 series engines with a power of 605 horsepower were equipped with strong torsion force, which far exceeded that of the small airframe. The safety margin that can be provided, this problem has a negative impact on the low-speed maneuverability and take-off and landing safety of the BF-109, but the urgent war at that time did not allow Germany to completely eradicate this problem.

As a result of the cooperation of several aviation design units, under the personal operation of Dr. Kurt Tank, who pays attention to driving experience, the operating system of the new fighter is also more advanced than all previous fighter models in the world. Its landing gear Like the FW190 of the previous plane, the weapon system adopts electric retraction, while the rudder surface adopts a set of mechanically driven non-return hydraulic resistance control system.

In this control system, the steel cable connected to the joystick does not directly pull the rudder surface, but pulls the valve control rocker arm of the hydraulic booster, and then relies on the force of the hydraulic booster to deflect the wing surface to the corresponding position.In this way, no matter how fast the aircraft is, and no matter how much the moment acts on the pivot of the rudder surface, the pilot needs to apply the same force to the joystick to obtain the same deflection angle of the rudder surface.

This is because the force that the pilot feels from the joystick is not from the rudder surface at all, but is simulated for them by Dr. Tan Ke using a set of spring-type force-sensitive centering device.Don't think that this system is very high-end. To put it bluntly, the pilot's control force pulls the roller arm connected to the force spring, driving the roller to move on a cam.

This means that the greater the deflection angle of the rudder surface, the greater the amount of control stick required; the more the spring will be pulled, the greater the pilot's sense of force.But if the pilot releases the control stick, the roller will automatically return to the depression in the middle of the cam under the action of the spring, and at this time all the control surfaces will return to the neutral position, and the aircraft will automatically return to level flight.

Although the structure of the system itself is not very complicated, it is a godsend for inexperienced and physically weak pilots!You say that you are a rookie, and you are not sure how much rod force you need to add at different speeds?It does not exist on the BF-109!You said that your arms are thin, and the speed of the plane is too fast to pull the lever, which does not exist on the BF-109!

All in all, on the control system of the BF-109 fighter plane in this plane, Dr. Tan Ke has basically reached or even surpassed his achievements on the FW190 in the previous plane.The control of the engine, the torque change of the propeller, and the retraction and retraction of the landing gear are all automated, and the pilot does not even need to worry about the torsion force of the propeller that is common in single-engine fighter jets in the last plane. In Dr. Tan Ke's own words It is: "On this aircraft, even a novice who is new to aircraft can quickly find confidence in flying.

If someone asks is such an OS worth it for a fighter?Helena's answer is that it is not only worth it, but it will definitely be worth the money in the future!This set of operating system that is extremely friendly to pilots, especially novice pilots, will increase the development and production costs on the surface, but it will greatly reduce the training costs of pilots in the future, shorten the training cycle of pilots, and allow pilots to spend more energy on Kill opponents instead of laboriously serving your own aircraft.

In this way, although the procurement cost of each aircraft will increase somewhat, if the pilot training cost, including money cost and time cost, is considered comprehensively, the overall cost will decrease instead.If coupled with the reduction of non-combat losses and the improvement of pilot combat efficiency and other favorable factors, improving the control system is definitely a very cost-effective deal.

Helena even thinks that the potential value of the improvement of the control system exceeds the improvement of the aerodynamic layout itself. Dr. Tan Ke also agrees deeply. He is even considering adding a gyro inertial navigation system to the aircraft designed in the future, so that the pilot can even climb to After presetting the altitude and calibrate the heading, let it go, and wait until the target airspace is reached before taking over the aircraft.

Chapter 2

Since Helena positioned the new fighter as a low-cost fighter with comprehensive performance, the German designers not only showed a strong sense of innovation, but also showed a relatively cautious side.For example, in the selection of the plane shape of the aircraft wing, the new fighter resolutely rejected the elliptical wing with the least induced resistance in theory; Some military fans regard it as a natural laminar airfoil.

