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Rotational Airflow (no forward movement)

Rotational Airflow (no forward movement). Tip Speed 700 FPS. Circular movement of the rotor blades…. …produces basic rotational relative wind. Maximum speed is at the tip of the blade and decreases uniformly to zero at the hub. Tip Speed 700 FPS. Typical Blade Tip Speeds.

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Rotational Airflow (no forward movement)

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  1. Rotational Airflow (no forward movement) Tip Speed 700 FPS Circular movement of the rotor blades…. …produces basic rotational relative wind. Maximum speed is at the tip of the blade and decreases uniformly to zero at the hub Tip Speed 700 FPS

  2. Typical Blade Tip Speeds Acft RPM TIP Speed AH-1F(Cobra) 324 746 (fps) OH-58(Kiowa) 354 656 (fps) AH-64(Apache) 289 726 (fps) UH-60(Black Hawk) 258 725 (fps) R-22(Snail) 530 672 (fps) EC-135(Bug) 395 695 (fps)

  3. Development of Induced Flow At flat pitch air leaves the trailing edge of the rotor in the same direction that it moved along the on to the leading edge. No lift is being produced On a asymmetrical airfoil, air following the curved upper camber will leave the trailing edge with a downward flow imparted on it. Each blade creates a greater downward column of air that is ingested by the next blade which produces more downflow and so on. The next slide will attempt to illustrate...

  4. Development of Induced Flow Still air Blade 1 Point A Blade 2 Point A Blade 3 Point A Blade 4 Point A Downward column of air

  5. Initial Velocity = 0 Velocity Induced Velocity Final = 2X Velocity Induced Since the air is disturbed each time that a blade passes a point in space, the air is accelerated downward until it either slows due to ground effect or is slowed by a high volume of undisturbed air well below the rotor (OGE)

  6. Rotor Tip Vortex At a hover, the rotor tip vortex (air swirl at the tip of the rotor blades) slightly reduces the effectiveness of the outer blade portions. Also, the vortexes of the preceding blade affect the lift of the following blades. If the vortex made by one passing blade remains a vicious swirl for some number of seconds, then two blades operating at 350 RPM create 700 long lasting vortex patterns per minute.

  7. Rotor Tip Vorticies cont. This continuous creation of new vortices and ingestion of existing vortices is a primary cause of high power requirements for hovering. These vortices are little more than the blade out running the high pressure below the blade seeking the lower pressure above the blade

  8. Rotor Tip Vorticies cont.

  9. Affects of Airspeed • Advancing Blade • Airspeed is added to the rotational relative wind speed • The greatest value will occur when the blade is at the 3 o’clock position • Increases the velocity along the span of the advancing blade by a velocity equal to the forward airspeed. • Retreating blade • Airspeed is subtracted from the rotational velocity • The minimum value will occur when the blade is at the 9 o’clock position • Decreases velocity across the span of the retreating blade • Produces three “NO LIFT” areas along the retreating blade

  10. Affects of Airspeed cont. Blades over the nose and tail are affected minimally by forward airspeed Development of lift areas around the rotor system in forward flight The entire advancing blade is producing lift, However, the retreating blade produces five distinct lift areas: • Reverse Flow: Airflow is from trailing edge to leading edge • Negative Stall: Airflow strikes blade from well above chord line • Negative Lift: Airflow also above chord line but lift produced under blade • Positive Lift: Airflow is below chord line. This is desirable • Positive Stall: Airflow well below chord line. More drag than lift Reverse Flow, Negative Stall and Negative Lift are the three “NO LIFT” areas discussed on the previous slide

  11. Positive Stall Reverse Flow Negative Stall Negative Lift

  12. Resultant Airflow (120 KTS) 800 FPS = Rotation +200 FPS = Fwd Airspeed 1000 FPS Tip Speed 800 FPS = Rotation - 200 FPS = Fwd Airspeed 600 FPS Tip Speed The forward velocity is added to the advancing blade…. …while it is subtracted from the retreating blade

  13. Dissymmetry of Lift The potential for unequal lift to develop between the advancing and retreating halves of rotor disk due to the differential velocity of wind flow across the advancing and retreating halves of the rotor system. The helicopter would become uncontrollable if dyssemmetry of lift were permitted to manifest itself in the rotor system. A means to compensate for, overcome or eliminate its effects must be available. Those means are:

  14. Blade Flapping The rotor system will compensate for dissymmetry of lift automatically, without pilot input, through blade flapping Upflapping As the relative wind speed of the advancing blade increases, it gains lift and starts flapping up. It reaches its maximum upflap velocity at the 3 o'clock position, where the wind velocity is at its highest. The upflapping velocity creates a downward flow of air across the blade. This has the same effect as increasing the induced flow velocity and reducing angle of attack, decreasing lift across the advancing blade.

  15. Downflapping As the relative wind speed of the retreating blade decreases, the blade loses lift and starts flapping down. It reaches its maximum downflap at the 9 o’clock position, where the wind velocity is the lowest. The downflapping velocity creates an upward flow of air across the blade. The upflow reduces the induced flow velocity and increases the angle of attack, increasing lift.

  16. Retreating half Advancing half Decreased velocity Increased velocity

  17. Due to gyroscopic effect the maximum upflap takes place 90° after its maximum upflap velocity. Since the maximum upflap velocity is at the 3 o’clock position, the maximum upflap displacement is at the 12 o’clock position. Likewise, because the maximum downflap velocity is at the 9 o’clock position, the maximum downflap displacement is at the 6 o’clock position. Upflapping and downflapping do not change the amount of lift produced by the rotor system. The blades flap to equilibrium. However, flapping changes the attitude of the rotor system (blowback) and therefore, the direction of the total lift vector. This reduces helicopter speed.

  18. + + - + - + - - Lift differences before considering phase lag (speed < ETL) Lift differences after the effects of phase lag are applied

  19. The flight profile of an aircraft experiencing BLOW BACK Initial cyclic input made to start forward momentum, then cyclic is held in place with no further corrective actions taken

  20. - + Cyclic Feathering Since blade flapping alone would limit directional velocities to around ETL, another means of compensating for dissymmetry of lift must be available. The pilot must be able to control the attitude of the rotor to attain the desired direction and velocity.

  21. While both cyclic feathering and blade flapping are used to compensate for dissymmetry of lift, cyclic feathering is the primary means of compensating for dissymmetry of lift in normal cruise flight. Other design features to reduce flapping: Forward tilt to the rotor reduces flapping to a minimum during normal cruise flight Synchronized elevator/stabilator help maintain the desired fuselage attitude to reduce flapping

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