Tiltrotor Fam 9 Discuss Items

=Performance Data from NATOPS= ...The information provided in this chapter is primarily intended for mission planning and is most useful when planning operations in unfamiliar areas or at extreme conditions. The data may also be used in flight to establish unit or area standing operating procedures and to inform ground commanders of performance/risk tradeoffs...

...The data presented cover the maximum range of conditions and performance that can reasonably be expected. In each area of performance, the effects of altitude, temperature, gross weight, and other parameters relating to that phase of flight are presented. In addition to the presented data, your judgment and experience will be necessary to accurately obtain performance under a given set of circumstances. The conditions for the data are listed under the title of each chart. The effects of different conditions are discussed in the text accompanying each phase of performance. Where practical, data are presented at conservative conditions. However, no general conservatism has been applied. All performance data presented are within the applicable limits of the aircraft...

...The primary advantage of the helicopter over other aircraft is the capability to hover and take off and land vertically (zero airspeed flight). To more rapidly calculate the performance tradeoffs in hover mode, hover ceiling charts have been included...

Standard Data

 * AIRSPEED CALIBRATION CHART The airspeed calibration chart (Figure 23-1) converts calibrated airspeed to indicated airspeed and vice versa. Calibrated airspeed (KCAS) is indicated airspeed (KIAS) as read from the airspeed indicator corrected for instrument error, plus the installation correction.
 * PRESSURE ALTITUDE Pressure altitude is the altitude indicated on the altimeter when the barometric scale is set on 29.92. It is the height above the standard datum plane at which the air pressure is equal to 29.92 inches of mercury.
 * DENSITY ALTITUDE CHART Density altitude is an expression of the density of the air in terms of height above sea level; hence, the less dense the air, the higher the density altitude. For standard conditions of temperature and pressure, density altitude is the same as pressure altitude. As temperature increases above standard for any altitude, the density altitude will also increase to values higher than pressure altitude. Figure 23-2 expresses density altitude as a function of pressure altitude and temperature...
 * TEMPERATURE CONVERSION CHART The temperature conversion chart (Figure 23-3) provides a means of converting temperature in degrees Celsius to degrees Fahrenheit and vice versa.
 * SHAFT HORSEPOWER VERSUS TORQUE CHART The shaft horsepower versus torque chart (Figure 23-4) provides a means of converting torque to shaft horsepower, and vice versa, for 100-percent rotor rpm.
 * TORQUE AVAILABLE CHARTS Both pressure altitude and FAT affect engine power production. Figures 23-5 and 23-6 show power available data at 5-minute power and maximum continuous power ratings in terms of the allowable torque as recorded by the torquemeter (percent Q). Note that the power output capability of the 250-C20J engine can exceed the transmission structural limit (85-percent Q) under certain conditions. Figure 23-5 is applicable for maximum power five minute operation at 100-percent Nf rpm. Figure 23-6 is applicable for maximum continuous power at 100-percent Nf rpm. Prolonged IGEhover may increase engine inlet temperature as much as 10 _C, therefore, a higher FAT must be used to correct for the increase under this condition.
 * FUEL FLOW CHART The fuel flow for this aircraft is presented in Figure 23-7. Sheet 1, fuel flow versus torque, shows fuel flow in gallons-per-hour versus torquemeter percent (percent Q) for pressure altitude from sea level to 20,000 feet and for 0 _C free air temperature. Sheet 2 gives fuel flow at engine idle and at flat pitch with 100-percent rpm. The primary use of the charts is illustrated by the examples to determine fuel flow. It is necessary to know the condition or torquemeter percent and the FAT as well as the pressure altitude. Fuel flow will increase about 2 percent with the reverse flow inlet (snow baffles) installed. Also a range or endurance penalty of 2 percent should be accounted for when working cruise chart data. A fairly accurate rule of thumb to correct fuel flow for temperatures other than 0 _C FAT is to increase (decrease) fuel flow 1 percent for each 10 _C increase (decrease) in FAT. These charts are based on JP-4 or JP-5 fuel and 100-percent rpm.