It's not that Helena doesn't know the advantages of elliptical wings and natural laminar wings, although these two technologies were compared in the British "Spitfire" fighter jets and the American "Mustang" fighter jets in the last plane. However, Helena and the designers agreed after weighing that, starting from the design positioning of the BF-109 fighter, neither the elliptical wing nor the natural laminar flow wing is a suitable choice for this fighter.

For example, in the last plane, the elliptical wing, which was famous for the "Spitfire" fighter jets, has a slightly lower induced drag than the conventional trapezoidal wing.It is also for this reason that the wing of the He-109 fighter that Heinkel took out to compete with the BF-112 before adopts this plane shape.However, if the wing of a fighter plane adopts such a complex plane shape, the production of each aircraft will cost about 1500 man-hours more than the model with a trapezoidal wing. This is still under the condition that there are enough skilled workers. Totally unacceptable for Helena.

As for the natural laminar flow wings used on the "Mustang" in the last plane, it is also not as perfect as many military fans imagined.The drag reduction effect of the laminar flow wing is mainly due to its relatively flat upper surface, and the position of the maximum thickness is relatively behind, so the airflow in the boundary layer accelerates relatively gently when flowing through the wing, and the position where the flow changes from laminar flow to turbulent flow is also closer to the surface. back.Since the frictional resistance of laminar flow is much smaller than that of turbulent flow, the laminar flow wing can theoretically greatly reduce the frictional resistance.

It's a pity that all this is just a theory. To maintain a larger range of laminar flow on the upper surface of the wing, the surface of the wing must have a good finish, otherwise the laminar flow will also be converted into frictional resistance when passing through the rough surface of the wing Greater turbulence.This not only requires the use of more sophisticated processing technology in production, but also requires more effort in maintenance to keep the surface of the wing clean.This is because the rain, snow, frost, and sand attached to the wing in the field environment will cause the laminar flow to be destroyed, so that the laminar flow wing loses the effect of reducing frictional resistance.

At this time, someone may say: In addition to reducing frictional resistance, laminar flow wings can also reduce aircraft pressure resistance and increase the critical Mach number of the wing. This does not sound like a big problem, but it still cannot withstand scrutiny.

The problem is that about 85% of the total flight resistance of a subsonic aircraft is provided by induced drag and frictional resistance, and of the remaining 15% of flight resistance, pressure differential resistance only contributes one-third of it one or so.That is to say, the pressure differential resistance of the entire aircraft only accounts for about 5% of the total resistance. Specifically, the pressure differential resistance generated by the wing, and then specific to the point where the laminar flow wing reduces the pressure differential resistance, is basically small enough to Ignore it.

As for the role of the laminar flow wing in increasing the critical Mach number, Helena is even more disapproving of it.With the level flight speed of 700-800 kilometers of propeller fighter jets during World War II, there are many ways to make the critical Mach number of the wing higher than this level.In fact, in World War II on the previous plane, the critical Mach number of the relatively thick wing of BF-109 was as high as 0.77Ma, and the critical Mach number of the relatively thin wing of the "Spitfire" fighter was as high as 0.89 Get up and down.Unless it is diving at full speed, it is basically impossible for the aircraft to reach such a speed.

In addition, the aerodynamic characteristics of the laminar airfoil itself also has some defects, such as the maximum lift coefficient under low speed conditions is lower than that of conventional airfoils, and for example, due to the small radius of the leading edge of the wing, the laminar airfoil can Down is easier to stall and so on.Although these problems are not unsolvable for Helena, for example, wing leading edge twisting technology can be used to optimize the stall characteristics of laminar wings, but doing so will further increase the difficulty of aircraft manufacturing.

Based on the above, the reasons why German designers do not favor laminar wings are very clear:

Although the laminar flow wing can reduce frictional resistance in theory, this advantage is based on higher requirements for production and maintenance, and the combat positioning of the BF-109 requires it to often take off and land at field airports with relatively harsh conditions , and the poor logistical conditions there make it difficult to ensure that the surface of the wing is free from ice, snow, mud and sand, so it is difficult to take advantage of the most significant advantage of the laminar flow wing to reduce the frictional resistance of the wing.

