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6.11 Power Requirements

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Drag

How is drag generated? Drag is the aerodynamic force that opposes an aircraft's motion through the air. Drag is generated by every part of the helicopter (even the engines). The term given to this is Parasite Drag.

Parasite drag is a mechanical force. It is generated by the interaction and contact of a solid body with a fluid or a gas. For drag to be generated, the solid body must be in contact with the fluid. Drag is generated by the difference in velocity between the solid object and the fluid. There must be motion between the object and the fluid.

We can think of drag as aerodynamic friction, and one of the sources of drag is the skin friction between the molecules of the air and the solid surface of the aircraft. Because the skin friction is an interaction between a solid and a gas, the magnitude of the skin friction depends on properties of both solid and gas. For the solid, a smooth, waxed surface produces less skin friction than a roughened surface. For the gas, the magnitude depends on the viscosity of the air and the relative magnitude of the viscous forces to the motion of the flow. Along the solid surface, a boundary layer of low energy flow is generated and the magnitude of the skin friction depends on conditions in the boundary layer.

This source of drag depends on the shape of the aircraft and is called form drag. As air flows around a body, the local velocity and pressure are changed. Since pressure is a measure of the momentum of the gas molecules and a change in momentum produces a force, a varying pressure distribution will produce a force on the body. We can determine the magnitude of the force by integrating (or adding up) the local pressure times the surface area around the entire body.

Drag Flow Chart

A major problem for helicopters is recirculation of its down wash. Aerodynamicists have named this component the induced drag. This drag occurs because the flow near the blade tips is distorted span-wise as a result of the pressure difference from the top to the bottom of the blade. Swirling vortices are formed at the blade tips, which produce a swirling flow which is very strong near the blade tips and decreases toward the blade root.

The local angle of attack of the blade is increased by the induced flow of the tip vortex, giving an additional, downstream-facing, component to the aerodynamic force acting on the blade. This additional force is called induced drag because it has been "induced" by the action of the tip vortices.

Profile Drag

Profile drag develops from the frictional resistance of the blades passing through the air. It does not change significantly with the aerofoil’s angle of attack, but increases moderately when airspeed increases. Profile drag is composed of form drag and skin friction. Form drag results from the turbulent wake caused by the separation of airflow from the surface of a structure.

Profile Drag

The amount of drag is related to both the size and shape of the structure that protrudes into the relative wind. Skin friction is caused by surface roughness. Even though the surface appears smooth, it may be quite rough when viewed under a microscope. A thin layer of air clings to the rough surface and creates small eddies that contribute to drag.

Profile Power

Profile Power can be thought of as "main rotor turning power," accounts for 15 - 45% of main rotor power in a hover and is used to overcome friction drag on the blades.

Profile Drag Curve

It remains at a relatively constant level of power required as the helicopter accelerates into forward flight, due to the compensatory effect of the decrease in profile drag on the retreating blade and the increase in profile drag on the advancing blade.

Induced Drag

Induced drag is generated by the airflow circulation around the rotor blade as it creates lift. The high-pressure area beneath the blade joins the low-pressure air above the blade at the trailing edge and at the rotor tips.

Induced Drag

This causes a spiral, or vortex, which trails behind each blade whenever lift is being produced. These vortices deflect the airstream downward in the vicinity of the blade, creating an increase in downwash. Therefore, the blade operates in an average relative wind that is inclined downward and rearward near the blade. Because the lift produced by the blade is perpendicular aircraft yaw.

Induced Drag Tip Vortex

It is easy to visualize the creation of form drag by examining the airflow around a flat plate. Streamlining decreases form drag by reducing the airflow separation, to the relative wind, the lift is inclined aft by the same amount. The component of lift that is acting in a rearward direction is induced drag

Induced Drag Curve

The formation of induced drag is associated with the downward deflection of the airstream near the rotor blade. As the air pressure differential increases with an increase in angle of induced drag increases. Since the blade’s angle of attack is usually lower at higher airspeeds, and higher at low speeds, induced drag decreases as airspeed increases and increases as airspeed decreases. Induced drag is the major cause of drag at lower airspeeds.

Induced Power

Induced Power is the power required to overcome Induced Drag. The power required is at a maximum when hovering out of ground effect (HOGE), due to high pitch angles and high-induced flow and reduces to a minimum as forward airspeed increases due to lower pitch angles.

