Aerolab Midterms PDF

TYPES OF FLOW BOUNDARY LAYER It is a very thin layer of air flowing over the INVISCID FLOW vs VISCOUS FLOW surface of a...

TYPES OF FLOW BOUNDARY LAYER It is a very thin layer of air flowing over the INVISCID FLOW vs VISCOUS FLOW surface of an object. As air moves past the wing, the INVISCID FLOW molecules right next to the wing stick to the surface. NO mass diffusion Each layer of molecules in the boundary layer moves NO viscosity (friction) faster than the layer closer to the surface. The NO thermal conduction greater the distance (n) from the surface, the greater the velocity (V) of the molecules. At the outer edge of the boundary layer, the molecules move at the same velocity (free stream velocity) as the molecules outside the boundary layer. Ludwig Prandtl revolutionized fluid dynamics when he introduced the boundary layer concept in the early 1900s. VISCOUS FLOW With mass diffusion Has viscosity (friction) Experience thermal conduction Theoretically, inviscid flow is approached in the limit as the Reynold’s number goes to infinity. BOUNDARY LAYERS may be either LAMINAR (LAYERED), or TURBULENT (DISORDERED) depending However, for practical problems, many flows with high but on the value of the REYNOLDS NUMBER. finite Re can be assumed to be inviscid. For such flows, the influence of friction, thermal conduction, and diffusion is limited to a very thin region adjacent to the body surface called the boundary layer, and the remainder of the flow outside this thin region is essentially inviscid. There are some flows that are dominated by viscous effects. For example, if the airfoil/wing is inclined to a high angle of attack, then the boundary layer will tend to separate from the top surface, and a large wake is formed downstream. For lower Reynolds numbers, o the boundary layer is laminar o the streamwise velocity changes uniformly as one moves away from the wall, as shown on the left side of the figure. For higher Reynolds numbers, o the boundary layer is turbulent o the streamwise velocity is characterized by unsteady (changing with time) swirling flows inside the boundary layer. STANDARD SEA LEVEL CONDITIONS (SSLC) FLOW IN THE DUCT The velocity increases in the convergent portion of the duct, reaching a maximum value V2 at the minimum area of the duct. This minimum area is called the throat. At the throat, the pressure reaches a minimum value P2. In the divergent section downstream of the throat, the velocity decreases and the pressure increases. FLOW IN A DUCT INCOMPRESSIBLE AND COMPRESSIBLE FLOW COMPRESSIBLE FLOW Flow in which the density of the fluid elements can change from point to point All real-life flows are compressible, but there are circumstances in which the density changes only slightly, that it becomes negligible INCOMPRESSIBLE FLOW FOR COMPRESSIBLE FLOW: Flow in which the density of the fluid elements is always constant It is a myth, since it can never occur in nature FOR INCOMPRESSIBLE FLOW: However, for those flows with negligible variation of density, it is convenient to make the assumption that it is constant. CONTINUITY EQUATION SAMPLE PROBLEM 1: From a physical principle of: mass can neither be Consider a convergent duct with an inlet area A1 = 5 m2. Air created nor destroyed, or the law of conservation of enters this duct with a velocity V1 = 10 m/s and leaves the mass duct exit with a velocity V2 = 30 m/s. What is the area of the duct exit? Also, what is the mass flow rate? Assume SSLC BERNOULLI’S EQUATION FOR RESERVOIR AND TEST SECTION: In fluid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously FOR TEST SECTION AND DIFFUSER: with a decrease in pressure or a decrease in the fluid's potential energy. The principle is named after Daniel Bernoulli, a Swiss AT VARIOUS LOCATIONS: mathematician, who published it in 1738 in his book Hydrodynamics. CONTINUITY EQUATION AND BERNOULLI’S EQUATION SAMPLE PROBLEM 2: The air density along the streamline is 0.002377 slug/ft3, which is standard atmospheric density at sea level. At point1 on the streamline, the pressure and velocity are 2116lb/ft2 and 10ft/s, respectively. Further downstream, at point 2 on the streamline, the velocity is 190 ft/s. Calculate the pressure at point 2. LOW SPEED WIND TUNNEL In essence, a low-speed wind tunnel is a large venturi where the airflow is driven by a fan connected to some type of motor drive.  