# Emergencies Generally speaking, aerodynamic emergencies can be avoided by removing at least one of the factors necessary for the emergency to occur. Pre-mission planning will identify aerodynamic profiles to avoid. Operating within appropriate common standards and task-specific standards significantly minimizes the risk. *** ## **Settling with Power** Settling with power (see figures 1-74 through 1-76) is a condition of powered flight in which the helicopter settles in its own downwash. This condition may also be referred to as vortex ring state. Under certain conditions, the helicopter may descend at a high rate which exceeds the normal downward induced flow rate of the inner blade sections (inner section of the rotor disk). Therefore, the airflow of the inner blade sections is upward relative to the disk. This produces a secondary vortex ring in addition to the normal tip vortex system. The secondary vortex ring is generated about the point on the blade where airflow changes from up to down. The result is an unsteady turbulent flow over a large area of the disk which causes loss of rotor efficiency although engine power is still supplied to the rotor system. Figure 1-74 shows normal induced flow velocities along the blade span during hovering flight. Downward velocity is highest at the blade tip where blade speed is highest. As blade speed decreases nearer the center of the disk, downward velocity is less.
Figure 1-75 shows the induced airflow velocity pattern along the blade span during a descent conducive to settling with power. The descent is so rapid, induced flow at the inner portion of the blades is upward rather than downward. The upflow caused by the descent has overcome the downflow produced by blade rotation and pitch angle.
If this rate of descent exists with insufficient power to slow or stop the descent, it enters the vortex ring state (figure 1-76, page 1-60). During this vortex ring state, roughness and loss of control occur due to turbulent rotational flow on the blades and unsteady shifting of the flow along the blade span.
### **Conditions for Settling with Power** The following conditions must exist simultaneously for settling with power to occur: * A vertical or near-vertical descent of at least 300 feet per minute (FPM). The actual critical rate depends on gross weight, rotor RPM, density altitude, and other pertinent factors. * Slow forward airspeed (less than ETL). * The rotor system must be using 20 to 100 percent of the available engine power with insufficient power remaining to arrest the descent. Low rotor RPM could aggravate this. ### **Flight Conditions Conducive to Settling with Power** The following flight conditions are conducive to settling with power: * Steep approach at a high rate of descent. * Downwind approach. * Formation flight approach (where settling with power could be caused by turbulence of preceding aircraft). * Hovering above the maximum hover ceiling. * Not maintaining constant altitude control during an OGE hover. * During masking/unmasking. ### **Recovery from Settling with Power** Recovery from settling with power may be affected by one, or a combination, of the following ways: * During the initial stage (when a large amount of excess power is available), a large application of collective pitch may arrest rapid descent. If done carelessly or too late, collective increase can aggravate the situation resulting in more turbulence and an increased rate of descent. * In single-rotor helicopters, aviators can accomplish recovery by applying cyclic to gain airspeed and arrest upward induced flow of air and/or by lowering the collective (altitude permitting). Normally, gaining airspeed is the preferred method as less altitude is lost. In most helicopters, lateral cyclic thrust combined with an increase in power and lateral antitorque thrust produces the quickest exit from the hazard. * In tandem-rotor helicopters, fore and aft cyclic inputs aggravate the situation. By lowering thrust (altitude permitting) and applying lateral cyclic input or pedal input to arrest this upward induced flow of air, the aviator can accomplish recovery. #### **Conclusions** Several conclusions can be drawn from figure 1-77, page 1-61: * The vortex ring state can be completely avoided by descending on flight paths shallower than about 30 degrees (at any speed). * For steeper approaches, the vortex ring state can be avoided by using rates of descent versus horizontal velocity either faster or slower than those passing through the area of severe turbulence and thrust variation. * At very shallow angles of descent, the vortex ring wake is dispersed behind the helicopter. Forward airspeed coupled with induced-flow velocity prevents the upflow from materializing on the rotor system. * At steep angles, the vortex ring wake is below the helicopter at slow rates of descent and above the helicopter at high rates of descent. Low rates of descent prevent the upflow from exceeding the induced flow velocities. High rates of descent result in autorotation or the windmill brake state.
