Monday, November 25, 2013

Bernoulli Versus Coanda or What Keeps the Ball Flying

There was an insurance commercial where a woman proclaimed that if you read it on the Internet it had to be true. The more I read on the Internet the more I question the validity of anything I read but that is true for all sources of information.  The path that lead me to the research on air flow started with purchasing a simple kit at Radio Shack known as the RadioShack Turbo Air Kit, cost under $10.

Air Stream Pointed Straight Up

This project assembles very quickly and no soldering is required, what it demonstrates is flying a foam ball in a stream of air. Many people have done this same experiment with either a vacuum cleaner reversed or a hair dryer and a ping pong ball.  Once you get the ball positioned correctly in the air stream it tends to stay in the air stream even if the column of air is tilted but at a certain point the ball will fall out of the stream if tilted too far.

Air Stream Titled at an Angle

What started the confusion and the controversy is when I tried to research physics behind this. Air pressure is pushing the ball upwards with more force than the force of gravity pulling it down, I think everyone would agree on that. The physics that keeps the ball centered in the column of air is where opinions differ.

 Many people cited the "Coanda Effect" for the explanation but other people said it was because of Bernoulli's Principle.  Really trying to simplify the Coanda effect is that it deals with fluid moving around a curved surface. Bernoulli's principle correlate an increase in air velocity decreasing pressure. 

Back of Spoon in Water Stream

The classic example of demonstrating the Coanda Effect is holding the back of a spoon in a stream of water. There is an article on Wikipedia that disputes this experiment as a valid demonstration but this article is labeled as questionable. When doing this research this leads into the discussion of how an airplane wing actually works which appears to be an even bigger can of worms.

Bill Kuhl


Friday, November 8, 2013

Experimenting with NASA FoilSim Website

I have been interested in the more technical side of my hobbies and projects lately and took another look at the FoilSim program on the NASA website.  Your computer needs an update to current version of Java to run and there could be a splash screen about a risk to accept on to get things started.  Through your web browser many aspects of airfoil performance can be simulated in an interactive manner. There is a disclaimer stating that it is for educational purposes and not for designing aircraft.

Link to FoilSim:

Opening Screen with Default Values

To start with I used default values which provide a wing of 100 square feet flying at 100 mph then I selected the flat bottom airfoil and the flat plate airfoil. Next I started increasing the angle of attack from zero in one degree increments, output data from this included amount of lift, drag, and L/D. To me it was interesting to note that L/D increased until about 3 degrees and then started to diminish. Which means at this point the drag of the wing is increasing faster than the lift.

Starting at 0 Degrees with Flat Bottom Airfoil

Increasing to 3 Degrees

The same data was entered for a wing that might be typical for a small model airplane with a wing area of only 1 square foot flying at 15 mph. Output was somewhat different but followed the same basic pattern. Calculations were also done for the flat plate airfoil.

Changing to Model Size Wing Section

For me this is interesting because I had read many books on model aircraft design theory and it was fun to see the theory simulated. For one degree value of the model size wing I changed the aspect ratio from 4 to almost 8 to see the change in L/D, it improved considerably as predicted.  My data and some of the output were entered in an Excel spreadsheet manually.  Take a look at this fun simulator.

Entering Data into Excel

Bill Kuhl

For an explanation of many aspects of simple aerodynamics with math problems check out my article Basic Aerodynamics With a Lesson