Terminology used for rotor Chord (1) Span (2) Vertical Hinge Pin (3) Horizontal Hinge Pin (4) Trunnion (5) Yoke (6) Blade Grip Retainer Bearings (7) Blade Twist
Airfoils in generalApplications: Sustentation (A Wing or Rotor Blade) For Stability (As a Fin) For Control (A Flight Surface, such as a Rudder) For Thrust (A Propeller or Rotor Blade) aerodynamic forces necessary to keep any body aloft are produced when air passes about the rotor blades and this is known as “Lift”
How still air is changed to a column of descending air byrotor blade action a three-bladed system rotating at 320 revolutions perminute passes a given point in the tip-path plane 16 timesper second
Airflow from rotation, modified by induced flow,produces the Resultant Relative Wind.
Centrifugal force & CentripetalForce Because of its rotation and weight, the rotor system is subject to forces and moments peculiar to all rotating masses. One of the forces produced is Centrifugal Force. It is defined as the force that tends to make rotating bodies move away from the centre of rotation. Centripetal Force. It is the force that counteracts centrifugal force by keeping an object a certain radius from the axis of rotation.
The effective diameterof the rotor disk withincreased coning isless than the diameterof the other disk withless coning. A smallerdisk diameter has lesspotential to producelift
Drag forcesTotal Drag produced by an aircraft isthe sum of the Profile drag, Induceddrag, and Parasite dragCurve "A" shows that parasite drag isvery low at slow airspeeds and increaseswith higher airspeeds.Curve "B" shows how induced dragdecreases as aircraft airspeed increasesCurve "C" shows the profile drag curve.Profile drag remains relatively constantthroughout the speed range with someincrease at the higher airspeeds.Curve "D" shows total drag andrepresents the sum of the other threecurves. It identifies the airspeed range,line "E", at which total drag is lowest.That airspeed is the best airspeed formaximum endurance, best rate of climb,and minimum rate of descent inautorotation.
TorqueThe helicopter fuselage tends to rotate in the directionopposite to the rotor blades. This effect is called torque.The torque effect on the fuselage is a direct result of thework/resistance of the main rotorCompensation for torque in the single main rotorhelicopter is accomplished by means of a variable pitchanti-torque rotor (tail rotor)the tail rotor produces thrust in a horizontal planeopposite to torque reaction developed by the main rotor.
From 5 to 30 percent of the available engine power maybe needed to drive the tail rotor depending onhelicopter size and design.A helicopter with 9,500 horsepower might require1,200 horsepower to drive the tail rotora 200 horsepower aircraft might require only 10horsepower for torque correction.
Translating Tendency During hovering flight, the single rotor helicopter has a tendency to drift laterally to the right due to the lateral thrust being supplied by the tail rotor. The pilot may prevent right lateral drift of the helicopter by tilting the main rotor disk to the left This lateral tilt results in a main rotor force to the left that compensates for the tail rotor thrust to the right.
Angle of attack The angle between the airfoil chord line and its direction of motion relative to the air (the resulting Relative Wind)
Angle of Incidence (or AOI)Angle of Incidence (or AOI) is the angle between theblade chord line and the plane of rotation of the rotorsystem.It is a mechanical angle rather than an aerodynamicangle:
T Rotational velocities in the rotor system when speed is doubled, lift is increased four times.
Lift variation along the span The lift at point "A" would be only one-fourth as much as lift at the blade tip Because of the potential lift differential along the blade resulting primarily from speed variation, blades are designed with a twist. This graphic compares a twisted versus an untwisted blades lift:
Dissymmetry of lift Dissymmetry of lift is the difference in lift that exists between the advancing half of the rotor disk and the retreating half. The blade passing the tail and advancing around the right side of the helicopter has an increasing airspeed which reaches maximum at the 3 oclock position. the airspeed reduces to essentially rotational airspeed over the nose of the helicopter. Leaving the nose, the blade airspeed progressively decreases and reaches minimum airspeed at the 9 oclock position.
Factors which can be used toovercome the dissymmetry of liftIn the lift equation, density ratio and blade area are the same for both the advancing and retreating blades The airfoil shape is fixed for a given blade. The only remaining variables are changes in blade angle of attack and blade airspeed. These two variables must compensate for each other during forward flight to overcome dissymmetry of lift
Blade flappingRight side of the helicopter Left side of the helicopter
Flapping VelocityFlapping Velocity, both upward and downward,must be of such a value as to increase or decrease theangle of attack so that the lift will remain constant.The force-displacement phase is 90 degreesthe maximum upward and flapping velocity isdirectly over the right side of the helicopter, themaximum displacement or actual flapping will takeplace over the nose of the aircraft. the maximum downward flapping velocity isdirectly over the left side of the helicopter, themaximum displacement or actual flapping will takeplace over the tail of the aircraft
Ground effect Ground Effect is a condition of improved performance encountered when operating near (within 1/2 rotor diameter) of the ground. Increased blade efficiency while operating in ground effect is due to two separate and distinct phenomena. The high power requirement needed to hover out of ground effect is reduced when operating in ground effect.
Reasons for less powerrequirement reduction of the velocity of the induced airflow. The result is less induced drag and a more vertical lift vector
a reduction of the Rotor TipVortexThe airfoil operating out-of-ground-effect is lessefficient because ofincreased induced windvelocity which reducesangle of attackThe airfoil that is in-ground-effect is moreefficient because it operatesat a larger angle of attackand produces a morevertical lift vector
The HoverHovering is the term appliedwhen a helicopter maintains aconstant position at a selectedpoint, usually a few feet abovethe ground (but not always,helicopters can hover high inthe air, given sufficient power.The rotor blades move largevolumes of air in a downwarddirectionThis is the air flow around ahovering helicopter (Note: it is out of groundeffect)
Retreating blade stallThe airspeed of the retreating blade (the blade movingaway from the direction of flight) slows down asforward speed increases.as the airspeed of the retreating blade decreases withforward aircraft speed, the blade angle of attack mustbe increased to equalize lift throughout the rotor diskarea.As forward airspeed increases, the "no lift" areas moveleft of centre, covering more of the retreating bladesectors.
The major warnings of approachingretreating blade stall conditions are: Abnormal Vibration Nose Pitch upWhen operating at high forward airspeeds: High Blade loading (high gross weight) Low Rotor RPM High Density Altitude Turbulent Air