*** ## Dynamic Rollover A helicopter is susceptible to a lateral-rolling tendency called dynamic rollover. Dynamic rollover can occur on level ground as well as during a slope or crosswind landing and takeoff. Three conditions are required for dynamic rollover—pivot point, rolling motion, and exceed critical angle. ### Pivot Point Dynamic rollover begins when the helicopter starts to pivot around its skid, wheel, or any portion of the aircraft in contact with the ground. When this happens, lateral cyclic control response is more sluggish and less effective than for a free hovering helicopter. This can occur for a variety of reasons including failure to remove a tiedown or skid securing device, the skid or wheel contacts a fixed object while hovering sideward, or the gear is stuck in ice, soft asphalt, or mud. Dynamic rollover may also occur if proper landing or takeoff technique is not used or while performing slope operations. If the gear or skid becomes a pivot point, dynamic rollover is possible if proper corrective techniques are not used. ### Rolling Motion The rate of rolling motion is vital. As the roll rate increases, the critical angle is reduced. In a fully articulated rotor system, all three control inputs (collective, cyclic, and pedals) can contribute to the rolling motion. ### Exceed Critical Angle To understand critical angle we must first discuss static rollover angle. Each helicopter has a static rollover angle that, if exceeded, causes the aircraft to rollover. The static angle is based on CG and pivot point. This angle is described as being the point where the aircraft CG is located over the pivot point. When a rolling motion is present the dynamic rollover angle is introduced and is called the critical angle. The dynamic angle varies based on the rate of the rolling motion of the helicopter. The greater the rolling motion the earlier (less bank angle) the critical angle is exceeded. If the dynamic rollover angle is exceeded, momentum carries the helicopter through the static rollover angle, regardless of corrections by the aviator. *** ## Types of Dynamic Rollover Certain factors influence dynamic rollover including right skid down, left pedal inputs (single-rotor aircraft), the effects of pilot input (lateral motion) in a rigid rotor aircraft, lateral loading (asymmetrical loading), crosswind, and high roll rates. Smooth and moderate collective inputs are most effective in preventing dynamic rollover as it reduces the rate at which lift/thrust is applied. A smooth and moderate collective reduction is recommended if the onset of dynamic rollover is encountered. There are three main rollover types normally encountered—rolling over on level ground (takeoff), rolling downslope (takeoff or landing) and rolling upslope (takeoff). ### Rolling Over on Level Ground A rollover condition can occur during takeoff from level ground if one skid or wheel is stuck on the ground. As collective pitch is increased, the stuck skid or wheel becomes the pivot point which sets dynamic rollover into motion. A smooth and moderate collective reduction is recommended lowering the aircraft back to the ground until the stuck skid or wheel is free. Then the aircraft may be picked up normally. ### Rolling Downslope A downslope rollover during landing (figure 1-78) occurs when the steepness of the slope causes the helicopter to tilt beyond the lateral cyclic control limits. If the steepness of the slope, a crosswind component, or CG condition exceeds lateral cyclic control limits, the mast forces the rotor to tilt downslope. The resultant rotor vector has a downslope component even with full upslope cyclic applied. To prevent downslope rollover during landing, the aviator slowly descends vertically until ground contact with the upslope skid/wheel occurs. At this point, aircrew members can better assess slope conditions. After stabilizing the helicopter in this position, the aviator smoothly reduces collective until the downslope skid/wheel contacts the ground or cyclic nears lateral limits. If the cyclic is near the lateral limit, the aviator must carefully evaluate remaining distance to ensure enough cyclic travel remains to land without exceeding aircraft limits. If not enough travel remain the aviator should abort the landing, return the aircraft to a hover, and select an area of lesser slope. A downslope rollover during takeoff (figure 1-78) can occur when the aviator lands the helicopter on too steep a slope, then attempts takeoff. If the upslope skid/wheel begins to rise first, the aviator should lower the collective to prevent a downslope rollover condition. If, with full cyclic applied, the resultant lift of the main rotor is not vertical or directed upslope enough to raise the downslope gear first, and then further takeoff attempts result in the mast causing resultant rotor lift to move further downslope and cause dynamic rollover. The aviator should consider some adjustments before making additional takeoff attempts. These adjustments include awaiting different wind conditions, changing the CG of the helicopter by moving/removing some of the internal load, or contacting a recovery crew.
