最近遇到一個飛友,我們一起討論飛機重心的相對關係,在聊天過程當中也發現很容易讓人誤解的地方,就是重心與升力中心的相對關係,很多人都誤認重心與升力中心位置是同一個地方,其實飛機重心(CG)與升力中心點(CL)是不一樣的,對於飛機的設計來說,都是盡可能努發展讓飛機的三軸穩定性與平衡性更高,尤其是針對飛機的X縱軸方面考量的因素最多,通常都是設計成不平衡性的設計概念,它的設計依據都是來自於以下的三個因素
1:升力中心(CL)與飛機重心(CG)的相關位置(參考圖一) 。
2:水平尾與重心(CG)的相關位置 。
3:水平尾的面積大小。
我們來分析穩定性的問題,飛機的任何操作向量都是繞著飛機的重心來達到平衡,來注意看一下機翼跟水平尾的力距關係,假設這個力距設計,初始時候是在一個平衡的狀態之下(CG跟CL都在同一點上),這種設計的飛機在飛行時會有忽然nosed up的現象發生..想像一下設計一種左右長度不一樣,但是左右兩邊的重量是一樣的蹺蹺板,當蹺蹺板的重心CG與CL都在同一點(中心),他是會達到平衡,但是呢以力學的角度來看,較長的這端往下10cm.可能另較短的一端已經翹起30公分了,簡略的把這個概念放在飛機上,這樣飛機就變得很難操縱,可能你只輕輕的帶桿2英吋,可能仰角瞬間增加10度,這樣的飛機設計是很難操縱的,尤其是在降落的時候,沒辦法抓到很微量的桿量,這樣的飛機也不好飛,所以機翼跟水平尾的力距設計就要改變,升力中心(CL)跟重心(CG)的位置就不能在同一個點上,根據計算兩者所受承受的force,必須是要提供一個不平衡的設計,當你做任何操作改平飛的時候,才不會有突然機頭上仰的情況發生,飛機升力的中心(CG).大部分都是設計成非對稱的頃向(請參考圖片1),有沒有看到重心(CG)位置被設計在機翼的前面,而升力中心(CL)位置卻在機翼上,當機翼的攻角增加時,升力中心(CL)會向前移動,這種先天性的升力轉移會帶給機翼一個所謂不平衡性的特性,讓飛機的俯仰更好操控。
假如飛機的速度減少,經過翼面的氣流速度變慢,這樣的結果會讓下洗流減少,導致經由水平尾的氣流變少,同時水平尾向下的壓力也變輕,(請參考圖片三),然而因為先天不平衡的設計讓機頭會變得較重了,為了維持高度,當速度逐漸遞減你會同時增加你的升舵來保持平飛,然而就會變成不斷的增加攻角飛行狀態,相對的當你速度增加的時候,.作用在水平尾上的下洗氣流也變強,因為力距關係所以機頭會上仰,進而減低飛行時的攻角,大家可以回顧一下是不是加速的時候機頭都會上揚呢?這些都是只針對俯仰穩定性的設計做討論~~下一章節會為各位繼續介紹LATERAL STABILITY (ROLLING)~~
In designing an airplane, a great deal of effort is spent in developing the desired degree of stability around all three axes. But longitudinal stability about the lateral axis is considered to be the most affected by certain variables in various flight conditions. Longitudinal stability is the quality that makes an airplane stable about its lateral axis. It involves the pitching motion as the airplane’s nose moves up and down in flight. Alongitudinally unstable airplane has a tendency to dive or climb progressively into a very steep dive or climb, or even a stall. Thus, an airplane with longitudinal instability becomes difficult and sometimes dangerous to fly. Static longitudinal stability or instability in an airplane,is dependent upon three factors: 1. Location of the wing with respect to the center of gravity。 2. Location of the horizontal tail surfaces withrespect to the center of gravity。 3. The area or size of the tail surfaces。 In analyzing stability, it should be recalled that a body that is free to rotate will always turn about its center of gravity.To obtain static longitudinal stability, the relation of the wing and tail moments must be such that, if the moments are initially balanced and the airplane is suddenly nosed up, the wing moments and tail moments will hange so that the sum of their forces will provide an unbalanced but restoring moment which, in turn, will bring the nose down again. Similarly, if the airplane is nosed down, the resulting change in moments will bring the nose back up.The center of lift, sometimes called the center of pressure, in most unsymmetrical airfoils has a tendency to change its fore and aft position with a change in the angle of attack. The center of pressure tends to move forward with an increase in angle of attack and to move aft with a decrease in angle of attack. This means that when the angle of attack of an airfoil is increased, the center of pressure (lift) by moving forward, tends to lift the leading edge of the wing still more. This tendency gives the wing an inherent quality of instability. Figure 3-12 shows an airplane in straight-and-level flight. The line CG-CL-T represents the airplane’s longitudinal axis from the center of gravity (CG) to a point T on the horizontal stabilizer. The center of lift (or center of pressure) is represented by the point CL. Most airplanes are designed so that the wing’s center of lift (CL) is to the rear of the center of gravity. This makes the airplane “nose heavy” and requires that there be a slight downward force on the horizontal stabilizer in order to balance the airplane and keep the nose from continually pitching downward.Compensation for this nose heaviness is provided by setting the horizontal stabilizer at a slight negative angle of attack. The downward force thus produced, holds the tail down, counterbalancing the “heavy” nose. It is as if the line CG-CL-T was a lever with an upward force at CL and two downward forces balancing each other, one a strong force at the CG point and the other, a much lesser force, at point T (downward air pressure on the stabilizer). Applying simple physics principles, it can be seen that if an iron bar were suspended at point CL with a heavy weight hanging on it at the CG, it would take some ownward pressure at point T to keep the “lever” in balance.Even though the horizontal stabilizer may be level when the airplane is in level flight, there is a downwash of air from the wings. This downwash strikes the top of the stabilizer and produces a downward pressure, which at a certain speed will be just enough to balance the “lever.” The faster the airplane is flying, the greater this downwash and the greater the downward force on the horizontal stabilizer (except “T” tails). [Figure 3-13] In airplanes with fixed position horizontal stabilizers, the airplane manufacturer sets the stabilizer at an angle that will provide the best stability (or balance) during flight at the design cruising speed and power setting. [Figure 3-14] If the airplane’s speed decreases, the speed of the airflow over the wing is decreased. As a result of this decreased flow of air over the wing, the downwash is reduced, causing a lesser downward force on the horizontal stabilizer. In turn, the characteristic nose heaviness is accentuated, causing the airplane’s nose to pitch down more. This places the airplane in a nose-low attitude, lessening the wing’s angle of attack and drag and allowing the airspeed to increase. As the airplane continues in the nose-low attitude and its speed increases, the downward force on the horizontal stabilizer is once again increased.
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