TRIMMED CONDITION ROTOR FORCE 16-6 Figure 16-10. If the CG is located in front of the rotor force line, the gyroplane is more stable than if the CG is located behind the rotor force line. Blade Flapping—The upward or downward movement of the rotor- blades during rotation. If the CG is in front of the rotor force line, a gust of wind, which increases the angle of attack, causes the rotor disc to react the same way, but now the increase in rotor force and  blade flapping decreases the moment. This tends to decrease the angle of attack, and creates a stable condition. Suppose the CG is located behind the rotor force line in forward flight. If a gust of wind increases the angle of attack, rotor force increases. There is also an increase in the difference between the lift produced on the advancing and retreating blades. This increases the flapping angle and causes the rotor to pitch up. This pitching action increases the moment around the center of gravity, which leads to a greater increase in the angle of attack. The result is an unstable condition.  Conversely, if the CG is above the propeller thrust line, the CG is usually located ahead of the rotor force line. Of course, the location of fuselage drag, the pitch iner- tia, and the addition of a horizontal stabilizer can alter where the center of gravity is placed. As was stated earlier, manufacturers use a combination of the various stability factors to achieve a trimmed gyroplane. For example, if you have a gyroplane where the CG is below the propeller thrust line, the propeller thrust gives your aircraft a nose down pitching moment when power is applied. To compensate for this pitching moment, the CG, on this type of gyroplane, is usually located behind the rotor force line. This location pro- duces a nose up pitching moment. Because some gyroplanes do not have horizontal stabi- lizers, and the propeller thrust lines are different, gyro- plane manufacturers can achieve the desired stability by placing the center of gravity in front of or behind the rotor force line. [Figure 16-10] nearly so there is no tendency for the nose to pitch up or down. [Figure 16-9] Figure 16-9. A gyroplane which has the propeller thrust line above the center of gravity is often referred to as a low profile gyro- plane. One that has the propeller thrust line below or at the CG is considered a high profile gyroplane. Propeller Thrust Propeller Thrust Center of Gravity  Center of Gravity  High Profile Low Profile R o t o r F o r c e R o t o r F o r c e