### Rolling Upslope An upslope rollover during takeoff (figure 1-79) occurs when the aviator applies too much cyclic into the slope to hold the skid/wheel firmly on the slope. If the aviator improperly applies collective, the helicopter then rapidly pivots upslope around the upslope skid/wheel. To prevent this, the aviator needs to cautiously apply collective while neutralizing the cyclic. When the cyclic is neutral and upslope skid/wheel has no side pressure applied, the aviator performs a vertical lift-off to a hover, then a normal takeoff.
*** ## Prevention of Dynamic Rollover **Human Factors in Preventing Dynamic Rollover** Dynamic rollover incidents often result from a combination of physical and human factors. While physical factors like main rotor thrust, center of gravity (CG), tail-rotor thrust, crosswind component, and ground surface characteristics play a significant role, human factors can also contribute to dynamic rollover incidents. Here are some human factors considered in preventing dynamic rollover: 1. **Inattention:** Dynamic rollover is more likely to occur if the pilot fails to pay attention to the aircraft's position and attitude during takeoff or landing, leading to a loss of situational awareness. 2. **Inexperience:** Most dynamic rollover accidents happen when inexperienced pilots are at the controls. Therefore, pilots in command must remain vigilant, especially during critical phases of flight. 3. **Failure to Take Timely Corrective Action:** Timely corrective action is crucial before a roll rate develops. Delayed or inadequate response to dynamic rollover cues can lead to a loss of control. 4. **Inappropriate Control Input:** Applying incorrect or inappropriate control inputs is a common cause of dynamic rollovers. Smooth and precise control inputs are essential for avoiding dynamic rollover situations. 5. **Loss of Visual Reference:** Loss of visual reference can result in the aircraft drifting unnoticed by the crew, particularly during critical phases of flight like landing. If visual reference is lost, pilots should rely on instrument techniques to maintain control. **Common Errors** In addition to the human factors mentioned above, several common errors contribute to dynamic rollover incidents: * Failure to detect lateral motion across the ground before landing. * Abrupt cyclic displacements, especially in fully articulated rotor systems, can exacerbate dynamic rollover tendencies. * Large and/or uncoordinated antitorque pedal inputs can destabilize the aircraft during critical phases of flight. * Performing slope landing or takeoff maneuvers while using rapidly increasing or decreasing collective control applications can increase the risk of dynamic rollover. Awareness of these human factors and common errors is essential for pilots to effectively prevent dynamic rollover incidents during helicopter operations. Vigilance, proper training, and adherence to standard operating procedures can significantly mitigate the risk of dynamic rollover. *** ## Retreating Blade Stall (RTS) The retreating blade of a helicopter eventually stalls in forward flight (figures 1-80 through 1-82). As the stall of an airplane wing limits the low speed of a FW aircraft, the stall of a rotor blade limits the high speed of a rotary-wing aircraft. In forward flight, decreasing velocity of airflow on the retreating blade demands a higher AOA to generate the same lift as the advancing blade. Figure 1-80 illustrates the lift pattern at a normal hover with distribution/production of lift evenly spread throughout the rotor disk.
Figure 1-81 illustrates the normal cruise lift pattern where the smaller area of the retreating blade, with its high angles of attack, must still produce an amount of lift equal to the larger area of the advancing blade with its lower angles of attack. This figure shows the advancing blade producing lift throughout its span while the retreating blade is producing lift in only part of its span due to effects of forward airspeed. When forward speed increases, the no-lift areas of the retreating blade grow larger, placing an even greater demand for production of lift on a progressively smaller section of the retreating blade. This smaller section of blade demands a higher AOA until the tip of the blade (area of the highest AOA) stalls.
Figure 1-82, illustrates the same disk at a critical airspeed with the retreating blade producing less than sufficient lift due to the no-lift area growing larger and effects of tip stall. Tip stall causes vibration and buffeting which spread inboard and aggravate the situation while the aircraft may roll left and nose pitches up. While this may be subtle, it worsens if aft cyclic is not applied or collective is reduced (altitude permitting). The effects of retreating blade stall in a tandem-rotor helicopter create a different response. With the forward and aft rotor systems turning in opposite directions, effects of retreating blade stall on the separate rotors tend to counteract themselves. The pitch-up of the nose will be insignificant. Blade stall probably occurs on the aft system first as it operates in the turbulent wake of the forward rotor system. The most likely effect is an increasing vibration which is easily reduced by slowing down and reducing collective pitch (thrust).
