To: hang-gliding Subject: how is flight possible From: Davis Straub <72147.3716@compuserve.com> Date: 26 Nov 93 04:42:41 EST I was pleased to see the recent posting regarding the nature of flight. There is a fundamental question that we all have as pilots. How is it that we are able to fly? Unfortunately, this question has not been answered well, at least IMHO, in the hundred or so aerodynamic texts that I have reviewed, read, or skimmed. Over the last two months I was able to bring two aerodynamists from the Boeing Company to make presentations to the Cloud Base Country Club here in Seattle Washington. They were charged with answering the questions - how do you explain flight to a lay audience of pilots. Martin Withington - speaking also on chipmunk powered flight - stated that the basic principal is that the wing forces the air down. Doug McLean - former world record holder in the penny class rubber band powered flight and touted by Martin as one of the deep thinkers at Boeing - stated that the basic principal is that the wing forces the air down. I hope that this discussion can continue and that we will be able to form a coherent and correct explanation of flight that we can pass along to our comrades at the hang gliding schools. To encourage the quest I include a short version of my best explanation and invite your critique: How is flight possible? - the real short story Davis Straub What are the fundamental physical reasons why a wing (and therefore a hang glider or paraglider) flies? A glider is an efficient device for gathering air molecules and forcing them down. The wing span is long in the direction opposite to the direction of travel in order to gather as many air molecules as possible. The wing is streamlined so that it can hold on to the air molecules and slowly force them down smoothly with as few swirls and eddies as possible breaking out near the surface of the wings. Because the wing can force large numbers of air molecules down, it can slow the gliders descent through the air. Because it presents a low profile to the air molecules as it moves forward, the wings forward progress is not unduly impeded. It therefore has the opportunity to encounter large quantities of air molecules that it will force down. How does a wing force the air down? Air molecules near the upper surface of the wing are pulled down following along the contour of the wing from the top most point on the wing down to the trailing edge. They therefore gain a net downward momentum. Air molecules near the lower surface are pushed downward by the approaching wing and by the air molecules that have already been disturbed by the wing. Wings at higher angles of attack generate greater lift because they are able to move the air molecules a greater vertical distance in a shorter amount of time. This can only continue up to a certain point as angle of attack increases, after which the air molecules are no longer able to be guided gently downward. Why cant I fly a 4x8 sheet of plywood as well as I can fly my glider? Leaving aside issues of control, center of pressure and center of mass, it is because the plywood sheet is not able to smoothly guide the air molecules that it encounters near its top surface down to its trailing edge. Because of its sharp edged nose the air over the top surface of the wing is turbulent and loses much of its downward momentum striking the upper surface of the plywood. To: hang-gliding , Dale Slechta Subject: how is flight possible From: Davis Straub <71603.1057@compuserve.com> Date: 03 Jan 94 00:31:48 EST I had intended to go on to the next step in explaining how flight is possible, i.e. starting from the fact that the wing forces the air near it downward, how does it accomplish this feat, but I had such a good time dealing with the response to my first claim that I thought I would tarry a while and provide further support for it as well as attack Bernoulli. This attack on Bernoulli is not frivolous although I could have great fun at the expense of the poor dead man's supporters. First let's start with Dr. Munk, as in, "The Principles of Aerodynamics" by Max. W. Munk, Ph.D, Dr. Eng. Max is a very unusual aerodynamist in that he writes clearly and with great vigor. As an important pioneer in the development of aerodynamic theory Dr. Munk is well known to all trained aerodynamists. Dr. Munk writes, "An aircraft flying through the air is also supported by the air, propelled by means of the air, and (unfortunately) retarded in its progress by the air." "An airplane would never be pressed upward by the air unless it first pressed the air down. In airplane flight, air must be deflected downward continuously. Fresh, resting, peaceful air is continually waked up from its slumber and set in motion down toward the ground. The air thus disturbed resists that motion, thereby pressing the airplane upward." "Let us consider the air, cubic foot by cubic foot, or pound by pound if you prefer. Find out what velocity component each pound of air had before it came into action, and determine the same component after the action. The air force of that pound, its component in the direction considered, is directly proportional to the product of its weight and velocity change, and inverse to the time or period during which the change took place...The airplane throws the air down;...The airplane is not placing itself on the back of a vortex