Critical Engine 嘅定義係:
“The critical engine is the engine, the failure of which would give the biggest yawing moment.”
“The critical engine is the engine, the failure of which would give the biggest yawing moment.”
以下落黎講嘅係關於conventional twin engine (clockwise propeller rotation)
**counter rotating twin engine 係冇critical engine* *
所有conventional twin engine 嘅Critical Engine 都係左邊~
構成Critical Engine 嘅原因分別係 “P A S T”
P – Factor
Accelerated Slipstream
Spiraling Slipstream
Torque
P – Factor (asymmetrical disc loading)
P – Factor 係講緊asymmetric thrust，由於 descending blade propeller 會比ascending blade propeller blade 產生更多thrust，呢個asymmetric 嘅情況會將 center of thrust shift 去propeller 嘅右手邊。由下而幅圖見到，右手邊嘅center of thrust 距離CG 嘅位置比左手邊長 (longer moment arm/greater leverage for the right center of thrust)，所以兩個engine produce 出黎嘅thrust 雖然一樣，但由於moment arm 長度唔同，右邊嘅engine 會有更大嘅向左 yawing moment~
Accelerated Slipstream
Accelerated Slipstream 會令兩邊機翼產生lift 嘅位置唔一樣， 啱啱講過同一個propeller 右邊嘅thrust 比左邊大，所以流經機翼嘅右邊airflow 比左邊更多更快。呢個結果令左右兩邊機翼嘅lateral center of lift 位置唔一樣，右邊較近CG; 左邊較遠離CG，造成moment arm difference 。因此右邊機翼有更大嘅tendency roll 向左邊~
Spiraling Slipstream
Spiraling Slipstream 可以增加longitudinal stability ，當Propeller rotate 嘅時候會帶動埋後面嘅airflow 一齊轉，基於Coriolis effect，airflow 會向右移。左邊propeller 向後移嘅airflow 會打到 vertical fin，所以用rudder 嘅時候會更加effective; 相反右邊propeller 向後移嘅airflow 唔會打中任何airfoil 所以對controllability 冇幫助~
Torque
Torque 係Newton’s Third Law。產生出黎嘅，當Propeller rotating clockwise 自然就會有一個anticlockwise 嘅force，呢個就係torque。而Torque 係唔同情況對directional control 會有唔同影響。
Scenario 1 (left engine failed)
如果left engine failed， Torque 會成為第三樣嘅force 令飛機有更大向左嘅tendency，因為呢個時候toque 同 P- factor, accelerated slipstream嘅force係同一個方向~
Scenario 2 (right engine failed)
如果right engine failed ，Torque 可以offset 部分嘅向左嘅tendency，因為toque 同 P- factor, accelerated slipstream 嘅force 係相反方向 ~
