Leroy R. Grumman Cadet Squadron, NER-NY-153

Civil Air Patrol - The official auxiliary of the United States Air Force

Rotary-Wing Aircraft (Helicopters) - Types

Typical single rotor - tail rotor design.

 

The twin-rotor (or tandem or side-by-side) configuration is used mainly with large helicopters. Because of the opposite rotation of the rotors, the torque of each single rotor is neutralized, and no tail rotor is required.

 

The twin-rotor synchropter is a system with two rotors that mesh into each other, much like a gearwheel. Like the tandem rotor, this configuration doesn't need a tail rotor because the torque is compensated for by the opposite rotation of the rotors.

 

The coaxial rotor configuration has one rotor is located on top of the other. The two rotors turn in opposite directions. Depending on which rotor produces more lift, the helicopter will turn to the left or right, because of the torque.  Again, no tail rotor is required.

Rotary-Wing Aircraft (Helicopters) - Controls

A helicopter pilot manipulates the helicopter flight controls in order to achieve controlled aerodynamic flight. The changes made to the flight controls are transmitted mechanically to the rotor, producing aerodynamic effects on the helicopter's rotor blades which allow the helicopter to be controlled. For tilting forward and back (pitch), or tilting sideways (roll), the angle of attack of the main rotor blades is altered cyclically during rotation, creating differing amounts of lift at different points in the cycle. For increasing or decreasing overall lift, the angle of attack for all blades is collectively altered by equal amounts at the same time resulting in ascents, descents, acceleration and deceleration.

 

A typical helicopter has three separate flight control inputs. These are the cyclic stick, the collective lever, and the anti-torque pedals. Depending on the complexity of the helicopter, the cyclic and collective may be linked together by a mixing unit, a mechanical or hydraulic device that combines the inputs from both and then sends along the "mixed" input to the control surfaces to achieve the desired result. The manual throttle may also be considered a flight control because it is needed to maintain rotor RPM on smaller helicopters without governors.

 

 


 

Control Heading

Almost all single-rotor helicopters produced use a tail rotor for the purpose of compensating the reaction torque and providing directional control. The tail rotor system is cumbersome, adding weight and complexity with its system of shafts and gearboxes to provide drive power and vary the pitch of the blades for directional control. Other rotor problems include its susceptibility to damage, the hazard it poses to bystanders, its power-saw whine, and the fact that it tends to lose much of its effectiveness in a tailwind. In addition, the tail consumes 5 to 10 percent of the overall engine power.

 

To get rid of the tail rotor, the NOTAR (No Tail Rotor) system was developed. The operating principle of the NOTAR system involves ejecting a jet of air from the tail boom to counteract the rotor’s reaction couple and provide directional control, just like a tail rotor does. The system has proved very successful since it solves the problems of mechanical vulnerability, danger to bystanders, and noise.


 

Collective Control


The collective pitch lever or stick is located by the left side of the pilot's seat and is operated with the left hand. The collective is used to increase main rotor pitch at all points of the rotor blade rotation. It increases or decreases total rotor thrust. The collective lever is connected to the swash plate by a series of push-pull controls. Raising the collective lever increases the pitch on the main rotor blade, lowering the collective lever decreases the main rotor blade pitch. The amount of movement of the lever determines the amount of blade pitch change. As the angle of attack increase, drag increases and Rotor RPM and Engine RPM tend to decrease. As the angle of attack decreases, drag decreases and the RPM tend to increase. Since it is essential that the RPM remain constant, there must be some means of making a proportionate change in power to compensate for the change in drag. This coordination of power change with blade pitch angle change is controlled through a collective pitch lever-throttle control cam linkage which automatically increases power when the collective pitch lever is raised and decreases power when the lever is lowered.

 

Cyclic Control


The total lift force is always perpendicular to the tip-path plane of the main rotor. When the tip path plane is tilt away from the horizontal, the lift -thrust force is divide into two components of forces that are, the horizontal acting force, thrust and the upward acting force, lift. The purpose of the cyclic pitch control is to tilt the tip path plane in the direction that horizontal movement is desired. The thrust component of force then pulls the helicopter in the direction of rotor tilt. The cyclic control changes the direction of this force, thus controlling the attitude and air speed of helicopter. The rotor disc tilts in the same direction of the cyclic stick was moved. If the cyclic stick is moved forward, the rotor disc will tilt forward: if the cyclic is moved aft, the rotor disc will tilt aft, and so on. The rotor disc will always tilt in the same direction that the cyclic stick is moved.

 

Schematic - Swash Plates and Control Rods

 

Swash plate in the resting position

 

A raised swash plate causing lower collective blade pitch. Note that the control arms are on the trailing side of the blades, causing the raised swash plate to lower the blade pitch

 

A tilted swash plate giving cyclic blade control. Note the change in pitch of the blades during rotation.

 

 

Rotary-Wing Aircraft (Helicopters) - Additional

In forward movement of the helicopter, the velocity from blade rotation and velocity from overall forward motion are added together on the advancing side of the rotor; on the retreating side they are subtracted from each other. This means that as the rotor turns, one blade is moving significantly faster than the other (in relation to the air around them). If the rotor blades were rigidly fixed to the shaft, the lift would vary cyclically and cause the helicopter to roll.

