Taiwanese Dragonfly

NAME_______________________________ PARTNER ______________________ DATE _______



To build a Traditional Taiwanese Dragonfly Toy and to use the toy to illustrate the concepts of rotation, as well as, study its ability to fly and determine how the pitch of the propeller blades affects its flight .




Digital photo of Dragonfly (Chinese Cultural toy), 2 pairs of wooden chopsticks, 3 disposable tongue depressors or Buster Bar sticks from Dairy Queen, small hacksaw, Power Drill, set of small drillbits, protractor, sandpaper, stopwatch, ruler, glue gun and glue sticks (optional)




When rotation of the Dragonfly has been imparted, the configuration of the propeller blades will be such that air causes an action of force. The interaction of force is developed by the displacement of air above and below the blades. We should also be able to observe and recognize the pressure differences between the upper and lower blade surfaces as an application of Bernoulli's principle. As the propeller spins, air passes across the blades at different velocities. This difference creates what is considered the lifting force on the upper surface necessary for flight. This lab can be used to observe and identify the relationship of energy, linear and angular momentum, forces, mass, and velocity. The equation which shows the relationship of angular momentum, mass, and velocity is L = mvr sinf . L represents the angular momentum, m is the mass of the moving object, and v is the speed of the mass moving in a rotating motion, and r is the radius of the propeller blades.




  1. Place all materials out on the lab table surface.
  2. Measure and make sure that your tongue depressor is at least 14-15 cm long.
  3. Draw a line in the middle of the tongue depressor along the long axis, now measure about 7-7.5 cm from each end to find the midpoint of the long axis and mark the midpoint of that line. Measure and draw a line 1.0 cm from each side of the perpendicular at the midpoint and long axis.
  4. Draw another line vertically bisecting the tongue depressor..
  5. Sand the tongue depressor on one side and do the same on the diagonal side of the top surface of the tongue depressor. Remember not to sand over the vertical line and the line 1.0 cm from either side of the marked midpoint.
  6. Drill a hole at the midpoint, just large enough for the diameter of the wooden chopstick to fit snugly through the tongue depressor.
  7. Cut the chopstick to the length of 16cm to 20cm long.
  8. Once the chopstick centering hole is drilled at the midpoint of the tongue depressor, use the glue gun and spot glue the tip of the chopstick. Now place the chopstick into the drilled centering hole of the tongue depressor and allow to dry for a few seconds.
  9. Change the length of the string to 60cm long.
  10. Repeat procedures 2 through 8 with varying sanding angles on the tongue depressor. Preferably one prop that is thin, one medium, and one thicker for comparisons.
  11. Check for drying and then proceed to fly your Dragonfly a few times before recording any data on the sample data table.
  12. When you have had enough practice runs or (trials), discuss the possible physical quantities that can be measured. Create the appropriate data table for measurable quantities. Allow one member of your lab team to do the flying and another teammate taking the data (i.e. timings for the duration of flight) for the homemade Dragonfly.
  13. Record all data in the data table

Sample DATA Table


Mass of the Dragongfy (g)


Angles of Propeller


Time (s)




Speed w and u (rad/s, m/s)


Angular Momentum (mvrsinf )




  1. After completing all of the trials in the lab, describe any relationship observed between the conservation of energy, linear and angular momentum, mass of the object, velocity of the motion, and the angle of pitch of the blades.
  2. Draw a graph comparing propeller angles of the Dragonfly and duration of the motion. Does it appear to represent a relationship of angle(A) = 1/2a t2 ?
  3. Draw a graph comparing speed of the motion and duration of the flight.
  4. Draw a graph comparing speed of the motion and propeller angles.
  5. Which Dragonfly had the potential to stay in the air the longest?
  6. Which Dragonfly had traveled the longest distance?
  7. Explain how the Dragonfly works.
  8. Can Bernoulli's Principle (P + 1 / 2 r n 2 +r gh) be used to explain the Dragonflys ability to fly?
  9. Draw a free-body diagram and label all external forces acting on the Dragonfly. Identify the regions of low and high pressure.
  10. Can the lift force of the Dragonfly be approximated by the standard airfoil treatment of lift supplied by a wing? Consider the Lift force approximation shown by: L = 1/2CLr u 2A when CL 2p sina 1 + 2 /A


A couple of current references are:

C. Waltham, "Flight without Bernoulli," Phys Teach. 36, 457-462 (1998)


H. Tennekes, "The Simple Science of Flight," MIT Press, Cambridge, MA 1996.

D. Oliver and T. Ng, "Rubber Band Driven Airplane Contest," Phys Teach.. 37, 108-109 (1998)