AOA and Blade Pitch. The blade pitch or pitch angle (also referred to as angle of incidence) is the angle between the chord line of a main or tail rotor blade and the plane of rotation of the rotor system (tip path plane). It is a mechanically defined angle rather than an aerodynamic angle (Figure 2-23). The AOA is the angle between the airfoil chord line and the relative wind (Figure 2-24). In the absence of an induced velocity, AOA and pitch angle are the same. Whenever relative wind is modified by induced velocity, the pitch angle and AOA are different. Collective and cyclic feathering (control input pitch changes) mechanically changes the pitch angle.
Therefore, a change in the blade pitch results in a change in the AOA, which changes the coefficient of lift of the airfoil.
Figure 2-23 Blade Pitch Angle
Figure 2-24 AOA
AOA and Stall. AOA is one of the primary factors that determines amount of lift and drag produced by an airfoil. Because the AOA is an aerodynamic angle, it can change with no change in the blade pitch, such as due to an increase in induced velocity decreasing the AOA. Aviators adjust the blade pitch through normal control manipulation and the aerodynamic forces change the AOA, yet even with no aviator input, the AOA will change as an integral part of the rotor blade traveling through the rotor disk arc. This continuous process of change is designed to accommodate rotary-wing flight. Aviators may have little control over blade flapping, blade
flexing, and gusty wind or turbulent air conditions, but cyclic feathering allows for pilot compensation.
Up to the point of aerodynamic stall, increasing the AOA causes greater acceleration of air atop the airfoil which results in a larger pressure differential between the top and bottom of the airfoil which, in turn, produces more lift. However, if the AOA is increased beyond a critical angle, the airflow across the top of the airfoil will separate (boundary layer separation). This creates a turbulent layer and causes static pressure on the upper surface of the wing to increase thereby reducing the net pressure differential between the upper and lower surfaces. When this occurs, lift rapidly decreases, and drag rapidly increases. The only factor that can cause a stall is exceeding the critical AOA for the specific airfoil (the airspeed may vary at which the critical AOA will be reached).
A symmetric airfoil at zero AOA, (Figure 2-25, left side), produces identical velocity increases and static pressure decreases on both the upper and lower surfaces. In the figure, arrows indicate static pressure relative to ambient static pressure. Arrows pointing toward the airfoils indicate higher static pressure; arrows pointing away from the airfoils indicate lower static pressure. The large arrows represent total force generated on each surface. At zero AOA the force on the lower surface is equal and opposite to that on the top surface. Since there is no pressure differential perpendicular to the relative wind, the airfoil produces zero net lift.
The right side of Figure 2-25 depicts what happens when an AOA is introduced to a symmetric airfoil. The streamtubes on the top surface get more compressed than those on the bottom surface, so the flow speeds up more. The faster flow on the top surface leads to higher dynamic pressure and lower static pressure. Notice that pressure on the lower surface is lower than ambient pressure also, but it is still higher than the pressure on the top surface. The lower static pressure above the airfoil creates a partial vacuum or suction which pulls the airfoil up. Some people prefer to think of it as the higher static pressure on the bottom surface pushes the airfoil up toward the lower pressure air on the upper surface and lift is generated.
Figure 2-25 Lift on a Symmetric Airfoil.
A cambered airfoil produces lift even at zero degrees AOA (Figure 2-26, left side). The positive camber makes the area in the streamtube above the wing smaller than the area in the streamtube below the wing. The airflow above the wing thus travels faster and causes a lower static
pressure. Higher static pressure below the wing pushes toward the lower pressure on the upper surface and generates lift. Also notice that putting a cambered airfoil at a positive blade pitch generates more lift than a symmetric airfoil would generate at the same angle.
Figure 2-26 Lift on a Cambered Airfoil 220. AERODYNAMIC FORCES
Aerodynamics involves the study of forces imposed on a body that is placed in an air flow. The air can be moving past the object at a given velocity, or the body can be moving through still air at the given velocity. The effect is the same. The velocity at which the air impacts the body is represented by a relative wind vector that has magnitude and a direction.
Recall the discussion of airflow and streamtubes earlier in the chapter. Air flows around a symmetric airfoil at zero AOA in a streamline pattern similar to that depicted in Figure 2-27. As the air strikes the leading edge of the airfoil, its velocity slows to near zero, creating an area of high static pressure called the leading edge stagnation point. Air that is flowing adjacent to the stagnation point separates so that some air moves over the airfoil and some under it. Air passing over and below the airfoil is bounded on one side by the airfoil surface and on other sides by pressure in adjacent streamlines, so that a sort of “streamtube” is formed. Airflow leaving the leading edge stagnation point is accelerated as it passes through the decreased area in each streamtube, in the same way it would in a tube of decreasing size.
Figure 2-27 Streamlines Around a Symmetric Airfoil
As it travels along the airfoil, the airflow on top and bottom surfaces reaches a maximum velocity at the point of maximum airfoil thickness (and minimum streamtube width). The
airflow velocity then decreases until the flow reaches the trailing edge where the upper and lower airflow meet. At the trailing edge, the velocity slows to near zero velocity, forming another area of high static pressure called the trailing edge stagnation point.
As previously discussed, Bernoulli’s equation explains what happens next as a result of the air’s flow from leading edge to trailing edge. Since we are not doing any work to the airstream, the total pressure remains constant and thus only the static pressure and dynamic pressure are allowed to vary.
PS + ½ V2 = constant
The increase in airflow velocity at any point over an airfoil causes dynamic pressure to increase and static pressure to decrease at that point. These changes in pressure, along with friction, are responsible for the aerodynamic force on an airfoil.
Now consider a cambered airfoil or a symmetrical airfoil at some positive AOA. As the air travels over the curved upper surface, there is a greater velocity increase and static pressure decrease over the upper surface than the lower surface. Therefore a pressure differential develops between the upper and lower surfaces. That differential, combined with the resistance of the air to the passage of the airfoil, creates a force on the airfoil. The sum of those forces is The Aerodynamic Force (TAF) on the body.