rily on the forward speed of the aircraft, with higher speed creating a larger region of reverse flow. To some degree, the operating speed of the rotor system also has an effect on the size of the region, with systems operat- ing at lower r.p.m. being more susceptible to reverse flow and allowing a greater portion of the blade to experience the effect. relative wind striking the advancing blade, and sub- tracted from the relative wind striking the retreating blade. To prevent uneven lifting forces on the two sides of the rotor disc, the advancing blade teeters up, decreasing angle of attack and lift, while the retreating blade teeters down, increasing angle of attack and lift. (For complete discussion on dissymmetry of lift, refer to Chapter 3—Aerodynamics of Flight.) The lower angles of attack on the advancing blade cause more of the blade to fall in the driven region, while higher angles of attack on the retreating blade cause more of the blade to be stalled. The result is a shift in the rotor regions toward the retreating side of the disc to a degree directly related to the forward speed of the aircraft. [Figure 16-5] The retreating blade stall in a gyroplane differs from that of a helicopter in that it occurs outboard from the rotor hub at the 20 to 40 percent position rather than at the blade tip. Because the gyroplane is operating in autorotation, in forward flight there is an inherent stall region centered inboard on the retreating blade. [Refer to figure 16-5] As forward speed increases, the angle of attack on the retreating blade increases to prevent dis- symmetry of lift and the stall region moves further  outboard on the retreating blade. Because the stalled portion of the rotor disc is inboard rather than near the tip, as with a helicopter, less force is created about the aircraft center of gravity. The result is that you may feel a slight increase in vibration, but you would not experi- ence a large pitch or roll tendency. As with any heavier than air aircraft, the four forces acting on the gyroplane in flight are lift, weight, thrust and drag. The gyroplane derives lift from the rotor and On a rotor system in forward flight, reverse flow occurs near the rotor hub on the retreating side of the rotor disc. This is the result of the forward speed of the air- craft exceeding the rotational speed of the rotor blades. For example, two feet outboard from the rotor hub, the blades travel in a circle with a circumference of 12.6 feet. At a rotor speed of 300 r.p.m., the blade speed at the two-foot station is 42 m.p.h. If the aircraft is being operated at a forward speed of 42 m.p.h., the forward speed of the aircraft essentially negates the rotational velocity on the retreating blade at the two-foot station. Moving inboard from the two-foot station on the retreating blade, the forward speed of the aircraft increasingly exceeds the rotational velocity of the blade. This causes the airflow to actually strike the trailing edge of the rotor blade, with velocity increas- ing toward the rotor hub. [Figure 16-6] The size of the area that experiences reverse flow is dependent prima- 16-3 ROTOR FORCE REVERSE FLOW RETREA TING BLADE STALL Figure 16-6. An area of reverse flow forms on the retreating blade in forward flight as a result of aircraft speed exceeding blade rotational speed. Figure 16-5. Rotor disc regions in forward autorotative flight. ForwardGLYPH<.notdef> Flight atGLYPH<.notdef> 42 kt 42kt 42kt 42kt 42kt 2' Area ofGLYPH<.notdef> Reverse flow 4 2 k t Rotor Speed 300 r.p.m. Forward Driven Region Driving Region StallGLYPH<.notdef> GLYPH<.notdef> Region RetreatingGLYPH<.notdef> Side Adv ancingGLYPH<.notdef> Side