Pages from mod 8.2 49-55.pdf

British Airways Global Learning Academy – Basic Aerodynamics Drag Polar Curve The drag polar is a curve that shows the relationship between the drag coefficient and lift coefficient for a full aircraft. This relationship is expressed by an equation that can be represented by a graph called drag pola...

British Airways Global Learning Academy – Basic Aerodynamics Drag Polar Curve The drag polar is a curve that shows the relationship between the drag coefficient and lift coefficient for a full aircraft. This relationship is expressed by an equation that can be represented by a graph called drag polar. From inspection of Figure 26 it is evident that both CDO (shown in the graph as CDP) and CDi vary with lift coefficient. However, the part of parasite drag above the minimum at zero lift is included with the induced drag coefficient. The equation that defines the drag polar of an aircraft can be obtained from the total drag generated in it. The total drag is obtained from the sum of parasite drag and the induced drag due to lift generation of the aircraft, thus the equation that defines the total drag of an aircraft as aerodynamic coefficients can be written to follows. CD = CDO + CDi Where;  CDO  CDi = Parasite Drag = Induced Drag The variation of parasite drag coefficient, CDP with lift coefficient, CL is shown for a typical aeroplane in Fig 26. As a reminder AR is the wing Aspect Ratio, which has a significant contribution to any induced drag coefficient CDi and Drag coefficient CD The minimum parasite drag coefficient, CDO min usually occurs at or near zero lift and the parasite drag coefficient increases above this point in a smooth curve. Figure 26 – Drag Polar Curves The induced drag coefficient is shown on the same graph for purposes of comparison. In many areas of aeroplane performance it is necessary to completely distinguish between drag due to lift and drag not due to lift. Module 08B ETBN 0492 October 2023 Edition 49 Basic Aerodynamics – Aerodynamics British Airways Global Learning Academy – Basic Aerodynamics Lift/Drag Ratio Since it is common to both the top and bottom of the equation we can remove it, giving: The first polar diagram that we plot (Figure 26) is looking at how the ratio of the lift to drag produced by an aerofoil varies as the angle of attack that the aerofoil is presented to the relative airflow changes. 1 × 𝜌 ×𝑉 ×𝑆 ×𝐶 𝐿𝑖𝑓𝑡 2 = 1 𝐷𝑟𝑎𝑔 × 𝜌 × 𝑉 ×𝑆 ×𝐶 2 If we first consider the lift equation from earlier: 𝐿𝑖𝑓𝑡 (𝑓𝑜𝑟𝑐𝑒) = 1 × 𝜌 ×𝑣 ×𝑆 ×𝐶 2 𝐿𝑖𝑓𝑡 𝐶 = 𝐷𝑟𝑎𝑔 𝐶 And the drag equation: 𝐷𝑟𝑎𝑔 = 1 × 𝜌×𝑣 ×𝑆 ×𝐶 2 Now we see that the ratio of Lift to Drag is the same of the ratio of the Co-efficient of Lift to the Co-efficient of Drag. Then to find the ratio of lift to drag all we do is to divide the first equation by the second: The flight efficiency of an aeroplane is based on its aerodynamic efficiency, in particular the lift/drag (L/D) ratio. 1 × 𝜌 ×𝑉 ×𝑆 ×𝐶 𝐿𝑖𝑓𝑡 = 2 1 𝐷𝑟𝑎𝑔 × 𝜌 × 𝑉 ×𝑆 ×𝐶 2 As you can see the term × 𝜌 × 𝑉 ×𝑆 This is known by dividing the coefficient of lift by coefficient of drag at each angle of attack. The wing is most efficient at an angle of attack which gives the maximum ratio of lift to drag. From the lift curve Figure 24, we saw that we get most lift at about 15°, from the drag curve and from Figure 25 we get least drag at about 0°. is common to both the lift and drag equation – this is because it represents the dynamic pressure around the aerofoil. Module 08B ETBN 0492 October 2023 Edition From figure 27 we see that neither 0° nor 15° gives the optimum flight condition. We need the best compromise between lift and drag which occurs at about 3° when the lift is nearly 24 times the drag. 50 Basic Aerodynamics – Aerodynamics British Airways Global Learning Academy – Basic Aerodynamics Aerofoil Contamination Ice Formation And Effects of Ice, Snow And Frost Icing on aircraft is caused primarily by the presence of super-cooled water droplets in the atmosphere. If the droplets impinge on the forward facing surfaces of an aircraft, they freeze and cause a buildup of ice which may seriously alter the aerodynamic qualities. This applies particularly to small objects, which have a higher catch rate efficiency than large ones, as small amounts of ice will produce relatively bigger changes in shape. The actual amount and shape of the ice build-up depends on surface temperature. When the temperature is less than 0ºC all the impinging water droplets are frozen and when it is above 0ºC none are frozen. Fig 27 – Lift/Drag Ratio Graph The final influencing factor of note is that icing does not occur above about 12,000 m (40,000 ft.) since the droplets are all frozen and in the form of ice crystals and will not adhere to the aircraft’s surface. We find that the lift/drag ratio increases very rapidly up to about 3°, at which angle the lift is nearly 24 times the drag, the ratio then gradually falls off because, although the lift is still increasing, the drag is increasing even more rapidly. Some