Takeoff
The hover charts (Figure 24-1, sheets 1 to 4) show the hover ceiling and the torque required to hover respectively at various pressure altitudes, ambient temperatures, gross weights, and skid heights. Maximum skid height for hover can also be obtained by using the torque available from Figure 23-5. Controllability during downwind hovering, crosswinds, and rearward flight has been demonstrated to be adequate during FAA certification test. The capability of the ship to hover in a 17-knot right crosswind and to fly rearward to 30 knots has been demonstrated. The primary use of the charts is illustrated by the charts examples. In general, to determine the hover ceiling or the torque required to hover, it is necessary to know the pressure altitude, temperature, gross weight, and the desired skid height.
 * HOVERING CHARTS
 * TAKEOFF CHART The takeoff chart (Figure 24-2) shows the distances to clear various obstacle heights, based upon a level acceleration technique. Takeoff distance is shown as a function of several hover height capabilities. The upper chart grid presents data for climbout at a constant 35-knot INDICATED airspeed. The two lower grids present data for climbouts at various TRUE airspeeds.

Climb

 * CLIMB PERFORMANCE CHARTS The climb performance charts (Figure 25-1) represent a synthesis of the cruise charts to ease estimation of the climb portion of the flight plan. The charts show relationships between gross weight, initial and final altitude and temperatures, and time to climb, distance covered while climbing, and fuel expended while climbing. The chart (Figure 25-1, sheet 1 of 2) is presented for climbing at maximum torque (5-minute operation) and the second chart (Figure 25-1, sheet 2 of 2) is presented for (continuous operation) climbing. Both charts may be used for all drag configurations.
 * CLIMB-DESCENT CHART The upper grid of the climb-descent chart (Figure 25-2) shows the change in torque (above or below torque required for level flight under the same gross weight and atmospheric conditions) to obtain a given rate of climb or descent. The lower grid of the chart shows the relationships between descent-climb angles, airspeeds, and rates of descent or climb.
 * SERVICE CEILING CHARTS Figure 25-3 presents service ceiling (100 fpm rate-of-climb capability) for continuous torque available. Data is presented for the range of operational pressure altitude and free air temperatures.

Cruise
The cruise charts... show the percent torque and engine rpm required for level flight at various pressure altitudes, airspeeds, gross weights, and fuel flows... The following parameters [are] contained in each chart... There is also a
 * Airspeed.
 * Torque
 * Fuel Flow
 * Maximum Range.
 * Maximum Endurance and Maximum Rate of Climb
 * TIME AND RANGE VERSUS FUEL CHART The time and range versus fuel chart (Figure 26-2) shows the en route time and the distance that the helicopter can cover while in level cruise with calm winds. The only information needed is the cruise fuel, the fuel flow, and the cruise true airspeed.

Endurance

 * HOVERING ENDURANCE CHART The hover endurance chart (Figure 27-1) is shown for out-of-ground effect at pressure altitudes of sea level, 4,000 feet, 8,000 feet, 10,000 feet, and 12,000 feet for various gross weights and outside air temperatures. Hover endurance can be determined if gross weight and fuel loading are known.

Emergency Operation

 * AUTOROTATIONAL GLIDE CHARACTERISTIC CHART
 * GLIDE DISTANCE CHART

Special Charts

 * RADIUS OF TURN AT CONSTANT AIRSPEED CHART Figure 29-1 presents turn performance.
 * WIND COMPONENT CHART Figure 29-2 may be used to obtain a wind component for various crosswind components and crosswind direction or azimuth.
 * AIRSPEED OPERATING LIMITS CHART Refer to Figure 29-3 for forward airspeed limits. Sideward flight limit is 25 knots. Rearward flight limit is 15 knots. Autorotation flight limit is 100 KIAS. With any doors removed, flight limit is 110 KIAS.
 * DRAG CHART The drag chart (Figure 29-4) shows the torque change required for flight because of drag area change as a result of external configuration changes.

=Power Required Exceeds Power Available= NATOPS procedure (14.5): Indications: Uncommanded descent with torque at maximum allowed Rotor Droop Loss of tail rotor effectiveness Procedures: If impact is imminent:
 * 1. Collective - Adjust as required to maintain Nr in operating range.
 * 2. Twist Grip - Full open.
 * 3. Airspeed - Increase/decrease to 50 KIAS (minimum power required airspeed).
 * 4. Angle of bank - Level wings.
 * 5. Jettison - As Required.
 * 6. Level the aircraft to conform to terrain.
 * 7. Cushion the landing.

=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).