801. LESSON TOPIC LEARNING OBJECTIVES

1. Terminal Objective: Partially supported by this lesson topic:

Upon completion of this unit of instruction the student will understand the factors that determine and affect power required and power available for flight.

2. Enabling Objectives: Completely supported by this lesson topic:

a. Describe how power required to hover is affected by changes in gross weight, altitude, DA, and rotor speed.

b. Use a chart to determine airspeed for maximum range and maximum endurance/rate of climb IAW NATOPS.

c. Describe effect of wind on maximum range airspeed.

d. Describe the physical properties that limit engine performance.

e. Use a chart to convert torque to shaft horsepower IAW NATOPS.

f. Use a chart to determine power (torque) available and power required to hover in ground effect (HIGE) and out of ground effect (HOGE) IAW NATOPS.

g. Use a chart to determine distance required to clear an obstacle on takeoff IAW NATOPS.

h. Describe conditions which can cause an aircraft’s power required to exceed its power available and recovery procedures.

i. State the environmental factor which most significantly affects power available.

j. State the effect the tail rotor will have on power available to the main rotor.

802. REFERENCES

1. Fundamentals of Aerodynamics, CNATRA P-202 PAT 2. Rotary Wing Aerodynamics for Naval Aviators

3. Fundamentals of Flight 4. Helicopter Aerodynamics 5. NAVAIR 01-H57BC-1 NATOPS 803. STUDY ASSIGNMENT

1. Review Chapter Eight.

2. Answer the Review Questions.

804. GENERAL

Helicopter aircraft and engine performance require an understanding of the power required curves, power available, and the relationship between them.

Power is required to overcome the drag produced by the two rotors and the fuselage. The power available to meet this power requirement is produced by a turboshaft engine with numerous rotor and stator blades designed using aerodynamic principles. After these principles are covered, an overview of various engine malfunctions with their aerodynamic causes is presented. A brief introduction to aircraft performance is included here to establish general principles. Turbojet aircraft produce thrust directly from their engines and do not turn a propeller or rotor. As such, jet aircraft performance charts only slightly resemble helicopter drag curves (Figure 8-1). For each pound of drag generated by the aircraft at a specific airspeed, a pound of thrust must be generated by the jet in order to maintain level flight. Thrust is the force opposite the acceleration of air particles by the jet engine. The amount of thrust produced by a jet engine is directly proportional to fuel flow and therefore endurance and range performance may be determined from an aircraft total drag curve.

The differences between the two types of performance curves can be attributed to the different contributions of profile and induced drag in the helicopter. The helicopter rotor also produces thrust, but the production of thrust is not directly related to the fuel flow for turbo-shaft engines.

Turbo-shaft fuel flow is more closely related to how much power is being produced by the engine. Accordingly, a total drag curve cannot be used in the same way as a performance chart for helicopters.

Figure 8-1 Total Drag Curve

Instead, “power required curves” (or more specifically, “fuel flow curves”) are presented in operator’s manuals for use in mission planning. Power is simply the rate of doing work. James Watt, the Scottish inventor who improved upon the steam engine, calculated that a horse can do 550 foot/pounds of work in one second. A mathematical variation of that calculation provides that if drag is multiplied by the aircraft velocity (and divided by 325, if knots are used), each point on an aircraft’s drag curve may be re-plotted as a point on a power required curve. Such a power required curve is shown in Figure 8-2 and although very similar to the drag curve, the points may not be identical. For this reason, most discussions of helicopter performance centers on power required curves rather than drag curves. Note that this is a “power vs. airspeed”

relationship (power is on the vertical axis) and not a “drag vs. airspeed” relationship. Power required curves can be plotted using “torque,” horse power or fuel flow since they are all related to power.

Figure 8-2 Power Required Versus Airspeed Curve

Most turbine helicopters are equipped with a gauge for measuring torque which may be viewed by the aviator in the cockpit. Since power equals torque times rpm, if the rpm remains constant, the torque is a direct representation of current engine power output. Further, a fuel flow scale is usually provided opposite the torque scale of a cruise chart, thereby enabling the aviator to convert torque directly to fuel flow.

It is important to note that the lowest point on the power required curve (Figure 8-2) is the point of minimum power required (best lift to drag ratio) and not necessarily the point of minimum drag (as is the lowest point on the total drag curve). The point of minimum power results in the lowest fuel flow and is therefore the airspeed for maximum endurance. The airspeed for minimum power is slower than the airspeed for minimum drag because a decrease in velocity to the minimum power airspeed decreases the power required even though flying at any airspeed below minimum drag actually increases drag. However, because the bottom of the drag curve is nearly flat, the slight increase in drag is more than offset by the decrease in velocity, which slows the work rate and therefore results in an overall reduction in power required.