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36-469: For many applications the Clark Y has been an adequate airfoil section; it gives reasonable overall performance in respect of its lift-to-drag ratio , and has gentle and relatively benign stall characteristics. The flat lower surface is not optimal from an aerodynamic perspective, and it is rarely used in modern designs. The Clark YH airfoil is similar but with a reflexed (turned up) trailing edge producing

72-591: A built-in flight computer that automatically prevents the aircraft from increasing the angle of attack any further when a maximum angle of attack is reached, regardless of pilot input. This is called the 'angle of attack limiter' or 'alpha limiter'. Modern airliners that have fly-by-wire technology avoid the critical angle of attack by means of software in the computer systems that govern the flight control surfaces. In takeoff and landing operations from short runways ( STOL ), such as Naval Aircraft Carrier operations and STOL backcountry flying, aircraft may be equipped with

108-445: A fixed wing aircraft are wingspan and total wetted area . One method for estimating the zero-lift drag coefficient of an aircraft is the equivalent skin-friction method. For a well designed aircraft, zero-lift drag (or parasite drag) is mostly made up of skin friction drag plus a small percentage of pressure drag caused by flow separation. The method uses the equation where C fe {\displaystyle C_{\text{fe}}}

144-448: A given flightpath, so that doubling the L/D ratio will require only half of the energy for the same distance travelled. This results directly in better fuel economy . The L/D ratio can also be used for water craft and land vehicles. The L/D ratios for hydrofoil boats and displacement craft are determined similarly to aircraft. Lift can be created when an aerofoil-shaped body travels through

180-479: A glider it determines the glide ratio , of distance travelled against loss of height. The term is calculated for any particular airspeed by measuring the lift generated, then dividing by the drag at that speed. These vary with speed, so the results are typically plotted on a 2-dimensional graph. In almost all cases the graph forms a U-shape, due to the two main components of drag. The L/D may be calculated using computational fluid dynamics or computer simulation . It

216-422: A high angle of attack and a gentle stall are also important. As the aircraft fuselage and control surfaces will also add drag and possibly some lift, it is fair to consider the L/D of the aircraft as a whole. The glide ratio , which is the ratio of an (unpowered) aircraft's forward motion to its descent, is (when flown at constant speed) numerically equal to the aircraft's L/D. This is especially of interest in

252-533: A more positive pitching moment reducing the horizontal tail load required to trim an aircraft. The Lockheed Vega and Spirit of St. Louis are two of the better known aircraft using the Clark Y profile, while the Ilyushin Il-2 and Hawker Hurricane are examples of mass-produced users of the Clark YH. The Northrop Tacit Blue stealth technology demonstrator aircraft also used a Clark Y. The Clark Y

288-418: A particular airspeed . The airspeed at which the aircraft stalls varies with the weight of the aircraft, the load factor , the center of gravity of the aircraft and other factors. However, the aircraft normally stalls at the same critical angle of attack, unless icing conditions prevail. The critical or stalling angle of attack is typically around 15° - 18° for many airfoils. Some aircraft are equipped with

324-410: A viscous fluid such as air. The aerofoil is often cambered and/or set at an angle of attack to the airflow. The lift then increases as the square of the airspeed. Whenever an aerodynamic body generates lift, this also creates lift-induced drag or induced drag. At low speeds an aircraft has to generate lift with a higher angle of attack , which results in a greater induced drag. This term dominates

360-435: Is also influenced by the wing shape, including its airfoil section and wing planform . A swept wing has a lower, flatter curve with a higher critical angle. The critical angle of attack is the angle of attack which produces the maximum lift coefficient. This is also called the " stall angle of attack". Below the critical angle of attack, as the angle of attack decreases, the lift coefficient decreases. Conversely, above

396-587: Is associated with increasing lift coefficient up to the maximum lift coefficient, after which lift coefficient decreases. As the angle of attack of a fixed-wing aircraft increases, separation of the airflow from the upper surface of the wing becomes more pronounced, leading to a reduction in the rate of increase of the lift coefficient. The figure shows a typical curve for a cambered straight wing. Cambered airfoils are curved such that they generate some lift at small negative angles of attack. A symmetrical wing has zero lift at 0 degrees angle of attack. The lift curve

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432-401: Is measured empirically by testing in a wind tunnel or in free flight test . The L/D ratio is affected by both the form drag of the body and by the induced drag associated with creating a lifting force. It depends principally on the lift and drag coefficients, angle of attack to the airflow and the wing aspect ratio . The L/D ratio is inversely proportional to the energy required for

