![]() Very high speed aircraft will also be equipped with a Mach indicator since Mach number is a more relevant measure of aircraft speed at and above the speed of sound. Gamma is the ratio of specific heats (Cp/Cv) for air. Where a sl = speed of sound at sea level and ρ SL = pressure at sea level. The airspeed indication system of high speed aircraft must be calibrated on a more complicated basis which includes the speed of sound: In theory, compressibility effects must be considered at Mach numbers above 0.3 however, in reality, the above equations can be used without significant error to Mach numbers of 0.6 to 0.7. It should be noted that the equations above assume incompressible flow and are not accurate at speeds where compressibility effects are significant. V I N D = V e = V S L = 2 ( P 0 − P ) ρ S L In this text we will assume that such errors can indeed be neglected and the term indicated airspeed will be used interchangeably with sea level equivalent airspeed. In dealing with aircraft it is customary to refer to the sea level equivalent airspeed as the indicated airspeed if any instrument calibration or placement error can be neglected. We discussed both the sea level equivalent airspeed which assumes sea level standard density in finding velocity and the true airspeed which uses the actual atmospheric density. In chapter two we learned how a Pitot‑static tube can be used to measure the difference between the static and total pressure to find the airspeed if the density is either known or assumed. Which can be rearranged to solve for velocity This combination appears as one of the three terms in Bernoulli’s equation In the previous section on dimensional analysis and flow similarity we found that the forces on an aircraft are not functions of speed alone but of a combination of velocity and density which acts as a pressure that we called dynamic pressure. How fast can the plane fly or how slow can it go? How quickly can the aircraft climb? What speed is necessary for lift‑off from the runway? Many of the questions we will have about aircraft performance are related to speed. ![]() ![]() It is also obvious that the forces on an aircraft will be functions of speed and that this is part of both Reynolds number and Mach number. We also know that these parameters will vary as functions of altitude within the atmosphere and we have a model of a standard atmosphere to describe those variations. We know that the forces are dependent on things like atmospheric pressure, density, temperature and viscosity in combinations that become “similarity parameters” such as Reynolds number and Mach number. Now that we have examined the origins of the forces which act on an aircraft in the atmosphere, we need to begin to examine the way these forces interact to determine the performance of the vehicle. Performance in Straight and Level Flight Introduction
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |