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How does helicopter make flight
Helicopters are awesome and always used to be my curious flying machine because of it's versatility and amazing performances. An increasing number of people like hoteliers, trauma carers, sight builders, rescue operators etc who have been concerned only with fixed wing aircraft, are becoming associated with helicopters for it's incredible maneuverability. Helicopters besides making a safe and efficient method of transportation from place to place it's capability to fly side ways, back words also to land and take off vertically makes the aircraft different from all other flying machines. Above all, helicopter have the ability to rotate 360 degrees on the spot and remain hovering above the ground, sea or mountain.
How does rotor generating lift
We know already an aircraft need to run fast through the runway for it's wing to generate lift. How does helicopter with out having a wing or wheels to run will generate lift? That is what the whole interesting thing about helicopter.
Rotor is a system of rotating airfoils located above the helicopter fuselage. Main component of the rotor is long airfoil shaped blades which are attached to a rotor head assembly that in turn coupled to a vertically mounted shaft. Engine turns the shaft and rotates the rotor blades. Airfoils as a result move through air in much the same way as the wing of an air plane, difference being the wing move forward and the rotor rotates. Since rotor on helicopter takes the place of a wing, helicopters are therefore known as rotary wing or rotating wing aircraft.
To make helicopter rise from the ground an upward force (lift) which is greater than the weight of helicopter is to be applied to the rotor head situated above the center of gravity of helicopter. We know lift is produced when air foil is moved through air and lift can be increased or decreased by altering the angle of attack of air foil (see the picture of air foil at different angles of attack).
Pitch or in simple term angle, of all the blades of the rotor is simultaneously controlled by a pilot's control known as collective pitch lever. Pulling the collective pitch lever up will increase blade pitch equally on all blades and helicopter will rise. Opposite happens when the lever is pushed down, pitch of all blades decreases and helicopter descends as the lift decreases. In some cases, on the same lever engine throttle control or some times known as "twist grip" is also incorporated for controlling engine power to maintain the rotor rpm.
Methods of varying lift
Lift generated by rotor depends on the angle of attack of the blade (pitch). Lift can be varied either by altering pitch of the blade or by varying speed (rpm) of the rotor. Since it takes time to alter speed because of inertia, lift is varied only by altering the pitch and rpm is kept constant.
Disc area of a Rotor
An imaginary circular disc formed when the rotor rotates or in more technical explanation, the total area of the circle swept by the main rotor of a helicopter as it rotates. The disc area is largest when no pitch is given to the rotor and disc area get reduced as the collective control is pulled to induce lift. More the lift induced smaller becomes the disc area.
What is Conning of Rotor?
Tip of each blade tends to go higher because of high lift available at the tip, as the tip is running at an increased speed than the root of the blade. As a result, a shallow cone facing upward will be formed when the blades are in full rotation and the conning will be further increased when the pitch is increased. Imaginary surface swept by the rotor blades during their rotation is called rotor disc. Lift and blade conning are indirectly proportional, When conning is less the disc area is more, and when conning is more disc area will be less (see the picture below). During flying the disc supports the weight of the helicopter as in the case of a wing does for the fixed wing aircraft.
Effect of Rotor Coning
What an interesting feature! We think when rotor rpm is less during flight, the blade will droop down due it's weight. No it is not true, because of the reduction in centrifugal force due to low rpm, the blade will go more up and will make excessive coning as the lift remains applied to the blade. This can end up in blades getting bent from the middle section and the whole helicopter can lose it's altitude considerably as the effective disc area is reduced.
When pilot pulls the collective lever up (take off) if weight of aircraft is more blade coning will be more because of high lift generated by the blade. Blade conning will go more for one more reason that is when rotor rpm (speed) goes down beyond limitation, because of the reduced centrifugal force. Forces acting on blades as you see are centrifugal force and lift. Lift will tend to fly the blade high or to cone increasingly but centrifugal force stretching the blades out ward from the center and preventing them from excessive coning, one is contributing to other.