 


 

 

 

Since the 'advancing' half is moving at a higher speed, more lift is created and the blades on that side will move upward.

 


 

 

 

To address the issues noted in the diagrams shown above, helicopters use an articulated rotor system consisting of three or more rotor blades. The blades are allowed to flap, feather, and lead or lag independently of each other. Each rotor blade is attached to the rotor hub by a horizontal hinge, called the flapping hinge, which permits the blades to flap up and down. Each blade can move up and down independently of the others. The flapping hinge may be located at varying distances from the rotor hub, and there may be more than one.

 

Each rotor blade is also attached to the hub by a vertical hinge, called a drag or lag hinge, that permits each blade, independently of the others, to move back and forth in the plane of the rotor disc. Dampers are normally incorporated in the design of this type of rotor system to prevent excessive motion about the drag hinge. The purpose of the drag hinge and dampers is to absorb the acceleration and deceleration of the rotor blades.

 

 

Drawing shows root attachment of rotor blade to an articulated hub. The flapping hinge permits each blade to rise and fall as it turns, and the vertically mounted drag hinge allows lead-lag motion.

 

The flapping hinge provides the blades with flapping freedom, which permits each blade to rise and fall, as it turns, so the tip rides higher or lower in its circular path. While the hinge may be located very close to the center of the rotor drive shaft, it is more frequently designed to be a short distance from this center-line. This is termed an "offset" flapping hinge, and it offers the designer a number of important advantages. The flapping motion is the result of the constantly changing balance between lift, centrifugal, and inertial forces; this rising and falling of the blades is characteristic of most helicopters and has often been compared to the beating of a bird's wing. One other point should be mentioned; the flapping hinge, in company with the natural flexibility found in most blades, permits the blade to droop considerably when the helicopter is at rest and the rotor is not turning over. During flight the necessary rigidity is provided by the powerful centrifugal force that results from the rotation of the blades; this force pulls outward from the tip, stiffening the blade, and is actually the only factor that keeps it from folding up.

Rotary-Wing Aircraft (Autogyros/Gyroplanes)

Modern "Pusher"-type Gyroplane

Gyroplane is an official term designated by the Federal Aviation Administration (FAA) describing an aircraft that gets lift from a freely turning rotary wing, or rotor blades, and which derives its thrust from an engine-driven propeller. Historically, this type of aircraft has been known as the autogiro, autogyro and the gyrocopter. These early names and their variants were filed as trademarks.  Gyroplanes derive lift from freely turning rotor blades tilted back to catch the air. The rushing air spins the rotor as an engine-driven propeller thrusts the aircraft forward.

 

Cierva C-6 Autogyro

 

 

 

Early gyroplanes were powered by engines in a tractor (pulling) configuration and were relatively heavy. Modern gyroplanes use a pusher propeller and are light and maneuverable. With the engine in the rear ("pusher"-type), the gyroplane has unobstructed visibility.

 

Gyroplane - Helicopter Comparison

 

 

Gyroplane

  • Thrust is produced by an engine-driven propeller
  • The un-powered, freely turning rotor is tilted back as the gyroplane moves forward
  • Oncoming airflow through the rotor causes it to spin, producing lift. This is called autorotation.
  • Always operates in autorotation, thus:
  • Cannot stall like fixed wing aircraft
  • Flies safely at low altitudes and low speeds, but cannot hover
  • No need for heavy main rotor transmission nor a tail rotor

 

Helicopter

  • The powered rotor produces both lift and thrust, and is tilted forward
  • Can hover, but a powered rotor requires:
  • Adequate forward speed and/or altitude to maintain flight in case of power failure
  • A heavy main transmission
  • Tail rotor to counteract the torque imposed on the aircraft

A gyroplane can fly more slowly than airplanes and will not stall. They can fly faster than helicopters but cannot hover. Since the rotor blades on the gyroplane are powered only by the air (autorotation), much like a windmill, there is no need for a tail rotor for anti-torque. The gyroplane is a stable flying platform. This is not so with helicopters, which pull the air down through engine-powered rotor blades making it possible to hover, but also making the aircraft very complicated and expensive to fly. Due to their inherent simplicity, gyroplanes are easier to operate and less expensive to maintain than helicopters.

 

Gyroplanes in flight are always in autorotation. If power fails in a gyroplane the autorotation continues so long as forward motion is maintained, and the aircraft settles softly to the ground from any altitude. The procedure to land after a power failure is nearly the same procedure as a normal landing, which requires no landing roll. Thus the gyroplane is a safer aircraft for low and slow flight, as compared with both helicopters and airplanes. The ability of gyroplanes to fly faster than helicopters and slower than airplanes makes it something of a hybrid, having the good qualities of the other two types of aircraft with little of the bad.

 

The single attraction of helicopters over gyroplanes is their ability to hover, which is necessary in some situations such as rescue or in sling load work. Helicopters at low altitude out of ground effect avoid hovering whenever possible.

"Tractor"-type Gyroplane

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