SUBJECT: Aeronautics
TOPIC: Circular airfoils
DESCRIPTION: A spinning paper tube generates lift as it travels forward.
CONTRIBUTED BY: Dale Bremmer, NASA HO
EDITED BY: Roger Storm, NASA Glenn Research Center

MATERIALS and TOOLS:
Center tube from paper towel roll 5 feet of surgical rubber tubing 3 ft of cloth ribbon (do not use slippery ribbon)
2 ft of bell or iron wire
C-clamp
Pliers

PROCEDURE:
1. Fold over about 1 inch of the rubber tubing and wrap it tight with a piece of wire. Twist the wire tight with the pliers and as doing so, form a loop about 1 inch in diameter in the wire.
2. Attach one end of the ribbon to the other end of the rubber tub with a piece of wire. Twist the wire tight.
3. Slip the wire loop over the screw shaft of the C-clamp. Tighten the clamp to the end of a table.
4. Lay the tube and ribbon across the table. Place the paper tube on top of the ribbon at its free end and roll it up snugly in the ribbon.
5. While keeping the ribbon from slipping, pull the paper tube back to stretch the rubber tubing. Release the tube. The tube will be spun as the ribbon is pulled off the tube while at the same time it is pulled forward by the contracting rubber tube. With enough speed and spin, the paper tube will lift off the table and may fly a loop through the air.

DISCUSSION:
When the paper tube is released, two motions are imparted to it. The tube is pulled rapidly through the air and at the same time it is spun. The combinations of motions generate aerodynamic lift. When seen from the side, air is made to flow over and under the tube as the tube moves forward. The tube's surface experiences a small amount of friction with the air. As it spins, the friction drags air in contact with the tube's surface around with it (see the figure). On the top of the tube, the air being dragged around with the tube and the air flowing over the tube are traveling in the same direction. Below the tube, the air being dragged around the tube and the air flowing under it are traveling in opposite directions. There, the air piles up and creates a small zone of relatively high pressure. On top of the tube the pressure is lower. If the tube is moving fast enough through the air and spinning rapidly enough, the over versus under pressure difference will be greater than the weight of the tube. The tube will begin flying and it will do so as long as its air speed and rotation are maintained. This is aerodynamic lift.

Rotating airfoils have been added to the wings of some high-performance airplane wings to increase lift under certain flying conditions. At least one early airplane designer unsuccessfully tried to build a plane with rotating cylinders instead of wings (see the film "Aeronautical Oddities").

What do you think would happen if the paper tube were rotated in the opposite direction as it moved forward? Place the ribbon on top of the paper tube and wind it up turning the top of the tube away from the rubber tubing. this should wind the ribbon in the opposite direction as the first trial. What happened when you stretched and then released the rubber tubing? Did the tube have any lift? Why not?

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Objects that Lift
Lift of Rotating Cylinder