### Conditions Producing Blade Stall In operations at high forward speeds, several conditions are most likely to produce blade stall in either single- or tandem-rotor helicopters: * High blade loading (high gross weight). * Low rotor RPM. * High-density altitude. * High G-maneuvers. * Turbulent air. * Recovering from blade stall. The following steps enable the aviator to recover from retreating blade stall: * Reduce collective. * Reduce airspeed. * Descend to a lower altitude (if possible). * Increase rotor RPM to normal limits. * Reduce severity of the maneuver. *** ## **Ground Resonance** Ground resonance may develop in helicopters having fully-articulated rotor systems when a series of shocks cause the rotor blades in the system to become positioned in unbalanced displacement. If this oscillating condition progresses, it can be self-energizing and extremely dangerous, potentially causing structural failure. Ground resonance is most common in three-blade helicopters with landing wheels. The rotor blades in a three-blade system are equally spaced (120 degrees), but are constructed to allow some horizontal lead and lag action. Ground resonance occurs when the helicopter contacts the ground during landing or takeoff (figure 1-83). If one wheel of the helicopter strikes the ground ahead of the others, a shock is transmitted through the fuselage to the rotor. Another shock is transmitted when the next wheel hits. The first shock causes the blades straddling the contact point to jolt out of angular balance. If repeated by the next contact, resonance is established setting up a self-energizing oscillation of the fuselage. Severity of the oscillation increases rapidly. The helicopter can quickly disintegrate without immediate corrective action. Corrective action may consist of an immediate takeoff to a hover or a change in rotor RPM to alleviate the condition and disrupt the pattern of oscillation. In the event takeoff is not an option, all personnel should remain in the aircraft until main rotors have stopped. Ground resonance usually occurs when the aircraft is nearly airborne (80 to 90 percent hover power applied).
The following conditions can cause ground resonance: * Defective drag dampers allowing excessive lead and lag and creating angular unbalance. * Improperly serviced or defective landing-gear struts. * Hard landings on one skid or wheel. * Ground taxiing over rough terrain. * Hesitant or bouncing landings. *** ## **Compressibility Effects** The effects of compressibility on airfoils can be severe, even to the point of structural failure. Aircraft performance charts will show the regions in which compressibility can occur. Avoiding these flight profiles will avoid blade damage from compressibility effects. ### **Compressible and Incompressible Flow** At low airspeeds, air is incompressible. Incompressible airflow is similar to the flow of water, hydraulic fluid, or any other incompressible fluid. At low speeds, air experiences relatively small changes in pressure with little change in density. However, at high speeds greater pressure changes occur causing compression of air which results in significant changes to air density. This compressible flow occurs when there is a transonic or supersonic flow of air across the airfoil. Because helicopters are being flown at increasingly higher speeds, aviators must learn more about coping with effects of compressible flow. The major factor in high-speed airflow is the speed of sound. Speed of sound is the rate at which small pressure disturbances move through the air. This propagation speed is solely a function of air temperature. Table 1-4, shows the variation of speed of sound with temperature at various altitudes in the standard atmosphere. *Table 1-4. Speed of sound variation with temperature and altitude* | Altitude | Temperature (ºF) | Temperature (ºC) | Speed of Sound (Knots) | | --------- | ---------------- | ---------------- | ---------------------- | | Sea Level | 59.0 | 15.0 | 661.7 | | 5,000 | 41.2 | 5.1 | 650.3 | | 10,000 | 23.3 | -4.8 | 638.6 | | 15,000 | 5.5 | -14.7 | 626.7 | | 20,000 | -12.3 | -24.6 | 614.6 | | 25,000 | -30.2 | -34.5 | 602.2 | | 30,000 | -48.0 | -44.4 | 589.6 | | 35,000 | -65.8 | -54.3 | 576.6 | | 40,000 | -69.7 | -56.5 | 573.8 | | 50,000 | -69.7 | -56.5 | 573.8 | | 60,000 | -69.7 | -56.5 | 573.8 | Compressibility effects are not limited to blade speeds at and above the speed of sound. The aerodynamic shape of an airfoil causes local flow velocities greater than blade speed. Thus a blade can experience compressibility effects at speeds well below the speed of sound because both subsonic and supersonic flows can exist on a blade. Differences between subsonic and supersonic flow are due to compressibility of supersonic flow. Figure 1-84, compares incompressible and compressible flow through a closed tube. In this example, the mass flow along the tube is constant. ### **Subsonic Incompressible Flow** The example of subsonic incompressible flow is simplified because density of flow is constant throughout the tube. As the flow approaches a constriction and streamlines converge, velocity increases as static pressure decreases. A convergence of the tube requires an increasing velocity to accommodate the continuity of flow. Also, as the subsonic incompressible flow enters a diverging section of the tube, velocity decreases and static pressure increases; density remains unchanged. ### **Supersonic Compressible Flow** The example of supersonic compressible flow is complicated because variations of flow density are related to changes in velocity and static pressure. The behavior of supersonic compressible flow is a convergence causing compression; a divergence causes expansion. Therefore, as the supersonic compressible flow approaches a constriction and streamlines converge, velocity decreases and static pressure increases. Continuity of mass flow is maintained by the increase in flow density accompanying the decrease in velocity. As the supersonic compressible flow enters a diverging section of the tube, velocity increases and static pressure decreases; density decreases to accommodate the condition of continuity.
