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Aerodynamics

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Aerodynamics AERODYNAMICS AIRFLOW DURING A HOVER IGE v.s. OGE POWER REQUIREMENT The maximum flow velocity in IGE is lower due to ground disruption of the airflow. Additionally, the rotor tip vortices formed are smaller resulting in a larger lifting surface along the blade span. As the aircraft is brought to an OGE hover, the flow velocity of the air is allowed to increase resulting in a greater induced flow and the tip vortices become increasingly larger decreasing span lifting area. Although the blade angle of incidence is larger due to collective increase, the angle of attack remains the same. Greater induced flow causes an increase in the angle of incidence while angle of attack remains the same. RETREATING BLADE STALL CAUSE, CONDITIONS, SYMPTOMS During forward flight, the advancing blade flaps up and increases in speed. The retreating blade must over come increased flow on the rear portion of the blade. It flaps down to increase its angle of attack and decreases speed to provide an equal amount of lift to that of the advancing blade. As forward speed increases, the 3 no- lift areas, Negative Lift, Negative Stall, and Reverse Flow, begin to move outboard from center. At the same time, the Positive Stall occurring on the rotor tip begins to move inboard. At a certain point the lifting area of the retreating blade will not be able to equal the advancing blade and Retreating Blade Stall will occur. Increased vibrations will be felt with the aircraft pitching up and rolling left. The corrective action is to reduce/correct any of the conditions that can or have caused it. These are: CAUSE CORRECTION • High Gross Weight • Reducing Power • Low Rotor RPM • High Density Altitude • Reducing Airspeed • Steep or Abrupt Turns • Reducing High “G” •T urbulent Air maneuvers • High Airspeed • Increasing RPM • Check Pedal Trim TOTAL AERODYNAMIC FORCE EXPLANATION Total Aerodynamic Force is the resultant force or action generated by airfoil when airflow (Relative Wind) is introduced over and under it. It primarily consists of two components, Lift and Drag. When the airfoil is given an angle of attack in relation to the relative wind this causes lift. A high pressure area is created under the airfoil while a low pressure area is created above the airfoil. At the same time drag is acting against the lift component. Drag types are Parasite, Profile, and Induced. Total lift must overcome total drag in order for the aircraft to fly. While lift is perpendicular to the relative wind or resultant relative wind when induced flow is introduced, drag is parallel to it. If a vector is drawn between both components, total aerodynamic force will the vector where they meet. AERODYNAMICS DRAG TYPES, EXPLANATION Drag is the force acting against lift on the airfoil. There are three types of drag, Parasite, Profile, and Induced. Parasite drag is the drag caused by the non-lifting components of the aircraft. Induced drag is caused by the production of lift (induced flow from tip vortices and main rotor downwash). Profile drag is a sub-component of parasite drag called skin friction. It is caused by the boundary layer of stagnant air covering the airfoil and at subsonic speeds is relatively low and constant. As airspeed is increased it is a bigger factor. Total Drag is the combination of all three of these drag components. The best performance of the aircraft can be found at the “bottom of the bucket”. This refers to the lowest portions of Total Drag. Referring to the Cruise Charts (drag charts turned sideways) you can see this at Max End. COMPRESSIBILITY CAUSE, CONDITIONS, SYMPTOMS At low airspeeds air is incompressible. Pressure waves are allowed to propagate ahead of the airfoil. These waves cause the separation of airflow around the airfoil ahead of the leading edge. This propagation speed is solely a function of temperature. As airspeed is increased and the speed of sound is reached (sooner at low temperature conditions), these waves cannot form and a compression wave forms at the leading edge. While airflow was influenced ahead of the leading edge before, it now has to make sharp and sudden changes in velocity and pressure. A shockwave is also formed disrupting airflow around the airfoil. This disruption as well as the change in pressure and speed of the airflow makes the Center of Pressure (normally on the spar at the 1st 1/3rd of the blade) to move aft causing a twisting moment. Rotor blades are not made structurally to handle this change and the result is catastrophic. Like Retreating Blade Stall, the conditions which get the aircraft into this are similar with the most notable difference being High RPM. With the exception of RPM control, the recovery technique is identical. CAUSE CORRECTION • High Airspeed • Reducing Power • High Rotor RPM • Reducing Airspeed • High Gross Weight • High Density Altitude • Reducing High “G” • LOW TEMPERATURE maneuvers • Turbulent Air • Decreasing RPM • Check Pedal Trim AERODYNAMICS DYNAMIC ROLLOVER CAUSE, CORRECTIVE ACTION Dynamic Rollover happens when the aircraft exceeds its critical roll angle. That angle is the angle at which corrective action will have no effect on the rolling moment. Corrective action prior to exceeding the angle is dependent upon control applications that caused the rolling moment to develop. If excessive cyclic input is making the roll occur, reducing the cyclic input will correct it. If excessive collective is causing the roll then opposite collective input will correct it. Pedal application in the UH-60 causes a rolling moment opposite the pedal applied. The conditions that cause this to occur are always the same; introducing a pivot point where a roll can occur and introducing a rolling moment. Rolling moments can occur with control inputs or movement of the aircraft during landing that encounters a pivot point. Care must be taken to ensure there is no lateral drift during landings. SETTLING WITH POWER CAUSE, CONDITIONS, RECOVERY Settling with Power is a condition where the helicopter settles in its own downwash and collective application is ineffective in arresting the descent rate. It is also referred to as the Vortex Ring State. During a normal hover, induced flow is greatest at the blade tip and at its lowest closer to the hub. When the aircraft gets into a high descent rate, airflow is disrupted inboard where induced flow is lower and allowed to flow upward through the rotor system. As the descent is continued vortices can begin to form inboard and continue to grow in size. This will disrupt normal airflow and result in a condition of little or no lift along the blade span. If collective pitch is applied, the inboard vortices will only increase worsening the situation. Airspeed and collective reduction to move out of and decrease this airflow pattern is the only corrective action. Conditions conducive to Settling with Power are; • Vertical or near vertical descent 300 fpm or greater. • Low forward airspeed. • Rotor using some or all of available engine power. • Insufficient power to stop the sink rate. TRANSLATING TENDENCY EXPLANATION, EFFECT Translating Tendency is the tendency for the helicopter to drift to the right due to tail rotor compensation for torque effect. The tail rotor produces thrust to the right to compensate for main rotor torque. This thrusting moment if not corrected would cause the helicopter to drift. To compensate the pilot must tilt the disk to the left or a mechanical design has to be installed to tilt the disk automatically. The helicopter will attempt to maintain equilibrium and follow the mast. This results in the aircraft fuselage being left side low at a hover. AERODYNAMICS TRANSVERSE FLOW CAUSE, CORRECTIVE ACTION Transverse Flow is a condition that results from the increased down-flow of air into the rear half of the rotor system. As the helicopter gains airspeed, a more horizontal flow of air enters the forward half of the rotor system resulting in a lower less induced flow and a higher angle of attack. The rear half continues to have an increased induced flow and a reduced angle of attack. This difference in lift is most notable at speeds between 10 and 20 knots resulting in vibrations felt by the pilot and a rolling moment to the right. DISSYMMETRY OF LIFT CAUSE, CONDITIONS, SYMPTOMS Dissymmetry of lift is difference in lift that exists between the advancing and retreating halves of the rotor system. During forward flight, the aircraft relative wind is added to the rotational relative wind. This results in increased lift on the advancing blade which corrects for it by flapping up. It reaches its maximum velocity and up flap at the 3 o’clock position. Due to gyroscopic procession (phase lag) this lift manifests itself at the 12 o’clock position by pitching the nose up (blowback). The pilot must overcome this by using cyclic feathering. The retreating blade flaps down to maintain a higher angle of attack to equalize the lift on the rotor system. The retreating blade must also decrease its speed to maintain equilibrium. The three no lift areas reduce the amount of lifting area on the retreating side, so the small area left must equal lift produced on the advancing side. Dissymmetry of lift is characterized by the helicopter pitching up due to blowback. Blade flapping and cyclic feathering correct for this. The tail rotor must also compensate for Dissymmetry of Offset Hinge Lift. As with the main rotor, the tail rotor incorporates flapping to overcome dissymmetry. To do this helicopters incorporate flapping hinges and hingeless systems. The two flapping hinges are the delta hinge Delta Hinge and the offset hinge. The delta hinge is not oriented parallel to the blade chord and is designed so flapping automatically causes the blades to feather.
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