发布时间：2021-01-08 来源：http://www.quanyimoxing.com 浏览量：
Force and resistance
Airplanes and model airplanes can fly because the lift of the wings overcomes gravity. The lift of the wing is caused by the air pressure difference between the upper and lower parts of the wing. When the model flies in the air, the air velocity on the upper surface of the wing increases and the pressure decreases; the air velocity on the lower surface of the wing decreases and the pressure increases (Bernoulli's law). This is the cause of the pressure difference between the upper and lower wings.
There are two reasons for the change of flow velocity: A. asymmetric airfoil; B. the angle of attack between airfoil and relative flow. An airfoil is the shape of the airfoil section. Most of the wing sections are asymmetric, the following arc is straight, the upper arc is upward curved (flat convex type) and the upper and lower arcs are upward curved (concave convex type). A symmetrical airfoil must have a certain angle of attack to generate lift.
The size of lift mainly depends on four factors: A. lift is directly proportional to wing area; B. lift is directly proportional to the square of aircraft speed. Under the same conditions, the faster the flight speed, the greater the lift; C. the lift is related to the airfoil, usually the lift of asymmetric airfoil is larger; D. the lift is related to the angle of attack, when the angle of attack is small, the lift (coefficient) increases linearly with the angle of attack, and when it reaches a certain limit, the angle of attack increases, but the lift decreases rapidly, which is called the critical angle of attack.
The wing and the horizontal tail produce not only lift but also drag, and other components only produce drag.
Level flight is called level flight. Level flight is the most basic flight attitude. The conditions for maintaining level flight are: lift equals gravity and pull equals resistance. Because the lift and drag are related to the flight speed, if the horsepower of an original model in level flight is increased, the pull will be greater than the drag, so that the flight speed will be accelerated. When the flight speed is increased, the lift will increase, and the model will gradually climb when the lift is greater than the gravity. In order to keep the model flying level at high horsepower and speed, the angle of attack must be reduced accordingly. On the contrary, in order to make the model keep level flight at low horsepower and speed, the angle of attack must be increased accordingly. Therefore, to control (adjust) the model to level flight is essentially a correct match between engine horsepower and flight angle of attack.
As mentioned earlier, when the model flies horizontally, if the horsepower is increased, it will turn into climbing. The angle between the climbing track and the horizontal plane is called the climbing angle. A new force balance may be achieved under a certain horsepower and a certain climbing angle, and the model will enter a stable climbing state (both speed and climbing angle remain unchanged). The specific conditions for stable climbing are as follows: the pulling force is equal to the backward component of resistance plus gravity (F = & quot; X + GSIN & theta;), and the lifting force is equal to another component of gravity (y = GCOS & theta;). When climbing, part of the gravity is borne by the pulling force, so a larger pulling force is needed, and the burden of the lifting force is reduced.
Similar to normal flight, in order to maintain a stable climb at a certain angle of climb, the proper matching of horsepower and angle of attack is also needed. Breaking this match will not maintain a steady climb. For example, the increase of horsepower will cause the increase of speed, lift and climb angle. If the horsepower is too high, the climbing angle will increase continuously, and the model will climb along the arc track, which is the common phenomenon of rollover.
Gliding is flight without power. When gliding, the resistance of the model is balanced by the component force of gravity, so gliding can only fly downward along the oblique line. The angle between the glide track and the horizontal plane is called glide angle.
The condition of stable glide (glide angle and glide speed remain unchanged) is that the drag is equal to the forward component of gravity (x = GSIN & theta;) and the lift is equal to another component of gravity (y = GCOS & theta;).
Gliding angle is an important aspect of gliding performance. The smaller the gliding angle, the farther the gliding distance at the same altitude. The ratio of glide distance (L) to descent height (H) is called glide ratio (k). Glide ratio is equal to cotangent glide ratio of glide angle, and is equal to the ratio of lift and drag (lift drag ratio). Ctgθ="1/h=k。
Gliding speed is another important aspect of gliding performance. The larger the lift coefficient of the model, the smaller the gliding speed; the larger the load of the model wing, the larger the gliding speed.
When adjusting a model aircraft, the angle of attack of the wing is changed by adjusting the lifting tab and moving the center of gravity back and forth to achieve the purpose of changing the gliding state.
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