Subject: Re: Newton From: "John R. Johnson" Date: 1996/08/26 Message-Id: References: <321CEA59.58F8@calmvs.ercb.gov.ab.ca>, <4vkh2k\$22g@newsbf02.news.aol.com> X-Sender: john@reliant Content-Type: TEXT/PLAIN; charset=US-ASCII Organization: Southern Illinois University - Carbondale Mime-Version: 1.0 Newsgroups: rec.aviation.student There is a great deal of confusion about how lift is generated. There is an particularly significant confusion about what "Newton" means when contrasted with "Bernoulli." Let me first give you a little background about me. I am 58 years old. I started flying when I was a teenager in the early fifties. I got my first real pilots license in 1956 while I was a student in Aeronautical Engineering at the University of Minnesota in Minneapolis, Minnesota. I have designed several airplanes, at least one of which has been built in fairly large numbers. I did the aerodynamic work for LaMar Steen on the Skybolt. LaMar did the structures work. Now, definitions first: Newton Explanation: This is an explanation of lift using the basic action/reaction principles of Newtonian Physics. These explanations are quite correct and do explain the lift produced by a wing or the thrust produced by a propellor. Both of these problems are similar in all their essentials, differing only is the more esoteric elements like interference from previous passages, and local velocities through the air. Bernoulli Explanation: This is an explanation of lift using the energy transer concepts of Bernoulli. He stated that the energy content of a packet of gas is constant unless energy is either added or removed from the packet. The energy can be either dynamic or potential ( due to static pressure ) and the sum is constant. This says then, that if we speed the packet up, the static pressure must decrease accordingly. This effect can be observed and does indeed happen. The lift produced by a wing is a product of differences in pressure created between the upper surface and the lower surface of the wing. The higher pressure below the surface tends to force the wing in the direction of the lower pressure above the surface creating "lift." In doing so, some air is forced downward creating the "reaction" to the "action" of lift. The quantity of air forced downward and its resulting velocity is such that the product of the mass and accelleration of the air forced down exactly equals the product of the mass and accelleration of the aircraft upwards. In level flight, this accelleration of the airplane exactly balances the accelleration downward usually called "gravity." We will defer for elsewhere the highly esoteric argument about the existence of a "force of gravity" and such other topics that would only add to the extant confusion. The real confusion comes about when we try to explain WHY the pressure is lower on the upper surface. There is no question but that it is lower. For low angles of attack the pressure difference is approximately linear with changes in angle of attack. The energy required to create this difference in pressure is considerable and increases as the angle of attack and the resulting pressure difference increases. This energy required to produce the lift we desire appears to the airplane as a source of "drag." We call this "drag" due to the production of "lift" by the term "induced drag.' This is to distinguish it from the "profile or parasite drag" which is the "drag" that results from what is essentially the "friction" of the air with the surfaces of the aircraft. Since "induced drag" depends on the pressure difference between the surfaces and total "lift" is a function of this local pressure difference and the area of the lifting surface and the "velocity" through the air, "induced drag" gets smaller as we go faster. This happens because, when we go faster we accellerate more air downward in the same time. Since the mass times the accelleration of the air is a constant equal to the mass of the aircraft time gravity, moving more air means the mass increases so the accelleration can decrease to give the same lift! Whew ... When the accelleration of the air downward decreases, we can reduce the pressure difference for the same lift. We do this by decreasing the "angle of attack." This is controlled by the fore and aft movement of the control. That is the reason we say "pitch determines speed." Every "angle of attack" has a unique "velocity" where the resulting "lift" is correct. The "Bernoulli Effect" is usually utilized to explain the lower pressure on the upper surface. This explanation serves for some people and does not serve so well for others. The explanation does NOT change what is going on around the wing. It is merely an attempt to describe it in a way you can visualize. We are describing basic physical principles that obey natural laws. These laws do NOT depend on our understanding of them. All explanations are equally valid, although some may come closer to the actual underlying reality! The basic "bernoulli" explanation says that the upper surface of the airfoil is longer than the lower. Packets that were adjacent in front of the wing want to be adjacent behind the wing also, so the ones going over the top of the wing have to hurry to catch up with their friends passing below the wing. This increases the dynamic pressure and caused the static pressure to decrease, making the pressure lower above the wing and generating "lift." I have never been able to understand why these packets of air have such an affinity for each other that they speed up on their own to lower the pressure, so I personally favor the "Newton" explanation for lift. In the "Newton" explanation you can visualize the wing as a surface moving through the air. If you incline the surface ( give it an angle of attack ) it will push some air out of the way as it goes along. The air beneath the surface is pushed together as the surface moves downward in its passage. This increases the pressure beneath the surface. Above the surface something very different is happening. The surface is moving away from the air that was there making an empty area. With no air in the area the static pressure is extremely low. Immediately air molecules start to be pushed toward the top of the wing because of the low pressure in the hole, and the relatively high pressure of the undisturbed air above the wing. The pressure never gets all the way back to ambient because of the pressure difference that must exist to cause air particles to accellerate toward the void left by the surface moving downward. Of course, this accelleration toward the hole left by the wing applies in ALL directions so the wing is also accellerated upward by this resulting pressure difference. This says that the pressure observed depends on how much the air molecules have to be accellerated to reach the vicinity of the top of the surface. This would indicate that the pressure would be lowest where the curvature of the upper surface is the greatest. This is indeed observed when the pressure distribution is measured around a surface in the wind tunnel. All of the air accellerated downward to fill the hole left behind when the wing moves forward continues downward after it has passed and makes its contribution to the total mass and accelleration of the air that is the reaction that matches and balances our "action" of lift. The greater the angle of attack, the more accelleration has to be imparted to the air molecules to stay close to the surface, to the greater the difference in pressure required to provide this accelleration, and the greater the lift for any given surface area. Also, the greater the induced drag produced by the forces required to create the hole in the air in the first place. This explanation is somewhat simplified and much less quantitive and technical than the explanation from the courses I used to teach. I hope it is simple and NOT simplistic. John