飛行器設計學習筆記(十)

Mon Lapin,

假期結束，我來更親啦~(≧▽≦)/~

一個多月沒複習，感覺前面的內容快忘光了，要重新開始咯(??????) ?

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入射角

入射角是指機翼安裝部位的角度。通常機翼是沿著機身呈直線的，但有時機翼會向上傾斜一個角度，這樣機翼尖端就會高於機身處的機翼底部。

?有了這個入射角，機翼就像傻兔子的耳朵翹起來的樣子，非常可愛噠~~~

為了更好的理解為什麼要這樣做，我們需要先看一下翼面的工作原理，以及它們會產生多少浮力。重點之一是攻角(the angle of attack)。

攻角是機翼和外部氣流之間的夾角。如圖所示，氣流方向為飛機飛行方向的相反方向。如果飛機向後傾斜爬坡，攻角會增大；如果飛機向前傾斜俯衝，攻角會減小。

快來跟我複習一下筆記（一）：【浮力是當飛機飛行在空氣中造成的，主要是由機翼產生的。機翼有一個特殊的形狀，稱作翼面。機翼上這樣特殊的形狀會讓上方流動的空氣比下方流動的速度要快。在機翼下方緩慢流動的空氣會產生更高的氣壓，這樣產生的浮力能讓飛機上升，同時平衡重力。】

———————————這周的我來啦~(≧▽≦)/~————————

敲黑板，跟我看圖片咯：

翼弦線是穿過機翼的頂端和尾部的一條直線。

當攻角變大時，機翼會產生更多的浮力。如果翼面發生彎曲，那麼即使攻角為零度，機翼也會產生浮力；但如果兩側對稱，那麼機翼就不會產生浮力了。這也是為什麼垂直的尾翼通常擁有對稱的翼面，因為這樣，即使飛機直線飛行時，也不會轉向。

飛行過程中，一架飛機通常會保持一定的浮力進行水平飛行。這有可能是機翼的翼面產生足夠的浮力，當攻角大於零度，這裡我們假設攻角為3度。為了保證浮力，整架飛機可能飛行的角度為3度，但這意味著在旅行的大部分時間裡，飛行員和乘客不得不以一個向後傾斜的角度乘坐飛機。這也可能使餐食推車更難在過道里移動，並且，這會減少飛行員觀察飛行情況的可視能力。另外一種方式是讓機翼的角度高於機艙3度。這意味著當機艙在水平位置0度時，機翼處在3度。這樣也能產生足夠的浮力飛行。這3度角就是入射角。

?這3度角就像是：為了和傻兔子有更多時間交流，我得減少娛樂時間或是壓縮零碎時間；除此之外，更好的方式就是提高做事效率，以此擁有更多的時間啦~~喜歡你就像3度角喲:)

讓機翼出於一個正入射角還能減少機艙產生的阻力。入射角通常由風洞測試來進行設置，這是一個複雜的過程，但最後入射角度通常僅有1度或是2度。機翼也會被扭曲，機翼的角度從底部到頂端會發生變化，這也是很複雜的，並且在飛機模型上很難實現。

?模型要比實際簡化了不少，這就像是文字的喜愛也道不出真實喜愛的萬分之一呀~

當我們製作泡沫飛機模型時，機翼是一個扁平的板子。這樣，飛機需要花費更多的時間保持在一個正角的角度飛行。這意味著把機翼安置在一個入射角是有用的。然而，實際上，製作零入射角的機翼更難。並且，模型飛機也不需要安置飛行員和乘客。額外的阻力對於模型飛機的速度來說並沒有太多影響，所以飛機以微小的角度飛行時也並不重要。

將引擎以向下的角度安置，會產生和入射角相似的作用，這一點我們以後再聊咯~

When making a foam model aircraft, the wing is a flat symmetrical plate. As such, the plane will need to spend most of its time flying at a positive angle. This means that it would make sense to put the wing at an angle of incidence. However, this would be much harder to make than a wing at no incidence angle. Also, there are no pilots or paasengers to upset, and the extra drag does not matter too much at model aircraft speeds, so it does not matter that the aircraft will fly at some slight angle. A similar effect to an incidence angle can be gained by angling the motor downwards, and this will be discussed in Learning Module 7.\

extbf{Incidence angle}\

Incidence angle is the angle at which the wing is installed. Ordinarily a wing would be straight along the body of the aircraft, but sometimes the wing is angled upwards, so that the front edge is higher on the fuselage than the back edge.\

To understand why this would be done, we need to have another look at how airfoils work, and how much lift they produce. An important part of this is the angle of attack. \

The angle of attack is the angle between the wing and the incoming airflow. The airflow generally comes from the direction the plane is flying. If an aircraft tilts back to climb, the angle of attack increases, and if it tilts forward to dive the angle of attack decreases. \

The diagram below illustrates the angle of attack. Note that the chord line is a line through the wing joining the front and back edges.\

Wings produce more lift when they are at a higher angle of attack. If an airfoil is curved, then the wing will still produce some lift when the angle of attack is zero, but if it is symmetrical it will not produce any lift. This is why vertical tails usually have symmetrical airfoils, as these will not try to turn the plane when it flies in a straight line.\

During cruise, an aircraft has to maintain a certain amount of lift to fly level. It may be that the airfoil chosen for the wing will only produce enough lift when the angle of attack is greater than zero, lets say 3 degrees. To do this, the whole aircraft could fly at an anlge of 3 degrees, but this would mean that the pilots and passengers would have to spend most of the trip sitting at a tilted back angle. This would make it harder for meal trolleys to move up the aisle, and would also reduce the pilots ability to see where the plane is going. The other alternative is to angle the wing at 3 degrees higher than the fuselage. This means that when the fuselage is level, the wing will be at an angle of attack of 3 degrees and will produce enough lift to fly. This angle of 3 degrees is called the incidence angle. \

Setting the wing at a positive incidence angle can also reduce the drag from the fuselage. The incidence angle is usually set by testing with wind tunnels, which is a complicated process, but in the end incidence angles are often only 1 or 2 degrees. Wings can also be twisted, where the angle of the wing changes from root to tip, but this is complicated and hard to do on model aircraft.\

When making a foam model aircraft, the wing is a flat symmetrical plate. As such, the plane will need to spend most of its time flying at a positive angle. This means that it would make sense to put the wing at an angle of incidence. However, this would be much harder to make than a wing at no incidence angle. Also, there are no pilots or paasengers to upset, and the extra drag does not matter too much at model aircraft speeds, so it does not matter that the aircraft will fly at some slight angle. A similar effect to an incidence angle can be gained by angling the motor downwards, and this will be discussed in Learning Module 7.\

Key idea: Wings generate more lift at higher angles of attack.\

Incidence angles can make flight more convenient for passengers and reduce drag.\

Incidence angles are hard to build in model aircraft and are not necessary.\