As for the other advantages of the laminar airfoil, such as reducing the differential pressure resistance and increasing the critical Mach number, either the effect is so small that it is almost negligible, or other means can be used to achieve it.Moreover, some shortcomings of the laminar flow wing itself somewhat violated Helena's original intention of requiring comprehensive and balanced performance of the new fighter.

After vetoing the elliptical wing and natural laminar airfoil, the geometric shape of the BF-109 wing on this plane is unpretentious compared to the ingenuity of the lift, exhaust and cooling system.

The plane shape of the wing is a trapezoid similar to the BF-109E of the last plane, but the chord length of the root of the wing is increased, and the wingspan is slightly increased (from 9.92 meters to 9.98 meters), making the wing area from 16.2 The square meter has increased to 17.25 square meters.The cross-section airfoil still adopts the conventional airfoil, but due to the increase of the wing chord length, the relative thickness of the wing is slightly reduced when the absolute thickness remains unchanged, and then the maximum thickness position of the wing is changed from the chord length The 30% position moves back to the 36% position.

These small changes reduce the high-speed drag while maintaining the low-speed lift of the BF-109 wing, and slightly increase the critical Mach number of the wing from 0.77Ma in the previous plane to 0.84Ma to improve the reliability of the aircraft when diving. manipulative.But more importantly, these changes will basically not increase production costs and maintenance difficulties.

Compared with the moderate or even conservative design of wing geometry, BF-109 has taken a path completely opposite to the conventional design concept of this era in the choice of propeller blade shape.

Its propeller blades are no longer the needle-shaped or dome-shaped blades that were most common during World War II on the previous plane, but a rectangular blade with rounded corners.The curvature of this rectangular blade gradually becomes smaller near the tip of the blade, and the relative thickness is also gradually thinned.The efficiency of this blade is similar to that of traditional blades at low speeds, but when the tip speed of the propeller is close to the speed of sound at high speeds, since the tip of the rectangular blade is designed to be thin and flat, it will have a higher critical Mach number.This allows the propeller blades to rotate at high speeds with less drag and more pull.

In contrast, propellers with traditional needle-tip or elliptical-tip propellers used in World War II on the last plane, although theoretically have the lowest induced drag when turning, their blade tips are shortened due to the chord length The relative thickness is relatively large, so once the speed of the propeller tip is close to the speed of sound, the airflow on the tip will exceed the speed of sound earlier and generate a shock wave.At this time, the engine power will be used more to overcome the shock wave resistance instead of providing pulling force for the aircraft, which will lead to a serious waste of engine power.

Rectangular blades are more efficient than needle-shaped or dome-shaped blades. This may not be in line with the common sense of design for many years for the designers of the same period on this plane, but it is indeed a fact confirmed after a long test in later generations.Over the years, Helena has spent a lot of time and experimental funds to let the German aerodynamics and design circles have a little understanding of this fact. After all, it is really not easy to change the thinking inertia of a group of people. things.

Although Helena knows that if the propeller blades are processed into a swept-back scimitar shape, the generation of blade tip shock waves can be further delayed and the propeller efficiency can be improved, but that also means that the processing technology of the propeller blades needs to start from a relatively simple two-dimensional The curved surface becomes a more complex three-dimensional curved surface, which is not conducive to improving production efficiency.In addition, the high-speed rectangular blades are more than enough for the piston propeller aircraft of World War II, so Helena didn't do anything extra for the illusory "advanced nature".

Chapter 2

On February 1935, 2, surrounded by the most elite aviation technical team in Germany, the third prototype of the BF-11 fighter plane on this plane, and also the first test aircraft, slowly drove out of Bavaria under the traction of a trailer. The newly built final assembly building of the aircraft factory.At the explicit request of Helena, the two prototypes that had been produced were used for static tests and fatigue tests respectively.After all, considering the technical span of BF-109 and its significance to the future Luftwaffe, all hidden dangers should be eliminated on the ground as much as possible.

