Fam 14 Discuss Items

=Mechanical versus Virtual Axis= Because the fuselage of the aircraft is suspended beneath the rotor system, it reacts to changes in attitude of the rotor disk like a pendulum. When the tip-path-plane shifts, the total aerodynamic force and virtual axis (the apparent axis of rotation) will shift, but the mechanical axis (the actual axis of rotation) and the center of gravity, which is ideally aligned with the mechanical axis, lag behind. As the center of gravity attempts to align itself with the virtual axis, the mechanical axis (which is rigidly connected to the fuselage) also shifts, and the aircraft accelerates.

In the case of high speed forward flight, the nose of the aircraft would be low due to the tilt of the rotor disk and moment due to fuselage drag. To compensate for this, a negative-cambered horizontal stabilizer is incorporated to provide a downward lifting force on the tail of the aircraft. Therefore, the aircraft fuselage maintains a near level attitude during cruise flight.

This misalignment of the axes is a principal cause of pilot instability during helicopter flight. Because the results of cyclic inputs are not manifested in instantaneous fuselage attitude changes, there is a tendency for pilots to initiate corrections with excessively large inputs. As the fuselage catches up with the tip-path-plane, the pilot realizes the gravity of his error and attempts to correct with an equal and opposite input, creating the same problem in another direction. Called "pilot induced oscillation," this situation can be described as "getting behind the motion." Since this phenomenon is unpredictable and does not always occur, the best advice to a pilot in this situation is: relax for a second and let the aircraft settle down.

=Dissymmetry of lift= To begin this discussion, we need to backtrack all the way to rotor system control. In a no-wind hover, the rotational velocity that each blade sees throughout each revolution is equal. In forward flight, the velocity distribution varies. The advancing side of the rotor disc sees a combination of rotor speed and forward airspeed (movement through the air mass) which is faster than the retreating side, which sees a combination of rotor speed and a "reduced" forward airspeed. For a given pitch setting, and angle of attack, an equal amount of lift will be produced throughout the rotor disc in a no-wind hover, but in forward flight, the advancing side will generate more lift, thus developing a rolling moment. The ingenious method of equalizing this dissymmetry of lift in forward flight is to allow the blades to flap. By connecting the blades to the hub by a method which allows a flexible up-down motion, the advancing blade, which encounters higher lift, begins to flap upward. The retreating blade, which encounters less lift, flaps downward. Flapping equilibrium is found at a point where the rotor system has an angle of attack which compensates for changes in airspeed throughout the rotor disk revolution.

=Blowback= As the aircraft moves forward, the advancing blade "sees" a higher airspeed, and the resultant dissymmetry of lift causes the blades to flap to a maximum 90 degrees later due to phase lag. This extra lift generated over the nose causes the nose to pitch up. Conversely, the nose will tend to pitch down as the aircraft decelerates. The combined effect of dissymmetry of lift and reduced induced velocity defines this transition to a more efficient flight regime, called translational lift. The pitch-up tendency of the aircraft as it accelerates and the pitch-down tendency as the aircraft decelerates are known as rotor blowback (figure 3-20).

Forward cyclic input in proportion to degree of blowback must be used to maintain a constant rate of acceleration. Aft cyclic will be required during deceleration.

As the helicopter transitions to a hover from a decelerating glide slope as in a normal approach, it often experiences an uncommanded nose-up tendency - not nose-down as described above. This is referred to as Pendulum Effect, and it occurs in response to increased collective pitch. Although collective blade pitch is increased proportionally, forward flight dissymmetry of lift is augmented. This overrides the effects of decelerating rotor blowback and causes the nose of the aircraft to pitch up (figure 3-20).

=Geometric Imbalance= Rotor blades also tend to move in the horizontal plane. The reason for this is angular momentum. Physics tells us that angular momentum must be conserved (MVR2=C). This concept is well illustrated by a spinning ice skater who increases his spin rate by pulling his arms toward his body. The same sort of thing occurs while the rotors are turning. As the blade flaps its center of mass moves with respect to the center of rotation. When the blade's center of mass is closer to the center of rotation it will tend to lead (move faster). If the blade's center of mass is farther away, it will tend to lag (move slower). Geometric imbalance occurs when rotor blade centers of mass are not equidistant from the center of rotation.

Rotor blades generally work best as a team, and the three combinations that you are most likely to encounter are the semi-rigid, fully articulated, and rigid rotor systems, all of which allow for flapping and compensate for geometric imbalance. These systems allow for pilot control of the rotor blades through use of the cyclic and collective controls.

The fully articulated rotor system incorporates more than two blades. Lead/lag is possible by use of vertical hinge pins. Horizontal hinge pins allow for flapping. The movement of each blade is independent of the other blades and independent in respect to the rotor head.

The term rigid as applied to rotor systems is generally misleading due to the considerable flexibility in the systems. "Hingeless" may be a better description in most cases. The hub itself bends and twists in order to provide for flapping, lead-lag, and pitch control.

The semi-rigid rotor system uses two rotor blades and incorporates a horizontal hinge pin only for flapping. Pitch change movement is also allowed. We will spend most of our time investigating this system, since it is the type that you will become most intimately familiar with first.

Semi-rigid rotor systems are attractive due to their simplicity. They are limited to two blades, have fewer parts to maintain, and do not use lead-lag hinges. So how does the semi-rigid system compensate for geometric imbalance? Recall that the semi-rigid system uses underslinging. This underslung mounting is designed to align the blade's center of mass with a common flapping hinge so that both blades' centers of mass vary equally in distance from the center of rotation during flapping. The rotational speed of the system will tend to change, but this is restrained by the inertia of the engine and flexibility of the drive system. Only a moderate amount of stiffening at the blade root is necessary to handle this restriction. Simply put, underslinging effectively eliminates geometric imbalance.

=Phase Lag= The advancing blade will encounter its highest rotational speed 90 degrees prior to a position over the nose of the aircraft, but does not experience the highest degree of flapping at this point. In fact, this maximum flapping occurs over the nose, 90 degrees later, due to a principle of a dynamic system in resonance. A system in resonance receives a periodic excitation force sympathetic with the natural frequency of the system. The flapping frequency of a centrally hinged system is equal to the speed of rotation. Therefore, maximum response occurs 90 degrees after maximum periodic excitation. This is termed phase lag. In order for a helicopter in forward flight to roll into a left turn, maximum lift must be realized at the right "wing" position and minimum lift must be realized at the left "wing" position. Therefore, maximum angle of attack must occur at the 180-degree position and minimum angle of attack must occur at the 360-degree position. To obtain the appropriate response 90 degrees after maximum excitation, logic tells us that forward cyclic is the appropriate input to initiate a left turn. No wonder helicopters are such a challenge to fly! Well, they are challenging, but not for this reason. Inputs are translated 90 degrees prior mechanically, thanks to some design engineers who had a little foresight.