or hanging itself under a vacuum at the beginning of the flight, like a rider on back of his horse. That is the correct idea; try to impress it into your mind. If you succeed, you have learned half of all aerodynamics and perhaps more than that. Do not feel badly if I have upset your ideas about the vacuum and the vortex." "The wing lifts 'because there is a vacuum on its top' sounds as absurd to me as saying 'an automobile needs no horse because its wheels turn by themselves'." "The wing is carried by the air, and a model vacuum is formed on its top surface because that air reacts to be accelerated downward, and kicks back." "Netiher of these two things, the larger velocity or the smaller pressure (referring to Bernoulli's equation), are the cause of lift, or of each other, but all are different symptoms of the same thing - lift caused by the change of motion of the air." On second thought I will wait a bit for the real onslaught on Bernoulli. To: hang-gliding Subject: Bernoulli From: Davis Straub <72147.3716@compuserve.com> Date: 01 Feb 94 23:25:25 EST Cc: "Raymond H. Kraft" , Martin Withington Bernoulli don't know lift and drag It is quite a burden that we have placed on this poor Swiss mathematician's shoulders - the explanation of the forces of flight. Actually the equation named for him is attributed to another Swiss mathematician of the seventeen hundreds, Leonhard Euler (for a more complete story see "Introduction to Flight " , John D. Anderson pages 154-156). And neither Euler nor Bernoulli applied it to the study of flight. Indulge me by letting me start off with a brazen statement: Bernoulli's equation determines that the lift and drag of a wing is zero. Therefore Bernoulli's equation and the phenomena that are said to arise as a consequence of the correctness of Bernoulli's equation cannot account for the aerodynamic forces (lift and drag). Don't believe me? I suggest that you check out "Fundamentals of Flight", Richard S. Shevell, Stanford University, 1985, page 123, or Kuethe and Chow, "Foundations of Aerodynamics: Bases for Aerodynamic Design", 1986, page 86. This s fact is given a name, "D'Alembert's paradox." Now let me step back a bit, and state that Bernoulli's equation is applied to "perfect" fluids, fluids that don't have any friction, inviscid fluids. When hydrodynamic theory (potential flow theory) is applied to air foils assuming that the air is an inviscid fluid, the resulting streamlines show a stall point on the upper surface of the wing. Applying Bernoulli's equation to these streamlines results in a calculation of zero lift and zero drag. As this is clearly not the case, this combination of hydrodynamic theory, Bernoulli's equation and the requirement of an inviscid fluid are not adequate to form the basis of a theory of flight. Bernoulli's equation is just an expression of the relationship between the pressure and velocity of an inviscid fluid. It is usually introduced in aerodynamics texts after the streamlines around a cylinder or airfoil are empirically illustrated to show how lift could be generated given these streamlines. It is not an explanation or theory as to why those streamlines ar e where they are. This is my first indictment against the use of Bernoulli's equation as an explanation. It is not sufficient to determine why the air flows where it does. My second indictment is that it is not necessary. If one has a theory of flight (say Navier-Stokes equations or Euler's equations combined with the Kutta condition, see below) which determines the position and velocity of the streamlines around an air foil, then you have determined everything that you need to know. You don't need the extra baggage of Bernoulli's equation (although it can be used quite conveniently to calculate the pressure distributuon on an air foil and therefore its lift). My third indictment is simply that the condition necessary to use Bernoulli's equation, inviscid air, is contradicted by the condition necessary to determine the streamlines around an airfoil, the Kutta Condition, which requires viscous flow. Re the Kutta condition, Kuethe and Chow state: "A body with a sharp trailing edge in motion through a fluid creates about itself a circulation of sufficient strength to hold the rear stagnation point at the trailing edge," and the "circulation is fixed by the imposition of an empirical observation." (page 86) Both these statements are quite unsatisfying from a theoretical point of view. They basically state that we don't know why lift and drag are generated, but they are, so let's get on with the work of calculating their strength (i.e. engineering and forget physics). It is possible to get around the basic contradiction above by stating that the air for the most part is invsicid and Bernoulli's equation can be applied with good accuracy in the most important range of angle of attack, i.e. not near the stall point, as long as we assume streamlines as determined from potential flow theory and the imposition of circulation required to get the stagnation point t o the trailing edge. To summarize - It is my feeling that using Bernoulli's equation to explain why a wing produces lift and drag is inappropriate. It is much more important to explain why the air flows where and at what speed that it does and once this is accomplished one may use either Bernoulli or Newton to explain why this flow creates lift and drag.