Induced Power

Induced power can be thought of as "pumping power," is power associated with the production of rotor thrust. This value is at its highest during a hover (60 - 85% of total main rotor power) and decreases rapidly as the helicopter accelerates into 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. Therefore, induced power decreases to ¼ hover power with an increase to maximum forward speed.

Parasite Drag

Parasite Drag is present all the time when a helicopter moves through air. This type of drag increases with airspeed. Because of its rapid increase with increasing airspeed, parasite drag is the major cause of drag at higher airspeeds.

Form drag and pressure drag are virtually the same type of drag. Form or pressure drag is caused by the air that is flowing over the aircraft. The separation of air creates turbulence and results in pockets of low and high pressure that leave a wake behind the helicopter (thus the name pressure drag). This opposes forward motion and is a component of the total drag. Since this drag is due to the shape, or form of the aircraft, it is also called form drag.

Parasite Drag

Streamlining the aircraft will reduce form drag, and parts of an aircraft that do not lend themselves to streamlining are enclosed in covers called fairings, or a cowling for an engine, that have a streamlined shape.

Helicopter non-lifting components that produce form drag include; (1) the fuselage, (2) tail surfaces, (3) nacelles, (4) landing gear, (5) fuel tanks and external stores, (6) engines, (7) and rotor mast, all contribute to parasite drag. Any loss of momentum by the airstream due to such things as vents for engine cooling creates additional drag.

Skin friction drag is caused by the actual contact of the air particles against the surface of the aircraft. This is the same as the friction between any two objects or substances. Because skin friction drag is an interaction between a solid (the helicopter surface) and a gas (the air), the magnitude of skin friction drag depends on the properties of both the solid and the gas.

For the solid helicopter, skin fiction drag can be reduced, and airspeed can be increased somewhat, by keeping an aircraft's surface highly polished and clean. For the gas, the magnitude of the drag depends on the viscosity of the air. Along the solid surface of the airplane, a boundary layer of low energy flow is generated. The magnitude of the skin friction depends on the state of this flow.

Parasite drag is simply the mathematical sum of Form Drag and Skin Friction Drag.

Parasite Drag = Form Drag + Skin Friction Drag

Parasite Drag Curve

Parasite drag varies with the square of the velocity. By doubling airspeed, parasite drag increases the by four times. 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 trade-offs of the high weight and cost of aerodynamically efficient designs versus structural requirements dictated by required stiffness, mechanical travel, and loads.

Parasite Power

Parasite Power is the power required to overcome Parasite Drag. It can be thought of as the power required to move the aircraft through air. 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. A smaller horizontal force is produced by the unbalanced profile and induced drag 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.

Total Drag

Total Drag for a helicopter is the sum of all three drag forces. As airspeed increases, parasite drag increases, while induced drag decreases. Profile drag remains relatively constant throughout the speed range with some increase at higher airspeeds. Combining all drag forces results in a total drag curve.

Total Drag Curve

The total drag curve represents the combined forces of parasite, profile, and induced drag; and is plotted against airspeed.

The low point on the total drag curve shows the airspeed at which drag is minimized. For most helicopters the speed for minimum total drag occurs at around 55kt. This is the point where the lift-to-drag ratio is greatest and is referred to as L/D max. At this speed, the total lift capacity of the helicopter, when compared to the total drag of the helicopter, is most favourable, with maximum lift and minimum drag providing the most efficient helicopter performance.

All of these types of drag must be accounted for when determining drag for subsonic flight. The total drag is the sum of parasite and induced drag.

Total Drag = Parasite Drag + Induced Drag + Profile Drag

But the net (or total) drag of an aircraft is not simply the sum of the drag of its components. When the components are combined into a complete aircraft, one component can affect the airflow around and over the helicopter, and hence, the drag of one component can affect the drag associated with another component. These effects are called interference effects, and the change in the sum of the component drags is called interference drag.

(Drag)1+2 = (Drag)1 + (Drag)2 + (Drag)interference

Interference drag can be minimized by proper fairing and filleting, which induces smooth mixing of air past the components. No adequate theoretical method will predict interference drag; thus, wind tunnel or flight-test measurements are required. For rough computational purposes, a figure of 5% to 10% can be attributed to interference drag on a total aircraft.