Updated: July 20, 2012 http://www.allstar.fiu.edu/aero/airflylvl3.htm Funded in part by
The third description, which we are advocating here, we will call the Physical Description of lift. This description is based primarily on Newton’s laws.
So, why has the popular explanation prevailed for so long? One answer is that the Bernoulli principle is easy to understand. There is nothing wrong with the Bernoulli principle, or with the statement that the air goes faster over the top of the wing. But, as the above discussion suggests, our understanding is not complete with this explanation. The problem is that we are missing a vital piece when we apply Bernoulli’s principle. We can calculate the pressures around the wing if we know the speed of the air over and under the wing, but how do we determine the speed?
The preceding is an article by David Anderson, Fermi National Accelerator Laboratory, and Scott Eberhardt, formerly of the Department of Aeronautics and Astronautics, University of Washington, now at the Boeing Company. The authors have given ALLSTAR permission to present this article on the ALLSTAR website. The authors have written an updated copy of this article and provide it for all to use.
The above is recent writing by those who took us from Bernouilli to Newton. They wrongly say there is nothing wrong with the Bernouilli approach. However, they are not clear about whether the air the plane flies through is stationary, or as in a wind tunnel. See their Figures 2, 3 and 4, which show the air having velocity before the plane arrives. Whether the “plane” or the air is stationary or moving is crucial. This is because the air approaching the wing in a wind tunnel has momentum. The situation is very different in a wind tunnel.
Ivor Catt. 19 March 2016.
“There is nothing wrong with the Bernoulli principle, or with the statement that the air goes faster over the top of the wing.” – but the air is not moving! IC sept 2016
National Accelerator Laboratory
of Aeronautics and Astronautics University of Washington
Latest to be found here
Theory of Flight
In The Daily Telegraph, 3may01, p3, Robert Uhlig refers to a report by Dr. David Anderson in New Scientist, which I have not seen. However, I have read the report here .
Having been a technician in the RAF, and having done an engineering degree at Cambridge, I felt very foolish at having accepted the conventional (Bernoulli) theory of flight. Just by reading the brief report in The Daily Telegraph on 3may01, the conventional theory collapsed.
Three points arise.
The report “A Physical Description of Flight” over-gilds the lily, and obscures the key points which were clearer in The Daily Telegraph, although even there, they were not as clear as they could have been. The key points are;
An opportunity is given to us to contemplate the two versions of Newton’s Second Law of Motion.
Here we can compare and contrast two apparently identical mathematical statements of the law, and we realise that a mathematical statement is not what it appears. Otherwise, the two versions would be equally appropriate, which they are not.
In order to create lift for the aircraft, downwards momentum has to be imparted on some air. Acceleration is not a relevant factor.
I have for many years wondered about the lack of interest in the two versions of Newton’s Law. This discussion gives us much food for thought.
I had to study fluid dynamics to some degree at college, and notice that the Bernoulli theory is far from the conventional approach in that field, which is to take a “hands off”, distant, macroscopic approach on the grounds that fluid flow in detail is not well understood. I now realise that the Bernoulli approach flies in the face of that general principle in fluid mechanics.
Ivor Catt 25apr02
I shall restate 3 above. When I studied fluid mechanics in Cambridge in 1957, the reigning approach was to say that we did not know enough about the detail, so we should take a macroscopic approach. Thus, in the case of designing the shape of a boat’s hull, we would experiment on a model and separate out drag resulting from friction from drag resulting from turbulence. No one noticed that in the case of theory of flight, this approach was being flouted. Bernoulli is a microscopic approach, dealing with the local pressure at the surface of each part of the wing. In 3 above, in the tradition of the discipline “fluid mechanics”, I take the macroscopic view using Newton’s Second Law of Motion, distancing myself from the minutiae of the situation. The overall effect is lift, drag, and downwards momentum delivered to the air (plus the initial problem, which is the weight of the aircraft). Approach closer to the wing will result in confusion and unnecessary complexity. Now, what we have to do is deliver lift to the wing by giving downwards momentum to the passing air. In the process, we must try to minimise drag, and so minimise the required engine thrust.
I was amused by a friend, who reported that his pilot friend said that in order to fly upside down, the wing shape had to be changed by putting out the flaps. This was the pilot’s effort in extremis to save the Bernoulli theory. It took me some time to realise that under that theory, a plane could not fly the right way up with its flaps out.