In this era when the ground test process of aircraft is not yet complete, most aircraft do not have special static testing machines and fatigue testing machines.Most of the design manufacturers conduct some simple ground tests and then directly put the first prototype into test flight.This mode is sufficient when designing a relatively simple aircraft, but as the aircraft becomes larger, faster, and more complex in structure, it is urgent to establish a more scientific ground test system.

At the same time, on the other side of the aviation industry track, Reginald Mitchell, the chief designer of the British Super Marlin, is working ecstatically in front of the drawing board. Now he has devoted himself to the "breathing fire" fighter jets are under development.Although Mitchell himself didn't like the name "Spitfire" very much, but because his boss Robert McLean had a soft spot for the name, Mitchell, whose arms couldn't twist his thighs, had to succumb to the boss's coercion.

At this time, Mitchell didn't know that he was engaging in an extremely unfair competition.When he was still rejoicing at the 109 pounds allocated by the British Royal Air Force, Helena had already spent dozens of times that amount on the development of the BF-[-], not including Germany's previous More than a decade of continuous investment in advancing aerodynamics.What's more, Mitchell can only mobilize the technical team of Supermarin, but Helena can mobilize the entire German technical team to contribute to the new fighter project.

Although Mitchell himself is also a talented aircraft designer, what he needs to face is the collaboration of a large number of equally outstanding design talents.In addition to the old pair of Luzel and Messerschmidt, first-class technical talents including Kurt Tank, Claude Dornier, Hugo Junkers, Walter Gunter, etc. Participated more or less in the design of BF-109. In a sense, the BF-109 project is a general mobilization of the German aviation industry.The staged result of this general mobilization was the successful first flight of the BF-2 prototype on the afternoon of February 15.

比起上个位面BF-109的原型机，此次首飞的原型机的主尺度要略大一些。其中机身长度从上个位面的8.54米增加到了9.56米，而翼展则从9.87米增加到了9.98米。但相比上个位面液冷战斗机家族中那些更加大型化的型号，比如美国的P-51D系列和德国的FW-190D系列，本位面的BF-109依然属于相对小巧灵活的类型。

During the consecutive days of test flight work, the prototype gradually demonstrated its extraordinary flight performance, so that Herman Ulstad, the chief test pilot of the Bavarian Aircraft Manufacturing Factory, had to fly in the next flight every time he completed a flight test subject. After the plane, everyone was hooked.Seeing what Ursta said so excitedly, Lieutenant Colonel Ernst Udet, who loves flying, finally couldn't sit still. He even proposed to a group of designers that he would personally drive the BF-109 fighter jet to complete the maximum level. Fly speed test.

Helena wasn't overly surprised by Udet's approach. This guy's love for various challenging flights has almost reached the point of obsession.In 1938 on the last plane, when he was a major general of the Air Force, he personally tested and flew the He-100 fighter jet developed by Heinkel Company, and recorded a speed record of 634.73 kilometers per hour.

Although the plane persuaded Udet to re-enlist the old boss Hermann Göring to become the head of the newly-built naval aviation, Udet did not go to the air force, but became a newly promoted lieutenant colonel , but this does not seem to prevent Udet's eccentric desire to fly all the flying things in Germany by himself.So Ruzel and other technicians finally had to agree to Udet's test flight after repeated persuasion was ineffective.

So on March 1935, 3, Lieutenant Colonel Udet finally realized his dream.On the pre-booked 6-kilometer circular closed route, Udet personally piloted the BF-100 prototype at an altitude of 109 meters and completed the flight mission at an average speed of 6400 kilometers per hour, creating an appalling but A world record that has not been permitted to be published.

要知道上个位面中，同样使用带一级增压器的1350马力发动机BF-109F极速只有624千米每小时而已。虽然这架BF-109原型机由于配套的20毫米机炮还没有研制完成，现在只在机鼻部位配置了两挺7.92毫米MG-17机枪，导致这个测试数据实际上存在一定的水分。但能在机体比上个位面的BF-109F稍大的情况下，用性能相当的发动机飞出快11%的极速，依然值得大书特书。更何况上个位面的BF-109F已经是英伦空战后期才出现的型号。