examples of ice formation at differing temperatures and their impact on a wings’ aerodynamic lift and drag performance can be found in Figure 28. Until at the stalling angle the lift may be only 10 or 12 times as great as the drag, and after the stalling angle the ratio falls still further until it reaches 0 at 90°. The chief point of interest about the lift/drag curve is the fact that this ratio is greatest at an angle of attack of about 3°, in other words, it is at this angle that the aerofoil gives its best all round results. Module 08B ETBN 0492 October 2023 Edition 51 Basic Aerodynamics – Aerodynamics British Airways Global Learning Academy – Basic Aerodynamics Fig 28 – Ice Formation At Different Temperatures Module 08B ETBN 0492 October 2023 Edition 52 Basic Aerodynamics – Aerodynamics British Airways Global Learning Academy – Basic Aerodynamics Ice on an aircraft affects its performance and efficiency in many ways. Ice build-up increases drag and reduces lift. It causes destructive vibration and hampers true instrument readings. Control surfaces become unbalanced or frozen. Fixed slots are filled and movable slots jammed. Radio reception is hampered and engine performance is affected. Ice, snow, rime formation or deteriorations of other kind having a thickness and surface roughness of a medium sandpaper on the leading edge and upper surface of the wing can reduce wing lift by around 30% and increase drag by 40% and increase overall aircraft weight and wing loading (which will be discussed in theory of flight). It is important to consider that other kind of contaminations or deteriorations also affect wing Lift and drag in the same way like explained for ice, rime and snow. Some examples of contamination and deterioration are:       Mis-rigging of control surfaces Absence of seals on movable sections Dents on surfaces (bird strikes, accidental ground damages) External repair (doubler) Paint peeling Lack of cleanliness The first 20% of the wing chord is particularly sensitive, because disturbances of air flow passing it can cause separation resulting in early stall. These changes in Lift and drag significantly increase stall speed, reduce controllability and alter flight characteristics. Fig 28a – Aerodynamic Impact of Surface Contamination Figure 28a illustrates some of the aerodynamic impacts of aircraft and wing contamination, severely reducing lift ability by increasing aircraft weight, wing loading and drag. Module 08B ETBN 0492 October 2023 Edition 53 Basic Aerodynamics – Aerodynamics British Airways Global Learning Academy – Basic Aerodynamics Types of Ice And Snow Glaze Ice Glaze/Clear ice is the glassy deposit that forms over the village pond in the depth of winter. On aircraft in flight, glaze ice forms when the aircraft encounters larger water drops in clouds or in freezing rain with the air temperature and the temperature of the airframe below freezing point. It consists of a transparent or opaque coating of ice with a glassy surface and results from the liquid water flowing over the airframe before freezing Glaze ice may be mixed with sleet or snow. It will form in greatest thickness on the leading edges of aerofoils and in reduced thickness as far aft as one half of the chord. Ice formed in this way is dense, tough and sticks closely to the surface of the aircraft. It cannot be easily shaken off and, if it breaks off at all, it comes away in lumps of an appreciable and sometimes dangerous size. Fig 29b- Glaze and Rime Ice Formation Rime Ice Rime ice is brittle and opaque and tends to grow into the airstream. It is formed as the droplets freeze immediately upon impact with the cold wing structure (Figure 29b) The main danger of glaze ice is still aerodynamic, but to this must be added that due to the weight of ice, unequal wing loading and propeller blade vibration. Pack Snow Normally, snow falling on an aircraft in flight does not settle, but if the temperature of the airframe is below freezing point, glaze ice may form from the moisture in the snow. Glaze ice is the most severe and the most dangerous form of ice formation on aircraft. It is formed when the droplets deform and/or flow along the surface prior to freezing (Figure 29b). The icing of the aircraft in such conditions, however, is primarily due to water drops, though snow may subsequently be embedded in the ice so formed. Glaze icing can be more serious to the aircraft than rime, since it tends to run back along the airframe, covering more surface area than rime icing, perhaps flowing onto and adhering to unprotected areas. Glaze icing can be hard to see from inside the aircraft, so that the pilot may be unaware of ice build-up. Module 08B ETBN 0492 October 2023 Edition 54 Basic Aerodynamics – Aerodynamics British Airways Global Learning Academy – Basic Aerodynamics Light Rime Ice Severe Glaze Ice Moderate Mixed Ice Supercooled Large Droplet Ice Fig 29c – Different Types Of Ice Module 08B ETBN 0492 October 2023 Edition 55 Basic Aerodynamics – Aerodynamics