OTHER MISC. MESSAGES ABOUT BERNOULLI/NEWTON LIFTING FORCE EXPLANATION Date: Tue, 30 Dec 1997 17:37:07 -0000 From: basil.scott@octel.com To: billbeskimo.com Subject: Bernoulli versus Reaction Theories of Flight Hi, Bill Just discovered your interesting web page with long correspondence concerning Bernouilli vs. Reaction explanations of flight theory. Discovered it via the Bad Science web page. Just a few observations. A. I was spoon fed the Bernouilli explanation all through my physics education (physics Bachelor's degree from Univ of Liverpool, England in 1966), including when studing Theory of Flight in the air force cadets. The reaction theory was never mentioned! I realised soon after I started reading the correspondence how prejudiced I was - and all my tutors clearly also had been - about it. Upside down flight (kind of subliminally) never quite seemed to fit into it, but I never questioned it properly, not even mentally to myself. Oh, the power and influence of prejudice! B. Rather ironically, the explanation of why a sailboat sail can drive a boat to windward used to be given as the "reaction" version - known as "the wedge effect". Then latterly, the influence of Bernouilli came in and said "No, the foresail acts like a wing flap, and the mainsail is like the wing - it's the same way an aircaraft flies!" Ho, hum! C. I wonder how much lift does a flat board experience when moved through an air mass at an angle - i.e. the limiting case of a thin aerofoil? (I can't do the maths, but maybe someone out there can, or already has done.) I guess for this case one can discount Bernouilli completely. Or can one? C. Nobody has mentioned kites! Long before the Wright brothers, people (back as far as Leonardo da Vinci at least) were experimenting with them, especially so in the 19th century right up until Kitty Hawke. There were no - or very few - curved aerofoils involved then. I remember being fascinated as a child by the good old box kite. All flat surfaces, yet they always flew well! Where is Bernouilli there? How do they relate to plane wings? D. What about those aerofoils I've seen where both surfaces curve in the same direction, looking a bit like convex-meniscus lenses? What does Bernouilli say about them, I wonder?! I believe aeroplane flaps are shaped like them! E. All this leads me to the following position: Maybe we are in the same position as regards really understanding flight as the Egyptians were about Pythagoras. They certainly knew *some* right-angled triangles worked - 3,4,5, and 5, 12, 13 etc., else how did they build the pyramids so true (very accurate right angles!). But they could never prove it for *all* such triangles. Maybe there's a "flight-Pythagoras" somewhere out there waiting to discover some real true unifying explanation? Meanwhile, we can still build our "pyramids-which-fly" from empirical knowledge alone. I'm not sure how to get this on to your web page, but if you think it appropriate, maybe you can add it. Happy New Year. Bas Scott #UM Test UK Lucent Technologies-OMD 3 Furzeground Way Stockley Park, Uxbridge, Middlesex UB11 1QA E-mail: Basil.Scott@octel.com (temporary only) or basilscott@lucent.com Phone: +44 (0)181 867 4016 Date: Fri, 7 Jun 1996 07:29:41 -0700 (PDT) From: William Beaty To: Martyn Harrison Subject: Re: Wings and things. On Fri, 7 Jun 1996, Martyn Harrison wrote: > Appreciative as I am for the misconceptions concept, I think maybe I do > not understand the one about the wings. Seems to me that when I was > doing aeronautics at school, we did some experiments on this, and it's > not merely displaced air "causing" the wing to lift. Hmm, how do I > explain my objection... > > Ok, take two large, ocean-going vessels. They obviously have about equal > shaped sides, port and starboard. Have them sail a parallel course with > neutral rudder, and if they are close enough, they will become sucked > together and collide. Don't take my word for it - oil tankers are > becoming increasingly cheap, especially if you don't worry too much > about safety andsuchlike. You're completely right on this one. When two surfaces are involved, the forces must be explained by the Bernoulli Effect. The force expressed upon one surface has its complementary reaction force expressed upon the other surface, and even though the fluid stream departs on a path parallel to the direction in which it entered the system, conservation of momentum is preserved. This arises in many other situations too: trying to blow a card off a spool, trying to blow a pingpong ball out of a funnel, blowing between two pieces of paper, constriction forces arising when air is forced through a venturi, etc. There is a fallaceous thought experiment associated with these above and with wings. It goes something like this: When air is forced through a venturi, it speeds