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Know-It-All Q&A - Rotorcraft

Q. What does the term “lateral balance” mean?

A. For most helicopters, it’s usually not necessary to determine the lateral CG for normal flight instruction and passenger flights. This is because helicopter cabins are relatively narrow and most optional equipment is located near the centerline. However, some helicopter manuals specify the seat from which you must conduct solo flight. In addition, if there’s an unusual situation, such as a heavy pilot and a full load of fuel on one side of the helicopter, which could affect the lateral CG, its position should be checked against the CG envelope. If carrying external loads in a position that requires large lateral cyclic control displacement to maintain level flight, fore and aft cyclic effectiveness could be dramatically limited.

Q. What’s collective pitch control?

A. The control for changing the pitch of all the rotor blades in the main rotor system equally and simultaneously, and consequently, the amount of lift or thrust being generated.

Q. What do the rudder pedals control on a gyroplane?

A. The rudder is operated by foot pedals in the cockpit and provides a means to control yaw movement of the aircraft.  On a gyroplane, this control is achieved in a manner more similar to the rudder of an airplane than to the antitorque pedals of a helicopter. The rudder is used to maintain coordinated flight, and at times may also require inputs to compensate for propeller torque. Rudder sensitivity and effectiveness are directly proportional to the velocity of airflow over the rudder surface. Consequently, many gyroplane rudders are located in the propeller slipstream and provide excellent control while the engine is developing thrust. This type of rudder configuration, however, is less effective and requires greater deflection when the engine is idled or stopped.

Q. What is a hovering turn?

A. A hovering turn is a maneuver performed at hovering altitude in which the nose of the helicopter is rotated either left or right while maintaining position over a reference point on the surface. The maneuver requires the coordination of all flight controls and demands precise control near the surface. You should maintain a constant altitude, rate of turn, and rpm.

Q. Why don’t helicopters use the standard airport traffic pattern at non-towered airports?

A. Helicopters and powered parachutes are required to avoid the flow of fixed-wing aircraft in accordance with FAR 91.126.

Q. What is a gyroplane prerotator?

A. Prior to takeoff, the gyroplane rotor must first achieve a rotor speed sufficient to create the necessary lift. This is accomplished on very basic gyroplanes by initially spinning the blades by hand. The aircraft is then taxied with the rotor disc tilted aft, allowing airflow through the system to accelerate it to flight rpm. More advanced gyroplanes use a prerotator, which provides a mechanical means to spin the rotor. Most prerotators are capable of only achieving a portion of the speed necessary for flight; the remainder is gained by taxiing or during the takeoff roll. There’s a wide variety of prerotation systems available, each with unique characteristics and techniques associated with their operation.

Q. Both gyroplanes and helicopters are rotorcraft; how are they different?

A. A fundamental difference between helicopters and gyroplanes is that in powered flight, a gyroplane rotor system operates in autorotation. This means the rotor spins freely as a result of air flowing up through the blades, rather than using engine power to turn the blades and draw air from above. Forces are created during autorotation that keep the rotor blades turning, as well as creating lift to keep the aircraft aloft. Aerodynamically, the rotor system of a gyroplane in normal flight operates like a helicopter rotor during an engine-out forward autorotative descent.

Q. Why do some gyroplanes have a horizontal stabilizer?

A. The horizontal tail surfaces on most gyroplanes aren’t controllable by the pilot. These fixed surfaces, or stabilizers, are incorporated into gyroplane designs to increase the pitch stability of the aircraft. Some gyroplanes use very little, if any, horizontal surface. This translates into less stability and a smaller envelope of maneuverability. When used, a moveable horizontal surface, or elevator, adds additional pitch control of the aircraft. On early tractor-configured gyroplanes, the elevator served an additional function of deflecting the propeller slipstream up and through the rotor to assist in prerotation.

Q. What is a Fenestron tail rotor design?

A. The Fenestron or “fan-in-tail” antitorque system uses a series of rotating blades shrouded within a vertical tail. Because the blades are located within a circular duct, they’re less likely to come into contact with people or objects.

Compared to an unprotected tail rotor, the Fenestron antitorque system provides an improved margin of safety during ground operations.

Q: How does thrust act on a gyroplane?

A: Thrust in a gyroplane is defined as the component of total propeller force parallel to the relative wind. As with any force applied to an aircraft, thrust acts around the center of gravity. Based upon where the thrust is applied in relation to the aircraft center of gravity, a relatively small component may be perpendicular to the relative wind and can be considered to be additive to lift or weight.

In flight, the fuselage of a gyroplane essentially acts as a plumb suspended from the rotor, and as such, it’s subject to pendular action in the same way as a helicopter. Unlike a helicopter, however, thrust is applied directly to the airframe of a gyroplane rather than being obtained through the rotor system. As a result, different forces act on a gyroplane in flight than on a helicopter. Engine torque, for example, tends to roll the fuselage in the direction opposite propeller rotation, causing it to be deflected a few degrees out of the vertical plane. This slight “out of vertical” condition is usually negligible and not considered relevant for most flight operations.

Diagram
Engine torque applied to the propeller has an
equal and opposite reaction on the fuselage, deflecting it a
few degrees out of the vertical plane in flight.

Q. What are the pedals on the floor of the helicopter control for?

A. The antitorque pedals, located on the cabin floor by the pilot’s feet, control the pitch and therefore the thrust of the tail rotor blades. The main purpose of the tail rotor is to counteract the torque effect of the main rotor. Since torque varies with changes in power, the tail rotor thrust must also be varied. The pedals are connected to the pitch change mechanism on the tail rotor gearbox and allow the pitch angle on the tail rotor blades to be increased or decreased.

Q. What does autorotation mean?

A. Autorotation is the state of flight where the main rotor system is being turned by the action of relative wind rather than engine power. It’s the means by which a helicopter can be landed safely in the event of an engine failure. In this case, you’re using altitude as potential energy and converting it to kinetic energy during the descent and touchdown. All helicopters must have this capability in order to be certified.

Autorotation is permitted mechanically because of a freewheeling unit, which allows the main rotor to continue turning even if the engine isn’t running. In normal powered flight, air is drawn into the main rotor system from above and exhausted downward. During autorotation, airflow enters the rotor disc from below as the helicopter descends.

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