этапы отбора и ответы. Этапы отбора и ответы. REV2. Этапы отбора в Аэрофлот
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Drag typeshttps://www.skybrary.aero/index.php/Drag Aerodynamicists will tell you that there are really only two sources of drag on an airplane: pressure drag and skin friction drag. That is, an airplane can only “feel” the aerodynamic retarding forces through air pressure (perpendicular to the surface of the airplane) or through skin friction, shearing force acting tangentially to the surface of the airplane. Aerodynamicists will also tell you that several physical mechanisms in the flow field contribute to the pressure drag. Dividing the pressure drag into “components”, according to how much of the drag is caused by each mechanism, is something we do all the time in practice. However, we should keep in mind that this division into components is not exact. Because the different flowfield mechanisms interact in complicated ways, it is not possible to rigorously define how much of the pressure drag is attributable to which mechanism. Pressure Drag The component of drag caused by the pressure distribution over the three-dimensional shape of the airplane is referred to as pressure drag. This term includes several different elements: induced drag, shock drag, and form drag. Pressure drag can be reduced by careful shaping of critical areas such as the cockpit and aft body closure. Components of Drag Here’s a complete list of the components of an airplane’s total drag, in alphabetical order, without showing the estimated contribution of each component to the total drag: • compressibility drag • excrescence drag • form drag • induced drag • interference drag • “other” drag • skin friction drag • trim drag induced drag Induced drag is the inevitable result of the difference in pressures between the lower and upper wing surfaces. The magnitude of a wing’s induced drag is largely influenced by its design. Ideally, a wing will have a large span in order to minimize this component of drag. In practice, however, large spans are problematic for commercial jet transport airplanes because they may result in airport gate compatibility problems. Angle of attack coefficientOne of the first things noticed is the fact that at an angle of attack of 0 °, there is a positive coefficient of lift, and, hence, positive lift. This is the case of most cambered airfoils. One must move to a negative angle of attack to obtain zero lift coefficient (hence zero lift). It will be remembered that this angle is called the angle of zero lift. A symmetric airfoil was shown to have an angle of zero lift equal to 0 ° as might be expected. Notice next that from 0 ° up to about 10 ° or 12 ° the "liftcurve" is almost a straight line. There is a linear increase in the coefficient of lift with angle of attack. Above this angle, however, the lift coefficient reaches a peak and then declines. The angle at which the lift coefficient (or lift) reaches a maximum is called the stall angle. The coefficient of lift at the stall angle is the maximum lift coefficient. Beyond the stall angle, one may state that the airfoil is stalled and a remarkable change in the flow pattern has occurred. Note that below the stall angle, the separation points on the airfoil move forward slowly but remain relatively close to the trailing edge. Near the stall angle the separation points move rapidly forward and the pressure drag rises abruptly. Past the stall angle, the effects of the greatly increased separated flow is to decrease the lift. Induced draghttps://www.skybrary.aero/index.php/Induced_Drag Previously, we have discussed several sources of aerodynamic drag: skin friction drag which results from viscous shear forces, and pressure drag, the result of separated turbulent wakes downstream of aerodynamic shapes. Induced drag sometimes referred to as drag due to lift, because it’s exactly that. Remember that the air pressure on the lower surface of a wing in flight is greater than it is on the upper surface. This results in a flow of air from the lower surface toward the upper surface, around the wing tips, as illustrated here.  That flow from below to above around the tips has an effect on the flow over wings. It induces a spanwise component of the flow direction: toward the wingtips on the lower surface, toward the wing root on the upper surface.  If you could visualize the airflow over the wings of an airplane in flight when looking at the airplane from directly ahead of it or behind it, you would see something like this:  Cause of induced drag. Aerodynamicists have several ways of describing the reason for induced drag. There is considerable energy in the wingtip vortices of an airplane. This is an energy loss to the airplane, which must be compensated for by added thrust. Hence it’s considered to be one component of the airplane’s total drag. The induced drag can be calculated from the following equation:  Where: L is the lift e is the “span efficiency factor” q is the dynamic pressure b is the wingspan The “span efficiency factor” is a function of planform. Elliptical wings have an e value of 1.00; for other planforms, e varies from 0.85 to 0.95. Note also that the induced drag is a function of the square of the lift. Notice also that the induced drag is an inverse function of the square of the wingspan, thus explaining why high-performance airplanes such as sailplanes typically have long wingspans. To express the induced drag in a dimensionless coefficient form, you need to recall that:  and that:   Playing around with the equations then will yield:  Equation 4 shows that induced drag is inversely affected by aspect ratio, and that therefore the best thing to do to achieve a wing having low induced drag is to maximize the aspect ratio. That’s true, but the increase of aspect ratio should be accomplished without a decrease in the wing area. Simply reducing the wing chord would increase the aspect ratio (good) but failing to increase the span at the same time so as to hold the same wing area will result in an increase in the lift coefficient (bad). Wingtip Treatments Аny real wing (as opposed to “two-dimensional wings” as may be used in wind tunnel testing of airfoil characteristics) will inevitably have some loss of efficiency because of the airflow pattern at the wingtips. The wingtip vortex condition represents a substantial loss of energy, equivalent to an increase of drag; as you now know, this is referred to as “induced drag”. Clearly, anything which can be done to reduce the intensity of the wingtip losses will ultimately translate into improved fuel efficiency. Since induced drag accounts for approximately 40 to 45 percent of total cruise drag, any reduction can obviously produce substantial fuel savings. Winglets and other wingtip treatments are designed and installed for the sole purpose of reducing wingtip energy losses. Probably the most frequently seen wingtip treatment is the addition of a winglet. The 767-400 has taken a different approach: instead of winglets, that airplane utilizes a raked wingtip. A winglet is, in simple terms, a small vertical airfoil attached to the tip of an airplane’s wing, and oriented at an angle to the longitudinal axis of the airplane so that it presents a slight lateral angle of attack to the airflow. The function of a winglet is to reduce the strength of the wingtip vortex, to redistribute the lift across the wing, and thereby to decrease the wing’s induced drag component. The addition of a winglet to an existing wing, however, is not without some penalty. First of all, the winglet adds weight, and it also adds skin surface area. Raked means that the wingtip is extended but the extension is raked back at a sweepback angle greater than that of the wing. This is not a winglet, simply a wingtip extension with a greater angle, but the goal is the same: reduction of the wingtip vortex and a redistribution of lift across the wing. The structural weight implications of a raked wingtip are less than for a winglet. The wingtips extend the airplane’s wingspan by approximately fifteen feet. The raked wingtip has been demonstrated to provide a cruise drag reduction of approximately five and a half percent. |