up and the pressure exerted on the inner surface of the venturi drops. To imagine the operation of aircraft wings, merely imagine the venturi pipe to be cut in half down the middle. The remaining half is an airfoil, with the curved side upward, flat side down, and lower pressure on the top surface creating the lift. ___________________________________________ |_____ _________| AIR : \___ ______----- : -----> : --_____------ : : : FLOW : _____ : : ___-- ------______ : :_____/ -----________: |___________________________________________| Venturi This sounds completely sensible, no? But it is wrong. In the above venturi, the pressure difference across the lower half causes a net upward force. By newton's 3rd law, there must be an equal downward force paired with this. And this of course is the force expressed upon the upper half of the pipe. If we slice the pipe down the middle and remove the upper half, then there is no physical object to couple to the other half of the force. In short, CUTTING THE VENTURI IN HALF IS A VERY SIGNIFICANT CHANGE IN THE SITUATION. Half a venturi is NOT a wing, the physics of a wing are significantly different. When air is directed over a wing, the other half of the force pair is expressed upon the air itself. If half of the venturi depicted above is placed in horizontally flowing air, then there will be forces pulling upward on the venturi section and pulling downward upon the air. This causes both to move. Or, from a more familiar "everyday" standpoint, the venturi section deflects the air downwards, and as a consequence it is forced to move upwards. Look closely at the right half of the lower venturi section. Yes, the bottom surface is flat, but doesn't the whole thing have an overall downward tilt? When air travelling along the top and bottom surfaces comes together, won't the combined flow after the trailing edge have a downward tilt? Yes, exactly. The leading edge of the venturi fools us into not noticing that the trailing edge is tilted downwards, and that the venturi section as a whole should deflect air downwards: ___ ---___ ---___ THIS ONE'S TILTED ---___ _______________________/ ------______ ------______ ______| THIS ONE ISN'T ______------ ------ ------ From the Kitfox mailing list, http://dunkin.Princeton.EDU/.kitfox/kitlist/ Date: Fri, 12 Jul 1996 08:43:17 -0500 (EST) From: 00nzwilliams@bsuvc.bsu.edu Subject: Re: Bernoulli lie |>Is this a good time to ask whether Bernouilli actually knew what he was |>talking about? How many still believe an airplane flies because of the |>hump on the top of the wing? Or that, according to Jeppesen/Sanders, the |>upper surface of the wing contributes 75% and the lower surface |>contributes 25% to the lift? |The qustion is really whether to look at lift from Newton's or Bernoulli's |point of view. The answer is that they are both saying exactly the same |thing, Newton describing particle motion and Bernoulli describing the |average aggregate effects of all particles. |Anyone who say Bernoulli didn't understand aerodynamics simply doesn't. Well, it was a good troll. Most people never heard of Newtonian aerodynamics; I hadn't until recently. I believe Bernoulli is incomplete and I prefer Newton. I'm no physicist - never even had a formal course, I'll say up front - but it makes as much sense to me to say that compression is the reason for lift as it does to say suction. A flat plate dime store balsa glider flies because it has a positive angle of attack and squashes air downward. The 75/25 ratio in the top paragraph cannot be defended. One surface can't exist without the other and lift is nothing more than the pressure differential between the two; that's 50/50. I came to my understanding of lift, limited as it is, back in the '60s after reading an article in Air Facts (Leighton Collins ?). Recently I met a physicist in our EAA chapter, who has written a 110 page book titled Newtonian Aerodynamics Fundamentals. He's given presentations at OshKosh and Sun/Fun on the subject and has taken on the FAA about its required instruction in aerodynamics for private pilots. He sent them his book and challenged their correctness. FAA turned to the chair of the physics department at UTexas for advice and that official sided with Gale Craig, the author. He has in his possession a letter from the FAA admitting that he is correct and that they will have to change their test questions on the written exam. Those interested in the argument can buy the book from Regenerative Press, 1900 Romine Rd., Anderson, IN at $15.75. ISBN 0-9646806-0-2. I didn't start out to write a commercial; sorry. I have no financial interest in this matter and Gale Craig unaware of this post; he might even correct my statements here. I don't understand all the mathematics in the book, but I was able to follow it. Nyal Williams