Retreating blade stall
A tendency for the retreating blade to stall in forward flight is inherent in all present-day helicopters, and is a major factor in limiting their forward airspeed. Basically, the stall of the wing limits the low airspeed
capabilities of the airplane. The stall of a rotor blade limits the high airspeed potential of a helicopter. The airflow over the retreating blade of the helicopter slows down as forward airspeed of the helicopter increases; the airflow over the advancing blade speeds up as forward airspeed increases. The retreating blade must, however, produce the same amount of lift as the advancing blade. Therefore, as the airflow over the retreating blade decreases with forward airspeed, the blade angle of attack must be increased to help equalize lift
throughout the rotor disc area. As this increase in angle of attack is continued, the retreating blade will stall at some high forward airspeed. The advancing blade has relatively low angles of attack and is not subject to blade stall. Blade stall occurs during powered flight at the tip of the retreating blade, spreading inboard as forward airspeed increases. Retreating blade stall does not occur in normal autorotations.
When operating at high forward airspeeds, stalls are more likely to occur under conditions of: 1 - High gross weight.
2 - Low RPM.
3 - High density altitude. 4 - Steep or abrupt turns. 5 - Turbulent air.
The major warnings of approaching retreating blade stall conditions in the order in which they will generally be experienced are:
1 - Abnormal 2 per revolution vibration in two-bladed rotors or 3 per revolution vibration in three-bladed rotors.
2 - Pitchup of the nose.
3 - Tendency for the helicopter to roll.
At the onset of blade stall vibration, the pilot should take the following corrective measures: 1 - Reduce collective pitch.
2 - Increase rotor RPM. 3 - Reduce forward airspeed. 4 - Minimize maneuvering.
When operating under flight conditions likely to produce blade stall, a helicopter may quickly advance into severe blade stall by a steep turn, pullup, or other abrupt maneuver. The stall reaction will be rapid and violent. The vibrations, pitchup, and roll tendencies of the helicopter will present a serious threat to helicopter control and structural limitations. When flight conditions are such that blade stall is likely, caution should be exercised when maneuvering.
As the altitude increases, the never-exceed airspeed (red line) for most helicopters decreases. Figure 65 shows a chart from the helicopter flight manual for one model from which the Vne (never-exceed) speed can be
MPH; and at 6,000 feet and 2700-2900 RPM, it is 78 MPH. This chart immediately points up the effect that rotor RPM has on the airspeed at which retreating blade stall is experienced.
Figure 65 - Chart showing never-exceed (Vne) speed limits.
Settling with power
This condition of flight is sometimes described as settling in your own downwash. It involves high vertical rates of descent, and the addition of more power produces an even greater rate of descent. The helicopter is
descending in turbulent air that has just been accelerated downward by the rotor. Reaction of this air on rotor blades at high angles of attack stalls the blades at the hub (center of the rotor) and the stall progresses outward along the blade as the rate of descent increases. The following combination of conditions are likely to cause settling with power:
1 - A vertical or nearly vertical descent of at least 300 feet per minute. Actual critical rate depends on the gross weight, RPM, density altitude, and other pertinent factors.
2 - The rotor system must be using some of the available engine power (from 20 to 100 percent). 3 - The horizontal velocity must be no greater than approximately 10 miles per hour.
A pilot may experience settling with power accidentally. Situations that are conducive to a settling with power condition are:
1 - Attempting to hover out of ground effect at altitudes above the hovering ceiling of the helicopter; 2 - Attempting to hover out of ground effect without maintaining precise altitude control; or
3 - A steep power approach in which airspeed is permitted to drop nearly to zero.
In recovering from a settling with power condition, the tendency on the part of the pilot to first try to stop the descent by increasing collective pitch will result in increasing the stalled area of the rotor and increasing the rate of descent. Since inboard portions of the blades are stalled, cyclic control will be reduced. Recovery can be accomplished by increasing forward speed, and/or partially lowering collective pitch.
Ground resonance
Ground resonance may develop when a series of shocks cause the rotor head to become unbalanced. This condition, if allowed to progress, can be extremely dangerous and usually results in structural failure. In general, if ground resonance occurs, it will occur only in helicopters possessing three-bladed, fully articulated rotor systems and landing wheels. The rotor blades in a three-bladed helicopter are equally spaced around the rotor hub (120° apart), but are constructed to allow some horizontal movement. This horizontal movement is called lead and lag (drag), and the vertical hinge that makes this possible is the drag hinge.
As the name implies, ground resonance occurs when the helicopter makes contact with the surface during landing or while in contact with the surface during an attempted takeoff. When one landing gear of the helicopter strike's the surface first, a shock is transmitted through the fuselage to the rotor. This shock may cause the blades straddling the contact point to be forced closer together. The spacing might then be 122°, 122°, and 116°. When one of the other landing gears strikes, the unbalance could be aggravated and become even greater. This establishes a resonance which sets up a pendulum like oscillation of the fuselage - a severe wobbling or shaking similar to the oscillations of a silver dollar, or similar object, when dropped striking the floor at an angle. Unless immediate corrective action is taken, the oscillations will increase rapidly and destruction of the helicopter will result. Corrective action could be an immediate takeoff if RPM is in proper range, or an immediate closing of the throttle and placing the blades in low pitch if RPM is low.