Welcome to this episode, as a "flap" scholar, today for you to take an in-depth analysis - how did the flap appear?
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Before talking about flaps, it is necessary to talk to you about the history of aircraft. The Wright brothers invented the first airplane in 1903, the plane flew four times in the air, the longest one was only a minute, this staying power is simply ... Even so, this famous scene is enough to go down in history. The first flap design was in 1908. That is to say, in these five years, there is no such thing as a flap. Why are flaps? Are flaps a necessity for aircraft design?
After all, the airplane is a means of transportation, and being able to pull more people and fly faster is its eternal pursuit. In this context, in order to achieve the requirements of safe ground separation and smooth landing, how to increase lift has become a problem that designers urgently need to overcome. To some extent, the emergence of flaps is an inevitable result of the development process of aircraft. We know the lifting effect of the flap, and if we want to understand its design principle, it is necessary to tell you how the lift is formed.
There are generally two theories about the popular interpretation of aircraft lift, Newton's third law and Bernoulli's law. Since the two names are a bit tongue-twisting, in a later video, I'll refer to them shortly as Niu San and Effort.
First of all, let's talk about The Three Bulls, which holds that the lift is derived from the reaction of the air to the bottom of the wing. On the surface, this line of thinking seems to be no problem, but if you think about it carefully, this is nothing more than a principle of inference from the result.
Let's talk about effort again, Bernoulli according to the law of conservation of energy: the sum of static pressure and dynamic pressure of the fluid is a constant, corresponding to the corresponding wing, the upper and lower air flow will reach the end point at the same time. Due to the special reasons of the airfoil construction, the upper surface has a long distance, so the airflow speed is faster. The dynamic pressure increases, the static pressure will inevitably decrease, and the difference between the upper and lower static pressure makes the aircraft form a lift. On the surface, this line of thinking seems to be no problem, but if you think about it more closely, why does the upper and lower surface airflow reach the end point at the same time? When you stacked paper airplanes, did you design a special distance difference?
In Tieba and forums, you can always see such replies: "It turns out that Bernoulli's law is indeed wrong, and I remember when I was in high school, I fought with the teacher in red face, and the editor of the textbook when I was a child was also very unprofessional." ”
This one... Bull three and effort are only narrow, not mistakes. In fact, the effect of airflow on aircraft lift is very complex, and although these two explanations are full of loopholes, they are very easy to understand to explain various types of flight control designs. Therefore, they are widely used in various university textbooks. In the textbook, there is actually another more accurate lift formula.
Knowing Bernoulli's law and the lift formula, now let's think about a problem. How to increase lift? In other words, how are flaps designed? The first idea is to change the curvature of the airfoil so that the upper and lower surfaces form a difference in airflow velocity; the second idea, according to the lift formula, increase the wing area.
Curiously, much of the early aircraft design lacked a connection with scientific and technological developments, and at that time, many designs even helped form theories in reverse. We are moving forward at an incredible speed in the contradiction between reason and sensibility.
One of the easiest ways to form a speed difference is to form a distance difference, that is, to bend the wing. In 1908, Farman proposed the first flap design in human history, designing the trailing edge of the wing as a small wing surface that can be rotated around the axis, so as to achieve the effect of bending the wing. When talking about this history, many people will think that backward materials science is the helpless choice for this kind of design. In fact, after the Wright brothers invented the airplane, they registered all the patents they could think of, and this small wing face with the hind edge turn cleverly broke through the patent barrier.
In 1914, Sarajevo gunshots ignited barrels of explosives in Europe, and World War I officially exploded. War is cruel, but it is a catalyst for social progress and science and technology. The aviation industry has developed rapidly, and strange flap designs have appeared in turn, but they have never been able to jump out of the circle of thinking with curved wings.
In 1919, the German pilot Rahman was recuperating in the hospital, and it suddenly occurred to him that if a gap could be designed at the leading edge of the flap, the air flow to the upper edge would form a greater pressure difference, and the open-seam design increased the lift by more than 60%. Rahman patented it, but could not say how it worked. In fact, the air flow is the same as people, when the distance gradually becomes longer, some air flow begins to choose to take a detour, so that it is impossible to increase the lift of the aircraft. If you can increase the airflow to the upper surface, the "push" effect can push part of the airflow back to the right track.
At this point in the story, you may have doubts. Why early aircraft designers were obsessed with increasing the speed difference rather than the wing area. From the previous lift formula, you will find that this formula also applies to resistance. Therefore, the increase in wing area is not conducive to high-altitude cruising, even if a deformable wing can be designed, it will increase the thickness of the wing.
In the human mind, an increase in wing thickness is not conducive to flight. Because, in my impression, even the fattest bird has a pair of slender wings. It wasn't until 1917 that scientists confirmed the superiority of thicker wings, and we, who thought we had conquered the sky long ago, were actually still living in the prying eyes of nature.
In 1924, the milestone came – the Fuller flap. When working, the flaps are deflected backwards and downwards on the other, and while the receding deflection is accompanied by convergent gaps, the accelerated airflow is blown upwards to the wing surface. This is the first time in the history of the flap that the three principles of increasing the wing area, increasing the curvature and controlling the attachment layer have been used at the same time, and the lifting capacity has ushered in a qualitative leap.
It turns out that a single gap is feasible, then the effect of two gaps will be better, and so on. Today, 100 years later, the creviced Fuller flaps are still everywhere, and the influence can be seen. Interestingly, designer Fuller once said that although birds can control the curvature of their wings at any time, human intelligence can grow another pair of wings.