I know that airplanes use rudder to turn, and use elevator for nose up and down.
How do helicopters change their altitude and how do they make turns?
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Airplanes do not use their rudder to turn; they do it by banking into the direction of the desired turn, then the wings "lift" the plane in that direction. The rudder is only used to maintain lateral trim, so that the airflow doesn't impinge on the side of the fuselage and drag is minimized. To initiate a turn, the pilot uses the stick or yoke to actuate the ailerons on the wingtips, causing the wing on the inside radius of the turn to go down and the wing on the outside radius of the turn to go up. A slight back-pressure of the stick or yoke raises the nose, causing a "climb" to be initiated, but since the plane is banked toward the center of a circle, the "climb" doesn't cause a change in altitude, instead it causes the plane to fly a circular path. Small, simultaneous changes in engine power and rudder deflection are needed in order to maintain proper trim and constant altitude in what is called a "coordinated turn".
Helicopters turn in a manner analogous to that of an airplane. In forward flight, the cyclic stick (between the pilot's legs) is pushed slightly in the direction of the desired turn, causing the rotor disc to tilt, just as the wings of an airplane are tilted in a banked turn. The collective lever and throttle (at the pilot's left side) are adjusted as needed to maintain altitude and the rudder pedals are adjusted to maintain a coordinated turn. Since the fuselage of a helicopter is suspended from the rotor hub like a pendulum, as the helicopter enters the turn, the fuselage tends to be flung outward, so it banks, just like the fuselage of the airplane that is rigidly connected to its wings. (In a hover, with no forward airspeed, a helicopter can turn in any direction with the rudder pedals alone, something an airplane cannot do. In this special case the fuselage remains level.)
To change altitude, it is necessary to add power to climb or reduce power to descend. This is true of any heavier-than-air aircraft, as, for a constant velocity, increasing altitude implies increasing potential energy and decreasing altitude implies reducing potential energy. In helicopters in forward flight, a climb is accomplished by first pulling back slightly on the cyclic stick to raise the nose and establish a climb attitude, then raising the collective lever and twisting the throttle grip on the collective lever to maintain constant rotor speed. As the collective is raised, the pitch of the main rotor blades increases, giving them more "bite" on the air and, therefore, more lift. A skilled helicopter pilot will perform these actions in a coordinated manner, such that everything appears to happen simultaneously. In the special case of a hover, a helicopter can climb and descend with the rotor disk in a constant horizontal attitude. As described above, the collective lever is raised and power is increased with the throttle to put more energy into the system, raising the aircraft; to descend the collective lever is lowered and power is decreased with the throttle. Directional control in a hover is maintained entirely with the pedals; if the helicopter is the most common type with an anti-torque tail rotor, moving the rudder pedals requires the throttle to be increased or decreased slightly to compensate for the varying power consumed by the tail rotor and to maintain constant altitude.
(If this sounds complicated to you, you're right. Rotary wing flight is considerably more difficult than fixed wing flight, and it took decades longer to perfect helicopters after fixed wing aircraft had taken to the skies.)
A helicopter has 3 separate flight regimes: normal flight, autorotation, and hovering.
In normal flight and in an autorotation, a helicopter pilot initiates a turn by applying left or right pressure on the cyclic, the control stick between the pilots legs. Through a series of control rods or actuators, the cyclic causes the swash plate to change the pitch of the blades of the rotor, depending on the position of the blade in its cycle.
To turn the heli, more lift is needed on the outside of the turn than on the inside. Taking into consideration gyroscopic precession, the swashplate adjusts the blades so that the blade pitch is highest on the outside of the turn and lowest on the inside of the turn, which causes the rotor disc as a system to tilt to initiate the turn. In a semi-rigid rotor system, the entire disc actually tilts on the teetering hinge
In a fully-articulating rotor system (via the flapping hinge) or on a rigid rotor system (flexible blades), only the individual blades move, although they all move in concert to generate the effect of the entire disk tilting.
Once the disk starts to tilt horizontally in the direction of the cyclic, the fuselage will follow it along, and the heli rolls into the turn. Once the desired bank angle is established, the cyclic can be neutralized, and the increased airflow over the rotary wing on the outside of the turn will maintain the aircraft in the turn.
Since all the controls on a helicopter are strongly-coupled, the pilot will need to adjust the pitch, yaw, and power in order to control the heli's altitude, fuselage heading, and airspeed, and to correct for ambient conditions such as wind and turbulence.
In hovering flight, the helicopter pilot turns the helicopter by use of the anti-torque pedals, which control the yaw axis of the aircraft. And indeed, one can "turn" a helicopter during a flight without initiating it with horizontal movement of the cyclic. To do this, slow the aircraft to a hover, yaw the aircraft to the desired heading, and re-initiate forward motion. Don't try this in your airplane!
