If an accumulation chamber is not provided between the fan and the place of measurement, to allow the pressures to equalise, the results may not be totally representative. The problem with such an arrangement is that the airflow through an axial fan varies across its diameter slowest at its centre and fastest at its periphery. The airflow generated in a wind tunnel is usually via an exposed axial fan. This is the reason test-flights tend to produce slightly different performance results to those achieved in wind tunnels. It is reasonable to assume that for all normal situations, relative velocities will be similar around a single airplane (for example) at any instant in time, which may not be the case in a wind tunnel. The aspect ratio (Aᵣ) of an airfoil is the ratio of its span-squared ( Fig 4 s²) to its total area (A): Aᵣ = s²/A Note: the general term 'wing-span' refers to the distance between wing-tips either side of a plane's fuselage. Fighter aircraft will use variations based upon b) to minimise lift effects, giving the plane's thrust and its pilot freedom to determine aircraft behaviour. Transport aircraft tend to use wings similar to example a) due to the greater lift capacity. The airfoil shapes displayed in Fig 4 give some idea as to the differences between the two requirements. In both cases, wings normally taper towards their tip, reducing the bending moments in the wing. When 2 is replaced with ' γ' and 'q' is replaced with 'pₐ' (atmospheric pressure), 'v' will be the speed of sound.īoth designs, however, will adopt a sweep-angle (θ) at the wing's leading edge to reduce drag by Cos(θ). Note: v = √ where 'q' is air pressure differential (δp) and 'ρ' is air density. Where C D is a ratio of lengths so now the units match (see Accuracy below). However, 'q' is actually equivalent to pressure (not equal to it), because their units are not the same. The principal factor in calculating the resultant lift (force) is called the dynamic pressure q = ½.ρ.v² However, for any given airfoil mass (m), there is a minimum velocity below which it will not lift. Therefore, as shown in Fig 1, b), the pressure at the upper surface must be less than the pressure below it, resulting in natural lift, even if the underside angle is zero. It isn't difficult to see that the molecules at the upper surface of this airfoil (side-1) must travel further than those at its underside (side-2) in the same period i.e. To induce a lateral force on a shape, the air velocity over one side of it must be greater than over the other side.įig 1 a), shows a typical airfoil travelling through a column of air in a sequence of positions from 1 to 5. We know from Bernoulli that the velocity of a gas is inversely proportional to its pressure. # The relative velocity of a shape is modified by adding or subtracting 'wind-speed' add for head-wind and subtract for tail-wind. In reality, a shape such as an aircraft wing (airfoil) moves through a stationary # air column, which is not the case in wind tunnels. A typical 'special action' is the deliberate induction of lateral force as a result of longitudinal movement. Manipulating aerodynamics means altering a shape to perform special actions as a result of such movement. Improving aerodynamics simply means altering a shape to enable it to travel through a gas with greater speed whilst requiring less energy to do so. Where v s is the velocity of the shape, and v g is the velocity of the gas in the direction of travel. All we are interested in is the relative movement i.e. our atmosphere) through which it is moving either a stationary shape within a moving gas, or a moving shape within a stationary gas, or a moving shape within a moving gas. In aerodynamics, 'air-speed' refers to the velocity of a shape relative to a gas (e.g. Aerodynamics Calculator (wings & spoilers)Īerodynamics is a term used to describe the performance of a shape travelling within a gas, albeit it more accurately refers to the behaviour of the gas through which a shape is travelling.
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