468-411: Is more separated and the airfoil or wing is producing its maximum lift coefficient. As the angle of attack increases further, the upper surface flow becomes more fully separated and the lift coefficient reduces further. Above this critical angle of attack, the aircraft is said to be in a stall. A fixed-wing aircraft by definition is stalled at or above the critical angle of attack rather than at or below

504-403: Is moving. Angle of attack is the angle between the body's reference line and the oncoming flow. This article focuses on the most common application, the angle of attack of a wing or airfoil moving through air. In aerodynamics , angle of attack specifies the angle between the chord line of the wing of a fixed-wing aircraft and the vector representing the relative motion between the aircraft and

540-456: Is only indirectly related to stall behavior. Some military aircraft are able to achieve controlled flight at very high angles of attack, but at the cost of massive induced drag . This provides the aircraft with great agility. A famous example is Pugachev's Cobra . Although the aircraft experiences high angles of attack throughout the maneuver, the aircraft is not capable of either aerodynamic directional control or maintaining level flight until

576-473: Is the Mach number. Windtunnel tests have shown this to be approximately accurate. Angle of attack In fluid dynamics , angle of attack ( AOA , α , or α {\displaystyle \alpha } ) is the angle between a reference line on a body (often the chord line of an airfoil ) and the vector representing the relative motion between the body and the fluid through which it

612-435: Is the equivalent skin friction coefficient, S wet {\displaystyle S_{\text{wet}}} is the wetted area and S ref {\displaystyle S_{\text{ref}}} is the wing reference area. The equivalent skin friction coefficient accounts for both separation drag and skin friction drag and is a fairly consistent value for aircraft types of the same class. Substituting this into

648-550: Is wingspan. The term b 2 / S wet {\displaystyle b^{2}/S_{\text{wet}}} is known as the wetted aspect ratio. The equation demonstrates the importance of wetted aspect ratio in achieving an aerodynamically efficient design. At supersonic speeds L/D values are lower. Concorde had a lift/drag ratio of about 7 at Mach 2, whereas a 747 has about 17 at about mach 0.85. Dietrich Küchemann developed an empirical relationship for predicting L/D ratio for high Mach numbers: where M

684-462: The lift and drag coefficients C L and C D . The varying ratio of lift to drag with AoA is often plotted in terms of these coefficients. For any given value of lift, the AoA varies with speed. Graphs of C L and C D vs. speed are referred to as drag curves . Speed is shown increasing from left to right. The lift/drag ratio is given by the slope from the origin to some point on the curve and so

720-445: The lift-to-drag ratio (or L/D ratio ) is the lift generated by an aerodynamic body such as an aerofoil or aircraft, divided by the aerodynamic drag caused by moving through air. It describes the aerodynamic efficiency under given flight conditions. The L/D ratio for any given body will vary according to these flight conditions. For an aerofoil wing or powered aircraft, the L/D is specified when in straight and level flight. For

756-426: The span efficiency factor , a number less than but close to unity for long, straight-edged wings, and C D , 0 {\displaystyle C_{D,0}} the zero-lift drag coefficient . Most importantly, the maximum lift-to-drag ratio is independent of the weight of the aircraft, the area of the wing, or the wing loading. It can be shown that two main drivers of maximum lift-to-drag ratio for

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792-477: The zero lift axis where, by definition, zero angle of attack corresponds to zero coefficient of lift . Some British authors have used the term angle of incidence instead of angle of attack. However, this can lead to confusion with the term riggers' angle of incidence meaning the angle between the chord of an airfoil and some fixed datum in the airplane. The lift coefficient of a fixed-wing aircraft varies with angle of attack. Increasing angle of attack

828-403: The aircraft of speed very quickly due to induced drag, and, in extreme cases, increased frontal area and parasitic drag. Not only do such maneuvers slow the aircraft down, but they cause significant structural stress at high speed. Modern flight control systems tend to limit a fighter's angle of attack to well below its maximum aerodynamic limit. In sailing , the physical principles involved are

864-579: The angle of attack or Lift Reserve Indicators . These indicators measure the angle of attack (AOA) or the Potential of Wing Lift (POWL, or Lift Reserve) directly and help the pilot fly close to the stalling point with greater precision. STOL operations require the aircraft to be able to operate close to the critical angle of attack during landings and at the best angle of climb during takeoffs. Angle of attack indicators are used by pilots for maximum performance during these maneuvers, since airspeed information

900-408: The atmosphere. Since a wing can have twist, a chord line of the whole wing may not be definable, so an alternate reference line is simply defined. Often, the chord line of the root of the wing is chosen as the reference line. Another choice is to use a horizontal line on the fuselage as the reference line (and also as the longitudinal axis). Some authors do not use an arbitrary chord line but use

