# In-Flight Force A number of forces act upon an aircraft while in flight. While it is not necessary to memorize the lift equation, an aviator benefits significantly from a thorough understanding of the forces described in this section. An aviator should strive to explain the relationships of these forces and the results on the aircraft. ## **Total Aerodynamic Force** As air flows around an airfoil, a pressure differential develops between the upper and lower surfaces. The differential, combined with air resistance to passage of the airfoil, creates a force on the airfoil. This is known as Total Aerodynamic Force (TAF) (figure 1-39). TAF acts at the center of pressure on the airfoil and is normally inclined up and rear. TAF, sometimes called resultant force, may be divided into two components, lift and drag.
## **Lift** Lift is the component of the total aerodynamic force on an airfoil and acts perpendicular to the relative wind. (FAA-H-8083-21B) (figure 1-40). The resultant relative wind is the referenced relative wind associated with lift.
### **Lift Equation** The lift equation, represented as $$L=CL×ρ2×S×V2L=CL​×2ρ​×S×V2$$, helps explain how lift is generated. Here's a breakdown: * $$LL$$ = Lift force * $$CLCL​$$ = Coefficient of lift * $$ρ22ρ​$$ = 0.5 times the density of the air ($$ρρ$$ - rho), measured in slugs per cubic foot * $$SS$$ = Surface area of the airfoil, measured in square feet * $$V2V2$$ = Airspeed, measured in feet per second The coefficient of lift ($$CLCL​$$) is determined by the shape or design of the airfoil and the angle of attack (AOA). Aviators have control over AOA but not over airfoil design. They cannot alter the density of the air ($$ρρ$$) or the surface area of the airfoil ($$SS$$). However, changes in rotor RPM have a greater impact on lift than changes in airspeed. *** ## **Drag** Drag is the net aerodynamic force parallel to the relative wind, comprised of induced drag and parasite drag. Here's an overview: * **Induced Drag:** Generated due to the production of lift. It is proportional to the square of the lift and inversely proportional to the aspect ratio of the wing. * **Parasite Drag:** Arises from non-lift-producing surfaces, such as the fuselage and various protrusions. It includes form drag (due to the shape of the aircraft) and skin friction drag (caused by the friction between the air and the aircraft's surface). ### **Drag Equation** Understanding the drag equation, $$D=CD×ρ2×S×V2D=CD​×2ρ​×S×V2$$, is crucial for comprehending how drag is influenced. Here's a breakdown: * $$DD$$: Drag force * $$CDCD​$$: Coefficient of drag, largely determined by the shape or design of the airfoil and the angle of attack (AOA). The aviator can control AOA but not airfoil design. * $$ρ22ρ​$$: 0.5 times the density of the air ($$ρρ$$), representing the air density. * $$SS$$: Surface area of the airfoil, unaffected by aviator input. * $$V2V2$$: Relative wind velocity or airspeed, the only factor an aviator can change. ### **Types of Drag** Total drag acting on a helicopter is the sum of three types of drag: 1. **Parasite Drag:** Results from non-lift-producing surfaces. 2. **Profile Drag:** Arises from the airfoil's shape and surface characteristics. 3. **Induced Drag:** Generated by the production of lift. Understanding the components of drag is essential for optimizing aircraft performance and efficiency. **Drag Equation** The drag equation, $$D=CD×ρ2×S×V2D=CD​×2ρ​×S×V2$$, breaks down as follows: * $$DD$$: Drag force * $$CDCD​$$: Coefficient of drag, determined by the shape or design of the airfoil and AOA. The aviator can control AOA but not airfoil design. * $$ρ22ρ​$$: Half the air density ($$ρρ$$), reflecting air density. * $$SS$$: Surface area of the airfoil, unaffected by aviator input. * $$V2V2$$: Relative wind velocity or airspeed, the only factor an aviator can change. ### **Parasite Drag** Parasite drag results from non-lifting parts of the aircraft, including form drag, skin friction, and interference drag from various components. It increases with airspeed and dominates at higher speeds. ### **Profile Drag** Profile drag arises from the frictional resistance of blades moving through the air. It remains relatively constant with AOA but increases at higher speeds, especially with blade stall or compressibility effects. ### **Induced Drag** Induced drag stems from the production of lift. Higher angles of attack generate more lift but also create downward velocities and vortices, increasing induced drag. In rotary-wing aircraft, induced drag decreases with increased airspeed. ### **Drag/Power/Airspeed Relationship** Figure 1-41 illustrates the relationship between drag, power, and airspeed, providing insight into how these factors interact.