In a helicopter, rotor rpm is normally kept constant at 100 percent. What is increased or decreased is the pitch of the blades. When weight is added more to the helicopter more lift is to be generated by the rotor. Pilot will pull collective pitch control lever more up eventually ending up in more coning of blade. So to avoid excessive conning he has to reduce the lift by lowering the collective to maintain the blade disc area.
Vertical movement from hovering
Since there are two or more blades in an assembly, the lift produced by each blade is concentrated to the center and resultant lift vector is available at the the shaft (mast). Helicopter will remain hovering when this lift vector is equal to the weight of the helicopter. When upward force is increased by adding lift, the helicopter will rise and if the force is decreased, the gravity will cause helicopter to descend.
Directional flight from hovering
Both horizontal force (thrust) and vertical force (lift) are available to the helicopter when it is making forward flight from hover. A propeller driven fixed wing aircraft moves forward by propeller pushing air backward to generate an equal and opposite forward thrust. Similarly, when the cone or the disc is tilted forward large quantity of air is accelerated backward to generate forward thrust. In addition to the forward thrust, lift is already generating by the lift vector since the cone is facing upward even though the cone is slightly tilted forward.
Cone, when facing upward, helicopter will fly vertically upward and when it is tilted forward helicopter fly forward. The cone can be tilted backward or side ways to achieve corresponding directional flights. This is accomplished by operating a pilot's control known as cyclic pitch control.
Cyclic pitch control and tilting of cone
Let us be sure at this point, the difference between collective pitch control and cyclic pitch control, two flight controls of the helicopter. Collective pitch control changes the pitch of all blades togetherby same amount and in the same direction. Cyclic pitch control changes pitch of each blade consecutively from a minimum to maximum during each revolution of the disc.
Cyclic pitch control is a stick located in front of the pilot and can be moved around 360 degrees from it's floor mounted position. In a two bladed rotor, when cyclic pitch control is moved forward, the blade travelling from front to the rear will gain maximum pitch causing the blade to ride up. So also, the blade travelling from rear to the front will attain minimum pitch causing the blade to ride comparatively down. The blade reaching towards rear is called retreating blade and the blade reaching towards front is called advancing blade. In a plain of rotation when retreating (rear) blade is riding up and advancing (front) blade is riding down, the cone or the disc will remain tilted to the front. The helicopter will now fly forward.
Similarly, the blade disc (cone) can be tilted back ward to fly the helicopter backward by moving the cyclic pitch control towards back. Now the advancing blade (front) will ride up and retreating blade (rear) will ride low resulting to the cone tilted backward. Disc therefore can be tilted (a must see tube) to any direction by the cyclic pitch control to make the flight consequently to that direction.This is accomplished by the movement of a part called swash plate located under the rotor head (blue color). Swash plate wobbles to all the sides when cyclic control lever is actuated and it simply slides up and down when collective lever is actuated. Watch tube carefully and understand collective and cyclic operation.
Tail rotor and pedal control
Tail rotor is a small rotor mounted vertically on to the side of the tail section of helicopter. One of it's purposes is to counter the torque induced by main rotor. With out tail rotor the helicopter will be simply spinning in flight. Tail rotors are not incorporated on a twin rotor helicopter, torque generated by front rotor is offset by the torque produced by a counter rotating rear rotor.
Second purpose of tail rotor is to correct "yawing" during hovering and during directional flights. Yawing is movement where the helicopters heading is changed by drifting the nose to one side or to the other side. This drift can be corrected by shifting tail to either side so as to get nose put in to right direction.
This pedal operated control located in front of the pilot enables to increase or decrease forward thrust produced by tail rotor like a propeller. An increase in thrust will pull tail section of helicopter to the side where the tail rotor is installed. Decrease in thrust will push helicopter tail section to opposite side. Pedal control primarily increases or decreases the pitch of the tail rotor.
Now watch this tube link to learn how does helicopter make flight.