936-531: The construction of wings on plans mounted on a flat construction board. Inexperienced modellers are more readily able to build model aircraft which provide a good flight performance with benign stalling characteristics. An inverted Clark Y airfoil was used on the spoilers of the Dodge Charger Daytona and Plymouth Superbird . Some of the better-known aircraft that use the Clark Y and YH: Lift-to-drag ratio In aerodynamics ,

972-401: The critical angle of attack, as the angle of attack increases, the air begins to flow less smoothly over the upper surface of the airfoil and begins to separate from the upper surface. On most airfoil shapes, as the angle of attack increases, the upper surface separation point of the flow moves from the trailing edge towards the leading edge. At the critical angle of attack, upper surface flow

1008-527: The design and operation of high performance sailplanes , which can have glide ratios almost 60 to 1 (60 units of distance forward for each unit of descent) in the best cases, but with 30:1 being considered good performance for general recreational use. Achieving a glider's best L/D in practice requires precise control of airspeed and smooth and restrained operation of the controls to reduce drag from deflected control surfaces. In zero wind conditions, L/D will equal distance traveled divided by altitude lost. Achieving

1044-562: The equation for maximum lift-to-drag ratio, along with the equation for aspect ratio ( b 2 / S ref {\displaystyle b^{2}/S_{\text{ref}}} ), yields the equation ( L / D ) max = 1 2 π ε C fe b 2 S wet , {\displaystyle (L/D)_{\text{max}}={\frac {1}{2}}{\sqrt {{\frac {\pi \varepsilon }{C_{\text{fe}}}}{\frac {b^{2}}{S_{\text{wet}}}}}},} where b

1080-407: The low density of air in the upper atmosphere as well as at low speed at low altitude where the margin between level flight AoA and stall AoA is reduced. The high AoA capability of the aircraft provides a buffer for the pilot that makes stalling the airplane (which occurs when critical AoA is exceeded) more difficult. However, military aircraft usually do not obtain such high alpha in combat, as it robs

1116-632: The low-speed side of the graph of lift versus velocity. Form drag is caused by movement of the body through air. This type of drag, known also as air resistance or profile drag varies with the square of speed (see drag equation ). For this reason profile drag is more pronounced at greater speeds, forming the right side of the lift/velocity graph's U shape. Profile drag is lowered primarily by streamlining and reducing cross section. The total drag on any aerodynamic body thus has two components, induced drag and form drag. The rates of change of lift and drag with angle of attack (AoA) are called respectively

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1152-513: The maneuver ends. The Cobra is an example of supermaneuvering as the aircraft's wings are well beyond the critical angle of attack for most of the maneuver. Additional aerodynamic surfaces known as "high-lift devices" including leading edge wing root extensions allow fighter aircraft much greater flyable 'true' alpha, up to over 45°, compared to about 20° for aircraft without these devices. This can be helpful at high altitudes where even slight maneuvering may require high angles of attack due to

1188-451: The maximum L/D is not dependent on weight or wing loading, but with greater wing loading the maximum L/D occurs at a faster airspeed. Also, the faster airspeed means the aircraft will fly at greater Reynolds number and this will usually bring about a lower zero-lift drag coefficient . Mathematically, the maximum lift-to-drag ratio can be estimated as where AR is the aspect ratio , ε {\displaystyle \varepsilon }

1224-444: The maximum L/D ratio does not occur at the point of least drag coefficient, the leftmost point. Instead, it occurs at a slightly greater speed. Designers will typically select a wing design which produces an L/D peak at the chosen cruising speed for a powered fixed-wing aircraft, thereby maximizing economy. Like all things in aeronautical engineering , the lift-to-drag ratio is not the only consideration for wing design. Performance at

1260-442: The maximum distance for altitude lost in wind conditions requires further modification of the best airspeed, as does alternating cruising and thermaling. To achieve high speed across country, glider pilots anticipating strong thermals often load their gliders (sailplanes) with water ballast : the increased wing loading means optimum glide ratio at greater airspeed, but at the cost of climbing more slowly in thermals. As noted below,

1296-428: Was chosen as its flat bottom worked well with the design goal of a low radar cross-section. The Clark Y has found favor for the construction of model aircraft, thanks to the flight performance that the section offers at medium Reynolds number airflows. Applications range from free-flight gliders through to multi-engined radio control scale models. The Clark Y is appealing for its near-flat lower surface, which aids in

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