1999-00 -- 05-06 Academic Years
Airplanes and Helicopters
29 February 2000: Bill Shanks (Joliet Junior College)
raised the question, What makes things fly? Following the usual textbook explanation, he sketched a wing cross-section, said that the air over the top of the wing had to travel farther than that on the bottom, and so had to move faster, resulting in lower pressure at the top wing surface than on the bottom - and so providing lift on the wing a la Bernoulli. But then Bill sketched a flat-looking wing (supersonic aircraft?) and pointed out there would be very little difference in path over top and bottom. Same for a kite. No asymmetry - no difference - no lift! But then Bill pointed out that - assuming a wing inclined upward relative to the air stream - air striking the bottom surface would be deflected downward. Then the change in its momentum would be downward, corresponding (Newton's 2nd Law) to a downward force on the air by the wing. But the air must be exerting an equal and opposite force (Newton's 3rd Law) on the wing, which would be upward - resulting in lift! So where does that leave Bernoulli? (or those who invoke his "law"?) Refreshing, Bill! For more information see the websites http://www.allstar.fiu.edu/aero/airflylvl3.htm and http://www.amasci.com/wing/airfoil.html.
09 December 2003: Don Kanner (Lane Tech HS Physics Teacher)
At our last meeting John Scavo was wondering how the number of blades on a propeller related to airflow and Richard Goberville had propeller whirligigs for all of us to take and experiment with. Don had collected all of the whirligigs from the other Lane Tech teachers at SMILE and modified them in an attempt to answer John's question. Here is the data table:
Helicopter Whirligig Data
|Number of Blades||Mass / grams||Result|
|2 blades||11.3||hits ceiling|
|2 blades||5.8||hits ceiling|
|2 blades||4.0||hits ceiling|
|2 half-blades||9.5||halfway up|
|1 blade (balanced)||9.5||hovers|
21 March 2006: Bill Shanks (Joliet
Whistle Balloon Helicopter
Bill shared with us a favorite physics toy -- the Whistle Balloon Helicopter http://www.toys2wish4.com/whbahe.html. It is also available as the Neato Balloon Helicopter. It is great for illustrating Newton's Third Law. A balloon is inflated and attached to a little nozzle on a rotor with three blades. As the air comes out of the balloon, it causes the rotor to spin and you get a little helicopter that flies up. Look for it at toy stores or museum stores.
And off we go! Thanks, Bill.