Mast Bumping
- Mast bumping occurs when the rotor exceeds its critical flapping angle and the underside of the rotor hub contacts (bumps) the rotor mast. If contact is severe, mast deformation can occur and cause mast structural failure.
- Excessive rotor flapping can also cause rotor blade contact with the tailboom or cockpit. Mast bumping generally occurs at, but is not restricted to, the extremes of the operating envelope.
The most influential causes are (in order of importance):
- Low g maneuvers (below +0.5g).
- Rapid large cyclic motion (especially forward cyclic).
- Flight near longitudinal/lateral cg limits.
- High−slope landings.
Less significant causes are maximum sideward/rearward flight, sideslip, and blade stall conditions.
Warning: Should mast bumping occur in flight, catastrophic results are highly probable. Since conditions causing rotor flapping are cumulative, improper pilot response/recovery techniques to flight situations approaching or favorable to mast bumping can aggravate the situation and lead to in−flight mast bumping and mast separation.
Vortex Ring State
- Vortex ring state is an uncommanded rate of descent caused by the helicopter settling into its own downwash. In this state, the flow through the rotor system is upward near the center of the rotor disk and downward in the outer portion.
This results in zero net thrust from the rotor and extremely high aircraft descent rates. - Vortex ring state is not restricted to high gross weights or high density altitudes. It may not be recognized and a recovery effected until considerable altitude has been lost. Helicopter rotor theory indicates that it is most likely to occur when descent rates exceed 800 feet per minute during vertical descents initiated from a hover and steep approaches at less than 40 KIAS.
Warning: Increasing collective has no effect toward recovery and will aggravate vortex ring state. During approaches at less than 40 KIAS, do not exceed 800 feet per minute descent rate.
- Indications: Rapid descent rate increase, Increase in overall vibration level, Loss of control effectiveness.
1. Collective — Decrease.
2. Cyclic — Forward to gain airspeed.
Warning: Increasing collective has no effect toward recovery and will aggravate vortex ring state. During approaches at less than 40 KIAS, do not exceed
800 feet per minute descent rate.
If impact is imminent:
- Level skids to conform to terrain.
Power Required Exceeds Power Available
When power required for a maneuver exceeds power available under the ambient conditions, an uncommanded rate of descent will result. Factors that can cause or aggravate this situation are:
- High g loading
- High gross weight.
- High density altitude.
- Rapid maneuvering
- Spool−up time from lower power settings to high power settings
- Loss of wind effect
- Change of wind direction
- Loss of ground effect
Power required exceeding power available becomes dangerous to the crew and helicopter when operating in close proximity to obstructions where the pilot may not have enough altitude/maneuvering space to recover prior to impacting an obstacle. This condition will be aggravated by rotor droop and loss of tail rotor effectiveness associated with excessive power demands. Pilots can avoid power required exceeding power available by:
- Preflight planning to calculate expected aircraft performance.
- Avoiding excessive maneuvering, particularly during high/hot and/or high gross weight/marginal power
available situations. - Avoiding high descent rates at low altitudes, which will require large power inputs to arrest the descent of the helicopter.
- Avoiding downwind landings and takeoffs.
- Maintaining awareness of windspeed and direction, especially during low altitude/low airspeed maneuvers.
- Maintaining awareness of the factors leading to power required exceeding power available and the associated effects on aircraft and performance.
Dynamic Rollover Characteristics
- Dynamic rollover is a phenomenon peculiar to helicopters and primarily to skid−configured/rigid gear helicopters.
It is an accelerated roll about a ground−attached point (i.e., landing gear or skid). This roll requires ground contact and occurs extremely rapidly in proportion to both roll rate and angle, allowing little opportunity for recovery. - During normal takeoffs and landings, slope takeoffs and landings, or landings and takeoffs with some bank angle or side drift, the bank angle or side drift can cause the helicopter to get into a situation where it is pivoting about a skid. When this happens, lateral cyclic control response is more sluggish and less effective than for the free hovering helicopter. Consequently, if the bank angle (the angle between the aircraft and the horizon) is allowed to build up past 15°, the helicopter will enter a rolling maneuver that cannot be corrected with full cyclic and the helicopter will roll over on its side. In addition, as the roll rate and acceleration of the rolling motion increase, the angle at which recovery
is still possible is significantly reduced. The critical rollover angle is also reduced for a right skid−down condition, crosswinds, lateral center−of−gravity offset, and left pedal inputs. - When performing maneuvers with one skid on the ground, care must be taken to keep the aircraft trimmed, especially
laterally. For example, if a slow takeoff is attempted and the tail rotor thrust contribution to rolling moment is not
trimmed out with cyclic, the critical recovery angle may be exceeded in less than 2 seconds. Control can be maintained if the pilot maintains trim, does not allow aircraft rates to become large, and keeps the bank angle from getting too large. The pilot must fly the aircraft into the air smoothly, keeping executions in pitch, roll, and yaw low and not allowing any untrimmed moments. - Collective is much more effective in controlling the rolling motion than lateral cyclic because it reduces the main rotor thrust. A smooth, moderate collective reduction of less than approximately 40 percent (at a rate less than approximately full up to full down in 2 seconds) is adequate to stop the rolling motion with approximately 2° bank angle overshoot from where down collective is applied. Care must be taken not to dump collective at too high a rate as to cause fuselage−rotor blade contact. Additionally, if the helicopter is on a slope and the roll starts to the upslope side, reducing collective too fast creates a high rate in the opposite direction. When the low slope skid hits the ground, the dynamics of the motion can cause the aircraft to roll downslope and over on its side. Do not pull collective
suddenly to get airborne, as a large and abrupt rolling moment in the opposite direction will result. This moment may be uncontrollable.
