Aerodynamics Helicopter Flashcards
Bernoulli’s Principle?
Is a statement of the law of conservation of energy states energy cannot be created or destroyed and the amount of energy entering a system must also exit. A simple tube with a constricted portion near the center of its length illustrates this principle. Fluid speeds up in direct proportion to the reduction in area. Venturi effect is the term used to describe this phenomenon.
Venturi Flow?
While the amount of total energy within a closed system (the tube) does not change, the form of the energy may be altered. Fluid flow pressure has two components—static and dynamic pressure. Static pressure is the pressure component measured in the flow but not moving with the flow as pressure is measured. Static pressure is also known as the force per unit area acting on a surface. Dynamic pressure of flow is that component existing as a result of movement of the air. The sum of these two pressures is total pressure. As air flows through a constriction, static pressure decreases as velocity (dynamic pressure) increases.
Angle of Attack?
The AOA is the angle at which the airfoil meets the oncoming air flow. When the AOA is increased, air flowing over the airfoil is diverted over a greater distance, resulting in an increase of air velocity and more lift.
Forces Acting on the Aircraft?
Thrust—the forward force produced by the power plant/propeller or rotor.
Drag—a rearward, retarding force caused by disruption of air flow by the wing, rotor, fuselage, and other protruding objects. Drag opposes thrust and acts rearward parallel to the relative wind.
Weight—the combined load of the aircraft itself, the crew, the fuel, and the cargo or baggage. Weight pulls the aircraft downward because of the force of gravity.
Lift—opposes the downward force of weight, is produced by the dynamic effect of the air acting on the airfoil, and acts perpendicular to the flightpath through the center of lift.
What are the three types of Drag?
Profile
Parasitic
Induced
Profile Drag?
Profile drag develops from the frictional resistance of the blades passing through the air. Profile drag is composed of form drag and skin friction. Form drag results from the turbulent wake caused by the separation of air flow from the surface of a structure. 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.
Induced Drag?
Induced drag is generated by circulation around the rotor blade as it creates lift. The high pressure area beneath the blade joins the low pressure area above the blade at the trailing edge and at the rotor tips. 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 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.
Parasitic Drag?
Created by anything moving through the air. Cowlings, openings, windscreens….
Airfoil?
An airfoil is any surface producing more lift than drag when passing through the air at a suitable angle. Airfoils are used for stability (fin), control (elevator), and thrust or propulsion (propeller or rotor). Certain airfoils, such as rotor blades, combine some of these functions. In some conditions, parts of the fuselage, such as the vertical and horizontal stabilizers, can become airfoils. Airfoils are carefully structured to accommodate a specific set of flight characteristics.
Blade span?
The length of the rotor blade from center of rotation to tip of the blade.
Chord line?
A straight line intersecting leading and trailing edges of the airfoil.
Chord?
The length of the chord line from leading edge to trailing edge; it is the characteristic longitudinal dimension of the airfoil section.
Mean camber line?
A line drawn halfway between the upper and lower surfaces of the airfoil. The chord line connects the ends of the mean camber line. Camber refers to curvature of the airfoil and may be considered curvature of the mean camber line. The shape of the mean camber is important for determining aerodynamic characteristics of an airfoil section.
Flightpath velocity?
The speed and direction of the airfoil passing through the air. For airfoils on an airplane, the flightpath velocity is equal to true airspeed (TAS). For helicopter rotor blades, flightpath velocity is equal to rotational velocity, plus or minus a component of directional airspeed.
Relative wind?
Defined as the airflow relative to an airfoil and is created by movement of an airfoil through the air.
There are two parts to wind passing a rotor blade:
• Horizontal part—caused by the blades turning plus movement of the helicopter through the air.
• Vertical part—caused by the air being forced down through the rotor blades plus any movement of the air relative to the blades caused by the helicopter climbing or descending.
Induced flow?
The downward flow of air through the rotor disk. As blade pitch angle is increased, the rotor system induces a downward flow of air through the rotor blades creating a downward component of air that is added to the rotational relative wind.
Resultant relative wind?
Relative wind modified by induced flow. This is inclined downward at some angle and opposite the effective flightpath of the airfoil, rather than the physical flightpath (rotational relative wind). The resultant relative wind also serves as the reference plane for development of lift, drag, and total aerodynamic force (TAF) vectors on the airfoil. When the helicopter has horizontal motion, airspeed further modifes the resultant relative wind. The airspeed component of relative wind results from the helicopter moving through the air. This airspeed component is added to, or subtracted from, the rotational relative wind depending on whether the blade is advancing or retreating in relation to helicopter movement.
Angle of incidence?
The angle between the chord line of a blade and rotor hub. It is usually referred to as blade pitch angle. Collective input and cyclic feathering change the angle of incidence.
Center of pressure?
The point along the chord line of an airfoil through which all aerodynamic forces are considered to act.
Airfoil types?
Symmetrical- is distinguished by having identical upper and lower surfaces.
Non-symmetrical- has different upper and lower surfaces, with a greater curvature of the airfoil above the chord line than below.
Blade Twist?
Because of lift differential due to differing rotational relative wind values along the blade, the blade should be designed with a twist to alleviate internal blade stress and distribute the lifting force more evenly along the blade. Blade twist provides higher pitch angles at the root where velocity is low and lower pitch angles nearer the tip where velocity is higher. This increases the induced air velocity and blade loading near the inboard section of the blade.
Rotational Relative Wind?
The rotation of rotor blades as they turn about the mast produces rotational relative wind (tip-path plane). The term rotational refers to the method of producing relative wind. Rotational relative wind flows opposite the physical flightpath of the airfoil, striking the blade at 90° to the leading edge and parallel to the plane of rotation; and it is constantly changing in direction during rotation. Rotational relative wind velocity is highest at blade tips, decreasing uniformly to zero at the axis of rotation (center of the mast).
In Ground Effect (IGE)?
Ground effect is the increased efficiency of the rotor system caused by interference of the air flow when near the ground. The air pressure or density is increased, which acts to decrease the downward velocity of air. Ground effect permits relative wind to be more horizontal, lift vector to be more vertical, and induced drag to be reduced.
Out of Ground Effect (OGE)?
Induced flow velocity is increased. A higher blade pitch angle is required to maintain the same AOA as in IGE hover. The increased pitch angle also creates more drag. This increased pitch angle and drag requires more power to hover OGE than IG.