AIRFOIL NOMENCLATURE 5-DIGIT AIRFOILS: NACA 23012 2 = camber 0.02c 30 = position of camber at (0.30/2) = 0.15c from LE 12 = maximum thickness of 0.12c Design CL = 0.15 x 1st Digit 6-DIGIT AIRFOILS: NACA 653-421 MEAN CAMBER LINE - The line joining the midpoints between the upper and lower surfaces of an airfoil and 6 = simply identifies the series measured normal to the chord line. 5 = gives the location of minimum pressure (0.5c) 3 = Cd near minimum value over a range of Cl of 0.3 CHORD LINE - The line joining the end points of the mean above and below the design Cl camber line 4 = design lift coefficient in tenths (0.4 in this case) 21 = maximum thickness in hundredths of chord THICKNESS - The height of profile measured normal to the (0.21c) chord line THICKNESS RATIO - The maximum thickness to chord 7-DIGIT AIRFOILS: ratio, t/c NACA 747A315 CAMBER - The maximum distance of the mean camber line 7 = series designation from the chord line 4 = distance of minimum pressure area on upper LEADING EDGE - Most forward surface of the airfoil surface (0.4c) 7 = distance of minimum pressure area on lower TRAILING EDGE - Most rearward surface of the airfoil surface (0.7c) A = serial letter to distinguish different sections having CHORD (c) - Measurement of length from the leading edge the same numerical designation but different mean line to the trailing edge of the airfoil or thickness distribution 3 = design lift coefficient in tenths (0.3) 15 = maximum thickness (0.15c) NACA AIRFOIL DESIGNATION National Advisory Committee for Aeronautics (NACA) AIRFOILS Formed on March 3, 1915, with a charter to "supervise and direct the scientific study of the A supercritical airfoil is an airfoil that, principally, has problems of flight, with a view to their practical been designed to delay the onset of wave drag in solution." the transonic speed range. With luminaries like Orville Wright as members, the Typical features of supercritical airfoils, when compared group was on the cutting edge of technology in the to traditional airfoil shapes, are a flattened upper early decades of flight, before eventually being surface, a highly cambered or curved aft section absorbed by NASA in 1958. (National Aeronautics and greater leading edge radius. and Space Administration) 4-DIGIT AIRFOILS: 1ST DIGIT = Maximum Camber in percentage of the chord 2ND DIGIT = Position of the Maximum Camber in tenths of the chord 3RD AND 4TH DIGIT = Maximum thickness of airfoil in percentage of the chord EXAMPLES: NACA 4412 4 = Camber 0.04c 4 = Position of camber is at 0.4c from L.E. 12 = Maximum Thickness is 0.12c NACA 2415 ADVANTAGES OF A SUPERCRITICAL AIRFOIL: A significant reduction in shock induced boundary layer separation The production of a smaller, weaker shock wave at a position further aft on the wing than traditional airfoils The potential for more efficient wing design (allows reduction in wing sweep or an increase in wing thickness without the corresponding increase in wave drag) SYMMETRICAL Mirrored upper and lower surfaces such that the chord line and mean camber line are the same producing no lift at zero angle of attack. Applications: most of light helicopters in their main rotor blades. ASYMMETRICAL OR UNSYMMETRICAL Also known as cambered airfoils Has different upper and lower surfaces such that the chord line is placed above with large curvature. Have different chord line and camber line. ADVANTAGES: lift-to-drag ratio and stall characteristics are better; useful lift is produced at zero angle of attack DISADVANTAGE: not economical; production of undesirable moments  HIGH-LIFT DEVICES SPLIT FLAP The flap forms part of the lower surface of the FUNDAMENTALS OF HIGH-LIFT DEVICES trailing edge of the wing, with the upper surface PURPOSE OF HIGH-LIFT DEVICES contour not affected when the flap is lowered. To reduce the distances from take-off and landing. This allows operation at greater weights from the provided runway lengths and allows carrying higher payloads. TAKE-OFF AND LANDING SPEEDS The distances of take-off and landing depend on the velocities required at the screen and these are set out in the performance regulations. SLOTTED FLAP OR MULTIPLE SLOTTED FLAPS The stalling speed is determined by the wing's When lowered, a slot or gap between the wing and CLMAX and the CLMAX, therefore, must be as high the flap is opened. as possible to achieve the lowest