### **Transonic Flow Patterns** In subsonic flight, an airfoil producing lift has local velocities on the surface greater than the free stream velocity. Compressibility effects can then be expected to occur at flight speeds less than the speed of sound. Mixed subsonic and supersonic flow may be encountered in the transonic regime of flight. The first significant effects of compressibility occur in this regime. Compressibility effects on the helicopter increase the power required to maintain rotor RPM and cause rotor roughness, vibration, cyclic shake, and an undesirable structural twisting of the blade. Critical Mach number is the highest blade speed without supersonic airflow. As the critical Mach number is exceeded, an area of supersonic airflow is created. A normal shock wave then forms the boundary between supersonic and subsonic flow on the aft portion of the airfoil surface. The acceleration of airflow from subsonic to supersonic is smooth and without shock waves if the surface is smooth and transition gradual. However, transition of airflow from supersonic to subsonic is always accompanied by a shock wave. When airflow direction does not change, the wave formed is a normal shock wave. The normal shock wave is detached from the leading edge of the airfoil and perpendicular to the upstream flow. The flow immediately behind the wave is subsonic. Figure 1-85 illustrates how an airfoil at high subsonic speeds has local supersonic flow velocities. As the local supersonic flow moves aft, a normal shock wave forms slowing the flow to subsonic. As supersonic air passes through shock wave, air density increases, heat is created, velocity of the air decreases, static pressure increases, and boundary layer separation may occur.
As the shock waves move toward the trailing edge of the airfoil, the aerodynamic center begins to move away from its normal location of 25 percent chord. By the time the shock wave has reached the trailing edge of the airfoil, the aerodynamic center has retreated to the 50 percent chord. This causes the leading edge of the airfoil to be deflected down, which may result in structural failure of the blade (skin deformation or separation). Because speed of the helicopter is added to the speed of rotation of the advancing blade, the highest relative velocities occur at the tip of the advancing blade. When the Mach number of the advancing blade tip section exceeds the critical Mach number for the rotor blade section, compressibility effects result. The critical Mach number is the free stream Mach number producing the first evidence of local sonic flow. The principle effects of compressibility are large increase in drag and rearward shift of the airfoil aerodynamic center *** ## **Adverse Compressibility Conditions** The following operating conditions represent the most adverse compressibility conditions: * High airspeed. * High rotor RPM. * High gross weight. * High-density altitude. * High G-maneuvers. * Low temperature. Speed of sound is proportional to the square root of the absolute temperature; therefore, the aviator more easily obtains sonic velocity at low temperatures. * Turbulent air. Sharp gusts momentarily increase the blade AOA and thus, lower the critical Mach number to the point where compressibility effects may be encountered on the blade. ### **Corrective Actions** Corrective actions are any actions decreasing AOA or velocity of airflow that help the situation. There are similarities in the critical conditions for compressibility and retreating blade stall, with notable exceptions—compressibility occurs at high rotor RPM, and retreating blade stall occurs at low rotor RPM. With the exception of RPM control, the recovery technique is identical for both. Such techniques include decreasing— * Blade pitch by lowering collective, if possible. * Rotor RPM. * Severity of maneuver. * Airspeed --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://229.gitbook.io/229-hub/knowledge/army-aviation-fundamentals/aerodynamics-of-flight/emergencies.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.