High Speed Flight (Essay Sample)

HIGH-SPEED FLIGHT
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Introduction
The concept of high speed like many other things in life is not absolute. Gabrielli and Von Karman pointed out the difficulty of measuring the value of speed for a considerable number of airborne and ground systems (1950). One result of their analysis is a chart speed versus power required, in a double logarithm plot (the Gabrielli-von Karman chart). It was found that all systems lie above this curve and at their most efficient points the vehicles touch this line. More recently, Lorentz developed a general power scaling technique for airplanes and Helicopters. Manufacturers quote a number of different aircraft speeds. The ground speed is the aircraft speed measured with respect to a fixed point on the ground. The Never-to-exceed speed (VNE) is determined by the structural limits of the plane. For a given aircraft and a particular gross weight, this rate depends on the flight altitude. The dash is a supersonic speed that can be maintained for a relatively short time (or distance), enough to escape a dangerous theater. After this time, the engines may overheat, and fuel consumption cannot be afforded because of the operation with after burning. A supersonic jet aircraft is said to be supercritical at all altitudes if it can maintain supersonic level flight at sea level and above. Supercruise is the ability to fly at supersonic speeds without afterburning thrust.
Aspect Ratio
Very high-speed flight does not favor the use of high aspect ratio planforms. Low aspect ratio platforms have structural advantages and allow the use of thin, low drag section for high-speed flight. The aerodynamics of transonic flight also favor short span, low aspect ratio surfaces. Thus, modern configuration of an airplane designed for high-speed flight will have a little aspect ratio planform with characteristic aspect ratios of two to four. The most significant impression that should result is that the typical modern configuration will have high angles of attack for maximum lift and very prodigious drag due to lift at low flight speeds. The fact is of importance to the Naval Aviator because the majority of pilot-caused accidents occur during this regime of flight during takeoff, approach and landing. Induced drag predominates in these regimes of flight.
The modern configuration of high-speed airplane usually has a small aspect ratio planform with high wing loading. When wing sweepback is coupled with low aspect ratio, the wing lift curve has distinct curvature and is very flat at high angles of attack. The drag curve shows extremely rapid rise at high lift coefficients since the drag due to lift is enormous. These effects produce flying qualities that are distinctly different from a more "conventional" high aspect ratio airplane configuration.
Important Ramification of Modern High-Speed Configurations
During a takeoff where the airplane must not be over-rotated to an excessive angle of attack. Any given aircraft will have some fixed angle of attack, which produces the best takeoff performance, and this angle of attack will not vary with weight, density, altitude, or temperature. An excessive angle of attack produces additional induced drag and may have an undesirable effect on takeoff performance. Take off acceleration may be severely reduced, and a significant increase in takeoff distance may occur. The initial climb performance may be marginal at an excessively low airspeed. There are modern configurations of airplanes of very low aspect ratio (plus sweepback) which cannot fly out of ground effect. With the more conventional aircraft configuration, an excess angle of attack has no sharply defined stall but develops as an excessive amount of induced drag. To be sure that it will not go unsaid, an extreme small angle of attack on takeoff creates its problems. Excess takeoff speed and distance are critical tire loads.
During an approach where the pilot must exercise proper technique to control the flight path. "Attitude plus power equals performance." The modern high-speed configuration at low speeds will have small lifting ratios due to the high induced drag and can require relatively high power settings during the power approach. If the pilot interprets that his airplane is below the desired glide path, the first reaction must not be just to ease the nose up. An increase in angle of attack without a rise in power will lower the airspeed and significantly increase the induced drag. Such a reaction could create a high rate of descent and lead to very undesirable consequences. The angle of attack indicator coupled with the mirror landing systems provides references to the pilot and emphasizes that during the steady approach "angle of attack is the primary control of airspeed and power under the central control of rate of climb or descent." Steep turns during approach at low speed are always undesirable in any airplane because of the increased stall speed and induced drag. Steep turns at low airspeeds in a small aspect ratio aircraft can create extremely high induced drag and can incur dangerous sink rates.
During the landing phase where an excessive angle of attack (or excessive low airspeed) would create high induced drag and a high power setting of control the rate of descent. A common error in the technique of landing modern configurations is a steep little power approach to landing. The steep flight path requires considerable maneuver to glare the airplane for tough down and necessities a definite increase in angle of attack. Since the maneuver of the flare is a transient condition, the variation of both lift and drag with angle of attack must be considered. The lift and drag curves for a high aspect ratio wing. The high aspect ratio is usually capable of flare without unusual results. The increase in angle of attack at flare provides the increase in lift to change the flight path direction without changes in drag to decelerate the airplane.
Effect of Tapper and Sweepback
The wing's aspect ratio, is the primary factor in determining the three-dimensional characteristics of the ordinary wing a...