possible distances. The slot guides higher pressure air over the flap from the lower surface, and re-energizes the CLmax AUGMENTATION boundary layer. One of the main factors that defines an airfoil section's The slotted flap gives CLMAX a greater increase CLMAX is the camber. than the plain or split flap, and much less drag, but has a more complicated design. Increasing an airfoil section's camber increases the CL at a given angle of attack and increases CLMAX. A cambered section is suitable for take-off and landing but this would give high drag at cruising speeds and require a very nose-down attitude. HIGH-LIFT DEVICES – FLAPS FLAPS A hinged portion of the trailing or leading edge that can be deflected downwards and thus create a camber increase. LOW SPEED AIRFOILS – flaps will only be on trailing edge HIGH SPEED AIRFOILS – leading edge might be FOWLER FLAP symmetrical or have a negative camber, flaps is on both the leading edge and the trailing edge The Fowler Flap travels backwards and downwards, giving initially an increase in wing area, and then an TYPES OF FLAPS increase in camber. PLAIN FLAP It can be slotted. It has a simple construction. Because of both increased area and camber, the Fowler flap gives the greatest increase in lift, and also Provides a reasonable increase in CLMAX, but with a gives the least drag because of the slot and reasonably high drag. decreased thickness: chord ratio. However the pitching moment adjustment is It is primarily used on low-speed aircraft and where bigger because of the chord's rearward extension. very fast take-off and landing are not necessary. LIFT CURVE SLOPES OF FLAPS VARIABLE CAMBER LEADING EDGE FLAP The camber of a leading edge flap can be increased to improve efficiency by offering a better leading edge profile. The leading edge devices are either fully extended (deployed) or retracted (stowed) unlike trailing edge flap. LEADING EDGE FLAPS ON LIFT HIGH-LIFT DEVICES – LEADING EDGE FLAPS The leading edge flap's main effect is to delay LEADING EDGE FLAPS separation, thus increasing the angle of stalling and the resulting CLMAX. The leading edge can have very little camber on However due to the increased camber of the airfoil high speed airfoil parts and have a tiny radius. section, there will be some increase in lift at lower This can offer flow separation at reasonably low angles of attack. angles of attack just aft of the leading edge. Using a leading edge flap which increases the leading edge camber can be remedied for this. HIGH-LIFT DEVICES – LEADING EDGE SLOTS LEADING EDGE SLOTS KRUEGER FLAPS A leading edge slot is a gap between the lower It is a part of the lower surface of the leading surface and the upper edge of the leading edge, edge which can be rotated around its forward edge. and can be fixed or formed by pushing forward part of Used on the inboard section to facilitate root stall the leading edge (the slat). on a swept wing, because they are less effective than the opposite variable camber. LEADING EDGE SLATS BOUNDARY LAYER CONTROL A slat is a small auxiliary airfoil fixed to the wing 's SUCTION leading edge. It forms a slot when deployed which allows air In the process of suction, air is drawn from the passage from the high pressure region below the boundary layer which has formed through minute wing to the low pressure region above it. holes into a plenum chamber formed by providing The slat forming a convergent canal contributes an inner skin over the upper surface of the wing. additional Kinetic Energy to the airflow through the slot. This decreases the thickness of the boundary layer and ensures that longer laminar airflow stays over the wing. At the trailing edge of the wing, the air that has been pulled into the plenum chamber is ducted overboard. DISADVANTAGE: It involves a multitude of minute holes through the skin panels of the wing (about 0.0025 inch diameter). These are quite easily blocked Some air transport aircraft that use the method of suction cover these panels with their slats' top trailing edge. LEADING EDGE SLATS AND SLOTS BLOWING Blowing is accomplished by jetting high-velocity air from vents through the upper surface of the wing just below the leading edge. This speeds up the airflow near the skin of the wing and makes sure the boundary layer stays thin. The blown method is easier, but either a dedicated pump or motor extracts air.