Ivor Catt 24feb03
The extraordinarily shallow thinking that prevails in this context, even when billions of dollars are at stake, is demonstrated by the ICBM. In this case, the error goes right back to the Germans’ choice of Peenemunde, in a flat area near the sea.
Work done = force x distance.
Power = force x velocity.
In the case of a rocket, the objective is to deliver velocity to that part of the rocket that remains after the propelling fuel has been lost. The error seems to have been to think that work done = force x time. In fact, delivering a force to a stationary rocket is of no help, as we can see in the hours before launch, when the earth below the rocket delivered just that, and the rocket remained stationary. When the propellant is ignited below a stationary rocket, it largely takes over the job of the ground, at great expense.
The error in choosing Peenemunde was repeated when flat Florida was chosen.
Consider a rocket just after lift-off. With vertical lift-off, massive amounts of fuel are expended in holding the rocket more or less stationary just off the ground. This is what should have been avoided at all costs. Thus, the use of fuel to increase the velocity of a rocket should only be used when the rocket already has velocity. The launch of a rocket should take place in a mountainous area, and gravity used to give the rocket as much velocity as possible before take-off. This involves having the rocket descend from a high point down guiding rails, and then having its direction reversed by a curve in the rails, so that it leaves the ground at high speed in a vertical direction. Only thereafter should rocket fuel be brought into play, to be used more efficiently on a rocket which already has significant forward velocity.
Recap. What gives additional speed to a rocket is not force (provided by the rocket fuel), but power, which is force x velocity. Thus, force should only be applied after the rocket has achieved velocity, I suggest by the use of gravity.
Although it is possible that Florida and Texas were chosen for the US space programme because they were politically more powerful than the mountainous states, I suggest that their choice resulted from lack or thought rather than their greater political power.
Ivor Catt 10feb03
Amazing repetition of http://www.ivorcatt.co.uk/x32a2.pdf From 2001
See “Newspaper Comment” above.
Daily Telegraph 19 March 2016
THE Wright brothers would never have left the ground had they listened to modern aerodynamics experts, a leading physicist claimed last week.
David Anderson, an American government researcher, believes that since the first powered flight 98 years ago near Kitty Hawk in North Carolina, generations of students and aircraft designers have been misled by an incorrect explanation of the forces that keep planes in the air. It has led to a fierce dispute between mathematicians and physicists over the best way to explain how aircraft wings work.
Dr Anderson claims in a report in New Scientist that mathematicians, whose theory has had the upper hand until now, are fundamentally wrong. He said: "The standard explanation of how we fly is mostly myth. It's just wrong, but it has such a life of its own that you even see it on Nasa websites and in physics books."
Dr Anderson says Isaac Newton's laws of motion, postulated in the 17th century, provide a much better explanation of why planes fly than the latest computer calculations of fluid dynamic analysis, a branch of advanced mathematics. The popular explanation of flight, known as the Bernoulli principle, is that wings are sucked upwards because air has to move faster over the longer, top surface than over the bottom surface of the wing. The faster moving air creates lower pressure above the wing than below it, and the wing is "sucked" upwards.
But Dr Anderson says there is a crucial flaw in this theory: "If the shape of a wing determined lift, you could never fly upside-down." His explanation is that the shape of the wing does not matter because the angle of the wing to the oncoming air determines how well it lifts the plane.
Wings are forced upwards because they are tilted, which deflects air down, Dr Anderson says. Even the air flowing along the top of the wing is pushed down because of a phenomenon called the Coanda effect, which causes air to stick to the wing's surface.
He said: "The shape of the wing is the least important factor. Many fighters now have wings that are almost flat." Using Newton's explanation of lift, planes can fly upside down provided the pilot makes sure that the angle of the wing ensures that air is deflected downwards.
Anderson said: "If you look at a plane flying upside down, that's what happens. The pilot pushes the nose upwards, so that the wing attacks the air as if the plane was flying the right way up."
Engineers have accepted both the Bernoulli and Newton explanations of how wings work, but Dr Anderson said they would now have to discard the Bernoulli principle. He said: "Like the Wright Brothers, most aeronautical engineers use more experience than calculation when designing an aeroplane. Bernoulli is useful in calculation, but it doesn't explain lift."