Tecnología Aplicada

Now that we have discussed how rotor blades and rotor systems work, let's investigate how they work with a helicopter fuselage and all of the forces that come into play. For a helicopter to remain in steady, level flight, these forces and moments must balance. These forces exist in the vertical plane (Figure 8-3), horizontal plane, and about the CG in the form of pitching moments.

Figure 8-3 Aerodynamic Forces Affecting Power Required

To begin the discussion of these forces, we will discuss the types of power required due to these various forces (Figure 8-4).

Figure 8-4 Power Required Curves Versus Airspeed

How much power does it take? In a hover, two types are necessary - induced and profile power.

Induced power, is power associated with the production of rotor thrust. This value is at its highest during a hover (60 – 85 percent of total main rotor power) and decreases rapidly as the helicopter accelerates into forward flight. During forward flight, the increase in mass flow of air introduced to the rotor system reduces the amount of work the rotors must produce to maintain a constant thrust. (This concept will be explained in greater detail in a later section). Therefore, induced power required continues to go down with increasing airspeed.

Profile power, which can be thought of as "main rotor turning power," accounts for 15 – 40 percent of main rotor power in a hover and is used to overcome friction drag on the blades. It increases slightly with increasing airspeed.

In forward flight, parasite power joins forces with induced and profile power to overcome the parasite drag generated by all the aircraft components, excluding the rotor blades. Parasite power can be thought of as the power required to move the aircraft through the air. This power requirement increases in proportion to forward airspeed cubed. Obviously, this is

inconsequential at low speed, but is significant at high speed and is an important consideration for helicopter designers to minimize drag. This is a challenging task due to design tradeoffs of the high weight and cost of aerodynamically efficient designs versus structural requirements dictated by required stiffness, mechanical travel, and loads.

In addition to the drag curves which are the basis for the power required curves, there is a fourth power requirement, labeled “miscellaneous” in Figure 8-4, which is taken into account when power required curves are developed for specific rotorcraft. This is the power required to run the tail rotor and accessories such as generators, hydraulics etc. Accessory power requirements remain relatively constant independent of airspeed while tail rotor power required tends to decrease with increasing airspeed, as previously discussed. Depending on the charts used, this additional power requirement is sometimes combined with the profile power requirement, creating a “total rotor profile power” required to maintain a given rotor rpm, taking into account the rotor profile drag as well as the tail rotor and accessory requirements.

The smaller horizontal force, H-force, is produced by the unbalanced profile and induced drag (or inplane drag in some books) of the main rotor blades. Tilting the rotor disc forward from a fraction of a degree at low speed to about 10° at max speed compensates for this. We will not be getting into additional discussions about H-force in this workbook.

Different flight regimes are performed more efficiently at different forward speeds. The bowl-shape of the power required curve graphically illustrates the reason why (Figure 8-5). Optimum speeds determined by this curve are maximum loiter time (endurance), minimum rate of descent in autorotation, best rate of climb and maximum range airspeed; although these optimal speeds are discussed in more detail in Chapter Ten, they are briefly introduced here.

Best rate of climb airspeed is formed at the point where the difference is a maximum between power required and power available. The bottom of the curve is called the bucket airspeed.

Since the goal of achieving maximum loiter time is making the available fuel last as long as possible, and since fuel flow is proportional to engine power, maximum loiter time should also be at this point.

Near this speed, minimum rate of descent in an autorotation is also found, since the power required to keep the aircraft airborne is at a minimum.

Figure 8-5 Optimum Airspeeds

The point on the power required curve corresponding to the point of minimum drag versus airspeed on the drag curve is at an airspeed greater than that for minimum power (bucket airspeed). This is the airspeed for maximum range and is where the ratio of fuel flow to velocity is at a minimum value. This point is shown in Figure 8-5 and 8-6 at the point of tangency of the power required curve and a straight line drawn from the origin, providing the best power or fuel flow to airspeed (thus drag) ratio. Maximum range speed is found on the fuel flow curve by drawing a line tangent to the curve from the origin (Figure 8-6). This ratio of speed to fuel flow shows the distance one can travel on a pound of fuel on a no-wind day. If there is a head wind, the line should be originated at the head wind value, which derives a higher speed and lower range. For a tail wind, the optimum airspeed decreases, but the range increases significantly. On generic charts like the one above, the speed for maximum range and

autorotation maximum glide distance sometimes appear to be the same. Best airspeed for

maximum glide in an autorotation is also affected by headwinds and tailwinds just like maximum range airspeed. However, when using aircraft specific charts, that is not usually the case and the two speeds are different, as you will learn for the TH-57. The TH-57 NATOPS has specific charts for determining the maximum glide range airspeed and minimum rate of descent airspeed.

These will be covered in a later chapter.

Figure 8-6 Maximum Range Airspeed Adjustment for Winds 806. POWER AVAILABLE

Air density (DA) is the environmental factor which most significantly affects power available.

Less dense air requires that the engine works harder to produce the same amount of mass flow.

Power available is directly affected by density to such a degree that power available at a given DA can be calculated by simply multiplying power available at standard sea level by the density ratio in the ambient conditions.

Poweravail = Powersea level x density ratio ()

This is generally true in regions of relatively normal temperature variation. However, in locations of extremely wide temperature variations such as the desert environment, the

temperature can have an extra degrading effect on engine power available. This information is found from close inspection of the NATOPS power available charts and requires a large temperature variation to realize.

Other Factors limiting power available. Operating conditions that affect fuel flow or airflow directly affect the ability of the engine to generate power. Some of those factors follow:

1. Fuel Flow Limitation (cold) - As temperature decreases the density of air increases so the fuel flow must increase in order to maintain the stoichiometric fuel/air ratio for complete

combustion. However, the amount of fuel flow through the fuel nozzles has a limit; therefore at cold temperatures the fuel/air ratio will not be optimum, and incomplete (lean) combustion will occur, resulting in less power available.

2. Turbine Temperature Limitation (hot) - The materials used to build turbines have definite stress and temperature limits. To avoid unacceptable creep or component failure, turbine temperature must be limited. Depending on aircraft manufacturer this can be called exhaust gas temperature (EGT), turbine outlet temperature (TOT) or turbine inlet temperature (TIT).

3. Ng-Gas Generator Limitation (hot) - As the OAT increases the density of air decreases, therefore the gas turbine has to rotate faster in order to deliver the same mass flow rate. This increased rotational speed required at higher temperatures can approach limits that have been established to counter centrifugal loads on the gas turbine blades.

4. Age of the engine - Compressor blades erode with time, and their degradation results in loss of blade area that will degrade engine performance.

5. Component rating degradation - Transmission components have material limitations, so their torque capacity must be considered.

6. Humidity/Moisture Effect - Increases in humidity/moisture have a counteracting effect in that the associated decrease in air density is detrimental while the reduced combustor inlet temp (T3) is beneficial. Hence humidity/moisture has a negligible effect on gas turbine engines.

7. Torque limits - drive train limits, including drive shaft and transmission.

8. Airspeed effects (ram air) - Airspeed increases the flow rate into the engine, but at the speed at which rotorcraft operate this effect is negligible.

NATOPS Charts. Familiarization with all performance charts in the NATOPS is of paramount importance. While the pilot must master interpretation of all the charts, this course will

introduce the student to the determination of power (torque) available/required to hover

IGE/OGE, distance required to clear an obstacle, and conversion of torque to shaft horsepower, among other things. Therefore, the student is expected to become familiar with all NATOPS performance charts prior to class.