### **Aircraft Performance and Power Curves** Drag is a critical element utilized alongside flight testing and performance data to create performance planning charts found in operator manuals. These charts allow aviators to calculate expected performance data based on various conditions, aiding in determining predicted airspeeds, torques, and fuel flows for different mission profiles. **Maximum Range Airspeed** The airspeed that enables the helicopter to achieve the furthest distance. It's determined where airspeed intersects the lowest total drag amount (point E on figure 1-41). Cruise charts are used to determine required torque and fuel flows for maintaining this airspeed. **Maximum Endurance Airspeed** The airspeed that allows the helicopter to remain flying for the longest duration. It's found on the power required curve of the cruise chart where power required is minimal, not necessarily where total drag is lowest. **Maximum Rate-of-Climb Airspeed** Achieved by combining the maximum endurance airspeed with the maximum available torque, enabling the fastest rate of climb. ## **Centrifugal Force and Coning** The rotating blades of a helicopter generate significant centrifugal loads on the hub and blade attachment assemblies. This force causes the blades to rise from the static position, leading to coning as lift is developed during takeoff and flight. Excessive coning, resulting from factors like low RPM or high gross weight, can lead to stress on components and decreased lift due to reduced effective disk area.
## **Torque Reaction and Antitorque Rotor (Tail Rotor)** According to Newton’s law of action and reaction, the turning rotor system generates action, causing the fuselage to react by turning in the opposite direction. This reaction to torque turning the main rotor is known as the torque effect. To maintain control of the aircraft, torque must be counteracted, which is the role of the antitorque rotor (tail rotor). In tandem rotor or coaxial helicopters, where two rotor systems turn in opposite directions, the torque effect is effectively canceled out. However, most rotary-wing aircraft with a single main rotor require a tail rotor or similar means to counteract the torque effect. The tail rotor, typically driven by the main transmission through a drive shaft arrangement, is designed as a variable-pitch antitorque rotor to accommodate the varying effects of the torque system. Engine power required to operate and control the tail rotor can be substantial, necessitating careful consideration during performance planning for different conditions and scenarios. Emergency procedures are established to address issues such as loss of engine power, insufficient engine power, or tail rotor malfunction, highlighting the critical role of the tail rotor in flight safety. In American-built single-rotor helicopters, where the main rotor typically turns counterclockwise, applying right pedal decreases pitch in the tail rotor, reducing thrust and allowing the nose of the aircraft to turn right. Conversely, applying left pedal achieves the opposite effect.
### **Heading Control** Different helicopter designs employ various mechanisms for heading control, crucial for precise aircraft control during maneuvers to prevent damage or injury. Aircrews need to comprehend and anticipate the forces affecting heading control. ### **Single-Rotor Helicopters** In single-rotor helicopters, heading control, alongside torque counteraction, is primarily achieved through the tail rotor and its control linkage. During taxi, hover, and sideslip operations, applying more pedal than necessary to counteract torque causes the helicopter's nose to swing in the direction of pedal movement (left pedal to the left). Conversely, applying less pedal than needed results in the helicopter turning in the direction of torque (nose swings to the right). Maintaining a constant heading requires aviators to use antitorque pedals to counteract torque adequately and hold a slip. Heading control during forward trimmed flight typically involves using cyclic control with coordinated bank and turn to the desired heading. Adjustments to pedal trim are necessary when making power changes. ### **Tandem-Rotor Helicopters** In tandem-rotor helicopters, heading control relies on differential lateral tilting of the rotor disks. Application of the directional pedal results in the forward rotor disk tilting in the same direction while the aft rotor disk tilts in the opposite direction, facilitating a hovering turn around a vertical axis between the rotors. For heading control in forward flight, lateral cyclic tilt on both rotors is coordinated for roll control, while differential cyclic tilt is used for yaw control. Minor changes in pedal trim are required for adjustments in longitudinal speed trim, descents, climbs, and autorotations. ## **Balance of Forces** Newton’s law of acceleration governs helicopter flight, indicating that motion is initiated, stopped, or changed when forces acting on the body become unbalanced. In helicopter flight, total force generated by the rotor system is perpendicular to the tip-path plane, divided into lift (supporting aircraft weight) and thrust (horizontal acceleration or deceleration). Aviators direct thrust by tilting the tip-path plane. At hover in no-wind conditions, all opposing forces are in balance, resulting in the helicopter remaining stationary.
To make the helicopter move in some direction, a force must be applied to cause an unbalanced condition. Figure 1-46, illustrates an unbalanced condition in which the aviator has changed the attitude of the rotor disk creating a lift and thrust vector, resulting in a total force forward of the vertical. No parasite drag is shown as the aircraft has not started to move forward.
As the aircraft begins to accelerate in the direction of applied thrust, parasite drag develops. When parasite drag increases to be equal to thrust, the aircraft no longer accelerates because the forces are again in balance (figure 1-47) as the aircraft has achieved steady-state (unaccelerated) flight.
To return the aircraft to a hover, the aviator changes the disk attitude to unbalance the forces (figure 1- 48). By tilting the rotor disk aft, the thrust force acts in the same direction as parasite drag and airspeed decreases.
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