Propellers

PROPELLERS

A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and air or water is accelerated behind the blade. Propeller dynamics can be modeled by both Bernoulli's principle and Newton's third law. A propeller is often colloquially known as screw both in aviation and maritime.

 Aircraft propellers convert rotary motion from piston engines or turboprops to provide propulsive force. They may be fixed or variable pitch. Early aircraft propellers were carved by hand from solid or laminated wood with later propellers being constructed from metal. The most modern propeller designs use high-technology composite materials.

AIRFOIL

An airfoil or aero foil is the shape of a wing or blade (of a propeller, rotor or turbine) or sail as seen in cross-section.

An airfoil-shaped body moved through a fluid produces an aerodynamic force. The component of this force perpendicular to the direction of motion is called lift. The component parallel to the direction of motion is called drag. Subsonic flight airfoils have a characteristic shape with a rounded leading edge, followed by a sharp trailing edge, often with asymmetric camber. Foils of similar function designed with water as the working fluid are called hydrofoils.

The lift on an airfoil is primarily the result of its angle of attack and shape (in particular its camber). When either is positive, the resulting flow field about the airfoil has a higher average velocity on the upper surface than on the lower surface. This velocity difference is necessarily accompanied by a pressure difference, via Bernoulli's principle for in compressible in-viscid flow, which in turn produces the lift force. The lift force can also be related directly to the average top/bottom velocity difference, without invoking the pressure, by using the concept of circulation and the Kutta-Joukowski theorem.

STRUCTURE OF AN AIRFOIL

·         Angle of attack is the angle between the lifting body's reference line (chord) and the oncoming flow.

·         The chord of an airfoil is the imaginary straight line drawn through the airfoil from its leading edge to its trailing edge.

·         Camber is the asymmetry between the top and the bottom surfaces of an airfoil.

·         The trailing edge is the back of the airfoil—the place at which the airflow over the upper surface of the airfoil joins the airflow over the lower surface of the airfoil.

·         The leading edge is the "front" of the airfoil—the portion that meets the air first.

Structure of an airfoil

PRINCIPLE AND WORKING

     The principle and working of a propeller is based on Bernoulli’s Principle & Newton’s Third Law. Bernoulli's principle states that for an in-viscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy. Newton’s third law states that every action has an equal and opposite reaction.

Air speed variations on an airfoil

    An aerofoil is shaped so that air flows faster over the top than under the bottom. There is, therefore a greater pressure below the aerofoil than above it. This difference in pressure produces the lift. Lift coefficient is a dimensionless coefficient that relates the lift generated by an aerodynamic body such as a wing or complete aircraft, the dynamic pressure of the fluid flow around the body, and a reference area associated with the body. 

AERODYNAMIC FORCES ACTING ON THE AIRFOIL

Forces acting on an airfoil

     Lift and drag are considered to be the two aerodynamic forces that are acting upon the airfoil as shown in the above figure.

    Lift is defined to be the component of this force that is perpendicular to the oncoming flow direction.

     Drag is defined to be the component of the surface force parallel to the flow direction. In fluid dynamics, drag (sometimes called air resistance or fluid resistance) refers to forces that oppose the relative motion of an object through a fluid (a liquid or gas).

PITCH OF A PROPELLER

Pitch of a propeller is normally described as the distance traveled per rotation, assuming there is no slip.Low pitch yields good low speed acceleration (and climb rate in an aircraft) while high pitch optimizes high speed performance and economy.

Pitch of a propeller

      Blade pitch or simply pitch refers to turning the angle of attack of the blades of a propeller or helicopter rotor into or out of the wind to control the production or absorption of power. Wind turbines use this to adjust the rotation speed and the generated power. A propeller of a ship uses this effect to control the ship's speed without changing the rotation of the shaft and to increase the efficiency of streaming fluids. In aircraft, blade pitch is usually described    as "coarse" for a high angle of attack, and "fine" for a low angle of attack. Blade pitch is normally described in units of distance/rotation assuming no slip.Blade pitch acts much like the gearing of the final drive of a car. Low pitch yields good low speed acceleration (and climb rate in an aircraft) while high pitch optimizes high speed performance and economy. Because the velocity of a propeller blade varies from the hub to the tip, they must be of twisted form in order for the pitch to remain constant along the length of the blade. This is typical of all but the crudest propellers. It is quite common in aircraft for the propeller to be designed to vary pitch in flight, optimizing both cruise and takeoff performance.

PROPELLERS USED

    We have used 3 Blade, 9x5 pitch rotating and counter rotating propellers. Benefits of using a 3 blade propeller over 2 blades is that we get more blade area because of which the blade can transfer more power onto the air, thus providing more lift. We are using two different kinds of blades one rotating in clockwise directions and other rotating in anti-clockwise direction, thus producing force in opposite directions.

3 Blade CR Prop 9 x 5.0 HD9050RX3