Dynamic Rollover
- Static rollover angle is approximately 31° - at that point, the helicopter will roll over, regardless of control inputs
- After the lateral control contacts the stop, roll angle can still be controlled with the collective. Down collective will level the helicopter and up collective will cause the helicopter to roll over immediately
- With full lateral control deflection - will encounter mast bumping when the roll rate reaches 10° per second. the TPP will be gyroscopically tilted forward or aft by the bumping forces - tilting blades into the ground
Warning: With one skid on the ground and thrust approximately equal to the weight, if the lateral control becomes sluggish or ineffectual, contacts the lateral stop, or if bank angle or roll rates become excessive (15° or 10° per second respectively), the aircraft may roll over on its side. Reduce collective to stopthe roll and correct the bank angle to level.
When landing or taking off, with thrust approximately equal to the weight and one skid on the ground, keep the aircraft trimmed and do not allow aircraft roll rates to build up. Fly the aircraft smoothly off (or onto) the
ground, carefully maintaining trim.
Slope Landings and Takeoffs
(Dynamic Rollover)
- Slope landings should be made cross−slope by descending slowly, placing the upslope skid on the ground first.
- If the lateral cyclic contacts the stop or if
rotor−to−ground clearance becomes marginal before the downslope skid is resting firmly on the ground, the slope
is too great and a landing should not be made. - Slope landings or takeoffs should not be attempted on slopes greater than the lateral control capability of the helicopter because the tip−path plane cannot be kept level. This angle is 7.5° for the TH−57.
Rotor Blade Stall
- Blade stall occurs when the angle of attack of a significant segment of the retreating blade exceeds the stall angle. When this condition occurs, increased blade pitch (or collective) will not result in increased lift and may result in reduced lift and increased rotor drag.
- One of the more important features of the TH−57 two−bladed, semirigid system is its warning to the pilot of impending blade stall. Prior to progressing fully into the stall region, the pilot will feel marked increase in airframe vibration and control vibrations. Consequently, corrective action can be taken before stall becomes severe.
The threshold of stall varies with the following:
- Airspeed.
- Gross weight.
- Density altitude.
- G loading.
- Rpm.
* Indications: Progressively increasing two−per−revolution vibrations, Loss of longitudinal control and severe feedback in the cyclic, Violent vertical nose oscillations independent of cyclic position
Recovery may be accomplished by one or a combination of the following:
- Severity of maneuver — Decrease.
- Collective pitch — Decrease.
- Airspeed — Decrease.
- Altitude — Descend, if flight permits.
- Rotor RPM — Increase.
Caution: Entry into severe blade stall can result in structural damage to the helicopter.
Loss of Tail Rotor Effectiveness
(Unanticipated Right Yaw)
The aircraft characteristics and relative wind azimuth regions that must be present for LTE are:
- Weathercock stability (120° to 240°).
- Tail rotor vortex ring state (210° to 330°).
- Main rotor vortex disk interference (285° to 315°).
- Loss of translational lift (all azimuths).
* The aircraft can be operated safely in the above relative wind regions if proper attention is given to controlling the aircraft; however, if the pilot is inattentive for some reason and a right yaw is initiated in one of the above relative wind regions, the yaw rate may increase unless suitable corrective action is taken.
Rotor Droop
- Droop is a term used to denote a change in power turbine speed (Nf) and rotor speed that occurs with a demand for increased power with the governor at a constant speed setting. Droop may be further categorized as either transient or steady state.
- Transient droop is the momentary change in power turbine speed and rotor speed resulting from an increased power demand, and it is compensated for by the power turbine governor control.
- Steady−state droop is the decrease in power turbine speed and rotor speed that results from an increased power demand when the engine is already operating at maximum gas producer speed. This condition should be avoided during normal operation.
Vibration Identification
- Low−frequency vibrations, one per revolution and two per revolution, are originated by the main rotor. One−per−revolution vibrations are of two basic types: vertical or lateral. A vertical one−per−revolution vibration is caused by one blade developing more lift than the other blade at the same point or, simply, an out−of−track condition. A lateral one−per−revolution vibration is caused by an unbalance of the main rotor because of either a difference or weight between the blades (spanwise unbalance) or the misalignment of the blades (chordwise unbalance).
- Medium−frequency vibrations at frequencies of four per revolution and six per revolution are other inherent
vibrations associated with the main rotor system. An increase in the level of these vibrations is caused by a change
in the capability of the fuselage to absorb vibrations or a loose airframe component vibrating sympathetically at that frequency. - High−frequency vibrations can be caused by anything in the aircraft that rotates or vibrates at a speed equal to or greater than that of the tail rotor.
They are much too fast to count and feel like a buzz. These frequencies may come from the engine, improper drive shaft alignment, couplings improperly functioning, bearings dry or excessively worn, or an out−of−track tail rotor. If excessive high−frequency vibrations exist, it is recommended that the aircraft land and a crewmember attempt to locate the source. The area where the highest amplitude of the vibration exists is generally the area from which the vibration is originating.