There are three controls in a helicopter:
This picture shows the result of moving the cyclic. As you can see, the whole rotor is turned, causing the helicopter to move in a certain direction, as shown in this image by AVstop
The collective determines the amount force generated by the main rotor
The anti-torque rotor is used to rotate the helicopter, as shown here, obtained from Gunschip Academy:
The main rotor will induce a torque on the cabin, making it spin. A force is needed to prevent this spinning. The tail-rotor provides this force, keeping the helicopter-cabin straight. However, if we want the cabin to rotate, we can adjust the blade pitch of the tail-rotor. This will change the force delivered by the tail-rotor. The imbalance in moments will then rotate the cabin.
It should be noted here, that all the movements are strongly coupled. Any input will always have additional side-effects, which need to be counteracted.
Think of a triangle of vectors.
When the rotor disk is exactly level, the thrust from the rotor acts vertically. If you tilt the rotor, you now have a slightly reduced vertical thrust (the thrust vector) and a horizontal component which will apply a force to the helicopter in the direction of that horizontal component.
As ROIMaison states, the cyclic control tilts the rotor disc as required.
Please excuse my lousy drawing skills.
Because the vertical component is now reduced (the total thrust is the sum of the vertical and horizontal component), the helicopter will descend slightly since the vertical thrust no longer balances the weight. Therefore, a small increase in power is required achieved by increasing collective pitch. The engine then creates more torque which must be balanced with opposite torque from the tail rotor, achieved by moving the opposite pedal forwards.
As in all aircraft changes, there is a primary effect (tilting the disc in the direction of travel) and secondary affects of increased power and increased torque. This is why flying a helicopter needs adjustment of all three controls together.
To climb, you adopt a climbing attitude, by pulling back on the cyclic, which causes the nose to rise. Because the horizontal component of the total rotor thrust is now reduced (since the disc is now titled forward less), your speed will reduce unless you increase power by increasing the collective pitch which increase the horizontal component. This will need an input on the pedals to balance the correct torque.
To descend, you reduce power, which reduces the total rotor thrust. Since the vertical component no longer opposes the weight, the helicopter will descend. Since power is reduced, the horizontal component of the thrust is reduced and the helicopter will slow down. You push forward on the cyclic to increase the horizontal thrust and maintain speed. Of course, a pedal input is required to counter the reduced torque from the engine.
In order to effect a turn a lateral acceleration (horizontally to the left or right relative to the direction of the vehicles motion) is needed. Consider a bicycle leaning towards the center of a turn. In an aircraft primarily two things happen.
The ailerons are used to bank the aircraft towards the center of the turn in order to tilt the lift vector.
Particularly in slow aircraft the rudder is used to adapt the rotation of the aircraft around the vertical axis to the changing direction. This ensures that no sideslip occurs, i.e. that the aircraft always points exactly in the direction of flight (in reference to the air).
Once a helicopter has significant forward speed the same principles apply. Via the stick the cyclic rotor control is used to bank the helicopter (slightly) towards the turn center. The torque pedals are then used to coordinate the turn (sync the change in direction with the turn speed of the helicopter).
In order to change the altitude the flight path needs to be increased or decreased. Also two things need to happen.
The flight path needs to be changed via a short vertical acceleration (to bend it).
Then the aircraft needs to be stabilized / trimmed to maintain this path.
For fixed wing aircraft there are actually different techniques to do this. The most intuitive is to change the pitch of the aircraft via the elevator. Upon descending or climbing the thrust (engine setting) needs to be adapted. (A glider will adapt its speed accordingly). Another technique with classical fixed wing aircraft is to reduce (sink) or increase (climb) the engine power / thrust level.
Helicopters make use of their direct control of the lift via the collective control. (Subsequently this results in a change of rotor torque which needs to be compensated via the pedals and the engine settings.)
I so far experienced,the co-axial helicopter turns in forward flight or in hovering 360 degree turn,both can be done by reducing pitch angle of the opposite rotor(Clock wise or Anti-clockwise rotor) of turning direction which will be acting as torque effect as like a single rotor helicopter without tail rotor.
- Can someone clarify the relationship between aircraft speed, torque, prop length and pitch and how this applies to a helicopter.
Mentioned all terms are inter-related Speed- depends upon the torque(Engine Power)-Length of the blade/prop-the -lifting capacity of the helicopter depends upon on the blade area/disc loading-subsequently-Torque-pitch-to increase lift and thrust also depends on the engine power(Torque)-and all these are applied by the cyclic, collective and correlator governor and co-ordinated by mixer control unit. to make some one understand,it might take a long lecture which might take several hours.
- In addition I want to know if the amount of lift a helicopter generates changes dependant on pitch, how much does the pitch change?
To determine the actual changes of pitch angle for a certain amount of lift is a mathematical calculation,if you are to find out this in definite figure,you are to know the helicopter aerodynamics with mathematical terms.In a nut shell,you can say,the more lift is required,the more should be the pitch angle and the more should be the engine power. Thanks